Thomas D. Barkand
Mine hoists and elevators have experienced accidents with the potential for injuring or killing numerous miners. These accidents occurred on counterweighted hoisting systems when the mechanical drum brake failed while the cage was empty. This allowed the counterweight to fall to the bottom of the shaft, causing the cage to overspeed and crash into the headframe.
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 emergency braking systems has been developed and applied to mine hoists.
This paper will discuss the United States mine hoist inventory, accident statistics, revised hoisting regulations, and emergency braking systems designed to provide ascending conveyance overspeed protection. The design and testing of a passive electrical dynamic brake installed on a slope hoist and a case-study report on a pneumatic rope brake installed and tested on a single rope, vertical shaft mine hoist will be discussed.
The United States metal and nonmetal mining production during the same year was valued at 30.8 billion dollars and employed a work force of 127,000.(1) The 116 United States metal and non-metal mines utilized 310 hoists. These hard rock mining industries reported 94 hoisting accidents resulting in 15 injuries.(3)
Accident: Coal Mine Friction Hoist
Accident: Coal Mine Elevator
Accidents: Commercial and Industrial Elevators
Several supplemental emergency braking systems have been applied to mine elevators. Some of the proven systems are counterweight safeties, electrical dynamic braking, and a pneumatic rope brake system. The applications of these braking systems to multiple rope elevators are discussed in other literature.(8),(9) This paper discusses the application of passive dynamic braking and a suspension rope brake on single rope mine hoists.
CASE STUDY: DYNAMIC BRAKING
Hoist Mechanical and Electrical Specifications
During normal hoist operation, the regenerative braking capacity of the drive motor is
utilized to slow the hoist down to creep speed. Safe limits for acceleration and
deceleration have been established by the U.S. Code of Federal Regulations for metal and
nonmetal mining. The maximum normal acceleration or deceleration shall not exceed 6
A dynamic braking resistor provides emergency backup protection in the event that the
mechanical braking system fails. As previously mentioned, the hoist motor can act as a
generator to slow the hoist down to creep speed during a normal stop. However, during
emergency stopping the motor's retarding effort is not utilized unless a dynamic braking
resistor is properly installed.
Dynamic Braking System Design
The loss of the control system power to the field is detected by the field loss (FL) relay. This transfers the drive system into the dynamic braking mode by de-energizing the field loss and the dynamic braking (DB) relay. The drum brake is also set during a control power loss. If during a control system shutdown, the drum brake should fail, the overhauling load will cause the motor to be driven as a generator supplying braking current into the dynamic braking resistor and field current to the shunt field. The dynamic braking resistor would dissipate the energy generated by the overhauling load, and the load would be lowered at a safe, slow speed. Thus a "runaway" condition would be prevented. It is important to remember that dynamic braking does not stop the load like the drum brake, but rather is designed to limit the speed to a small percentage of the base motor speed.
Dynamic Braking Tests
The most demanding test of the dynamic braking system occurs when the drum brake fails after a prolonged power interruption has occurred. The dc hoist motor must generate its own shunt field current. For this test, the control power was removed while the conveyance was at rest. Then the drum brake was released and the conveyance accelerated down the slope at an initial rate of 0.4 ft/s2. The data obtained from this test procedure is shown in Fig. 2.
and Supply Car (24,000 lbs. total), Down
As the hoist approached base motor speed, 300 ft/min, armature current began to develop as the armature windings rotated rapidly through the weak residual magnetic field of the permanently magnetized field poles. The armature current, in turn, supplied current to the motor field through the dynamic braking resistor. The field quickly built up to 90 percent of its normal operating level. Once the field was established, the hoist rapidly decelerated to 100 ft/min. The final dynamic braking speed of this system was 33 percent of the base motor speed. This dynamic braking system effectively provided additional backup protection to the hoist. It would activate in the event of multiple system failures and prevent a high speed runaway condition on the hoist.
The dynamic braking system begins retarding the hoist immediately, since the motor contactor connects a resistor across the motor armature instead of opening the circuit and allowing the conveyance to accelerate downward. The dynamic braking effort diminishes as the speed decreases. Therefore, dynamic braking is an excellent system to assist the mechanical brake, since the dynamic brake produces the greatest deceleration during the drum brake dead time and provides very little retardation as the drum brake brings the hoist to a stop.
The electrical dynamic braking system is inherently unaffected by the number of suspension ropes and has been successfully applied to single rope mine hoists. However, the application of a rope brake on a single rope hoist has presented technical challenges. The next case study will discuss the control, design, and testing of the world's first application of a suspension rope brake to a single rope mine hoist.
CASE STUDY: HOIST ROPE BRAKE
Hoist Mechanical and Electrical Specifications
Rope Brake Design
The pneumatic design of the model 580 rope brake is illustrated in Fig. 4. When the rope brake is activated, a set of magnetic valves direct pressurized air from the compressor tank into 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 rope is clamped between the two brake pads. The rope brake is released by energizing the magnetic valves, which vent the pressurized rope brake cylinder to the atmosphere through a blowout silencer. The brake pads are forced open by six coil springs.
The force exerted on the suspension rope 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 in2 of surface area. The working air pressure is set at 120 lbf/in2. The corresponding force applied to the suspension rope is 49,123 lbs.
Rope Brake Installation
Rope Brake Modifications
A suspension rope guide, shown at the top of Fig. 6, was added above the rope pulse tachometer to prevent the slack rope from damaging the rope tachometer. A slack rope condition is generated by the two-month safety catch test, required by Mine Safety and Health Administration regulations contained in the Code of Federal Regulations, Title 30 §75.1400.
The thickness of each rope brake lining was also increased from 3/8 inch for typical elevator installations, to 3/4 inch for this hoist installation. The increased thickness of the brake lining material was required to allow for the greater wear demand due to increased rope abrasion, limited contact area on a single rope, and increased conveyance load. The additional brake lining also reduces the possibility of the 1-1/2 inch hoist suspension rope becoming damaged if the brake lining wears completely down to the brake pad mounting plates, since the combined thickness of the two linings is 1-1/2 inches. The brake lining wear characteristics will be discussed later.
The final modification was the addition of the brake pad guide shown in Fig. 7. The "L" shaped, pivoting steel plates on each side of the rope brake insures the brake pads remained parallel to each other. This prevents the original guides from binding or the brake piston from seizing when the brake is applied to the single suspension rope which is not directly in the center line of the rope brake.
Rope Brake Tests and Results
A series of tests were then conducted to evaluate the performance of the rope brake under extreme loading conditions and multiple control system faults. The compound braking effect on the deceleration rate was also evaluated. Compound braking occurs when the rope brake and the hoist brake operate simultaneously.
Average Hoist Deceleration Rates The rope brake and the compound braking deceleration rates are shown in Table 1. The four test conditions represent the extreme hoist loading conditions for all possible directions of travel. The braking systems were activated at the rated speed of 600 ft/min. The rope brake experiences an inherent brake dead time of approximately 300 msec, which is similar to the hoist drum brake.
The deceleration rates of each braking system cannot be added algebraically to yield the compound deceleration rate because of the mechanical time delay and the non-linear characteristics of the rope brake deceleration. For example, the machine brake will stop the empty cage traveling downward at an average deceleration rate of 7.3 ft/s2. Table 1 shows the rope brake deceleration under this condition is 4.0 ft/s2. However, the compound braking deceleration rate is 8.7 ft/s2, not 11.3 ft/s2.
The accepted maximum value for emergency deceleration is 16.1 ft/s2 (0.5g). Limiting emergency decelerations to rates less than 0.5g will minimize the possibility of injuring the passengers on the hoist. As shown in Table 1, the compound braking effort did not exceed 16.1 ft/s2 (0.5g) for any possible condition.
Typical Test Recording Fig. 8 shows the thermal array chart recording when the rope brake was applied, while the mechanical brake was held off. The cage was empty and traveling at the rated speed of 600 ft/min, in the ascending direction.
Empty Cage, Ascending Direction, Rated Speed
The hoist speed and armature voltage signal demonstrate the initial acceleration that typically occurs when the armature current is interrupted before the rope brake develops sufficient braking effort to decelerate the falling counterweight. The curved speed deceleration profile shows the braking force is inversely related to the speed of the hoist. This observation is consistent with previously reported findings on multiple rope elevators.(9)
The oscillation in the cage acceleration signal typically occurs when the rope brake stops the hoist. The oscillations are attributed to the elasticity of the suspension rope. The oscillations are also reflected into the hoist drum as shown by the drum acceleration signal. The inverse speed braking effort is indicated by the increasing deceleration shown in the drum acceleration signal.
Rope Brake Hold Test A one-hour rope brake hold test was conducted to evaluate the static braking capacity and to verify the integrity of the pneumatic system. The test was conducted by first setting the rope brake while the hoist was stationary with 125% of rated load (13,437 lbs.) on the cage. The air pressure in the compressor tank was then completely discharged. The rope brake remained set, due to a check valve in the air supply line. The rope brake must hold the load for one hour with no more than a 1 percent air pressure drop in the rope brake cylinder.
Slack rope was generated above the cage by operating the hoist in the down direction with the rope brake set. The rope brake held the static overload (cage + 125% load + rope = 28,692 lbs.) for one hour. During that time, the rope brake cylinder air pressure dropped from 104 to 103 psi.
The force applied by the rope brake at 103 psi is 42,164 lbs. This easily held the static load of 28,692 lbs. The rope brake should be able to hold the static load with only 70 psi of cylinder air pressure.
Rope Brake Lining Wear The testing sequence called for the four empty cage tests in Table 1 to be performed followed by the remaining four rated load (10,750 lbs.) tests. After the four empty cage tests were performed, a total of 0.5 inch of the brake lining material had been worn away. The remaining four load tests wore away 0.344 inch. (The rope brake only test, 10,750 lbs., down, 600 ft/min. accounted for 0.281 inch of the brake wear.) The remaining 0.656 inch of brake lining material was insufficient for the rated load, overspeed test which is discussed next.
Rope Brake Overspeed Tests The most demanding test on the agenda was the overspeed activation of the rope brake at rated cage load (10,750 lbs.) in the descending direction. The hoist speed was increased gradually until the rope brake sensed the overspeed and activated the rope brake at 702 ft/min. The hoist accelerated to 790 ft/min. before the rope brake began to develop braking effort. The hoist began to decelerate at a rate of 1.8 ft/s2. When the hoist slowed down to 280 ft/min., the 0.656 inch of brake lining had worn completely away and the brake pad surfaces made contact. Without the braking effort, the hoist began to accelerate until the hoist brake was set at 750 ft/min.
The brake linings were replaced with a different material and the test was repeated on February 3, 1992. The hoist stopping distance was 75 ft. with an average deceleration of 1.1 ft/s2. However, about 90 percent of the 1-1/2 inch thick brake lining material was worn away by the rope during the test. The replacement brake lining material may not have provided the same wear characteristics as the original material.
Rope Lubrication Since this hoist is not friction driven, the suspension rope can be protected by applying lubrication without causing rope slippage on the hoist drum. The lubrication presents a technical challenge not previously encountered when the rope brake was installed on friction driven elevators. The rope lubricant may have increased the stopping distances, which resulted in additional heat generation as the rope pulled through the applied rope brake.
The Pennsylvania Bureau of Deep Mine Safety has initially denied approval of the rope brake installed on the mine hoist. The decision was based on brake lining wear criteria established on previous elevator installations.
CONCLUSIONS Mining industry accidents indicate a strong need for supplemental ascending overspeed protection on personnel hoisting systems. Passive dynamic braking is a proven method of providing the supplemental overspeed protection on DC mine hoists. Additional emergency braking systems must be developed to improve the safety of mine hoisting systems.