which properties do YOU think are important?

personally, I think that the inability of plastics to resist abrasion from rubbing would make them short in service in typical terrestrial applications and at high rpms or heavy load due to cutting and brinelling compared to heat treat steel, but tribology is probably the least of the design worries.

So what properties do you think make plastics a substitute for steel?

milo "I'm somewhat conversant on steel properties and applications"

There are some interesting plastics such as polycrystaline materials reinforced with carbon fiber, or you might try researching Turcite. The success of using a plastic for a motor shaft would depend on the size and requirements of the motor. You might have to design the motor with a larger diameter shaft and redesign the other armature components accordingly. I think it could work in light duty applications or where longevity is not an issue but weight is an issue. Turcite is used as a bearing material and is very hard and slippery. These factors to me would make it a good candidate for experimentation. It is also expensive, so if you want to save money it is probably not a good idea, but if you want to save weight and cost is less of a factor, it might work.

Plastics have to be molded, extruded, or machined. Costs for engineering plastics are much higher per pound than most steels. The tooling to mold parts would also be much higher cost than screw machine tooling. Turned metal shafts can more easily be held to tight tolerances than molded parts. The real savings from plastics are from either weight reduction, or being able to mold complex shapes that are difficult to machine or stamp, or where electrical insulating properties are required. Turning a metal motor shaft on a screw machine is generally a lower cost method of manufacturing than molding or extruding and machining. While technically feasible for some motors, probably low-load applications, the costs are probably prohibitive.