Can someone tell me why the need for specific brass nuts and washers in this step of building the actuator?
Brass on steel is lower friction than steel on steel, this is why brass is often used for things like bearings and leadnuts. Also as the softer material it should wear the nut not the screw and then the nut can be replaced. With steel, both would need replacing from wear.
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.