# Measuring pitch



## Glenn Brooks (Apr 13, 2018)

Ok, so all the tutorials say gear pitch is measured as teeth per inch (TPI). But so far, I’ve never read how to actually measure TPI.

Are the number of teeth per inch measured along a 1”chord, or around the outside diameter of the gear, or something else?

I would like to determine pitch of a gear we need to replace.

Thanks much,
Glenn


----------



## RJSakowski (Apr 13, 2018)

Glenn Brooks said:


> Ok, so all the tutorials say gear pitch is measured as teeth per inch (TPI). But so far, I’ve never read how to actually measure TPI.
> 
> Are the number of teeth per inch measured along a 1”chord, or around the outside diameter of the gear, or something else?
> 
> ...



I believe that gear pitch is specified as diametral pitch which is the number of teeth in the gear divided by the pitch diameter in inches.  Pitch diameter id about halfway between the minimum diameter at the root of the tooth and maximum diameter at the crest of the tooth.  Diametral pitch is usually a nice number so you can usually determine it with an approximate measurement of the pitch diameter.

All gears with the same diametral pitch and pressure angle will mesh.  Pressure angles are typically 14.5º and 20º.

Metric gears are determine4d by module or modulus.  Module is equal to the pitch diameter in millimeters divided by the number of teeth in the gear.  Gears with the same module and pressure angle will mesh.


----------



## Glenn Brooks (Apr 13, 2018)

RJ,  thanks much.  I knew that!  What was I thinking??  Anyway, easy to figure out what I need now.

Many thanks.

Glenn


----------



## BtoVin83 (Apr 14, 2018)

Every once  in a while you will run across a gear that is odd like an oversize pinion that doesn't match the math. Gear designers will do this to match existing center distance or ratio but design a hunting tooth ratio to spread wear through all the teeth. You can also drop the number of teeth on small pinions using the same center distance to minimize undercutting the tooth profile


----------



## tq60 (Apr 14, 2018)

There is a trick where you do basic math and add a tooth or 2.

Not sure but have seen it a few times in discussions. 

Try Google and look for "determining gear pitch" for starters.

Sent from my SAMSUNG-SGH-I337Z using Tapatalk


----------



## Asm109 (Apr 15, 2018)

OD of gear = N+2/DP   Where N is number of teeth and DP is diametral pitch


----------



## 69Z/28-Nut (Jun 19, 2018)

To find the pitch of any gear, measure O.D. of gear. 
Then count the teeth and add 2. 
Next divide this number by the O.D. of the gear. 
Just the reverse of finding the diameter of a gear blank where you take the number of gear teeth needed add 2 and divide by the D.P. of the gear you are wanting to cut.

You can also purchase a gear tooth pitch set; Common sizes are
3,4,5,6,8,10,12,16,20,24,32 & 48 DP.

Uncommon set would be;
3-1/2,7,9,11,13,15,17,18,19,22,26,28,30,36 & 40 DP.

I dont owna metric set but the can be found on the internet.

They also come in coarse pitch, fine pitch, combination stub 20° PA, module pitch, circular pitch & involute spline 30° PA.

A real nice program is GearDXFSetup.exe if you can locate it on your internet.

The program does Standard and Metric Gears


----------



## BROCKWOOD (Nov 13, 2018)

Adding to the [U]69Z/28-Nut[/U] list is a Dp = 14. That is common for at least B&S & Van Norman Dividing Head (DH) Change Gears. There could be other DH manufacturers that also use that Dp for their change gears.


----------



## Bob Korves (Nov 13, 2018)

RJ was correct, as usual.  Diametral pitch is the number of teeth of the gear divided by the pitch diameter.

The "plus 2 teeth" thing does enter into some other equations, but not for determining diametral pitch.


----------



## BROCKWOOD (Nov 14, 2018)

Asm109 said:


> OD of gear = N+2/DP   Where N is number of teeth and DP is diametral pitch



Yes, but written more correctly: (N + 2) then / by Dp.  Or (N + 2) / Dp. Order of operation is the most common cause of people not liking math.

Try this chart for all your calculations.


----------



## 69Z/28-Nut (Nov 28, 2018)

B


BROCKWOOD said:


> Yes, but written more correctly: (N + 2) then / by Dp.  Or (N + 2) / Dp. Order of operation is the most common cause of people not liking math.
> 
> Try this chart for all your calculations.
> 
> View attachment 279727


Wish this chart were larger, at my age I can no longer see small print.


----------



## BROCKWOOD (Nov 28, 2018)

I can do that when I get home tomorrow morning Mr Z Man.


----------



## BROCKWOOD (Nov 30, 2018)

PM sent Mr Z Man. Hobby Machinist has this resizing gizmo that automatically makes my info too small no matter how large or clear it was when I uploaded it.  Here is a pic I made for diameter. 




Perhaps this pic will be more useful.


----------



## Bob Korves (Nov 30, 2018)

BROCKWOOD said:


> PM sent Mr Z Man. Hobby Machinist has this resizing gizmo that automatically makes my info too small no matter how large or clear it was when I uploaded it.  Here is a pic I made for diameter.
> 
> View attachment 281079
> 
> ...


The original poster is asking for the diametral pitch of the gear, not for the diameter.


----------



## BROCKWOOD (Dec 1, 2018)

OK then solve for P.




That example worked out a little too smoothly!

GIVEN:  N = 59, Do = 7     (Just random numbers)

(59 + 2) / 7 = 8.714~

That's messy. If this were an existing gear in a drivetrain:  I'd be checking the mating gear, my math, the tooth pictures in Machinery's Handbook & the Black Book. 

If I had to have 59 teeth in order to maintain a certain ratio with the next gear, I would check diameter with P = 8 & with P = 9 & adjust the rest of the design to fit.


----------



## benmychree (Dec 24, 2018)

Easiest is to just purchase a set of gear pitch gaged, made from stamped steel; they have 14 1/2 deg on one side and 20 deg on the other and are not expensive,


----------



## chips&more (Dec 25, 2018)

benmychree said:


> Easiest is to just purchase a set of gear pitch gaged, made from stamped steel; they have 14 1/2 deg on one side and 20 deg on the other and are not expensive,


For what they are. Just stamped metal. They can be very expensive. I would instead just do a search on the net for “gear tooth profile”. You will get enough head spinning info there for free. Even pics of 14 1/2° & 20° examples…Dave
https://www.amazon.com/Fowler-Involute-Gear-Tooth-Holder/dp/B00B5KQIKK


----------



## benmychree (Dec 25, 2018)

I am quite sure that they are available for much less than that ------ Amazon, instant gratification at a price!


----------



## chips&more (Dec 26, 2018)

Yeah, on fleabay from India. It amazes me on how some people value their dollar.

https://www.ebay.com/itm/Gear-Tooth...=item2abd5b8d72:g:gncAAOSwYwhba8nq:rk:16:pf:0


----------

