Spinning Cathead?

[MikeHalloran]

[ A cathead is the odd sprocket used to engage link chain, most easily visible on anchor windlasses. ]

In this instance, I'm designing parts of a bridge crane. Because it should not see regular use, the owner has asked for the bridge travel to be unpowered, by means of a cross shaft between the trucks, a cathead sprocket, and a loop of chain hanging down toward the floor. The hoist is specified as electric.

The hoist trolley is described in the written specifications as electric, controlled by the same pendant that controls the hoist.

.... However, the bid drawings show two chain loops hanging from the bridge, one near a truck as expected, and one hanging from what should be the hoist trolley motor.

The owner's PE has apparently chosen to interpret what I consider a drawing error to be some kind of combined specification, and is insisting that the hoist trolley should be electrically powered _and_ operable by hand.

It's not a military installation, it's just a maintenance hoist for big water pumps. There should be no need for hoist slewing in the absence of electrical power, and it would be a waste anyway, because the hoist is solely electric.

Besides which, I think it's a really bad idea, because it present the possibiity of the chain sprocket being driven by a motor. The cathead itself will be 20 feet in the air, so there's not a pinch hazard, but I can't help imagining the damage that a long loop of heavy stainless steel chain flailing about could do.

I get the sense that this guy only accepts arguments with chapter and verse, so I went looking for a regulation that prohibits driving a free loop of hand chain with a motor. I couldn't find anything that was really on point. I found a little about automatic brakes and how they differ on powered and nonpowered hoists and such, but in this case, the hoist load doesn't enter into the picture; this is just the left/right drive for the hoist.

Citations, please?

Thanks.

Mike Halloran
Pembroke Pines, FL, USA

 

[GregLocock]

Real world experience says you are right. The roller doors on our workshop had a chain for use if the electricity was out. Generally young engineers and apprentices who lazily 'forgot' to disengage the chain after using it only got a kick up the arse from the foreman.

You do not have a 'chain', you have an electrically powered moving device near an operator. As such it will need guards and interlocks.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[berkshire]

MikeHalloran (Mechanical)
I cannot give you a citation, I can however give you some anecdotal evidence.
I worked in a factory in England that had 3 bridge cranes on a common track. Two newer ones electric remote controlled,one at each end of the building and an old hand operated crane, in the middle of the building.
If the old crane was in the way, the guys would simply bring the bridges together, then push the hand operated crane with the power crane. The chain on the hand powered crane would flail and precess forming strange shapes in the air. People working around the area would give it a very wide berth.
B.E.

 

[unclesyd]

We had the same problem as berkshire in our machine shop until it snatched a water line loose. There was major effort to resolve the problem over the weekend. I don't know the particulars but some type overrunning or ratchet type clutch was installed.

 

[MikeHalloran]

Thanks for the anecdotes; they may be sufficiently persuasive.

I can see hypothetically how to add a clutch. I can't see how to assure that it will always be disengaged when it should be, or even how to prevent a disengaged clutch from seizing during years of inactivity.



Mike Halloran
Pembroke Pines, FL, USA