Methodology
These experiments were carried out in tribotechnology laboratory at UCLan on standard pin-on-disc tribometer under dry lubrication conditions at different loads and sliding speeds.
Pin-on-disk test is used to measure and benchmark the frictional performance and wear rate of X number of specimen materials, which are most commonly used in journal bearings.
The arm or lever presses the pin specimen against the disk at a specified load.
Friction force/2??? if the load is 2 N. Why?
Labview is used as a tool for tribology experiments and condition monitoring.
On a journal bearing test, the pin-on-disk apparatus has an oscillating mode that transforms rotation into translation. In a tribometer, the rotating module and the counter face i.e., the arm is fixed or it can move. In this case, the arm goes down and makes contact with the sample on the rotating module.
Parameters:
Spherical A6000 tribology ball
Brass substrate
Specific grade, if known
What is the sample made of?
Labview gives the option to put in friction coefficient of 0.2 (change depending on the lubricant. Use https://www.engineeringtoolbox.com/friction-coefficients-d_778.html if helpful). The temperature and humidity are recorded as well into the test.
20 mm track 100 m length
Depending on the radius of the track, Labview also considers a different sliding speed. The maximum sliding with these input parameters is 629 mm/s and the minimum is 12 mm/s. Setting the sliding speed of 600 mm/s, the test is conducted within 2 min and 47 sec. The equipment can also calculate the action pressure contact just by knowing the humidity and pressure.
The maximum load that can be applied in dry condition to the machine is 12 N. For instance, a tungsten carbide specimen is quite dense and heavy and has a weighty feel on the arm whereas aluminium reduces the dead weight on the arm. Hence, for the latter, the system calculated 4.5 load on each side + 1N. The load is added on the arm. So, we can adjust the position of the arm and also there is a knob on the back to adjust the position as well. Everything must be balanced before we do a test with the pin specimen. This is done by a lever scored at 1 inch to level the arm.
If the arm is not balanced, it means that you are putting extra load at the starting point. You might have an offset in the data. When the test is running, we also have a cover to be safe to protect the user from the movement in the system. A magnetic sensor tells the system that the lid is closed. The experimenter needs to ensure that the sensor is responding to what it is set to do.
Green line: friction force
Blue line: friction coefficient
We can custom create a new test, open an old test and export using the appropriate function on Labview.
We can edit the material list. We can correct the wrong parameter. For example, if I’ve done a test and I’ve realised that it was not a lubricant, it was a grease, I can modify the information that has been wrongly entered after the test.
Calibration: – To calibrate the main drive
30 mm is setup for the machine.
The shaft needs to be properly aligned with the system.
The tachometer measures the number of dashes as the bottom part rotates. The tachometer measured the rotating speed. As the part rotates at a higher speed, a higher number is recorded. The following measurements are recorded on the measuring device 0.905 ± 0.005 RPM, 566 RPM, 944 RPM, 1321 RPM and then 1699.5 ± 0.1 RPM.
To calibrate the forces:
The first test is the no-load friction test. At 0 position, four sensors are measured and the sensor offset is compensated. The forces should be zero because no load is applied to the arm. A well-known calibration load with a dead mass of 342 g is inserted to a pulley for the force calibration. The mass is set in isolation. The voltage changes due to the slight mass fluctuations, giving a variation in the force measurement. Perhaps waiting would help as things become more stable. Wait a few seconds to make sure it is stable. The machine measures the voltage on the load cell corresponding to the mass. A known second load of 317.3 g is added to 342 g. Wait a few seconds until it is stabilised and start the test. It again measures the voltage on the load cell. The same procedure is repeated to calculate the movement by changing the direction of the load. The load and the pulley are removed and attached to the opposite side to calibrate the system. The same procedure is carried out with the first and the second dead mass added onto the first. These are the reference values for the measurement. The machine tells you 0.23 deviation between the right and left side which means that one side is more accurate than the other. The sum of the two values needs to be >700 g. The system then measures the measuring gap between the arm’s open and closed position. The load on the arm is then adjusted depending on the weight of the specimen and the friction coefficient entered in the system. Start the test.
White light interferometer
White light interferometer based on fibre optics is used for scanning and analysing the surface structure and roughness measurements at or very close to the point of interest. Contact free displacement measurements based on white light interferometer can achieve nanometer accuracy. The white light interferometer contains a white light beam and detection electronics. The interferometer sensor head, connected by fibre optics, contains no electronics that can induce heat to the system. The output of the optical fibre is split into 2 beams. https://www.smaract.com/picoscale-interferometer The beam splitter has a mirror on one side that serves as a stable reference arm. The other is the measurement beam that is directed at the target surface. The beams recombine after reflection at the beam splitter and the resulting interference pattern contains information on the position of the target with respect to the reference mirror. https://www.smaract.com/advantages-of-michelson-interferometers The optical response of a typical white light interferometer forms a sinusoidal curve. Consequently, the interferometer measures the displacement in the surface structure and roughness of the specimen via the mirror.
The pin was analysed prior and after each test.
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