Tribo-Screening Methods For Performance Testing Of Lubricants By Optimol Instruments

Monday, 2 August 2021

Testing the tribological properties of lubricants is a challenging task. Tribometers that represent tribocontacts in a model environment are able to both accurately simulate the operating conditions of high-quality lubricants and provide reliable results to differentiate their performance profiles.

Oscillation In Tribotesting

The SRV®, developed by a German manufacturer of industrial lubricants in the 1960s, introduced oscillating test motion into tribotesting. This made it possible to create stressful test conditions and achieve distinguishable results. The SRV® has since established itself worldwide as an effective industry standard in tribological model testing.

SRV® Test Parameters

• The basic SRV® equipped for oscillation has an electromagnetic linear drive, which generates – as the relative motion of the test contact – a periodic sinusoidal translational movement in the frequency range from 0.001 Hz to 500 Hz with strokes of 0.01 mm to 5 mm.
• The normal load is adjustable in 3 different ranges up to 2500 N.
• Ambience conditions, such as test piece temperature (-40°C–1000°C), humidity (30% RH–90% RH) or atmosphere (gases, steam, etc.) can also be controlled.

Reliable Results Through Ready-To-Use Standard Methods

High-precision test results that can be reliably transferred to real-world applications are of paramount importance in tribological testing. Standardization of SRV® testing has been a continuous process since 1988, resulting in 29 ISO, ASTM, DIN and SAC methods to date. The use of standardized methods is an effective means of speeding up lubricant development.

Investigating The Performance Of Oil And Grease Formulations With Standardised Methods

Three SRV® standard methods are available for differentiating oils in terms of friction, wear and extreme pressure properties.

• ASTM D7421-19 – Standard Test Method for Determining Extreme Pressure Properties of Lubricating Oils Using A High-Frequency, Linear-Oscillation (SRV) Test Machine, included in ISO 19291

• ASTM D6425-19/DIN 51834-2:2017-05 – Standard Test Method for Measuring Friction and Wear Properties of Extreme Pressure (EP) Lubricating Oils Using SRV Test Machine

• ASTM D7755-16 – Standard Practice for Determining the Wear Volume on Standard Test Pieces Used by High-Frequency, Linear-Oscillation (SRV) Test Machine

Load carrying capacity of hydraulic oils

Four different hydraulic oils were tested in accordance with ASTM D7421. The graph below shows clearly distinguishable results in regard to load carrying capacity.

Customized Test Setups

Based on a sound standardization background, customized test setups pave the way for tribological model tests that are both reliable in the precision of their results and very practice-oriented in their design. The FZG setup for SRV® is an example.

Pre-screening of gear oils, alloys and coatings

FZG is a well-known application-oriented gearbox test rig. Our motivation was to develop a fast and cost-effective pre-screening method for gear lubricants and related additive packages that can also be used for new alloys and coatings. Figure 4 shows the configuration of the special setup developed by Optimol Instruments for testing gear oils.

FZG Test setup in the SRV®

Approach: The SRV® can perform tests with increasing test load fully automatically. Surface pressures of the FZG load steps 3 to 14 can be transferred to typical normal force values between 50 N and 1500 N with this geometry.

Lower specimen: Standard SRV® disk (24 x 7.8 mm, 100Cr6 rolling bearing steel. Lapped surface (Ra ≈ 0.05 µm)

Upper specimen: Crowned cylinder (6 x 8 mm) with a support length of 4 mm. Surface polished. Body inclined at an angle of 10°.

Rolling Friction And Wear Measurement Offers New SRV® Test Opportunities

The future of grease development is moving towards energy efficient greases. Optimization of bearing lubrication is predicted to improve energy efficiency.

In addition to the linear oscillating motion of the ball on the disc with SRV®, a new adapter has recently been developed to simulate rolling motion. This special method can distinguish between rolling friction and the resulting wear of the formulated grease. Rolling friction and wear can be simulated and measured to the 5th decimal place.