Involute Gears - Identification of Pressure Angle.

[Pete W.]

Hi there, all,    

N.B.:  this is NOT a 'for sale' posting!

I am having a 'spring clean' in my workshop                     and have discovered a number of gearwheels that are surplus to requirements.  I am about to compile a description prior to listing them on a well-known auction site and I like my listing descriptions to be complete and accurate.

I know how to determine the D.P. (they are Imperial) of these gears and measurement of the thickness and centre hole dimensions is straightforward.  I would like to include the pressure angle in my description.  However, I have no idea how to measure that parameter.                   

Wikipedia states that 20° is the usual value but that 14½° is a less common value.  I have read that both values are found on various lathe gearwheels.

So, my question is: is there a simple measurement procedure, using equipment I am likely to have, that would enable me to distinguish between 20° and 14½° pressure angle?  (I do not possess a gear-tooth vernier gauge.) 

[DMIOM]

Pete - not sure if they're any use but have a look at this or this (courtesy Mr Google)

Dave

[Pete W.]

Hi there, Dave,

Thank you for your reply.                   

I had done a Google search but I missed those links. 

The first one looks like the way I should go - just a vernier caliper, a pencil, the back of an envelope and, if a senior moment blitzes my arithmetic, break glass and deploy the slide rule!           

[Lew_Merrick_PE]

Wikipedia states that 20° is the usual value but that 14½° is a less common value.

Which should warn you about Wikipedia -- 14-1/2° PA was the primary standard for inch-based gear well into the 1970's.  20° PA entered usage during WWI, but did not become common for gear until the early-/mid-1960's.  One of my premiere screw-ups as an apprentice was failing to recognize the difference (because apprentice texts of that time assumed 14-1/2° PA as a matter of course).

[Pete W.]

Hi there, Lew,

Thank you for that warning.

[awemawson]

Take the gear, roll it in Plasticine to form a rack. Cut along the length of the rack so you can see the profile in elevation. Measure the angle

[drmico60]

I have looked at ways of identifying the pressure angle of gears. The simplest procedure is to roll it in plasticine, slice through the impression and measure the angle. When I tried this it did not work very well for two main reasons:
1. On the gears I was trying to identify (mini-lathe change gears) the plasticine would not pull away cleanly from the gear  when trying to make an impression. Often material was left between the teeth of the gear or the impression was distorted.
2. Cutting through the impression also causes distortion and the measured angle can differ depending on the direction of cut.
In the end I resorted to a photographic method which is outlined on this webpage:
http://mikesworkshop.weebly.com/change-gear-pressure-angle.html
This method seems to give good results and will positively differentiate between 14.5 degree and 20 degree gears.
Mike

[BillTodd]

One quick way , if accuate id is not important i.e. a beer bet, is to look at the root of the teeth, if the root is square shaped it usually means it's a 14.5PA gear .

Bill

[awemawson]

I have looked at ways of identifying the pressure angle of gears. The simplest procedure is to roll it in plasticine, slice through the impression and measure the angle. When I tried this it did not work very well for two main reasons:
1. On the gears I was trying to identify (mini-lathe change gears) the plasticine would not pull away cleanly from the gear  when trying to make an impression. Often material was left between the teeth of the gear or the impression was distorted.
2. Cutting through the impression also causes distortion and the measured angle can differ depending on the direction of cut.
In the end I resorted to a photographic method which is outlined on this webpage:
http://mikesworkshop.weebly.com/change-gear-pressure-angle.html
This method seems to give good results and will positively differentiate between 14.5 degree and 20 degree gears.
Mike

Talcum powder on your gear will cure the sticking. Placing the impression in the freezer before cutting will cure the distortion.


(I do the same with side of bacon before slicing them works wonders. A side of bacon needs about an hour in the freezer but I can't quote a time for plasicine !!)

[Pete W.]

Hi there, Andrew,

Thank you for your suggestion.

Take the gear, roll it in Plasticine to form a rack. Cut along the length of the rack so you can see the profile in elevation. Measure the angle

It had me flummoxed for a few minutes - where on the rack would I measure the angle?          

Then the penny dropped.  You don't wrap the Plasticene around the gear - you roll the gear along the Plasticene. 
So you're regenerating the rack appropriate to that DP gear, which for involute gears is a straight sided rack!!!          

OK, I've got it!!!  Thanks.

Thank you also to everyone else that has contributed.    

[tom osselton]

My father use to use a slide rule then the calculator came along and now the slide rule = shite and downhill! 

[awemawson]

Still got my sliderule. It's an Aristo Scholar and I had to use it throughout my training despite calculators having just come on the scene. (Glowing leds or green discharge tube!) But then I also have an abacus hanging on a nail in my office. I joke that it's the back up if the computer goes pop