What's The Purpose of a Key into a coupling ?

Question

Apparently I had an argument with my senior Fitter he always doesn't take me serious and today we had an argument well maybe he's right but it's best to ask here.

we have a booster fan that we use to cool the pay reel motors on the basement apparently there's one motor that drives the fan happen to die or burn out.
So we had a spare motor but the shaft of the motor is 34.5mm and the diameter (hole) coupling of the booster fan/impeller is 38mm so the difference is huge here.
So he insisted we have to fit the motor on the coupling and machine a key that will fit between the two and it will hold them together. So I told him that it won't be possible even if we can fit a key cos the key will break and the motor will run alone. So I had some question

1.what's the purpose of a key in a coupling ?
2.what will happen if you fit a coupling without a key?
3. What will happen if we fit the key in this situation ?

Thanks.

Answer 1

Let's start with a 3.5 mm difference between shaft and fan coupling. Too much. .15mm is plenty of clearance. The fan has to rotate concentrically about the shaft and will not with that much difference in diameter. Not only will it not work right, it will be dangerous to turn on the first time. Get a different fan or motor that fits. Would you put an 18" tire on a 16" wheel? This is equally stupid. The purpose of the drive key is to maintain rotational orientation and transmit torque reliably, at least that's what the equations say. You still want a slip fit of the shaft to the Fan coupling with key ways in both

Answer 2

I am not going to try and argue the details given in the 3 somewhat conflicting answers, but a key has the purpose of locking the outer surrounding material (sleeve, coupling, pulley, or machined part) to a shaft. Unless purposely done to provide an emergency shear - like on a propeller shaft on an outboard motor, the key is as strong as the shaft and is not going to break. The purpose is to keep the shaft fixed in position with what it is turning without attempting the damage a setscrew can do trying to tighten it down. The key spreads the force over a much wider area than a set screw.

Answer 3

Linery movement .

In purpose having acceleration and at the same time shifting position in a linear move this method was invate .

Other type of key and coupling is to luck the shaft inside of the coupling ,where pressure shook of the engaging does not cause coupling rotate freely around the shaft .

Quiet example of combination of both method is in radial drill shaft .

Answer 4

A drive coupling almost always has a key It slides into a single spline. No they are not that strong, but a fan will not strip it. Now what you really need is a shaft bushing and you can buy that very inexpensive, it will still have a key and you should use it, Tighten the allen screw and you are set.

Answer 5

That's a bit tricky. The difference is 3.5 mm, so the bushing would only be 1.75 mm thick, which will be tricky to turn, because it will collapse if you cut too deeply. You would have to be very, very gentle with the tool. And yes..you need two keys now, because the rotor can slip on the bush, and the bush can slip on the shaft. I think you need a new motor with a 38 mm shaft, or fix the old motor.

Answer 6

1. The key transfers the torque of the drive to the pulley or gear.



2. Unless you have a shrink fit, the shaft will spin but the pulley or gear will tend to drag on the shaft and you will have a failure.



3. Because the diameters are not matched the coupling will not be centered on the shaft and even with a key you will likely have vibration because of the coupling is off center.

Answer 7

You could probably machine a sleeve with a slot in it, then use a key that is 1.75mm thicker than the old one was. However a custom sleeve and custom key would be kind of an expensive solution, much easier to find the right motor for the job or replace both the motor AND the fan.

Answer 8

Do the shaft and Fan have key-ways in them?

Either way you may need to make a coupler.

A key-way and key prevent relative slippage between a shaft and the hole it fits into.

Some Keys are designed to shear in the event of an overload.