Also, we’ve found that using two washers between the gear and the main body is quite important for longevity; with only one washer, or with no washers, the plastic wears quite quickly. With two washers, most of the sliding friction is between the two steel washers, which will last much, much longer.
Using a steel nut will work initially, but will fail more quickly, particularly if you are running long, automated experiments. We found during fatigue testing that correctly specifying the nut/washer/screw and using light oil as directed increased the lifetime from about 24 hours of continuous operation to many months.