What is maximum safe height (minimal overlap) for a 2 stage elevator?

We are trying to determine the maximum safe height for our 2 stage elevator by determining the overlap between stages. We have found 1 recommendation that we keep 10% overlap between stages but are looking for more data. Is there a solid recommendation out there?
Our elevator is about 31" overall height at start
Thanks

The answer might depend on how the elevator is built. How about some design details?

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We are using approximately 12" of overlap on our multi-stage elevator this year, with 37" stages. We are using the REV linear bearing system, which allows you get the bearings pretty tight against the extrusion. YMMV if using a different system, but I don’t realistically find 10% adequate.

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We will hopefully be using the Andy Mark elevator bearing kit with standard 1x2 tubing hopefully cut and drilled on our CNC mill. Is there more information that would help?

It all comes down to slop. If you can manufacturer in such a way that the bearings remain tight against the stock, then you can get by with less overlap. OTOH, if the bearings don’t fit tight, then the stages will begin to sway without adequate support, and your precision at the upper limits will be lessened. We haven’t used the AM elevator bearing kit, so I can’t speak for its capabilities, but from the pictures I see on their website, their doesn’t appear to be any mechanism to taking out slack. I could be wrong.

I am guessing the only slack would be between the brackets bearings and the tubing unless I am mistaken. If so, we have no control over either. Is that correct?

I’ve heard the rule of thumb is about 20 percent overlap.

It depends a great deal on how you anticipate the elevator being loaded. A light duty telescope fully within the frame perimeter can get away with very little overlap. An elevator you expect to keep working after bashing into a wall…needs a bit more.

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It might help to draw free body diagrams of the parts of the elevator, at different amounts of overlap. You might find that as the overlap is reduced, the load on the rollers and tubes increases dramatically.

Something about “leverage”.

We are using 6” of overlap with a total of 3 stages going up 7 feet

An addage from FRC past was 30% overlap. Granted, this is from 15 years ago and the manufacturing quality of FRC teams has risen significantly since then. 1712 got away with substantially less than 30% on our elevator (built with the AndyMark bearing kits) in 2018.

That being said, 10% is a very aggressive design. You may want to target somewhere between those two extremes.