worm gear drive

[csrc]

Does anyone have any good reason not to use a 20:1 or 10:1 worm gear drive attached directly to a motor and a rack/spur gear drive for linear motion? I am motivated to use this solution by the guideways I have collected and the fact that the commercial machines I have seen rely on racks. The worm gear primary reduction appears to have many advantages, not the least of which is static stability. I feel I may be missing something, as few people seem to be doing it this way. In as much as I will have to make my own gearcases for the drive, I should be able to adjust out a certain amount of backlash in the worm gear assy. As for the racks, It seems to me that's how the big boys do it (Trumpf). My apologies if this has been covered, the website is very large!

http://www.cnczone.com/gallery/showfull.php?photo=4312

[RICHARD ZASTROW]

csrc, Worm / wormwheel gear reductions are good for high gear reductions. They do not like speed. This arrangement is not really "gearing". It is actually functioning as a screw. Gears roll usually on an involute surface Whereas screws and worms slide against nuts or wormwheels.

If what you need is slow, accurate and fairly backlash free motion, worm gear reductions are fine. Some examples are the workpiece rotation in fairly slow rotary motion like gear hobbing / shaping or rotary tables etc.

The only linear motion using worm reductions I remember was the slow movement of very large boring mills etc.

Hope that helps

[csrc]

Thank you for your comments. Boston Gear rates these gears for at least 1800rpm which is twice my max. I was hoping to create a compact gear reduction so that everything can be mounted on the gantry to allow maximum adaptibility of the frame (with a fourth axis and continuous cutting in mind). I could adapt the torque transfer shaft to a ballnut/rack design, but I wish to retain the narrow gantry.Belts would also work, but my linears are very robust and I guess I would need reinforced belts to max the cut. Perhaps I will design the frame to accept all possibilities.

[CarbideBob]

Worm gears (except for some very special designs) must have backlash to operate. Something in the range of .006 at the pitch dia. Their other limitation is top speed (sliding speed between the worm and gear faces) which is why they are not used very often in servo drives. Since speed is not a problem if you can live with the backlash it should work for you. Cone drive makes zero backlash, spring loaded worm gearsets but these are very expensive. Be sure to use the synthetic gear lube made for worm gears to minimize wear (this lesson I learned the hard way).
Bob

[jcc3inc]

Worm gear efficiency is low. Typically it runs less that 40% depending on the worm ratio. Therefore it will take more motor power to generate the force or acceleration you may want.

An advantage of worm drives is that they do not back-drive except at very low ratios.

I prefer spur gearing.

Regards,
Jack C.

[csrc]

Thank you for your input. I think I should concentrate on adapting ballscrews to my design.