INDIAN INSTITUTE OF SPACE SCIENCE AND TECHNOLOGY

THIRUVANTAPURAM

A STUDY REPORT ON:-

Walschaerts steam valve gear

Done by :-

Kundrapu Raghava (SC10B038).

In the guidance of :-

Prof. Kurian Isaac sir.

ABSTRACT

Walschaerts valve gear was the most popular valve gear

which was extensively used in steam locomotives from the late 19th

century until the end of the steam era. The Walschaerts valve gear

enables the engineer to operate the steam engine in a continuous

range of settings from maximum economy to maximum power.

While initially unpopular, the Walschaerts gear was

probably applied to more of the twentieth century's steam

locomotives than any other, replacing the former favorite gear, the

Stephenson valve gear as locomotives got larger. This is one of the

basic model design of the valve gears which we are using now. In

this we made an analysis on kinematic view of the mechanism. We’ll

see how it was designed and its working principles and its

mechanism.

INTRODUCTION

WHAT IS VALVE GEAR ?

Valve gear on a steam locomotive is used to control the

admission of steam into the cylinders of a locomotive. Specifically, the

valve gear controls the timing or duration of time boiler pressure steam

is allowed into the cylinders.

The function of the locomotive valve gear is to regulate the

movement of the valves so that steam is admitted and exhausted from

the cylinders in relation to the position of the pistons. It also allows the

driver to alter the length of steam admission, known as cut-off, and to

reverse the locomotive.

Walschaerts valve gear

In 1844, Egide Walschaerts invented a valve gear that by

1848, appeared very much as it does today.

The primary reason for a more universal adoption of

Walschaerts’ gear was its general suitability for mounting outside the

frames, convenient for maintenance and permitting more strongly

braced frames. It's main advantage was that it was located completely

outside the wheels and therefore was much easier to maintain than

Stephenson valve gear.

Figure 1 :- Walschaerts valve gear of a steam locomotive.

1. Eccentric Crank; 2. Eccentric Rod; 3. Reach Rod; 4. Lifting Link

5. Lifting Arm; 6. Reverse Arm & Shaft; 7. Link (Expansion Link);

8.Radius Bar; 9. Crosshead Arm; 10.Valve Stem Guide; 11. Union Link

12. Combination Lever; 13. Valve Stem; 14. Valve Spindle.

Typical analysis of links and joints

From the above kinematic skeleton diagram,

1. Total no. of links (n) = 15

2. Total no. of joints with DOF as 1 (j2) =20

3. Total no. of joints with DOF as 2 (j1) =1

Hence,

Degrees of freedom = 3(n-1)-2(j2)-(j1)

= 3*(15-1)-2*20-1

= 1

LET’S SEE HOW IT WORKS?

The motion is taken primarily from the locomotive driving wheel.

At the end of the crankpin, a return crank is fitted on the main

driver. The other end of this return crank is at a point just over 90

degrees ahead of the crankpin around the driving axle and closer

to the wheel's center.

The return crank drives the return crank arm, which in turn is

fastened to the lower end of the expansion link - this is a curved

piece (the concave side facing forward) of metal with a slot in it,

pivoted at the center so it can rotate.

The return crank arm is fastened to the bottom end of it, and as

the driving wheel rotates it therefore rocks the expansion link

back and forth around its center.

In the slot of the expansion link there fits a slider called die block.

This is moved up and down in the expansion link by the reversing

shaft i.e., in reverse direction.

If the die block is in the center of the expansion link, it does not

move in a longitudinal sense and only rotates as the expansion

link is rotated back and forth.

Figure-3

1. Valve spindle; 2.Combination lever; 3.Radius rod;

4. Expansion link; 5. Crosshead; 6. Eccentric rod; 7.Return crank;

8.Crank axle; 9. Reversing rod; 10.Connecting rod.

The further it is moved from that central position the more it

moves; if moved downward, it moves in the same sense as the

return crank arm, while if moved upward it moves in the reverse

sense.

The die block is attached to a rod named the radius rod, which

provides the large part of the valve motion to the piston or slide

valves. The expansion link and die block are used to control the

direction of movement of the locomotive, and the amount of

valve travel (as equivalent as the gearbox in an automobile).

In this Walschaerts’ gear, to make it a serviceable valve gear for a

locomotive it was discovered that by adding a small proportion of

the piston's movement to the valve motion produced better valve

events. Thus, instead of the radius rod being connected directly to

the valve spindle, it is connected to a more-or-less vertical rod

called the combination lever.

The radius rod is connected to the top of this lever; the valve

spindle is connected to a position only slightly down the lever,

since the radius rod's motion is the large proportion of the desired

valve motion.

The other end of the combination lever is connected to the

crosshead at the end of the piston rod by a horizontal rod named

the union link - the purpose of this was simply that the crosshead

was not normally close to the location of the combination lever

but rather substantially further back.

The combination lever, thus, combines the movements of the

radius rod and the crosshead in approximately a 10:1 proportion,

adding only a small amount of the crosshead's motion to the valve

motion.

The movement of the expansion link is obtained from an

eccentric rod attached via the crank axle. Adjustment to the

length of valve travel is made by raising or lowering the position

of the radius rod within the expansion link. This is achieved by

operation of the reversing rod from in the cab. The length of

travel of the radius rod, and therefore of the valve spindle,

depends on the rod's position within the expansion link.

Maximum valve travel giving maximum steam admission is

obtained when the radius rod is positioned furthest from the

centre of the expansion link. Moving the radius rod from one half

of the expansion link to the other, reverses the movement of the

locomotive by admitting steam into what otherwise would have

been the exhausting side of the piston's cycle.

Working principle to obtain maximum power & economical :-

Any valve gear has to satisfy the following two conditions:

1. At the instant when the space on one side of the piston starts to

expand, i.e. at the very start of a stroke, the valve opens to admit

steam from the boiler into that space. The pressure of this steam

provides the driving force.

2. At the instant when the space on one side of the piston starts to

contract, the valve starts to release steam from that space to the

atmosphere, so as not to impede the movement of the piston.

Steam is admitted to the expanding space for only part of the

stroke; at an instant of time, the intake is cut off. Since the

exhaust is also shut, during the rest of the stroke the steam that

has entered the cylinder expands in isolation, thereby its pressure

decreases.

For maximum economy, the cutoff point should be carefully set so

that, when the exhaust valve opens, the steam is down to exactly

atmospheric pressure. Thus, all the mechanical energy available

from the steam (in the absence of a condenser) is used.

The Walschaerts valve gear enables to change the cutoff point

without changing the points at which intake starts and at which

exhaust starts.

Economy also requires that the throttle is wide open, so that no

energy is wasted pushing steam through a constriction, and that

the boiler pressure is at the maximum safe level to maximize

thermal efficiency.

For economy, a steam engine is used of a size such that the most

economical settings yield the right amount of power most of the

time, such as when a train is running at steady speed on level

track.

When greater power is necessary, e.g. when gaining speed when

pulling out of a station and when ascending a gradient, the

Walschaerts valve gear enables to set the cutoff point near the

end of the stroke, so that the full pressure of the boiler is exerted

on the piston for almost the entire stroke. With such a setting,

when the exhaust opens, the steam in the cylinder is near full

boiler pressure.

The pressure in the steam at that moment serves no useful

purpose.It is wasted driving a sudden pulse of pressure into the

atmosphere, but this waste is compensated by maximized

economy at other times.

1. The motion must be adjusted with the crank on the dead centers by

lengthening or shortening the eccentric rod until the link takes such

a position as to impart no motion to the valve when the link block is

moved from its extreme forward to its extreme backward position.

Before these changes in the eccentric rod are resorted to, the length

of the valve stem should be examined, as it may be of advantage to

plane off or line under the foot of the link support which might

correct the length of both rods, or at least only one of these would

need to be changed.

2. The difference between the two positions of the valve on the

forward and back centers is the lead and lap doubled and it cannot

be changed except by changing the leverage relations of

the combination lever.

3. A given lead determines the lap or a given lap determines the lead,

and it must be divided for both ends as desired by lengthening or

shortening the valve spindle.

4. Within certain limits, this adjustment may be made by shortening or

lengthening the radius bar but it is desirable to keep the length of

this bar equal to the radius of the link in order to meet the

requirements of the first condition.

5. The lead may be increased by reducing the lap, and the cutoff point

will then be slightly advanced. Increasing the lap introduces the

opposite effect on the cutoff. With good judgment, these qualities

may be varied to offset other irregularities inherent in transforming

rotary into lineal motion.

6. Slight variations may be made in the cutoff points as covered by the

preceding paragraph but an independent adjustment cannot be

made except by shifting the location of the suspension point which

is preferably determined by a model.

How this gear is advantageous over others ?

Another class of competitors were Stephenson valve

gear, baker’s valve gear, the poppet valve gears, including the Caprotti

valve gear, the Franklin Oscillating Cam valve gear, the Franklin Rotary

Cam valve gear and others. Theoretically much more efficient, all of

these suffered from the dual problems of not handling wear very well

and the worse one of complexity, and were not widely adopted.

The Walschearts gear could be used with three or four

cylinder locomotives, too. One set of valve gear could be installed per

cylinder, of course. This either required the center gear(s) to be

mounted inside, or for two sets of valve gear to be mounted on the

same side of the locomotive (normally driven from 2 different driving

wheels).

Some four cylinder locomotives had the cylinders set

up so that they operated in linked pairs, and thus needed only two sets

of valve gear. It was also possible to use levers to derive the valve

motion for a third cylinder from that of the other two, as was done

after in the Gresley - Holcroft Conjugated valve gear.

References

www.mekanizmalar.com/walschaerts_valve_gear.html

www.steamlocomotive.com/appliances/valvegear.php

en.wikipedia.org/wiki/Walschaerts_valve_gear

http://www.railway-technical.com/st-glos.shtml

Walschaerts valve gear - YouTube www.roundhouse-eng.com/pdf/wvg.pdf

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Thank you