Axle Splines - What's the science behind?

Just looking for a good engineering theory behind axle spline count and why more is better.

more splines=more surface area to spread load

More splines = more diameter!

RCP Phx said:

More splines = more diameter!

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but arent 29 and 27 spline 8.25 shafts the same diameter?

1990JEEPXJ said:

but arent 29 and 27 spline 8.25 shafts the same diameter?

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Parts of it are!

I'm only familiar with D44 shafts, but as you go from 30 to 33 to 35 the shafts get much thicker.

RCP Phx said:

More splines = more diameter!

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Half marks.

That should read, "Assuming the same diametral pitch, more splines mean a larger shaft.

For the last 20-25 years, it's been common to us a 24-pitch spline in automotive applications (meaning that a shaft 1" in diameter will have 24 splines.)

This ain't always been so - there are D44 and D60 shafts that have down around 16 splines, and the ends of the shafts, in section, look like spur gears.

The "involute spline profile" is also a help. Splines used to be "square cut," which resulted in a straight shear load on the spline edge and actually made it a shade weaker. The involute profile (also properly used in gear teeth) is a slightly convex curved profile that serves to direct stress either into the centre of the shaft (external spline) or into the core of the gear/wheel (internal spline,) thereby reinforcing the spline without having to add a significant amount of material.

One more little factoid to bear in mind - with any splined shaft, the outer working diameter of the shaft should be slightly less than the root diameter of the splined section. Therefore, if the splines stand .100" tall, the working OD of the shaft should be not more than (spline OD - .100"), and should be enough less for the difference to be visible. This actually helps to reduce shear loading on the splines, again effectively increasing the strength of the shaft (as well as giving the shaft a "fuse" section - if the shaft fails in torsion, it will fail at the narrowest section. If the narrowest section is just outboard of the splines, you'll have an easier time removing the broken stub than you would if the shaft failed in torsion at the splined section!)

Make sense? For a more detailed discussion, I refer you to either Differentials (don't recall who wrote it, but it's a good manual on the "hows and whys" of automotive differential drives) or Machinery's Handbook (pretty much any edition) by Industrial Press.

A copy of MH is essential to any fabricator, engineer, machinist, or just someone who wants to know about how things work and why. MH is currently on the 29th or 30th edition, but my favourite (out of the ones I own) is the 12th, published around 1945. It does a bit better job of explaining the "hows and whys" than the later editions do - the later editions are the same size, but have to cover CNC, ISO thread profiles, the new alloys, SAE/ASTM specs, conversion of the old steel numbering to the current AISI numbering, and the like.

But whatever edition you get, it's worth every damned cent you pay for the thing!