they are cleaning spillage or accumulations of ore from idlers and pulleys. Nip angles between belts and their idlers and pulleys should be effectively guarded.

To minimize spillage, the belt must be uniformly loaded and alined. Self-alining idlers, side rollers, and mechanical pulley cleaners help keep the belt in alinement and reduce the side-friction fire hazard. A paddle switch hung on the feed chute just outside the trajectory of the material feeding onto the belt has been used successfully to stop the belt when the chute becomes plugged or overflows (fig. 35).

Belt circuits should always be wired electrically to start and stop in the proper sequence.

Belt fires caused by the friction of slippage at the drive pulley can best be minimized by installing a centrifugal roller switch which derives its rotation from an idler pulley on the return side of the belt. As long as the belt is operating at normal speed the roller switch is closed; should the belt slow for any reason the switch opens, and the belt stops.

Other sources of belt fires are trash and debris loaded onto the belt. Powder boxes, paper, and the like, when hung up on the beltline, may ignite from friction.

Provision should be made for cutting off the power and preventing accidental starting of the conveyor during repair and cleaning. Crossovers should be provided at intervals to remove the temptation for a workman to climb over a moving belt. Adequate clearance on one side is a necessity, and a railing between the walkway and the belt is desirable. Where working clearances are not provided on both sides, lubrication connections should be extended to the clearance side. Riding the belt should be prohibited.

FIGURE 35.-Underground Conveyor Controlled by Paddle Switch.
(Courtesy National Safety Council.)

One of the difficulties in conventional belt-conveying practice is adherence of particles of material being conveyed 29 to the carrying surface of the belt. When sticky materials are transported this often causes buildup of material on the return idlers, which may result in difficulty in training the belt.

Much attention has been given to the problem of cleaning the surface of belt conveyors, and many devices have been developed. Even the best of these has not been entirely effective, and most of them involve troublesome maintenance. One solution of the problem may be to twist the return belt through 180° so that, on its return run, the clean surface of the belt is carried on the return idlers. The belt is inverted again just before the return pulley to restore it to its normal position for the forward run. The method has been successfully used to stockpile over 1,000,000 tons of ore in Michigan. Here it has been found that, for successful use, all belts should have vulcanized joints. The method is feasible and has certain advantages. Water sprayed on the ore-carrying side of the belt as it is twisted underneath to the return idlers lessens the fine spill under the beltline. The experiments in Great Britain and a few tests in the United States indicate that this method is feasible and has certain advantages.

The above paper outlines the possibilities of twisted and inverted gate belt conveyors, and indicates how, in addition to providing greater safety and improved conveying conditions from a dust point of view, it is possible to employ twisted belts with advantage in connection with bottom belt conveying.

29 Bloomfield, G. W., Prevention of Dust Spillage From Belt Conveyors: Read before the North of England Branch, N. A. C. M., May 20, 1953; pub. in Iron and Coal Trades Rev., Aug. 7, 1953.

HAULAGE CREWS

Selection, training, and supervision of employees are important factors in accident prevention. Haulagemen have considerable responsibility to perform their duties without injury to themselves and other employees. They should be capable, careful, and alert, with special emphasis on agility and good eyesight. A haulage applicant's previous experience is not necessarily proof that he is capable, and he should not be required to perform hazardous tasks until he has been trained and proved his ability in actual service. Close, direct supervision of haulagemen is often impractical, but positive disciplinary supervision is essential.

Realizing the importance of haulage employees, some companies proceed in their selection and training as follows:

1. Select the most promising applicants.

2. Train them in safe haulage practices and operating procedures.

3. Put them on probation with capable haulage employees.

4. Issue certificates and give regular assignments to those qualifying.

A safe plant is one where there is respect for discipline, which in its positive sense is continuous in every phase of the operation and applies to supervisors and workers alike; good safety discipline cannot be attained otherwise. Strict, positive discipline of haulage employees is probably more important than for other types of mine workers, because close, direct supervision is impractical.

One of the best approaches to proper supervision is formulation of a code of safe-operating practices (rules), augmented by frequent, short meetings of haulagemen, conducted by a haulage supervisor, at which rules, practices, and procedures are discussed.

MAN-TRIPS

Men are being handled with greater care and safety than in the past. Better-built cages and man-cars are in use for taking men into and out of the mine. Loading, unloading, and movement of mantrips should be under the control of a supervisor who will see that safety rules are observed.

Many mines have waiting rooms fitted with lights, seats, and heat, if needed, so that the men will not sit or stand along the track while waiting for a man-car or man-trip. For sectional use man-cars are operated (fig. 36). These cars may be operated to the shaft station or to a central man-trip station. If the haul is long the cars are enclosed and provided with seats, especially if operated underground and on the surface (fig. 37).

Safe operation of man-trips or man-cars depends upon many factors, including the type and condition of equipment, the control of loading and unloading, clearance above cars, the number of persons permitted in each car, care by motormen or hoisting engineers, speed, and type of waiting stations.

Man-trips should come to a full stop before men load and unload, and the men should proceed in an orderly manner to and from mantrips. Only tools and supplies that can be carried entirely inside a separate car or cars should be handled on man-trips; blasting supplies should not be hauled with workmen. When riding in a car not suit

ably covered on a trolley haulageroad, men should ride on the side opposite the trolley wire. Men in man-trips should not ride in the car next to the trolley locomotive unless such a car is specially designed to provide front-end and overhead protection.

If there is any danger of men coming into contact with the trolley wire at man-trip stations, the power should be cut off while they are getting on and off cars; moreover, the wire should be adequately guarded at these stations. Man-trips should be operated at safe speeds, and they should not be operated behind loaded trips on ascending grades or in front of loaded trips on descending grades. Man-trips should never be pushed.

Man-trips or man-cars on mechanical or gravity slopes or inclined planes should be provided with safety ropes connected to the main

rope above the regular hitching and extending around or through all cars. In addition, they should be equipped with devices for stopping if the main rope fails.

SIGNAL AND COMMUNICATION SYSTEMS

Transportation on many haulage levels now is frequently controlled by a dispatcher who transmits messages to motormen over a trolley mine-telephone system. Intercommunication among motor operators is possible; thus collisions and delays are prevented.

To increase efficiency and safety in haulage, several large mining companies have installed dispatcher systems with voice signals relayed over radio-communication equipment on each locomotive and at other strategic locations. Voice communication on a two-way basis has done much to relieve congestion and hazards in transporting ore underground.

A system installed by the Tennessee Coal & Iron Division of the United States Steel Corp. in Alabama 30 consists of frequency-modulation transmitters and receivers. Units were installed in each of the mainline locomotives; 1 each in a dispatcher's office at rotary dumps, the underground maintenance shop, and motor pit; and 1 on a materialhandling locomotive. Units operate on 100-kilocycle frequency modulation (FM); power is from the trolley wire. Current passes through a voltage divider, which gives 120 volts to the transmitter and 35 volts to the receiver. This system is operated to augment and not replace block-signal systems.

The dispatcher should have absolute control over the movement of trains, because if his orders are violated by a motorman who moves his train without clearance, a wreck may result. In addition, the dispatcher serves as a clearing house and relay station for messages that come from all parts of the mine by way of the locomotive-mounted units. Supervision can thus be applied more effectively; calls for materials can be relayed to surface; and trains can be more safely routed. Such equipment can also greatly expedite the removal of men from a mine in case of a mine fire or other emergency.

Some block signals are operated automatically; others are manually made and broken by locomotive operators.

Even in the smallest mine, locomotives can be equipped with headlights and gongs and trips with taillights and warning devices.

The foregoing systems are applicable to safe and efficient transportation in mines. As an emergency signal system to warn all employees, in the least possible time a stench-warning system in the compressedair line and ventilation currents has no equal.

CHUTES

Car loading at chutes leads to accidents, chiefly from rolling rocks, falling chute lips, and unsafe use of point bars. Often car loaders (who frequently are haulagemen) have very little room to work in and about a chute so that they must climb into the car or bend around it to start or stop the flow of ore. The hazards are greatly reduced with

30 Engineering and Mining Journal, Trolley Haulage Speeded by Better Communication: vol. 153, May 1952, p. 81.