emergency braking systems for mine elevators

8/10/2019 Emergency Braking Systems for Mine Elevators

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EMERGENCY BRAKING SYSTEMS FOR MINE ELEVATORS

Thomas D. Barkand

U.S. Department of Labor

Mine Safety and Health Administration

Pittsburgh Safety and Health Technology Center

P.O. Box 18233Pittsburgh, Pennsylvania 15236

ABSTRACT

Investigation of several serious hoisting accidents identified machine brake

failure as the most common cause. The recent revelation of this potential

hazard has prompted regulatory authorities to require an additional,

independent, emergency braking system. As a result of this initiative, a new

generation of braking systems have been developed and applied to mine

elevators.

This paper will discuss the design and testing of an electrical dynamicbrake, pneumatic rope brake, and traction sheave brake. The effect of

compound braking on hoisting systems equipped with multiple brakes will

also be addressed.

INTRODUCTION

Mine elevators and personnel hoists provide a lifeline for miners at more

than 240 coal mines nationwide.[1]The hoisting system transports mine

personnel through an isolated corridor during routine operations or life

threatening emergencies. The potential risk of injury is great if the hoisting

system fails. Therefore, a safe, reliable hoisting system is essential to the

well being of the miners.In coal mining history there have been two well documented investigations

of mine personnel elevators crashing in the upward direction.[2][3]These

accidents occurred on counterweighted hoisting systems when the

mechanical brake failed while the cage was empty. This allowed the

counterweight to fall to the bottom of the shaft, causing the car to

overspeed and crash into the overhead structure. The accidents were

initially believed to be isolated incidents. However, research covering a 5-

year period, showed there were over eighteen documented cases of

ascending elevators striking the overhead structure.[4]

Rules and regulations applying to elevator safety have come under reviewin response to these accidents. The Canadian Elevator Safety Code and the

Pennsylvania Bureau of Deep Mine Safety have already revised their

regulations to require ascending car overspeed protection. This paper will

discuss new emergency braking systems designed to provide ascending car

overspeed protection.

ELEVATOR DESIGN

In a typical elevator, the car is raised and lowered by six to eight motor-

driven wire ropes attached to the top of the car at one end, travel around a

pair of sheaves, and attach to a counterweight at the other end as shown inFigure 1.


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Fig. 1. Mine Elevator

The counterweight adds accelerating force when the elevator car isascending and provides a retarding effort when the car is descending so

less motor horsepower is required. The counterweight is a collection of

metal weights that is equal to the weight of the car containing about 45

percent of its rated load. A set of chains are looped from the bottom of the

counterweight to the underside of the car to help maintain balance by

offsetting the weight of the suspension ropes.

Guide rails run the length of the shaft to keep the car and counterweight

from swaying or twisting during their travel. Rollers are attached to the car

and the counterweight to provide smooth travel along the guide rails.

The traction to raise and lower the car comes from the friction of the wire

ropes against the grooved sheaves. The main sheave is driven by an

electric motor.

Most elevators use a direct current motor because its speed can be

precisely controlled to allow smooth acceleration and deceleration. Motor-

Generator (M-G) sets typically provide the d.c. power for the drive motor.

Newer systems use a static drive control. The elevator controls are designed

to vary the motor's speed based on a set of feedback signals that indicate

the cars position in the shaft. As the car approaches its destination, a switch

near the landing signals the controls to stop the car at the floor level.


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Additional shaft-way limit switches are installed to monitor overtravel

conditions.

The worst fear of many passengers is that the elevator will go out of control

and fall through space until it smashes into the bottom of the shaft. There

are several safety features in modern elevators to prevent this fromoccurring.

First is the high-strength wire ropes themselves. Each 5/8 inch diameter

extra-high strength wire rope can support 32,000 pounds, or about twice

the average weight of a mine elevator filled with 20 passengers. For

safety's sake and to reduce wear, each car has six to eight of these cables.

Elevators also have buffers installed at the shaft bottom that can stop the

car without killing its passengers if they are struck at the normal speed of

the elevator.

In addition, the elevator itself is equipped with safeties mounted

underneath the car. If the car surpasses the rated speed by 15 to 25

percent, the governor will trip and the safeties will grip the guide rails to

stop the car. However, the inherent design of the safeties render them

inoperative in the ascending direction.

In the upward direction, the machine brake is required to stop the cage

when an emergency occurs. Under normal operation the machine brake

serves only as a parking brake to hold the cage at rest. However, when an

emergency condition is detected, modern elevator controls rely solely upon

the machine brake to stop the car.

Several emergency braking systems are available to back-up the machinebrake and provide ascending car overspeed protection. These systems can

be installed on existing elevators. Three recently developed braking

systems are presented here for consideration.

DYNAMIC BRAKE

A new solution which is used in the United States mining industry is the

application of passive dynamic braking to the elevator drive motor as shown

in Figure 2. As mentioned earlier, most elevators use direct current drive

motors which can perform as generators when lowering an overhauling

load. Dynamic braking simply connects a resistive load across the motorarmature to dissipate the electrical energy generated by the falling

counterweight. The dynamic brake can safely lower an overhauling load the

entire length of the shaft. Dynamic braking is applied every time the

machine brake is set. A passive dynamic braking control can be designed to

function when the main power is interrupted. Dynamic braking does not

stop the elevator, but limits the runaway speed in either direction, so the

buffers can safely stop the conveyance.


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Fig. 2. Passive Dynamic Braking

CASE STUDY:

DYNAMIC BRAKING INSTALLATION AND TESTING

Performance testing of dynamic braking has been conducted on several

mine hoisting systems.[5]Two case studies of service elevators that had

dynamic braking installed, and were recently tested, will be presented forillustrative purposes.

History

An elevator accident occurred on February 4, 1987 at a western

Pennsylvania coal mine due to a mechanical brake failure. The

counterweight fell to the bottom of the 400-ft shaft, causing the cage to

overspeed and crash into the headframe. The cage was unoccupied at the

time of the accident. The elevator was out of service for several months due

to the severity of the damage.


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The governor tripped and attempted to set the safety catches. However, the

wedge design of the governor jaws and safeties rendered them ineffective

in the upward direction.

Dynamic braking was installed on the main elevator drive to prevent a

reoccurrence of this type of accident. Dynamic braking was also installed onthe auxiliary elevator to provide the same degree of safety.

Dynamic Braking Installation

A passive type of dynamic braking system was installed on both elevators

servicing the mine portal. The main elevator was a gearless design and the

auxiliary elevator was geared. The equipment needed for the modification

of each elevator included a three-pole loop contactor, a dynamic braking

resistor, a single-phase rectifier bridge, and a drive fault relay. A simplified

schematic diagram of the dynamic braking control circuit is shown in Figure

3.

When the mechanical brakes were called to set, the M contactor dropped

out and disconnected the armature from the power supply, and also applied

the dynamic braking (db) resistor across the motor armature. When the

field power supply was operative, the drive OK (DROK) relay was picked-up

and the field was supplied with normal standing field current.


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Fig. 3. Dynamic Braking Circuit

When a total power loss occurred, the dynamic braking resistor was

connected across the armature, the DROK relay dropped out, andregenerative braking current was supplied to the motor field. A rectifier

bridge in the regenerative field power supply (not shown) insures the

generated amp-turns added to the residual magnetic field for either

direction of cage travel.

Dynamic Braking Tests

The dynamic braking tests were designed to demonstrate the response of

the hoisting system to various emergency conditions. The dynamic braking

systems were recently tested under the following conditions:

1.

machine brake failure, power supply operative, starting at rest,


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2.

machine brake failure, power failure, starting at rest,

The armature current, armature voltage, field current, and motor speed

were recorded on a thermal array recorder during the tests.

Geared Elevator Testing

The auxiliary service elevator was geared at approximately a ratio of 20 to

1. It operates at 350 ft/min with a rated capacity of 4500 pounds. The

nameplate ratings of the elevator shunt wound drive motor were: 50 Hp,

1150 r/min. armature; 500 V, 81.1 A, field; 2.60 A, 89.8 ohms at 25 C.

The motor armature was powered by a three-phase, full wave, reversing

SCR converter. The motor field current was supplied by a SCR-controlled

single-phase, half-wave rectifier.

Test Condition 1: A sample recording for test condition 1, no load with 2.40

ohm dynamic braking resistance, is shown in Figure 4. The speed signal

shows the cage accelerated slowly to 140 ft/min without overspeeding.

Fig. 4. Test Condition 1: Dynamic Braking, Geared Elevator


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Gearless Elevator Testing

The main elevator was a gearless drive roped 2:1. It operated at 600 ft/min

with a rated capacity of 9000 pounds and was 40 percent over-

counterweighted. The nameplate ratings of the elevator were: 115 hp, 127

r/min (600 ft/min cage speed), armature; 407 V, 234 A, field; 17.1 A, 8.61ohm at 25C. The motor armature was powered by a three-phase full-wave

reversing SCR converter. The motor field current was supplied by an SCR-

controlled single-phase half-wave rectifier.

Test Condition 2: A sample recording for test condition 3, no load with 1.60

ohm of dynamic braking resistance, is shown in Figure 5. An empty cage

with 40-percent over-counterweighted provided 3600 lb of upward cage-

accelerating force. When the mechanical brakes were defeated, the cage

accelerated, the armature windings rotated rapidly through the weak

residual magnetic field of the permanently magnetized field poles. Thus a

small amount of armature current was generated, which divided betweenthe dynamic braking resistor and the field winding.

Fig. 5. Test Condition 2: Dynamic Braking, Gearless Elevator


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The field current increased the strength of the magnetic field, which in turn

increased the generated armature current. This positive feedback

continued, causing the field current to build on the generated armature

current until a sufficient retarding torque was developed at 735 ft/min and

the car began to decelerate. The field and armature currents then began to

decrease as the car decelerated. The car slowed down to a steady-statespeed of 220 ft/min with an underdamped response.

The peak speed reached during the self-excitation process was primarily a

function of the time constant of the inductive motor field winding and the

acceleration rate of the cage. The inductance of the field winding was fixed;

however, the acceleration rate of the cage was a function of the load inertia

and the imbalance between the cage and counterweight. The maximum

acceleration rate for personnel load conditions occurs when one person is

transported. As more persons are added to the cage (up to the rated

personnel capacity) the load imbalance between the cage and

counterweight is reduced, thereby reducing the acceleration rate and thepeak speed.

Case Summary

These dynamic braking systems were designed to safely lower an

overhauling load, even under simultaneous failure of the mechanical brakes

and the main power supply. The simple dynamic braking system is an

economical method for providing ascending car overspeed protection.

ROPE BRAKE

A pneumatic rope brake has been developed by Bode Elevator

Components1which grips the suspension ropes and stops the elevator

during emergency conditions.[6]A typical rope brake installation is shown in

Figure 6.
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Fig. 6. Rope Brake Installation

The rope brake guards against overspeed in the upward and downward

directions and provides protection for uncontrolled elevator car movements.

The rope brake is activated when the normal running speed is exceeded by

15 percent as a result of a mechanical drive, motor control system, or

machine brake failure. The rope brake does not guard against free fall as a

result of a break in the suspension ropes.

Standstill of the elevator car is also monitored by the rope brake system. If

the elevator car moves more than 2 to 8 inches in either direction when thedoors are open or not locked, the rope brake is activated and the control

circuit interrupted.

The rope brake also provides jammed conveyance protection for elevators

and friction driven hoists. If the elevator car does not move when the drive

sheave is turning the rope brake will set and the elevator control circuit will

be interrupted.

The rope brake requires electrical power and air pressure to function

properly. The rope brake sets if the control power is interrupted. When the

power is restored the rope brake will automatically release.


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Typically, elevator braking systems are spring applied and electrically

released. Therefore, no external energy source is needed to set the brake.

The rope brake requires stored pressurized air to set the brake and stop the

elevator. Therefore, monitoring of the air pressure is essential. If the

working air pressure falls below a preset minimum, the motor armature

current is interrupted and the machine brake is set.

CASE STUDY:

ROPE BRAKE TESTING AND EVALUATION

The first pneumatic rope brake was installed in the United States at a

western Pennsylvania coal mine on September 8, 1989. Since then, two

additional rope brakes have been installed. The largest capacity Bode rope

brakes, model 580, were installed on the coal mine elevators. The rope

brake installations were tested extensively by Mine Safety and Health

Administration engineers from the Pittsburgh Safety and Health Technology

Center.[7]Several mechanical and electrical modifications were required to

make the rope brake suitable for mine elevator applications. A summary of

the findings will be presented in this study.

Pneumatic Design

The rope brake system is shown in Figure 7. Starting from the air

compressor tank, the pressurized air passes through a water separator and

manual shut off valve to a check valve. The check valve was required to

insure the rope brake remains set even if an air leak develops in the

compressed air supply. A pressure switch monitors for low air pressure at

this point and will set the machine brake as mentioned earlier. The air

supply is split after the check valve, and goes to two independent magnetictwo-way valves. The air supply is shut-off to the brake cylinder and directed

to port A while the magnetic valve coil is energized. When the magnetic

valve coil is de-energized, the air supply is directed to the B port which is

open to the rope brake cylinder. The air pushes the piston inside the rope

brake cylinder and forces a movable brake pad toward a stationary brake

pad. The suspension ropes are clamped between the two brake pads. The

rope brake is released by energizing the magnetic valve which vents the

pressurized rope brake cylinder to the atmosphere through a blowout

silencer on port S.


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Fig. 7. Rope Brake System

The force exerted on the suspension ropes equals the air pressure

multiplied by the surface area of the piston. The rope brake model number

580 designates the inner diameter of the brake cylinder in millimeters. This

translates into 409.36 square inches of surface area. The working air

pressure varies from 90 to 120 psi. The corresponding range of force

applied to the suspension ropes is 36,842 to 49,123 pounds. Typically, theforce experienced by the ropes as they pass over the drive sheave under

fully loaded conditions is about 35,000 pounds. Therefore, the ropes

experience a 5 to 40 percent greater force during emergency conditions

than normally encountered during full load operation.

Dynamic Performance Tests

The retarding capacity of the Bode rope brake model 580 was tested at the

first mine site installation on three occasions over a six month period.

During the test procedure, the elevator motor armature current, field

current, armature voltage, speed (analog tachometer feedback) and rope

brake cylinder air pressure were monitored and recorded on an 8-channel

thermal array recorder.

Rope Brake Test: Approximately 100 deceleration tests were conducted

over the six month period. Increasing rope brake retarding effort was

observed during the final tests. The increase in rope brake effectiveness

may be attributed to the grooves worn into the brake lining by the

suspension ropes. After approximately 125 operations of the rope brake,

the groove wear-in becomes self limiting.[8]

Initially the rope brake lining is flat and smooth, grooves are worn into the

brake lining after the rope brake has repeatedly stopped the elevator.These grooves conform to the contour of the suspension ropes which


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greatly increases the braking surface area. The increased surface area

dissipates the heat more effectively and therefore, reduces the peak

temperatures generated when the brake is applied. Lower brake lining and

suspension rope temperature increases the coefficient of friction and

consequently generates a greater braking effort.

Another factor which would increase the braking effort was the cleaning

effect the application of the rope brake would have on the suspension

ropes. The repeated application of the rope brake over the testing period

would have stripped the dirt and grease accumulations off a majority of the

suspension ropes. If the rope brake was applied on a cleaned portion of

suspension ropes, the braking effort would improve.

Low Air Pressure Tests: A series of tests were conducted with the air

compressor motor disconnected from the power source to determine the

number of times the rope brake could stop the elevator from the stored

pressurized air in the compressor tank. The tests were conducted with nocar load in the upward direction. The elevator was stopped by the rope

brake twelve times from rated speed with the air compressor power supply

disconnected as shown by the dashed line in Figure 8. Then the air pressure

fell to 52 psi, the pressure switch tripped and opened the elevator control

fault string and prevented operation of the elevator. The pressure switch

contact was temporarily bypassed to allow further testing. The rope brake

was activated eight additional times and the corresponding air pressure and

stopping distances are indicated by the solid line in Figure 8. The rope

brake was activated at speeds ranging from 640 to 680 ft/min. The

stopping distances were calculated from the actual deceleration rates based

on an initial speed of 600 ft/min. As expected, the stopping distance

increased as the available air pressure decreased. The rope brake was able

to effectively stop the elevator in 82 feet with as little as 30 psi in the air

compressor tank. After the rope brake set, only 22 psi was available in the

air compressor tank. The slight distortion in the curve may be attributed to

the varying condition of the suspension rope surface and initial speed

fluctuations.


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Fig. 8. Rope Brake Retarding Effort During Low Air Pressures

Compound Braking: The effect of compound braking is always a concern on

hoisting systems equipped with multiple brakes. This elevator is equipped

with three independent braking systems; the machine brake, dynamic

brake, and rope brake. Each system must be individually capable of

retarding the elevator. However, excessive deceleration rates should not

occur when all the braking systems are activated simultaneously.Analysis of the data showed the greatest deceleration rates were observed

when the machine, dynamic, and rope brakes were activated with no car

load in the down direction. This compound braking produced a deceleration

rate of 13.8 ft/s2, which is considered to be a safe stopping rate.

To better illustrate the compound braking effect, speed curves from 4

separate mine site tests are shown in Figure 9. The first three curves show

the machine brake, dynamic brake, and rope brake independently activated

under no load in the ascending direction. The combined response of all

three braking systems acting together, under the same test conditions, is

shown on the compound braking curve.


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Fig. 9. Brake System Response, No Cage Load, Ascending Direction

THE MACHINE BRAKE provides a linear deceleration rate of 2.5 ft/s2.

However, a slight fading of the braking effort was observed as a result of

the temperature rise in the brake lining during the final 400 milliseconds.

There is also an initial increase in speed while the overhauling

counterweight accelerates the car upward, prior to the machine brake

setting. The retarding effort of the drive motor is interrupted immediately

by opening the M contactor. However, there is an inherent 440 millisecond

time delay before the machine brake sets.


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THE DYNAMIC BRAKING produces a retarding force proportional to the

speed, with an initial deceleration rate of 2.7 ft/2. The dynamic braking

system begins retarding the elevator immediately since the motor contactor

connects a resistor across the motor armature, instead of opening the

circuit and allowing the counterweight to accelerate downward. The

dynamic braking effort is reduced as the speed decreases until anequilibrium is reached between the retarding effort and the load forces,

resulting in a steady state speed.[6]

THE ROPE BRAKE produced an inverse speed response and developed a

deceleration rate of 7.14 ft/2. This is the greatest retarding effort produced

by any of the three independent braking systems. The rope brake retarding

effort increases as the rope speed decreases to produce the observed

convex shaped speed curve. This brake also suffers from an inherent time

delay before actuation, similar to that of the machine brake, which results

in an increase in the initial speed.

THE COMPOUND BRAKING response produced a slightly "S" shaped curvewith an average deceleration rate of 9.09 ft/2. The initial 200 millisecond

deceleration response was proportional to the speed (concave speed curve)

as a result of the dynamic braking effort. After the inherent 200 millisecond

time delay in the mechanical braking systems, the braking curve exhibited

an inverse speed response as a result of the combined effort of the linear

machine brake and the dominant inverse speed rope brake.

Dynamic braking is an excellent system to assist the mechanical brake since

the dynamic brake limits the initial overspeed conditions without having a

significant compound braking effect.

Case Summary

Extensive mine and laboratory tests were conducted on the rope brake's

mechanical and electrical system to determine if the rope brake would

operate reliably in the mining environment to provide ascending car

overspeed protection. As a result of the testing and evaluation, several

modifications were required to enhance the reliable operation of the

emergency rope brake in the mine environment and during fault conditions.

TRACTION SHEAVE BRAKE

Northern Elevator Limited1has developed and tested a device which would

fulfill the new Canadian code requirements. This device is expected to becost effective and be easily retrofitted to the existing line of Northern

machines and possibly others, as well.[9]The device is the "Traction Sheave

Brake"1 or, as it is nicknamed, the "Sheave Jammer." The Northern

"Traction Sheave Brake" assembly is mounted on the driving machine so

that its braking pads are in close proximity to the rim face of the traction

sheave on the opposite side to which the suspension ropes ride and the load

is applied as shown in Figure 10.

The sheave brake is not applicable to elevators with suspension ropes

double wrapped around the traction sheave and deflection sheave. The

device will engage and apply braking force directly to the traction sheaverim face in either direction of the traction sheave rotation (car travel). The
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applied braking force is sufficient to cause the car to decelerate and be

brought to a stop, from either a high speed or low speed condition without

any assistance from the machine brake.

Fig. 10. Traction Sheave Brake

The applied braking forces exerted on the traction sheave rim face are less

than the loading forces imposed via the suspension ropes. The "Traction

Sheave Brake" is held in the released or normal running position by a

solenoid coil which is normally energized. When the solenoid coil is de-

energized, the carrier and frictional plate assemblies will be forced against

the traction sheave rim face by the action of compression springs. If the

traction sheave is rotating during or after the frictional plate has made

contact with the sheave rim, the frictional plate will be pulled by the

rotational movement of the traction sheave to the engaged braking

position. The movement of the frictional plate assembly during the


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engagement operation, will be horizontal as well as vertical because of its

wedge-shaped profile. The vertical component of this movement, against

the calibrated disc spring sets create the force necessary for braking.

The compression stroke of the disc spring sets is controlled by the

horizontal stroke of the frictional plate assembly, which is controlled(limited) by the stroke adjustment stop bolts. Once engaged, the brake is

self-locking and can only be released by re-energizing the solenoid coil,

then rotating the traction sheave slowly in the opposite direction from which

the device was applied until the device is again centered. At this point, the

device is reset and the required running clearance is re-established to allow

normal operation of the drive machine.

The device is equipped with a safety switch which causes power to be

removed from the driving machine and brake, when the device is in the

engaged position, to prevent further operation of the elevator equipment.

The power for the "Traction Sheave Brake" solenoid coil is supplied from a

battery backed-up power supply and controlled by a series of monitor

circuits, designed and arranged to de-energize the solenoid in the event of

either low-speed uncontrolled movement of the elevator away from the

landing with its doors open in either direction of travel or in the event of an

ascending car over-speed condition (where no counterweight safeties are

provided). Additional circuitry and battery back-up are provided to prevent

nuisance engagements due to power failure, door lock clipping, safety

circuit, stop button activations and/or other like occurrences. Circuitry is

provided to delay enabling of the drive machine during power-up to ensure

the device is in its normal running position and clear of the traction sheave,before allowing the machine to start.

The sheave brake is the newest emergency braking system to be

developed. Otis Elevator Company1 has recently been assigned a patent for

a similar "sheave brake safety" design on December 18, 1990.[10]

At this time, a sheave brake has not been installed on a coal mine elevator.

Therefore, dynamic performance test data was not available when this

paper was written.

CONCLUSIONS

Elevator accidents have indicated a strong need to provide ascending car

overspeed protection. Three new emergency braking systems have been

developed to meet this need. The time has also come to review the current

elevator safety equipment, and incorporate these new technologies in the

field of elevator safety.

REFERENCES

[1] "Mine Hoist Inventory," (MSHA Data - June 1988), Compiled by the

U.S. Bureau of Mines, Pittsburgh Research Center.

[2] W.J. Helfrich, "Island Creek Coal Company V.P.-5 Mine," MSHA, MineElectrical Systems Division Investigative Report No. C080978, August


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1978.

[3] T.D. Barkand, "Investigation of the Accident and Installation and

Testing of Dynamic Braking on the Main Elevator at Duquesne Light,

Warwick Mine, #3 North Portal," MSHA, Mine Electrical Systems

Division Investigative Report C-052287-12, May 1987.

[4] A17 Mechanical Design Committee Report on Cars Ascending into the

Building Overhead, ASME, September 1987.

[5]T.D. Barkand, W.J. Helfrich, "Application of Dynamic Braking to Mine

Hoisting Systems," IEEE Transactions on Industry Applications,

September/October 1988.

[6] J.A. Nederbragt, "Rope Brake: As Precaution Against Overspeed,"

Elevator World, July 1989.

[7] T.D. Barkand, "Ascending Elevator Accidents: Give the Miner a

Brake," IEEE Transactions on Industry Applications, May/June 1992.

[8] J.A. Nederbragt, "Report of Test Results" conducted by TNO-IWECO,Delft Technical University, June 7, 1989.

[9] "Traction Sheave Brake" Elevator World, December 1990.

[10] Patents - "Sheave Brake," Elevator World, pg 108, April 1991.

http://www.msha.gov/S&HINFO/TECHRPT/HOIST/PAPER5.HTM

emergency braking systems for mine elevators

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