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Involute sprocket diameter


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#1 ronbourd

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Posted 17 May 2020 - 12:03 PM

Can someone help me find the true diameter of a involute sprocket with 2.86 pitch 8t. In a perfect world, would they be the same diameter for a one ply track and for two ply track. I had a shaft with sprockets on the lathe, and the two sprockets were not exactly the same diameter...Thank you for your reply Ronald. 



#2 wileo

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Posted 17 May 2020 - 01:15 PM

Should be 2.86 x number of teeth / pie
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#3 ronbourd

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Posted 17 May 2020 - 07:24 PM

wileo, I tried your formula, but this is not correct. Part of the thickness of the track must be part of the formula.  There must me some one on this forum with the knowledge of this rolling circomference of an involute sprocket.Thank you for your reply,Ronald.



#4 mile9c1

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Posted 19 May 2020 - 10:36 PM

Why does track thickness matter if you're just trying to match the sprocket to the drive lugs? The pitch on the track surface that touches the sprocket is 2.86". The top of the track doesn't touch the sprocket. So I'd agree with wileo. I'd probably try to make both sprockets the same diameter, or at least make them perfectly round.

#5 Rev-man

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Posted 20 May 2020 - 10:30 AM

https://www.dootalk....ockets-on-axle/



#6 ronbourd

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Posted 20 May 2020 - 07:37 PM

Mile9c1, the factory must have calculated an ideal size of this sprocket. This is the size i am looking for.  8 tooth time 2.86 gives 22.86"divided by 3.1416 equals=7.28"diameter. The diameter of a 8t is less than that number...Ronald, thanks for your reply!



#7 SnowMN

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Posted 21 May 2020 - 09:41 AM

The formula (Pitch x Number of teeth) / Pi  is going to give you the Pitch Diameter of the pulley.  The pitch diameter is the theoretical centerline at which the track rolls over used for calculating ratios or speed. 

 

I think what you are looking for is the Outside Diameter of the pulley.  If you google Outside Diameter I'm sure you will be able to find formulas used for belts and pulleys.  The trick in getting an accurate number is understanding the belt design (track in this case) and how it effects the pulley design.  In the case of snowmobile tracks you can look in the window cutouts to see the tensile cords located approximately in the center of the track's thickness.  Belts are typically designed so their tensile cords are on their pitch line.  My recommendation to you is to use the formula you have been using for Pitch Diameter and subtract off 2 x distance between the tensile cords and the inside of the track.  So do: (8 x 2.86) / 3.1416 - (2 x .125) = 7.033.  The 7.033 should be very close to the Outside Diameter you are looking for. 

 

To complicate matters for you further please keep in mind the 7.033 diameter is likely not consistent across the entire width of the drive sprocket.  I haven't measured a BRP drive sprocket, but am guessing the sprockets or at least the portions facing the outside on each side are slightly convex to assist in tracking.  Good luck!



#8 ronbourd

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Posted 21 May 2020 - 07:31 PM

SnowMN, you did understand my search! The involute sprockets ( 8t, 2.86"pitch) that I machined on the lathe came down to 7,140"diameter. I had shaved about 0.060"before they became perfectly round. We are learning every day...the answer is near... Thank you for your help! Ronald.


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#9 Boomer

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Posted 22 May 2020 - 10:30 PM

Is this what you're looking for?.......

 

https://wahlracing.c...kets-hex-shaft/



#10 djm

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Posted Yesterday, 02:24 PM

SnowMN, you did understand my search! The involute sprockets ( 8t, 2.86"pitch) that I machined on the lathe came down to 7,140"diameter. I had shaved about 0.060"before they became perfectly round. We are learning every day...the answer is near... Thank you for your help! Ronald.

 

Thanks snowMN and ronbourd for this bit of ino.  I have never designed sproketss for tracks adn have always used pitch diameter in my calculations.  Interesting to learn more!  Much appreciate the lesson!    The secret machinists number of 7.14" comes pretty close to the Wahl Brothers 7.125.   Thank you for the info!  


Edited by djm, Yesterday, 02:25 PM.


#11 ronbourd

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Posted Yesterday, 06:36 PM

DJM, and to gain the maximum... the thickness of the track has a small influence. The track with more thickness would need a smaller diameter sprocket!


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#12 wileo

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Posted Yesterday, 09:20 PM

DJM, and to gain the maximum... the thickness of the track has a small influence. The track with more thickness would need a smaller diameter sprocket!


I thought the bigger the drivers the better?

#13 djm

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Posted Today, 03:22 AM

I thought the bigger the drivers the better?

  

 

 

That is still true.. But it means you need one with more teeth to change or increase the overall diameter.  What he is saying if you have a thicker belt / track, that you need to take a little more off the diameter of the base driver.   But a 9 tooth on a given pitch is more efficient that an 8 tooth.   A 10 tooth is more efficient than a 9 tooth.   The pitch diameter is always constant = number of teeth x track pitch divided by Pi,  The given pitch diameter is constant but the base diameter of the sprocket is sized for the thickest track.  There is a lot of  leeway here as you can run a 3.0 pitched track on a 2.86 pitch sprocket.  Even I have done that and it works ok.  But you can't run a smaller pitched track on a larger pitched sprocket.  That does not work.   


Edited by djm, Today, 03:38 AM.





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