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Sleeve bearings' tolerance for low-volume oil feed lets designers simplify lubrication setups and cut power loss.
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It's surprising how little lubricant oil-film sleeve bearings really need. Millions of bearings in electric motors, machine tools, pumps, and industrial fans run without a hitch on lubricant feeds that can't develop full hydrodynamic films of oil. In fact, at low loads and speeds, less oil cuts power loss and simplifies o il -feed -system designs. Often ring oiling, oil mist, wicks, or droplet feed will work in sizable slow-speed machines.
However, engineers need to understand load and speed limits and the damping characteristics of starved bearings to ensure safe operation. A good starting point is to follow manufacturers' guidelines. Engineers should also know how to calculate minimum oil-film thickness and power loss, as well as predict temperature rise from reduced oil supplies.
Minimum feed consequences
What's the worst that can happen to a lubricantstarved sleeve bearing? If the lubricant film doesn't separate bearing surfaces, they will wear and scuff, leading to surface destruction and bearing failures.
Even in cases where lubricant film is just below the minimum thickness, bearings can overheat and even seize if loads or speeds stray above the bearings' design limits. And starved bearings are less able to damp vibration from external components or misalignments.
In his 1984 book, Theory und Practice of Lubrication for Engineers, D.D. Fuller gave the following expression for a general estimate of the minimum oil feed, Q, (in.Vsec) necessary to lubricate a 360° sleeve bearing:
Q - (0.0043 + 0.0000185 P) × UCL
where P - radial unit loading, psi; U = surface velocity, in./sec; C - radial clearance, in.; and L = bearing length, in.
So a 5-in. -diameter, 5-in.-long bearing operating under a 100-psi unit load at 500 rpm (U= (500 rpm/60 sec/ min) × 5 in. × π = 131 ips), needs an oil-feed Q of 0.00015 in.Vsec. This is equivalent to one 0.008-in.3 drop of oil every 13 sec, 2 to 3% of the full feed rate.
Field observations further prove little oil is needed at low to moderate speeds: A 16-in. journal bearing ran without failure under load at 3,600 rpm for 15 min after it lost its supply...