Does too much backlash weaken a gear pair? 3

[Windward]

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?

 

[MikeHalloran]

Yes.



Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

Windward,

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

 

[CDGear]

The standard normal tooth thickness (without backlash) for these gears would be 1.57mm (0.062"). Backlash is typically chosen by application, I typically use a backlash of approx 1% of the normal thickness for a new design so 0.016mm (0.0006") per gear which equals 0.032mm (0.0013") backlash in the gear set. I would consider scraping a gear at somewhere around 5% tooth thickness loss, which would be 0.078mm (0.003") per gear or 0.156mm (0.006") backlash in the set.

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

 

[Windward]

Thanks for the good answers Terry and Chris. I realize now that the backlash I mentioned is not the backlash of the gear pair, it is the backlash of the whole gearbox, which is three stages. The center distance is a little high in all three stages, but from what you have said, probably not enough to weaken any of the gears very much. Good thing I don't plan to run the gearbox very long.

Would a tooth most likely fail at the root, or further toward the tip?

George

 

[dinjin]

By changing the center distance, you are loosing contact ratio. How many of these are you testing? I think the backlash difference will have minimal change in the life
of the gearbox tests.

 

[Windward]

Dinjin, I will test only one gearbox, because of the cost. I want to prove a load-sharing device but don't have a grant or other assistance. The test will show only whether the torques are equally distributed. This shouldn't take more than one hour. From your comment and others, I understand that the strength of the gears should not be reduced very much by the excessive center distances, only the life, which will not matter in this case.

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.

 

[mfgenggear]

Windward

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.

 

[Windward]

mfgenggear, I measured the angular displacement of the shaft as it rotated back and forth freely in the mesh, then calculated the arc length to get the backlash. I didn't need high accuracy, so my measurement of the angular displacement was crude. However, it seems to me that this method should work to any degree of accuracy, but I am not a gear engineer and don't know how it is done in a shop.

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.

 

[hydtools]

Most likely failure would be at the root, the location highest bending stress.

Ted