Efficient Plant June 2022 - 20

feature | lubrication solutions
friction, thereby reducing wear, the Stribeck curve and, in particular,
the Hersey number help us to understand the primary lubricantselection
question: What viscosity of oil should I use to minimize
friction? As an example, let's consider lubricant selection for a plain
(journal) bearing under high load.
BASIC VISCOSITY SELECTION
When the shaſt is at rest on the bearing (N=0), the system operating
condition is to the far leſt of the Stribeck curve. As the shaſt starts
to turn within the bearing, N (speed) is low, and P (load) is high.
In other words, the overall friction between the shaſt and bearing is
high, assuming the machine is not equipped with any form of hydrostatic
liſt ing mechanism. Th is corresponds to area A in the Stribeck
curve, which is referred to as boundary lubrication. Under boundary
lubrication, the specifi c fi lm thickness (λ), which is defi ned as the
ratio of the average separation between the moving surfaces (Fig. 2,
p. 21) is less than one.
As speed increases, the operating condition transitions from
boundary (the A regime in Figure 1) to the B regime, referred to as
mixed fi lm. Under these conditions, the specifi c fi lm thickness (λ) is
close to 1.0, meaning the average separation of surfaces and composite
surface roughness are roughly equal. Th is is the minimum speed
for the applied load and viscosity for which true metal-to-metal
separation occurs due to oil viscosity.
Th e last regime is full fi lm, denoted as regime C in Figure 1. Fullfi
lm separation is only achieved at very high speeds and/or very low
loads and corresponds to λ values exceeding 3.0. Under full-fi lm
conditions, there is complete separation between moving surfaces
inside the machine. Interestingly, even under full-fi lm separation,
friction does not equal zero and, in fact, continues to rise as speeds
increase (Figure 1). While this may seem counterintuitive, increased
friction under high-speed/full-fi lm separation is due to friction
within the lubricant, caused by sliding motion between discrete oil
molecules under laminar fl ow. Th is is referred to as fl uid friction
and is one of the reasons why even properly lubricated components
generate heat.
Based on the Stribeck curve, we can now start to make sense of
many of those OEM viscosity recommendations. For low-speed
applications, we tend to use higher viscosity. In high-speed conditions,
we choose a lower viscosity.
For journal bearings, such as those found in turbomachinery,
lubricant selection is straightforward. Choose a lubricant that, under
normal operating loads, speeds, and temperatures, allows the system
to operate under full-fi lm hydrodynamic conditions, where λ>3.0.
Doing so will minimize mechanical friction and maximize machine
life. Most journal bearings of this type use rust- and oxidation-inhib20
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Fig. 1: Stribeck curve
B
A
Speed x Viscosity
Load
= Hersey number
The Stribeck curve illustrates how friction and fi lm thickness interact
in the three Hersey number regimes: A is boundary lubrication, B is
mixed fi lm, and C is full fi lm.
ited (R&O) oil with an ISO viscosity grade in the 32 to 68 cSt range.
So lubricant selection is simple: Choose a lubricant such that fl uid
viscosity at operating temperature corresponds to a Hersey number
and hence the specifi c fi lm thickness that is in the full-fi lm range
(regime C in Figure 1)?
Not so fast! While this may work for journal bearings, for other
applications we need to consider factors such as how the lubricant
is supplied to the lubricated components and how the machine
functions.
COMPLEX CONDITIONS
As an example, think about a gear reducer driven by a 1,740-rpm
motor, with an output shaſt speed of 70 rpm. With such a wide
variation in speed, how do you chose the right viscosity? While the
(high-speed) input-shaſt bearing may demand a low viscosity, the
slow-speed output-shaſt bearing and associated gearing needs a
higher viscosity.
But not too high. Since most smaller gear reducers are splash
lubricated, we need a lubricant to be fl uid enough to be adequately
distributed to the internal lubricated components.
In reality, lubricant selection for a gear reducer is typically based
on the pitch-line velocity of the slow-speed gear and is a trade-off
between viscosity and fl uidity. While it's true that, for gearboxes,
we select a higher viscosity base oil to try to drive the system to the
right-hand side of the Stribeck curve, the reality is, at the low-speed
end, most gear reducers are operating under boundary lubrication.
Recognizing this problem, gearbox OEMs usually recommend
gear oils fortifi ed with extreme pressure (EP) additives designed to
JUNE 2022
C
Film thickness
Friction
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Efficient Plant June 2022

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