TRANSPORT AND ROAD RESEARCH LABORATORY Department of Transport

RESEARCH REPORT 134

SLOW SPEED "DYNAMIC" AXLE W E I G H E R S :

EFFECTS OF SURFACE IRREGULARIT IES

by J Prudhoe

The views expressed in this Report are not necessarily those of the Department of Transport

Vehicles and Environment Division Vehicles Group Transport and Road Research Laboratory Crowthorne, Berkshire RG11 6AU 1988

ISSN 0266-5247

CONTENTS

Page Page

Abstract 1 5. Results of the tests

1. Introduction 5.1 Test 1: Weights of the 2-axle vehicle,

measured statically and at slow speed

2. Operation of the slow speed axle weigher 5.2 Tests 2 to 6: Calculation of weight

difference

2.1 Description of the weigher

2.2 The procedure for check-weighing vehicles

2.3 Principle of operation of the weigher

3 5.3

5.4

Tests 2 to 6: Ranges of weight difference

Tests 2 and 3: Effects of height differences between apron and weighbeam

9

13

3. Preparations for the tests

4.

3.1 Verification of the apron's constructional standards

3.2 Verification of the weigher's calibration

The tests

5

5

5.5

5.6

5.4.1 Tests with the 2-axle vehicle

5.4.2 Tests with the articulated vehicles

Test 4: Effects of raising part of the approach or exit apron

Test 5: Effects of placing a narrow board across the approach apron

13

13

16

16

4.1 The vehicles tested at TRRL

4.2 Test 1: Static and slow speed weighings of the 2-axle vehicle

4.3 Tests of effects of large area deformations

4.3.1 Test 2: Raising the weighbeam

4.3.2 Test 3: Raising the apron

5.7

5.8

5.9

5.10

Test 6: Effects of placing discs on the approach apron

Test 7: Repeatability of the reference readings

Test 8: Results from the weigher at Crick

Test 9: Results from the weigher at Towcester

17

17

18

• 18

4.4

4.3.3 Test 4: Raising part of the approach or exit apron

Tests of effects of small area deformations

6. Summary of test results

7. Discussion

7.1 Effects of variations in apron profiles

19

19

19

4.4.1 Test 5: Placing a narrow board across the approach apron 7

7.2 Effects of variations in axle weights during weighing 20

4.5

4.4.2 Test 6: Placing discs on the approach apron

Test 7: Comparison of repetitive slow speed weighings

8 7.3 The performance of the weigher

8. Recommendations

9. Conclusions

20

21

21

4.6 Tests at Department of Transport axle weighers

4.6.1 Test 8: Weighings using the instrument at Crick

4.6.2 Test 9: Weighings using the instrument at Towcester

10. Acknowledgements

11. References

© CROWN COPYRIGHT 1988 Extracts from the text may be reproduced,

except for commercial purposes, provided the source is acknowledged

21

22

Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on 1 st April 1996.

This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.

i

'1

SLOW SPEED "DYNAMIC" AXLE WEIGHERS: EFFECTS OF SURFACE IRREGULARITIES

A B S T R A C T

Enforcement officers check-weigh goods vehicles using slow speed ('dynamic') axle weighers. Each weigher consists of a weighbeam which is installed in a length of concrete (the 'apron') laid to a precision of _+3 mm and which measures the loads imposed by axles as they pass over it axle by axle at a speed of not more than 2.5 mile/h.

TRRL has test-weighed goods vehicles on three axle weighers to determine how the precision of each apron's construction influenced the accuracy of weighing vehicles. This report describes the test results and their implications on the accuracy with which vehicles can be weighed.

The verification procedure that is used to check a weigher's accuracy is discussed.

1 I N T R O D U C T I O N

Until about 1975, the weights of goods vehicles were checked by weighing entire vehicles on a whole- vehicle weighbridge. However, road wear is more directly related to the loads imposed by axles than to vehicles' total weights so 'slow speed axle weighers' have been introduced. These instruments, commonly referred to as 'dynamic axle weighers', are now installed at about 60 Department of Transport sites in Great Britain and more are planned. They are used by enforcement officers to ensure that the gross weights and the axle loads of goods vehicles are within the legal limits.

Each weigher consists of a weighbeam which is set into a length of precision-laid concrete (the 'apron') and which measures the loads imposed by axles as they pass over it axle by axle at a speed of not more than 2.5 mile/h; see Figure la. The weighbeam is connected to a remotely-located reading and printing console; see Figure lb. The use of the weigher enables vehicles to be weighed more rapidly than by static means, minimising delays to drivers.

Proving trials conducted on these weighers commenced in 1976, firstly using a machine on a lay- by adjacent to the A30 Staines Bypass (Kilsby 1976) and later on two other weighers. The vehicles tested were rigid vehicles with 2, 3 and 4 axles and articulated vehicles with 3, 4 and 5 axles. In almost all of the trials, the sum of the axle weights of each vehicle agreed with its total weight, previously established on a local conventional weighbridge of known accuracy, within _+100 kg per axle multiplied by the number of axles of the vehicle.

Based on the results of the proving trials, a Code of Practice (Department of Transport 1981)was prepared. It describes the procedure for enforcement weight checks and defines the standards of construction of sites and periodic verif ication of performance for the weighers. These standards are high. The apron has to be constructed to be flat and level to within a level tolerance of _+3 mm for a distance of 8 m on either side of the weighbeam. In the verif ication checks (currently conducted every six months) vehicles are test-weighed on the weigher and, for each vehicle, the sum of the recorded axle weights has to agree with the total vehicle weight, established via a whole-vehicle weighbridge, to within a tolerance of _+100 kg per axle multiplied by the number of axles of the vehicle.

Constructing and maintaining the weigher's concrete apron to the required level tolerance and periodically re-verifying the weigher is costly. It has been suggested that the standards are unnecessarily high and that they should be relaxed provided that weighing accuracy is not impaired. Accordingly, in 1984, TRRL was asked to assess the performance of a DTp-approved 'Weighwri te ' slow speed axle weigher recently installed on the Laboratory's Research Track.

The principal objective of the assessment was to determine the extent to which the surface of the apron could be made more uneven before the resulting weighing inaccuracies exceeded the Code's verification standards. Addit ional objectives were to establish the accuracy of the weigher to weigh vehicles both statically and at slow speed when in its normal state of use, and to determine the effectiveness of the Code's verif ication procedure.

This report describes the weigher at TRRL and the procedure for using it in routine weighing. Subsequent sections describe the methods used to temporari ly deform the apron when weighing vehicles and the results obtained. Also described are the weighing of vehicles on the TRRL weigher and on two other Department of Transport axle weighers when in their normal state of use and the results obtained. Finally, the report discusses the implications of these results for relaxing the construction and verif ication standards and presents conclusions.

2 O P E R A T I O N OF THE S L O W SPEED A X L E W E I G H E R

This section describes a typical installation (the one at TRRL), its principle of operation and the way in

Weighbeam

Concrete apron ~ Concrete apron

Control console

Fig. l a Plan of apron and weighbeam (not to scale)

Concrete apron

Weighbeam

cell !-i

Electrical connecting cable

Digital display Printer

Control console

Fig. l b Details of weighbeam and console

2

which it would be used to check-weigh goods vehicles. The design and operation of other systems may differ in detail.

2.1 DESCRIPTION OF THE WEIGHER The apron is built on a stable foundation and is about 40 m long and 3.5 m wide. In accordance with the Code, its surface is made to be nominally level and free from surface defects to within a level tolerance of ___3 mm for a distance of 8 m on either side of the weighbeam. In this respect, the apron's constructional standards are considerably tighter than those for the country's public highways so that the loads imposed on the apron by the slow-moving axles being weighed will be reproducible and fully representative of the loads imposed on actual highways under the same circumstances. (Weight readings are not necessarily reproducible on surfaces which are more irregular than that of the apron.)

The weighbeam is 0.76 m long by 3.05 m wide and sits in a transverse pit cut into the centre of the apron so that its top surface is nominally flush with the apron's surface (see Figure la and Plates 1 and 2). Each corner of the weighbeam rests on a load cell. These are connected by an electrical cable to a remote control/display console (see Figure lb). The equipment is mains-powered and is normally left switched on when not in use so that it can be used at any time without having to wait for the equipment to warm up. When 'Automatic' (dynamic) mode is selected, the weigher displays and prints the weight of an axle moving slowly across the weighbeam at the instant when its weight is evenly distributed on the beam.

2.2 THE PROCEDURE FOR CHECK- WEIGHING VEHICLES

This section describes the procedure for weighing goods vehicles recommended in the Code of Practice; this method was used when weighing vehicles during the tests described in this report.

Before weighing a vehicle, the apron is inspected and, if necessary, swept to remove debris (stones etc). Two console settings are then checked and adjusted if necessary. The first of these, the 'zero setting', controls the reading that the console displays when there is no load on the weighbeam. The other, the 'calibration setting', controls the reading of an internally-generated signal simulating the application of a 16 tonne static load. These two settings are checked on each occasion that a vehicle is weighed.

The vehicle to be weighed is stopped about 6 m from the weighbeam and the driver is instructed 'to drive across the weighbeam at a steady speed not exceeding 2.5 miles per hour; during this run the driver must neither accelerate nor use his brake. This can normally be achieved by engaging lowest

forward gear and driving at a tick-over speed over the weighbridge.' As each axle of the vehicle passes over the weighbeam, the console displays a digital reading in tonnes to two decimal places corresponding to the load imposed on the weighbeam. The same reading is also printed (in black) on a paper roll. If the vehicle's speed exceeds 2.5 mi le/h, the reading is printed in red and such readings are not admissible as evidence of overload in judicial proceedings. If the speed is too high, the driver is asked to make another pass at a lower speed. When the operator sees that the vehicle's last axle has passed over the weighbeam, he presses a button causing the sum of the loads imposed by the individual axles to be printed.

2.3 PRINCIPLE OF OPERATION OF THE WEIGHER

The weighing procedure described above is commonly known as 'dynamic weighing'. If the vehicle passes over the weighbeam suff iciently smoothly, its speed (at not more than 2.5 mi le/h), is slow enough not to excite substantial oscillation of the vehicle's suspension which would give rise to dynamic variations of an axle's weight as it passes over the weighbeam. If any small oscillations do occur, several cycles will elapse during the t ime for which the axle is being weighed. Hence, the mean value of the signals from the load cells displayed will be representative of the static load imposed by the axle.

When the weigher is new or after it has been repaired, standard metal weights (traceable to national standards) are placed on the weighbeam and the static weight indicated has to be within a tolerance of ___10 kg of the true applied weight. When a vehicle's axle passes slowly and steadily over the weighbeam ie at a speed of not more than 2.5 mi le/h, the weigher can be expected to indicate the load transmitted to the weighbeam with an accuracy close to this same _+10 kg static tolerance (as explained above). However, the load transmitted by each individual axle, particularly by those forming part of a bogie, may vary with the motion of the vehicle due to the complex forces generated in the suspension system. These forces are related to the design and condition of the suspension system and its interaction with the surface profile of the road or apron. As a result, the load imposed by an axle on a surface continually changes as the axle moves over the surface.

Thus, the load imposed by a particular axle may vary between successive weighings. (These variations in load may occur even when the vehicle is stationary when weighed, since the forces imposed on the axle by its suspension system during the vehicle's movement may not be ful ly released when the vehicle stops.) Axles comprising part of a bogie, particularly one with an ineffect ive load-compensating mechanism, are more likely to exhibit these variations

i

Neg. no. CR457/84/8

Plate 1 Raising the height of the w e i g h b e a m (Test 2)

Neg. no. R554/84/1

Plate 2 Rais ing the he igh t of the a p p r o a c h and ex i t a p r o n s (Test 3)

than are independently-sprung axles. To accommodate these variations, the Code currently accepts that a tolerance of _+100 kg per axle has to be assumed in respect of each weighing for verification purposes and a greater allowance of _+150 kg per axle is given for the purposes of enforcement. Air suspended or hydraulically suspended bogie axles are less likely to display large fluctuations in load than are mechanically suspended axles.

3 P R E P A R A T I O N S FOR T H E TESTS

Shortly before tests of the weigher at TRRL commenced, the apron was surveyed and the weigher's static and slow speed weighing calibrations were checked to ensure that the Code's requirements were satisfied. The static and slow speed checks were repeated six months later in 1985.

3.1 VERIFICATION OF THE APRON'S CONSTRUCTIONAL STAN DARDS

The apron of the weigher at TRRL was designed to be level in a longitudinal direction ie along its 40 m length. However, it had to be constructed with a crossfall (about 1 in 75) to match that of the lane of the Laboratory's Research Track in which it was installed. This crossfall has no effect on the load that each axle imposes although it will cause the loads imposed by the tyres at opposite ends of each axle to be slightly different.

The apron of the weigher had been comprehensively surveyed shortly after its construction in 1983 and the results were found to satisfy the Code's level requirements. However, a second survey was conducted before the tests commenced. In this survey the levels on the apron were measured at 1/3 m intervals along longitudinal reference lines extending 5 m on either side of the weighbeam; this distance was greater than the spacing between the first and last axles of any bogie of the test vehicles. The levels along each line were found to be within the Code's level tolerance of _+3 ram.

3.2 VERIFICATION OF THE WEIGHER'S CALIBRATION

The static calibration checks were made in accordance with the Code's procedure. In these checks, a load of 16 tonnes was loaded onto the weighbeam in 1 tonne increments using weights traceable to national standards, to confirm that the weighing tolerance was within the Code's stated limits. The static calibration was also checked before each day's use of the weigher. This check consisted of statically weighing each axle of a loaded 2-axle vehicle ie when it was stationary with an axle on the

weighbeam. The sum of the indicated axle weights (the vehicle's total weight) was generally within 30 kg of the true total weight of about 16 tonnes.

Slow speed verification checks were conducted after the static calibration tests. The weigher was verified in both directions of movement across the weighbeam following the procedure specified by the Code. In this, laden goods vehicles of accurately- known weight were test-weighed to check that the scatter between weighings was within ___100 kg per axle.

For the initial verification check made soon after the weigher had been installed in 1983, the equipment had to be adjusted until the indicated readings of weight agreed with the required readings.

The verif ication checks made in 1984 and 1985 showed that no further adjustments were necessary because, for each of the 3 goods vehicles used for verif ication (a 2-axle rigid vehicle, a 4-axle rigid vehicle and a 4-axle articulated vehicle), the weighing tolerances were within the l imits of ___100 kg per axle for both directions of movement across the apron. These results confirmed that the crossfall of the apron has no detectable effect on the accuracy with which vehicles can be weighed by the slow speed method.

4 T H E T E S T S

One set of tests consisted of weighing vehicles on the TRRL axle weighecand on two other Department of Transport axle weighers when the instruments were in their normal state of use. These tests were intended to determine the accuracy of weighing when the weighers were being operated normally. The other set of tests was intended to reveal the effect of uneven surfaces over the weighing area on the load imposed by an axle on the weighbeam. Accordingly, each of these second set of tests comprised two sets of weight readings ie with and wi thout plywood sheets (or .discs) placed on the apron or weighbeam, to produce a controlled degree of unevenness. To reduce experimental error, each set consisted of f ive readings of each measurement, from which the average was calculated. Placing sheets (or discs) on the weighing area before weighing vehicles was only done for test purposes; it would be incorrect to weigh vehicles for verif ication or enforcement purposes with the sheets in position.

In each test, the plywood sheet (or disc) was placed in position (see Figure 2) and then the vehicle being assessed was driven slowly and smoothly over the weighbeam five t imes in succession at a speed of not more than 2.5 mi le /h and in accordance with the requirements of the Code of Practice (see Section 2.2). For each run, the indicated weight readings for the loads imposed by each axle and all axles (the

E x i t apron Weighbeam Approach apron

/ Direction of test vehicle

Test 2 P lywood sheet laid on weighbeam to raise its height relative to apron

Test 3 P lywood sheets laid on approach and ex i t aprons to raise height o f apron relative to weighbeam

Test 4 P lywood sheet laid on approach apron (and subsequently on exi t apron) and progressively moved away f rom weighbeam

.. .... ' ~ rT f7 / / iiii iiiiiiii!j/f,, ,, : i : i : i i i : ~ yL~ / : / , /

:/:_:J / : _// / ~

#.j" // // '/ // // i

/ / / / i Z J z J • (

Test 5 Narrow board laid across approach apron and progressively moved away f rom weighbeam

,::!::::::::: : : : : : : : : : : : : : : : : : : : : : : : : : '

.:,,~iiiiiiiiii~iiiiiiiiiiiiii~: .... , :,,:iiiii!iiiii~iiiiiiiiiii;i~': .... ,

.... :iiiiiiiiiiiiiiiii~ii!iiiiiiiiii!iiiii:~ ..... " - ~ - * -

/

/ / 4 ~ - 1.4m

/

/ Test 6 P lywood discs (up to 4 in number) posit ioned on approach apron, 1.4m f rom centre of weighbeam

Not to scale Fig. 2 D e t a i l s o f t he tests

indicated total weight of the vehicle) were recorded. The sheet was then removed and the vehicle was again driven five times in succession over the weighbeam to provide reference weight readings for the axles.

At the end of each day of tests, the vehicle (with driver) was weighed on at least one whole-vehicle weighbridge, the accuracy of which was routinely checked by Trading Standards Officers.

4.1 THE VEHICLES TESTED AT TRRL Tests at TRRL, referred to as Tests 1 to 7, employed six laden goods vehicles representing a range of types in common use. These included three of the four types of vehicle specified in the Code's slow speed 'verification procedure ie a 2-axle vehicle and articulated vehicles of 4 or 5 axles. The remaining type specified (a 4-axle rigid vehicle) was not tested.

Five of the six vehicles had conventional steel leaf- spring suspensions. They were a 2-axle vehicle weighing about 16 tonnes and four articulated vehicles weighing 32-38 tonnes. These four vehicles were a 2-axle tractor towing a 2-axle trailer, a 2-axle tractor towing a 3-axle trailer, a 3-axle tractor towing a 2-axle trailer and a 3-axle tractor towing a 3-axle trailer. Each of these four articulated vehicles consisted of a conventionally suspended tractor coupled to a trailer having a bogie of interconnected leaf-spring suspended axles. The sixth vehicle was a 6-axle combination weighing about 32 tonnes. Its tractor had leaf-spring suspension and the trailer bogie had three inter-connected air suspended axles.

Four of the five combinations had a 'bogie spacing' (the spacing between the centres of adjacent axles of the bogie on the tractor or trailer), of 1.3 m-1.4 m (a commonly occurring range). The exception was the 4-axle vehicle--the spacing between axles of its 2-axle trailer bogie was about 2.0 m.

4.2 TEST 1: STATIC A N D SLOW SPEED W E I G H I N G S OF THE 2-AXLE VEHICLE

During this test the axles of the 2-axle vehicle were weighed twice; firstly when the vehicle was stationary with no brakes applied and secondly, as it moved at slow speed and in accordance with the Code of Practice (see Section 2.2). Ten sets of weighings were obtained. During the test the axle weigher was in its normal state of use (ie without any material on the weighbeam or apron) and shortly after the static calibration had been checked and found to be within ___10 kg.

4.3 TESTS OF EFFECTS OF LARGE A R E A D E F O R M A T I O N S

This section describes the positions and thicknesses of the plywood sheets used for Tests 2, 3 and 4. As

stated earlier, sets of indicated and reference weights were recorded for each location of the sheets. All six goods vehicles were used for Tests 2 and 3 but only three of them were used for Test 4; see Section 5.

4.3.1 Test 2: Raising the weighbearn A large 2 mm thick sheet was laid over the weighbeam to raise its height relative to the apron (see Plate 1). The test was repeated using 4 ram, 6 mm and 9 mm thick sheets.

4.3.2 Test 3: Raising the apron The height of the apron was raised relative to the weighbeam by placing large sheets, 2 mm thick, on both the approach and exit aprons, the test vehicle passing over a sheet immediately before and after crossing the weighbeam (see Plate 2). The sheets covered the apron's 3.5 m width and extended along the apron's length for a distance of about 3 m on either side of the weighbeam. The test was repeated using 4 mm, 7 mm and 9 mm thick sheets.

4.3.3 Test 4: Raising part of the approach or exit apron

This test and each test of small area deformations (see Section 4.4) was confined to the articulated vehicles, since they show the effect on bogie axles of placing sheets of different thicknesses near the weighbeam.

A large sheet, 9 mm thick, was placed on the approach apron close to the weighbearn so that the test vehicle passed over the sheet immediately before crossing over the weighbeam. The sheet covered the apron's 3.5 m width and extended along its length for a distance of about 3 m. The gap between the sheet and weighbeam was then increased in stages until it had increased to 9 rn. Then the sheet was transferred to the exit apron and the above test repeated.

4.4 TESTS OF EFFECTS OF S M A L L A R E A D E F O R M A T I O N S

This section gives details of the positions and thicknesses of the board and discs used in Tests 5 and 6 respectively. As in Tests 2 to 4, sets of indicated and reference weights were recorded for each position of the board or discs. Only three of the six goods vehicles were used for these tests, as stated in Section 5.

4.4.1 Test 5: Placing a nar row board across the approach apron

A narrow board was used to simulate a ridge running across the apron, raising the height of all wheels on a single axle as the vehicle passed over it. The board, which was 9 mm thick and 3.5 m by 300 ram, was placed across the approach apron so that its

3.5 m long side was parallel and adjacent to the 3 m side of the weighbeam. Hence, all wheels of a vehicle's axle passed simultaneously over the board before passing over the weighbeam. The distance between the board and the weighbeam was then noted, being the distance from the centre of the board to the position on the weighbeam at which the weigher measures the load imposed by an axle. This position is 110 mm past the centre of the weighbeam and is termed the 'effective centre' of the weighbeam in this report. The board was then moved in stages further away from the weighbeam until the distance had increased to about 3 m. Readings were also obtained using a 3 mm thick board.

4.4.2 Test 6: Placing discs on the approach apron

In these tests, 150 mm diameter discs were placed on the approach apron in order to simulate deformations of similar area to a tyre's footprint. The discs were positioned on the apron with their centres about 1.3 m from the "effective centre' of the weighbeam. An observer aligned each disc to lie under the wheels of each successive axle as the vehicle passed by. At a distance of 1.3 m, the wheels of a bogie axle would pass over the discs at the same t ime as the wheels of an adjacent bogie axle passed over the "effective centre' of the weighbeam. (The 4-axle articulated vehicle which had a 2 m spacing between the axles of its trailer bogie was not tested.)

4.5 T E S T 7: C O M P A R I S O N OF R E P E T I T I V E S L O W S P E E D W E I G H I N G S

The previous tests required each vehicle to be driven over the weighbeam, with no sheets or discs in position, up to 65 times in a single day in order to provide reference weights. The true total weight of each vehicle did not alter during the day (apart from a small loss in weight due to the fuel used) so these reference results were compared to enable their repeatabil i ty over several hours of use to be established.

4.6 T E S T S A T D E P A R T M E N T OF T R A N S P O R T A X L E W E I G H E R S

Additional tests were conducted at Department of Transport axle weighers at Crick and Towcester in Northamptonshire in order to test the accuracy of the installations when weighing a variety of vehicles in current use. A t both sites, recent surveys had confirmed that the level tolerances were within the ___ 3 mm required by the Code of Practice.

4.6.1 Test 8: Weighings using the instrument at Crick

Northarnptonshire Trading Standards Department had conducted a routine 6-monthly verif ication in April

1986 in which three laden goods vehicles with conventional suspension were used; a 2-axle vehicle, a 4-axle rigid vehicle and a 5-axle articulated vehicle consisting of a 2-axle tractor coupled to a 3-axle semi-trailer. The respective bogie spacings (the distance between the centres of adjacent bogie axles) for the rear bogie of the 4-axle vehicle and the trailer bogie of the 5-axle vehicle were 1.54 m and 1.37 m.

Five additional laden goods vehicles were hired by TRRL and used in a further verification check in the following month. These vehicles were 4-axle and 5-axle articulated vehicles, consisting of 2-axle and 3-axle tractors coupled to conventionally suspended 2-axle semi-trailers of less commonly occurring bogie spacings, the values ranging from 1.25 m to 2.05 m.

4.6.2 Test 9: Weighings using the instrument at Towcester

This axle weigher had recently been recommissioned after completion of repairs to the recorder house and the verification procedure involved the same three vehicles that had been used at Crick.

TRRL conducted a goods vehicle survey at this site in May 1986. During the exercise, goods vehicles were weighed at slow speed by means of the axle weigher. They were then weighed statically on an adjacent level stretch of road using calibrated "Haenni' wheel load scales which are portable static load measuring devices (usually referred to as weigh pads) and are designed to measure the imposed loads of vehicle wheels sitting on the pads.

The method involved placing pads in front of the wheels of the vehicle to be weighed. The vehicle was then driven onto the pads so that the indicated weights imposed by the wheels could be noted. Eight pads were available enabling any vehicle with up to four axles to be weighed in one operation.

5 RESULTS OF THE TESTS

The results for Tests 1 to 9 are described in this section.

5.1 TEST 1: W E I G H T S OF THE 2 -AXLE V E H I C L E , M E A S U R E D S T A T I C A L L Y A N D A T S L O W SPEED

The results for the 10 sets of readings are shown in Table 1. The individual axle weight readings made at slow speed are within about ---30 kg of the average values measured with the vehicle stationary. The ranges of load variation recorded for the axles are 30 kg (axle 1) and 40 kg (axle 2) for slow speed weighing and only 11 kg and 18 kg respectively for the static weighing. For the slow speed weighings, it seems likely that the variability in the load imposed on the weighbeam by a particular axle is greater

TABLE 1

Results of Test 1

Loads imposed by axles of the 2-axle vehicle (kg)

10 static weighings:

Type

10 slow speed weighings:

True vehicle weight (from

of weighing

minimum weight average weight maximum weight

minimum weight average weight maximum weight whole-vehicle weighbridge)

Axle 1

6 065 6 069 6 076

6 040 6 055 6 070

Indicated Weight (kg)

Axle 2

9 757 9 764 9 775

9 750 9 776 9 790

Axle weight

s u m

15 825 15 833 15 851

15 810 15 831 15 860 15 815

No tes The minimum (or maximum) total weight of the vehicle is not necessarily the sum of the two minimum maximum) weights of the axles shown in the Table.

(or

because of variations in the motion of the vehicle over the weighbeam. These results confirm the comment made in Section 2.3 that, during slow speed weighing, the instantaneous load imposed by each axle is indicated with an accuracy close to the _+10 kg tolerance for static weighing, provided that the vehicle is driven in accordance with the Code of Practice.

Individual determinations of the vehicle's total weight, obtained by summing the weight readings of the two axles, exceed the true total weight by only 36 kg (static weighing) and 45 kg (slow speed weighing). The ranges obtained for the indicated total weight of the vehicle are 26 kg (static weighing) and 50 kg (slow speed weighing) respectively. The test results show that, for vehicles which do not have multi-axle bogies, the axle weigher is significantly more accurate than is called for in the Code of Practice.

5.2 TESTS 2 TO 6: CALCULATION OF WEIGHT DIFFERENCE

Tests 2 to 6 involved placing plywood sheets (or discs) on the apron or weighbeam as shown in Figure 2. For these tests, sets of measures of a weight difference A were calculated for comparison purposes. For each axle of a vehicle, A is the average of the five indicated weights in a particular test condition minus the average of the five readings in the reference condition. For a bogie, A is the average of the five loads that the combined axles of the bogie imposed in the test condition minus the corresponding average load imposed in the reference condition. For the total vehicle weight, A is the average of the five sums of individual axle weight readings in the test condition (ie the vehicle's

average indicated total weight) minus the true vehicle weight, as determined by a whole-vehicle weighbridge.

For each vehicle tested, a value of A and a range representing the scatter amongst readings were calculated for each axle, the bogie (or bogies) and all axles (the vehicle weight). In the graphs (see Figures 3 to 6), the weight differences measured in the tests are shown in terms of kg per axle. For a bogie the value of A divided by the number of axles in the bogie is plotted and, for the total weight of a vehicle, A divided by the number of its axles is plotted.

5.3 TESTS 2 TO 6: RANGES OF W E I G H T DIFFERENCE

The repeatability of the test readings is indicated on the graphs by bands representing the range of A at a particular condition in terms of kg per axle. When the range is small (about 30 kg or less) it is represented as a single point.

During the tests it was found that the range of A per axle at a given condition is least for air suspended trailer axles--most values do not exceed 20 kg (see Figure 3a). For individual axles of the bogies with leaf-spring suspensions the ranges of A per axle are much higher, being up to 300 kg for one of the trailer axles of the 5-axle articulated vehicle with the 2-axle trailer. (The results for the individual axles of this vehicle are not plotted.) For sets of coupled axles acting as bogies, ranges of A per axle of up to 77 kg and 185 kg for 3-axle semi-trailer and 2-axle tractor bogies respectively are found; these results are within the 200 kg per axle range of the verification tolerance of _+100 kg per axle. (Figure 3b shows results for trailer bogies; results for tractor axles are not shown.)

9

• • • /~ and its range

/~ per axle = (average indicated weight of axle w i th p l ywood in pos i t ion) -- (average indicated weight o f ax le, when apron and weighbeam are level)

A == v

<3

oJ

" O

E= O3

0 ;

<

1200 --

1100 - -

1000 --

900 --

800 --

700 - -

600 - -

500 - -

400 - -

300 - -

200 - -

100 --

0

- - 100

-- 200

-- 300

- - 400

-- 500

-- 600

-- 700

- - 800

-- 900

- -1000

- -1100

- -1200

- -9

Ax le 2 ~ ~ ~ . . ~ - . - - ' 1 ~ ' ~ " ~ - ~ J ~

)-- Ax le1 / O / / , / ~ / s ,

/ / / / , , ,' / / / , , ,'

Ax le 2 / ¢ J

• /

m j s

s o s •

S p

t /

i" Axle a I I I I I I I I

Vehicle w i th convent ional ly Axle 1 -suspended trai ler .,

I I

S o /~,~Axl e 2 Y/////...;:;' s xle 3 / . ; "

/ . 4" s

s

Vehicle wi th air suspended trai ler

I I ! I I I

4 5 6 7 8 - - 8 - - 7 - - 6 - - 5 - - 4 - - 3 - - 2 - - 1 0 1 2 3

Change in weighbeam height (mm)

Change in weighbeam height is posi t ive i f the height of the weighbeam, relative to the apron, is increased

Fig. 3a Effect on axle weight difference A (for individual trailer axles of 6-axle articulated vehicles) of changing the height of the weighbeam relative to the apron (Tests 2 and 3)

10

v = x m

t, CL

<~

" 0

r ~

1 6-axle artic, with 3-axle air suspended trai ler 2 4-axle artic, with 2-axle convent ional ly suspended trai ler 3 5-axle artic, with 2-axle convent ional ly suspended trai ler 4 5-axle artic, with 3-axle convent ional ly suspended trai ler 5 6-axle artic, with 3-axle convent ional ly suspended trai ler

0 • a and its range

a per axle = [(average indicated weight of bogie with plywood in posit ion) -- (average indicated weight of bogie when apron and weighbeam are level)-] number of bogie axles

1100

1000

900

800

700

600 ~ 2

500

400

300

200

1 O0

0

- - 100'

- 200

-- 300

__~-__~_

-- 400

-- 500

-- 600

700

-- 800 / "

-- 900 5 / 2

-1000 / ,

--1100 b s ~ " 3

--1200 r I I I I

--9 --8 --7 -6 --5

l I l I I I I - 4 --3 - 2 --1 0 1 2 3

Change in weighbeam height (mm)

I I I ! I 4 5 6 7 8

Change in weighbeam height is positive if the height o f the weighbeam, relative to the apron, is increased

Fig. 3b E f f ec t on bogie w e i g h t d i f f e r e n c e a ( f o r t r a i l e r bog ies o f al l a r t i c u l a t e d veh ic les ) o f changing the he igh t o f the w e i g h b e a m re la t i ve t o t he a p r o n (Tests 2 and 3)

11

1 6-axle artic, with 3-axle air suspended trailer 2 4-axle artic, with 2-axle conventionally suspended trailer 3 5-axte artic, with 2-axle conventionally suspended trailer 4 5-axle artic, with 3-axle conventionally suspended trailer 5 6-axle artic, with 3-axle conventionally suspended trailer

0 • • A and its range

A per axle = [(average indicated weight of vehicle with plywood in position) -- (true weight of vehicle)] / number of axles of vehicle

X

$ t~

<3

600

500

400

300

200

100

0

"O J~ O3

- 1 0 0

-g >

- 2 0 0

- - 3 0 0

- -400

- -500

/

p

f / " ~ / s , " so S

s •

1

--9 - -8 --7 - -6 - -5 - -4 - -3 - -2 --1 0 1 2 3 4 5 6 7 8

Change in weighbeam height (mm)

Change in weighbeam height is positive if the height of the weighbeam, relative to the apron, is increased

Fig. 3c Effect on vehicle weight difference A (for total vehicle weights of all articulated vehicles) of changing the height of the weighbeam relative to the apron (Tests 2 and 3)

12

Results for 6-axle artic, with 3-axle convent ional ly suspended trai ler

Results for 6-axle artic, with 3-axle air suspended trai ler

Results for 5-axle artic, with 2-axle convent ional ly suspended trai ler

• - - O - - • a a n d its range

A p e r a x l e = [(average indicated weight of bogie with p lywood in position) - (average indicated weight of bogie when apron and weighbeam are level)'] / number of bogie axles

Exit apron

9 8 7 6 5 4 3

Distance of 9mm thick sheet f rom weighbeam edge (m)

2 1 0 0 1 2 3 100 100 . . . .

\ \ \

0 0

--100 ~ --100

- 2 0 0 <3 - 2 0 0

g

--300 --300

--400 ~ --400

- 5 0 0 - 5 0 0

- 6 0 0 - 6 0 0

//

Approach apron

4 5 6 7 8 9

Fig. 4 Effect on bogie weight d i f f e r e n c e a ( f o r t r a i l e r bog ies o f 3 articulated vehicles) o f p lac ing 9 m m thick sheet on the approach and exit aprons in turn and moving i t progressively away from the weighbeam (Test 4)

5.4 TESTS 2 A N D 3: EFFECTS OF HEIGHT DIFFERENCES BETVVEEN APRON A N D W E I G H B E A M

The results for the tests in which plywood sheets were used to alter the weighbeam's height relative to the apron are presented below.

axles of the 2-axle vehicle. The change in weight A due to placing 9 mm thick sheets on the weighbeam or apron is about 70 kg for each axle and 30 kg for the total vehicle weight (ie the sum of the weights of the two axles). Test 7 (in which the results of 65 reference runs were compared) was the only other test conducted with this vehicle.

5.4.1 Tests with the 2-axle vehicle Altering the height of the weighbeam relative to the apron has little effect on the indicated weights of the

5.4.2 Tests w i th the art iculated vehicles Figures 3a, 3b and 3c show graphs of A per axle plotted against the height of the weighbeam relative

13

Results for 6-axle artic, with 3-axle conventionally suspended trailer

Results for 6-axle artic, with 3-axle air suspended trailer

Results for 5-axle artic, with 2-axle conventionally suspended trailer

• • • Aand its range . . . . . . i

Aper axle = [(average indicated weight of vehicle with p lywood in position) -- (true weight of vehicle)~ / number of axles of vehicle

Exi t apron

9 8 7 6 5 4 3 2

Distance of 9mm thick sheet from weighbeam edge (m)

Approach apron

\

\ \

\

\ \

1 0 0 1 ~ 40 40

- 20 20 -

0 0 ,~.

- --20 --20 i ~' P

- --40 --40 .-

- --60 --60 f - --80 - 8 0

--100 --100

--120 ~ --120 x

\ --140 ~ --140

- 160 <3 - 1 6 0

--180 ~ - 1 8 0

--200 ~ --200

--220 "~ --220

--240 .~ --240

--260 > --260

--280 --280

- --300 --300

2 3 4

/ /

/

5 6 7

\

-- --320 --320

-- --340 --340

- --360 --360

- --380 --380

--400 --400

\ \

Fig. 5 Effect on vehicle weight difference A (for total vehicle weights of 3 articulated vehicles) of placing 9ram thick sheet on the approach and exit aprons in turn and moving it progressively away from the weighbeam (Test 4)

14

O • A a n d its range

Aper axle = ['(average indicated weight o f vehicle with p lywood in posit ion) -- (true weight of vehicle)] /number of axles of vehicle

Distances are measured from the point on the weighbeam at which axle weights are recorded ie. 1 lOmm after the wheels have passed over the centre of the weighbeam. This posit ion is termed the "effective centre" of the weighbeam.

x

CL

<3

- o

u

>

20

0

- 2 0

--40

--60

- 8 0

- 1 0 0

- 1 2 0

- 1 4 0

- 1 6 0

- 1 8 0

- 2 0 0

- 2 2 0

Distance of centre of 9mm thick board f rom 'effect ive centre' o f weighbeam (m)

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

, s

- - 4-axle articulated i ~ I I "

- - 2 axl t i I o

- - / S

B # I

t I I

- - ~ I i I

Distance f rom 'effective ~ ' ] centre' o f w e i g h b e a l t t~ / j • /

vehicle with ~ ~ conventional ly ~ suspended trailer t

5-axle articulated vehicle with 3-axle conventional ly ~ ~ / • suspended trai ler ~ t / •

t I ~ S I

~£_ Distance f rom 'effective centre' o f weighbeam ~ J i • - - o f second axle of 3-axle trai ler bogie 9 "~ Distance f rom 'effective centre' o f weighbeam = I ~(~/1 - - of second axle of 2-axle trailer bogie r l

o f third axle of 3-axle trai ler bogie

The indicated positions of the second and third trai ler axles occur when the f irst trai ler axle is being weighed at the 'effective centre' o f the weighbeam

Fig. 6 Effect on vehicle weight difference A (for total vehicle weights of 2 articulated vehicles) of placing a 9mm thick board across the approach apron (Test 5)

T A B L E 2

Results of Tests 2 and 3

Changes in load imposed by axles due to a 3 mm height change of the weighbeam

Type of weighing

Individual tractor axles Individual trailer bogie axles Total weight of trailer bogie Axle weight sum (the indicated weight of the vehicle)

Absolute value of A per axle for articulated vehicle of stated type (kg)

4-axle 5-axle 5-axle 6-axle 6-axle artic artic artic artic artic

2-axle convent.

trailer

40 355 290 145

2-axle convent.

trailer

55 585 450 195

3-axle convent.

trailer

75 390 305 200

3-axle convent.

trailer

250 530 435 290

3-axle air suspended

trailer

140 60 45 65

No tes Each of the above figures is the greater of the 2 values obtained by raising the weighbeam or the apron by 3 mm, following a verification test.

15

to the apron. Figure 3a shows the results for the individual trailer axles of the air suspended and leaf- spring suspended 6-axle articulated vehicles. The results for the trailer bogies and total vehicle weights of all five articulated vehicles are shown in Figures 3b and 3c respectively. The graphs show that A per axle is roughly linearly related to the difference between the heights of the weighbeam and the apron.

Table 2 summarizes the values of weight difference that would occur if the weighbeam's relative height were to be artificially changed by 3 mm, following a verification test. (The values given are the average of those obtained by using the 2 mm and 4 mm thick sheets and each figure shown in the Table is the greater of the two values obtained by raising the weighbeam or the apron by 3 mm.) For the vehicles with conventional suspension, A per axle is much greater for the individual axles of trailer bogies than for the individual axles of the tractors. The effect of subsequent tests on tractor axles will not, therefore, be discussed further.

These extreme results demonstrate the sensitivity of the indicated weights of conventionally suspended bogies, and hence of total vehicle weights, to the height of the weighbeam relative to the apron. At a height difference of 3 mm from the original setting of the weighbeam, the total weights of the four vehicles with the conventionally suspended trailer bogies have values of A of up to 1745 kg. In comparison, A for the total weight of the vehicle with the air suspended trailer bogie is only about 380 kg.

The Code recommends the use of a 4-axle or a 5-axle articulated vehicle for verification purposes. For the vehicles tested, the indicated weights of the 5-axle vehicles were found to be more sensitive than those of the 4-axle vehicle to height differences between the weighbeam and the apron (see Table 2). For a 3 mm height difference from the original setting of the weighbeam, the difference A between the true weight of each 5-axle vehicle and its total weight (the sum of the weight readings of individual axles) is about 1000 kg ie about 400 kg greater than that for the 4-axle vehicle.

5.5 TEST 4: EFFECTS OF R A I S I N G P A R T OF THE A P P R O A C H OR EXIT A P R O N

This test was applied to the two 6-axle vehicles and the 5-axle vehicle with the 2-axle trailer. In Figures 4 and 5, values of weight difference A for the trailer bogies and for the total weights of the vehicles respectively are plotted against the distance of the edge of the 9 mm thick sheet from the edge of the weighbeam. Figure 4 shows that the indicated weights of the two conventionally suspended trailer bogies change by up to 1725 kg (575 kg per axle x 3 for the 3-axle trailer bogie) when the sheet is next to the weighbeam. These changes reduce to less than

300 kg when the sheet is moved further than 3 m away from the edge of the weighbeam.

For the vehicle with the air suspended trailer bogie, the sheet causes smaller changes in indicated weight. The indicated weight of this vehicle's trailer bogie did not change by more than 210 kg even when the sheet is next to the edge of the weighbeam. When the sheet is moved 2 m further away from the weighbeam, the value reduces to about 75 kg. Unfortunately, the changes in vehicle weight (see Figure 5) are probably inaccurate because the 'true' weight established for the vehicle is believed to be incorrect: the public weighbridge used to weigh the vehicle failed subsequent Trading Standards checks.

5.6 TEST 5: EFFECTS OF PLACING A N A R R O W BOARD ACROSS THE A P P R O A C H A P R O N

This test was conducted using the 4-axle articulated vehicle and the 5-axle articulated vehicle with the 3-axle trailer. The change in the vehicles' indicated total weights due to a 9 mm thick board is shown in Figure 6, A per axle being plotted against the distance of the centre of the board from the 'effective centre' of the weighbeam.

For both vehicles, sharply-defined negative peaks in the weight difference A are observed in the graphs for certain distances of the board from the weighbeam. For example, for the 4-axle vehicle, A per axle is about--200 kg ( -785 kg for the complete vehicle) when the board is 2.05 m from the 'effective centre' of the weighbeam. (When the 3 mm thick board is in Position, the resulting negative peak is about--270 kg for this vehicle. These negative peaks occur because the centre of the board is positioned at a distance from the 'effective centre' of the weighbeam equal to the spacing between adjacent bogie axles. As a result, the second bogie axle of each vehicle mounts the board as the first bogie axle passes over the weighbeam. Load is then transferred from the first bogie axle (on the weighbeam) to the second one (on the board). When the indicated weights of successive axles on the weighbeam are summed the total is less than if no board was present. Two negative peaks are observed for the 3-axle bogie, corresponding to board-to-weighbeam distances of one axle spacing and two axle spacings respectively. Neither of these negative peaks coincide with that for the 2-axle bogie because the spacings between axles of the 3-axle and 2-axle bogies are different (about 1.3 m and 2.0 m respectively).

When the centre of the 9 mm thick board is about 2 m from the 'effective centre' of the weighbeam, the reduction A in the sum of the indicated axle weights of the 4-axle vehicle (its total weight) is about 785 kg. For the 5-axle vehicle, the reduction is only about 10 kg, the board having little or no effect.

It was found that, when a 3 mm thick board is positioned on the apron with its centre 2.0 m from the 'effective centre' of the weighbeam, the indicated

16

TABLE 3

Results of Test 6

Changes in load imposed by axles due to passing over discs

Value of A for total weight of vehicle of stated type (kg)

Number and thickness 5-axle artic (2-axle 6-axle artic (3-axle 6-axle artic (3-axle air of discs conventional trailer) conventional trailer) suspended trailer)

4 mm thickness: 1 disc 2 discs 4 discs

9 mm thickness: 1 disc 2 discs 4 discs

3 mm thickness: 1 disc 2 discs 4 discs

- 5 - 160 - 385

- 80 -210 - 580

- 2 5 - 7 0

- 1 9 5

- 1 1 5 - 140 - 355

- 290 - 425 - 920

- 9 5 - 140 - 305

- 8 5 - 120 - 2 3 0

- 1 3 0

- 185 - 295

- 4 5 - 6 0 - 100

Notes The results for the 3 mm thick discs are derived from those of the 9 mm discs by interpolation.

total weight of the 4-axle vehicle differs from its true weight by about 510 kg. The corresponding difference when no board is present on the apron is 210 kg. The difference between the indicated and true weights of the vehicle increases to about 650 kg when 3 mm thick boards are positioned on both the approach and exit sides of the apron with their centres 2.0 m from the 'effective centre' of the weighbeam.

5.7 TEST 6: EFFECTS OF PLACING DISCS ON THE A P P R O A C H APRON

The test was conducted with the two 6-axle vehicles and the 5-axle vehicle with the 2-axle trailer. Firstly, a 4 mm thick disc was placed on the apron in the offside track of the wheels of each vehicle being examined. With the disc in place, the sum of the axle weights decreased by up to 115 kg (see Table 3). Secondly, the test was repeated with two discs on the apron, both in the offside wheel track and positioned so that as each axle passed by, both offside wheels on double wheeled half-axles mounted the discs simultaneously. It was found that the reduction in indicated weight roughly doubled. Finally, the test was repeated with 4 discs, ensuring that all four wheels passed over the discs at once. The entire procedure was then conducted with 9 mm thick discs.

Table 3 summarises the results obtained with 4 mm and 9 mm discs and also shows the expected results

for 3 mm discs obtained by interpolation. The reduction in the sum of the axle weights was found to be roughly proportional to the thickness of the discs and the number of them placed on the apron at the same time, positioned as described. The corresponding reductions were generally less for the vehicle with the air suspended bogie than for the others. The test with the four discs demonstrates the change in indicated vehicle weight which would occur if surface defects were to develop over a period of t ime. These, although individually small in area, would have a similar effect to a continuous ridge extending across the apron's width, since they affect all wheels on an axle at once. When the distance between the centre of the four 4 mm thick discs and the "effective centre' of the weighbeam is equal to the spacing between adjacent axles of a vehicle's bogie, A is about half the verif ication tolerance of ___100 kg per axle for vehicles with conventionally suspended bogie axles.

5.8 TEST 7: R E P E A T A B I L I T Y OF THE REFERENCE R E A D I N G S

Table 4 presents a summary of the results of 65 reference runs made during one day by each of four of the vehicles. The variation in the sums of the axle weights (the total weights) for the vehicles is within the verif ication limit of _100 kg per axle times the number of axles of the vehicle. The result for the 6-axle vehicle with the air suspended trailer bogie is suspect since the true total vehicle weight was probably not established correctly (as has already

17

TABLE 4

Results of Test 7

Comparison of 65 reference runs over weighbeam

Axle

Tractor: 1 2 3

Bogie

Trailer: 4 5 6

Bogie

Axle weight s u m

True vehicle weight

2-axle vehicle

Average axle wt

(kg)

5 951 9 720

D

15 671

15 690

Variation (kg)

81 5O

m

100

120

5-axle artic (2-axle cony. trailer)

6-axle artic (3-axle conv. trailer)

6-axle artic (3-axle air susp. trailer)

Average axle wt

(kg)

5 674 5 620 3 907 9 527

5 362 5489 5 470

16 321

31 522

Average axle wt Variation

(kg) (kg)

5 765 125 9 006 154 6 029 71

15 035 185

8 786 294 7 949 252

16 735 225

37535 285

37 450 290

Average axle wt Variation

(kg) (kg)

5 977 83 5 337 97 4 569 111 9 906 85

5 180 180 6 772 202 7 800 220

19 752 162

35 635 177

35 700 240 (2)

31 410

Variation (1)

(kg)

16 31 17 27

43 49 11 51

63

160

Notes (1) The variation is defined as the maximum difference between average and individual values of weight. The

variation for the true vehicle weight is the maximum difference between the true value from the whole- vehicle weighbridge and the individual values from the sum of the axle weights.

(2) The value of 31 410 kg for the true vehicle weight is suspect, the whole-vehicle weighbridge on which the vehicle was weighed failed subsequent Trading Standards checks.

been mentioned). The variation in the readings of load imposed by individual axles or bogies for the two articulated vehicles with conventionally suspended trailer bogies sometimes exceeds the verification limit.

The results show that load is distributed less equally among axles of conventionally suspended trailer bogies than among those of air suspended bogies. The type of trailer suspension also appears to affect the variation in the indicated axle weights of the tractors, though this result may merely reflect differences between individual vehicles. The two 3-axle trailers were coupled to the same make and model of tractor although not to the same individual unit. For each of the three tractor axles, the variation in the indicated axle weight is only about 30 kg for the tractor coupled to the air suspended trailer but is about 110 kg for the unit coupled to the conventionally suspended trailer.

5.9 TEST 8: RESULTS F R O M THE W E I G H E R A T CRICK

The runs made by the verification vehicles were conducted in both directions of travel across the

weighbeam. All differences between indicated vehicle weights (by summing axle weights) and true vehicle weights were found to be within the Code's verification tolerance of +_100 kg per axle. The runs made by the five additional vehicles which had less commonly occurring bogie spacings were also conducted in both directions of travel across the weighbeam and their weighing errors were also found to be within the Code's verification tolerance.

5.10 TEST 9: RESULTS FROM THE WEIGHER AT TOWCESTER

About 100 4-axle vehicles with bogie spacings ranging from 1.1 m to 2.1 m were weighed both at slow speed using the weigher and statically using weigh pads. The difference between the indicated weights of vehicles for the two methods was more than _+100 kg per axle in only six cases. In five of these cases, there was evidence to suggest that the weigh pad scales had been misread by 0.5 tonnes or 1 tonne; the bogie spacings of the vehicles involved were not unusual. In the remaining case, the driver insisted on leaving his cab as soon as the vehicle was on the weigh pads and so his weight was not recorded.

18

6 SUMMARY OF TEST RESULTS

The slow speed weighing results for the 2-axle vehicle (see Section 5.1) show that, during slow speed weighing, the instantaneous load imposed by each axle is accurate to a tolerance of about _30 kg, compared to the __.10 kg tolerance for static calibration. To obtain this result the vehicle has to be driven in accordance with the Code of Practice. The results also indicate that the sum of the weight readings of the 2 axles obtained statically agrees closely with the vehicle's true total weight. If the axles are statically weighed one after the other, no brakes being applied during the process, the sum of the axle weights exceeds the true total weight by only about 40 kg. It follows that summing the individual static weighings of each axle of a 2-axle vehicle provides a convenient check of the weigher's static calibration.

The weigher's static calibration may also be checked by using the slow speed weighing results for a 2-axle vehicle. During the slow speed weighings involving such a vehicle described in Section 5.1, the sum of the axle weight readings agrees with the vehicle's true weight to within _45 kg. The same agreement was found for 90 per cent of the vehicle's 65 reference runs (see Section 5.8). However, for one of the 65 runs the difference between the sum of the axle weight readings and the vehicle's true weight is 120 kg (see Table 4). In addition, the greatest difference between the mean indicated axle weight and any individual result is about 80 kg (axle 1). The ranges recorded for the first and second axles are 150 kg and 80 kg respectively, compared to the values recorded during the test results described in Section 5.1 of 30 kg and 40 kg respectively. Nevertheless, the results for the 65 reference runs are within the _+100 kg per axle verification tolerance.

Of the six vehicles tested, the indicated weights of those with conventionally suspended trailer bogies are most affected by differences in relative height between the weighbeam and the apron. Conducting a verification test and then artificially changing the height of the weighbeam by 4 mm from its original setting causes a change in the sum of the axle weights of between 760 and 2190 kg for these vehicles (see Figure 3c). The corresponding changes in total weight of the 2-axle rigid vehicle and the 6-axle articulated vehicle with the 3-axle air suspended bogie are much less ie between about 10 kg and 360 kg respectively. Figures 4 and 5 show that the changes in the sum of the bogie weights or the sum of all the axle weights are due to differences in height between the weighbeam and the 3-4 m section of apron on either side of it. The board and disc tests demonstrate that these results are due to the difference in height between the bogie axle on the weighbeam and the bogie axle (or axles) on the apron (see Figure 6 and Table 3).

The results of the board test also suggested that the relation between the bogie spacing of a vehicle and

the distance from the weighbeam of an irregularity of sufficient size may affect the performance of the weigher. However, the results of tests at the Crick and Towcester axle weighers show that multi-axle vehicles with a wide range of bogie spacings were weighed to accuracies within the verification tolerance of ___100 kg per axle.

7 DISCUSSION

The main objective of the work described in this report was to examine the feasibility of reducing installation and maintenance costs by relaxing the standard of construction of the apron. Nevertheless, the tests have revealed strengths and weaknesses of the slow speed weighers which were not appreciated by TRRL at the outset. These will be discussed below.

7.1 EFFECTS OF V A R I A T I O N S IN APRON PROFILES

The tests with large sheets placed on the approach or exit aprons (Test 4), have shown limited scope for relaxing the standards of construction of the apron without worsening the correspondence between the true total weight of a vehicle and the sum of its axle weights as indicated by the weigher.

The agreement between these two figures is critically dependent on the level tolerances over the central section of the apron adjacent to the weighbeam. The Test 4 results have shown that the tolerance of ___3 mm which is currently specified for the 8 m length of apron on either side of the weighbeam, must be maintained over the portions of the apron which are within about 4 m from the weighbeam to uphold the instrument's performance, provided that the spacing between the first and last axles of bogies of vehicles being weighed does not exceed about 3m.

The instrument's performance is not significantly impaired by the effects of small Iocalised bumps of up to 9 mm high (even when they are 1 m from the weighbeam) provided they are less than about 150 mm in diameter, are few in number and do not form a pattern. However, it is found that performance is worsened--possibly to an unacceptable extent-- i f 2 or more wheels on an axle mount such bumps at the same time (see Table 3).

To prevent the apron of an enforcement weigher from having such irregularities, it would be advisable for a detailed survey of the apron to be undertaken when the instrument is commissioned or repaired so that patterns of irregularities which might be present may be detected. The work would involve determining levels at the intersections of a grid of

19

closely-spaced reference lines. If a pattern of bumps or depressions located parallel to the long edge of the weighbeam is detected, then remedial action to remove them may be necessary.

Clearly, the selected grid spacing has to be a compromise. The use of a very close spacing of (say) 100 mm would provide very detailed information about levels but the data would be very tedious to collect and analyse. Conversely, a very wide spacing of (say) 1000 mm would require far fewer readings but the information obtained would probably be inadequate. A reasonable compromise would seem to be to use a grid spacing of about 400 mm. The grid would start at the long edge of the weighbeam and would extend for a distance of 8 m on either side of the weighbeam. The range of distances (measured from the centre of the weighbeam) of about 800 mm to about 2800 mm would provide level information about the apron at locations where bogie axles would be when the first axle of a 2-axle or 3-axle bogie was on the weighbeam. For intersections whose levels were found to differ by more than ___2 mm from the chosen datum level for the apron, the irregularity's extent could be found by taking further levels in the vicini ty of these intersections at a reduced grid spacing.

A verif ication procedure is fol lowed when the instrument is commissioned or refurbished and at 6-monthly intervals during regular use. This is designed to ensure that for each vehicle weighed during an enforcement check, the weigher performs within the required tolerances. When the weighbeam is installed, it is customary to 'tune out' the effects of small surface defects by making final adjustments to the equipment. This procedure may be effective in optimising the weigher's performance with the vehicles used for verification. Nevertheless, it seemed possible that these adjustments could worsen the weigher's performanc e in determining the total loads of other vehicles with different bogie spacings even though the apron's variations are within the currently specified tolerance of ___3 ram.

However, this was not the case. The results of the tests conducted at Crick and Towcester show that vehicles with bogie spacings unlike those of the three verif ication vehicles were weighed to accuracies within the Code's verif ication tolerance of _+100 kg per axle. On the basis of these tests, the present three-vehicle verif ication procedure is adequate provided that the articulated vehicle used for verif ication has a 3-axle semi-trailer. There is a remote possibil ity that the normal verif ication procedure may be inadequate at some other site not tested by extra verif ication vehicles but the results of Test 5 (in which a narrow board was used) suggest that vehicles would still be weighed to accuracies

w i th in the enforcement tolerance of ___150 kg per axle.

7.2 EFFECTS OF V A R I A T I O N S IN AXLE W E I G H T S DURING W E I G H I N G

Fluctuating stresses are set up in the suspension systems and their supporting structures of all vehicles whilst they move. Even on an apron which is absolutely flat, these stresses and friction-locked deflections of the suspensions are sufficient to cause the share of a vehicle's load borne by one of its axles to vary randomly as the vehicle moves over its surface. The size of the variation is greater on an uneven surface. The effect of the variation is to add a random component to the sum of the weights of a group of axles when they are weighed in sequence. The loads imposed by bogie axles suspended on steel leaf-springs are shown in Tests 2 and 3 to change significantly if the axle on the weighbeam is raised or lowered by a few millimetres relative to the other bogie axle (or axles) on the apron. (See Section 6.) The only reason for specifying a tight tolerance on variations in apron profile is to minimise these effects. As mentioned previously, the variation in bogie axle loads is much less significant with bogie suspension systems that compensate effectively, but the inferior performance of a badly maintained or poorly designed system may result in substantial inequalities in the axle Ioadings imposed by the vehicle's bogie.

7.3 THE P E R F O R M A N C E OF THE WEIGHER

On the basis of the Test 1 results for a 2-axle vehicle (see Section 5.1), the weigher performs well as an instrument for determining the load imposed by a single independent axle either when the axle is at rest on the weighbeam's surface or whilst the axle passes slowly and smoothly over it. During the test, the instrument's accuracy and repeatability were found to fall consistently within limits of _30 kg-- considerably better than the _150 kg which enforcement officers are currently asked to presume. During a later test (see Section 5.8), the instrument's repeatability for determining the load imposed by a single independent axle of a 2-axle vehicle, although still within the verification tolerance of _100 kg per axle, deteriorated to about _+80 kg (see Table 4). Presumably, during the later test the vehicle did not pass over the weigher as consistently as had been achieved during Test 1. During slow speed weighing, the axles are weighed sequentially whilst the vehicle is moving so the degree of smoothness with which the vehicle passes over the weighbeam may affect the indicated weights, and the Code of Practice specifies a method of driving over the weighbeam to minimise this effect. The results for the 2-axle conventionally suspended vehicle confirm that the tolerance of _+100 kg per axle is a realistic limit for verification vehicles.

The variations observed in the indicated weights of bogie axles are not due to any inadequacies in the

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performance of the axle weigher but are due, to the motion of the vehicle and the poor compensation performance of many suspension systems.

For the reasons already given, the weigher's performance is less precise when used to determine the total load imposed by a group of axles comprising a bogie or an entire vehicle, especially one that is equipped with a bogie (by summing the weights of all axles in the group) than when weighing the loads imposed by individual axles. This is because the axles have to be weighed one by one and, particularly for the axles of bogies, errors can be introduced whose magnitude can not be determined reliably. However, the measurements made at Crick and Towcester indicate that the errors for total vehicle weight (and hence for total bogie load) do not normally exceed _+100 kg x the number of axles. These results are in line with the requirements of the Code of Practice and are confirmed by numerous verification tests of Department of Transport slow speed weighers made since they were commissioned.

8 RECOMMENDATIONS

On the basis of the tests conducted, two recommendations are made; and have already been adopted. The aim of these is to improve the safeguards for operators, reducing still further the chance that a vehicle may be wrongly judged as overloaded, but taking advantage of the instrument's precision in measuring axle weights.

(1) Vehicles used to verify the weigher: The vehicles should be fitted with conventional suspension systems and should be fully loaded. They should comprise a 2-axle vehicle with a plated gross vehicle weight of about 16 tonnes, a 4-axle rigid vehicle and a 5-axle (or 6-axle) articulated vehicle with a 3-axle semi-trailer.

(2) Using a reference grid when surveying the apron: When the apron is surveyed in order to detect any irregularities, levels should be obtained at the intersections of a grid of longitudinal and transverse reference lines, spaced 400 mm apart, for a distance of 8 m on either side of the weighbeam. Further readings should be obtained (using a closer grid spacing) in the vicinity of intersections whose levels are found to be more than _+2 mm from the datum level so that the irregularity's extent can be determined.

9 CONCLUSIONS

Tests have been conducted on the slow speed axle weigher installed on the TRRL Research Track and

on other Department of Transport slow speed weighers. The fol lowing conclusions are obtained on the basis of the test results:

(1) The observed error of the instrument in measuring the weight of an independent axle at slow speed (ie not exceeding 2.5 mile/h) is within ___100 kg and this can also be presumed to apply to any axle forming part of a bogie. However, the load carried by the axle of a bogie during its passage across the weighbeam depends on the height of the weighbeam relative to the apron.

(2) The value of the sum of the axle weights of a vehicle weighed at slow speed is affected by differences in height between the weighbeam and the apron. In some circumstances when weighing vehicles with ineff icient or poorly maintained suspension systems, a difference in height of 1 mm can cause the sum of the axle weights to change by up to 100 kg per axle. Conversely, the size of this change is greatly reduced by eff icient suspensions; for example it is about f ive t imes less for semi-trailers with air suspended bogles than for those with bogies suspended on steel leaf-springs.

(3) If close agreement between the sum of the axle weights and the true gross" weight of a vehicle is required then an overall level tolerance of ___3 mm must be maintained for the section of the apron extending to 4 m on either side of the weighbeam. This requirement has to be met in order to accommodate the varying compensation performance of different suspension systems. However, even within this central 8 m region, significant irregularities in level (up to ___9 mm) on the surface can be tolerated provided that they are confined to small areas (less than 150 mm in diameter), are few in number and do not fall on lines at right angles to the path of the vehicle across the weighbeam. Outside the central 8 m region (4 m on either side of the weighbeam) the _+3 mm level tolerance can be relaxed wi thout degrading the performance of the weigher.

(4) When the weigher is used to weigh a 2-axle vehicle at slow speed, the total weight of the vehicle derived by summing its axle weights is consistently recorded to within _+120 kg of its true value. This agreement is preserved even when the weighbeam is raised or lowered in level by 9 mm relative to the apron.

10 ACKNOWLEDGEMENTS The work described in this Report was carried out in the Vehicles and Environment Division of the Vehicles Group of the TRRL. The author gratefully acknowledges the help of everyone who assisted in the work. Particular thanks are given to the members

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of the joint Department of Transport/LACOTS Working Group on slow speed 'dynamic' axle weighers and to members of individual local authorities. (LACOTS is an abbreviation for 'Local Authorities Co-ordinating Body on Trading Standards'.)

1 1 REFERENCES

DEPARTMENT of TRANSPORT (1981). Code of Practice for Dynamic Axle Weighers. H M Stationery Office, London.

KILSBY R E (1976). Test Report for Department of Transport on Dynamic Axle Weighing. Surrey County Council, Trading Standards Department. Epsom, Surrey.

Printed in the United Kingdom for Her Majesty's Stationery Office (3301/87) Dd8222700 2/88 C.8 G426 10170

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