Tire uniformity testing

#The invention relates to the field of tire uniformity testing machines and, in particular, to a technique for improving the repeatability of uniformity testing machines.

#An established part of the manufacture of tires is testing the tires for uniformity. After the tires have been manufactured, they are routed to a uniformity measuring machine. One example of a tire uniformity testing machine is described in U.S. Pat. No. 6,016,695, “Tire Uniformity Testing System”, which is incorporated herein by reference. A typical uniformity machine will automatically convey the tire into the machine, chuck it between two half-rims, inflate it, load it against a drum to simulate the weight of a vehicle, proceed with steps to test it, then mark the tire and sort the tire into one of several output conveyors. Loadwheel assemblies found in currently available uniformity testing machines are described in U.S. Pat. No. 5,979,231 “Loadwheel Assembly for Tire Testing Systems Having Conical Support Plates”, and U.S. Pat. No. 4,704,900 “Apparatus and Method for Imposing a Desired Average Radial Force on a Tire”, both of which are incorporated herein by reference. Some uniformity machines perform additional steps, such as grinding the tire to improve its characteristics and measuring geometric parameters of the tire.

#The testing steps on a typical uniformity machine consist of rotating the loaded, inflated tire, measuring the forces exerted by the tire on the drum as a function of the rotational position of the tire, performing calculations on those measurements to obtain measurements of the characteristics of the tire, reversing the rotation of the tire (a description of a spindle assembly that is used to mount and rotate the tire during testing is found in U.S. Pat. No. 5,992,227 “Automatic Adjustable Width Chuck Apparatus for Tire Testing Systems” which is incorporated herein by reference), repeating the measurement process, comparing the measurements of the characteristics in one or both directions to preset limits to obtain grades for the tire's uniformity and determining the output conveyor for further processing, placing these grading and sorting results into memory for later use in marking and sorting, and stopping the rotation at the correct orientation so that a mark can be placed at a required angular location on the tire, usually corresponding to the high point of the first harmonic of radial force variation.

#FIG. 1 is a flowchart representation of a method for operating a tire uniformity testing machine in accordance with an embodiment of the present invention; and,

#FIG. 2 is a flowchart representation of a method for operating a tire uniformity testing machine in accordance with an embodiment of the present invention.

#One cause of reduced repeatability in uniformity testing machines is that when the direction of rotation of the tire is reversed under load, the tire takes a “set” during this reversal, and this “set” affects the measurements made in the second direction of rotation, especially the measurement of radial force variations. It is believed that this effect is predictable based on the angular position of the tire at which reversal occurs, ranging from an increase in radial force variation, through little change, to a decrease in radial force variation. Since the radial forces in the two directions of rotation show a high degree of similarity, it is possible to control the reversal based on measurements made in the first direction such that little change in radial forces in the second direction occurs. This allows the machine to provide the true measure of the radial forces in the second direction, on a very repeatable basis.

#According to a preferred embodiment, the uniformity machine is operated according to the steps outlined in FIG. 1. In step 110, the radial force variation is measured in the current direction of rotation. The first harmonic of this force variation is calculated in step 120 and a tire position corresponding to either zero crossing of the first harmonic is determined in step 130. In step 140, the tire position is monitored until a zero crossing is reached. The rotation direction is reversed at the appropriate location in step 150 and the test is continued.

#While this technique will cause a slight increase in the cycle time due to delaying the reversal, this increase is more than made up for by allowing the “warmup” in the second direction to be reduced.