Choosing woodruff key vs. square keystock

David J. · Sep 28, 2012

I'm repairing a shaft and drive hub ( 1in. dia. steel ) that had a 1/4in
hi profile woodruff key holding the shaft to the hub along with 2 setscrews.
The shaft is the main rotor of a wood chipper so it's subject to plenty of
shock loads. The hub is almost 1.5 in long and after broaching can take
square key stock the full length. I can either cut a full length slot in the
shaft or redo it like the manufacture did.
Was it done with a woodruff key just for quicker production or am I
missing something?
I'm leaning to the longer square keystock unless somebody has a
good reason to do it the original way.
Thanks
David

CalG · Sep 28, 2012

I recently had to rework a similar input shaft on a chipper shredder. The 1/4 X 14 x2 key had sheared, and distorted the shaft and drive coupling bore so as to make disassembly a bear. My solution was to drill the shaft at the key way position to accommodate a shear bolt.

Bolts are easily replaced without damage to the mating parts.

Square key stock is much better than a woodruff key ;-)

Oh! by the way, a shear pin should be about 40% of the shaft diameter if made of similar material,

and

a 1 inch shaft is good for about 4.7 Hp per 100 rpm.

You can use the mat to determine if the application is over loaded.

A 50 HP engine at 500 rpm is a show stopper!

mjk · Sep 28, 2012

Woodruff keys eliminate any possibility of the key escaping &
A square key will give more support driving the hub.
Did the original woodruff key shear?
If yes, I'd replace as origionally built, it may save another part of the assembly, and with a soft key

The other reason a full slot is not used is if the inner race of the bearing fitted to the shaft is such that the slot will case the inner race to deflect over the slot. If the bearing has an inner collar, this isn't a problem. I found this out the hard way trying to make an assembly easier by slotting the shaft and sliding a square key in after the gear was in place.
The bearing inner race failed within a short time
Mike

ratbldr427 · Sep 28, 2012

It is hard to tell.Had one machine where the square keys always were loose,eat up at tear down;replaced with woodruff keys and they were always in good shape at tear down.However I have seen thousands of tight square keys that were 30-40 years old.Once the shaft to bore gets to much clearance they will all get eat up.Never see bad keys on taper lock hubs so the culprit is loose shaft/bore working on the key.It can be a bitch disassembling press fit shafts OTH keys and keyseats don't get eat up.
Randy

atomarc · Sep 28, 2012

David,

I worked in the sawmill industry and never, ever saw a Woodruff key. IMHO..they are a pain the arse to mill and I fail to see the sense in them. Maybe, as Mike has noted, they are captured. A straight key of the appropriate size with the adequate setscrews will transfer all the load and last forever. You can even make the key-seat a few tenths shallow and it will be a light hammer fit.

They do have their place and I think that is in machine tools.

Stuart

ToolPost · Sep 28, 2012

As mjk stated, a Woodruff key prevents the key from sliding out of the assembly. Woodruff keys are also specified in applications where a milled square key would be cut up to a shoulder on the shaft, so as not to cause a weak point in the stress-concentration region of the shoulder.

If you've ever taken apart many Woodruff key and square key assemblies, you'll also no doubt notice that a Woodruff key allows for better concentricity of the flywheel/whatever assembly, which is important in high revving situations.

I'd replace the key with an original design; the shock loads in the chipper should be dealt with by sufficient mass of the flywheel.

PeteM · Sep 28, 2012

My understanding is that one advantage of a Woodruff key is that it avoids a stress concentration in a shaft. If there are high cyclical loads and fatigue failure is a possibility (as it would be in a chipper), that might be a consideration in the original design? However, captured keys (square with a rounded profile at the end) as seen for many milling adapters would also likely reduce stress concentration.

On edit: looks like ToolPost was posting as I was typing.

abarnsley · Sep 28, 2012

"Avoids stress concentrations in shaft" NOT HARDLY

Reason for using Woodruffs... Cheap... one shot to cut.

My BP backgear shaft failed right at the woodruff key cut.. That deep cut is a great place for a stress crack to start.

Round ended straight keys work well. Don't run slot under a bearing race surface...

A good tight fit is required for any keyed or serrated joint to survive...

David J. · Sep 28, 2012

Thanks for the replys.
It has plenty of room so the keyway clears the shoulder and bearings, easily capturing the
key stock with a bit of weld on the outboard end of the hub. I think in this case the length
of the hub vs. bore size should keep things pretty well squared up. I can bore the hub
( have to make new hub ) for a tight fit to the shaft. As far a failing the key vs. something
else.....the shaft/rotor is about the most expensive part on this thing and a "ate out"
key way and the wear of the hub slipping on the shaft is what I'm repairing now.
Sounds like a full length square key is the way to go.
Thanks again....great to get second, third etc. ideas.
David

Nick Mueller · Sep 29, 2012

I have seen woodruff keys only in places where they had not to transmit torque, but were use for locating the two parts in a exact position to each other. So you often do find them on conical shafts.
Wherever substantial torque had to be transmitted, square keys were used. Or even better splined (is it called that way? like PO shafts on a tractor etc.) shafts.

Nick

PeteM · Oct 1, 2012

To add, the limited torque capabilities of a Woodruff key are used by some designers as a sort of mechanical fuse. The notion being the key will fail before something more costly or dangerous.

Gary E · Oct 1, 2012

PeteM said:
To add, the limited torque capabilities of a Woodruff key are used by some designers as a sort of mechanical fuse. The notion being the key will fail before something more costly or dangerous.

VERY good point....

I was going to ask him ...
If you make this joint stronger.... what other connection BREAKS ?

Gary E · Oct 1, 2012

PeteM said:
To add, the limited torque capabilities of a Woodruff key are used by some designers as a sort of mechanical fuse. The notion being the key will fail before something more costly or dangerous.

VERY Good point......

I was going to ask him....

If you make THIS connection stronger.... what other connection BREAKS ?

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Choosing woodruff key vs. square keystock

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