How much preload can I apply to rack and pinion?

[AustinT]

Hi,

Does anybody know how much preload I can put on a 20 degree rack and pinion without wearing it out quickly? I can find specs for the tangent force, the force that shears the teeth off.
On the x axis I would like to use 2 v bearings and two spur gears. The v bearings would be below the spur gears and they would push the spur gears into the rack. One spur gear would be free wheeling bearing, the other spur gear would move the gantry.
The preload serves 2 purposes. First it holds the V bearings tight on their track. Second it reduces backlash.

Thanks
Dan

[NC Cams]

You need to be more specific.

When you say "too quickly" what is "too quickly"???

10 cycles, 1,000,000 cycles or something inbetween???

Are gears brass, steel, aluminum???

What sort of surface contact stresses are you generating???

Will the rack carry load or just move the system axially???

If the stresses exceed the yield point of the material at the load application points, the r&p won't even turn a few times let alone 100.

PIC supplies a lot of prototype rack and pinions. They usually have design guides in their manuals/catalogs. These can be handy in designing a r&p for minimum wear/max durability. Martin and Boston Gear have design aids as well in their older catalogs - don't know if their new ones still have the info.

Chances are and especially in a direct drive situation, friction to move/turn the rack will become more critical than wear. The nice part about high friction is that the system will tend to stay in position and, since it will be so hard to turn, one will tend NOT to try/want to use it thus sliding wear will not be a problem and the only concern is "will the gears deform??".

The next probem will be the support "bearings" as in ball or roller type. If you load these too heavily, they will not live.

Even though they may roll and even with grease, if the bearing speed is too low and loads too high, you'll punch a hole in the grease and make metal to metal contact. This will cause 'false brinnelling' and thus premature wear of the rolling elements and/or raceway.

Making rolling bearings live in r&p's is a hard thing to do for this very reason - oscillatory load. This is why bronze bushings are oftern used in these applications, in spite of the fact that friction penalty is higher.

[AustinT]

Hi, Thanks for the post

Its hard for me to put a number on the exact number of cycles of wear, I dont know how how fast the gears wear out without preloading.

The gears will be hardened steel or stainless I have not purchased them yet so it is still flexible.
After reading several posts in the forums I can see that almost all the rack and pinon applications are using some preload, even if it is only a spring applying the load.

"Chances are and especially in a direct drive situation, friction to move/turn the rack will become more critical than wear. "Even though I am using some reduction this is a concern .


The rack will face down it will not carry the weight of the gantry. The gantry weight will be carried by the v bearings. The bearings have an eccentric bushing that will allow me to push the spur gear up into the track.

Thanks for your help , I know I am asking a substantial number of questions on the forums right now but I am working on more than my own table.

I would like to finalize a good solid design this of course will take some practicing with my own table. I have access to a rapid prototype machine and an aluminum foundry. It would be nice if the Diy Cnc guys had access to low cost parts that would make the project easy and affordable

Thanks again

[NC Cams]

At this point, I'm confused.

What do you mean by "preload"???

If your are looking for a "preload" to counteract the separation force of the gears as they mesh, that is one thing. If you are looking for a "preload" to deal with gear backlash, that is something else entirely.

Before you get comitted too far, it might be easier to recommend something to address what you are trying to accomplish than to provide you with something you've already conceived.