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kclement16 (Electrical) (OP)
18 Jan 11 15:23
Hello,

I am very new with gears, but here is my problem:

I have been presented with the task of determining exactly when a pinion/gear set fail.  I figured the best way to do this was measure the nominal backlash and backlash after the components have been used.  I have come across a rule of thumb for backlash requirements:

min backlash = 0.03 / diametral pitch
avg backlash = 0.04 / diametral pitch
max backlash = 0.05 / diametral pitch

Do these equations hold true?

I have various spur, worm, rack and pinion, and bevel gears that were sent in for wear testing.  The measurements were returned and the circular tooth thickness was measured at the pitch diameter.  Is there a way I can approximate backlash with nominal and worn circular tooth thickness?  I have looked through various PDFs online as well as the Machinery Handbook and have had a very hard time.  Any helpful information that I could find mostly related to spur gears.

Overall, any information as to how to determine when gears "fail" (even if the machine using the gears is working) would be greatly appreciated.  Thanks in advance!
Helpful Member!  hydtools (Mechanical)
18 Jan 11 17:04
Sounds like you first need to determine your failure criteria.  Then you will know what to look for and measure.

Does whoever presented you with the task know what failure mode for which they want the test run?

Ted

plasgears (Mechanical)
19 Jan 11 12:08
In our shop we assigned a regular backlash regardless of gear size. It was in the form of increased center distance, usually .005 in. or less. This was for subfractional gearboxes. In plastic gearsets less backlash can be assigned because of gear and shaft flexibility.

My method was to avoid stress beyond 1/2 yield. Fatigue is the consideration.
dinjin (Mechanical)
21 Jan 11 14:17
I go with Hydtools.  Each application should have its own criteria.  Some applications allow half of the tooth to be gone or worn away before it is replaced.
kclement16 (Electrical) (OP)
24 Jan 11 14:37
I guess it is rather hard to determine a failure mode, as I was not presented with one.  A lot of the gears show around 35% wear, 55% wear, and one even shows about 76% wear.  However, this machine is set to sense when all components have returned to their starting position.  With this in mind, I suppose this machine could theoretically run until something physically breaks and it cannot anymore.  Thanks for all of your responses!
geesamand (Mechanical)
5 Feb 11 9:13
Failure can be determined by many things and it depends on the application.
What is the typical failure mode for the gearing it it's application?
Transmission error, vibration, tooth breakage, spalling, micropitting, etc?

David
bradleyelwood (Mechanical)
2 Mar 11 23:56
Helicopter transmissions typically have chip detectors built into oil sumps.  The detectors look like an outboard-motor-spark-plug except that they are magnitized.  When a chip or several chips short circiut the center electrode to ground, the pilot knows he has transmission chips, the electrical circuit lights a warning.

You might look at something similar.

If you wait until backlash changes, wear has progressed where most gear designers would say its too late.

tbuelna (Aerospace)
5 Mar 11 1:49
kclement16,

It is very uncommon for gearsets that are properly designed, manufactured, and maintained to experience "wear" as you describe it.  Specifically, a loss of material from the tooth working surfaces.  In most cases, a properly designed gearset would not operate in contact conditions that produced the fretting/galling action which results in material removal from the tooth flank surfaces.

As geesamand notes, the more typical failure modes for gear teeth are surface contact fatigue spalls and/or bending fatigue failures.

Hope that helps.
Terry

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