Tribometry in Automotive Applications

In the automotive sector, test standards are an important factor where clutches, brakes, tires, engines, seatbelts, etc. are checked for their material properties to comply with a number of standards such as ISO, ASTM, DIN, JIS, and others. Original equipment manufacturers usually request for standard and custom tests; however, an appropriate test equipment is not available to conduct these tests. To solve this problem, Bruker has launched the UMT Tribometer that can execute a number of tests on a single platform. In this application note, an illustration of various testing techniques of automotive materials and parts using tribometer is discussed in detail.

Figure 1. Photos of clutch pieces (a) concentric- (b) diamsond- and (c) square-grooved.

Experiment

Friction Test of Clutch Plates with Different Surface Preparations

For the test, three clutch samples with different surface preparations were taken to equate their static and dynamic friction properties. These samples were placed in a container on the test platform and locked to prevent any movement. Then, automatic transmission fluid was applied evenly to the samples. On the upper carriage, a torque sensor and a 200N force sensor were mounted to calculate torque (Tz) and normal force (Fz). Then, a steel disk measuring 1” in diameter was placed under the torque sensor. Static and dynamic friction tests were then carried out at 200N load at 30rpm. Coefficient of friction (COF) data were collected from the Fz and Tz data. For each specimen, static friction tests were performed thrice.

Figure 2. Test setup for friction test of clutch pieces.

Table 1. Static and dynamic friction data of clutch pieces

Lubricant Test on Friction Plates Used in Automotive Clutch Assemblies

Another instance of tribology testing involves the assessment of four different grades of transmission fluid. The fluids were tested with actual clutch components to complement real-world applications. One friction plate was attached to the lower rotary drive platen.

Figure 3. Friction plates

In this test configuration, an acoustic emission sensor was also used which identifies the sound waves caused by the interaction between the two surfaces. Among the four lubricants examined, sample 0 was a control sample whereas the other three samples, i.e. A, B and C, had additives to alter friction properties. The closed-loop control of the Z-carriage helped in maintaining a constant normal load (Fz). Later, tests were performed at four different normal loads and readings were then calculated at varied rotational speeds.

Figure 4. Graphs of friction coefficient vs. load-to-speed ratio.

The figures shown below show the changes in friction coefficient as a function of the load-to-speed ratio for four lubricants. In these figures, the curves show an identical trend. The friction coefficient remains stable at the lower ratio and increases where the ratio is higher. Lubricants containing lubricant samples A, B, and C marginally reduce the friction at the lower ratio.

Figure 5. Static COF of (a) concentric-, (b) square- and (c) diamond-grooved clutch pieces.

Figure 6. Dynamic COF of (a) concentric-, (b) square- and (c) diamond-grooved clutch pieces.

In all the instances, the acoustic emission increased with respect to speed. All the four tested lubricants revealed similar results at low speed (<50rpm). Since the testing was done on real automotive parts, materials scientists could be sure of the results and can choose only those materials that showed promise and hence can considerably bring down the time devoted for bench testing on actual drive train assemblies.

Adhesion Test on Coated Tire Samples

A test was performed to measure the adhesion strength of a coating used on the base tire material. Three different types of materials were tested and the results are outlined in the table below.

Table 2. Critical Load on different samples, scratch with a WC-blade

From this table, it can be seen that the critical force matches quite closely and sample #2 showed maximum adhesion strength as determined by each sensor. In the case of electrical resistance measurements, the coating thickness determines the resistance and will stay comparatively stable till the point at which adhesion failure occurs. On the other hand, acoustic emission was found to be constant during the prior part of the test but revealed multiple peaks, which occurred as a result of microcracking during adhesion failure.

Tribology Test on Automotive Components

A tribology test on automotive components was performed on cylinder liner and compression ring samples at different speeds on a linear reciprocating drive. The SAE 30 lubricant was used to infuse the liner sample at ambient temperature. By utilizing force sensors in the upper carriage, the COF was calculated. The normal load, Fz, remained stable while the reciprocating frequency was variable. While the test was performed in the reciprocating mode, the data was accounted as rpm because this convention is standard in automotive applications. Additionally, at 25 rpm, the COF remained almost same between the two surface finishes. This particular test can be employed to test material finishing processes or other similar properties.

UMT Tribometer: The Ultimate Choice

All the tests demonstrated above were performed on Bruker's UMT Tribometer platform. This platform can be configured as a macro-tribometer or micro-tribometer. Test samples of different shapes and sizes can be used on this instrument. Moreover, the tool can be used to conduct all standard lubricant tests, such as disc-on-disc, ball/pin-on-disc, 4-ball, pin-in-vee block, block- on-ring, drill-in-hole, nut-in-screw, shaft-in-bushing and mill-on-block. Linear and rotary drives promote nearly any combination of linear and rotary motions of test samples.

Users can configure the UMT system to conduct different types of lubricant tests. Using a liquid container, lower samples are partially or fully submerged in the lubricant sample. The liquid container from Bruker features an anti-splash design which prevents lubricant leakage.

Multiple Test Configurations to Match Specific Applications

Bruker offers the UMT Tribometer with four test configurations to suit specific application needs.

Figure 7. Lower drive and modules for lubricant test: (a) Rotary Module, (b) Linear-Reciprocating Module, (c) Block-on-Ring Module, and (d) 4-Ball Module.

For lubrication testing, a liquid container can be installed to submerge the disk or plate in the lubricant sample. This container, located beneath the ring specimen, supplies the lubricant. Load is applied in the downward direction via the upper sample against the lower sample.

Conclusion

The UMT Tribometer provides a variety of options, from evaluating the durability of clear-coats, paints and other finishes to measuring recovery of seal materials and wear testing of wiper blades. The tool is capable of programming and controlling complex motions and forces via servo-control and hence proves a suitable tool for automotive applications.

This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.

For more information on this source, please visit Bruker Nano Surfaces.