Making a gantry...need help.

At that span, consider a pair of 2x8s laminated together, capped with 2x4 flanges top and bottom of the middle half, to spread the concentrated load and endure the chafing from the hoist.

Or, three 2x6s side by side. The outer two don't really have to be full length; 3/4 of the span should keep the middle one from twisting.

In addition to glue and screws to hold the laminations together, consider wrapping the assembly at intervals with hose clamps, Band-It strapping, or metal package strapping.

I assume that the Unimog's cab normally tilts by means of some mechanism that is now inoperative. There's probably a torsion bar near the hinge, to take the load off the lifting mechanism. It may go over- center as you lift it past a certain point, so be prepared for your hoist to suddenly go slack and the cab to execute an uncontrolled rotation. Once it's come to rest, beware of energy stored in the torsion bar.

If at all possible, use a powered hoist with a remote control. This operation has the potential for a variety of bad outcomes.

-Mike-

Isn't the cab tilt "equalibrated" by springs or something? like the hood on some cars or a garage door?

Before you do (or have done) any gantry beam design you need to know your expected load.

How are the cabs pivoted normally?

Is this a long term tool or just a one time use?

Depending on the load, you might get by with only 100 lbs for the beam.

Sounds like an application for a light weight steel frame

cheers Bob

and...

First off, thanks to both of you for your replies. You both asked if there was some kind of springload assist to tilting the cab: nope. The normal means of tilting the cab is to have the Unimog in a shop that has an overhead hoist. Most large truck repair facilities have these. There are D-rings on the back of the cab, and in the front you simply attach two brackets between the cab subframe and the chassis frame, on which the whole pivots.

I am trying to estimate the expected load. If I understand correctly, the maximum load on the hoist will be when the cab is first coming off the frame, and that load should be close to half the total weight of the cab. Is that right?

Regarding the use of the tool, I HOPE to not have to use it often. ;-) But I will be keeping it, in case it will be used again.

Bob, you mentioned a light weight steel frame. Can you elaborate a bit on what might be sufficient?

Thank you again!

George Reiswig North by Northwest Expedition

You have all the makings of a good disaster here. But we've all been there so here goes. When wood fails, it does so by splintering. At that point it looses it's strength and collapses. You get very little warning. If you, or any part of you, is under 1200 lb, the results are nasty. The answer is to proof test whatever you build. That is to apply a weight up to50% greater than you will be lifting. The easiest way is to just take the weight of the cab on the hoist, then you and a couple of friends add their weight. If things fall, they will only drop a couple of inches. If it passes, away you go. When the cab gets high up, it may go over centre and fall forward. This can be dangerous as well. Ask someone who has raised these cabs if this will happen. Good luck

John

I agree that I don't want this to fall apart in any way. And I will test it as you suggest. But I still don't know whether it would be better to have a piece of 4x4" box steel, or a wood beam made as Mike Halloran had suggested. Any thoughts?

If I was going to use two laminated 2x8s for a beam, I might also want to put a piece of sheetmetal flashing sandwiched between them....I did that to a floor support beam in a house I was remodeling, and it eliminated the need to put in an additional post.

Let's suppose that timber can take 1000 lb/sq in in compression. As it happens that's low side for high density balsa, so it should serve for the white woods you'll be sourcing.

Two verticals at 4X4 = 16 X 1000 = 16000 lb each ultimate say So they won't fail in compression, but could buckle.

A usual makeshift for a gantry is a cross piece in compression, and a chain hanging from the beam ends in tension A strut of two 2 X 6 beams fastened together would be good for 24000 lb ultimate compression and the chain would also be stressed for 12 tons tension. If the chain drooped at 20 degrees to the hoist from each end it should be good for 4 ton ult, a safety factor of 6 or 7 which will do for temporary private use if you keep out from under.

An Unpaid opinion..... Brian Whatcott

Adding metal is a good idea.

I'd go one better, and tightly wrap the laminated beam with flashing, with a tight lockseam joint. Putting the metal on the outside improves the beam's resistance to buckling and abrasion.

-Mike-

I'd be more concerned about the uprights buckling.

Since you only need one lifting point at which to attach the hoist, you can eliminate the beam entirely by constructing a 'shearlegs', or A-frame. Or do what you can to reduce the span; a Unimog is not 12 feet wide.

But on reflection, I don't think I'd use 4x4s for shearlegs or for uprights, even if I knew where to get sound 4x4s that long. Got access to telephone poles?

-Mike-