Electricity + Control July 2017

DRIVES, MOTORS + SWITCHGEAR

Applying Speed Reducers to Mechatronic Systems Glyn Craig, Techlyn

The term mechatronics unites various engineering disciplines such as mechanics, electrical, electronics and computer engineering.

A t the forefront of successful designs lies the correct specification of the mechani- cal components used. This article concen- trates on the key interface, the speed reducer which couples a driving motor to a driven load.

• Figure 1 shows a typical leadscrew arrange- ment. Screws are usually steel (ferrous or stainless) with nuts made from bronze or self-lubricating plastics such as Acetal. The screw thread can be rectangular, but trapezoi- dal types are more common. Efficiencies are in the range of 35% to 80% depending on mate- rial used and lubrication. • Figure 2 shows a ballscrew and nut. In this case the sliding contact between the screw and the nut is replaced by the rolling action of the recirculating balls in the nut. Efficiencies are now in the range of 85% to 95%

Speed reducer functions • Match motor top speed to load

• Match motor rotor inertia to load inertia • Isolate drive from load environment From this, it will become apparent that reducer se- lection is not a trivial process. Reducer types • Convert motor rotary motion to linear motion. Many machines such as machine tools and gantry robots require this type of motion. The required speed conversion is always part of the process. Common methods include lead- screws, rack and pinions, belts and chains • Drive rotary loads with suitable speed change. Applications include rotary tables, machine spindles and robot arms. Apart from gearhead boxes, motors can drive loads via belts (usually toothed) or chains Linear drives A small number of applications suit the use of linear motors. The moving carriage interacts with stationary guideway magnetically. At present, ap- plications are restricted to high speed low inertia systems such as pick and place machines. Leadscrews These are widely used to convert rotary input to motion linear displacement.

Figure 1: Lead screw.

Figure 2: Ball screw.

10 Electricity + Control

JULY 2017