Saturday, January 7, 2012

Mechanical Load Brake Operation and Inspection

There are many misconceptions of mechanical load brakes on lifting equipment. I have heard from many customers that they think the load brake is designed to hold the load during normal hoisting operation. This is not completely correct. I have also been told from customers that load brakes are required to be inspected on an annual inspection. This too is incorrect. There are two basic reasons hoist have load brakes. The mechanical load brake has to control the speeds when lowering the load, and has to be able to hold the load when the motor is not running. The load brake operates very similar to a nut and bolt. Let me introduce you to load brakes and give you a brief description of how the load brake works and see if I can help you understand the load brake functionality and inspection frequency for them.

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 electric motor 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.
Let me explain my statement when I said that it is not completely true that the load brake is designed to hold the load during normal hoisting operation. The load brake actually works together with the hoisting 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.

The other misconception is that the load brake requires an annual inspection. This is incorrect. All load brakes are self-adjusting and minimum maintenance is required. There is however cases the load brake require an inspection. If you hear clicking sounds coming from your load brake then an adjustment is required and can be done externally with the adjustment nut. However if it has been several years since the gear case has been opened and inspected than it is a good idea to open the gear case to inspect the brake linings and other components. Lubrication to the load brake is also highly important. Every time pressure is applied to the load brake the heat is dissipated by the lubricant. If not enough lubrication is in the gear case than the brake will over heat destroying the brake liners. In some cases forced cooling is required for heavy loads. The lubrication should either be changed out annually or lab tested. The viscosity of the lubricant is vital to control the operating temperature of the gear case. Old or dirty oil will change the original viscosity of the oil. Like anything oil too has a shelf life even if it is clean. If the viscosity or oil level is too high or too low it can cause overheating, excessive power consumption, and foaming. It is recommended to inspect the gear case and load brake every 4 years or per your OEM. Each OEM will have a different recommendation so check with your owner’s manual for inspection frequency. It is recommended as well to test the load brake annually at 25% of the load capacity. Not following your OEM recommendations could result to premature wear of your load brake.

I hope this small amount of information has been helpful to you. The inspection process although not annually is very important in safety of your crane and in extending the life of it. It may be a costly investment but it is vital in preserving the life of your crane and the safety of those that work under it.

8 comments:

  1. Well written. One minor correction though: There are some older hoists out there whose load brakes are designed to be able to hear the clicking. The KITO or Harrington ES series and some Coffing EC hoists allowed the pawl to lightly touch the ratchet.

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  2. Thank you Mr.Mike.

    You are correct that their are a few style load brakes that they are designed to hear the ratchet. Typically these are light duty hoist. If the ratchet and pawl are always hitting each other, eventually it will wear out. Harrington is a good hoist but you do want to be careful load brakes that are designed to hit the pawl and ratchet assembly. For the most part load brakes are designed so the pawl does not hit the load brake ratchet.

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  3. I just stumbled across your blog yesterday. Great find! Thanks for doing such a great job. I've already passed of the link to your blog to some of our inexperienced techs.

    Touching on what mrmikeb wrote. I've been trying to wrap my head around how a clicking pawl can tell you that the load brake needs to be adjusted. Can you clarify what model of hoist that it's on? It's the first I've heard of this.

    For example, Shawbox, a very common brand doesn't use ratchet pawls in their hoists. Nor does Whiting, Detroit, or R&M. A clicking ratchet in a Yale EW series usually means that the friction spring in the pawl has broken. The same goes for P&H Hevi-lift.

    Also, I think it might be a great idea to touch on the newer package hoists coming out that don't have load brakes due to better friction gearing and squirrel cage motors. And of course eddy brakes and VFD dynamic braking.

    Again, thanks for doing this and keep up the good work!

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  4. Thank you Matt for sharing my blog. My intent on this blog is to better inform people in the industry and companies that have cranes in their facility.
    You are correct that the many hoist on the market today do not have a load brake but only a motor brake. Like you said, because of the friction gearing that is on the market today there is not a need to have load brakes.
    The load brakes I was discussing are Weston style load brakes.Weston style load brake is what you see on P&H or Shepard Niles. There are other types of load brakes out there. Like you stated Shawbox has a different style load brake. Shawbox uses an electric solenoid. I focused on Weston because this is a traditional style load brake which is very common in the industry. I will see what I can do about putting something together to explain all the different load brake styles. That is a good idea. Thank you for posting Matt.

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  5. Thanks for the info. And can you please explain some more briefly about how this ratchet and pawl mechanism are controlled during hoisting and lowering the load?

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  6. What about a load brake chattering while lowering say shepard niles for example?

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  7. sorry for the anonymous above. You have great info on your blog.

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  8. Do you have Manual / Procedure ( Precaution to be taken ) for overhauling Weston Load Brake for 20 Ton Hoist.

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