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Uniformity Measurement

The RFP-5 System can measure geometry and uniformity at a TU-Machine (Tire Uniformity Machine) at the same time. The uniformity measurement ascertain the forces that occurs between the tire and the road wheel. The road wheel simulates the road. Because of this reason the forces referring to the real forces between tire and road. Basically you make a distinction between lateral and radial forces.

All parameters necessary for the uniformity measurement will be administrated in the inspection plans and functional sequence plans of the RFP-5 System. It can totally control the whole operation of a TU-Machine. Besides it supports the calibration of the load cells.

Measuring principle for uniformity measurement

The load cells for the force distribution uniformity are connected via a special measuring amplifier to the RFP-5 Measuring Computer. For the lateral and radial force 2 measuring channels each are available. Each measuring amplifier converts the analog signal of the load cells directly to a 20 bit digital signal. This high resolution puts aside an otherwise usual similar load separation. Thus the complete force range is analyzable with highest resolution. The data transmission of these 4 raw signals to the RFP-5 Measuring Computer is made by a fiber-optic cable system of the company Beckhoff („II/O Lightbus“). Only in the computer these are linked together and filtered. The digital signal is filtered first with 16 Hertz (rotational speed: 60 min^-1) and then converted to the measured variables described in the following chapter. The analyses of the uniformity are based on 512 measured values per tire turn. The RFP-5 Datamanagement is able to display also the raw signals. Advantage: One can detect electrical and mechanical influences of noise of the machine in the raw signal.

Analysis of uniformity measurement

Lateral forces

Figure 1: Road wheel - tire

The rolling tire will affect lateral forces to the road wheel. The analysis of the lateral forces needs a determination of the sign in relation to the tire. This assumes the allocation between tire and load wheel.

The tire will be mounted with the DOT on the top. A lateral force that affects to the direction of the DOT is negative.

The direction of rotation refers to the DOT side and is defined as cw = right-hand rotation / ccw left-hand rotation.

Figure 2: Lateral forces

LSFT

The tire runs on the road wheel. After a calming down period it comes to a lateral force which stabilizes to a mean value. The mean lateral force established on the circumference is called Lateral Shift. You have to distinguish between LSFT.cw and LSFT.ccw.

LFPP

The designation for lateral force variation of uniformity measurement is called LFPP Lateral Force Peak to Peak. LFPP describes the lateral force variation that develops around the mean lateral force LSFT. LFPP is the difference between the biggest and smallest value of this variation. Also it is possible to calculate the angle of the high or low point of LFPP. You have to distinguish between LFPP.cw and LFPP.ccw.

LFHx

It can be done a FFT-Analysis (Fourier Transformation) for the course of lateral force variation LFPP. The established harmonic value is called LFHx Lateral Force Harmonic x. The „x“ means the 1^st to 10^th harmonic. Also it is possible to calculate the angle of the high and low point for LFH1 Lateral Force Harmonic 1 . You have to distinguish between LFHx.cw and LFHx.ccw.

CONY

The lateral forces LSFT of the tire can be distinguished concerning the direction of rotation (CONY Conicity). This can be caused by a conical manufactured tire or uneven spring constants of the tire.

CONY = (LSFT.cw + LSFT.ccw)/2 + conicity-offset

CONY.cw = CONY.ccw

Formula 1: CONY

PLYS

The lateral tire forces LSFT can be distinguished concerning the direction of rotation . This can be caused by a conical manufacurted tire or uneven spring constants of the tire. The reference direction for the PLYS Plysteer is the direction of rotation to the right. A positive PLYS means that the lateral force that affects the rotating tire would always point to the direction of a right-hand turning screw.

PLYS = (LSFT.cw - LSFT.ccw)/2

PLYS.cw = - PLYS.ccw

Formula 2: PLYS

Radial forces

Figure 3: Radial forces

LOAD

A road wheel is moved to the tire in the TU-Machine. This movement will be stopped as soon as the preselected force has been adjusted. Then LOAD is the average radial force (wheel load) that results during the run of the tire on the road wheel.

RFPP

The designation for the radial force variation for the uniformity measurement is called RFPP Radial Force Peak to Peak. RFPP describes the radial force variation that develops around the average radial force LOAD. RFPP is the difference between the biggest and the smallest value of this variation. It is possible to calculate also the angle of the high and low point of LFPP.

RFHx

It can be done a FFT-Analysis (Fourier Transformation) for the course of the radial force variation RFPP. The established harmonic value is called RFHx Radial Force Harmonic x. The „x“ means the 1^st to 10^th harmonic. Also it is possible to calculate the angle of the high and low point for RFH1 Radial Force Harmonic 1.

PEAK

The PEAK Analysis is based on the course of the curve of the RFPP over 360°. The algorithm uses an adjustable measuring window from 2° to 40°. The result is output as amount of the force value (peak to peak), that is determined within the measuring window. Further the output of the angle for the maximum value of PEAK is possible.

PDEPTH

Penetration Depth: By means of a rotation transducer, which controls and measures the traversing path of the load wheel, the penetration depth can be measured in mm. That is the way, that the load wheel moves toward the center of the tire starting from achieving the touching threshold. The touching threshold is to be determined in the inspection plan. It is the value of the radial force, in order to detect surely the "contact" of the tire by the load wheel.

Measuring accuracy for force distribution uniformity

The measurement uncertainity of the measuring system (load cell / measuring amplifier / RFP-5 Measuring Computer) without the mechanical influences of the machine is shown in following table:

Truck Tire Uniformity Machine

		measurement uncertainity
radial force	50.000N	+/-20N
lateral force	5.000N	+/-15N

Table 1: Measurement uncertainity at truck tire uniformity machine

Passenger Car Tire Uniformity Machine

		measurement uncertainity
radial force	8.900N	+/-0,3N
lateral force	4.400N	+/-0,15N

Table 2: Measurement uncertainity at passenger tire uniformity machine

For machines that comply mechanically to the manufacturer's specifications, the measuring accuracies are improved clearly by using of the RFP-5 Measuring System. In particular digital signal processing directly behind the load cells provides for an improved dissolution and intercarrier noise suppressor. There are no potentiometers etc.. A calibration of similar electronics is omitted. The digital system does not have problems with the longterm stability.
The measuring accuracy of the RFP-5 Measuring System is situated clearly over the mechanical possibilities of the machine. Therefore the data of repetition accuracy, standard deviations etc. is crucial, which you can take from the specification of the machine manufacturers.

Example: Retrofit of an Akron Standard D70

Original Specification from Akron Standard:

Specification 108-313-113
Model D70
Tire Uniformity Machines
Revision 4
September, 1988
Table A (page - 9) / Addendum I (page -17)

* Machine repeatability will be demonstrated by running 8 tires through the machine 10 times. The standard deviation (S) for each parameter for each tire shall be calculated using the equation shown below.

is defined as average of standard deviations (S) of 8 tires with 10 runs (N).

(Xj = individual value, X = arithmetic mean of all Xj)

Formula 3: Machine repeatability

Positioning, Marking, Grinding

Positioning

The position of the maximum values, or - as the case may be - the minimum values, can be output for all suitable measuring parameters. These positions can be used, for example, when turning the tire on the rim for a marking device.

Labeling/sorting tires

In line with the analytical results in each particular case, outputs can be set for the purpose of labeling or sorting the tires by grade.

Grinding

Size-controlled grinding of the tread is possible. An existing radial runout can be minimized by grinding. The grinding is kept under control by means of ongoing radial runout measurements and brought to an end as soon as a preselected target value is attained.