The problematic "symptom" that I try to fix, is the static friction of two sliding plastic parts.
This is about a keyboard key switch that was made during the 80s. The problem/issue is that after I applied Grease #1, then when the slider is pressed under a small angle, it gets strong binding issues (i.e. it is stuck and needs a lot more force to move it) and it also exhibits a substantial "stick-slip" after it starts moving, i.e. irregular dynamic friction, which instead of a steady velocity and a smooth sliding it slides in a starting-stopping way.
When I applied Grease #2, the aforementioned issues got greatly reduced (not completely eliminated the static friction though), resulting in a much better and smoother sliding.
However, this is contrary to what I have expected. On paper, Grease #1 is much more viscous than Grease #2. And according to the consensus saying how much viscosity matters, it should be the #1 that should make it smoother and not the other way around.
These 2 different greases have a big difference in their base oil viscosity at 40C. Their Techincal Data Sheets claim that grease #1 is 9500 mm2/s viscous and grease #2 is 1500 mm2/s viscous at 40C.
Another important difference is this:
- Grease #1 is a grease with a Polydimethylsiloxane (PDMS) "silicon oil" as Base Oil, and an "inorganic" Thickener (probably silica)
- Grease #2 is a grease with a Polyalphaolefin (PAO) "synthetic hydrocarbon" as Base Oil, and Silica as a Thickener.
After a simple study of the basics of Tribology and by reading various articles on the subject, I have came to know that viscosity is regarded as the number one factor for lubricating two sliding surfaces and that higher viscosity results in lower static and dynamic friction and more importantly eliminates the unwanted stick-slip phenomenon*. The higher the viscosity the better.
According to the above, for my use case, grease #1 should be much better at both reducing friction and eliminating stick-slip sliding. However, when applied and tried empirically the exact opposite is true. #2 is actually lubricating where #1 is probably making the friction and the stick-slip even worse than without applying any grease at all.
- Why this happens instead? Is the viscosity rated in the TDS wrong? Is there another explanation?
- What property exactly of #2 is responsible that provides the desired lubricating properties, which #1 lacks?
- Is there another property of a lubricant (grease) that would be better for this use case?
Use case: Two plastic parts sliding against each other. They are parts of a keyboard switch. The plastics are Nylon (the Slider) and the second (Housing) is probably ABS. More information: https://deskthority.net/wiki/Alps_SKCL/SKCM_series (the parts are the Slider and the Shell)
Some relevant and helpful information:
The switch was made during the 1980s, therefore this means that they must have used oils/lubricants/greases that used the technology that was available at that time. The switch's Slider came pre-applied with a grease/spray/solid/whatever from the factory, which strongly suggests that the manufacturer knew that without it, the slider would not slide smoothly. And this is a fact that have been verified, if this is cleaned off, the switch gets a high static friction.
One more important thing: Another grease, Grease #3**, actually worked very well with this switch and completely eliminated any static friction, even at obtuse angles. It has Polyalphaolefin (PAO) as Base Oil and Silica as thickener, however it is extremely viscous. It is considered a heavy "dampening" grease instead of a "lubricating" one, which makes the sliding slower, but also makes it very precise and smooth. These kind of greases are used with binoculars, microscopes, rotary dials and controls that need a very precise and smooth movement, and they are regarded as highly desirable qualities for these use cases.
My guess is that the factory applied grease/solid/whatever, is probably a "dampening" one too. Switches in a good condition from that series, have that precise and smooth slide, which is a characteristic behavior that differs significantly from the behavior of other switches applied with usual lubricants.
Technical details of the greases:
Grease #1 Technical Data Sheet: https://www.oks-germany.com/product_downloads/pdf/PI/en/PI_OKS_1110_110645_EN.pdf
Grease #2 Technical Data Sheet: https://www.oks-germany.com/product_downloads/pdf/PI/en/PI_OKS_477_113696_EN.pdf
*. "The test results show that, by increasing the viscosity of the lubricant, the potential for stick slip is greatly reduced." - How to Measure, Prevent, and Eliminate Stick-Slip and Noise Generation with Lubricants
**. Grease #3 Technical Data Sheet: NyoGel 767A "Dampening" grease