A couple of questions about splines.

Mark Rand · Oct 18, 2018

In the next few days I'll be making a replacement for the hollow splined shaft that drives the spindle on my Beaver milling machine. The spindle has got 9° of slop in the splines, 98% of it in the female, driving spline and this makes for hard work when using any larger end mills or shell mills. I've got two questions that folks probably know the answers to:-

The spindle is hard, but the current hollow splined shaft that drives it appears to be dead soft. I'm going to be making the replacement from EN24T==4340PH. Might it be a good idea to get this gas nitrided to take the surface hardness up from about 34HRC to 50HRC. There's not a lot of sliding motion on a mill's splines, but I don't know if it's normal to have both parts hardened.
Is there any benefit at all, in having a close fit on the inside or outside of the splines if you are aiming at as little backlash as possible so the sides of the splines ought to centre the two shafts?

Thanks in advance for any help.

PS:- Here's the original. The original broached dimentions are still visible in the slight lands either side of the spline teeth!

HuFlungDung · Oct 18, 2018

Consider making the female spline out of some type of plastic, something that is quite stable through various humidities. The actual amount of power transmitted is tiny, so no real need for a heavy duty steel spline. I suppose that there is more damping of transmitted shock when the metal hits plastic than when it bounces off another chunk of metal.

But hardening both components will result in a longer life. And a controlled fit on the OD of the male, ID of the female can help stabilize the joint when it does get a bit looser. But this can be difficult to achieve unless you are broaching the female spline and custom fitting it to the shaft.

Mark Rand · Oct 18, 2018

The power is 2hp at speeds from 3000rpm down to 80rpm on a 2" length of 1" OD six tooth spline. I think that's why the original spline is so whaled out

.

I'll be 'single point' slotting the female spline using the mill's slotting head and the rotary table. I expect the original was a single push broach, but I don't have that sort of tonnage, especially with pre-hard stock. It will be made to fit, even if I screw up and need to throw a failure away. I've got the choice of fitting to the inner diameter, the outer diameter, neither or both. Although I suspect It could be hard to use the quill feed with some of those combinations.

10 fingers · Oct 18, 2018

RE: "Is there any benefit at all, in having a close fit on the inside or outside of the splines...……"

Hell yeah. Unlike involute spines, the only thing keeping these components coaxial is their major dia fit. I do a fair amount of these repairs. If the male component is OK, EDM a new female part. Make the fit on the snug side, then using fine lapping compound remove any interference.

john.k · Oct 19, 2018

The driveline repaireres used to dip big truck tailshaft splines in a green plastic liquid,and then insert back into the male spline.....The hard plastic lasted as well as the original steel spline fit....Dont know any details,but had it done on a couple of my trucks ....only used to cost a fraction of replacements .

farmersamm · Oct 19, 2018

I had a question when I first read this post, but was afraid to ask it (might show my ignorance)

Then I figured "Hell, might as well...might correct my ignorance if I ask"

Is there a problem when running hard against hard, if the components are a matched hardness? Just about everything I've seen (actually observed), or read, points to soft running against something harder. Galling being the issue when the components have the same hardness.

Also, have seen a "sacrificial" situation too. The most easily replaced component is generally softer.

Had to ask. Hate to sound like a moron.

farmersamm · Oct 19, 2018

Oh yeah, I was just stepping out the door...……..

One case on point, I think.

The original design on the NP203 transfer case had an output shaft that was case hardened, and ran on caged needle bearings. They gave up the ghost in relatively few miles. The case hardening wore through, exposing the soft inner core, and you wound up having to replace the output shaft.

Later designs incorporated a standard roller bearing with inner, and outer races. This was the better design. The shaft then rode inside the inner race, and didn't turn against anything like it had before.

Outta here, thanks

Mark Rand · Oct 19, 2018

This was one of the things I wasn't sure about. I know that mill spindle splines don't see the same degree of sliding movement you'd get in a bearing, but I don't know if fretting is still a problem when both sides are hard. What are PTO shafts and couplings like?

10 fingers · Oct 19, 2018

RE: "Is there a problem when running hard against hard?"

Not a yes or no answer. Need to appreciate a bit about abrasive and adhesive wear. Also have a modest understanding of PV values and lubricants. Modern transmission design has drawn cup and caged needle roller bearings running directly on properly hardened shaft journals. The trend in transmission design is for minimal oil to prevent churning loss. The demand on such an oil application is critical. For other applications, bronze and cast iron against hardened steel works well. Not a simple subject.

Gordon Heaton · Oct 19, 2018

Personally, I'd leave the replacement coupler soft, then next time you can replace it again instead of replacing a coupler and a spindle.

Mark Rand · Oct 20, 2018

Gordon Heaton said:
Personally, I'd leave the replacement coupler soft, then next time you can replace it again instead of replacing a coupler and a spindle.

That might be the best answer Gordon! Ok, I'm 60 and the mill isn't much younger, but I can hope to last that long

.

I'll stay with the EN24/4340 pre-hard in it's current condition, about 30-34 HRC and not go for any further heat treat.

Thanks everyone for the help.

awake · Oct 20, 2018

HuFlungDung said:
Consider making the female spline out of some type of plastic, something that is quite stable through various humidities. The actual amount of power transmitted is tiny, so no real need for a heavy duty steel spline. I suppose that there is more damping of transmitted shock when the metal hits plastic than when it bounces off another chunk of metal.

But hardening both components will result in a longer life. And a controlled fit on the OD of the male, ID of the female can help stabilize the joint when it does get a bit looser. But this can be difficult to achieve unless you are broaching the female spline and custom fitting it to the shaft.

I had wondered about plastic when I read the original post - particularly since I have just been experimenting with 3d printing of POM (Delrin). Interesting results - very strong, and with the lovely slick properties that one expects of Delrin, but when I've tried printing a part 20mm x 44mm x 22mm, it starts warping like crazy within the first mm of height. That's with a heated bed but without a heated chamber. However, I can print thin-walled pieces, 20mm diameter x 25mm height, ~1mm thick walls, with no trouble whatsoever. Might be interesting to see how an internal spline would come out.

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A couple of questions about splines.

Mark Rand

Diamond

HuFlungDung

Diamond

Mark Rand

Diamond

10 fingers

Hot Rolled

john.k

Diamond

farmersamm

Cast Iron

farmersamm

Cast Iron

Mark Rand

Diamond

10 fingers

Hot Rolled

Gordon Heaton

Titanium

Mark Rand

Diamond

awake

Titanium

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