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MTM (Mini Traction Machine)
/ Accessories

Electrical Contact Resistance (ECR) Option (Part code: MTMECR)

Overview

The ECR (Electrical Contact Resistance) is an add-on to the standard MTM2 system. The electrical resistance is measured between the disc and the upper specimen (ball, pin or roller).

An electrical potential is applied to the ball.

When the upper specimen is fully separated from the lower specimen (disc), the ECR reading will be 100%. When direct metal-to-metal contact is made between the specimens, the contact will be a short circuit and the ECR reading will be 0%.

During a test, the lubricant between the specimens is normally non conductive, therefore the ECR signal (value between 0 and 100%) gives an indication of the surface interaction between the specimens. This is particularly useful when studying additive performance in the mixed and boundary lubrication regimes. The ECR signal has proven valuable in the study of scuffing by detecting the sudden film collapse at the onset of scuffing.

How does the ECR option work?

A balance resistor is placed in series with the ball-disc contact and an electrical potential of approximately 15mV is applied across both. The contact resistance and balance resistor then act as a voltage divider.

When the ball and disc are fully separated, (open circuit) the voltage at the disc will be the same as the applied voltage (15mV), so the potential across the balance resistor will be 0V. This will represent 100% film.

When the ball and disc are in direct metal-to-metal contact, the voltage at the disc will be 0V, since this is a short circuit to ground. The entire voltage drop (15mV) will therefore occur across the balance resistor. This will represent 0% film.

The schematics below show the ECR set up on the MTM and the equivalent circuit:

The ECR reading is proportional to the voltage (V) across the balance resistor:

The ECR gives an indication of the surface interaction:

• When the upper specimen is fully separated from the disc, the full potential will remain across the contact and the ECR reading will be 100%.
  V = 15 mV; ECR signal = 100 x 15/15 = 100 %
• When direct metal-to-metal contact is made between the upper and lower specimens, there is no contact resistance: it will be a short circuit. The potential will then be reduced to 0V and the film reading will be 0%.
  Rc = 0 ; ECR signal = 100 x [ 0 / (0 + Rb)] = 0 %
• When a lubricant separates the specimens, the contact will have a resistance because of the non-conductivity of the oil that partially (some asperities are in contact) separates the surfaces.
  If Rc = Rb, ECR signal =100 x [Rb / (2Rb)] = 50 %
  If Rc < Rb, ECR signal > 50 %
  If Rc > Rb, ECR signal < 50 %

Contact resistance

The contact resistance is a function of:

• The contact area: the smaller the area, the greater the resistance so the higher the ECR reading. The contact area is dependant on the specimens’ geometry and material (Young modulus) as well as the applied load.
• The inherent electrical resistance of the materials in contact
• The inherent electrical resistance of the lubricant
• The film thickness/asperity interaction. This is directly related to the lubrication regime of the contact.

The plot below is a Stribeck curve with illustrations of the lubrication regimes. When the entrainment speed and/or viscosity are high (hydrodynamic HD and elastohydrodynamic EHL lubrication regimes), the surfaces in the contact are fully separated by the lubricant film. The ECR reading will be equal (or close) to 100%.

As the speed and/or viscosity decrease, the film thickness decreases and the contact enters the mixed lubrication regime: where surfaces asperities are only separated thin surface boundary films. With increasing asperity/boundary film contact the ECR reading drops.

The graph below shows a MTM2 result for a ‘Stribeck’ type of test using the ECR set up.

Specimens

The standard MTM specimens are a disc and a ¾" ball both made of steel. However other materials and geometries are available and compatible with the ECR set up.

Some compatible materials that PCS Instruments can supply are: copper, bronze, aluminium, tungsten carbide.

Other geometries: roller, pin, ½" ball.

Setting the ECR option

The ECR option is normally fitted to the mechanical unit at PCS. In the case of a purchase after installation of the rig, the user may have to set its PC.

The ECR hardware

The ECR set up comprises a balance resistor. There are 4 different balance resistors/channels available:

The user chooses the channel to use when building the test profile.

Choosing the ECR Channel in a Profile

To choose an ECR channel in the profile editor, select it from the drop down menu. The ECR channel has to be selected for each step.

Calibrating the ECR

The ECR setup cannot be calibrated if the reciprocating kit is fitted underneath the rig.

To calibrate the ECR setup, the user needs a clean steel ball and a clean steel disc to be fitted prior to calibration and the ball and disc in the idle position with the lid lock in place.

Running a test with the ECR setup

The test setup when using the ECR is the same as for a standard test:

• The rig should have been recently calibrated
• Place the specimens and sample as recommended in the MTM2 manual
• Select a profile, a test name and press Start

The ECR signal cannot be logged when running a reciprocating test as the ECR cable is disconnected from the disc shaft when fitting the reciprocating kit. The specimens used must be conductive and the lubricant non conductive for the ECR to work. Some lubricants are conductive due to the metal-based additives they contain.

Results

During a ‘TIMED STEP’ step, the ECR signal is plotted on the graph on screen in red, on a 0 to 100% scale on the right side axis, as shown on the next page. During a ‘STRIBECK CURVE’ or a ‘TRACTION CURVE’ step, the ECR signal is logged but not plotted on the graph. Whatever type of step the user is running, the ECR results (%) are written in the columns 5 and 14 in the test results file.

Channel selection

Ideally the balance resistor will be of a similar magnitude to the resistance of the contact, although this is not always possible. As a guideline:

If the ECR reading during a test is around 100%, a higher balance resistor should be selected.

If the ECR reading during a test is around 0%, a lower balance resistor should be selected.

The graph below plots together ECR results obtained for the same test set up and profile but for the different channels. For this particular example, the user should choose to use:

• The channel C or D if the speeds of interest are above 1000 mm/s (EHL regime)
• The channel A or B if the user wants to study the mixed and boundary regimes (speed below 1000 in this example)

Applications

Study of additives

When testing in the mixed or boundary lubrication regime, the ECR readings give an indication of asperities interactions and therefore can be used to compare the effect of different additives concentrations or packages.

Scuffing test / study of starvation

The ECR can be used to detect the sudden film collapse at the onset of scuffing. Scuffing can occur:

• Under high load and/or high sliding conditions for a fully flooded contact
• In a contact starved of lubricant

Example of results

Below is an example of results obtained running a scuffing test procedure: the speeds are kept constant while the load increases in stages. The graph plots the friction coefficient (left axis) versus the load stage as well as the ECR signal (%) and load (N) on the right axis. Each point on the graph is the average value over each 1 min period. When the load reaches 20 N, the ECR signal drops dramatically until scuffing happens, as shown by the sudden increase in friction.

Specification

Compatibility of the ECR option

PCS Instruments. Tribology Test Equipment: Traction Machines: Testing Fuels
Tel: +44 (0)20 8600 9920, Fax: +44 (0)20 8600 9921, Email: info@pcs-instruments.com