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# Involute Spline - Pressure Angle

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 ISUSteve (Mechanical) 4 Aug 12 22:31
 I have an old sprocket off a 40's tractor and want to go to a smaller sprocket to increase the tractor's top speed. I need to match the internal spline of the old sprocket, but I can't accurately get the pressure angle. It is a flat root, 12 tooth, 5/10 pitch, internal spline. The major and minor diameters are within 1% of spec. When I take 2 pins (actually two 3/8 diameter drill bits which measure at .375" and .374") I get the distance between pins as 1.885". When I do the calculation, I should see 1.773" with a 30* pressure angle. This is about 5% off spec. When I do the calc for a 45* pressure angle, I'm closer to the pin to pin spec. I'm withing 1% of spec on the major diameter, but 5% off of spec on the minor diameter. How do I figure out the pressure angle? Also, Machinery's Handbook list di (diameter of pin) as 1.7280/P. Which gives di as .3456". I used a di of .375". Could my calcs be off because I used 3/8" pins instead of the .3456" pins?
 TheTick (Mechanical) 6 Aug 12 9:48
 I do love geeking out on gear geometry... Your pin-to-pin measurement can be deceiving. Variations in the amount of backlash space can change the diameter. So can a number of other factors, including wear. I d not consider this a sound method for reverse-engineering old gears. Most gears are made with 14.5° or 22° pressure angles. For some reason, these are de facto standards for gear teeth. You might also check some of the other gears to see if there is any indication of pressure angle used in the design. Consult Machinery's Handbook for old gear and spline specs. If you can, find an old version. http://en.wikipedia.org/wiki/Involute Involutes (the curve used on gear teeth) are a beautiful thing. Done correctly, they can be very forgiving. Even if you are off on the pressure angle, you will still have smooth engagement, which is usually far more important than nailing the pressure angle. Mathematically, the pressure angle is important because it is used to derive the base diameter from the pitch diameter. The base diameter is the diameter used to actually generate the involute curve. Honesty may be the best policy, but insanity is a better defense. http://www.EsoxRepublic.com-SolidWorks API VB programming help
 mfgenggear (Aerospace) 6 Aug 12 18:57
 Steve unless you have the necessary equipment to verify the involute. it's not very easy. my best advise is seek a gear shop that can help you with reverse engineering. If you do it your self, the sprocket that will require shaping or cheap push pull broaching. The shaping can be done for a one pc job simply shaping with a mill. it would be cheaper if you buy an after market sprocket for that tractor with the spline machined. do a google search with base pitch verification this may get you close. Mfgenggear
 ISUSteve (Mechanical) 6 Aug 12 20:23
 This is an internal involute spline, so I just just need to differentiate between a 30, 37.5, or 45 degree pressure angle. The sprocket is a No 100 roller chain, that's an easy one. There's a program from Frenco that draws involute splines. The demo version only lets you use prime numbers. Does anyone have this program and would be willing to post to generated picture? Here's a picture of the spline:
 ISUSteve (Mechanical) 6 Aug 12 20:58
 The tractor is a 1942 Case. I've been using B92.1-1970 to do my calcs. I can't find much on earlier standards and it doesn't fit the Old American Standard. Maybe I'm using too modern of specs. I was hoping to draw this up and send it out to a sprocket machine shop with capabilities to form an internal spline to have it manufactured. There are no smaller aftermarket sprockets.
 mfgenggear (Aerospace) 6 Aug 12 21:35
 Steve an EDM house that has a gear program can do this for you. they can use your current part to make electrodes. check fit of the electrode to the internal spline. use the mating part as a gage to make sure it fits. It's funny , L/O your picture I get 20 Deg PA. Mfgengear
 ISUSteve (Mechanical) 9 Aug 12 20:13
 How did you arrive at a 20 degree PA and what does L/O mean?
 mfgenggear (Aerospace) 9 Aug 12 21:00
 ISUsteve L/O =Lay Out Take a black or red marker & mark the face of the existing sprocket hub. scribe a line to center line of the tooth space. then scribe a line to the approximate angle of the spline flank. now measure the included angle with a protractor. I measure 20 deg. It just an approximation. who ever cut this internal spline has to match it precisely. Mfgenggear

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