How can I calculate the optimal viscosity for a lubricant?

How can I calculate the optimal viscosity for a lubricant for the motor’s internal gears (at every stage) and for the bearings (at different loads)?
Also, what temperature do the internal gears reach during normal operation?
How should grease be applied?

It varies wildly on the type of lubricant and the temperature.

Doing a quick google search on viscocity charts I came across this that explains viscocity.

This one from SKF had the viscocity rates for lithium grease in the food safe section.

No idea the temperatures but I think in the motor library threads you can find some temperatures used to estimate the PTC trip…](

Another interesting question!

Just to explain the basics, viscosity is a measurement of how a liquid flows. If it flows easily, it is “thin” and it has a low viscosity, for example water. If it hardly flows, it is “thick” and it has a high viscosity, for example honey.

Viscosity is very much affected by temperature. As the temperature of a liquid is raised, the viscosity lowers, sometimes drastically, as shown below.

My question is, how do you define optimal viscosity for use with the internal motor gears? Is the optimal viscosity the one that provides the least wear of the gear teeth or is it the one that provides the least resistance to the motor turning? A luxury car owner might go for the former while a race car driver might go for the later?

The temperature of the internal gear lubricant in a VEX motor module is not a fixed value, as it will be different at the start of a match to the end of a match, so the lubricant chosen would ideally perform well over a range from ambient temperature ~ 20°C to whatever it reaches in a hot motor, my guess ~ 50°C.

We have used a small amount of relatively thin silicone lubricant on the gears - whether this is optimal, I have no idea, but I am reasonably convinced that this is better than not adding any lubricant. :slight_smile:

Cheers, Paul