Pin On Disc Tribometer – Measuring Friction and Wear in Sliding Motion

What are Tribometers?

The word “tribometer” is a generic term and is used for the equipments which are used in tribology to measure friction, wear and lubrication. Before briefly knowing about tribometers let us know about tribology. Tribology is the branch of science which deals with friction, wear and lubrication. Tribology plays an important role in our daily life. From opening a tap to the flight of rockets, everything involves tribology, hence it becomes very important to evaluate the materials to understand their properties with respect to their behavior  and performance to friction, wear and lubrication. Thus, laboratory scale studies using various equipments which indicates about the materials performance to friction, wear and lubrication are used. These equipments are commonly known as Tribometers.

General function of a tribometer (not limited to):

1. Simulate the type of contact for specific applications
2. Evaluate the friction and wear resistant properties of various materials
3. Evaluate the performance of lubricants: anti-wear properties, load bearing properties, rolling contact fatigue analysis etc
4. Evaluating the bearing materials
5. Evaluate the performance of coolants in machining

Pin on disc tribometers are the most common types of tribometers being used to investigate the friction and wear properties. A typical pin on disc tribometer consists of a stationary pin and a rotating disc. A normal load is applied on the pin and the friction readings are recorded using a load cell. The wear rate is recorded using a LVDT. The pins attached are generally hemispherical, however flat faced pins are also used. The pins can be of varied shapes flat, triangular etc however, the most commonly used pins are cylindrical or spherical. The main advantage of using a pin on disc is that a variety of materials can be tested. The only requirement is that the specimen is to be made as per the requisite dimensions and it must withstand the stresses which the pins withstand during the tribo-tests.

The material properties such as the indentation hardness, micro structures, compositions, surface finish, treatments (process) and specimen dimensions are taken into consideration during the tests.

A schematic diagram of pin on disc tribometer is shown in Figure 1. As per ASTM G 99 standards a typical spherical or cylindrical pin diameter ranges between 2mm to 10 mm and a typical disc diameter range between 30 to 100 mm. The thickness of the disc is generally between 2 to 10mm. The surface roughness also plays a major role in controlling the wear, and hence, ground surface roughness (average) of 8µm or less is recommended. Additionally, pins having any sub surface defects should not be used during the tribo test.

The test parameters of a tribo test using pain on disc tribometer is shown in Figure 2.

A tribo test is designed as per the test parameters listed below:

• Load (Newtons)
• Relative speed (m/s)
• Distance covered (m)
• Temperature (contact temperature of either or both contacting surfaces)
• Atmosphere (presence of humidity, various temperature of environment, presence of gases, humidity, lubricants etc.)

Figure 3 briefly explains the procedure of using a pin on disc tribometer:

1. The samples and the disc are to be thoroughly cleaned. It is better to use non-chlorinated, non-film forming agents and solvents.
2. Inspect for any loose metal fragments entrapped and remove them.
3. Avoid magnetized steel specimens.
4. Measure the specimens and weight them prior to test after cleaning.
5. The disc is to be fixed securely perpendicular to the axis of resolution.
6. Start the motor and fix the number of revolutions as desired.
7. Fix the pin in the pin holder firmly such that the pin is perpendicular to the disc when in contact.
8. Weights are to be added as per the requitement of the test conditions.
9. Start the test under the loaded conditions.
10. The test is stopped when the required revolutions are reached.
11. Remove the specimens and inspect for any type of damages, deformations, discolorations, micro cracks, dislocations, protrusions.
12. Weigh and measure the specimens after tests upto 0.0001g and 2.5µm respectively. The test is to be repeated till a statistical significant result is obtained.

Important points to consider during the tests:

1. Thorough cleaning of the pin and disc is to be done.
2. The surface roughness should be within a desired roughness range for all the pins. Different roughness values of a similar material will result in different wear and friction values. A high roughness will indicate high wear and high friction during the test.
3. Do not use any sub surface damaged pins.
4. Do not restart or interrupt any test
5. The pin surface should not touch the disc till the desired disc speed is reached.
6. Do not stop with a single reading. Repeat the tests till a statistically significant reading is reached.

Calculating the volume losses:

The following formula is used to calculate the volume losses of a spherical pin with radius R (assuming no disc wear):

For disc wear:

The wear scar diameters are to be measured at two different locations on the pin surfaces and the disc surfaces and the average of the results is to be reported.

In several cases the wear track depth is also measured using a stylus profilometer. The readings using a profilometer gives the exact reading about the shape of the wear surfaces, thereby enabling accurate readings of the wear volume. In order to find the disc wear volume, the average wear track profile can be integrated to obtain the track cross section area and multiplied by the average track length [1]. In case of the pin two orthogonal results are taken and the reported result is the average of the two results.

The mass loss method is also used to report the results. This method reports the results in terms of volume loss and hence, eradicating the problems due to variable densities.

In case of materials, which experiences high transfer between the materials (specimens), the above volume loss should not be considered.

It is always recommended to mention the frictional coefficient and its nature, for example initial stage or steady stage.

Conclusion:

This article briefly describes bout pn on disc tribometer. The modern pin on disc tribometer consists of several attachments such as lubrication unit, humidity unit, gas unit, high temperature as well as sub zero temperature, thus enabling the tribologists to approximately simulate the real time applications.

Reference:

1. Standard Test Method for Wear Testing with a Pin-on-Disk Apparatus. Designation: G 99 – 95a