Working out the pitch of gears

[Barry]

After Andy_n mentioned that he didn't know what pitch his spur/pinion gears are, it got me thinking. Perhaps it's not that common knowlege what the "pitch" of a gear is, or how it's worked out. So here's a nice quick tutorial on how to work out what pitch your gears are

Firstly: the pitch of a gear can (in a very round about sort of way) be thought of as how small the the teeth on the gear are.

A better way to explain it would be as the number of teeth on the gear divided by the diameter of the gear.

So, an example...

Say I have a gear that I found in a parts bag, down the side of a sofa or some other unlikely place, and I didn't know what pitch it was. Here's what I'd have to do:

Firstly, I'd need to know how many teeth it had on it. If it wasn't stamped on the side, I'd have to count them; using a fine marker pen to put a dot on the side of each tooth as I counted it, as to not loose track of which teeth I had counted, and which teeth I had not. Lets say it has 33 teeth.

The next thing I'd need would be the diameter of the gear. Now, this next bit is important! The diameter of the gear is not measured from the ends of the teeth, it's measured from the gaps between them. To save me explaining what I mean, I made a pretty picture.

This pretty little thing represents a 33 tooth pinion gear. The red line with arrows on shows where to measure in order to find the diameter I'll need to calculate the pitch. Because the majority of RC cars use imperial pitch, I'll want to measure the diameter in inches.

Now, I'd used a digital caliper to measure my imaginary pinion gear. But if you don't have one: a good steel rule will do the job well enough, as long as you have a sharp eye. Or perhaps a freind with a sharp eye.

So let's say my imaginary pinion gear turned out to have a diameter of 0.515 inches. To work out the pitch is a nice easy calculation, and it would go a little something like this:

number of teeth / diameter of gear = pitch

33/0.515 = 64.077669902912621359223300970874dp

So, I can tell that my gear is near as dammit 64dp. The 0.7766 etc difference between my answer and 64dp can be attributed to engineering tollerances and error in my measurements.

If you're using a steel rule rather than a digital caliper to measure your diameter, you should expect your answer to be further out from mine.

Measuring my pinion gear with a steel rule would have probably given me diameter of 0.5 inches, and therefore a pitch of 66. But because the following are the most common pitches used in RC cars: 64dp, 48dp and 32dp I can pretty much assume that my pinion must have a pitch of 64dp.

Hopefully all of that's helpful to someone.

Reply with any questions

[JamesChatz]

Very good guide, thanks for sharing

[Turner Boy]

Ahhhhh, so thats how you do it, thanks dude, been wondering that for a while

[tdriver]

very helpful Barry,thanks!

two questions though,

what if the gear is metric? (modulus)

it seems this calculation doesn't apply,cause i got a number of 2.5227, (33\13.081)

and we now that 64 pitch = about 0.4 module (actually 0.396 module)

.....and another one,

how can we find out if a gear is in module or pitch...

[lilllad83]

I know this is an old thread but would the same ruling apply to spur gears? Trying to figure out what my lst xxl spurs pitch is.

Found a few threads via google but all have different answers provided so I'm more confused than when I started looking.

[Davidpal92]

I know this is an old thread but would the same ruling apply to spur gears? Trying to figure out what my lst xxl spurs pitch is.

Found a few threads via google but all have different answers provided so I'm more confused than when I started looking.

Short answer... Yes

A spur gear is basically a pinion gear with more teeth, the maths works the same way.

David

Edited September 26, 2012 by Davidpal92

[Madmini]

Trying to use this to help me work out what I need for my merv but the picture is missing... Any ideas folks?

[David Allen]

Although the pitch of a gear is strictly measured half-way up the tooth profile, in practice you can just measure the outside diameter of the gear, it will get you close enough to the real pitch.

 

For an imperial (dp) gear, you calculate teeth/diameter (in inches) to get the pitch..

 

For a metric (module) gear you calculate diameter/teeth (in mm) to get the module.

[Madmini]

Although the pitch of a gear is strictly measured half-way up the tooth profile, in practice you can just measure the outside diameter of the gear, it will get you close enough to the real pitch.

For an imperial (dp) gear, you calculate teeth/diameter (in inches) to get the pitch..

For a metric (module) gear you calculate diameter/teeth (in mm) to get the module.

I don't know if it's metric or imperial however I have done the calculations for both

My old pinion is 18tooth and measures 10mm from edge to edge

So that gives me either

45.8 dp imperial

Or

0.55 metric

Any idea which is correct? Or will both be the same?

Cheers

[jake1*2011]

48dp and 0.5 mod are very close but are not the same. you can use 48dp with 0.5 mod gears but will go through gears quicker

[Madmini]

48dp and 0.5 mod are very close but are not the same. you can use 48dp with 0.5 mod gears but will go through gears quicker

So any ideas which one I have?

[David Allen]

Because measuring the OD overestimates the number of teeth, you'll find that that is a 48dp gear. Very unlikely to be a 0.5mod gear as the only one I can think of was released with the Tamiya Super Astute and is very rare indeed.

 

Here are a couple of better formulae I have found, basically the outside diamater give you the pitch of a gear two teeth larger than the one you actually have.

 

Diametral Pitch (P) Outside Diameter (OD) and the Number of Teeth (N) P = (N+2)/OD

The outside diameter of the gear.

OD = (N + 2) x MOD

[Madmini]

Thanks guys you have been a great help

[akhtar507]

I'm usually pretty good with my maths but struggling with this one:

Pinion gear of a brushes motor. Marked 21t and confirmed. Outside diameter is 12mm or 0.48".

Halfway down the teeth is 10.9mm or 0.42".

Base of the teeth measures at 10mm or 0.4"".

All measurements are rough but I get:

OD 0.57p (mm), 43.74p (")

Halfway 0.51p (mm) 50p (")

Base 0.47p (mm) 52.5p (")

Which one should I go with? Either way, it conflicts with Barry's OP regarding common pitches.

Also, while I'm here, what is the correct size for the little screw that hold the pinion onto the motor shaft?

Sent from my GT-I9300 using Tapatalk 2

Edited June 26, 2013 by akhtar507

[tezpellet]

someone sent me this link from here a few weeks back as i had a load of pinions and didnt know which was which, 

 

i already had a precision calliper which cost about

[RadioFish]

Verry helpfull thank you i have a box of pinions, from where/what i have no clue but i can hopefully find one or two a home

Sent from my LG-H870 using Tapatalk