Electric Chain Hoist Limits

[benelmo]

Okay, I had a long-standing belief that a motor's limit was affected by running the motor out of phase. I basically thought that the more chain you ran through while the motor is out of phase, the more the limits were being thrown off.

 

Some years later, when I started disassembling and repairing chain motors, I realized that the limit shaft is mechanically linked to the lift wheel. To me, this meant that it didn't matter what phase it was running in; the limit switches wouldn't know the difference. At this point, I was under the belief that the only way the limits got messed up was due to mechanical failure in the spindle (missing teeth, stripped spindle, etc).

 

Now, ten years later, I have no idea. We precisely set about 20 nice, new CM Lodestar motors this time last year. We check them, and half of them are up to 12" off the mark. There is no mechanical problems in any of the limit sections. WHAT GIVES???

 

I don't think our brakes or clutches are slipping either, we don't overload them.

 

WHAT MAKES THIS HAPPEN??? ANYBODY????

[Joe Betts]

Okay, I had a long-standing belief that a motor's limit was affected by running the motor out of phase. I basically thought that the more chain you ran through while the motor is out of phase, the more the limits were being thrown off.

 

Some years later, when I started disassembling and repairing chain motors, I realized that the limit shaft is mechanically linked to the lift wheel. To me, this meant that it didn't matter what phase it was running in; the limit switches wouldn't know the difference. At this point, I was under the belief that the only way the limits got messed up was due to mechanical failure in the spindle (missing teeth, stripped spindle, etc).

 

Now, ten years later, I have no idea. We precisely set about 20 nice, new CM Lodestar motors this time last year. We check them, and half of them are up to 12" off the mark. There is no mechanical problems in any of the limit sections. WHAT GIVES???

 

I don't think our brakes or clutches are slipping either, we don't overload them.

 

WHAT MAKES THIS HAPPEN??? ANYBODY????

 

You were right the first time.

The limits are 2 switches at either end of a shaft.

2 plates are set at a specific spacing to give the hoist its top and bottom limits, and when the shaft turns the

plates move along it in the direction of the rotation untill one hits a switch and stops the movement of the hoist.

 

When the hoist is mis phased, and is run out to the limit, pressing up will trigger down, and the hoist will "run past the limit" meaning

the plate will be screwed tightly against the switch, and potentially skip a few threads to stay in position, this alters the position of the limit on that end, and the same applies the other way around.

 

Over several uses in this manner the differance could be very obvious.

 

This is why its important to check and correct phasing before using the hoists.

[Seano]

I don't think our brakes or clutches are slipping either, we don't overload them.

 

We're talking Lodestars here, right?

 

If you run beyond the 'top' limit until the hook meets the body of the motor, you do - the chain wheel is effectively jammed and from that point until the motor stops the load protector will be slipping. It doesn't take long for a slipping load protector to cook itself. But that's by the by.

 

When the hoist is mis phased, and is run out to the limit, pressing up will trigger down, and the hoist will "run past the limit" meaning

the plate will be screwed tightly against the switch, and potentially skip a few threads to stay in position, this alters the position of the limit on that end, and the same applies the other way around.

Quite so. It also bends the nuts, and doesn't do the microswitches any favours either.

 

This is why its important to check and correct phasing before using the hoists.

More importantly are the consequences of running the motor with no effective limits - at one end the hook can run into the body and ultimately destroy the load protector (see above).

 

At the other end you can break the soft link and run the chain out of the motor all together. This is an inconvenience if you've already landed the motor in its box and you're just running it out, but if its rigged motor-up it'll mean dumping the load chain on the floor (if you're lucky - dumping it on someone's head if you're not).

 

If (god forbid) you're doing a staged lift or something whacky that involves reaching the end of the chain before the load touches down (very rare, but it happens) - the whole load gets dropped. (shudder)

 

Lodestars are available with a 'smart phasing' device which effectively makes it impossible to run them with their phase rotation incorrect, possibly a worthwhile investment if you have motors going out on hire and being run beyond their limits. (Pretty sure they can be retro-fitted to older hoists.)

[Snailtrail]

You don't say whether your motors are toured or in an install. I believe that the limit switches can spin a little due to vibration in transit... If they've been toured heavily over the year then this could possibly be the reason.

 

Sam