bearing question

I am making a davit that is going to be 14' tall. I am probably going to make it out of pipe or 2" square tube. It will be L shaped, the L upside down. There will be a diagonal brace on the top leg of the L. I will use it to mount a hoist. At top, I will put a 1/2" pin to fit into a receiver. On bottom, I was wanting to mount it to a bearing to make it easier to turn, since it needs to move about 270 degrees.

What kind of a bearing would I use? Would it have to have tapered rollers, so that the inside would support the davit, and the outside would be mounted to the base plate?

Or should I just go simple, and mount a couple of big washers face to face and keep them lithium greased?

How would you do it? The davit will weigh what about eighteen feet of two inch square by quarter inch thick steel will weigh, plus a 110 hoist of about thirty pounds, plus the weight of the cargo. So, that much weight would be riding on the bottom swivel point.

Steve

Reply to
SteveB
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The handbook numbers I've seen for slow-moving steel-on-brass bearing pressure run around 1500-2000PSI. I measured a few lifts and pulleys to see what their designers had used and found about the same values. This pressure is load / (length * diameter).

I like to have at least enough drag in the boom bearing to keep the load from swinging as soon as it clears the ground.

jw

Reply to
jim.wilkins

I'd think a couple big truck axle bearings would be a good choice for cheap tapered roller bearings with a high load capacity. I'm not a structural engineer, but your 2" square tube seems way small for this application. Please check with someone qualified or at least compare with some commercial units of the expected capacity.

Pete C.

Reply to
Pete C.

I'm not sure how long the upper horizontal portion of the "L" is going to be, but it sounds like there's a pretty big moment around the base.

Jim

Reply to
jim rozen

But Jim, it only has to handle a twenty pound load.

John Martin

Reply to
John Martin

Pete, your point about 2" tubing is right on!! I built a similar system except my horizontal arm was an I beam with a trolley which held the hoist.

For the vertical column I used an 12" I beam. First time I used it (lifting my TIG unit, probably 300 lbs) into the back of the truck, it almost killed me. The 12" beam deflected considerably, the horizontal beam became "un-horizontal" and the trolley just rolled (slammed) out against the stop and the load knocked me over.

Obviously needed a larger column. Checking with my structural buddies they all said that the beam need to be "boxed". Got some 12"x2" channel and "boxed" the i-beam. No more problems.

A 2" COLUMN IS WAY TOO SMALL!!!!!!!!!!!!!

Ivan Vegvary

Reply to
Ivan Vegvary

What everyone seems to be missing is that the column is pinned *both top and bottom*, in other words, no moments at the connection points. Depending on the details of the design, such a hoist made of 2" tube may be capable of lifting significant loads.

Personally, I think the rolling bearing at the base is overkill. A nice greasy thrust washer should work fine.

Ned Simmons

Reply to
Ned Simmons

Ivan Vegvary wrote: ...

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I have one of these and have loaded it to 700 Lbs with no problem. The boom felt like mild steel when I drilled it to mount a winch.

jw

Reply to
jim.wilkins

I'm sure it works. Mine was 22 feet tall. It's a matter of slenderness ration which varies with length (height).

Ivan Vegvary

Reply to
Ivan Vegvary

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