Size ‘Em Up: How to Properly Select and Size an Air Motor for your Needs
You’ve probably heard people say that if you oversize an air motor and load it lightly, you’ll save money and enhance the durability of the motor.
That’s not true!
Air motors are most efficient when loaded up to 95% of their capacity.
Besides, an over-sized air motor presents challenges such as erratic tool operation, reduced capacity, and high energy costs per unit. It can also cause sludge and rust build up in the main and branch lines, which can shorten the service life of your motor.
So how do you select the right air motor for your needs?
Here are six main things to consider.
The portability of an air motor determines its size. There are two options in this regard – stationary and portable air motors.
Stationary air motors are typically larger in size and should be used in a spacious garage or workshop. They pack a high amount of horsepower and come with a huge storage tank for prolonged usage. They also have a vertical orientation to cut on the amount of floor space they take.
Portable air motors, on the other hand, are smaller in size and come in different styles including:
- Pancake compressors: These have round, flat gas storage tanks that are mounted at the bottom of each unit. This design creates a more stable set up that takes up less space than most portable air motors.
- Hot dog compressors: These come in a horizontal design with one cylindrical tank.
- Wheelbarrow compressors: These compressors have two cylindrical tanks. They also have wheels and handles for easy portability.
- Twin-stack compressors: This type of portable compressors come with two cylindrical storage tanks. Their stacked design allows them to pack more air without taking up too much space.
2. Required Air Flow
Airflow in pneumatic motors is measured using two merits. These include Pounds per Square Inch (PSI) and the deliverable Cubic Feet per Minute (CFM).
Collectively, these measurements dictate the effectiveness of an air motor in different scenarios.
When choosing a compressor, let the CFM and PSI requirements of your tools guide you. If your tasks need you to use one air tool at a time, let the CFM and PSI requirements of your highest rated tool dictate your choice. If you use several air tools simultaneously, you’ll need your air motor to match the total sum of their CFM requirements.
In less technical terms, choose a compressor that meets the CFM and PSI airflow demands of your tools so you don’t end up being underpowered.
For best results, go for air motors with 1.25 to 1.5 times the CFM airflow requirements of your highest rated tool. This will allow you to maintain optimal performance of your tools without overloading the compressor. It’ll also increase the efficiency of your air tools and the compressor.
The load speed dictates the amount of load your air motor can handle, and to some extent, the amount of power produced by your motor.
Generally, the speed of a pneumatic motor drops as you increase the load. The power of the motor, on the other hand, increases as the load increases.
As a general rule, always try to size your motor such that it runs at 40-50% of its idle speed. Depending on the amount of load, the working speed can be as low as 10% of its idle speed.
4. Force Output
Another key factor to consider when selecting an air motor is the amount for force generated by its cylinder. You can determine this from the bore size and the air pressure of the cylinder.
For high-friction and vertical tasks, the force output should be two times the load. In some cases, you may need extra force to compensate for extra friction.
The two types of torque you need to think about when selecting an air motor are the stall torque and the torque under load.
Stall torque simply means the maximum amount of torque generated by a motor. The amount of stall torque in your motor should match your application. For light applications such as clamping and screwing, you need the least stall torque possible.
In applications that require maximum stall torque, always ensure that the torque you apply doesn’t exceed your system’s mechanical resistance because this can easily cause breakage.
The torque under load is especially useful if you use your motor to lift loads. This kind of torque is variable and depends on the position of your motor’s vanes at startup time. It can also vary according to the quality of lubrication you use on your motor.
6. Power Reserve
If you want your motor to maintain a steady speed even after a torque increase, you may want to consider the power reserve. This means that you’ll have to choose a motor whose working point speed is higher than the speed it runs at maximum power. In case of unstable speed, you may need a torque limiter to prevent your motor from reaching maximum torque.
You may also want to choose a motor whose speed at the optimal working point is less than its speed at normal working point.
Choosing the Right Air Motor for your Needs
If you keep the above factors in mind, you’ll most certainly end up with the right air motor size for the job. When selecting an air motor it’s critical to check the valve, filters, and regulators to ensure they’re working properly.
Also, check out our guide to choosing the right air compressors so you can make a more informed decision.