Shafting and bushing/bearing issues
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Q. We recently experienced a problem with our Tea weighing system. There was tea leakage from the weighers, whereby they were overhauled. Previously we were using normal Mild Steel shafts with phosphorus Bronze bushes. During overhaul, however these shafts were replaced with Chrome plated MS shafts.
When the machines were put back into operation most of these shafts seized. Upon analysis it was revealed that the chrome plated surface fused to the bronze bush upon heating - and restricted its movement.
My question is whether this was a correct hypothesis ?
Should this have happened?
How could it have been prevented ? Increasing lubrication, changing material etc. ?
Khawaja Abdullah- Manufacturing - Karachi, Sindh, Pakistan
Something about that doesn't sound right. Were the clearances the same as before?
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Jeffrey Holmes, CEF - Spartanburg, South Carolina |
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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.
Guillermo MarrufoMonterrey, NL, Mexico
March 20, 2012
Q. I am having bushing vibration issues while using a 50C hardened 440C stainless shaft (~20 microinch finish) with an SAE spec graphite impregnated bronze bushing. The 5/8" ID bushing is running at a .0005" - .001" diametral clearance. The bearing is stationary while the shaft spins. The bushing supports a 1.5 lb hanging mass. It appears that the bushing/mass system is vibrating while the shaft spins, creating the appearance of high friction and making noise. Has anyone had a similar problem? What can be done to fix it (different shaft material, plating, or clearance?). Thanks in advance!
Brandon Richardson- Palo Alto, California, USA
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.
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Jeffrey Holmes, CEF - Spartanburg, South Carolina |
