Friction Testing at Rubber Consultants
At Rubber Consultants we have the capability to measure the friction between two materials in a wide range of conditions. The frictional force between two materials depends on a large number of different factors factors and is intractable to modelling except for a few, well definied, conditions. It has to be measured experimentally.
Friction is a very important property of rubber and is connected to the origin of its name, from one of its earliest applications, to rub out pencil marks. The largest use of rubber is in tyres, where the friction between the road and the tyre is vital. Many other uses of rubber are affected by its friction behaviour.
For most materials and conditions, the force required to slide two materials is directly proportional to the force holding them together. Accordingly, friction is usually quantified by a value known as the coefficient of friction which is the ratio between the force required to make the materials slide and the force pressing them together. However, for rubber, this relationship does not hold and the coefficient of friction falls with increased pressure. It is therefore necessary to measure the friction properties under conditions as realistic as possible. Although, it has been found that, with a number of measured results, an empirical equation called Thirion’s law could be used to find the behaviour at other loads (Measurement and Modelling of Friction of Rubber; J. Gough, A.H. Muhr, M. Kwong; in Constitutive Models for Rubber II, proceedings of The 2nd European conference on constitutive models for rubber, ed D Besdon, R H Schuster & J Ihlemann, publ. Balkema.
In order to accommodate the varying conditions encountered in service, Rubber Consultants possesses three different friction testing machines. The first is a general purpose machine designed by Plint. The second is a pendulum skid tester. The third is a pneumatically loaded high pressure rig.
The Plint machine is a general purpose computer controlled machine. It is capable of speeds from 0.01-10mm/sec and loads of up to 30N. It incorporates a heated stage for high temperature testing. It has been used for a wide range of situations, from pen nibs to Formula 1 tyre materials.
The pendulum skid tester is used to perform tests according to British Standard BS 7976. It can be used to compare different floor or road surfaces, against a standard rubber, or different rubber compounds, against a standard surface.
The high pressure rig used a pneumatic system to achieve loads of up to 5,000N. With the designed rubber mouldings, it can achieve pressures of 15 MPa. It is designed for situations such as bridge bearings.