The invention herein described relates generally to friction test machines and methods and, more particularly, to a friction testing machine and method for measuring the coefficient of friction between a rubber specimen or a tread element and different friction surfaces at different speeds, contact pressures and orientations.
When developing new compounds for tires, a prior practice was to build a tire and subject the tire to various traction tests. Because of the cost and time needed to build test tires for the purpose of optimizing traction properties, small samples of tread compounds have instead been used to determine the traction characteristics of the compounds. By using small samples for testing, a large number of different tread compounds can be made in small batches for screening to determine which compound has the best properties. The small samples could also be tested with different friction test surfaces representing different road surfaces and different conditions, whereby the traction properties of the compounds may be observed to determine which compound has the best traction on a specific road surface under specific conditions.
In U.S. Pat. No. 5,113,688 there is disclosed an apparatus and method for predicting tire traction characteristics of tread compounds using small test samples. The apparatus causes a rotating relationship between the sample and friction surface. The torque between the sample and friction surface is measured and a torque versus time curve is established for the sample. Such apparatus, however, does not appear to be easily movable as may be desired for testing at different locations, nor is such apparatus suitable for evaluating the performance of different tread patterns. It would be advantageous to have a portable friction testing apparatus that not only can measure the coefficient of friction of plain tread elements, but also can measure the coefficient of friction of tread blocks, shoulders, etc., of existing tires. A further desirable advantage would be an apparatus that permits one to view the interaction between the friction surface and the specimen in the same way as the present invention.
The present invention provides a friction testing machine and method for measuring friction characteristics between a test sample and a friction surface. The machine and method are particularly suited for measuring the coefficient of friction between a rubber specimen or a tread element and different friction surfaces at different sliding velocities, contact pressures and orientations. A preferred embodiment of machine is self-contained and portable, configured for easy and quick changing of the friction surface, and provides for rotating the test sample about an axis normal to the sliding surface and the direction of movement of the sample relative to the friction surface.
In general, a friction test machine according to the invention comprises a sample holder configured to hold a sample in frictional engagement with a friction surface, and a motion device for effecting relative movement between the friction surface and sample holder in a first direction. Also provided is a variable weight loading device on the sample holder for loading the sample holder such that a selected load can be applied to the sample normal to the friction surface, and a force measurement device for obtaining a measurement indicative of the frictional force resisting such relative movement between the sample holder and the friction surface effected by the motion device.
Preferably, there is provided a second motion device for effecting relative movement between the sample holder and friction surface in another direction different than the first direction, such as rotation about an axis perpendicular to the friction surface and/or translation in a direction transverse to the primary direction of movement.
In one embodiment of friction test machine according to the invention, there is provided a base on which the friction surface is supported, and a carriage mounted on the base and guided for movement in a first direction parallel to the friction surface. The motion device is connected between the carriage and base for moving the carriage in the first direction, and the sample holder is mounted to the carriage for movement therewith and configured to hold the sample in frictional engagement with the friction surface during movement of the carriage in the first direction. In another embodiment of the friction test machine, the motion device causes relative movement between the carriage and the friction surface in a second direction opposite to the first direction so that the force measurement device can obtain measurements indicative of the frictional force in forward and reverse directions.
Preferably, the friction surface is provided on a friction element, and the friction element is removably supported on the base, whereby the friction element can be interchanged with other friction elements for providing respective different friction surfaces. In a preferred embodiment, the base includes a recess for removably holding the friction element, and the base is supported on a cart for easy transport of the machine from place to place.
In another embodiment of friction testing machine according to the invention, there is provided a carriage fixed to the base, and the sample holder is mounted in the carriage for vertical movement perpendicular to the friction surface. A table is mounted to the base for rotation about a vertical axis, and the friction surface extends annularly around a radially outer peripheral portion of the table, whereby the friction surface is moved while the sample remains stationary.
In still another embodiment of friction testing machine according to the invention, there is provided a base adapted to rest on a surface against which a sample is to be tested, such as a road surface. A carriage is mounted on the base and guided for movement in a first direction parallel to the friction surface. A motion device is connected between the carriage and base for moving the carriage in the first direction, and a sample holder is mounted to the carriage for movement therewith and configured to hold the sample in frictional engagement with the friction surface during movement of the carriage in the first direction. Preferably, the base is provided with an aperture through which the sample holder extends for positioning the sample against the friction surface disposed beneath the base.
In a preferred embodiment of the invention, the variable weight loading device includes a weight platform on which removable weights can be stacked and removed to selectively vary the load applied to the sample. Alternatively, the variable weight loading device comprises a fluid cylinder that is controlled by the processor to vary the load applied to the sample. It also is preferred to provide the friction surface on a friction element that is easily and quickly interchangeable with other friction elements for providing respective different friction surfaces.
In a preferred embodiment of the invention, a temperature chamber encloses the friction surface and the sample whereby temperature can be controlled to simulate different temperature conditions. Additionally or alternatively, the machine includes a water source and a pump which transfers water from the source onto the friction surface whereby the measurement device can obtain measurements indicative of the frictional force in wet conditions.
In a preferred embodiment of the invention, a processor controls the motion device, controls the variable weight loading device, controls the temperature chamber, and/or records the measurements obtained by the force measurement device. The processor further analyzes the measurements and compiles the results in the form of tables and/or graphs and an output device is provided to allow a readout of the compiled results. Preferably, the processor provides a library of pre-defined tests and a selection device for selecting a particular test.
The present invention accordingly provides a method of predicting tire traction characteristics of a tread component. The method comprises the steps of preparing a sample of the tread component; attaching the sample to the sample holder, setting the variable weight device to apply a certain load to the sample, and using the force measurement device for obtaining a measurement indicative of the frictional force resisting such relative movement between the sample holder and the friction surface effected by the motion device. During this method, the friction surface may be replaced with another different friction surface to predict tire traction characteristics with respect to different road surfaces. Also, water may be transferred onto the friction surface to gather measurements indicative of the frictional force in wet conditions. The temperature surrounding the friction surface and the sample may be controlled to simulate different temperature conditions, such as winter conditions and summer conditions.
The foregoing and other features of the invention are hereinafter fully described and particularly pointed out in the claims, the following description and the annexed drawings setting forth in detail one or more illustrative embodiments of the invention, such being indicative, however, of but one or a few of the various ways in which the principles of the invention may be employed.