Help Missing gear

[oncewas]

I have a missing gear on a starter motor, what I need to find out is the number of teeth and also the size of the blank from which to cut the gear. The gear that remains is an involute 20 deg. 38 tooth on a 1.578 in. outside diameter that is on a centre to centre with the missing gear CTC 1.069 in.
Any advice or help would be gratefully received. Thanks.

 

[francist]

Hmmm, I ran some numbers through a gear calculator I have and the closest I can come up with that matches your existing criteria is a Module 1 gear with 16 teeth and an outside diameter on the blank of 0.709”

Here’s what I did:
I ran numbers for 38 teeth with your specified OD . Diametral gears weren’t getting very close, but a module 1 gear gave me 1.575” or 40.00 mm diameter.
So I then used a Module 1 gear and kept plugging in different teeth until a 16 tooth gear size gave me a close CTC distance. In this case, 1.063” or 27mm.
Then I simply calculated the OD for a 16 tooth Module 1 gear and calculation said 0.709” or 18.0mm

Ive attached my screen shots of the three calculated steps. Maybe someone else has a better way?

-frank

 

[RJSakowski]

Hmmm, I ran some numbers through a gear calculator I have and the closest I can come up with that matches your existing criteria is a Module 1 gear with 16 teeth and an outside diameter on the blank of 0.709”

Here’s what I did:
I ran numbers for 38 teeth with your specified OD . Diametral gears weren’t getting very close, but a module 1 gear gave me 1.575” or 40.00 mm diameter.
So I then used a Module 1 gear and kept plugging in different teeth until a 16 tooth gear size gave me a close CTC distance. In this case, 1.063” or 27mm.
Then I simply calculated the OD for a 16 tooth Module 1 gear and calculation said 0.709” or 18.0mm

Ive attached my screen shots of the three calculated steps. Maybe someone else has a better way?

-frank

Frank, I think that you hit it.

Here's how I did it.

Using the formula (no.teeth + 2) x mod = o.d. or (38 + 2)/(1.578 X 25.4) = .9980 =~ 1. The center to center distance is the sum of the pitch diameters/2 or 2 x center to center = 2 x 1.069 x 25.4 = 54.3 =~54 = p.d.1 +p.d.2. and 54 - 38 = p.d.2 = 16. The center to center distance is slightly more than the 1.063 for half the sum of the pitch diameters to allow for some clearance for the meshing gears.

 

[oncewas]

Hi Frank, and RJ , thanks for the info your numbers really do make sense I am ashamed to tell you the method that I was using which brought out my final answer which was 14 teeth. I truly stand corrected. Thanks for all your help. Cn.

 

[francist]

All I did was plug numbers. RJ’s way has me winded just thinking about it!

How did you arrive at 14T? It’s not that far different.

-f

 

[oncewas]

Here i go, I took the OD of the known gear and multiplied it by PI , then divided that by 38 to find the requirement for each tooth, ( all rounded out to 3 decimal places ) I then drew a circle representing the known gear overlapped that circle by the measured depth of the known gear using the CTC dimension and drew a second circle measured the second circle OD and multiplied again by PI to find circumference then divided by the each tooth requirement and came out with !4.
OK confession over, but I will accept any and all criticism. Thanks to everyone, Colin.

 

[brino]

Hmmm, I ran some numbers through a gear calculator I have

Frank is that something you can share or is it a paid app.?
Either way I'd like to know the name.
Thanks!
-brino

 

[francist]

Frank is that something you can share or is it a paid app.?

Yeah for sure, it’s an app called Spur Gear Calculator by (I think) Spiral Arm Consulting in UK. It’s still available on the App Store as well as a few more perhaps more involved ones by the same company. I only have the basic one on my iPhone and iPad but it works great, at least for what I need. It was and still is free.

There’s another one I sometimes use called Gear Generator but I don’t have that in app form, just website. It’s really cool because you can animate the gears and run compound ratios, but it doesn’t do calculations for CTC distances as far as I could see.

-frank

 

[brino]

Yeah for sure, it’s an app called Spur Gear Calculator by (I think) Spiral Arm Consulting in UK. It’s still available on the App Store as well as a few more perhaps more involved ones by the same company. I only have the basic one on my iPhone and iPad but it works great, at least for what I need. It was and still is free.

There’s another one I sometimes use called Gear Generator but I don’t have that in app form, just website. It’s really cool because you can animate the gears and run compound ratios, but it doesn’t do calculations for CTC distances as far as I could see.

Thanks Frank!