Characterization of various liquid lubricants in sliding conditions

• Test conditions – 5 N mass, 8 cm/sec slide rate for short-term tests, 10 cm/sec slide rate for long-term tests

• Ball specimen – 6mm 440C

• Disk specimen – Inconel

• Lubricants – WD-40, 3-in-1 household oil, Pro Link chain lubricant, Purple Extreme chain lubricant, Pedros Extra Dry All-Purpose Lube

Tribological testing was performed of various commercial lubricants to determine their effectiveness at lowering sliding friction in a standard pin-on-disk test machine. Testing consisted of first characterizing the dry sliding friction between 440C and Inconel, which is shown in Figure 1. Wear quickly develops and sliding friction converges to a value of approximately 0.8. After testing, a wear track was evident in the test specimen.

Figure 1. Sample friction time history of 440C on Inconel with no lubrication.

The wear track developed in the dry sliding test was used for each short-term wet test. Prior to applying a lubricant, a short test was performed to verify friction was on the order of 0.7 to 0.8. Then, a liberal amount of lubricant was added to the Inconel surface in the wear track. The specimen was cleaned, using methyl ethyl ketone and isopropyl alcohol, between tests. Figure 2 shows the performance of various lubricants.

Averages and deviations for the data shown in Figure 2 were computed for the various lubricants. “Best performance” ranges were used. For example, for Purple Extreme, data prior to a decrease in performance was used. Thus, the values represent the most ideal friction coefficients.

• Pedros – s=0.1381, m=0.0163
• Purple Extreme – s =0.1119, m =0.0140
• 3-in-1 – s =0.1028, m =0.0017
• WD-40 – s =0.0961, m =0.0007
• Pro-Link – s =0.1106, m =0.0015

Figure 2. Short term testing of various lubes.

Since the above tests were performed with fairly liberal applications of lubricant, selected lubricants were retested with longer term testing and an application that would be typical of bicycle chain maintenance procedures (applying lubricant, allowing time to “seep in” and lightly wiping surface with shop rag). New wear surfaces were used for each test. The specimen was cleaned prior to the next lubricant test. Figures 3 and 4 show the performance of the lubricants tested (Figure 4 magnifies the initial stages). It can be seen from these figures that WD-40 and Purple Extreme quickly broke down and high sliding friction ensued. Figure 5 shows the performance of 3-in-1 oil, which suffered to breakdown and provided a consistent low sliding friction coefficient. It should be noted that in the 3-in-1 test, the slider slid over 9 km relative to the specimen in the same wear track (r=12.18 mm).

Figure 3. Long-term testing of some lubricants.

Figure 4. Enhanced view of long-term test.

Figure 5. Performance of 3-in-1 oil.