# How to measure/determine gear pressure angle

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Is it possible to determine the pressure angle of a gear by simple measurements and if so what do I measure? Its a 24 tooth gear that is missing a few teeth, from measurements of the OD I think it's probably 16DP.

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Try:-

and look for gearpa.zip:-

Mark Rand RTFM

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Recently I needed to figure out PA on a gear for a DaeJung engine lathe. I plugged numbers into gearspec from this link and by process of of elimination determined I had a 105T 20 degree pa, 1.5 mod gear.

Hope this helps,

Wes

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I have used plasticine.

Roll a snake of the stuff. Lay it on a flat surface.

Roll the gear across it, squeezing it down evenly.

Slice off the mushy edge, leaving a clean(ish) rack form molded off the gear.

Compare angle to protractor.

Works OK to separate 14.5 from 20 from 25 degree angles.

Cheers Trevor Jones

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Nice one :-)

Mark Rand RTFM

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Thanks, I measured around 0.477", the gearpa gave the following output.

Number of teeth on gear [30] ? 24 Diametral pitch of gear [6] ? 16

Pressure angle = 14.50 deg...Chordal span over 3 teeth = 0.4833 in Pressure angle = 20.00 deg...Chordal span over 3 teeth = 0.4823 in

I've found a copy of Machinery's Handbook with the formula that gearpa uses, but assuming the program is written correctly I'm not sure if it's going to give me any better results! Even with a brand new gear I doubt I could measure accurately to 1 thou across 3 teeth and thereby tell the difference between 14.5 and 20 deg PA.

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Surely thats the helix angle and not the pressure angle?

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I must admit that I had wondered, having seen the state of some of the gears in the Hardinge apron that I'm still working on.

Mark Rand RTFM

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It should form a "rack" with teeth that have the same pressure angle as the gear did. Same helix angle as well, but you don't need to measure that!

Mark Rand RTFM

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Mike , What is the gear off, that could give us a clue.

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Take a look at this link which shows it nicely...

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I continue to be amazed that something that presumeably will prevent unlike gears from functioning together should be so difficult to measure! I wonder if it would actually matter in a slow-running, adjustable meshing, train such as change gears? Anyone done it?

Of course, I have seen photos of a functioning change gear made by passing a strip of shim brass between two gears to corrugate it, lapping and soldering the ends together, and epoxying it to the perimeter of a wooden disc turned to fit!

Don Young USA

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You get helix if you measure the angle with the protractor flat on the table, with the base parrallel to the axis of the gear, and the indicator aligned with the impression . Stand it up on edge.

The teeth impress a rack form into the plasticine. Rack is a straight tooth, similar to an acme thread form. Pressure angle is equal to the angle, either side of 90 degrees.

Cheers Trevor Jones

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An oil pump drive on a 1950's BRM Formula 1 engine.

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Thanks, I'll give it go at the weekend.

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