Thursday, June 3, 2010

Limit Switches
When do you check a Limit Switch? Many people understand this regulation completely wrong. There are many others that think that they don’t need to check the limit switch, and of course others who don’t want to check it because they are worried about it failing. This of course is the exact reason why you should be checking. So when do you check a limit switch? Well, if you said daily you are wrong again. You should be checking it each shift! That is the correct answer. If your facility has more than one shift then it needs to be checked at the beginning of that shift. Checking it only once a day opens the door for something to potentially to happen because the next operator has no idea what was done on the first shift. So, please for your own good check the limit switch on the beginning of each of your facility’s shift.
Another idea that floats around that I hear when I visit my customers is that the limit switch needs to be tested under a load. That answer is absolutely false. You should never check the limit switch under a load. For that matter you should never check any part of the crane or hoist under a load. The only time a load should be on the hook during a test is during a load test and that should be done by professional crane technicians.
Many companies are very worried and tell their crane operators to skip the limit switch inspection. They worry that it may get stuck or does not work. I tell you if there ever is a time you want to find that out, it is during an inspection, you don’t want to find out while you’re making a vital pick. To inspect the upper limit you should be inching the load block into the limit switch. Do not go right into high speed. Go in slow speed or if you have a single speed hoist then jog the pendant so the load block is inched into the limit switch.
Many manuals will suggest you to go into high speed. Me personally, I leave that to the operator. You can after you jog it into the limit switch. However be prepared that the load block may not stop at the same point that it stopped when you jogged it into the limit switch. Just like a car, the faster you go the longer it will take to slow down; at least if you have a VFD that is how it would work. So be careful!
A second fable that exists in the crane world is pretty much the exact opposite of what I just told you. I have talked to many customers that think that they can use the limit switch at an operational device. When I say operation device I mean that they think they can use the limit switch every time they operate the hoist. There are many applications that would require the load block to reach its upper most limit in order to get the most height out of the load hook as you possibly can. However unless you have two limit switches on your hoist this is not allowed or recommended. A limit switch is designed as a safety device only; so constant operation was not applied to the engineering of the device. This can decrease the life of the limit switch since many are made out of plastic. If your crane is equipped with two limit switches than you should check with your overhead crane contractor to see if you can utilize the limit switch as an operating device.
You should be having a crane company come in quarterly, semi-annually, or annual depending on the usage of your crane. During this time it is a good time to talk to your crane contractor too see what they recommend for performing your daily, and monthly inspections. Depending on your crane you may want to inspect it differently from what I have suggested.

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