Air and hydraulic motors also are other types of actuators that turn fluid power energy into rotary output. They are pumps in reverse and can come in as many varieties. Unlike cylinders that are rated in pounds force thrust, motors produce twisting or torque that turns a shaft. Motors are rated in torque output usually stated in pound-inch (lb-in.), pound-foot (lb-ft.) American; or Newton-meters, (N-m) Metric.

Figure 15-20. Formula for figuring torque

The example in Figure 15-20 shows how torque is measured and applied to shafts. With a 500-lb weight hanging from the end of a 2-ft arm rigidly attached to a shaft, the shaft would have to resist 1000 lb-ft of torque (500 lb X 2 ft) to keep from turning. This is only true when the weight is at right angles to the shaft. Changing arm length or the amount of weight changes the amount of torque. As noted in the example, the metric figure is in Newton-meters and one Newton-meter equals 8.851 lb-in. and 0.7375 lb-ft.

Why use a fluid motor?

The main reason cited for using air or hydraulic motors is that they have high torque in a small package. Air drills of 1/2 to 3 hp for many rotation speeds and torqus require less than one-third of the space a comparable electric motor setup would use. This means space at the work is less cluttered and/or more units can be applied. Hydraulic motors are even more compact especially in higher horsepower units. The main reason for the small size is no reduction gearing is required for hydraulic motors and small planetary units work well with air motors.

    Other reasons for choosing fluid motors are:
  • They have instant or almost instant reversing capabilities. Because these motors are so compact and have little inertia to overcome, they can reverse rotation quickly without damage. Some motors act as oscillators that never make a full revolution.
  • Variable speed capabilities of fluid motors are simple and result in little change in torque when it is a low-speed/high-torque hydraulic motor. A change in flow with flow controls or variable volume pumps requires little sophistication when minor speed changes can be tolerated. Sophisticated servo controls allow speed changes and can maintain tight control at any speed.
  • Overload protection is part of any fluid power system and motors have the same ability. When a motor circuit meets resistance it cannot overcome, it stalls and holds torque without damage to the circuit or machine. Fluid motors are capable of stalling for long periods without over heating or damaging themselves. There is some internal leakage while stalled but this can be minimized with the right motor selection.
  • Another place where fluid motors shine is in wet or explosive atmospheres. These motors have no electrical input so they pose no threat from sparks or overheating. A hydraulic motor can operate underwater with bio-degradable fluids without any of the problems electric motors have in this application.

Pneumatic motors

Air motors are often very inefficient and with air compressor inefficiency added there is often no more than 20% utilization of input power. This high inefficiency is offset somewhat in most applications because the air motor only has to run while work is taking place. This could mean the air motor must start and stop often but this is not a problem for fluid motors.

There are a variety of pneumatic motors in different configurations and with different attributes. The vane type shown in Figure 15-21 is one of the oldest designs. They are usually high to very high speed from 1000 to 30,000 rpm for everything from sanders and grinders to dentist drills. These motors may offer unidirectional or bidirectional rotation according to their design use.

Figure 15-21. Vane-type air motor

The cutaway shows vanes in a rotor that is off center in a cam ring. Air at the inlet acts against the vanes halfway around, forcing the rotor to turn while spent air exhausts during the other half revolution. Only one vane in this unbalanced design is producing torque. The remaining vanes on the inlet side have pressure on both sides so they have no force.

The vane motor in Figure 15-21 is for medium-to-high-speed applications at low torque. Several manufacturers add reduction gears to give low-speed/high-torque capabilities.

Figure 15-22. Radial-piston air motor

The radial piston motor in Figure 15-22 does not require reduction gearing for most low-speed/high-torque applications. Force from half the pistons is driving the crankshaft to turn it while the remaining pistons are exhausting. Inlet-outlet ports connected to a rotary valve driven by the crankshaft send 40 to 100 psi air through cored holes in the body and cylinder bores to and from the pistons. Most radial piston air motors run at less than 500 rpm due to air energy waste at faster speeds. These motors can be physically large and take a lot of mounting space. Some manufacturers make axial or in-line piston motors while a few have experimented with gerotor designs.