SAE Standard Spline Torque Capacity

[mtipg]

I have a question about the Torque Capacity of SAE Standard Splines. When I referenced my Machinery's handbook (27th edition - page 2185) for SAE Standard Spline tolerances and torque capacities, i found the numbers hard to understand.

For a spline of the same nominal diameter:
A "permanent fit" tolerance has a smaller torque capacity then a "to slide under load" tolerance.

This seems counter intuitive...

My question is this:

Does this "d" value represent the ID of the Spline sleeve that would fit over the shaft, or the OD of the actual shaft inside?

If this is the ID of the of the spline sleeve. Why are the "d" values for permanent fit larger then the slide under load "d" values? (I would think a smaller ID would provide a tighter tolerance, as you would expect in a permanent fit.)

If this is the OD of the shaft inside. Why aren't the torque ratings proportional to the tolerance increase in the permanent fit. (ie: a tighter tolerance should give a better torque rating?)

Any insight would be welcome.

- Dazed and Confused

 

[tbuelna]

mtipg,

Designing a spline joint is not as simple as it might seem at first.

Depending upon the loading and life cycle requirements required of the spline, your analysis techniques will differ. A spline that sees high torques and only a few load cycles should probably be analyzed for root shear. But a spline that sees low torque with lots of load cycles should be designed for surface contact fatigue.

For a spline limited by contact, the relative motion probably helps to lubricate the contacting surfaces. A surface contact condition that has boundary or mixed regime properties will be more durable in fatigue life than one that has metal-to-metal contact.

That's my theory any way.
Regards,
Terry

 

[edendebrock]

Many of the hydraulic pump manufacturers have extensive specs on the torgue limitations on the smaller splines sizes from 5/8 through 1 3/4. These are all sliding fits.

 

[mtipg]

Anyone have any insight? I appreciate the responses but I don't feel that this question has been addressed... What are the values in the Machinery Handbook representative of? What should I use for shaft dimensions if this dictates the sleeve dimensions? I am lost.