Shrink Fit - Part Hardness Problem 2

[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,

 

[israelkk]

What is wrong with press fit?

 

[Baheej]

The shaft is part of an electric motor and we are concerned that a press fit might load the bearings in a nonfavorable way. I don't recall the pressure needed but I doubt that the motor allows that kind of thrust pressure on it bearings.

 

[PeterCharles]

Support the non-drive end of the rotor shaft so your loads are confined to the rotor shaft and do not pass through the bearings into the frame of the motor ??

 

[FOETS]

How practical would it be to "freeze" the shaft with dry ice or liquid nitrogen. You then dont need as much heat on the bush. Obviously depends on relative sizes if you can get the amount of shrink you needed with shaft size. You may need to do a combination of all the above to reduce the thermal effects.

 

[israelkk]

Maybe Baheej will be so kind to let us know the actual shaft diamter and free length and the spline dimensions? This way we will not have to speculate and invent solutions that are not practical.

 

[israelkk]

Shaft torque and tolerances and spline inner dimensions and tolerances too will be nice to know.

 

[Baheej]

israelkk, the shaft OD is 48mm. The free length is 135mm. The splined bsuhing OD is 60mm. Torque is 740 N.m. The bushing is made from 8620 steel. The interference amount is 0.002".
PeterCharles,We cannot support the other end of the shaft due to the nature of the assembly.
Foets, we are concerned that freezing the shaft might result in humidity being introduced into the connection. Is that a legitimate concern or are we being too cautious?

 

[israelkk]

How long is the splined bushing?

 

[Baheej]

The bushing is as long as the free length: 135mm.

 

[MikeHalloran]

That seems like a relatively thin bushing. You checked the hoop stress from the interference, right?



Mike Halloran
Pembroke Pines, FL, USA

 

[israelkk]

You could machine the 48 mm OD to say 30 mm at the end of the motor shaft to 10 mm length. You can machine a new spline with an "inner" flange (inside diameter of 30 mm) at the end of spline. At the flange drill few holes for bolts and in the 30 mm motor shaft drill and tap threads to accommodate the flange bolts. To assist the bolts to hold the 750 N-m you can either use dowel pins or a key in the flat edge of the motor shaft and the flange.

You may even be able to press fit the spline using the flange bolts. In this option you don't even need to turn the 30 mm diameter just build the spline with one end closed. Through this side drill for few bolts. At the end of the motor shaft drill and tap threads to accommodate the bolts of the flange. Using the bolts you can press fit the spline on the motor shaft. You may need to make the motor shaft free length in the amount of the flange thickness.

Third and simpler option is to use the same spline. drill and tap few threads at the flat edge to the motor shaft. Use a disk with a little bit larger outside diameter than the spline outside diameter. Drill holes for bolts/screws to accommodate the threads in the flat edge of the shaft. Now you can push and press fit the spline on the shaft using threaded shafts and nuts where the threaded shafts length is larger than the 135 mm of the spline plus the disk thickness plus the nut thickness.

 

[FOETS]

with dry ice or liquid nitrogen there is no water as it is CO2 or nitrogen which "boils" off so humidity should not be a concern.
Dry ice is a method of cleaning electrical switches so I am pretty sure that humidity is not a concern, but you may want to ask the electrical gurus to make doubly sure.

FOETS

BEER: It's not just for breakfast anymore.

 

[ornerynorsk]

You should be able to shrink fit your assembly with no problem. Cool the motor shaft with cold water or ice if nothing else is available. Allow it to chill for as long as possible so that the core temp is as low as possible. The trick to an assembly like this is to install the heated collar or bushing AS RAPIDLY AS POSSIBLE ! A special fixture made up to align the 2 parts and facilitate the rapid installation necessary is very helpful. I have done numerous shrink fits and have never had a problem. You do not need to worry about hardness degradation if your temp can be achieved rapidly, which it should due to the thinness of the bushing. The resultant "quench" that occurs when installed to the shaft is all that you need to finalize the assembly. I would not pour water or any other quench material on it. Good luck.

 

[Baheej]

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.