Round Key vs. Square Key for disk/shaft fit 4

[poetix99]

(This will be posted identically on Mech. Engr'g (#404) and also on Gears and Pulleys forum.)

I am trying to get any design references about the use of shaft keys with a round cross-section.

We are now trying to analyze this configuration because the original design work is 30+ years old; anything that was written down (if anything WAS written down) is gone. We are pushing the envelope of our known applications, and would like to better know where we're going.

It seems clear that the original rationale was the idea of a lower stress concentration that one would obtain with a semi-circular key slot in the disk. I have questions (suspicions?) about torque capacity and any tendency for the disk to want to "cam" over the keyway as it transmits torque to the shaft.

We use a fairly heavy shrink fit in addition to the key, so the shrink is really intended primarily for torque transmission. The key is, in a meaningful sense, a back-up.

There are also some thermally induced (hoop) stresses in the disk during normal operation.

Any written design calculations have long since disappeared, and we are re-inventing the wheel, so to speak.

Thanks in advance.

 

[poetix99]

Some "clarifying" comments based on a post at the other thread:

The disks are mounted along the length of the shaft. The key slots are ball-end milled into the shaft. The mating disk slot can be broached, or machined in some other (conventional) way. As I've implied above, there may be several disks on a shaft. The round keys aren't improvised repairs; they're as-designed, but the original design documentation does not exist. The shrink is not a press fit; it is a "shrink", with heated disk and/or cooled shaft. Therefore, keyway alignment is not a particular issue. Torque is developed on the disk(s) rim and transmitted to the shaft.

I'd like to relax the shrink that we use and therefore would impose some torque transmitting "responsibilities" upon the round key.

Therefore, I would like to know of any application experience and/or design consideration that you folks might be willing to share.

Thanks.

 

[tschmidt]

I'm not sure I understand the entirety of your situation but I will attempt to answer some of your questions anyway.
I agree the former designer used a round key because of lower stress concentrations in the shaft and disk. The strength of the key in maintaining the integrity of the joint is based on the shear force required to cut it in half. The disk can't cam over unless it is very soft compared to the key. The keyways in both disk and shaft may deform slightly also during the shearing process.

Your other point about reducing the shrink fit to allow more load to fall on the key would be a mistake I believe. You won't save anything in terms of cost and you will now add another pathway to failure. Instead I would look at whether the key is really necessary. Most of the time, in these situations, the key is used as a locator for timing purposes. Rarely, if ever, are keys used to actually carry any load. The reason is that most loads are not steady and the constant load, unload cycling movement will eventually cause the key to fail even if it isn't overloaded at first. I would evaluate the amount of shrink you are using, the other stresses from the rpm and temp. induced hoop stresses, and design the joint for shrink fit only. If necessary add some type of adhesive to make the joint more permanent.

 

[GregLocock]

". Rarely, if ever, are keys used to actually carry any load. "

YES

yet on many agricultural pieces of machinery you see a shrink fit and a keyway. Is this in some fond hope that if friction over an area of several square inches won't hold the load then a 1/4 square piece of steel in shear will?

Cheers

Greg Locock

 

[dougholmes]

You might want to consider modifying the design to use keyless locking assemblies. There are several manufacturers of this kind of device.

Bikon Corp., West Mombasha Rd, RD1 Box 660 Monrow NY 10950 (914)782-5650
Ringfeder (no address or phone or web info at hand here)
Peter -

http://www.peterdrivecomponents.com

There are probably many more I don't know of.

The devices of the type I refer to can have a higher torque rating than any keyed or splined shaft can achive, higher even than can be reasonably achived with a press fit.

 

[dougholmes]

Greg,
I suspect the key in addition to shrink fit (if the key not needed for defining the rotational position on the shaft) is of the "Well if the shrink loosens up, maybe the key will take care of the situation" type thought.

 

[GregLocock]

Yes, I'm sure that's the reasoning used after the fact, but it doesn't really add up does it? We used to use Fenner taperlocks for this job - they work very well, at a cost. Other options - Loctite 200 if you think you might want to disassemble it, or 600 if you are sure you don't.

Another nasty little idea, which may have some merit, is to use a sliding fit on the bore and then a grub screw pressing down on the key. You might get a bit of friction capacity from this, and the grub screw is galling the key, not the shaft, which makes disassembly easier. I suppose it is essentially the same as a tapered key.





Cheers

Greg Locock

 

[dougholmes]

Greg,

Is Loctite 600 more "grippy" than 271? We use 271 on screws inside our machinery rather than threading locking wires through the screw heads. The info on 271 talks of it locking up so tight that power tools would be necessary to break the grip. Typically, the guys in the shop end up using impact air wrenches.

There are a number of places where I would like to be able to have the NF nut lock onto the shaft so hard it would require breaking the 3/4" Grade 8 bolt to undo the connection.

Doug

 

[GregLocock]

Yes, 600 series is almost unusably strong. I've had to cut nuts off threads before now, admittedly on valuable parts rather than just bolts. Watch out for the activator spray - it increases the speed and success rate of the joint, but at a cost in strength.

Cheers

Greg Locock

 

[butelja]

How about an expandable taper lock device and no keyway? I'm familiar with the ones that Ringfeder makes, and there are likely many others available too.

 

[plasgears]

If you are working with fractional HP, consider generating a spline in the following manner. The gear or outer member is hardened PM with semicircular lobes repeating 30-40 pattern on the ID. The pattern is sturdy and repeatable for many impressions on the PM press.

The mating shaft is relatively softer, and a collar is machined or formed on the OD. The outer member and the shaft are aligned, and the outer member broaches the spline during assy. Now you have an intimate spline fit, round pattern with precision on one side and simple mfg on the other. This was used in a fridge icemaker crusher.

 

[dugal]

I would think that round keys are good for hubs and shafts that undergo a lot of cycling stresses such as in steam turbines. On these machines you avoid sharp corners for fear of stress cracking.

 

[automatic2]

Don't be reducing the shrink fit. This will increase the torque applied to the keyway. Round keys are typically used to reduce production costs, and have limited capacity.

 

[jive]

I pressumed the design was intended to hoop stress reduction and maintenance accesibility... why don't you try to analyze... the engagement location between the disk and key with respect to keyway... because I don't think keyway broaching on he mounting shaft will reduce such hoop stress...

 

[sbi]

NOte## if this is on a piece of Ag equipment,
remember, if it can be fixed in the feild with basic tools, it will sell and be around for decades, if it takes special equipment to disassemble and repair, very few will be around in five years
We were the biggist jobshop in central Nebraska for decades till we came up with our own product line and switch to mfg 15 yr ago.
I can speak from excp. laying out in a feild at 15 obove 0 with a tourch and a 100 ton puller trying to remove a broken part that was sweated on at the factory, when it failed the unit could not bemoved and had to be jerry riged in the feild just to bring it in, needless to say after we fixed it the farmer traded it off for a piece of green & yellow that could be worked on anywere even though it cost twice as much!
morral of story, if its cheap and a throw awy, shrink fits ok, if it quality, make sure it can be fixed with a pair of pliers, some wire, and a hammer!

 

[ornerynorsk]

In my experience, woodruff keys will cause shaft failure more often than key failure. I personally would want to replace a key, rather than the shaft. All production cutting of woodruff keys involve sharp corners in the bottom of the slot, so where are you gaining anything? You just have more linear distance of sharp edge because of the arc segment. As a machinist and as someone who has repaired plenty of ag and non-ag machinery, I can tell you that square keys are genrally the way to go when your talking about transmitting high load.

 

[plasgears]

Stoffel Polygon Systems (914-961-2000) has a smmoth lobular system that drives with min stress concentrations. They come in bilobe (ellipse), trilobe, and four lobe versions. They claim twice the shear strength of involute splines.