# Generating a Stribeck Curve in a Reciprocating Test (HFRR/SRV-type  test)

The term Stribeck curve is used to describe a plot showing the frictional characteristics of a liquid lubricant over conditions usually spanning the Boundary, Mixed and Hydrodynamic regimes. Each  regime is defined by the ratio of the film thickness to the surface roughness, or the λ ratio (Figure 1). Such  curves are often used to evaluate the effect of changes to the lubricant’s viscosity or to the lubricant’s additive package, or the effect of surface roughness. The primary  requirement for generating a Stribeck curve is a fluid that  can be drawn into a converging gap,  thus creating a pressure increase to support the load (Figure 2).

The classic work  of Thurston1, Martens2, Stribeck3 and Hersey4 were done using  a journal-bearing geometry, which possesses all of the  above requirements (Figure 3). An instrumented journal-bearing test allows  for evaluation of the  friction over  a range of relative velocities and/or loads. In a Stribeck curve the  key parameter, against which the  coefficient of friction (COF) is plotted, is called  the Hersey number. The Hersey number is the  dimensionless number obtained from  the  velocity  (m/s) times the  dynamic viscosity (Pa∙s  = N∙s/m2), divided  by the  load per unit length of bearing (N/m).

The simplest method to obtain  a Stribeck curve and the method most commonly used, provided one  has  the appropriate converging gap geometry, is to keep two variables fixed (e.g.,  load and viscosity) and vary the third (e.g.,  velocity) over  a suitable range so that  the contact interface goes through the  region  of asperity contact (boundary), as well as full fluid-film separation (hydrodynamic). In the  laboratory this is most easily  done in a unidirectional manner using  a pin-on-side against a rotating disk (POD) under flooded lubrication, with  the pin-end contact geometry creating the  converging gap.

## Reciprocating Motion for Stribeck Curve Generation

Recently, however, there has  been considerable interest in evaluating lubricant frictional  characteristics using  a reciprocating test mode. Such  test rigs are  sometimes referred to as a high-frequency reciprocating rig (HFRR) or SRV-type test. (SRV is an acronym for the  German expression Schwingung Reibung Verschleiß, which  meansreciprocating friction and wear). An example of how  a reciprocating test mode might  be used is to simulate the lubrication condition of a piston ring in a cylinder  in an automobile engine.

The challenge for developing a Stribeck curve in the reciprocating mode is to develop enough velocity,  over a long enough stroke length before reversal, to build up the pressure and film thickness required to reach the hydrodynamic lubrication regime. This note describes such a test using  the  geometry of a polished cylindrical dowel pin- on-side against a highly polished flat plate, under conditions of high-frequency reciprocating motion. A Bruker  UMT-3 (Figure 4), equipped with  a heated high-speed reciprocating stage, was set up for this work.

In such a high-frequency reciprocating rig (HFRR), the  30 Hz conditions give an average velocity  of 0.42  m/sec and a maximum at mid-stroke of 0.59  m/sec, high enough to reach the hydrodynamic lubrication regime. A standard cylindrical steel dowel pin (9.5 mm  diameter x 15.5  mm  long, roughness 19 µm Ra ) was held  in a pivoting  self-aligning holder  and loaded against a highly polished 52100 steel plate  (0.013  µm Ra ). Figure  5a shows the samples in the  test chamber. As an alternative to the  cylindrical pin-on-side and a flat plate, samples fabricated from actual  components also  can  be used (Figure 5b).

The stroke length was fixed at 7 mm  and the  load at 20 N for all tests, and the  frequency was varied  from  0.1 Hz up to 30 Hz. Three  different viscosity poly-alpha  olephin  oils (PAOs) were used, namely PAO-2, PAO-10 and PAO-40 (the dash number refers to the  viscosity in cSt at 100°C). Tests were conducted at room  temperature. Figure  6 shows the  resulting Stribeck curves from  tests of these three viscosity lubricants.

## Additional Standard Reciprocating Test Capabilities

While there is currently no standard published test method for generating a Stribeck curve, either in reciprocating or unidirectional motion, a number of other lubricant performance-related tests can  be conducted with  the  same reciprocating motion for the  lower  sample. By exchanging the  cylinder-on-side upper sample for either a ball or a flat-pin, and adjusting the  frequency and/or  stroke-length, the  same setup can  be used to run 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, linear- oscillation (SRV) test machine

ASTM  D7755-11: Standard practice for determining

the  wear volume on standard test pieces used by high- frequency,  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

## Author

Steve Shaffer,  Ph.D.

Bruker Senior Applications Scientist

## References

1. Thurston, Robert H., Determination of the  Laws  and Co-efficient of Friction by New  Methods and with  New Apparatus, Trübner and Co, London, 1879.

2. Martens, Adolf, Schmieröluntersuchungen [oil studies], Mitteilungen aus  den  Königlichen Technischen Versuchsanstalten zu Berlin, Ergänzungsheft III [releases from  the  Royal Technical Testing Institutes of Berlin, complementary volume III], Julius Springer, Berlin, 1888, 1–37.

3. Stribeck, Richard,  Die Wesentlichen Eigenschaften der Gleit- und  Rollenlager [the  main  characteristics of the  sliding and roller bearings], Zeitschrift des Vereins Deutscher Ingenieure [Journal of the  Association of German Engineers] 36 (Band 46), 1902, 1341– 48,1432– 38, and 1463 –70.

4. Hersey, Mayo,  The Laws  of Lubrication of Horizontal Journal Bearings, Journal of the Washington Academy of Sciences, 4, 1914.

5. Society of Tribologists and Lubrication Engineers — www.stle.org/resources/lubelearn/lubrication/;