Heinz P. Bloch articles
Lubrication Update for Electric Motors
by Heinz P. Bloch, P.E.—[email protected]*
API-610, the most widely used pump standard in the petrochemical and refining industries,
includes experience-based recommendations for lubricant application and one of these
relates to oil mist. The API standard asks for oil mist to be routed through the bearings,
Figure 1, instead of past the bearings, Figure 2. Although intended for pumps, this same
recommendation will work, equally well for electric motor rolling element bearings. The
resulting diagonal through-flow route guarantees adequate lubrication, whereas oil mist
entering and exiting on the same side might allow some of the mist to leave without first
wetting the rolling elements. Through-flow is thus one of the keys to a successful
installation.
Major electric motor manufacturers, including the former Reliance Electric “”Company
(Cleveland, Ohio), were fully aware of this fact. Representing “Best Technology,” their mid1970s bearing housings were configured for through-flow. Moreover, a very wide oil mist
suitability range is documented for electric motors. Industrial giant Siemens has, years
ago, published technical bulletins showing pure oil mist as a superior technique for electric
motors ranging in size from 18 to 3,000 kW.
Bearing size constraints and synthetic lubricants for electric motors. Decades of
experience confirm the success of oil mist for rolling element bearings in the operating
speed and size ranges found in motors for process pumps. Since about 1960, empirical
data have been employed to screen the applicability of oil mist. The influences of bearing
size, speed, and load have been recognized in a rule-of-thumb oil mist applicability
formula, limiting the parameter “DNL” (D= bearing bore, mm; N= inner ring rpm; and L=
load, lbs) to values below 10^9, or 1,000,000,000. An 80 mm electric motor bearing,
operating at 3,600 rpm and a load of 600 lbs, would thus have a DNL of 172,000,000--less than 18% of the allowable threshold value. As of 2014, approximately 26,000 oil mist
lubricated electric motors are operating flawlessly in reliability-focused user plants.
Capitalizing on this favorable experience, the procurement specifications for both new
projects and replacement motors (with rolling element bearings) at many of these plants
require oil mist lubrication in sizes 15 kW and larger.
Although it was well known that synthetic lubes reduce friction, little quantitative work had
been done before 1980. Morrison, Zielinski and James (Ref. 1) quantified how diester
fluids reduce the frictional power losses of industrial equipment; their findings are
summarized in Tables 1 and 2. The potential cost savings through power loss reduction
are quite substantial. It has been estimated that industrial machines consume 31% percent
of the total energy in the United States (Ref. 2). As much as 5% of the mechanical losses
of these machines could be avoided through a combination of improved equipment design
and lubricant optimization.
Motor sealing. Motor sealing and mist drainage are well understood. Although oil mist
will neither attack nor degrade the epoxy insulation on electric motor windings
manufactured since the mid-1960’s, mist entry and related sealing issues merit inclusion
in this overview. Regardless of motor type, i.e. TEFC, X-Proof or WP ll, cable terminations
in junction boxes should not be made with conventional electrician’s tape. The adhesive in
this tape will last but a few days and then become tacky to the point of unraveling. Inferior
products are replaced by superior materials; these are often Teflon®-based. For
termination leads (“T-Leads”) competent motor manufacturers use an irradiation crosslinked polymeric insulation system that is highly resistant to oil mist. To date, irradiation
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cross-linked polymeric insulation systems have consistently outperformed the many other
“almost equivalent” systems.
Similarly, and while it must always be pointed out that oil mist is neither a flammable nor
explosive mixture, it would be unsightly to allow a visible plume of mist to escape from the
junction box cover. The wire passage from the motor interior to the junction box should,
therefore, be sealed with 3M Scotch-Cast Two-Part Epoxy potting compound. Doing so
will exclude oil mist from entering the junction box.
Finally, it is always good practice to verify that all electric motors have a small (3 mm) weep
hole and that XP-motor drains are given closer attention. The latter are furnished with
either an explosion-proof rated vent or a suitably routed weep hole passage at the bottom
of the motor casing or lower edge of the motor end cover. Intended to drain accumulated
moisture condensation, the vent or weep hole passage will allow coalesced or atomized
oil mist to escape. Note, however, that explosion-proof motors are still “explosion-proof”
with this passage. Reasoning on the issue should convince us that a motor with its interior
slightly pressurized by non-explosive oil mist cannot ingest any explosive vapors from a
surrounding atmosphere. The suitability of oil mist for Class 1, Group C and D locations
was specifically re-affirmed by Reliance Electric in July of 2004.
TEFC vs. WP ll Construction. On TEFC (totally enclosed, fan-cooled) motors, there are
documented events of liquid oil filling the motor housing to the point of near contact with
the spinning rotor. Conventional wisdom to the contrary there neither were, nor will there
be, detrimental effects with the oils used in normal industry. The motor could have run
indefinitely! TEFC motors are suitable for oil mist lubrication by simply routing the oil mist
through the bearing, as has been explained in a comprehensive text on lubrication. There
are numerous other references, including API-610. No special internal sealing provisions
are needed with pure oil mist filling a TEFC motor as long as the pressurized mist keeps
dirty atmospheric air from entering.
On weather-protected (WP ll) motors, merely adding oil mist has often been done and has
generally worked surprisingly well. In this instance, however, it was found important to lead
the oil mist vent tubing away from regions influenced by the motor fan. Still, weatherprotected (WP ll) electric motors do receive additional attention from reliability-focused
users and knowledgeable motor manufacturers.
Air is constantly being forced through the windings and an oil film deposited on the
windings could invite dirt accumulation. To reduce the risk of dirt accumulation, suitable
means of sealing should be provided between the motor bearings and the motor interior.
Since V-rings and other elastomeric contact seals are subject to wear, low-friction face
seals are considered technically superior. The axial closing force on these seals could be
provided either by springs or small permanent magnets (Ref. 3). Also, many modern
motors use advanced rotating labyrinth seals with closure O-rings that travel axially (see
Refs. 4 and 5).*
*It should be noted that the author does not advocate rotating labyrinth seals with O-rings
that could potentially make contact with sharp-edged grooves. For best performance Orings should move in the axial direction as in a LabTecta™ Bearing Isolator
As is so often the case, the user has to make intelligent choices. Some low friction axial
seals (face seals) may require machining of the motor end caps, but long motor life and
the avoidance of maintenance costs will make up for the added expense. Double V-rings
using Nitrile® or Viton® elastomeric material are sometimes used because they are
considerably less expensive than face seals.
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Sealing to avoid stray mist stressing the environment. Even when still allowed under
prevailing regulatory environmental regulations (e.g. OSHA or EPA), air quality and
environmental concerns make it desirable to minimize stray oil mist emissions. It is helpful
to recall that state-of-art oil mist systems are fully closed, i.e. are configured so as not to
permit any mist to escape. The various bearing housings are sealed with the magnetic
seals incorporated in the motor end bells in Figure 1; alternatively, advanced rotating
labyrinth seals could be installed (Refs. 4 and 5).
Combining effective seals and a closed oil mist lubrication system has, for many decades,
represented a well-proven solution. The combination not only eliminates virtually all stray
mist and oil leakage, but makes possible the recovery, subsequent purification, and re-use
of perhaps 97% of the oil. These recovery rates make the use of more expensive, superior
quality synthetic lubricants economically attractive.
For many years PAO and diester-based “synthetic” lubricants have proved to embody
most of the properties needed for extended bearing life and greatest operating efficiency.
These oils excel in the areas of bearing temperature and friction energy reduction. It is
not difficult to show relatively rapid returns on investment for these lubricants, especially
when the system is closed and the lubricant is being re-used after filtration (Ref. 3)..
Closed systems and oil mist-lubricated electric motors give reliability-focused users
several important advantages:



Compliance with actual and future environmental regulations
Extended bearing life and reduced electric motor maintenance budgets
The technical and economic justification to apply high-performance synthetic oils
PAO and diester-based “synthetic” lubricants embody most of the properties needed for
extended bearing life and greatest operating efficiency. These oils excel in the areas of
bearing temperature reduction (Figure 3) and friction energy reduction (Figure 4).
A composite plot of different changes and power reduction percentages is given in Figure
5. It is not difficult to show relatively rapid returns on investment for these lubricants,
especially for closed systems where the lubricant is re-used after filtration.
Converting grease-lubricated electric motors to pure oil mist. When converting
operating motors from grease lubrication to oil mist lubrication, consider the following
measures in addition to the above:
a.
Perform a complete vibration analysis. This will confirm or rule out pre-existing
bearing distress and will indicate if such work as re-alignment or base plate
stiffening is needed to avert incipient bearing failure
b.
Measure the actual efficiency of the motor. If the motor is inefficient, consider
replacing it with a modern high efficiency motor, using oil mist lubrication in line
with the above recommendations. This will allow capture of all benefits and will
result in greatly enhanced return on investment
c.
Evaluate if the capacity of the motor is the most suitable for the application.
“Most suitable” typically implies driven loads that represent 75% to 95
% of
nominal motor capacity. The result: Operation at best
Note that converting an overloaded, hot-running electric motor to oil mist lubrication will
lead to marginal improvement at best
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The required volume of oil mist is often expressed in “bearing-inches”, or “BIs.” A bearinginch is the volume of oil mist needed to satisfy the demands of a row of rolling elements in
a one-inch (~25 mm) bore diameter bearing. One BI assumes a rate of mist containing
0.01 fl. oz., or 0.3 ml, of oil per hour. Certain other factors may have to be considered to
determine the needed oil mist flow and these are known to oil mist providers and bearing
manufacturers. The various factors are also extensively documented in several references;
they are readily summarized as:
a.
b.
c.
d.
e.
f.
Type of bearing. The different internal geometries of different types of contact
(point contact at ball bearings and linear contacts at roller bearings), amount of
sliding contacts (between rolling elements and raceways, cages, flanges or
guide rings), angle of contact between rolling elements and raceways, and
prevailing load on rolling elements. The most common bearing types in
electrical motors are deep groove ball bearings, cylindrical roller bearings and
angular contact ball bearings.
Number of rows of rolling elements. Multiple row bearing or paired bearing
arrangements require a simple multiplier to quantify the volume of mist flow.
Size of the bearings, related to the shaft diameter---inherent in the expression
“bearing-inches.”
The rotating speed. The influence of the rotating speed should not be
considered as a linear function. It can be linear for a certain intermediate speed
range, but at lower and higher speeds the oil requirements in the contact
regions may differ from straight linearity.
Bearing load conditions (preload, minimum or even less than minimum load,
heavy axial loads, etc.)
Cage design. Different cage designs may affect mist flow in different ways. It
has been reasoned that stamped (pressed) metal cages, polyamide cages, or
machined metal cages might produce different degrees of turbulence. While
different rates of turbulence may cause different amounts of oil to “plate out” on
the various bearing components, the concern vanishes when oil mist is applied
in through-flow mode.
Using the right bearing and a correct installation procedure. Very significant
increases in bearing life and overall electric motor reliability have been repeatedly
documented in the half century since 1960. Of course, oil mist cannot eliminate basic
bearing problems. It can, however, provide one of the best and most reliable means of
lubricant application. Bearings must be:







Adequate for the application, i.e. deep groove ball bearings for coupled drives,
cylindrical roller bearing to support high radial loads in certain belt drives, or angular
contact ball bearings to support the axial (constant) loads in vertical motor
applications
Incorporating correct bearing-internal clearances
Mounted with correct shaft and housing fits
Carefully and correctly handled, using tools that will avoid damage
Correctly assembled and fitted to the motor caps, carefully avoiding misalignment
or skewing
Part of a correctly installed motor, avoiding shaft misalignment and soft foot, or
bearing damage incurred while mounting either the coupling or drive pulley
Subjected to a vibration spectrum analysis. This will indicate the lubrication
condition as regards lubricating film, bearing condition (possible bearing damage)
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and general equipment condition, including misalignment, lack of support (soft
foot), unbalance, etc.
Sealing to avoid stray mist stressing the environment.
Closed systems and oil mist-lubricated electric motors give reliability-focused users
several important advantages:



Compliance with actual and future environmental regulations
Convincing proof that oil mist lubrication benefits electric motors and the
maintenance budget
The technical and economic justification to apply high-performance synthetic oils
Since the 1980’s, modern additives technology has further strengthened wear protection
and offers reduced energy consumption with other synthetic base oils and without requiring
reductions in viscosity. All are worthy of your consideration.
*This translation from an original article in Hydrocarbon Processing was authorized by Heinz P.
Bloch, 11 January 2015, for publication in Brazil by www.tecem.com.br
References
1.
Morrison, F.R., Zielinsky, J., James, R., “Effects of synthetic fluids on ball bearing
performance”, (1982) Transactions of the ASME, Journal of Energy Resource
Technology, Vol. 104, pp.174-181
2.
Pinkus, O., Decker, O., and Wilcock. D.F. “How to save 5% of our energy,”
Mechanical Engineering, September 1997
3.
Bloch, Heinz P.; “Practical Lubrication for Industrial Facilities”, 2nd Edition
(2009),The Fairmont Press, Lilburn, GA 30047
4.
HP In Reliability Column, Hydrocarbon Processing, July 2006
5.
Bloch, H. P.; “Pump Wisdom: Problem Solving for Operators and Specialists,”
(2011), John Wiley & Sons, Hoboken, NJ 07030
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Figure 1: Oil mist routed through electric motor bearings
Figure 2: Oil mist applied to the center of a bearing housing is not providing optimal
lubrication; much of the mist is simply flowing from entry to drain
Average temperature rise plot for the ball bearing test
80
AVG. TEMP. RISE (ºK)
70
60
50
40
30
20
10
0
MN 68
OIL SUMP
SYN 32
OIL MIST
Figure 3: Average temperature rise plot for the ball bearing test of Ref. 1
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Power loss per bearing (KW)
L = 8.9 KN (2000 lbf)
MIN 68
SYN 32
Oil Sump
0.271
0.254
Oil Mist
0.192
0.169
Table 1: Overview of power loss with different oils and application methods (Ref. 1)
Power Loss plot of ball bearing test
POWER LOSS PER BERING (KW)
0,300
0,250
0,200
0,150
0,100
OIL SUMP
MIN 68
OIL MIST
SYN 32
Figure 4: Power loss plot for the ball bearing test of Table 13-9.
Note that two different oils are used at two different viscosities (Ref. 1)
Change
D Power loss per bearing Total reduction
Sump:MIN 68 to SYN 32
0.017
6%
Mist: MIN 68 to SYN 32
0.022
8%
Sump MIN 68 to Mist MIN 68
0.080
29%
Sump SYN 32 to Mist SYN 32
0.085
31%
Sump MIN 68 to Mist SYN 32
0.11
38%
Table 2: Overview of power loss and loss reduction percentages with
different oils and application methods (Ref. 1)
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Total reduction
40%
38%
35%
29%
30%
31%
25%
20%
15%
10%
6%
8%
5%
0%
SUMP:MIN 68
TO SYN 32
MIST: MIN 68
TO SYN 32
SUMP MIN 68
TO MIST MIN
68
SUMP SYN 32
TO MIST SYN
32
SUMP MIN 68
TO MIST SYN
32
Figure 5: Plot of different changes and power reduction percentages that resulted
(Ref. 1)