Posted by Ron V on February 06, 2006 at 19:45:06:
In Reply to: Re: Helical Gear Problem posted by Stuart on February 06, 2006 at 10:17:01:
Before going any further it's best to work out what the base pitch is so as we can work out the pitch and pressure angle.
Base pitch is the circular pitch at the base circle. Base pitch = module x 0.12368 x (cos P.A.). So if you know what the pitch and P.A. is you can work out the B.P. or if you know what the B.P. is you can work out the pitch and P.A.
To measure a gears BP is simple. Use a good set of verniers, or gear tooth mics, in the same way you would when taking span measurements in the normal plane. Open up the verniers to the number of teeth that best fits between the jaws. What this means is that you don't want the tips of the jaws to interfere in the root nor do you want to be measuring across the tips of the teeth. There will be a certain number of teeth to span across where the jaws contact the active part of the tooth flanks. For example the 65 teeth gear in question should give you a good fit at around 8 teeth. Record the dimension and then take the same measurement, but this time take it across one less tooth e.g. 7 teeth. Record this dimension also. Subtract the second result from the first and what you end up with is something very close to the base pitch. Once again using the gear in question, if itís 7module, 20 deg P.A. the base pitch you measure should be around 20.66mm. If you have know idea what the pitch and P.A. is of a ďmystery gearĒ then just transpose the equation above and insert the measured B.P. I have a set of B.P. charts Iíll email you to make the process a bit easier for you. This process is by far the quickest, easiest and most accurate way I know of to find out what a mystery gear is. It can also be used to work out mystery involute splines. The other great thing about it is that the base pitch doesnít change even if the thicknesses of the gears teeth have changed e.g. a badly worn gear. As long as the wear is even you should still be able to get a fairly good idea of what the data of the gear is.
Good luck and donít forget to post your results for others to see.
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