Is there a reason to use a woodruff key instead of a straight key?

[Averagesupernova]

TL;DR Summary

I need to cut keyways in a new shaft to replace the broken old shaft. It's easier for me to cut straight keyways into the shaft than a woodruff.

I have a broken 1.25 inch diameter shaft. One end has a large pulley attached I'm guessing around 16 inches in diameter with a pair of setscrews and a straight key to hold it to the shaft. The key is a square 5/16 inch key.
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The other end of the shaft has a hub with a keyway cut into it the entire length of the bore. This is where the woodruff key is used also with a pair of setscrews. The width of this key is also 5/16 inch. This hub is attached to a sprocket with two bolts used as shear bolts. The sprocket is able the freewheel on the shaft in the event that the bolts shear. The hub is near the end of the shaft. It is easier for me to cut a straight keyway in rather than a keyway for a woodruff key so that is what I'd like to do. Opinions?

 

[Baluncore]

The advantage of a woodruff key is that it stays where the semicircular pocket is sunk, while the position is still adjustable. The woodruff key maintains index but will not transfer much torque. Some form of clamp must be used to lock the two parts after assembly.

Unless it is held positively, a normal rectangular key can creep or slide along and out of the slot in the shaft and the pulley.

 

[Averagesupernova]

Unless it is held positively, a normal rectangular key can creep or slide along and out of the slot in the shaft and the pulley.

That's the only reason I can see for it in this case. By the pictures you can see it is close enough to the end of the shaft so that is a possibility. Honestly I'm not to worried. On a production scale the way they made that was likely the sensible choice. As a replacement shaft, a drop of loctite in the shaft keyway should prevent movement. A ding in the bottom of the keyway with a center punch just past the end of the key would prevent it also.
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If you notice where the shaft broke you'll see it is right next the the hub. Not even enough stub shaft sticking up for the press to push on to get it started moving without a spacer. The shaft didn't break perfectly square so the spacer didn't set square. I won't press on anything set up like that due to safety unless the spacer is very short so it was a slow process.
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To me, the break in the shaft is the weak point in the whole design. The break right next to the pulley hub indicates the shaft is too weak for the load. The shear bolts never gave. So my change to a square key is not likely to cause any more trouble than the OEM one did.

 

[Baluncore]

A roller chain sprocket always presents a side force, and if under too much tension, a hammering as each roller passes. That could quickly reach the cyclic fatigue limit of the shaft.

It would be interesting to examine the sprocket tooth profile for wear on the working face, and test the chain for stretch.

 

[Averagesupernova]

The overall length of the shaft is around 32 inches. The break in the shaft is next to the large pulley. The sprocket is quite close to the end of the shaft. There is a sealed ball bearing right next to it. About 24 inches over from that is the break in the shaft right next to the large pulley. Opposite side of the pulley is another sealed ball bearing. My opinion is the shaft is too small for the load in combination with the length. I think the flex in the shaft in combination with the large pulley hub acting like a shoulder cut into a shaft. Often times the shoulder causes a weak spot.

 

[Baluncore]

I only see four pictures in post #1.

 

[phinds]

Honestly I'm not to worried.

I think you are overly optimistic. I had a straight key creep out on a floor model belt sander once. The sander WAS heavily used over several years, but still ...

 

[Averagesupernova]

I only see four pictures in post #1.

Anything specific you want to see?

 

[Averagesupernova]

I think you are overly optimistic. I had a straight key creep out on a floor model belt sander once. The sander WAS heavily used over several years, but still ...

I won't say it can't happen. I've seen a lot more straight keys stay put than moved.

 

[phinds]

I've seen a lot more straight keys stay put than moved.

Well, of course. it IS rare, but still ...

 

[jack action]

I would think Woodruff keyway minimizes the stress concentration in the shaft. So if your shaft is already weak...

Also:

Where & Why woodruff key is used?
It is mainly using to fix the tapered shaft into the hub or gear. It gives a good fixing area for the shaft as its top flat surface gets to engage with the hub/gear to get better fixing.

 

[Averagesupernova]

I would think Woodruff keyway minimizes the stress concentration in the shaft. So if your shaft is already weak...

There's no doubt that there's less material removed to make the key seat with the woodruff. I'll take a few more pix and post. @Baluncore implied there should be more pix. Never replied to my question about what specifics he wanted to see.

 

[Baluncore]

Never replied to my question about what specifics he wanted to see.

I need to see the pulley and the pulley key, plus the position of bearings on the shaft relative to pulley position. Is the pulley for a v-belt, single/multiple, or flat? How much belt tension?

 

[Averagesupernova]

Not in a position to get a pic of it right now but will later. I can tell you the pulley is mounted on the shaft directly to the right of the break. You can see the dark colored part where there is no paint in the pic. That's where the pulley was mounted. You can see the length of the keyway and the key didn't protrude out from under the hub of the pulley. Specific measured lengths I can't tell you until I get a tape measure and check it. Post #5 gives a basic description. The belt tension I have no idea since a hydraulic cylinder moves an idler pulley to engage and disengage rotation of this shaft. There is spring tension to take up the excess so it's not likely to be tightened to the point of damage. Single belt, 7/8 inch width.

 

[Averagesupernova]

@Baluncore
Here are some pix as promised.
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The key on the pulley:




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Bore of the pulley hub:

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Positioning of the bearing and pulley and shaft break. Length of the pulley bore can be noted where there is an abrupt lack of paint:

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Sprocket end. Nothing broke here. The rectangular hub is a press fit to the shaft. The sprocket will freely turn on the shaft in the event of sheared bolts between the hub and sprocket:


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Distance to sprocket end. Tape measure wasn't moved for all pix. Hub is installed next to the bearing. Again, note the missing paint.

 

[Dullard]

I agree with most of what's been posted - I haven't seen any mention of:
One good reason to use a Woodruff key is as a "mechanical fuse." They may be sized (and of proper material) so that they fail before anything else. That approach obviously requires some room between the 'running' and 'damage' torque conditions of the system - It can't fix under-sized basic hardware.

 

[Baluncore]

I haven't seen any mention of:
One good reason to use a Woodruff key is as a "mechanical fuse."

The Woodruff key is often used to index shaft phase during assembly, typically the timing gears in engines or on fuel injection pumps. The shaft and sprocket will be gently tapered, with an axial bolt being used to lock the sprocket onto the tapered shaft.

A Woodruff key will fail if the axial bolt is insufficiently torqued, when the key must transfer the full working torque, not just the assembly torque. A Woodruff key will often flog out the keyway before the key is sheared. It is very difficult to know if the key has sheared or lost index, since the key is hidden in a short pocket in the shaft, covered by the sprocket hub, and the unit will need to be disassembled.

A Woodruff key would make a poor mechanical fuse because it would probably damage the shaft when it failed, and then remain hidden. As a fuse, it would need to be used on a parallel shaft, without the locking taper or axial bolt, so it would then flog out. If the key had sufficient section to carry the working torque, then the pocket in the shaft would significantly reduce the strength of the shaft at that point.

 

[Averagesupernova]

One good reason to use a Woodruff key is as a "mechanical fuse."

I doubt that was the intent in this case. As stated in the first post of this thread:

The other end of the shaft has a hub with a keyway cut into it the entire length of the bore. This is where the woodruff key is used also with a pair of setscrews. The width of this key is also 5/16 inch. This hub is attached to a sprocket with two bolts used as shear bolts.

The shear bolts held just fine.