Measuring the impact of lubricants on static friction
Figure 1 Schematic of the ball-on-three-pins setup along with the reference images of the measuring shaft and the sample holder.
Background
The selection of lubricants for a particular application depends upon various factors ranging from the contact pressure, motion conditions, temperature, etc. and is crucial for the overall performance of a machine or component. With ever increasing demand for improvement in the effi ciency of systems, selection criteria have also become quite stern.
A critical lubricating property is typically the potential of lubricants to form hydrodynamic fi lms, which reduce friction and protect from wear. These fi lms form above certain speeds, as described by the Stribeck curve for that particular system. However, many applications do not run constantly, but are operated with start-stop type motions. Critical factors are the forces and energies required to overcome the static frictional resistance of the tribological system and to set it into macroscopic motion.
Typical applications where this parameter could be of great signifi cance are seat regulators, sliding guides, bearings, gears, etc. While in most cases a low break-away force is desired, it must also be noted that a certain amount of resistance is still required to inhibit involuntary movements.
The article at hand presents a method to determine the break-away force of greases in a particular tribological system and the speed-dependent change of these forces with one measurement.
Experimental setup
The accurate determination of break- away force requires highly precise control and measurement of forces such as the applied normal force and torque, and quantities like angular displacement on the nano-scale. Therefore, for this study, the tests were carried out on a tribometer employing a measuring drive from a MCR 502 rheometer from Anton Paar. The test confi guration used here was ball-on-three-pins, as shown in Fig. 1, employing a Peltier heated tribology cell in combination with a Peltier heated hood for precise temperature control. The specimens, both balls and pins, were made from 100Cr6 steel. While the diameter of the ball was 12.7mm, the
cylindrically shaped pins measured 6mm in both diameter and height. The sample surfaces were rinsed in isopropanol prior to the tests to remove organic matter and impurities from the surface.
Two greases, #1 and #2, with minor differences in composition, were tested here. Once the pins were inserted into their positions in the specimen holder, grease was applied on their surfaces with the help of a spatula just before beginning the test. The amount of grease used in each test was around 2 mL, which was just enough to cover the surface of the pins without fi lling the cavity at the bottom of the sample holder.
Figure 2 Test profi le showing logarithmic ramp of the rotational speed with respect to time. Continued on page 26
24
LUBE MAGAZINE NO.123 OCTOBER 2014
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65
