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One for the Mechanical Engineers and Machinists - Help!
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Author:  Rick Cowan [ Tue Aug 03, 2010 11:50 am ]
Post subject:  One for the Mechanical Engineers and Machinists - Help!
Woefully lost in jargon and need some translation help for the mechanically inclined.

I'm finally getting to that thickness sander build that I have been trying to get to for three summers now! Too many honey-do jobs! I have a height adjust system in mind which requires two miter gears. Essentially, I need to transfer the rotational force of a crank system 90* to the threaded height adjust rod. It's very simple - two miter gears (1:1 ratio) with 1/2" bore diameters. My confusion is immersed in the nomenclature used to describe the gears I'm looking for. I can get gears locally from a supplier who sources from Martin Gears (I'm looking at pages 51-55 in their catalog http://www.martinsprocket.com/2001/SecG.pdf#G50).

My questions:
  • For my intended use hardened vs. unhardened teeth? (My guess is unhardened will be fine for the degree of torque applied in a thickness sander.)
  • Plain bore vs. keyed and set screw (if I go plain bore could I tap them and install a set screw? If not, how do you secure a plain bore gear to a shaft?)
  • I get No. of Teeth (I presume more is smoother) but what do they mean by 4 Pitch, 5 Pitch, 6 Pitch ....? I understand pitch to be the distance from the leading edge of one tooth to the leading edge of the next tooth - so what unit is 4 or 5 or 6 etc.?

So what should I be looking for?????

Thanks in advance for your help!

Cheers
Rick
Author:  Tom West [ Tue Aug 03, 2010 3:01 pm ]
Post subject:  Re: One for the Mechanical Engineers and Machinists - Help!
Rick: Unhardened gears with a plain bore, then drill and tap for a set screw should do. The pitch you quote is the diametral pitch and it is the number of teeth per inch of pitch diameter. Not to get too concerned about. I would suggest m1218 as a suitable choice. Lets see what some of the others have to say before you dash of and order. Good luck in your project.
Tom
Author:  Rick Cowan [ Tue Aug 03, 2010 5:12 pm ]
Post subject:  Re: One for the Mechanical Engineers and Machinists - Help!
Thanks Tom - appreciate the guidance.
Rick
Author:  Bob Garrish [ Tue Aug 03, 2010 6:16 pm ]
Post subject:  Re: One for the Mechanical Engineers and Machinists - Help!
Rick Cowan wrote:
  • Plain bore vs. keyed and set screw (if I go plain bore could I tap them and install a set screw? If not, how do you secure a plain bore gear to a shaft?)
Cheers
Rick


Plain bores are designed for a friction fit, usually a shrink fit.
Author:  Tom West [ Tue Aug 03, 2010 7:20 pm ]
Post subject:  Re: One for the Mechanical Engineers and Machinists - Help!
Bob: How much interference?
Tom
Author:  Parser [ Tue Aug 03, 2010 8:25 pm ]
Post subject:  Re: One for the Mechanical Engineers and Machinists - Help!
you usually need .0015" of interference for every inch of bore diameter (I believe AGMA standards are setup this way). I would recommend the keyed connection as the easiest to deal with. If you have to adjust something, it's a lot easier to just loosen the set screw over the key instead of trying to move something that is shrunk onto a shaft.

Trev
Author:  David Malicky [ Tue Aug 03, 2010 8:37 pm ]
Post subject:  Re: One for the Mechanical Engineers and Machinists - Help!
Rick Cowan wrote:
  • For my intended use hardened vs. unhardened teeth? (My guess is unhardened will be fine for the degree of torque applied in a thickness sander.)
  • Plain bore vs. keyed and set screw (if I go plain bore could I tap them and install a set screw? If not, how do you secure a plain bore gear to a shaft?)
  • I get No. of Teeth (I presume more is smoother) but what do they mean by 4 Pitch, 5 Pitch, 6 Pitch ....? I understand pitch to be the distance from the leading edge of one tooth to the leading edge of the next tooth - so what unit is 4 or 5 or 6 etc.?

- Unhardened teeth are fine for your use and many heavier uses.
- Plain bore will save $. Yes, drill-tap-setscrew works fine for light duty. My understanding is that plain bores are usually intended for later machining to a custom shaft size. I've not seen a diameter tolerance on a plain bore, so I'd be hesitant to rely on it for the precision required in a press fit... +/- 0.0005" or so. As Trevor said, press fits are a pain for sizing and service. For light duty, if the price is low enough, I often get the keyed & set screw type and just ignore the keyway.
- Tom has good advice on pitch. Higher pitch = smaller teeth (and smaller $ and face width, usually). See pages G-93 to G-95 of that catalog. You want large enough pitch so that the gears don't slip or bind if they aren't aligned just right, but not so large that the gear pair is rough to turn. 12 to 16 pitch sounds about right. 16 pitch will probably be lower $, but requires a bit more precision for shaft alignment.

All that said, for a light duty app like yours, you could probably get away with nylon gears. McMaster 7297K18 is $4.
Author:  Tom West [ Wed Aug 04, 2010 11:21 am ]
Post subject:  Re: One for the Mechanical Engineers and Machinists - Help!
Rick: If you have a way of sizing you parts and accurately measuring the same,you should be able to make a shrink fit as Bob suggested. Your gear bore should be exactly the same size as the shaft to plus .0002 on the shaft. If you heat the gear in your oven to a temp of 450 degrees F the hole will expand enough to slip on the shaft. The mating surfaces should be smooth ,no burrs,slight chamfer on both and you have to move quickly. When the gear cools it will grab on the shaft so you have to have it positioned correctly. If you have a machinist friend they should be able to help you with this. Myself I think it would be easier to use set screws. As David suggested it also may be worthwhile exploring the nylon gears. Again good luck with the project.
Tom
Author:  Rick Cowan [ Wed Aug 04, 2010 5:25 pm ]
Post subject:  Re: One for the Mechanical Engineers and Machinists - Help!
Gentlemen,

Thanks so much for the advice and feedback. I continue to be amazed at the knowledge the members of the OLF have and their willingness to share.

I have to say I was stunned to find out how expensive these things are! Just got a quote on the M1218 (unhardened teeth with setscrew) -- $40 Cdn ea. OUCH! I saw the McMaster nylon versions at $4 US but am nervous about the wear.

Again, thanks to each of you!

Regards
Rick
Author:  David Malicky [ Wed Aug 04, 2010 5:55 pm ]
Post subject:  Re: One for the Mechanical Engineers and Machinists - Help!
Rick,
You're welcome! Yeah, gears can be $$. The plain bore will be quite a bit less.
Nylon is a great gear material, actually -- tough, relatively strong, and low friction. If I understand your design correctly, the load on the gears should be quite low -- just that to turn a threaded rod and lift a few kg table (nice design, btw... I crank an upside down handle for mine). That should be no problem for nylon. If they somehow don't survive, you could replace them with 16 pitch steel.
Good luck!
Author:  Tom West [ Wed Aug 04, 2010 7:09 pm ]
Post subject:  Re: One for the Mechanical Engineers and Machinists - Help!
Rick: I concur with David. Have fun and take care.
Tom
Author:  Ziegenfuss [ Thu Aug 05, 2010 11:56 am ]
Post subject:  Re: One for the Mechanical Engineers and Machinists - Help!
Just a couple of thoughts....

Before you start looking at anything, you have to qualify what ballpark you are in in terms of forces, then you can begine to present an argument on materials, torque transfer, etc...

How much to you anticipate the sanding head will weigh? what is the thread angle of the lifting screw? With these numbers we can draw up and FBD and get a general idea of the expected torque we will see at the output shaft - and then can work to the force required at the input shaft...Then you can look at nylon vs. steel vs. hardened steel etc...Friction fits can be nice - but they are a full commitment. Also, they are very dependent upon material choice. You can quickly calculate the expected dimensional change based upon the thermal coefficient of the material - AND what I would suspect is that the tolerance required to make this work would be more expensive than getting a broached keyway. At least it was most recently in a project for me with ASTM 1013 steel in about 1" diameters.

Also - as this is a homemade prototype - there will be assembly and disassembly - DO NOT PUT A SET SCREW RIGHT ONTO A SHAFT! You will screw your self before will have gotten started. You will ineveitably throw up a burr on the shaft makeing it nearly impoosible to take the gear off, and subsequently get back on...Either grind a flat in the shaft where the set screw will sit, or go with a keyed approach.


Just some thoughts from experience.
Stephen
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