How to figure chainsaw sprocket pitch?

[dsell]

From time to time, I get sprockets that I can't figure out the pitch because I don't have a matching chain. Does anyone know the math behind it? I understand chainsaw chain pitch is the distance between 3 rivets divided by two. For roller chain, the pitch is determined by where the center of the roller rides in the sprocket. I mocked this sprocket up in CAD going with 1/2 distance from ID and OD for pitch.

 

[SimonHS]

Would it be as simple as (pitch diameter x pi) / number of teeth?

In your drawing the pitch diameter is 1.918.

It should be easy to measure a known sprocket and see if this works out OK.

 

[dsell]

You mean try the same measurements and formula on a 3/8 pitch sprocket? I'll try that tonight.
So:
((Sprocket OD - Sprocket ID) / 2) + Sprocket ID = Pitch Diameter
then:
(Pitch Diameter x pi) / number of teeth = Chain pitch

 

[Franny K]

For a rim sprocket the outer diameter is where the bottom of the tie straps in the vicinity of the rivet contact. For a spur sprocket the "diameter" is the bottom of the tie straps basically half way between the rivets. Note for 3/8 (really 0.367" + - ) the Picco rim drive sprockets are larger in diameter than the standard 3/8 ones. I would think take the pitch, multiply by two, multiply by number of tooth drive (are we doing nose or drive sprocket?) use that number to get a circumference or diameter at the rivet center. Then adjust for the offset of the rivet from the tie strap contact place.

I did not really proof read it.

Nose sprocket seems to jam in between the drive links.

What ones are a problem? 9 tooth 1/4 pitch is pretty close to .325 7 tooth in rim sprocket. 8 tooth .325 is probably pretty close to 3/8 7 tooth.

One would think there is some engineering set in stone however I have some 9 spline 9 tooth 1/4 inch pitch sprockets that differ somewhat in diameter than the 9 tooth ones in 7 small spline.

 

[dsell]

The Stihl 325-8 is the same diameter as the Stihl P7 which is 3/8LP. 1.435 diameter

 

[dsell]

This is a Wright Burns 3/8-8 Spur Drum 4832 which is 1.667 OD and 1.063 ID
So:
((1.667 - 1.063) / 2) + 1.063 = 1.365
then:
(1.365 x pi) / 8 = .536 chain pitch which is incorrect if my math is correct.

The 3/8-8 rims above measured 1.605 OD.

Notice the chain preset fits perfectly in the spur but doesn't fit in the rim.

Another thing to note, the drive links bottom out in the rim and the side straps are not touching the outer diameter.

Also, I can move the chain back and forth when wrapped around the rim but I can't with the spur.

The spur angles fit the drive links perfect and the drive links are not bottomed out in the spur.

 

[dsell]

Let's reverse engineer the equation.

Chain pitch is .730 / 2 = .365 or known as 3/8 pitch.

This is a Wright Burns 3/8-8 Spur Drum 4832 which is 1.667 OD and 1.063 ID

(PD x pi) / 8 = .365 chain pitch

(.929 x pi) / 8= .365 so pitch diameter using that formula would be .929, which doesn't work.

I'm no closer to figuring out the pitch of that sprocket. The angles match a 1/2-7 spur.

 

[Franny K]

Sprocket acquisition disease?

I seem to recall the Stihl mini spline 7 tooth in .325 measured a bit different than the Oregon small spline 7 tooth .325.

There are two pitch increments spacing between drive links. The drive link holes need not be the same value as the tie strap pins.

I can see that the spur sprocket outer "diameter" is considerably closer to the rivet "diameter" than the diameter of a comparable rim sprocket.

 

[Woodslasher]

Let's reverse engineer the equation.

Chain pitch is .750 / 2 = .375 or known as 3/8 pitch.

This is a Wright Burns 3/8-8 Spur Drum 4832 which is 1.667 OD and 1.063 ID

(PD x pi) / 8 = .375 chain pitch

(.929 x pi) / 8= .365 so pitch diameter using that formula would be .929, which doesn't work.

I'm no closer to figuring out the pitch of that sprocket. The angles match a 1/2-7 spur.

3/8 chain is .375 chain, not .365

 

[Franny K]

100 feet * 12 inches per foot / 1640 drive links per roll =0.732 inches between drive links. rounded off .7317......


For 7 tooth rim
0.732 * 7 = 5.124 inches circumference at rivet centers

5.124 divided by pi 3.14159(....... ) =1.631 inch diameter at rivet center circle

For rim drive I see 1.380" for 3/8 7 tooth subtract from 1.631 and divide by two = 0.1255 or real close to 1/8 inch which is realistic for the sliding surface of 3/8 chain to rivet center I would think.

Not sure if the shape of the tie strap might complicate things to a minute amount.

For the 8 tooth spur, I did the math for 7 by accident first so will do for 8
5.856" circumference at rivet center
1.864" diameter at rivet centers which should be measurable.
(1.864-1.667)/2=0.093" for offset notch at tie strap center to rivet center.

 

[dsell]

Sprocket acquisition disease?
Aren't we all diseased on this site?

I seem to recall the Stihl mini spline 7 tooth in .325 measured a bit different than the Oregon small spline 7 tooth .325.
Not much, 1.252 vs 1.248 in the pics above.

There are two pitch increments spacing between drive links. The drive link holes need not be the same value as the tie strap pins.

I can see that the spur sprocket outer "diameter" is considerably closer to the rivet "diameter" than the diameter of a comparable rim sprocket.
Yes, if you drew a line across the bottom of the drive link bores it would touch the points of the spur.

 

[christylleam]

Are you looking for a chainsaw?

 

[dsell]

100 feet * 12 inches per foot / 1640 drive links per roll =0.732 inches between drive links. rounded off .7317......


For 7 tooth rim
0.732 * 7 = 5.124 inches circumference at rivet centers

5.124 divided by pi 3.14159(....... ) =1.631 inch diameter at rivet center circle

For rim drive I see 1.380" for 3/8 7 tooth subtract from 1.631 and divide by two = 0.1255 or real close to 1/8 inch which is realistic for the sliding surface of 3/8 chain to rivet center I would think.

I measured .15 to .16 from the (od of a 3/8-7 1.383) to the center of the rivet.

Not sure if the shape of the tie strap might complicate things to a minute amount.

For the 8 tooth spur, I did the math for 7 by accident first so will do for 8
5.856" circumference at rivet center, yes
1.864" diameter at rivet centers which should be measurable. Yes, that measures good, they rock a bit.
(1.864-1.667)/2=0.093" for offset notch at tie strap center to rivet center. That looks correct as best I can see with a caliper.

 

[dsell]

Are you looking for a chainsaw?

I'm not sure I understand the question. I'm trying to figure out how to tell the pitch of the sprocket in the first post. That sprocket fits a chainsaw or a harvester, I'm guessing an old gear drive.

 

[christylleam]

I think it is.

 

[buzz sawyer]

Wouldn't it be easier to keep some short sections of different types of chains on hand and see which fits the sprocket?

 

[dsell]

Wouldn't it be easier to keep some short sections of different types of chains on hand and see which fits the sprocket?

Yes, but I don't have .354, 7/16, 1/2, 9/16, and 3/4 pitch chain.

 

[buzz sawyer]

Yes, but I don't have .354, 7/16, 1/2, 9/16, and 3/4 pitch chain.

Start collecting.

 

[buzz sawyer]

3/8 chain is .375 chain, not .365

Actually, is it .365. Just measured two 3/8 chains and got 0.730 across three rivets. One chain was a used Stihl chain from m MS361

 

[Philbert]

Not sure if this helps. They measure with a caliper about half way through:

Sprocket acquisition disease?

It's a nice collection!

Philbert