A plot illustrating the frictional characteristics of a liquid lubricant, under conditions such as mixed and hydrodynamic regimes and boundary, is known as the Stribeck curve. Each of the regimes is defined by the ratio between the surface roughness and the film thickness, and is also known as the λ ratio (Figure 1).
Figure 1. Schematic of Stribeck curve.
Stribeck curves play a vital role in evaluating the effect of alterations in the lubricant’s additive package or the lubricant’s viscosity, and the effect of surface roughness. The principal condition for generating a Stribeck curve is the ability of a fluid to be drawn into a converging gap to create an increase in pressure to support the load (Figure 2).
Figure 2. Geometry to create hydrodynamic pressure and lift.
Historical/Traditional Test Geometry
Hersey, Martens, Stribeck, and Thurston carried out their classic work by using a journal-bearing geometry possessing all the aforementioned requirements (Figure 3). Friction can be evaluated over a wide range of loads and/or relative velocities by carrying out an instrumented journal-bearing test. The coefficient of friction (COF) is plotted against the Hersey number, which is the key parameter in a Stribeck curve. The Hersey number is dimensionless and can be derived by multiplying the velocity (m/s) with the dynamic viscosity (Pa.s = N.s/m2), and then dividing the product by using the load per unit length of bearing (N/m).
Figure 3. Converging gap of a journal bearing.
The most common and simple technique used to acquire a Stribeck curve is by maintaining two variables (e.g. load and viscosity) and varying the third (e.g. velocity) over an appropriate range so that the contact interface passes through the asperity contact region (boundary) and through the full fluid-film separation (hydrodynamic). However, this technique is appropriate only if the proper converging gap geometry is known. This technique can be easily carried out in the laboratory in a unidirectional manner using a pin-on-side against a rotating disk (POD) under flooded lubrication, where the pin-end contact geometry creates the converging gap.
Reciprocating Motion for Stribeck Curve Generation
However, in recent times, significant interest has been shown in evaluating the frictional characteristics of a lubricant by employing a reciprocating test mode. These types of test rigs are often called high-frequency reciprocating rig (HFRR) or SRV-type test, where SRV stands for Schwingung Reibung Verschleiß, which is a German expression that means reciprocating friction and wear. One way of using a reciprocating test mode is the simulation of a piston ring’s lubrication condition in an automobile engine cylinder.
One of the challenges faced in the development of a Stribeck curve in the reciprocating mode is the need to develop adequate velocity over a long enough stroke length before reversal to increase the pressure and the film thickness needed to achieve the hydrodynamic lubrication regime. This article presents a test method employing the geometry of a polished cylindrical dowel pin-on-side against a highly polished flat steel plate (Figure 4) under high-frequency reciprocating motion conditions. A Bruker UMT-3 tribometer fixed with a heated high-speed reciprocating stage was utilized to carry out this test (Figure 5).
Figure 4. (A) Cylindrical pin-on-side vs. polished steel plate. (B) Piston ring and cylinder liner segments.
Figure 5. Bruker UMT Tribometer
For all of the tests, the load and the stroke length were fixed as 20 N and 7 mm, respectively. The frequency was varied from 0.1 to 30 Hz. Poly-alpha olephin oils (PAOs) with three different viscosities (PAO-2, PAO-10 and PAO-40) were utilized. Here, the number after the dash denotes the viscosity of the oil in cSt at 100°C. The tests were performed at room temperature. Figure 6 illustrates the Stribeck curves obtained from tests carried out on the lubricants with three different viscosities.
Figure 6. Stribeck curves generated using reciprocating test mode.
Additional Standard Reciprocating Test Capabilities
At present there is no standard published test method that can be used for generating a Stribeck curve, either in unidirectional motion or in reciprocating motion. However, a variety of lubricant performance-related tests can be carried out with the same reciprocating motion for the lower sample. The cylinder-on-side upper sample can be exchanged with a ball or a flat-pin, and the stroke length and/or frequency can be adjusted to enable the same setup to be used to carry out the following standard ASTM-, DIN- or ISO-issued reciprocating tests:
- ASTM D5706-11: Standard test method for determining extreme pressure properties of lubricating greases using a high-frequency, linear-oscillation (SRV) test machine
- ASTM D5707-11: Standard test method for measuring friction and wear properties of lubricating grease using a high-frequency, linear-oscillation (SRV) test machine
- ASTM D6425: Standard test method for measuring friction and wear properties of extreme pressure lubricating oil
- ASTM D6079-11: Standard test method for evaluating lubricity of diesel fuels by the high-frequency reciprocating rig (HFRR)
- ASTM D7688-11: Standard test method for evaluating lubricity of diesel fuels by the high-frequency reciprocating rig (HFRR) by visual observation
- ASTM D7594-11: Standard test method for determining fretting wear resistance of lubricating greases under high Hertzian contact pressures using a high-frequency, linearoscillation (SRV) test machine
- ASTM D7755-11: Standard practice for determining the wear volume on standard test pieces used by highfrequency, linear-oscillation (SRV) test machine
- ASTM G133-10: Standard test method for linearly reciprocating ball-on flat sliding wear
- ASTM G203-10: Standard guide for determining friction energy dissipation in reciprocating tribosystems
- ASTM G206-11: Standard guide for measuring the wear volumes of piston ring segments run against flat coupons in reciprocating wear tests
- DIN 51834: Determination of friction and wear data of lubricating oils
- ISO 12156-1:2006: Diesel fuel — assessment of lubricity using the high-frequency reciprocating rig (HFRR), Part 1: Test Metho
Evaluation of lubricants and surface treatments using a Stribeck curve reciprocating motion test is quite challenging, compared to classical Stribeck curve generation performed using a unidirectional test. The adjustable stroke length and frequency range options and high-speed linear oscillatory stages of the Bruker’s UMT systems, as well as their companion software data reduction algorithms enable such characterization to be efficiently performed. The Stribeck curves generated through the reciprocating test method can be directly applied to lubricant development and evaluation in the field of automotive engine systems and various critical engineering systems.
This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.
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