Fine-pitch worm gear questions 1

[wiyosaya]

This is not my main field, so I am hoping that someone here will be able to help me out with this.

I'm working (in my spare time) on a metering structure for small astronomical telescopes (16" diameter main mirrors and less), and I have come to the conclusion that using fine-pitch worm gears will give the kind of precision adjustment that I would like to have in the assembly. I have looked at some of the references for gear design cited in previous messages in the group like Dudley for one.

I note that Buckingham and Ryffel in "Design of Worm and Spiral Gears" state that fine-pitch standard screw threads can be used for the worm. Based on that, I am considering using 1/4" - 56 thread (Machinery's Handbook does list this as a standard thread size) for the worm. For the worm gear, I am thinking of using a 60-degree circular arc with a six-inch radius. I would also like to have the gear "single-enveloping". As a worst-case scenario, I believe that the torque at the gear threads will be approximately 450-in lbs; however, the gear will only have to bear this load while the adjustment is made. After the adjustment is made, there will be a clamping mechanism holding the shaft that will bear this torque.

I am looking to have as minial a weight for the finished structure as possible, therefore, I'm thinking that 7075-T7351 for the gear could possibly be used, however, since the worm will have much less mass, I have yet to decide what to use but I am thinking maybe the same material, or maybe grade 2 Titanium.

Given all that background, my main question is is it possible to make the gear using currently available gear making methods?

What would be the ideal thickness for the gear based on using a 1/4" dia worm?

Is there a better choice of material than 7075, if so, what?

Thanks in advance.

Best Regards,
Matthew Zenkar

 

[insideman]

I recall using a 1/4-20 screw ( modified by filing on the lathe) as a worm against a 64dp 14 1/2 pitch die cast wheel.

Worked beautifully for my needs, but Idoubt it would serve as a telescope drive. The loads you cite seem rather high....

 

[wiyosaya]

Thanks for your reply. It suggests to me that assuming I am able to find a shop that can hob the gear with an equivalent DP, I should be able to do this.

I'm citing an absolute worst-case load that I doubt will ever be seen in real-life, and here's how I'm figuring the load. The heaviest amateur 16" telescope I expect the structure to carry weighs in at approximately 120-lbs. I'm assuming that the center of gravity of this assembly will be 30" from the axis being adjusted giving 3600-in-lbs. As the gear spurs will be 6" from that same axis, 450 lbs will be required to counter act the load from the telescope assembly. Opps , I mixed up the units on this; it should have been lbs not in-lbs. I am assuming, though, that more than one thread will be required to bear this load during adjustment.

In reality, the center of gravity of the telescope will probably lie around 15" from the axis being positioned which would give about 225 lbs of force required to position that axis.

Thanks again.

Best Regards,
Matthew

 

[wiyosaya]

Ah well, mucked up my math again . The load should be 600 lbs at 30" & 6" and 300 lbs at 15" & 6"

Best Regards,
Matthew

 

[Philrock]

If you had a hardened steel pinion running on a high-performance bronze gear, they'd be good for maybe 10 lbs. force at the pitch line. On top of that, both aluminum and titanium are extremely prone to galling.

I suggest an entirely different approach. Get lots of reduction with a multi-stage planetary gearbox. (Maxon makes really nice ones.) Put a short arm on the output shaft of the gearbox. Put a longer arm on the telescope pivot shaft. Have the short arm drive the long arm with a link. Spring preload the telescope to eliminate backlash - note that this will add to the load on the drive system.

 

[wiyosaya]

Thanks for your reply. This is puts things in perspective for me after looking at the size and weight of the planetary gearboxes designed to handle this kind of torque and realizing that this is a *large* torque. To reach my design goals, I will have to return to the method I started out with which was using a threaded rod in a tripod type arrangement to deflect the same axis rather than driving it directly with the worm gear.

Thanks again.

Best Regards,
Matthew

 

[Compositepro]

All the telescopes I've ever seen have counter-weights so that they can be precisely balanced arround their axes of motion. If you are not balanced even a heavy duty gear will not maintain position. With a balanced telescope a very light drive will work fine.

 

[wiyosaya]

Thanks for the tip Compositepro. Indeed, most telescopes are counter-balanced well, and balanced on their primary axes of rotation, however, this assembly will not be metering those axes. The assembly will be used to align those primary axes. Unfortunately, no counter-balancing is possible without unjustifiably complicating the assembly. My solution will be to deflect the loaded axes with a threaded rod.

Thanks again.

Best Regards,
Matthew

 

[Compositepro]

I think I get what you are trying to do. You want to be able to make a precise adjusment to align something on a machine, and this will be done infrequently or only once. This is actually common on many types of machines and is done with opposing screw mechanisms or opposing nuts on a screw. Start with both screws clamped tightly on what you are adjusting. Then back one off a fraction of a turn and tighnten the opposing screw by the same amount. This is sort of analogous to a worm gear with adjustable backlash.