Why are involute splines involute? 4

[MarkEIT]

Just a general question. Why are involute splines 'involute'? I can understand gear teeth being involute, so that the contact area of the two gear teeth does not move relative to one another. But involute splines do not rotate relative to eachother, so why does the spline profile have to be involute?

Thanks,
Mark

 

[ivymike]

Are you sure that they don't rotate relative to each other? That would imply a perfect fit and totally rigid splines, wouldn't it?

 

[MarkEIT]

The two splines fit together (male and female). The fit is going to be pretty good. If there is any slippage (perhaps from alternating loads), it'll tend to make the spline teeth slide relative to one another, meaning that there is sliding contact. Which goes against the reason to make something involute (at least in the case of an involute gear). I don't know, maybe I'm missing something.

 

[ivymike]

a quick web search gives:

http://www.gearsolutionsonline.com/articles/involute0304.html

are designed and made with an involute form, due to the ease of manufacture and its self-centering attribute.

http://www.tradexinstruments.com/splines.htm

Involute Splines are favoured over
Straight Sided Splines because of
their greater strength and the fact
that for any given pitch the tooling
can cut any number of teeth
resulting in a more cost effective
production method.

http://www.markwilliams.com/technical/schmaxles/

All OE axles, differentials, and so on, have involute splines, which means that the faces of the splines are slightly curved to provide optimum contact and even pressure distribution during engagement.

 

[MarkEIT]

I don't know why I couldn't find any of that! I did do a web search, and found nothing.

I thought the involute profile was expensive to produce. I guess with the amount of development that has gone into it, it must be a lot cheaper now.

Thanks for that!

 

[magnograil]

I expect it is because they are cut on a hobbing or shaping machine. A machine shop will cut splines with a mill cutter and indexing head which leads to variances in the spline depth and indexing. A hobbing machine turns both the cutter and part so depth and indexing is constant for all the splines on a part. It does not matter what the hob cutter profile is so an involute is just as easy to cut as a straight sided spline. The involute hob is probably cheaper than one that cuts trapezoidal splines since the tooth profile on the hob is trapezoidal to cut an involute and an involute to cut a trapezoid.
You can cut an involute with a special mill cutter (such as gear cutters) but you are mostly limited to 14.5 and 20 degree tooth pressure angles (common gear forms).

 

[MikeHalloran]

Male involute splines are rolled, not cut.

Remember how you could roll a ball of modeling clay between your palms to turn it into a cylindrical rod?

Now imagine your palms are hard steel platens with v-profile teeth cut straight across them, and they are held at a fixed distance from each other while sliding relative to each other inside a very, very sturdy metal structure.

Stick a semi- hard round rod in the space between the racks while they are axially displaced so they don't face each other, then stroke the racks toward and past each other, and they roll/ crush a beautiful spline in the o.d. of the rod.

If the workpiece has a rough- turned finish, that minimizes the shock load on the rack teeth, also leaving a rough finish on the crest of the male spline, and the racks will last for millions of cycles. A cycle takes a couple of seconds.

That's how they did axle splines at Ford, thirty years ago.

WAY faster than hobbing.

Male threads are rolled in similar fashion.

Female splines are still broached, I think.

A subtle trick: the racks for pinion splines were angled by a fraction of a degree so the male spline had a little twist in it. Made it a bitch to assemble a straight broached drive flange over it, but prevented fretting.

Normal pressure angle for involute splines is 45 degrees.



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[swall]

Splines also rolled at GKN for axle shafts and CV joint outers. Mike--you answered something I had always wondered about--helix angle. Now I know why.

 

[GregLocock]

we use the helix to improve the concentricity and parallelism of the joint.

Cheers

Greg Locock

 

[gearguru]

My 2 pennies:
the most common pressure angle for involute splines is 30 deg, it is interesting that it is the only pressure angle standardized by DIN. 45 deg is used mostly on rolled splines. Our German customers use 45 deg splines also, but as I mentioned they use different standard, not DIN . Of course the external splines can be also machined, but the rolling is the most cost effective manufacturing method if there is a lot of them to make, the machine and tools (racks) are not cheap. Larger pressure angle (45) makes the rolling racks stronger.