Hydrocarbon Processing - May 2021 - 13

Reliability

HEINZ P. BLOCH, RELIABILITY/EQUIPMENT EDITOR
Heinz.Bloch@HydrocarbonProcessing.com

Pushing the limit can have unexpected consequences
A prominent manufacturer rediscovered the old axiom,
" pushing the limit can have unexpected consequences, " after
experiencing many bearing failures in its 40-hp air blowers.
These lifetime-lubricated (sealed) bearings had been properly
installed on 20-mm shafts, and it can be assumed that premium
greases were being used. However, the blower rotors spin at
22,000 rpm, and the radial ball bearings did not last very long.
As one engineer noted, these bearings failures became a costly
repeat-warranty problem.
The manufacturer had selected these bearings based on
load-related information, and the operating loads were well below the rated or permissible values. Chances are, however, that
the manufacturer had not consulted FIG. 1, which is related to
re-lubrication intervals.
One of the characteristics of mechanical components is their
designed-in range of application. SKF America published the
recommended re-greasing intervals in FIG. 1; they are a function
of bearing bore diameter, shaft speed (rpm) and bearing type.
The highest rpm for a 20-mm bearing is shown as 16,000 rpm.
According to FIG. 1, this bearing would need to be re-lubricated
every 10 hr. Since it is lifetime lubricated and sealed, it cannot
be re-greased, although it will probably survive a few months of
operation on a sealed-in charge of premium grease. The curve
for the same 20-mm bearing operating at 22,000 rpm is off the
chart. It is reasonable to believe that its L-10 life (the number of

FIG. 1. Recommended re-lubrication intervals. Source: SKF.1

operating hours at which 10% of the bearings will have failed)
may be only a few weeks.
SKF recommends a " speed factor limit " of 500,000 mm/min
for either of the angular contact bearings in FIG. 2.1 The unsatisfactory blower bearings had similar contact angles, as depicted
on the left side of FIG. 2. Bearings with a variation in contact angles are shown on the right side of FIG. 2; these probably would
have given marginally better service. The contention that grease
lubrication will disappoint is based on the fact that the actual
speed factor for the bearings at issue is ([20 mm] × [3.14]) mm/
revolution × [22,000] revolutions/min = 1,381,600 mm/min.
Considerable research by bearing experts at FAG/Kugelfischer Georg Schäfer in the early 1970s argues against lifetimelubricated sealed bearings long before speeds approach 22,000
rpm.2 Its authors concluded that re-lubrication, even for open
bearings and well below 22,000 rpm, would need to be scheduled in intervals measured in weeks, not months. A small, dedicated, closed-circuit oil mist module would be well suited for
this blower and its 1,381,600 speed factor.
LITERATURE CITED
SKF, SKF General Catalogue, " 5,000 E, " June 2003.
2
Eschmann, P., L. Hasbargen and K. Weigand, Ball and Roller Bearings-Theory,
Design, and Application, 1st Ed., John Wiley & Sons, Hoboken, New Jersey, January
1985.
1

HEINZ P. BLOCH resides in Montgomery, Texas. He retired
as Exxon Chemical's Regional Machinery Specialist for the
U.S. and has authored or co-written more than 780
publications, among them 23 comprehensive books on
practical machinery management, failure analysis, failure
avoidance, compressors, steam turbines, pumps, oil mist
lubrication and optimized lubrication for industry. Mr. Bloch
holds BS and MS degrees (cum laude) in mechanical engineering from the
Newark College of Engineering (NCE). He is one of 10 inaugural inductees
into NCE's Hall of Fame, which honors its most distinguished alumni.

FIG. 2. Competent bearing manufacturers design contact angles to
ascertain favorable rolling motion and minimize skidding. Source: SKF.1
Hydrocarbon Processing | MAY 2021

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