HOW TO MEASUREMENT OF FRICTION

Friction is defined as the force of resistance to motion that occurs when a solid body moves tangentially with respect to the surface of another body that it touches. The friction force acts in a direction opposite to that of motion. Even when an attempt is made to initiate the motion, the friction force exists. The friction force required to initiate the sliding is called the static friction force and that required to maintain sliding is called the kinetic friction force, the value of which is usually lower than the former for the same combination of material and other parameters. The basic principle of any friction measuring instrument is to place two specimens together under a specified normal load and in relative motion while the tangential force resisting motion is measured. Many methods of varying specimen geometry, loading condition, and resisting force measurement are available. Different researchers use many ingenious setups to investigate different specific cases.

1. Inclined-Plane Rig

The simplest arrangement is the inclined-plane test. A specimen is placed on a flat plane whose inclination with the horizontal is gradually increased until the specimen on it starts sliding.

If the inclination at this moment is θ, then,

𝜇s = tan θ.

Obviously, this method is not capable of evaluating friction in continuous sliding.

2. Pin-on-Disc Rig

In continuous-sliding cases, the rig based on pin-on-disc configuration is used. The pin is held stationary under a normal load while the disc is made to rotate. The loading can be provided by simple dead weight or by spring loading or hydraulic or pneumatic pressure.

The friction force is measured with the help of the calibrated tangential movement of a capacitive or inductive transducer mounted on the stationary specimen. For a multiple-pass arrangement the pin is held at a constant radial distance from the center of the disc, but in a single-pass arrangement it is moved radially during the experiment. Other standard arrangements such as pin-on-cylinder, crossed cylinder, and reciprocating arrangement.

3 Conformal and Nonconformal Geometry Rig

The test rigs can be classified into two groups depending on the test geometry: conformal and nonconformal. In the conformal geometry test, the profiles of the two contacting surfaces are matched carefully before the experiment is started. In this case the contact pressure is mode rate and normally held constant throughout the experiment. The test may then be used to simulate the situations such as brakes, thrust bearings, plane bearings, face seals, and clutches. On the other hand, in the nonconformal geometry test (with spherically profiled pin) contac tpressure is initially high because on first loading contact is made at a single point and with time pressure reduces due to development of small wear scars. This can be used to simulate the heavily loaded contacts such as gear teeth or to provide accelerated tests of friction and wear of a number of candidate material pairs for specified applications.

4 Environment Control

For accurate investigation the friction test must be carried out in an enclosed environment having simulated environmental conditions. Different friction pairs are susceptible to the presence of liquid lubricants, water vapor, gases, and so on. If pin-on-disc tests are carried out in the presence of liquid lubricant, the results vary due to poorly controlled hydrodynamic conditions at the interface. Sometimes the small deformations of the rig caused by the thermal loading or pressure loading may give rise to experimental scatter. So many other forms of test rigs have been developed for specific applications involving different environmental conditions.

* For example, space environments are simulated by the use of high-vacuum conditions where the entire test rig is installed inside the vacuum chamber.

5 Techniques for Friction Force Measurement

Two basic types of devices are commonly used for the measurement of friction force, viz.the piezoelectric force gauge and strain gauge transducer. Piezoelectric force gauges give a direct measurement of friction force as an electrical impulse that is recorded electronically.

Piezoelectric gauges operate by elastic deflection of a piezoelectric crystal and are sensitive to temperature, vibration, and corrosive agents. Piezoelectric gauges are also relatively expensive. Strain gauge beams are comparatively cheaper. In this case, friction force is usually measured from the bending of a beam arranged perpendicular to the direction of the friction force and strain gauges are mounted on the beam to record the deflection of the beam. Strain gauge beams are effective in recording steady friction forces where piezoelectric force gauges are unsuitable. The only difficulty with the strain gauge beam is its failure to record rapid change in friction force.

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