Fine-pitch worm gear questions
Fine-pitch worm gear questions
(OP)
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
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





RE: Fine-pitch worm gear questions
Worked beautifully for my needs, but Idoubt it would serve as a telescope drive. The loads you cite seem rather high....
RE: Fine-pitch worm gear questions
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
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
RE: Fine-pitch worm gear questions
Best Regards,
Matthew
RE: Fine-pitch worm gear questions
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.
RE: Fine-pitch worm gear questions
Thanks again.
Best Regards,
Matthew
RE: Fine-pitch worm gear questions
RE: Fine-pitch worm gear questions
Thanks again.
Best Regards,
Matthew
RE: Fine-pitch worm gear questions