Shaft Diameter Of Motor

Bending force?

How will the shaft be supported?

Is there any additional torsional load?

I am an electrician, we had 3 instances when the shaft bent, mechanical engineers had made no progress regarding this, common sense tells me we need to support the shaft or increase its diameter, but just wanted to hear from someone else.

That would be the logical answer. Some manufacturers that offer extended shaft motors do recommend the method.

http://www.freestudy.co.uk/c103/t3.pdf

On back-of-envelope estimate, it looks as though you would need 54mm, but that would need to extend all the way through the motor, which might not be possible. An outboard mounted bearing is probably better.

The relevant term here is overhung load, which is somewhat related to torsional load.

Dear Mr.eltech,

From resonance point of view - the extension should be analysed - what will be the impact is to be assessed.

Then the bending moment impact is to be compensated in terms of dia.

DHAYANANDHAN.S

Without more information about the application for this motor, I am afraid all these responses are more or less shots in the dark. Some analysis needs to be made as to why the problem exists in the first place, what was modified when, and when did the problem(s) appear?

Is this a stationary mounted motor, or is it on moving equipment with heavy overhung loading? Is it a mixing motor for a large reactor? See? the game of 20 questions has already begun.

The only way the shaft would be in a bending mode is when the load is moved from the 110 mm location to the 300 mm location. That assumes the load is a sheave (s) driving a belt. If that is the case, where could you place an additional bearing? Between the motor and the sheave? not a good idea. You might want to increase the length of the shaft so a pillow block could be added at the outboard end.

If you must extend the shaft, then do as several others have suggested; add additional length if necessary for a bearing on the far side of the gear, sprocket, or pulley.

If possible, a better choice would be to change the motor mount so you can move the motor and leave the motor shaft as is.

What are the characteristics of the load ? Radial,axial, or a combination of the two( most likely). Also is there a gear, sprocket or coupling on the shaft ? If so overhung load could be an issue.

To try to add to what others have said - I have a motor catalogue which gives permissible radial loads (and axial, but it's the radial that's causes your problem). As the electrical guy you should be able to get hold of that and give the figures to your mech engs.

18.5kW 2-pole is frame 160L. Various figures, but picking ball bearing and 20000 hours life, force = 2980N applied at bottom of shaft (closest to body), 2310N applied at end, pro-rata in between. These are based on what the bearings can take, if the shaft is bent it sounds like it's been grossly overloaded.

The catalogue doesn't say how to calculate permissible load if it's applied further out as you're doing, but could be estimated using the distance between the bearings, ~ 500mm.

Just for interest, I worked out the radius the force would need to be applied at, if it was all due the motor power, eg a V-drive. Came to 20mm, ~ shaft radius. Typical V-pulley more like 75mm radius, so force lower. Of course there might be external forces which must also be allowed for.

Beefing up the shaft (external to the motor) wouldn't help.

As others have said, best to ensure the load applies little or no radial load, eg by adding an extra bearing. Then the shaft extension can be as ling as you like.

<...extend the length to 300mm...> How - weld a longer bit on (rhetorical question - NNTR)? How would one know that the centre of rotation of the extended shaft and the existing shaft were contiguous (rhetorical question - NNTR)?

It would be far better to design the housing on the driven equipment to accommodate the motor shaft. In that respect it would be of larger diameter_{, which was what was asked}.