Many older electric wire rope hoists have a mechanical or Weston-type load brake that was introduced in the 1930’s. Often the working of this secondary brake is a mystery to most of us so here is a great explanation of how the device works.
The Load Brake is called an automatic load break because it is automatically set by the loaded itself and holds the load until released by the motor. The brake consists of an alloy cast steel ratchet with cast iron wearing rings, riveted to the sides and machined true after riveting. Between those wearing rings, and the driving and driven heads are hard bronze friction liners, forming the friction element of the break. The ratchet is held in the lower direction by to harden steel pawls, so mounted that one is always within half a tooth space of engagement, and should one fail to engage, the other will catch. This arrangement affords safety and cuts backlash to a minimum. Two bronze plugs with a spring between, are placed in board holes in each pole. These plugs bear against an extension to the side plates on the ratchet. In lowering, the plugs pull the pawls into positive engagement, and in hoisting, they throw the pawls back clear of the ratchet, so there is no rattling. Bosses on the case form stops at the rear end of the pawls so as to provide about 1/8 inch clearance over the ratchet. If this clearance is increased much, there may be some backlash before the pawls engage. The brake should be examined through the observation hole in the top of the case occasionally. If the ratchet ends of the pawls become battered, the burrs should be removed as otherwise the pawls may be wedged backward and either the pawl or stop maybe broken.
When the load is suspended it tends to turn the drum and drum gear which turn the drum pinion. This tends to revolve intermediate gears and intermediate pinions. Pinions, the driving head, and the clutch are all keyed to the load brake shaft. The driven head runs free on the shaft. If the motor is not revolving, the motor pinion, motor gear, and the drilling head remain stationary and when the load brake shaft turns, the clutch runs up to the high part of the helix so as to force the driven head remain stationary, squeezing the ratchet between the two friction liners.
The ratchet cannot revolve in the lowering direction as it is held by a pawl. The load cannot descend unless the brake slips, however, the pitch of the helix in the clutch and the friction of the brake or so proportioned that the pressure produced by any load will cause friction enough to sustain that load. Therefore, the brake automatically holds any load within the capacity of the machine. If the motor runs in the lowering direction, it turns the driven head toward the lower part of the helix and thus releases the pressure on the brake and allows the load to descend, but it can only descend in proportion to the speed of the motor, for as soon as the load gets ahead of the motor it runs the driven head of the helix and tightens the brake. The speed of lowering is determined entirely by the speed of the motor. The brake is set just as hard in hoisting as in lowering, but the ratchet runs free, and the brake does not slip, for any slippage would simply screw the clutch up tighter and tighter, until the driven had turned with the load brake shaft.