How to increase tightness between splined pieces?

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KellyJones
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Re: How to increase tightness between splined pieces?

Post by KellyJones » Sat Aug 01, 2020 11:49 am

I designed splines for aircraft applications for 30 years, most were required to handle up to 5000 in-lbs. (~410 ft-lbs).

You seem to have two problems: 1) how to eliminate the backlash, and 2) ensure the spline can safely carry the required torque.

I often found these two requirements at odds with each other. That is, methods of increasing torque often do not lend themselves to lower backlash, and methods of reducing backlash often cause a reduction of load carrying capacity.

First, you need to work out how safe it must be. If human safety is an issue, then you probably want a high factor of safety (maybe 5 or 10). Strength is a function of material and spline geometry. Some materials could easily carry the required load, but may require expensive alloys or heat treatment.

There are 4 basic failure modes for splines: tooth shear, shaft shear, contact stress and rim bursting. For not rotating parts, you can probably ignore contact stress. Darle Dudley wrote the definitive paper on spline sizing in 1957:

When Splines Need Stress Control
https://pdfslide.net/documents/when-spl ... udley.html

Most of the article can now be found in Machinery's Handbook.

The second issue is backlash control. Several ideas have been suggested here: fill the space with sealant, RTV, Loctite, or some other compound, welding after assembly, using a different kind of joint, and so forth. Here are a couple of ideas I have used:

1) cut a ~.015" wide slot in the female piece such that the slot runs radially through one of the tooth roots. This splits the female housing on one side. Install a bolt 90 degrees to the slot outside the spline diameter and use it to clamp up on the external spline. Fixes backlash, and only minimally compromises spline strength.
2) use a standard internal spline (like you are doing) and machine the external spline yourself. Cutters are available through McMaster-Carr. By cutting the external spline slightly less deep (not plunging the cutter as deep) you will thicken the external teeth. We're not talking much here, a couple thou. Not a terribly difficult job if you have a dividing head.
3) Drill a hole lengthwise through the shaft with the external spline. Then, cut the shaft with the external spline in half, approximately in the center of the spline. the trick is that the cut must be at an angle (~45 degrees) and not 90 degrees to the centerline of the shaft. Assemble and install a bolt and nut through the shaft. When you tighten the bolt, the two external spline halves will "ride up the ramp" and consume the backlash. The only caution is that the length of the external spline carrying the load must me sufficient to carry the load by itself. The other half only creates the backlash control.

Hope some of this helps.

Kelly
Kelly Jones, PE
A life spent making mistakes is not only more honorable, but more useful than a life spent doing nothing.
George Bernard Shaw
(1856-1950)

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Bill Shields
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Re: How to increase tightness between splined pieces?

Post by Bill Shields » Sat Aug 01, 2020 11:52 am

even more concern before you choose what to do:

What happens if your 'spline fix' fails?...and the shaft is allowed to spin inside of the bracket?
Does something 'fall' and is a person likely to get hurt?....

Choose wisely....
Too many things going on to bother listing them.

10 Wheeler Rob
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Re: How to increase tightness between splined pieces?

Post by 10 Wheeler Rob » Sat Aug 01, 2020 12:15 pm

Is the splinded shaft
Large enough to drill and tap with NPT thread. Spilt the shaft with 2 or 3 saw cuts and then put on a pipe plug to rap and the shaft into the female part. There are flush pipe plugs that would not stick out when assembled.

Chickens
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Re: How to increase tightness between splined pieces?

Post by Chickens » Sun Aug 02, 2020 10:09 pm

There is a product called Belzona that is made for this purpose. It is a 2 part epoxie. They have a parting agent that is applied to one surface that prevents the two parts from being permanently glued together. I had a Kubota 4x4 with worn splines on the rear axles that I used it on 3 years ago. It's used every day and is still going.

JohnnyK81
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Re: How to increase tightness between splined pieces?

Post by JohnnyK81 » Mon Aug 03, 2020 11:20 am

Thanks for all the ideas guys! I appreciate them.

(Preface: I believe the female spline plate I have is used, so I have ordered a new one to see if the fit is any tighter).

No, there is no human safety issues with this. This is pretty much an experiment for my own curiosity.

yes, the splined have to allow for disassembly, and movement.

Unfortunately a lot of the ideas regarding doing something to the face of the splined shaft will not work, as I need a hex hollow, or some other way to adjust preload if necessary.

I think the best ideas for this particular scenario are to either use a filler (silver solder? This Belzona you speak of?) and use the shaft to broach it. OR, find a way to shrink the female plate slightly: Ideally I would like to cut a slot and push the plate slightly closer together and weld it up, but I assume I will lose a spline, making this idea unfeasible? Or is there a way to avoid that? (I could experiment of course, but maybe it's been done before)

(I think the bigger challenge may be finding my 200ft-lb, 90 deg torsion spring, so the whole idea may be moot!
Torsion drawing.jpg

Chickens
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Re: How to increase tightness between splined pieces?

Post by Chickens » Mon Aug 03, 2020 2:02 pm

There are companies that stock any spring imaginable or can custom make one to your specs. Onfortunatly as a hobby the cost factor enters into play. Belzona would work. There again it't hard to buy in very small quantities and is very pricey. Sometimes it's better to just redesign things if that's at all possible. I have bought splined shafting and I think you can buy splined bushings. If you have some machining/welding ability you should be able to come up with something.

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Bill Shields
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Re: How to increase tightness between splined pieces?

Post by Bill Shields » Mon Aug 03, 2020 3:29 pm

filler with the shaft to broach it is not a good idea...it will just peel off as you push the shaft through.

been there - done that - will not do it again.

it it is only an experiment, why not just replace the spline with a square key???
Too many things going on to bother listing them.

JohnnyK81
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Re: How to increase tightness between splined pieces?

Post by JohnnyK81 » Sat Aug 08, 2020 8:15 am

Well since this is a 3/4-36spline shaft, maybe I will buy a 3/4 - 36 spline coupler, make the mating plate with a hole in the center, and weld in the (cut off) coupler to remake the square mating plate? Thoughts?

My other thoughts, this shaft is a Borgeson I believe. Would other brands of 3/4 - 36 spline steering couplers be a tight fit? Or is my safest bet to buy a Borgeson coupler?

Thanks again guys.

John Evans
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Re: How to increase tightness between splined pieces?

Post by John Evans » Sat Aug 08, 2020 3:52 pm

If the spline is made to a standard ,anyone's made to the sane standard should work. But, I think I would buy from Borgeson to be sure. And when you weld the 2 pieces together also cross pin the joint or rosette weld also from the side to make a mechanical joint as well as the weld. Belts and braces you see !
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