effect of backlash on durability of straight gears 1

[gio1]

Hi,

I am analysing a gear train consisting of straight gears. Briefly the method I am using is as follows:

1- Dynamic simulation of the entire system (torsional model of engine) to calculate critical speeds corresponding to peaks in crankshaft / camshaft /geartrain torsional amplitudes.

2- At the critical speeds, perform FEA on each gear applying loads calculated at each crank angle and build a stress history.

My findings are that reducing the backlash between gears greatly improves durability (fatigue safety factors). I don't have a lot of practical experience with gears and I wonder whether any of you could confirm (or not) this

Thanks

Gio1

 

[plasgears]

Providing excess backlash will introduce dynamic impact, which is undesirable. Also, excess backlash robs tooth thickness from the set.

As a refinement consider helical gears and thicker gear teeth on the smaller gear, the pinion. This is putting more thickness where needed.

 

[kmount]

I agree with plasgears. Going to helical gears will provide better entry engagement & therefore less backlash.

 

[diamondjim]

If you are decreasing your center distance
to achieve smaller backlash, theoretically
you are getting a greater contact ratio.
I assume this increases your durability
ratio.

 

[diamondjim]

If you are decreasing your center distance
to achieve smaller backlash, theoretically
you are getting a greater contact ratio.
I assume this increases your durability
ratio. The converse is true. I would think
that the change would be marginal.

 

[gio1]

Thank you for your suggestions.
Helical gears are not possible due to other design constraints, but reducing the backlash looks like the way to go

 

[israelkk]

Backlash is needed for proper lubrication of the teeth. To minimize backlash a more accurate gear is needed the more accurate the gear the lower the runout and gear teeth errors. The minimum backlash can not be lower than the rounout and additional backlash is needed for proper lubrication.

 

[diamondjim]

Have you looked at long addendum
pinions with short addendum gears?
Sometimes called recess action
gearing. Higher strengths for the
pinions and hidden life effects.

 

[EnglishMuffin]

There is a make of lathe known as a Binns roll turning lathe. These are very big machines used for turning very large rough blanks. They have zero backlash main drive gears, and I think this is achieved by preloading the gears slightly by reducing the center distance. This of course runs completely counter to conventional wisdom, at least with high power gearing, and few people are aware that it can be done. I remember reading a technical article about this a long time ago, but cannot remember where. I think it mentioned that when Jack Binns started to do this, there was a lot of skepticism. Of couse, for this application, torsional drive stiffness is very important because of the intermittent cuts at the beginning of the roughing operation, and I think this tends to support gio1's findings.

 

[ornerynorsk]

To be sure. This is what I've witnessed in the real world, excess backlash greatly accelerates wear, bearing troubles, and other maladies on down the drive train.