Help needed with pinions for internal gears

[Brian HParticipant @brianh50089]

HELP, wanted, some clever person to help me!

[Brian HParticipant @brianh50089]

Some years ago I made the internal gears that bolt onto the inside of the hind wheels for my Burrell-Boydell engine using the method outlined below;>>

"The engine is driven by pinions on the ends of the crankshaft, which mesh with internal gears fastened to the inside of the hind wheels. The method of engaging gear is to use two handwheels (not visible on the picture) to raise the rear of the boiler so putting the pinions into mesh.>>

The problem arises as to how to machine these gears, which are over 8 ½ inches in diameter. After discarding the idea of cutting them as racks and curving them in the same way as the steering quadrant, it was decided to cut them from solid as the picture below shows. Lady luck was smiling as I found 2 flame-cut rings in the scrap box that were just right.>>

The slots were cut through using a 1/8 in slot drill (well, 3 actually) and then the boring head was set up as shown to curve the flanks over three teeth to produce a reasonable shape to the teeth. A ring will then be silver soldered to the outside to provide the ‘bottom of the teeth.>>

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I'm now looking to cut the pinions to mesh with these and, looking at things afresh, realise that my original thinking of approximately 16DP was incorrect. >>

Looking at the gear form it looks more like 14 DP and I will need to make a cutter.>>

The snag is that I cannot work out the maths with any degree of certainty, hence this plea for help. Perfection is not required, just the hope that the gears mate sufficiently to provide drive.>>

The internal gears are 8.5" ins. PCD with 108 teeth with a pitch of 0.240" ins. The OD of the pinion needs to be approx 7/8 ins. (because of the way it all fits together) and I make the number of teeth required on the pinion to be 9, which is outside the calculations in the books I have.>>

Any assistance would be gratefully received and acknowledged.>>

Brian>>

[D HannaParticipant @dhanna35823]

Formula for spur gears

OD = (Number of teeth +2)/DP

OD = 11/14 = 0.786″

Hope that helps

Edited By D Hanna on 04/11/2017 21:37:43

[Anonymous]

Before we can look at the design of the pinion (my books do cover 9 teeth) we need to sort out the parameters for the internal gear. Some of the numbers don't seem to stack up. If we take the number of teeth, 108, and divide by the PCD of 8.5" we should get the DP of the gear. That gives a DP of 12.7? The circular pitch is the circumference of the PCD divided by the number of teeth. I make that 0.247", close but not exactly 0.240".

The next question is what pressure angle was the gear designed for, 20°PA is fairly common for internal gears?

Andrew

[John ReeseParticipant @johnreese12848]

Posted by D Hanna on 04/11/2017 21:36:02:
Formula for spur gears

OD = (Number of teeth +2)/DP

OD = 11/14 = 0.786"

Hope that helps

Edited By D Hanna on 04/11/2017 21:37:43

Not sure that works for internal gears. Too lazy to get off my recliner and look it up.

I think I recall that the addendum was reduced on the internal gear and increased on the pinion. Maybe I got it backwards.

I believe there is software that will generate CAD files for gears and pinions. Gearotic comes to mind. Probably others.

If you don't have Ivan Law's book on gear cutting, I highly recommend it. Not sure if he covers internal gears. If you have access to Machinery's Handbook it contains a lot of information on gearing. If there is a UK equivalent to MH I would be grateful if someone could enlighten me.

You might also try using CAD to check your pinion profile against the gear profile.

While this post might not be helpful in itself I hope I have pointed out a couple potential issues and a couple resources.

[Anonymous]

Posted by John Reese on 04/11/2017 22:18:52:

I think I recall that the addendum was reduced on the internal gear and increased on the pinion. Maybe I got it backwards.

I believe there is software that will generate CAD files for gears and pinions. Gearotic comes to mind.

It depends on the number of teeth on the internal gear and pinion. As these get closer together a number of interference problems arise. They can, to a certain extent, be ameliorated by using stub teeth. For the gears being discussed I don't think addendum modification is needed.

Even with help from the late JohnS I never managed to get Gearotic to work properly.

The book by Ivan Law does not cover internal gears.

The formula given for the OD is not relevant for internal gears, as the concept of an outside diameter is meaningless.

Andrew

[D HannaParticipant @dhanna35823]

Agreed Andrew but OP was looking for a formula for the mating pinion along with the suggestion of 14 DP.

As he has the circular pitch of the internal gear he would be better off calculating diameters of a mating circular pitch pinion. Either way he’s looking at grinding a fly cutter to cut the teeth which is still only a guestimation unless someone has a Fellows gear shaper or similar who could cut both gears for rolling accuracy.

[Brian HParticipant @brianh50089]

Many thanks to all who answered especially the suggestion to use CAD. This is what I did; copy one tooth form and produce a circular array within the 7/8"in diameter of the proposed pinion using different numbers of teeth until a 'best' fit was obtained. The 'best ' fit was 11 teeth.

The 108 tooth internal gear was not produced using modern methods; it came from an old book showing how a gear could be set out on the drawing board and in the pattern shop for producing a casting. The tooth flanks were produced using a circle over 3 teeth once the number of teeth and their depth had been decided.

Although crude, this method produces gears suitable for low speeds where strict accuracy is not vital.

I have a 1911 book entitled "American Machinist Gear Book" by Charles H Logue that is full of tables and formulas for designing and cutting all types of gears but reading it is hard work!

Brian

[Anonymous]

Posted by D Hanna on 05/11/2017 02:06:21:
Agreed Andrew but OP was looking for a formula for the mating pinion…………..

Fair point.

I used 3D CAD when I designed my internal gear, the teeth are based on a true involute curve, imported from a spreadsheet:

You don't need a Fellows shaper to make an internal gear, although some form of slotting head is useful:

The gears mate very smoothly, the pinion was made on the CNC mill, because I was too idle to set up the dividing head, and too mean to buy a cutter:

Good luck with the gears.

Andrew

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