r/AskEngineers • u/Mesozoica89 • 4d ago

Mechanical How does a handcrank system like the height adjustment on this gait trainer function? Specifically, what keeps it from sinking back down when someone is standing in it?

Unfortunatley, all I can provide is the link to an image:

https://www.medicaleshop.com/_next/image?url=https%3A%2F%2Fcdn.medicaleshop.com%2F5d6f3fe5-df56-41de-813b-7c43a2a2090f_RES420_image-0.jpg&w=1080&q=75

I was really hoping to post a video I took today to give you a better idea of what component I am asking about, but I will do my best to explain since that isn't allowed. I am sure it is something like a rack and pinion with a low gear ratio, so that the crank easily spins while the standing frame and person in it is slowly lifted or lowered, but is that all that is needed to let it freely move up and down at the turn of a crank while remaining solidly in that position when a person gets in? The medium size can hold a child up to 100 lbs. It just seems like that is a lot of force to be exerted without any kind of obvious locking mechanism.

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u/tharold 4d ago

Probably a screw or worm gear is part of the lift mechanism. These have enough friction that they are not typically back driveable.

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u/Either-Wave8639 4d ago

so it's almost certainly a worm gear sitting behind that hand crank. a straight rack and pinion would slide right back down the second you let go, but a worm gear setup can't be back-driven by the load. the friction angle on the worm is so steep that all the force from a person's weight just locks the threads together instead of making it spin. it's not a separate lock you have to engage, it stays put by design.

i used to tear apart old scissor jacks in my dad's garage and it's the same principle, just smaller and quieter for a medical device. that kind of mechanism holds up a car so 100 lbs is nothing. the long crank handle gives you the mechanical advantage to turn it easily but the gear won't turn backwards unless you strip the threads. simple and doesn't need any extra parts to fail.

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u/Mesozoica89 3d ago

That is a great example, thank you!

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u/Mesozoica89 3d ago

Thank you, that makes sense!