When should you check limit switches? Many people completely misunderstand this rule. There are also many who think they don't need to check a limit switch, and of course some who don't want to check it because they're worried it will fail. This is of course the exact reason you should check it. So when should you check limit switches? Well, if you said daily, you're wrong again. You should check it every shift! That's the correct answer. If your facility has multiple shifts, then you need to check it at the beginning of the shift. Checking it only once a day can cause things to happen because the next operator won't know what the first shift did. So, do yourself a favor and check the limit switches at the beginning of every shift in your facility.
Another popular opinion I hear when visiting customers is that limit switches need to be tested under load. This answer is absolutely wrong. You should never check a limit switch under load. For that matter, you should never check any part of a crane or hoist under load. The only thing that should be on the hook during testing is a load test, which should be done by a professional crane technician.
Many companies get really worried and tell their crane operators to skip the limit switch check. They worry that the limit switch may be stuck or not working. I tell you, if you want to find out, it's during the inspection, you don't want to find out when you are making an important selection. To check the upper limit, you should move the load block slowly into the limit switch. Do not go straight into high speed. Run at a low speed, or if you have a single speed crane, wiggle the pendant so that the load block moves slowly into the limit switch.
Many manuals will advise you to go into high speed. Personally, I leave that up to the operator. You can jog it into the limit switch after that. However, be prepared that the load block may not stop in the same spot it did when you jogged it into the limit switch. Just like a car, the faster you go, the longer it takes to slow down; at least if you have a VFD, that's how it works. So, be careful!
The second myth that exists in the crane world is almost the exact opposite of what I just said. I have spoken with many customers who think they can use limit switches on their operating devices. When I say operating devices, I mean that they think they can use limit switches every time they operate their hoist. There are many applications that require the load block to reach its upper limit in order to get as much height as possible from the load hook. However, unless you have two limit switches on your hoist, this is not allowed and is not recommended. Limit switches are designed to be a safety device only; therefore, constant operation is not appropriate for the engineering of the device. This may reduce the life of the limit switch because many limit switches are made of plastic. If your crane is equipped with two limit switches, you should check with your crane contractor to see if you can utilize the limit switches as operating devices.
You should have your crane company come in for inspections quarterly, semi-annually, or annually, depending on how your crane is used. At this time, you can talk to your crane contractor to see what they recommend for your daily and monthly inspections. Depending on your crane, you may want to inspect it differently than I recommend.
