Friction, Friend or Foe?
Friction is rarely talked about in motion control circles (pun intended for those paying attention). It is the “waste” energy in mechanical systems. We spend a lot of time and sometimes cost, trying to eliminate it. Many times we just ignore it.
This was the case when a friend of mine was designing a material handling system for newspaper bundles. A very exotic conveyor system with 8 servo driven belts and a design that involved 10 pages of hand calculations of inertia. We shipped the servos and sent out a field engineer to start up the project only to find out that the motors and drives were too small. The designer had forgotten to account for friction. In this case the frictional load was 3 times the mechanical load due to the unique belt and roller configuration.
So the first lesson is; don’t forget to look at friction as 1 of 3 components of the torque load. The three being; steady state torque, torque of acceleration and friction.
Then there is the fanciful wishing that there wasn’t any friction to worry about. Kind of like doing experiments in the space station and having no gravity. It’s fun to think about, but there are few real world situations where this is likely to work. The only exception is air bearings. Of which there are a few.
If you have ever played air hockey, air bearings are like that. Parts in motion tend to stay in motion when there is no friction to worry about. And that would be good in a lot of applications. No friction will generally result in smaller servos, so there are savings in the hardware requirement. No friction means no mechanical wear, nothing to service as the machine runs up cycles. No friction also means high speed motion is a lot easier to achieve.
Cars coast to a stop because of friction. That’s a good thing. Without friction, parts would end up flying off the conveyor instead of going where you want them to go. In conveyor belt applications there is usually a lot of friction and that helps the system slow down and stop.
So the second lesson is; friction can be your friend.
In between systems with friction and systems with no friction, there are rolling bearings. Systems like the Bishop Wisecarver “Vee Guide” are among many products on the market are examples of this. Rolling element bearings have very low coefficients of friction, so losses are low and therefore the energy needed to overcome them is very low. This also results in very low wear, so maintenance on this type of mechanism is also low.
The are dozens of linear actuators on the market and each vendor has developed unique bearing solutions, whether sliding or rolling, that perform well at varying price points. There are no universal rules for selection. The typical criteria are move speed, positioning accuracy, life expectancy and cost.