Durability Testing of Electrical Contacts
Introduction
Electrical contacts are found almost anywhere electrical equipment is used, in applications such as automotive, consumer appliances, and public transportation. Because of the nature of their function, electrical contact materials are subject to wear, and durability testing is an important part of the contact's design. The UMT Series Micro-Tribometer has been developed to perform durability tests of electrical contacts. It is the only instrument of its kind in the world that can simultaneously measure contact electrical resistance (or capacitance), acoustic emission, and forces and displacements in three directions. The objective of this Application Note is to describe how the UMT Series Micro-Tribometer is used to perform, for example, durability testing of a carbon brush/copper contact
Instrumentation
During durability testing, the UMT series, Micro-Tribometer provides precision translational, rotational, or reciprocating motions to the friction pair specimens, with speeds ranging from 0.1 µm/s to 10 m/s, thus simulating their dynamics under actual working conditions. A normal load is applied by a closed-loop servo mechanism in the Micro-Tribometer's upper stage, and can be kept constant or linearly increasing, ranging from 0.1 mN to 1,000N. Specimen environmental temperature and pressure can also be controlled. Friction force (Fx), normal load (Fz), electrical contact resistance (ECR), and acoustic emission (AE) can all be measured and recorded at a total sampling rate of 20 kHz. Wear depth, electrical capacitance, and digital video are also readily available. All these parameters, in addition to the calculated coefficient of friction, are recorded and may be displayed in real time during the test.
Specimens in a wide variety of shapes and dimensions can be accommodated. Common motion modes used for durability tests include reciprocating and rotational.
Test Description
Wear is a key gauge of the durability of friction pairs. The UMT Series Micro-Tribometer was developed to determine this wear. In addition to the parameters mentioned above, the instrument measures specimen wear in-situ during a durability test, which can last any length of time. A pictorial schematic of the Micro-Tribometer used in durability testing of electrical contacts is shown in Figure 1 and a photograph of the Micro-Tribometer with test specimens in place is shown in Figure 2.
Figure 1: Micro-Tribometer Provides Ideal Test Platform for Durability Testing of Electrical Contacts
Figure 2: Photograph of Micro-Tribometer Showing Test Specimens in Place.
For this particular durability test, a carbon brush and a mating copper contact were used as the test specimens. The copper cable had a diameter of 0.375” and a length of 2”. The brush length was 0.5”. Three sample pairs (brush/contact) were tested.
The copper cable was attached to the lower flat base of the UMT Series Micro-Tribometer, and designated as the lower specimen, while the carbon brush was attached to the upper holder of the instrument, and designated as the upper specimen. The curvature of the end of the brush matched that of the round cable.
Test Parameters
Parameters used for the durability test were as follows:
Normal Load: 5 lb (22.2 N)
Reciprocating frequency: 10 Hz
Stroke length: 0.5 inch (12.7mm)
Duration: 48 hours
Test temperature: Ambient, 22oC
Test Description
During the tests a constant normal force was applied by the upper specimen holder, which remained stationary. The lower specimen holder had the reciprocating movement. The effective wear length after two days of testing was about 27 miles for the brush, and about 13.5 miles for the copper cable. The brush wear length is the product of distance traveled by the brush per stroke and the total number of strokes. The cable wear length is shorter because the copper contact length is only half that of the brush during a stroke.
Results
Figure 3 shows wear depth for the three samples. Wear depth was calculated as the difference of the upper carriage vertical (Z-direction) position at the beginning and the end of the test. The wear depth included the combined wear of the brush and cable with a 2µm accuracy. Weight loss determined with an analytical balance, confirmed the wear depth results.
Figure 3: CETR's Micro-Tribometer Determines Wear Depth in Durability Tests
Coefficient of friction is calculated as the ratio of the tangential (friction) force to the normal (loading) force and is shown in Figure 4 for the three sample brushes during the course of the test. The coefficient of friction for the Sample No.3 brush was lower and more stable than that for the other two brushes. This correlates with the previous data wherein Sample No.3 had the lowest combined weight loss and lowest total wear depth.
Figure 4: Coefficient of Friction Indicates Durability of Different Samples
Other tests were performed to investigate the effect of electrical contact resistance. For these tests, the contact electrical resistance was determined with an imposed constant current and measured voltage drop (see Figure 1). An example of these test results is given in Figure 5 for coefficient of friction and in Figure 6 for wear depth, both as a function of contact voltage drop. Figure 5 shows the COF of either brush sample is only slightly dependent on the contact resistance, and there is not much difference between samples.
Figure 5: The UMT Series, Micro-Tribometer Determines Durability Sensitivity To Contact Resistance
However, the UMT Series Micro-Tribometer shows very different results when durability (wear) is also considered, as in Figure 6.
Figure 6: The UMT Series, Micro-Tribometer Determines Wear During Durability Testing
Here, Sample B shows an order of magnitude less wear compared to Sample A. This shows the importance of using the UMT Series Micro-Tribometer for simultaneous electrical and wear measurements in durability testing.
Conclusions
The UMT Series Micro-Tribometer is fully capable of measuring simultaneously and in-situ, the characteristics of friction, wear, and electrical resistance (or capacitance) for electrical contacts under simulation of actual operating conditions. The UMT Series Micro-Tribometer provides the customer with a quantitative solution for evaluating the durability of electrical contacts.