To freeze or not to freeze?

I´m a newbie with a problem: I have a 50 kW electric motor that I´d like to attach into a transmission. I must make a coupler to join the two shafts. However, the motor´s shaft makes this difficult. The coupler should have inner helical gears, and that kind of coupler is expensive to make. So, another option that comes into my mind is to use the freezing/heating method that is often used to install bearings on shafts. The male coupler would go into the motor shaft hole.

So, my question is if this a proper method in this case? Is it strong enough to prevent rotation between the shafts? How to determine the coupler diameter? Is there a risk the coupler brakes the motor shaft when it expands?

Any other ideas how to solve my problem?

Appreciate your help, Oskari

Hi Oskari

I'd advise anything except that "interference fit" where one part is heated to expand while the other is cooled to contract. The bearing race installed that way didnt anticipate any torque. If the motor spins at something like 1,700 RPM, the torque can be considered to be around 200 ft lbs. This motor, spinning and fully loaded would get my attention if it was near by. I sure wouldnt want to be close to it if that flex joint failed. So, I'd design for a significant safety factor. If this was a problem for me to solve, I'd consider disassembly of the motor and welding a shaft section to the motor's rotor shaft. Then I'd buy the best flex coupler I could afford to join the motor to the load.

Jerry

There is not a chance in the world that a freeze joint will take the torque the motor puts out. The groves in the male shaft will just cut similar groves in the female coupler, spin out under load.

Usual procedure is to find the female mating end in a junk part from whatever applicaton where it was orignally used, machine that part down into a suitable coupling. As someone else mentioned, you can also pull the motor down, machine a suitable shaft and key into the motor rotor. That will be tough on this one given the hollow end and no metal to work with.

Your questi> Hi All,

This is what, around 70 HP? A shrink fit basically relies on friction, unless your motor shaft is in the feet-across area, you're going to want a positive mechanical link between shaft and whatever you're trying to drive. Looks like some kind of dedicated traction motor to me, I've never seen that sort of coupling area on a general- purpose motor, looks almost like a short starter Bendix. Maybe for some sort of automated clutch arrangement? AC or DC? Just interested.

Stan

Problem is, nothing mated to that shaft. That shaft is the "sun gear" of a planetary set. You COULD weld the planets on the planetary and connect the shaft to the planet carrier.

Loctite is your friend.

Mark Rand RTFM

I agree with Jerry. Have a stub shaft welded to it..preferably one wth a spline on it and have a hub with the same spline to match, with a "lovejoy" type coupler welded to it. That way it will slide on the shaft, and yet everything will still be removable for service.

Perhaps a splined section of driveline from an automobile or truck could be hunted down at a wrecking yard and easily modified.

Gunner

"Liberalism is a philosophy of consolation for Western civilization as it commits suicide"

- James Burnham

Thank you guys,

even with my limited understanding of the subject I was quite sure the shrink fit isn=B4t ment for this kind of forces. Just wanted to make sure.

"This is the classic "if you have to ask the question, you really don't want to know the answer" "

I don=B4t quite get this... I asked this because I knew I can=B4t rely on my own judgment here. I=B4m very happy I asked even if the answer was negative! I have no desire to risk my own or others health with this. I think I have to forget this motor.

"Looks like some kind of dedicated traction motor to me, I've never seen that sort of coupling area on a general- purpose motor, looks almost like a short starter Bendix. Maybe for some sort of automated clutch arrangement? AC or DC? Just interested. "

You guessed right. It=B4s an AC traction motor for an electric vehicle. There was an integrated gearbox attached to the shaft originally:

-Oskari-

If you have the original gearbox and want a high speed ungeared output could you fill the gearbox with epoxy?

Not quite that simple. But I DID give a possible solution. Only thing is, you need parts of the original transmission, which the OP likely does NOT have.

Sounds like a typical hydraulic pump type coupler-look for power transmission equipment in the phone book, or giggle hydraulic pumps

I don=B4t have the gearbox. I=B4d have to attach the motor with the original gearbox of the ICE car I=B4m going to convert. Actually, I=B4ve opportiny to buy several of these motors and to sell the converted cars, but the motor must be quick, easy and cheap to install, and that =B4s why I=B4m not so keen on disassembling the motor either...

-Oskari-

No Jerry, it is a PLANETARY DRIVE application. This has already been established. The original application has the motor shaft as the sun gear of a planetary reduction gear. Perhaps the planets and carrier of an automotive automatic trans would mesh with that gear. I'd be trying a few - C6 Ford, FMX, TH400, etc to see if any fit. If so, weld the planets to the carrier and get the spline to fit the carrier to adapt to the new transmission.

As I said (cryptically) previously, Loctite is the OP's friend. Turn a coupling that is a close slip fit in the hole in the gear and Loctite it in. An Oldham coupling or similar on the end of that shaft may help in the OP's application. Unless the OP wants to find an internal helical gear and a couple of helical pinions to fit, the above would be the simplest solution.

Mark Rand RTFM

But would it work with that size motor? I doubt it. At 4mm wall thickness, that gear would not sustain an interference fit to accept that hp, whether it is loctite induced or shrunk fit. That bore is there to accept a bearing.

Russ

commits suicide"

If you weld on the shaft, you will never get that stub shaft to run true. If the shaft doesn't run true, you will get excessive vibration and eventually fracture the shaft or coupling.

John

Hi John

Why would the shaft "never run true"? All I have is a lathe and a welder. I thought I could weld a stub that would run true. How do you know I couldn't make the stub run true?? I wonder if you have been looking at some of the stuff I've been building lately. Or, maybe you know something I dont.

Jerry

It depends on how true you want the shaft to run. Any weld will pull a shaft one way or the other. IF you put a shaft on centers and check for runout after welding it I will garantee there will be some runout. One way to eliminate the runout is to re machine the shaft after welding. Another way is to straighten the shaft in a hydraulic press. On stainless shafts you can straighten them with some heat and some ice, properly applied.

If I were doing that job, I would cut a keyway in the spline after plugging the center hole. Then I would make a shaft with an internal keyway to go over the whole thing. I would have a keyway on both sides of the shaft.

John

Lots of ideas, thanks...

IF can find a metal shop to do the inner helical gear coupler that joins the motor shaft and the tranny shaft (in a reasonable price), would it work ok? I mean the helical splines would propably force the coupler to move towards the tranny when the rotating to a certain direction (sorry my english) - actually I haven=B4t yet checked if it=B4s the forward direction or not.

So, would that strain the tranny too much? The shaft isn=B4t designed to have forces in that direction I suppose. Should I try to lock the coupler in its place somehow?

-Oskari-

The gear will sustain the torque (otherwise It would have been bigger in the first place :-). An interference fit is not needed, a slip fit is. The torque would be about 450lbft for a 1500 rpm motor. This isn't an outrageous amount.

Remember that when it was working as a sun gear, the load would have been concentrated at two, three or four points, depending on the number of planet gears. With a coupling Loctited in, the load would be distributed evenly. If the motor needs an outboard bearing, that can run on the outside of the coupling shaft. Simple job!

Mark Rand RTFM