EMERGENCY BRAKING SYSTEMS FOR MINE ELEVATORS
Thomas D. Barkand
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 dynamic brake, pneumatic rope brake, and traction sheave brake. The effect of compound braking on hoisting systems equipped with multiple brakes will also be addressed.
Mine elevators and personnel hoists provide a lifeline for miners at more than 240 coal mines nationwide. 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. 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.
Rules and regulations applying to elevator safety have come under review in 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.
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 in Figure 1.
The counterweight adds accelerating force when the elevator car is ascending 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. 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 from occurring.
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 machine brake and provide ascending car overspeed protection. These systems can be installed on existing elevators. Three recently developed braking systems are presented here for consideration.
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 motor armature 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.
DYNAMIC BRAKING INSTALLATION AND TESTING
Performance testing of dynamic braking has been conducted on several mine hoisting systems. Two case studies of service elevators that had dynamic braking installed, and were recently tested, will be presented for illustrative purposes.
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.
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 on the 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.
When a total power loss occurred, the dynamic braking resistor was connected across the armature, the DROK relay dropped out, and regenerative 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:
The armature current, armature voltage, field current, and motor speed were recorded on a thermal array recorder during the tests.
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.
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, 8.61 ohm at 25°C. 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 between the dynamic braking resistor and the field winding.
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-state speed 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 the peak speed.
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.
A pneumatic rope brake has been developed by Bode Elevator Components1 which grips the suspension ropes and stops the elevator during emergency conditions. A typical rope brake installation is shown in Figure 6.
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 the doors 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.
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.
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. 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.
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 magnetic two-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.
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, the force 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.
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 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 no car 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.
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.
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.
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 an equilibrium is reached between the retarding effort and the load forces, resulting in a steady state speed.
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 curve with 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.
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.
Northern Elevator Limited1 has developed and tested a device which would fulfill the new Canadian code requirements. This device is expected to be cost effective and be easily retrofitted to the existing line of Northern machines and possibly others, as well. 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 sheave rim face in either direction of the traction sheave rotation (car travel). The 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.
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 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.
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.
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.