Too much backlash between gears

[cdeane44]

I am working on a gear box using a worm and worm gear. (boston gear HDSH-1 and bostonGear D1124). In the manual it says to have the center distances be 1/2 the pitch diameter of the worm plus 1/2 the diameter of the worm gear. In this case the distance was 0.792" and when I assembled the gearbox together there was still a good amount of backlash between the gears. Is there a different way to calculate the center distance or a better way of aligning them? Thanks

 

[BobM3]

The only way I know of to adjust the backlash is to change the center distance. See the "gear train alignment" thread a few threads back for more info.

 

[israelkk]

They should have specify the tolerance on the cemter distance too.

All gears should have some backlash for a free running and a space for the oil film. The backlash and tolerances on the center distance are function of the gears accuracy. If you need less backlash you need to specify more accurate gears. If Bostom gears uses the AGMA gear system then they should specify the AGMA accuracy class for the worm and the worm gear.

 

[eromlignod]

Your calculation looks good. If the gear and worm are located precisely at that center distance, they should perform well with very little backlash.

Have you considered the play in your bearings? How accurately was the housing machined to this center distance?

Don
Kansas City

 

[gearcutter]

Your calculation for working out the center distance is incorrect and explains why you have ended up with too much backlash. It should be half the working pitch diameter of the worm plus half the working pitch diameter of the gear.

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

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

 

[BobM3]

I agree with Don. Your calculations are correct.

 

[gearcutter]

BobM3,
I don't understand how you can claim that cdeane44s' calculations are correct because the worm gear's diameter is being used and not the worm gear's pitch diameter. I must be missing something as I have never seen a center distance equation that uses the diameter of a gear.
From Dudley's Gear Handbook second edition, page 2.45, table 2.10; to find the center distance of a worm and gear having the pitch diameters of both parts; C = (D + d) / 2.

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

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

 

[dinjin]

I agree with gearcutter, the equation should read:

In the manual it says to have the center distances be 1/2 the pitch diameter of the worm plus 1/2 the (PITCH) diameter of the worm gear.

 

[israelkk]

gearcutter, dinjin, BobM3

cdeane44 is correct and he was asking a good question he just missed the word "pitch" for the worm gear. A quick check in Boston gear catalog shows that the worm pitch diameter is 0.333" and worm gear pitch diameter is 1.25" therefore, the theoretical center distance is 0.7915". Your debate is off course. The gears diametral pitch is 48.

Here is a note from Boston gear catalog "Note: Worm & Worm Gears do not have AGMA Class listings,
however Boston Gear manufacturing tolerances relate to
AGMA 8"."

 

[cdeane44]

Thanks for the help, I did miss the word "pitch". Thank you though, sorry about the mistake!


Chris

 

[eromlignod]

Right. I looked at the catalog too, to verify that he meant to write "pitch diameter" (use your head, guys). If he were off by an entire addendum he'd have a lot more than a simple backlash problem!

I'm guessing that the center distance is not ending up exactly .792", or varies, either due to machining tolerances, bearing play, or bearing-to-housing/bearing-to-shaft fit (or a combination of these factors). If you really need precision, I would recommend using precision bearings (not needle bearings or cheapies). Put some tighter tolerances on your hole centers and ensure that the bearings fit snugly in the housing and on the shaft, according to the manufacturer's recommended tolerances. And, as israelkk said, you want a little backlash in a worm-gear pair since they rub rather than roll.

Don
Kansas City

 

[eromlignod]

Oh, it just occured to me that there could be another cause: axial play of the worm. Often a preload washer is included at one end of the worm to remedy this.

Don
Kansas City