Friday, April 21, 2017

Overhead Cranes and Load Brakes... Why You Don't Need One!

 Since I have been selling and consulting with customers on overhead bridge cranes, I have received several quote request for hoist with load brakes. Mostly these are government or military specifications, but also a few private companies request the same. There are only three reasons why this would be listed in someone’s specifications today. One, it is the same spec they have used for decades so the easiest decision for those that know little about cranes to just continue with this decades old specification. Two, someone actually believes that is a requirement set by CMAA or other governing body. Or three, they honestly believe they are getting a safer more reliable crane with a load brake Well… all three are wrong and I will explain why.

First off you need to understand what the load brake is for. The mechanical load brake controls the speeds when lowering the load, and helps to hold the load when the motor is not running. The holding brake actually works together with the holding brake. The Weston Style brake only holds approximately 25 to 30% of the load while the holding brake does the rest.This is really what the load brake is for but I honestly don't feel that is why you see in many government specifications.  You can check out my more popular post Mechanical Load Brake Operation and Inspection. Here you will get a basic introduction to the load brake and learn about a few misconceptions about it.

Most hoist today do not have mechanical load braking technology in them. This is because it is outdated and does not fit the technology of our electric motors today.  A load brake does not make your crane safer. Many manufacturers who used to have load brakes in their equipment stopped decades ago. Yet, here we are still with new specifications requesting load brakes. The load brake was designed to control the load in it’s decent. In an emergency it should hold the load as well. This form of technology worked great back when all the hoist motors were wound rotor. A wound rotor motor cannot stop acceleration of the load from gravity. Therefore, load brakes were required for hoist and you certainly wouldn’t want to use a hoist without it. Today’s technology uses Squirrel Cage Motors. See video below for a two minute introduction. Squirrel cage motors are fully capable of regenerative braking. This technology meets all CMAA Spec 70 requirements. 

What about Variable Drives? Not only are load brakes not required with today’s technology, today’s technology does not work with load brake hoists.  If your hoist has a load brake you can forget about variable drives. You cannot get the speeds accuracy in a load brake hoist. Variable drives do not work with load brake hoist. You will have constant issues with your load brake and will become a safety and efficiency issue. If you need speed control on your hoist, and you have a load brake hoist, you will be out of luck.

So why do we need load brakes in all these government and military specs? If you ask me,.. I think it is a complete waste of money to invest in such outdated technology. It would be like going to Ford and asking them to remove power steering because you feel it would be more reliable without it. Just makes no sense. Don’t forget all the cost that is associated with your load brake hoist. That baby will need to be opened up at least every four years to verify the load brake is functioning properly, and re-adjust the air gap. The load brake is buried in your gear case; it is a very costly inspection. The brake works off of friction so it has pads or liners that wear causing it to need replacement periodically. Typically, it is best just to change them out during the inspection anyways. The labor to change them is more expensive than the liners so it is the perfect time. Your oil in the gear case will also be contaminated with the lining wear and dissipated heat from the load brake which will also affect your entire gearing in your gear box reducing the life of those gears.

Let’s also not forget that when you have high capacity cranes, or in process cranes you do not want a load brake. They create a lot of heat when stopping a load so it is very impractical for a process crane. This would also apply to any crane that would require a permanent below the hook device. If a below the hook device is connected to the load block permanently, the load brake is going to seize up causing costly downtime and repairs.

Today’s motor themselves are the secondary control braking function that is required by OSHA. Because people don’t really understand this process they disregard it altogether. The squirrel-cage motor functions using regenerative braking if the load tries to pull faster than the motors rated speed. Regenerative braking does not generate a lot of heat so it is a safer application for large capacity cranes and process cranes. 

Hopefully in the near future we will start to see the industry change to where there will be no more request for outdated technology.  It all starts with the best overhead crane companies training their customer's about the latest safety features available for their crane. You will get the safest cranes on the market using today's technology. 

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