Since the tubeless tires and especially cast alloy wheels are used, the air tightness of wheels is an important factor of the automobiles quality. Based on specification of the car industry that up to 10% decrease of the prescribed nominal tire pressure during a time of six-month is allowed, the requirements presented in specifications and norms are treated and validated. The practical experience and influences on the wheel tightness control are discussed and the data presented in a report of a wheel manufacturer, concerning the replacements of wheels in service due to air leakage are evaluated. Summarizing the results of analyses, a proposal is made for the testing of the cast aluminum car wheels to meet the requirements for a reliable and economical air tightness control in modern test facilities.
Wheels are safety-related components which have paramount importance for vehicle safety and function. Although they are loaded in a complex manner and are highly stressed during operation, a light-weight and a car related individual design is the primary requirement. Taking into account the possible lowest wheel weight and production costs as well as the possibility of a sophisticated design, the light alloy cast wheels are one of the best options especially for the premium class cars.
To prove the wheel durability, standardized tests like “rotating bending test” (for approval of the wheel disc), “rim roll test” (for approval of wheel rim) and “test under service-like loading in the biaxial test facility” (for approval of the entire wheel) are used; to approve the strength of light cast alloy wheels under special event loading, like hitting a curbstone, also the tests under impact loading must be made [
Introducing the casting technology for the production of automotive wheels, the air tightness of rims became an important factor concerning their quality assurance. The air tightness of wheel rim is not influencing the car safety but it influences its function and usage disposition. For the product reliability [
Similar problem (and recall actions) occurred also with the wheels from aluminum alloys with large rims and low profile tires for premium class cars as shown in
For the approval of cast aluminum alloy wheels it was important to check the micro-homogeneity of the rim area, not detectable by used X-ray inspection, which can lead to the tire inflation pressure decrease at usage. Based on agreement of car manufacturer for the leak proof, it was settled that a 10% decrease of the prescribed nominal tire pressure is allowed during a time of six month. To assure this requirement a certain number of wheels were controlled by means of so-called “water bath bubble test” [
the “water bath bubble test” (tests not accurate and depend on examiner, not corresponding to the operational conditions, not controlling the total rim leakage, etc.) and the need of the relatively long test time, the procedure using the helium gas was introduced, shown schematically on
For the quality control a permissible leakage rate of ≤3.2 × 10−4 mbar l/s was determined, which correspond to the requirement of 0.2 bar pressure decrease at 2 bar tire overpressure (3 bar inside the tire and 1 bar outside) and 25 l tire volume in a time of 6 month. Thus an integral leak rate as the sum of total leakage is used which allows, assuming corresponding calibrated test facility, a reliable and more accurate quality control of cast wheels than it was the case using the “water bubble test”. It must be mentioned that in the case of tires which have a volume of more than 25 l, a permissible leakage rate of ≤3.2 × 10−4 mbar l/s will correspond to a tire air pressure decrease of 10% in a time longer than 6 month (e.g. for a tire with a volume of 35 l to a time of about 8.4 months or for allowable 10% decrease in 6 month the allowable leakage rate will be 4.5 × 10−4 mbar l/s).
Requirements for the air tightness control, described in specific topics of individual car companies specifications and norms [
Concluding this chapter it can be said that most of car companies, based on the test procedures in water bath used almost more than 40 years ago, as the usage of cast light alloy wheels in the car industry was introduced, require in their specifications or company norms that the air tightness is controlled on unpainted wheels. Meanwhile manufacturing of these wheels is significantly improved, from casting technology, painting procedure, used materials and corresponding control (X-ray radioscopy of all wheels, geometrical control, durability control under operational loading, validation under impact loading simulating dynamic, accident like loads at special event in service, etc.) and the air tightness must be carried on 100% wheels at highly increased production. To prove the influence on air tightness control before or after the painting different investigation were carried out. The scope of these investigations was to prove what would be a most economical, reliable procedure to control the air tightness of cast wheels as their production was increased in individual companies to several millions of wheels pro year.
Based on specific conditions at testing the rim leakage by helium gas and the requirements for quality control on finished product, the question arisen, should the control be made as required in individual specifications before painting or after the wheels are finally manufactured including painting?
To respond this question different investigation were carried out. Their results will be treated in following chapters.
To analyse the influences on the accuracy and reliability of the testing on unpainted and painted wheels a specific measurements program was carried out [
a) Measurement of leakage rate on 25 wheels after machining but before painting.
b) Measurement of leakage rate on the wheel with highest leakage and on the wheel with lowest leakage repeating the measurement 25 times.
c) Measurement of leakage rate on same 25 wheels after painting.
d) Measuremnt of leakage rate on the wheels under point b. after they were painted repeating measurement 25 times.
The results of the measurements under point a. and d. are presented graphically in
From the measurements presented in
Similar results were determined with the wheel with highest leakage rate (wheel 11,
On wheel 11. the result of measurements for unpainted state have a mean value MW = 5.9 × 10−5 (s = 0.44) and for painted state MW = 4.0 × 10−5 (s = 0.23) as shown in
On wheel 12 the mesured values in case of unpainted state are MW = 4.1 × 10−5 and scatter s = 0.44 and for painted state MW = 2.1 × 10−5 with a scatter of s = 0.21.
A generalized evaluation of these analyses show that with unpainted wheels the mean value of measured data as their scatter is slightly higher when compared with data of same wheels after they are painted. Obviously the lacquer layer covers and closes the microporosities in the wheel rim leading to a lower leakage and also lower scatter of measured data. That means that individual measurements on painted wheels deliver more reliable results. To take into account the difference between the tests on unpainted and painted wheels at leakage rate validation the allowable leakage rate on painted wheels should be decreased to a value of ≤2.0 × 10−4 mbar l/s.
To prove the behaviour of painting layer under cyclic deformations as occurring on the rim base under operational loading, tests were made in the Fraunhofer Institute for Structural Durability (LBF)-Darmstadt [Report No. 18 8271, Sept. 2003] with specimens fabricated from rims of cast wheels (material AlSi9Mg) before the painting. On these specimens microholes were drilled comparable to the porosities generating the rim leakage. These test specimens were than painted under similar conditions and same lacquer as used for wheels. Finally the tests were made with these specimens in the servohydraulic test machines simulating stresses as they occur on the rim base due to tire inflation pressure and superimposed wheel loads in service. The test specimens were controlled after the tests were finished, proving their leakage under air pressure up to 5 bar in a special test facility, and the state of lacquer layer concerning possible cracks or other damage. The result was that no cracks or damage of the lacquer layer occurred and no measurable air leakage existed.
To control the most severe situation of wheels which in unpainted condition had been rejected at the “water bubble tests” and additionally controlled and rejected at helium gas tests due to the measured leak rate between 1.3 × 10−3 and 8.4 × 10−4 mbar l/s but after the painting they passed the tests, having the leak rate lower than allowable: <3.2 × 10−4 [
During these tests the inflation pressure was controlled and no significant decrease was found. After the durability tests were finished, the rim leakage was checked again by helium test procedure without any negative results related to the air tightness (leak rate <3.2 × 10−4). These wheels were stored, with tires mounted and inflated, and the air pressure controlled periodically without registering a significant (higher than 10%) decrease [
To prove the situation in the practical usage the data presented in the report [
also a decrease of the necessary replacements in the years 2014 and 2015, which should be connected with the improved accuracy of the air tightness control in the modern test facilities. Also it can be seen that nevertheless that the amount of tested wheels after the painting in the company was increased, the necessity to replace the wheels due to the rim leakage is decreased influenced by specific improvements in the manufacturing.
The assumption that by painting the leakage rate of wheels will be decreased is correct but it is important that the air tightness of a wheel does not change under operational usage. Otherwise it is found during investigations that the wheels approved in unpainted state (leakage rate lower than allowable) have not satisfied the requirement after painting; this was influenced by manufacturing steps after the leakage test on unpainted wheel, which include chemical and thermical treatment of the wheel up to temperatures of 240˚C, which can generate a change of the microstructure and open some porosities. Based on the results of the presented investigations and taking into account that the air tightness control on painted wheels save costs and time (not necessary to clean the wheel twice, after machining before helium gas testing and again before painting) and decrease the amount of waste due to the cleaning and due to it the ecological impact on environment, the testing of the wheel tightness after painting is better option. Testing of painted wheels as the finished product is fulfilling furthermore better the product liability requirements.
It will be proposed that for the modern, fully automated and properly calibrated helium gas test facilities, the tests should be carried out on finally manufactured, painted wheels using a permissible leakage rate of ≤2.0 × 10−4 mbar l/s. To take into account the increased inflation pressure for low profile tires, it is proposed that the leakage approval is based on an overpressure Δp ≥ 2.5 bar.
Grubisic, V. (2017) Air Tightness Control of Passenger Car Wheels. Engineering, 9, 171-180. https://doi.org/10.4236/eng.2017.92008