Introducing Our Floating Linear Gearbox

We recently have discovered the need to exert a large pulling force over a relatively short distance. (500 lbs for 4" to be exact). Here is our solution. It is a linear actuator of sorts, except that it only would work in the tension direction. We call it the PullBox

It uses a 5/16-24" eye bolt (connect the thing you need to pull to the eye. This passes through a 60T GT2 Pulley with 1/2" long couple nut which is conveniently 5/16" threaded inside and .5" hex outside. The 60T pulley is driven by a 12T pulley on a 775 Pro/Redline. The Gear box frame has bearings on two of the 3/8" bolts to increase the tooth engagement on both pulleys.

We say the gear box “floats” because it is suspended between cables (the two loops of cable on the side to anchor it, and the cable attached to the eye bolt. It could be bolted to something to make it “non floating”.

We haven’t built this yet, so use at your own caution. Also we haven’t actually checked to make sure that belt length exists, and may have to tinker with the spacings slightly.

We calculate about 800lbs of force if 20% efficient, and are looking for ways to increase efficiency (But would like to stick with the eyebolt as opposed to a threaded rod).

Critiques appreciated.

Have you looked at how the bearings on your cascade lift will handle the forces exerted on them while lifting a robot?

A 500lb axial load on a single 1/2" hex bearing might be a teeny bit excessive. You may want to look into adding a thrust washer between the nut and plate. A gearbox plate with the thickness pictured might also be too thin to handle a 500lb load without yielding.

One other concern: I suspect that if this gearbox is floating between two cable attachment points, the output torque will contribute more to twisting the cables than drawing in the threaded rod. Most linear actuators have some method of constraining the rotation of the two linearly moving parts (the eye bolt and gearbox, in this case).

It does look like a simple gearbox, so if you have the ability to make it and test it quickly, then go for it! I suspect you might run into a few issues on the way though, so be ready.

We haven’t built it yet, but are prepared to add needle roller bearings if needed.

Have you done any testing to see if the heat buildup from the thread interface will be an issue? It might be fine, depending on how frequently you’re using it and if it’s lubricated, but I’d be worried about frequent use causing the nut to seize on the threads.

Any suggestions in lubricants that would be effective on threaded rods?

I would try a general purpose grease, like white lithium or moly. You can get a tub of wheel bearing grease at an auto parts store or walmart for 5-10 dollars, or use an extra tube from the kit gearboxes if you have one.

I’m not sure what your using it for, but you stated 500 lbs. of force. Consider welding the eyebolt closed, so it doesn’t bend open in your application.

I am no expert, but please look into dissimilar metals (i.e. steel & brass) for the thread & nut respectively. Some super annoying failure modes may be avoided by adopting this design philosophy. If you can find/choose to go with a bushing* material for the nut you may not need to lubricate as many of these bushing materials are “self lubricating”. Adding lubrication in some of these cases may be a detriment, like putting grease on your car brakes…

Furthermore, try to avoid like materials (steel on steel) in order to prevent galling. You want one component to “wear in” to the other, usually the internal thread “wears in” external thread of the bolt (External thread is tougher & harder than the internal thread). I have heard 18-8 stainless on 18-8 stainless can cause some nasty thread lock and galling in higher speed applications.

Material science is such a vast world… everything is made of something.

  • Common bushing materials: Bronze(s), Brass, Cast Iron, Acetyl Plastic