Does too much backlash weaken a gear pair?
Does too much backlash weaken a gear pair?
(OP)
I am building a gearbox with Mod 1 helical gears from Dodge APG (American Parallel Gear) gearboxes. Helix angle is 15º. I believe the normal pressure angle is 20º. I don't know whether the teeth are modified. There are 22 teeth on the pinion and 50 teeth on the gear. Face width of the gear is 0.42", pinion is wider. I don't know the gear quality but they look very good. I believe the teeth are ground, not shaved.
The center distance with no backlash would be about 1.467". I used a center distance of 1.473" and there is a lot of backlash, maybe 0.040".
I am going to load this gearbox to destruction at low speed, so it will operate in one direction only for less than an hour. Failure will be when a tooth breaks, because all other parts are much stronger than necessary. Will the excessive backlash cause a tooth to break sooner than it would with normal backlash?
The center distance with no backlash would be about 1.467". I used a center distance of 1.473" and there is a lot of backlash, maybe 0.040".
I am going to load this gearbox to destruction at low speed, so it will operate in one direction only for less than an hour. Failure will be when a tooth breaks, because all other parts are much stronger than necessary. Will the excessive backlash cause a tooth to break sooner than it would with normal backlash?





RE: Does too much backlash weaken a gear pair?
Mike Halloran
Pembroke Pines, FL, USA
RE: Does too much backlash weaken a gear pair?
Assuming, as you state, that you have accurate, high quality gears, the only real problem with excessive backlash is that the teeth might be thinner, and thus somewhat weaker in bending. So yes, they will likely break sooner if the teeth are thinner than standard. But if the teeth are of standard thickness, and the excessive backlash is simply due to a non-standard mounting distance, then the tooth bending strength should not be significantly affected.
With regards to your non-standard center distance, if the gears are an involute form, they will not mind too much. That's why the involute gear form is widely used. The mesh may experience some excessive sliding, but if you test at low pitch line velocities it should not result in a scoring failure.
Of course, I don't believe that an extra .006 inch (1.473" vs. 1.467") in the mounting distance would produce an additional backlash of .040 inch in an otherwise normal 20deg PA gearset. So there may be other issues. Your gears are somewhere around 24DP, so a .040 backlash excess would equate to about 60% of a standard 24DP tooth thickness. That's a huge error.
Hope that helps.
Terry
RE: Does too much backlash weaken a gear pair?
From this the backlash that you state would be very excessive. Are you sure that you have the correct center distance, gear data etc? again to re-state what Terry has said the 0.006" additional center distance would not give this much additional backlash. If these gears are truly this thin then I would have to question the bending strength.
Chris
RE: Does too much backlash weaken a gear pair?
Would a tooth most likely fail at the root, or further toward the tip?
George
RE: Does too much backlash weaken a gear pair?
of the gearbox tests.
RE: Does too much backlash weaken a gear pair?
If the load sharing device works, I will build more gearboxes to demonstrate cost, life, noise, weight, volume and efficiency. For these units, I will know how to get the center distances right.
RE: Does too much backlash weaken a gear pair?
Is it physically possible to measure the backlash with feeler gages between the mating gear teeth.
or a magnetic base & a dial indicator would be more accurate. put the indicator approximate center of the gear tooth. I would be courious to see what is there.
RE: Does too much backlash weaken a gear pair?
My device is a load-sharing mechanism for multiple branch shaft gearboxes. I didn't need to know much about gears to develop it. As a result of this project, I have concluded that one lifetime is probably not long enough to acquire a complete understanding of the art and science of gears.
RE: Does too much backlash weaken a gear pair?
Ted