Splined Bushing Teeth Hardness

[Baheej]

We are trying to shrink fit a splined bushing onto a rotating shaft. To be able to transmit the application torque thru the shrink fit connection, both shaft and bushing are machined to very close tolerances.
The bushing needs to be heated as high as possible (no more than 300C/570F) to achieve the needed bore expansion. However, doing this for any reasonable time will result in degradation of the hardness of the spline teeth! We thought of quenching the bushing after it is installed on the shaft to restore part of the teeth hardness. But we are unsure if this really helps. Does anybody have any experience with such an application? Can we achieve the needed quenching effect using a water bath?
Regards,

 

[redpicker]

We will need to know what material the bushing is made from. In general, quenching from 570 F will not affect hardness of most materials.

rp

 

[CoryPad]

Why are you using both splines and a shrink fit?

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[EdStainless]

I assume that this is smooth ID and splined OD.
What material is the bushing and the shaft?
Make sure that your machined surfaces are not too smooth. All machine marks need to run longitudinal, not around.

My first thought is to cool the shaft and heat the bushing less. I have used liquid nitrogen for this.

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[Baheej]

EdStainless, your assumption is correct; the ID is smooth and the OD is splined. Shaft is steel 41CrS4v and the bushing is 8620. The shaft OD is 48mm.
redpicker, heating the hardened bushing to 570F will bring the hardness down to around Rc53. The spline teeth start at least at Rc 58 and we'd like to keep them up there.

We are concerned that cooling the shaft will result in humidity being introduced into the connection. Is that a legitimate concern or are we being too cautious?

 

[tripleZ]

We used to have this problem when we were shrink fitting carbide inserts into hardened steel die cases and steel inserts into steel die cases. It was really evident on pieces requiring an excessive shrink fit (customer mandated). If the surface contact area isn't too long, the method I used is as follows:
1) Get a table oven preheated. In your case, I'd guess around 400 (make it well below your tempering temperature).
2) Put your shaft in a freezer in the lunchroom.
3) Put your bushing in the table oven and let it heat thoroughly, maybe 30 minutes or so.
4) Now comes the tricky part. Not knowing the length of your part, I'd say you'll need two propane torches. The tips should yield a dispersed flame rather than a concentrated one. Watch your oxygen content also as you can really soot up a part if it's too high. Preheat a plate or flat surface with the torches. Not red hot but hot to touch.
5) Make sure your freezer with the part is nearby. You'll likely need two people for this process until you get the hang of it. Then get a telescoping gage and set it for roughly .002"-.003" greater than your shaft diameter.
6) Pull the bushing out of the oven and start to heat the outer and inner surfaces with the torch. You want to keep the flames constantly moving over the entire surface. Have your 2nd guy there to check the ID with the telescope gage and make sure the shaft is handy.
7) Now the tricky part. Once you've torched the bushing to get the ID expanded enough, insert the shaft. If your parts aren't the same length, you're going to have to come up with a method for holding them in position. If they are the same length, a heavier flat plate or scrap piece of steel works well.
8) Immediately after inserting and positioning that shaft, you want to spray the outer surfaces of the bushing with a cold air gun. Regular shop air will work, but the cold air gun is better as it will give you a faster quench. Your parts should be solid after about 30 seconds. After they're set, keep hitting the surfaces with cold air until they're cool to touch.

If you had your flame right and you got your parts cooled quickly enough, you should notice no degradation in your macro-hardness. Essentially, you heated the part but didn't give it enough time to transform the microstructures significantly.

When you get good enough at wielding two torches and fast quenching, you can often cut out the preheat step. It depends on your part sizes. Note that your outer surfaces WILL be discolored, but that's common.

 

[EdStainless]

If heating the bushing back to the temper temperature (450F ??) and chilling the shaft doesn't give you enough clearance then I would question the fit.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[GRoberts]

If you need a 500F degree difference (570Ffor bushing - 70F RT for shaft), try cooling the shaft in liquid nitrogen (-320F), and then the bushing would only need to be heated to about 250F. We use the liquid nitrogen mostly for inserting bushings into holes, but it works very nicely.

 

[tripleZ]

Baheej:

I re-read your post concerning humidity. You can forget the freezer process for cooling and still shock heat & cool the bushing sleeve material. It just may take a little more "play" time with the torches.

Hopefully you're not going after this in a large-scale production environment??? My method works for one-of-a-kind, but isn't a great idea for production quantities.

 

[Baheej]

Actually we are considering this solution for 50 units. However, if the first few assemblies don't go well, we'll have to find another solution.

We'll be attempting the first assembly on the third week of December. I'll let you know how the assembly goes. Thanks for all your suggestions and ideas.

 

[Tmoose]

shaft OD ?

ringfedder, tapered shaft/bore assembled by axially displacing the splined hub?