Measure gears/existing geared mechanism, to make or specify replacements

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David K
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Measure gears/existing geared mechanism, to make or specify replacements

Post by David K »

In order to get myself tuned up for a fun but probably complicated (to me) geartrain/gearcase design for some scale helicopters, I wanted to run thru a small project to bring me more up to speed with gears/gearing etc.

As an exercise in gear metrology and then design or standard specification, I am curious about the best way to measure an existing gear or geared mechanism, to make or specify replacements efficiently.
I am guessing, once I have the metrology, I guess I would just scour catalogs, see if anyone has anything close, that can be used as-is, or after some basic processing?  If not, then try to make it...

Here?s the project:
The 8-way power seats in my truck have 2 worm gear sets that provide height adjustment (at the front and back, thus 2 sets).  The worms are I think a brass, with a square bore.  The wheels are made of some kind of plastic, housed in a small gear case that provides the thrust force control.  These plastic gears are prone to failure.
There is somebody out there that makes the gears for this vehicle, and I will purchase a set, but I wanted to see if I could get thru the exercise of figuring out how to do it, to help prepare me for other projects.


The picture of the upside down 8-way seat mechanism, arrows pointing to the up/down height adjustment worm gear cases:


Attachments
updowngearcases.png
David K
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Re: Measure gears/existing geared mechanism, to make or specify replacements

Post by David K »

Does anyone have any tricks to measuring gears?
Are these the physical characteristics that I need to capture?

For the worm wheel:
1) Identify physical characteristics of worm gear
    a. Right handed
    b. Straight Face (does not look hobbed to me, but what do I know?)
    c. A non-throated worm gear set, thus, a Helical gear
2) Count the teeth
3) Measure the outside diameter, face width
4) Measure the hubs (diameters, bores, any mechanical mounting)
5) Measure the helix angle?


For the worm (pinion):
1) Identify physical characteristics of worm gear
    a. Right handed
    b. Straight (cylindrical)
    c. A non-throated worm gear set
2) Count the starts
    a. looks like 1 start for the worm in question
3) Measure the worm outside diameter, overall length of gear & length of worm section
4) Measure the hubs (diameters, bores, any mechanical mounting)
5) Measure the worm axial pitch (will calipers allow a good enough measurement here?)
6) Measure the worm helix angle (what kind of precision is needed here?)
David K
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Joined: Sat Jan 14, 2012 8:42 pm

Re: Measure gears/existing geared mechanism, to make or specify replacements

Post by David K »

this post has a few pictures to help

A case split open, with a 3rd party worm/wheel replacement solution made from brass:

A view of a gear case with it?s worm, showing it?s square bore.  The square drive shaft is for the other split gear case:

A view of the plastic wheel, and a 3rd party replacement worm/wheel set:

A view of the wheel shaft (will need a slip fit, and a bonder - the 3rd party solution uses JB Weld)  The shaft is mounted in a way that is permanent, so removal of the shaft from the seat adjustment frame isn?t really possible, unless you want to rebuild it:
Attachments
wheelshaftplasticgearremoved.PNG
damagedplasticwheel&3rdpartwormwheelset.png
viewofsplitgearcaseswithworm&squarebore.png
3rdpartyworm&wheelreplacement.png
David K
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Re: Measure gears/existing geared mechanism, to make or specify replacements

Post by David K »

I read thru a bunch of hobby-machinist and PM threads, and watched some youtube vids.  I think I understand how it will work, and can figure out if it's DP or Module based contingent upon which calculations have the "cleanest" numbers, near standard pitches.

I also heard from KHK, and they sent me the following (see pic below), so we can see if they have something I can use

I have a desktop mill that is CNC'd, but I don't have a dividing head or anything
I've got a Craftsman 101 that I just finished restoring, so maybe with it's headstock index pin I will hopefully be able to make something work.

seems like theres a bunch of videos that can help :)


Attachments
khk_gearmetrologyguidance.PNG
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ArtF
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Re: Measure gears/existing geared mechanism, to make or specify replacements

Post by ArtF »

Hi David:

  Worm gears come in many flavours. In some , its a standard involute gear used as the wheel, 
with a worm of the same mode. With others, the gear is a non standard fit to the worm. You can
usually tell which by if the wheel has a helix on it, if it does, its probably a normal involute gear.

  The amount of helix is something we can calculate if we know the module of the worm and its
radius. Some worms are hypogloboidal, so like an hourglass glass shape, others are flutes, same
radius in the center as the top and bottom.

  So, if you know the diameter of the gear, and the worm, the number of teeth of the gear should
be all you need to make such a thing. Gearotic will post code for the gear, but not the worm. Though
its close to release a version to give you the worm as well as a model, though not as a toolpath.

  If you can tell me the tooth count and diameter I can probably tell you the numbers you need for the
gear to make one.

Art






 


David K
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Re: Measure gears/existing geared mechanism, to make or specify replacements

Post by David K »

Thanks Art!  I may have the gears in hand this weekend, so I'll know more when I get them and can look closely.

to be candid, given this application, if the # of teeth on the worm, tooth type (e.g., helix etc.), gear diameter, face width and bushing/collar measurements can be gathered, I think a set could be used that was "close".  i.e, it's not important as long as it's close, because it's only linear motion that isn't terribly sensitive to speed etc... (it's just a power seat actuator :)
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