Four Ball Tribometer: To determine the lubricating properties of oils and greases.

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What are Tribometers?

The word “Tribology” is derived from the Greek words “tribos” meaning “wear” and “logos” meaning “logic,” so, in Tribology, we try to find out the logic behind wear. One of the primary ways to reduce friction is using the correct lubricant grade.

Now how to measure friction and wear?

So, A tribometer is used to investigate or simulate the conditions of friction, wear, and lubrication. Examples include a pin on disc tribometer, four-ball tribometer, scratch tester, reciprocating tribometer, FE 8 and FE 9, etc. This article will briefly introduce a Four-Ball Tribometer, which is commonly used to investigate the lubricating performance of lubricants.

Introducing Four Ball Tribometer:

A Four Ball Tribometer is used to characterize the lubricants to primarily understand the lubricant’s anti-wear and extreme pressure properties. Other tests such as Rolling Contact Fatigue Analysis and Viscosity Shear Stability (transmission fluids) can also be done using a Four-Ball Tribometer. Figure 1 shows a schematic diagram of a four-ball tribometer.

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Figure 1 A schematic diagram of a four ball tribometer

Construction of a four ball tribometer:

A standard four-ball tribometer contains the following:

  1. 1 Hp Single Speed or Variable Motor.
  2. Mechanical / Pneumatic loading systems.
  3. Controller: test duration control and temperature control.
  4. Ball pot with heating attachment (temperature controlled) (for anti-wear and extreme pressure properties). Ball chuck, Ball cup tightening base, torque wrench (0-25lb inch).
  5. Special ball pot: for rolling contact fatigue analysis and viscosity shear analysis.
  6. AISI 52100 steel balls diameter 12.7mm (four). These balls are used during the testing of lubricants.
  7. Friction measurement kit (load cells and air bearing assemblies) and data acquisition system.
  8. Emergency stop assembly.
  9. Electrical requirements: 220V, 60 Hz, 3 phase or 440V, 50Hz , 3 phase.

Four ball Anti-wear test:

The anti-wear test of the lubricants is performed to investigate the wear-resisting properties of the lubricants [1]. The tests are carried as per standards at a specific temperature (75 deg Celsius) and a specific load (392 N). About 30ml lubricant is used during the test. Three balls are tightly fixed in the ball pot, and the fourth ball is fixed to the ball chuck. Lubricant is poured into the ball pot such that all the three fixed balls are submerged in the lubricant. The fourth ball is rotated at 1200 rpm. Generally, the test is run for an hour, and then the wear scars on the bottom balls are observed in an optical microscope. The wear scars of all the three bottom balls are measured, and the average of these wear scars is recorded. These steps are being followed for all lubricants that are to be tested. The frictional force is also recorded and can be analyzed to find any sharp peaks during the test duration. The anti-wear test parameters can also be selected based on the application demand. In several situations, long-duration tests (72 hours) are also conducted. Check ASTM 4172 for more details.

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Figure 2 A typical wear scar on one of the fixed balls after anti-wear test

The following formulae are of interest for the readers [1,2].

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The frictional coefficient (FC) is given by 

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From the wear scar diameter, the flash temperature parameter can be determined. Higher the flash temperature parameter, better is the lubricating film formation.

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The contact pressure at the contact zone is given by

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The relationship between contact load and applied load is given by

image 3

From Hertzian Contact theory, the maximum contact pressure perpendicular to the contact zone can also be calculated.

Four Ball Extreme Pressure test:

The extreme pressure test using a four-ball tribometer helps to determine the performance of the lubricant at extreme pressure conditions [3,4]. About 30ml lubricant is used during the test. The test is run for 10 seconds at room temperature. Loads are gradually applied until the lubricant film ruptures and all four balls are welded (catastrophic failure). In this test, the user is interested in the following: Hertz line, compensation line, Hertz diameter, last non-seizure load (LNSL), load wear index (LWI), and weld load. The LNSL is the load at which the measured scar is less than the compensation load. The initial seizure load is the first load beyond LNSL.

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Figure 3 A schematic plot to determine the LNSL, ISL and Weld Load, wear scar diameter Vs applied load (ASTM D2783)

The compensation diameter is approximately 1.15 times the Hertz diameter. The load wear index indicates the ability of a lubricant to resist wear at different loads. It is the average of ten corrected loads just before the weld point. The ultimate failure load, known as the Weld Load (WL), is the load at which the lubricant film breaks fully and all four balls weld together. However, weld load is also considered when either of the following situations arises:

  • High motor noise
  • High vibration
  • Smoke from the ball pot
  • A sharp movement of the indicator pen (transverse)
  • A drop in the lever arm
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Please check ASTM D 2783 for further details.

Table 1 indicates the common test standards that are performed using a Four Ball Tribometer.

Table 1: Various test standards.

Test StandardLubricant TypeTest
ASTM D2266GreasesWear preventive properties
ASTM D2596, ASTM D2783 GreasesExtreme pressure properties
ASTM D4172Lubricating fluidsWear preventive properties
ASTM D5183Lubricating fluidsFrictional coefficient
IP 300Lubricating fluidsRolling contact fatigue
CEC L-45-A-99 (KRL)Lubricating fluidsViscosity shear stability of transmission lubricants

Four Ball Rolling Contact Fatigue Test:

Damages due to rolling contact fatigue are predominant in mechanical applications such as gears, bearings, wheels, etc. [5]. A good lubricant should protect the mating surfaces from sliding wear and protect them from the damages occurring due to contact fatigue. IP 300 standard is followed during the test. To conduct this test, a specially designed ball pot is used.

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Figure 4 A schematic diagram of rolling contact fatigue set up in a four ball tribometer.

Unlike the anti-wear and extreme pressure tests, the three balls placed in this special ball pot are allowed to rotate freely (Figure 4). The fourth ball fitted to the ball chuck is rotated with 1450 rpm. Generally, the test is conducted at room temperature. Sometimes a provision of re-circulation of the lubricant is also provided. Loads are applied gradually from 100N to 5886N. An accelerometer is placed on the ball pot to detect the abnormal vibration that occurs at the initiation of pitting due to surface fatigue. As soon as pitting is formed on any of the ball surfaces, there is a change in the signal received by the accelerometer due to an increase in vibration. The signal of abnormal vibration is sent to the controller, which stops the four-ball tribometer immediately. The data acquisition system then records the duration of the test.

Using various statistical methods, such as the Weibull plot, the bearing steel balls’ life cycles (L10, L50, and L90) are determined and plotted. The Rolling Contact Fatigue Test is an important test related to the damages caused due to fatigue in mechanical elements such as the gears, bearings, wheels, etc. Figure 5 shows a microscopic image of a bearing ball surface that experienced high plastic deformation due to surface fatigue during the rolling contact fatigue test in a four-ball tribometer, even in the presence of a lubricant. Also, check IP 300 for more details.

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Figure 5 Microscopic image of a bearing steel ball after rolling contact fatigue test under lubricated condition.

Viscosity Shear test (KRL test)

The four-ball tribometer can also determine the shear stability of lubricants, particularly the transmission lubricants.

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In these, the spherical bearing steel balls are replaced by tapper rollers, and the lubricant to be tested is poured into the oil pot. The test is conducted at 60 degrees Celsius under a constant load of 5000N. The roller bearings rotate at 1450 rpm. The test duration is 4 Hrs or 8 hrs or 20 hrs or even more. The decrease in viscosity at 100 degrees Celsius is the output of the KRL test.

Summary

This article briefly introduces the most commonly used tribometer: Four Ball tribometer. Various tests to evaluate the lubricant performance can be conducted using a four ball tribometer. However, before each test it is always advisable to check the following:

  1. The supply current to the equipment. Check for the voltage variations.
  2. Check whether the load cell is functioning properly.
  3. Check the heater coil in the ball pot to avoid over heating
  4. Check the balls roughness and hardness.
  5. Better to check the ball composition too if possible to avoid spurious balls.
  6. Always clean the ball pot and the balls before starting the test using hexane and dry them thoroughly.
  7. Check the torque wrench and set the correct torque mentioned in the standards.
  8. Check the tightness of the balls in the ball pot.
  9. Ensure the equipment is having a safe electrical connection with proper earthing connection.
  10. Check the belt of the motor before starting.
  11. Keep the data recorder ready.
  12. Do not forget to wear gloves and goggles to avoid contact with lubricants. A lab coat is also recommended.

Once ready, start the equipment and run your tests safely.

References:

  1. Bhaumik, S., Datta, S., & Pathak, S. D. (2017). Analyses of Tribological Properties of Castor Oil With Various Carbonaceous Micro- and Nano-Friction Modifiers. Journal of Tribology, 139(6), 061802.
  2. Yan,Z.G.,2000, ―Technical Manual for Lubricant Performance Testing‖, Petroleum Industry Press, Beijing
  3. Hernandez Battez A., Gonzalez R., Fergueroso D, Fernandez J.E., Fernandez Rocio del Ma, Garcia M.A., Penuelas.I.,2007, ―Wear prevention behavior of nanoparticle suspension under extreme pressure conditions,Wear,263,pp. 1568-1574.
  4. Dobson G.R.,1978, ―A re-examination of the four ball test‖ , Tribology International, 11(1), 59-62.
  5. Bhaumik, S.; Paleu, V. 2021, Wear and Rolling Contact Fatigue Analysis of AISI 52100 Bearing Steel in Presence of Additivated Lubricants. Metals11, 907.

About the author:

Dr. Shubrajit Bhaumik is a mechanical engineer and has been working in the area of tribology for more than 15 years. Dr. Bhaumik is highly experienced in handling various tribometers. He has been working in the area of nano lubrication since 2012. Dr. Bhaumik has published several technical article in reputed journals of tribology. He has successfully implemented the nano lubricant technology in several industries, resulting in significant reduction in wear, temperature and power consumptions particularly in gear boxes, bearings, compressors, spinning mills, automotive applications etc. He is a member of ASM International (EC- Chennai Chapter), Malaysian Tribology Society and Tribology Society of India.

For more discussions, please contact at shubrajit.projects@gmail.com

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