Efficient Plant June 2022 - 21

feature | lubrication solutions
Fig. 2: Calculating film thickness
R
λ =
Average separation (A)
Composite surface roughness (R)
Under boundary lubrication, the specific film thickness (λ) is defined
as the ratio of the average separation between the moving surfaces.
A
film separation through appropriate viscosity selection? Again, the
situation is a little more nuanced. In hydraulic systems, too high a
viscosity causes lower mechanical efficiency, leading to increased
friction and heat generation that can result in sluggish operation
and even poor lubrication due to reduced flow rates. Likewise, some
of the parts we are trying to lubricate, for example the ring-to-vane
clearance in a vane pump, are a pressure seal, meaning that the separation
between surfaces needs to be small to generate pressure and
prevent internal leakage.
As such, viscosity selection for a hydraulic system is a balance
protect moving surfaces under boundary-lubrication conditions
where λ is less than 1. For the same reason, gearboxes with high
reduction ratios usually run hotter because the viscosity " mismatch "
between the high speed and low speed shafts generates significant
amounts of fluid friction at the high-speed end.
What about component types such as hydraulics? Hydraulic
pumps operate at high speeds so, presumably, we can achieve fullbetween
operational considerations (operating pressures, flow rates,
and component types) and the lubricating properties of the oil. In
the case of hydraulics, operational considerations usually mean that
we need to use an ISO VG 32, 46, or 68 fluid. That means most
hydraulic vane or piston pumps operate under mixed-film conditions
(regime B in Fig. 1). For this reason, most hydraulic fluids
contain anti-wear (AW) additives designed to provide increased
lubricity when λ is close to 1. EP
Mark Barnes, CMRP, is Senior Vice President at Des-Case Corp.,
Goodlettsville, TN (descase.com). He has 21 years of experience in
lubrication management, oil analysis, and contamination control.
The steel industry's top performing calcium
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daubertchemical.com | 1.866.719.4332
JUNE 2022
EFFICIENTPLANTMAG.COM | 21
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Efficient Plant June 2022

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