Selecting the right motor first time saves a lot of messing around. The starting point for any motor selection is the required speed and either the power of the motor or the torque required. The other factors need to be considered, for example, air consumption, service life, start torque, stall torque and shaft loading.
When comparing motors from different manufacturers, it is necessary to check that the specifications are all made at the same air pressure. A 1kW motor rated at 6bar will have 20% more power than a 1kW motor rated at 7bar.
Power
The most straight forward way of selecting a motor is to determine the power requirement. Once this is worked out, you simply go to the motor range with at least the calculated power requirement. If the motor is overpowered, it may be necessary to adjust the sped using a combination of exhaust throttling and air pressure regulation.
The required power is easily calculated as long as you know the torque and speed requirements. It is calculated using the following formula:
Power (kW) = Torque (Nm) x Speed (rpm) / 9550
Speed
An important consideration in choosing an air motor is the fact that an air motor’s speed will slow according to the load applied. A 1200W air motor may have a no load speed of 9000rpm, as the load increases, the speed will drop, but the power will peak, and is measured at the nominal speed of 4500rpm. We normally try to size motors so that they will run at 40 to 50% of their idle speed. Depending on the load, the working speed could be as low as 10% of the idle speed. To correctly determine the final speed, we need to know the working torque. The motor’s final speed can be adjusted by using a combination of gearing, pressure regulation and throttling. The use of a motor with integrated gearing can often be cheaper than using a motor coupled to another brand of gearbox. Check both options.
Air Consumption
When comparing different air motors, always check the air consumption. Compressed air can be expensive, and over the lifetime of the motor it can pay to use the most efficient motor possible. Careful motor selection can also reduce air consumption. Firstly the smallest possible motor should be chosen, and ideally it should be running at about 40 to 50% of it’s idle speed.
Shaft Loading
It is important to check if the radial and axial loading on the motor shaft is going to be within the manufacturer’s guidelines, otherwise costly premature bearing wear and possible gear damage can be expected. Some motors will not tolerate any axial load at all. Either another motor should be selected, or some kind of external shaft support should be added to the design.
Shaft Type
Other than the standard keyed shafts, some manufacturers offer other options. Deprag offer threaded, tapered and even collet shafts on many of their motors and short runs of special shafts can be accommodated.
Service Life
If the motor is going to be used constantly, correct motor sizing can increase the time between service intervals. In general a motor should be chosen so that it is oversized as well as running near its nominal speed for maximum lifetime. If necessary, the speed should be reduced by throttling the air flow or reducing the air pressure.
