How Do You Determine Gear Tolerance and Surface Roughness?

[hanson]

Hi all.
How do one design the gear tolerance and the surface roughness of a gear?
I know there are some quality numbers in AGMA, but is quotaing the number is enough for specifying the tolerances of a gear?

about the surface roughnessm i saw many things like remove mostly 3.2mm etc...How do i decide all these?

 

[FredGarvin]

I know you're not going to like this answer, but ...it depends. This is a pretty specialized field that has a lot of independent research conducted. You really need to be around someone experienced in the specification of gears.

Let's put it this way...you have a general area that the gears are going to operate in as far as speed, load and possibly accuracy of loaction (i.e. stepper motors, motion control). Things like AGMA quality will be referenced off of your judgement of how imprtant these things are. Obviously, the balance is going to be the requirements of the design and the cost of making the gears. If you are running low speed and relatively low power, then something off the shelf, forged/cast would probably do you just fine. However, if you start to have to look into actually specifying a gear for manufacture, there's a lot that has to be considered.

You, as the designer, have to look at the overall design. Can you live with a lower tolerance in the gear, i.e. can your stack ups, etc... handle having a less controlled profile? Can you handle possibly greater runout and backlash? Things like surface roughness will effect the effectiveness of the lubrication as well as the noise generated by the gears. It can also effect the strength of the gear teeth and their longevity. Can you live with a bit more noise? Do you have sufficient cooling? It goes on and on.

I can tell you that, in my applications, we don't look below AGMA 7 and more frequently specify 10. However, we do high speed applications with extremely tight tolerances. We also have to have a long lasting gear so that also includes a lot of secondary machining and possibly plating processes.

Are you in the position to sit down with someone who has experience with gears, that can take a look at what you are doing? If not, can you post what your objectives are?