Worm meshing with a spur gear 1

[NoobieAKP]

My textbook (Machine Elements in Mechanical Design 4th ed. by Robert Mott, ISBN: 0-13-061885-3) says that a low cost method of creating a wormgear set is to use a worm with a spur gear. However, it doesn't say how to do this. I'm assuming it must have the same pitch but other than that, I'm not sure. Dutton-Lainson makes a worm gear winch which looks like it uses this concept (see the 2nd image and the dimensional drawing at

http://www.dutton-lainson.com/proddetail.php?prod=11011)

but I noticed that the worm is at an angle...not sure if this helps the two mesh?

 

[MikeHalloran]

The pitch and pressure angles should match, and you have to skew the axis of one of the gears, so that the teeth of the spur gear run parallel to the teeth of the worm where they mesh.

It's one of those things that works just well enough to say that it works, and I've done it on a one-off light duty lab thing, but I can't recall seeing it in production.





Mike Halloran
Pembroke Pines, FL, USA

 

[gearcutter]

See this thread for more info -

http://www.eng-tips.com/viewthread.cfm?qid=356479

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[NoobieAKP]

MikeHalloran and gearcutter

Thank you both for the insight, that explains the worm being at an angle for the Dutton-Lainson worm gear winch. Now, I'm sure this changes the calculations for the stress at the teeth. Is this something that can easily be calculated by hand (as with the standard worm and worm gear stress equations) or does this require FEA?

 

[MikeHalloran]

If you sketch in the _far_ side of the worm teeth in the D-L drawing, you'll see they're pretty much horizontal, so they can mesh with the spur gear.

I'm not convinced that FEA will help much in analyzing such an assembly, because the contact area is concentrated at the mesh, so the gears will Brinnell and/or wear against each other rapidly at first, effectively changing the contact geometry until the area becomes large enough to support the rated load.

I don't know if an FEA package can help you figure out what that eventual contact shape will be, and it will probably give odd/misleading results regardless of what you guess/assume about the contact geometry.

The D-L product looks like the sort of thing you'd develop by building a few prototypes and trying to map the effective lifetime against the applied load, hoping to find a load at which the wear plateus for a while, and calling that the rated load for a rated lifetime.

Or maybe D-L actually used FEA or some other toolset to help them arrive at a satisfactory design. Call them and ask.







Mike Halloran
Pembroke Pines, FL, USA

 

[3DDave]

When the screw thread is tilted relative to the spur-gear axis the effective pressure angle changes. 20 degrees is common in gears and 30 degrees is common in screw threads. Of course, the PA on spur gears is as measured at the nominal pitch diameter; it is always 0 at the base diameter and increases with increasing radius. Somewhere is a match between the two.