Author Topic: Making replacement gear  (Read 849 times)

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JeffB

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Re: Making replacement gear
« Reply #15 on: January 07, 2022, 12:01:10 PM »
+1
Without knowing the pressure angle and the pitch circle diameter, I'm afraid one is still guessing.  The pressure angle is what determines to tooth tangent to pitch resulting in tooth curvature; OD and number of teeth is simply not enough to know the true mesh.  As the pressure angle decreases the contact face of the tooth becomes more pitched.

Also one cannot measure the diameter of an ODD number of teeth.  Liken it this way, if you measure a three legged stool, you will measure from the tangent of one leg to a line between two tangents of the other two legs, I.e, a flat spot, which will skew the measurement.

Not saying you can't come close, but you really are just guessing, and there really is no way to determine if the mesh is English or Metric without an optical comparator, or measuring across pins of a very specific diameter.

Yes, I used to design Ford Tractor (now New Holland) transmissions many moons ago.

See the image below to truly understand all the the dimensions that are necessary to 3D model a properly meshed (teeth actually touching at the pitch circle) spur gear.


You're not wrong, but it doesn't require that much precision.  A fairly close measurement of the OD is enough, plus the number of teeth plus 2 is all you need.  We're talking about models here, not precision transmissions, gearheads or the like, with high loads and long lifespans.

The only time you run into trouble if the gear is some mongrel pitch (DP or Mod), or a non-standard tooth profile or just plain manufactured poorly.  Which is entirely possible.

Some might also argue that helical gears (RH and LH, sometimes called "worm and reverse worm" gears) require a complex calculation...  Maybe it does, but I've built many gearboxes using both and the simple calculation for gear spacing, plus a little clearance has always worked without issue. 

Jeff

Lemosteam

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Re: Making replacement gear
« Reply #16 on: January 07, 2022, 12:12:22 PM »
0
You're not wrong, but it doesn't require that much precision.  A fairly close measurement of the OD is enough, plus the number of teeth plus 2 is all you need.  We're talking about models here, not precision transmissions, gearheads or the like, with high loads and long lifespans.

The only time you run into trouble if the gear is some mongrel pitch (DP or Mod), or a non-standard tooth profile or just plain manufactured poorly.  Which is entirely possible.

Some might also argue that helical gears (RH and LH, sometimes called "worm and reverse worm" gears) require a complex calculation...  Maybe it does, but I've built many gearboxes using both and the simple calculation for gear spacing, plus a little clearance has always worked without issue. 

Jeff

In reality a helical gear is not much more than a spur gear with a twist on is axis for its width, and a lot more difficult to machine.  Not so sure I agree with you on the accuracy here in N.  Lash (tooth mesh gap) is a huge contributor to perceived locomotive noise, as well as slow speed performance, not to mention any clearance from the center of the hole to the pin its riding on.  Everything is negatively impacted in N scale when variation and clearances are loose. 

My main point was this- if you cannot reasonably accurately determine what gear you have in your hands, it just makes it more difficult to find a replacement that is not form a second, same model.

woodone

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Re: Making replacement gear
« Reply #17 on: January 07, 2022, 01:01:05 PM »
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To answer a few questions—
Pete- I think we were talking about how to figure the gear ratio of a planatary gear set up. Lots of info but I do not recall tooth design.
I have sleep since then and did not go back and read that post again-
I learned about making gears 60 years ago at an industrial art school, and I do not think the math has changed.
I can get the OD by making a go-no-go gauge so getting the correct OD does no present a problem.
The gear I need made has been removed from the gear case. There are two of the same gear used in the drive and only one is destroyed.
I can measure six ways from Sunday of said gear. Except the root diameter. I guess that I could use the wire method and a micrometer
but handling such a small gear with wires takes more hands than I have.
SkipGear you stated you had made some gears, would you care to take a stab at doing one of these for me?
I can furnish all the data you said you needed.
Thanks for all the feed back.

peteski

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Re: Making replacement gear
« Reply #18 on: January 07, 2022, 01:33:41 PM »
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Well then Jerry, sounds like you knew exactly what you've talking about all along. Good luck with the replacement gear.
. . . 42 . . .

turbowhiz

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Re: Making replacement gear
« Reply #19 on: January 07, 2022, 10:35:02 PM »
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I'd look into the several openscad files available for generating gears if I was trying to come up with a 3d gear model for printing.

Put in the parameters for your gear, and it generates your model.

Openscad I have used with good success. Its a totally programmatic way of designing 3d models, and for certain applications, its really super powerful. Obviously good if you have programming experience.

Gears seem like sort of design application it would do well with. You shouldn't need any programming experience just to take an existing gear generating program and run it for your purpose, assuming that it satisfies your requirement out of the box.

Again, not attempted using this for generating gears on N scale models, but I'd start there...




SkipGear

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Re: Making replacement gear
« Reply #20 on: January 07, 2022, 11:39:28 PM »
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To answer a few questions—
Pete- I think we were talking about how to figure the gear ratio of a planatary gear set up. Lots of info but I do not recall tooth design.
I have sleep since then and did not go back and read that post again-
I learned about making gears 60 years ago at an industrial art school, and I do not think the math has changed.
I can get the OD by making a go-no-go gauge so getting the correct OD does no present a problem.
The gear I need made has been removed from the gear case. There are two of the same gear used in the drive and only one is destroyed.
I can measure six ways from Sunday of said gear. Except the root diameter. I guess that I could use the wire method and a micrometer
but handling such a small gear with wires takes more hands than I have.
SkipGear you stated you had made some gears, would you care to take a stab at doing one of these for me?
I can furnish all the data you said you needed.
Thanks for all the feed back.

message me with a couple good pictures of the gear that is still in tact, measurement of overall diameter and the diameter of the compound smaller gear, overall thickness, and the thickness of each gear in case there is a bushing/standoff built in, and finally the bore. I can see what I can come up with.
Tony Hines

woodone

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Re: Making replacement gear
« Reply #21 on: January 10, 2022, 11:29:33 AM »
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SkipGear—- sent you a PM

railnerd

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Re: Making replacement gear
« Reply #22 on: January 15, 2022, 01:52:40 AM »
0

-Dave