Shafting and bushing/bearing issues

A. Something about that doesn't sound right. Were the clearances the same as before?

A. The hypothesis is not only right, but I'm afraid to say is obvious. If the bronze was welded to the chromed shaft it had to be due to temperature, what else? Now, the situation sounds right to me. A chromed surface does not "wet" well with oil. For that reason, chromed surfaces requiring good lubrication must not be highly polished. As a matter of fact, they have to be provided with adequate oil "pockets" by leaving the ground finish a bit rough or applying porous chrome, such as the one used in engine piston rings.

A. Assuming the shaft is straight, it sounds like a harmonic vibration. Changing almost anything will stop it:

Shorter, longer, lighter or heavier shaft. More rigid bearing mount. Less rigid bearing mount. Moving the bearing to a different place on the shaft. Longer or shorter bearing length. Lighter or heavier load on the bearing. Change in rotational speed.

Try whatever is easiest, change one thing by 20% and see if it goes away.

A. Hi Dave. Bronze bushings run best on harder material, so weld it if you need 40 more years out of it. But brazing will probably hold for a decade :-)

But if it's just a plain shaft it seems easier to put in a replacement length of shafting.

Regards,

 Thanks for getting back so quick Ted,

Its my Grandfathers Hayter 56 mower that has been with him since new; he's now 84 years old. It's more a case of getting it back to a sensible state of repair so it actually drives itself. She's a fully aluminium body with a triple roller on the rear, one driven and the other 2 are on special one way bearings (like a bicycle rear cog).
The shaft has multiple key ways, etc., so would be a nightmare to source or remake -- that's why I asked :)
I think he's been pushing it for the last few years (though he won't admit it), so a quick repair with braze will I'm sure be sufficient for the few years use that it needs to be capable of doing.

A. Hi Greg. I think you'll need to send a sketch for people to understand you.

A properly deburred keyway probably wouldn't harm a bronze bushing unless the loads were so great that the whole idea was questionable anyway :-)
But the thing is, where there's a keyway there's supposed to be a key, and the key can tear up the bushing. A key is not supposed to be shorter than the keyway, and it's very likely to "walk" if it is. Running a keyway all the way to the end of a shaft makes it easier to machine but less secure.

A keyed taperlock bushing is the most secure way to hold wheels & gears & sprockets & couplings to a shaft. If the loading is light, a non-keyed taperlock bushing can work.

Maybe you're saying you want bushings on the end of shafts that have unused keyways on their ends? Adding a machined collar around the shaft, and using the O.D. of the collar as the bearing surface (if I'm understanding you) would increase the both the speed and distance traveled at the sliding surface between steel and bronze, and thus sounds like a bad idea. Good luck.

Regards,

A. Hi again Greg. Although I had a fair amount of experience with bronze bushings in my career, I don't actually have any personal experience with the 180° half bushings that you describe, although they sound okay.

Unfortunately, I do have some experience with keys 'walking' when they are shorter than the keyway and only held in place by a set screw in the key and one at 90° into the shaft. A key that is shorter than the keyway is quite possibly a design flaw that you really should try to fix. The ideal is probably rounded-end keyways with rounded-end keys (feather keys).

Please be sure to carefully think through whether any mishap or malfunction could overcome gravity and dangerously lift the shaft off those half bearings.

Pics and sketches can be e-mailed to

Regards,

(pic courtesy of couplinganswers.com)

A. Hi again Greg. Things can be held in place on shafts with setscrews in this fashion if there is no loading. For example if you have wheels on an axle and the axle is on bearings so the wheels and shaft turn together. That may be the situation you are describing with your roller.

But you can't use setscrews to transmit torque for a number of reasons, including that they are made of very brittle hardened steel which can snap. If you need to transmit torque from a shaft to a wheel or from a gear to a shaft, as a minimum the connection must be "keyed".

Regards,

 Thanks Ted,

Appreciate your advice - will look to redesign and avoid this issue.