Inspecting hoisting brakes for proper functioning is a key element of a proper periodic inspection and important to ensure that the load does not drop unexpectedly creating a potentially fatal safety hazard.
THE TWO HOIST BRAKING FUNCTIONS
Hoists must have two braking functions, Holding and Controlled Lowering. The holding brake maintains the load in a stationary position when the load is suspended and no power is applied to the motor. The controlled braking function must ensure that the load does not overhaul or overspeed the motor when the load is being lowered.
Many older style hoists use a friction type brake for the holding brake and a mechanical (Westin Style or other) brake for the controlled lowering. Newer style hoists often use the friction brake for both the holding and controlled lowering and provide a brake that is capable of 150% of full load hoisting torque. There are other braking setups employing newer or older technologies, however virtually all are based off a friction type brake for the holding the load.
HOW A WESTIN BRAKE WORKS:
During the hoisting mode the load brake is unlocked and freewheeling. The purpose of a “load brake in a hoist application is to keep the suspended load from falling without power to the hoist and independent of the motor brake.” When the load is being lowered the clutch locks stopping the load brake system from rotating. As soon as the motor stops driving the load, or the load travels faster than the motor then pressure will be applied to the friction disc. If the holding brake fails the load brake will completely lock to hold the load. You will also be able to lower the load to the ground. This is done by the self-adjusting clutch. The brake will tighten slowing the load, and the load brake loosens if it is going too slow. This is a rapid succession that keeps the load controlled. If you hear a clicking sound when the load is being raised, this is the pawl hitting the ratchet teeth. You should not be hearing this noise if the load brake is adjusted properly.
it is not completely true that the load brake is designed to hold the load during normal hoisting operation. The mechanical load brake (control brake) actually works together with the holding brake. The Weston style load brake (pawl & ratchet) holds approximately 25 to 30% of the load being picked up. The holding brake does the bulk of the work. This is why it is vital to have your holding brake inspected frequently so the load brake does not end up doing all the work causing it to wear faster.
It is imperative that both the operator and the Inspector be informed on the types of braking used in a particular hoist and trained in determining whether both the Holding and Controlled Lowering are in proper operation.
The Operator must test the functioning of hoist brakes on near capacity loads
How and when does an overhead crane operator test the functioning of hoist brakes? Federal regulation OSHA 29CFR1910.179(n)(3)(vii) states that: “The operator shall test the brakes each time a load approaching the rated load is handled. The brakes shall be tested by raising the load a few inches and applying the brakes.” So how exactly does one apply the brakes on an overhead hoist? Overhead hoist brakes are applied automatically when hoisting motion stops by releasing the control. This braking action, referred to as holding brakes, requires specific brake torque settings outlined in OSHA 29CFR 1910.179: “Holding brakes for hoist motors shall have not less than the following percentage of the full load hoisting torque at the point where the brake is applied. 125 percent when used with a control braking means other than mechanical. 100 percent when used in conjunction with a mechanical control braking means. 100 percent each if two holding brakes are provided.”
Testing hoist brakes is not so simple!
To test the holding brakes, we simply raise the load a few inches and stop hoisting. Holding brakes set automatically.
Now we need to test the control lowering. Control braking exists to prevent the load from accelerating in the lowering direction. If the load is very high and a mechanical load control brake fails when you attempt to stop during load lowering, will the holding brakes still be able to stop the load safely? Probably not. We’ve seen this occur with very heavy loads where the holding brakes were no longer functioning properly and were unable to stop the load from accelerating beyond the braking capability of the holding brakes resulting in a loss of load control (dropped load).
Now that we understand how an overhead crane’s holding brakes and control brakes combine to provide safe load handling, the process for checking control brakes is simple. After checking to ensure that the holding brakes are functioning properly, hoist the load a little higher, maybe a foot or so more. Allow the holding brakes to stop and hold the load. Next, lower the load about halfway and again stop the load.
For a hoist equipped with a mechanical load control brake, this procedure tests both the holding brakes and the control brakes. If an overhead hoists’s brakes are going to fail, they’re more likely to fail in the lowering direction. This procedure should be followed with the first load of the shift, and anytime a substantially heavier load is handled later in the shift. You can now see why holding brakes, when used in conjunction with control braking means other than mechanical, must be set to 125% of maximum hoist motor torque. Non -mechanical control brakes eddy current or flux vector only control descent speed while the hoist is moving, and are unable to assist with any load holding.
Operators are the first line of defense in accident prevention. Understanding how overhead crane brakes function, and conducting proper shift inspections and testing goes a long way toward ensuring a safer and more reliable operation. Other standards that all overhead crane users should be familiar with include the ASME B30.2, ASME B30.11, ASME B30.16, and ASME B30.17. Also, read and be thoroughly familiar with your overhead crane user’s manual, and check local and State requirements to make sure you comply with all laws. Tests are an attempt at a controlled failure. Operators must follow all safe work practices and ensure their body is never in a position to be contacted by the load.
Operators must follow all safe work practices and ensure their body is never in a position to be contacted by the load.