worm gearing using ACME shaft? 1

[subsearobot]

Hello.
I am looking at feasibility of a design where I want to use an acme shaft as a worm. I am wondering what type of gear(s) will mesh "acceptably" with the acme threads. Our manual lathe uses a similar setup for thread-milling.

I am not transmitting power with the worm gear. I am using the AMCE shaft to lift a load (100 lbs). the load will be coupled to the shaft with a half-nut, which will dis-engage after ~40" of lift, allowing the load to free fall. After impact, I need to engage the half-nut again to repeat the cycle.

I am thinking of using the worm gear as a timing device to ensure accurate thread engagement. This will avoid thread tip collisions between the acme shaft and the half nut.

Basically, I envision the worm gear acting on the half-nut with a scotch-yoke type of motion, where it will allow the half nut to move radially into the shaft with the correct timing.

the shaft will rotate at 3600 rpm.

Any input is appreciated- I have little experience with gear-train design.

thanks!

 

[drawoh]

subsearobot,

An ACME thread is sort of a worm. The teeth are at 29°, which meshes with a 14.5° worm wheel. Threads are sized to the major diameter, and worms are sized to the pitch diameter, so you almost certainly have a problem.

Why can't you just order a worm and matching worm wheel? If you make the worm custom, you can have it integrated with your shaft.

JHG

 

[subsearobot]

JHG,

I am trying to avoid custom manufacturing if possible(beyond what we can do in our shop with cnc lathes and mills). this is a one-off project for a science instrument. the budget is limited, and there is not much of a market for additional pieces of equipment to amortize NRE costs.

the main purpose of the ACME shaft is to raise the load using an acme half-nut. the gear will simply function as a timing device. Not much power required here, but I suppose I do need minimal backlash.

Can I use a 14.5 deg spur gear as a worm wheel?

 

[Compositepro]

3600 rpm on such a long shaft sounds like a problem. You could probably accomplish what you want far cheaper and more easily using sprockets and roller chain. A ratchet pawl can hold onto a chain like a split nut will hold onto a threaded shaft, but you wont have all the friction. Run the chain on a rail for lateral support.

 

[subsearobot]

compositepro,
I will look at the chain concept- it definitely has merit.

regarding shaft speed, I ran a calculation that indicated I am well below critical speed. I will double check, though.

thanks

 

[drawoh]

subsearobot,

Consider buying a worm and worm wheel set. These are cheap. You can attach the worm to your shaft with our without ACME thread using adhesive. I have had excellent results with Loctite RC65.

JHG

 

[MikeHalloran]

If you intend to engage the half-nuts while the shaft is spinning, you'd be better off with a chain and a hook.



Mike Halloran
Pembroke Pines, FL, USA

 

[plasgears]

We used to say in our shop that all gears are special. In car seat actuators the emphasis is on plastic gear and worm. The worm had an acme-like profile. Unlike metal worm gear sets, plastic worms are the soft pliable component. We had the best experience with a nylon 6 worm engaging an acetal gear. [No glass].

 

[tbuelna]

subsearobot,

If the acme threaded shaft and half nut are somehow mechanically synchronized to ensure a smooth re-engagement, then I don't believe the device will be able to "free fall" on its return stroke. It would seem to be necessary for it to translate back down in the opposite manner as it was driven up, no?

An acme thread nut will produce a conformal contact with a similar acme thread shaft. If instead you want a conjugate type of contact (like with two external helical gears), then two similar acme thread shafts should work. This conjugate type of contact will have much higher localized contact stress, but if designed properly it can be much more efficient.

If I understand your post correctly, I believe what you want is something similar to a "planetary roller screw". This linear translation device is a set of externally threaded "planets" with a system of synchronizing gears arranged such that they achieve a true rolling motion at their contact with the center screw shaft, as they orbit about it and thus translate along its length. This rolling contact is very efficient, much more so than an acme jackscrew or worm gear.

Hope that helps.
Terry