Selection of spline module

[pmbrunelle]

I have a shaft of a given diameter, and I want to make a spline joint with a female mating part. The joint is subject to torque which is always changing direction, so fatigue is a concern.

I'm using DIN 5480 as a reference, but I don't think it matters too much for this discussion.

I can use many small teeth, or fewer large teeth.

What are the pros and cons of small versus large teeth?

So far, I see a few pros for small teeth:
1. Less material removal in machining.
2. Root diameter of shaft is bigger, so the shaft remains stronger in torsion.

What factors would push one towards using bigger teeth?

 

[Steven_D_P]

For gears increasing the module increases the tooth width and therefore the tooth root bending strength. I assume the same thing applies to splines.

However with splines increasing the module would increase the load on each tooth (because of the decrease in amount of teeth) which is not an issue with gears. So the increase in bending strength of each tooth might be offset by the increased load each tooth has to bear because of less teeth.

DIN 5466-1 probably has useful information. If you put the formula's in a spreadsheet you can check to see how the module would affect the strength of the splines.

 

[pmbrunelle]

So far, I've looked at the article When Splines Need Stress Control by Dudley in order to evaluate strength.

Due to the wide and stubby nature of spline teeth, Dudley does not suggest evaluating bending strength of spline teeth.
Instead, he suggests evaluating for the possibility of shearing on the pitch circle, fretting on tooth faces, shear torsion failure of the shaft, and tensile hoop stress of the hub.

In my first prototype iteration for this product, the prototype was tested fatigue-tested to failure, and the shaft broke in torsion... so probably if the shaft broke that way, the spline teeth are not a limiting factor, but the root diameter of the shaft splines may be the limiting factor.

I didn't know about DIN 5466-1. I'll look into getting a copy, though getting things in the purchasing pipeline here can be tricky.

 

[Steven_D_P]

DIN 5466-1 only deals with the strength of the splines themselves and not the fatigue strength of the shaft. In that case I would look into not just how the cross-section of the shaft changes with the module of the splines, but also the fatigue notch factor if you can find some information about this.

Carburizing the shaft will also help. According to DIN 743 when a shaft with involute splines is carburized the fatigue notch factor becomes 1. DIN 743 also has fatigue notch factors when the shaft isn't carburized, but they only account for the strength of the shaft's material and not the specific geometry of the spines (the geometry of the splines might be neglectable).

I've also found this free book about the strength of splines. I haven't read it so I don't know if it's useful.

https://link.springer.com/content/pdf/10.1007/978-3-658-36961-3.pdf?pdf=button

Edit: Alternatively you can also see which spline leads to the highest stresses in FEA. Idk about other programs but in Inventor you can generate a spline according to DIN 5480. Just make a shaft that has a spline along it's whole length, fix it on 1 end and apply a torque on the other end. That was you can easily compare the fatigue notch factors of different splines to each other.

 

[mfgenggear]

Hi Guys I am for getting important information. but here goes.
there would be two different calculations, one for the spline or gear, and a free body for the shaft using bearing journals and the root diameter of the gear or spline. AGMA and DIN both have great information, as well as post by professors on you tube about s/n curves, torque, rpm, and shear values. to much to post here. plus you tube links are frowned upon. it has gotten me IN trouble.