Rolling Element Bearing
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The flow and friction behavior of lubricants strongly influence the
lubrication performance. Rheology and tribology are established
techniques for the characterization of lubricants in their early state
of development. Both techniques give insights into the lubricant’s
behavior during the application.
The Physica MCR rheometer, equipped with suitable
accessories, enables rheology and tribology with one single instrument
and allows direct performance measurement of rolling element bearings.
Mineral oils are newtonian liquids which means that the viscosity
is constant independent of the applied shear rate. Greases exhibit
viscoelastic flow behavior, the apparent viscosity is therefore
dependent on the applied shear. A Physica
MCR rheometer measures the viscosity and determines the
viscoelastic properties allowing you to design oils and greases which
have the desired consistency, viscosity and flow behavior at given
Diagram: Amplitude sweep on greases of NLGI classes
0 (blue), 1 (red), 2 (black). NLGI class 2 has the highest structural
strength, and NLGI class 0 the lowest.
The Physica MCR rheometer can be equipped with a
tribological accessory which turns the instrument into a
high-performance tribometer. The system is based on the
ball-on-three-plates principle in which a ball is pressed on
self-centering plates with a given normal load up to 70 N. The
instrument can be run in speed-controlled and torque controlled mode,
allowing the determination of Stribeck curves and static friction in
the temperature range from -40 to 200 °C. Further test types can be
designed, programmed and displayed in the flexible software.
Diagram: Stribeck curve measured with a steel ball
on dry (blue) and lubricated (red) Polyoxymethylene (POM). Stribeck
curve measured with a steel ball on lubricated (green) steel plates.
Rolling Element Bearing
The best way to check the performance of a lubricant is during the
application because it is almost impossible to simulate all influencing
environmental factors within a model system. The reliability of the
result increases the closer a model system represents the application.
Greases are used in various types of rolling element bearings under
different conditions. A ball bearing holder for the tribology device
was therefore designed so that lubricated bearings are directly
measurable. It allows direct performance measurements of the lubricant
in the bearing within the temperature range from -40 up to 200°C.
Diagram: Speed ramp of a grease (NLGI 0) lubricated
bearing at –40 °C (red), 25 °C (blue) and 60°C (black).
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