Keyways

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Bob D.
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Location: Saco, ME. USA

Keyways

Post by Bob D. » Sun May 20, 2018 7:43 pm

I'm at the point in a large scale gas/electric critter project where I need to machine several keyways into my 1-3/16" steel axles. So that got me thinking on keyway sizes and styles.
What are the pros and cons, design cosiderations, fitment, etc, differences between straight keys and woodruff keys? If a shaft requires four keys in different locations along the length should they be lined up or radially spaced?
I've worked with and machined both but what reasons would favor one over the other? I guess a hybrid of the two is a straight key in a pocket.....
Thanks for any input!

Bob D.
3/4" Juliet II 0-4-0
3/4" Purinton Mogul "Pogo"
3/4" Hall Class 10 wheeler
3/4" Evans Caribou/Buffalo 2-8-0
3/4" Sweet Violet 0-4-0
3/4" Hunslet 4-6-0
3/4" Kozo A3. Delayed construction project

1 1/2" A5 Camelback 0-4-0

10 Wheeler Rob
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Re: Keyways

Post by 10 Wheeler Rob » Sun May 20, 2018 8:46 pm

Woofruff keys suck in my opinion. They do not assemble and dissassembly well. The key tends to rock and bind, which can make really tough to get apart. You also need a special cutter to make the slots as well.

Straight keys work well, you can mill the slots with either a 2 flute end mill (low cost) or with a key cutter. They work well for assembly and disassembly in either slip or press fits.

If you use set screws, normally there are 2, one on the key and one indexed 90 degrees from the key. Also I recommend to file or machine a flat spot under the set screw that contacts the shaft. This elimates a raised burr on the shaft that makes dissassembly tough to do.

That's my experance.

For your axle size I would probably use a 3/16 or 1/4" key. Key stock can be purchased in a slip fit and tight fit tolerances.

Rob

Harold_V
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Re: Keyways

Post by Harold_V » Mon May 21, 2018 1:49 am

If the proper Woodruff cutter is used, the key shouldn't present any problems, as it should be a snug press fit in the slot. They offer a real advantage for some situations, as the key doesn't present any problems by moving or dropping out.

If you choose to cut your keyways with an end mill, do keep in mind, they deflect. A four flute is not desirable, but even two flute end mills (or key slot cutters) tend to cut a little off center. That offers the opportunity for slots that are likely to be off center. If you like a proper fit (key should be a snug fit in the slot, to prevent movement and the resulting wear), use an end mill that is smaller than the target width. Rough the slot on location, then open the slot by taking cuts along each edge. You can control size that way, and keep location on center. Yeah, it's a little slower, but the quality of the slot is greatly improved, especially if you lube the cut and take the finish passes by climb milling.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

MrWhoopee
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Re: Keyways

Post by MrWhoopee » Mon May 21, 2018 10:16 am

Always use the STANDARD size key in a shaft unless there is some compelling reason to do otherwise. For a 1-3/16 shaft that would be a 1/4 keyway.
http://www.micro-machine-shop.com/Shaft ... _Sizes.pdf

If you are using a standard, helical flute endmill, rough the keyway undersize to eliminate the problem of cutter deflection creating an off-center/oversize keyway.

Straight flute keyway cutters significantly reduce the problem of deflection. When the cutting flute is in line with the centerline of the keyway (the point of greatest deflection), there is no flute in contact with the sides of the keyway. When the cutting flutes are in contact with the sides of the keyway, there is almost no deflection force.

Keys should be snug in the shaft, but not so snug that you need a hammer to put them in. Put they keyways in line on the shaft. AFAIK this is more a matter of aesthetics than anything strength related. It just shows that you care about the work.

Always face, chamfer and CENTERDRILL the ends of shafts. Someone may need to put a puller on it later. And, again, it shows you care about the work.

edit: I suppose I should correct myself. The proper term for the feature in a shaft is "keyseat". A keyway is the corresponding feature in a bore.
Last edited by MrWhoopee on Mon May 21, 2018 10:44 am, edited 1 time in total.

John Hasler
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Re: Keyways

Post by John Hasler » Mon May 21, 2018 10:41 am

> Always face, chamfer and CENTERDRILL the ends of shafts.

I wish motor manufacturers would go back to doing this.

Bob D.
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Joined: Thu Jun 21, 2012 10:43 pm
Location: Saco, ME. USA

Re: Keyways

Post by Bob D. » Mon May 21, 2018 11:48 am

Is there any advantage to having the key the full length of the hub? Is there a rule of thumb for key length for a given width?
I imagine that with the given length of a standard woodruff key you have achieved max strength. Some woodruffs have straight wings on the ends which I guess is to improve position stability.
If a 1/4" straight keyseat was cut for a length of 8" on a 1-3/16" shaft what are the odds this might affect the shaft straightness?
Is there an advantage setting up a Bridgeport with a right angle head and using an arbor support center to support a woodruff cutter that is in an R8 collet? I've never done it this way but now have the gear to do that. Never had an issue cutting spindle vertical, but horizontal would give full access to the shaft length.

Thanks for the replies.
Bob
3/4" Juliet II 0-4-0
3/4" Purinton Mogul "Pogo"
3/4" Hall Class 10 wheeler
3/4" Evans Caribou/Buffalo 2-8-0
3/4" Sweet Violet 0-4-0
3/4" Hunslet 4-6-0
3/4" Kozo A3. Delayed construction project

1 1/2" A5 Camelback 0-4-0

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SteveHGraham
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Re: Keyways

Post by SteveHGraham » Mon May 21, 2018 12:04 pm

I suspect you can learn a lot about the key width, keyseat length, and key length by looking at the horsepower you're dealing with. A NEMA chart will tell you these dimensions.
Every hard-fried egg began life sunny-side up.

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SteveHGraham
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Re: Keyways

Post by SteveHGraham » Mon May 21, 2018 12:05 pm

Here's something interesting. NEMA chart with key information.

http://www.esrmotors.com/Literature/Ref ... 202011.pdf
Every hard-fried egg began life sunny-side up.

Harold_V
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Re: Keyways

Post by Harold_V » Mon May 21, 2018 3:41 pm

Bob D. wrote:
Mon May 21, 2018 11:48 am
If a 1/4" straight keyseat was cut for a length of 8" on a 1-3/16" shaft what are the odds this might affect the shaft straightness?
Pretty much 100%. It's just a matter of how much. Any time you remove metal unevenly, you can expect movement. That's especially true if you interrupt cold finished materials, where internal stress tends to be higher than on hot finished materials.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

TRX
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Location: Central Arkansas

Re: Keyways

Post by TRX » Thu Jul 05, 2018 9:36 am

I used to be in the engine rebuilding business. I can't even guess how many times I had a Woodruff key tilt when trying to install a harmonic balancer. Since many balancers are a press fit as well as keyed, sometimes you don't know there's a problem (the key is behind the timing cover) until the balancer gets munged or cracks. I hates Woodruffs, I does.

johnfreese
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Re: Keyways

Post by johnfreese » Fri Jul 06, 2018 11:13 pm

If you use a Woodruff cutter to mill a straight keyseat there is no need to mill undersized then open up the keyseat to dimension. The woodruff cutter controls the width. Use this technique only on a keyseat at end of a shaft. If you do it in the middle of a shaft it can't be disassembled. The key rides up on the radius at the end of the keyseat and jams the hub on the shaft.

stevec
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Location: N.S. Canada

Re: Keyways

Post by stevec » Sun Jul 08, 2018 5:42 am

If you do it in the middle of a shaft it can't be disassembled. The key rides up on the radius at the end of the keyseat and jams the hub on the shaft.
That hasn't been my experience. :roll:

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