Keyed shaft coupling

[thrusterpump]

I'm evaluating the torque rating of a rigid keyed shaft coupling. Various references I have omit the case of a hollow "tube" with the keyway cut all the way through for evaluating the stresses. Can anyone shed some light on how to account for the keyway? Thanks in advance

 

[rb1957]

i guess if yu cut all the way thru the wall it's no longer a key-way. i'd look at the torque you're transmitting, the torque would be trying to shear the "key"s ...

 

[thrusterpump]

Thanks for the reply, by all the way through I meant thru the length of the coupler, not through all the way to the O.D of the coupler. Sorry, probably should have been clearer. So this is a keyway. However, the case rb1957 presents has been defined by references and I guess would be the worst case? I don't know the torque, but I'm trying to find knowing the allowable stresses, what the torque would be at a given rotational speed.

 

[sms]

Is this something that you built yourself? If you bought it, you can go to OEM and get the torque rating from them. It should be on the data sheet. What brand is it?

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[rb1957]

then i guess you have two failure scenarios ... failing the key (unlikely?) or failing the coupler at the net section.

you must have something in mind, 'cause i would know how to calculate the allowable stress without being able to define the applied load (the torque) ... allowable stress on what (area)? i'd expect to apply a torque to the structure, and then calculate the stresses, and decide if they are acceptable or if there needs to be a change to the structure. 'cause the allowable stresses are material dependent (ftu, fsu, endurance limits, etc)

i guess you could apply a unit torque ...

 

[thrusterpump]

rb1957, ok lets say i know the torque, how do you evaluate the stress with the method you know for the crosssection i described earlier. For however you plan to input the torque value, essentially that is the value I would like to solve. I work for the OEM, and there is no design documentation for this coupler, and I need to evaluate if it is properly sized.

 

[rb1957]

the two obvious calc's to me are ...
1) shearing the key ... allowable torque = key cross-section*(key Fsu)*radius (say to the middle of the key); and
2) shearing the coupler around the key ... maybe (coupler Fsu)*net area of the coupler*radius

this assumes you're looking for a static load, and of course ... cavet emptor

 

[desertfox]

Hi thrusterpump

Have a look at this site:-

http://www.utm.edu/departments/engin/lemaster/Machine Design/Lecture 19.pdf

regards

desertfox

 

[EdDanzer]

There are many types of rigid shaft coupler failures depending on the type. Split clamped couplers tend to work loose if subjected to reversing loads or over hung loads. Solid with set screws may move end ways or work loose if subjected to reversing loads. Typically key shear is the lease common failure if a standard inch size cold finished shaft and key are used.
If you post the shaft, key and hub dimensions and material along with torque and rpm and how it is to be used more specific detailed information could be posted

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[vims]

Look at pages 19-21 in the document desertfox posted. These calculations only applies to static loads, so if you have lot of dynamic loadings then be careful useing these formulas.

 

[electricpete]

It seems to me that the fit between the coupling and the shaft plays a very important role.

If it's a very loose fit, then most of the torque is transmitted by the key as I think you are assuming.

If it is a very tight fit, then most of the torque is transmitted by the fit, not the key. But there are still stresses created by virtue of the interference.

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