Everyone knows that a pair of gears needs some backlash so that the teeth aren't "pinched" (running with both flanks touching). Many people, though, firmly believe that the amount of backlash in a gear set is a measure of gear quality. What is the real story on backlash? Figure 1 shows it.
Why do we need backlash? If we don't have it, two teeth will be trying to occupy the same space at the same time. Since there's no room for that, something has to give! Either the bearings, the gear supporters, or the teeth themselves will have to bend to allow the teeth to roll past each other. That increases the load on everything and leads to premature failure.
How much backlash is enough? There has to be enough so that under any operating condition of speed, load, and temperature there is always some clearance between the unloaded sides of the teeth. All gears and bearings are a little out of round, so the runout of both gears must be accommodated. The other factors are thermal growth and housing and foundation distortion.
AGMA suggests a minimum (in inches) of:
For industrial gear sets where:
- B="feeler gauge backlash" (see Figure 2)
- C=center distance
Pnd=normal diametrical pitch
This is the minimum backlash. The maximum can be quite a bit more, depending on the tooth thickness tolerance of the gears and their accuracy. More-accurate gears (which cost more to make, of course) have less variation from minimum to maximum backlash than less-accurate (cheaper to make) gears.
If you'd like more detailed information on backlash, copies of the AGMA standard "Tooth Thickness Specification and Measurement" can be ordered from their Web site, which is [www.agma.org].
On the subject of gear quality, there are two parts to be considered: materials and geometry. Most people find geometry easier to measure and understand, so they emphasize that and ignore material. The most accurate gear is a waste of money, though, if its material is weak or brittle, Only the best materials warrant the time and effort necessary to make an accurate gear. What makes a gear material bad?
- It's too soft on the job
- It's hard enough, but the wrong crystal structure (microstructure)
- It's the right structure, but there are too many non-metallic inclusions
Cracks, holes, seams, and laps
In past issues of "Tooth Tips" we've pondered photomicrographic records of the core material of a failed gear tooth. The light-colored areas are "blocky ferritic." They show that the gear was hardened in a separate reheat/quench/temper process after carburizing and cooling, and that it wasn't thoroughly reheated before quenching. Blocky ferrite is weaker than the desired "tempered martensite" structure, and is not permitted in highly loaded carburized gears.
How can buyers know what they're getting? After all, the gears look, weigh, and measure the same. The difference is invisible, like good character in an individual, but it's there and will become obvious with time.
If you could look inside a well-made carburized gear, the case microstructure would show a uniform martensitic structure, free from defects.
Material quality is difficult to measure on a finished part because the critical areas are inaccessible. Quality is maintained by controlling the manufacturing process and checking the results each step of the way, from the ingot to final heat treatment and inspection for hardness and surface defects.
Some purchasers has strict material specifications and internal quality control, allowing them to verify the quality of the parts they buy. Others don't have such in-house capabilities, so they buy from vendors who have internal quality controls. The "qualified vendors" utilize quality standards and inspection expertise to get the right materials and processes into the gears as they're made. The remaining buyers take their chances with lowest bidder.
AGMA develops industry standards for gear quality, both in geometry and materials. The right quality level can be specified by reference to those standards, avoiding the need to write and maintain in-house documents.
AGMA sponsors a voluntary self-certification program, allowing participating companies to certify that their products are made to the appropriate AGMA standards.
To learn more, visit the AGMA Web site at [www.agma.org].