When should you use fluid power motors?

The evolution of rotational power has favoured the electric motor and the fuel fed reciprocating engine, each for seperate reasons. These devices, however popular, have some shortcomings which limit their use in some industrial applications. The problem areas, most of which can be addressed by fluid power motors, include the following.

Pneumatically driven motors have the ability to stall and/or instantly reverse repeatedly without any heat generation or detrimental effects whatsoever. Hydraulic systems, using a relatively non-compressible fluid as their power transfer medium, will generate heat in the fluid as the inertial and fluid forces are absorbed. Good system design will generally minimize these problems.

The internal braking forces developed in a motor during stalling or reversing are easily controlled within the limits and capabilities of the system. Crude acceleration and deceleration profiles can easily be achieved by balancing the load inertia against the braking or relief force. A typical application of this capability is a conveyor drive where a long belt full of heavy product is indexed from position to position.

The very small size-to-power ratio of fluid power motors can be extremely advantageous in a number of industrial applications. Pneumatic motors are used extensively in industrial hand tools such as drills and screwdrivers where light weight and small size are extremely important factors.

Fluid power motors can very often be an economic solution for rotary power use in hostile or explosive environments. Hydraulic motors are completely enclosed and self-contained and, barring leaks, can be operated submerged or used in many other hostile areas. Pneumatic motors are excellent for use in explosive environments and may even be run on an inert compressed gas such as nitrogen.

Several types of hydraulic motors normally are used in industrial applications. They all provide a relatively constant, predetermined speed throughout their pressure range. Their speed rapidly falls off when the maximum torque setting is reached and the hydraulic fluid bypasses the motor through a relief valve. System heat generation can be minimized through the use of a pressure compensated, variable volume pumping system in place of the relief valve.

Vane motors are the most popular general-purpose hydraulic motors. They have some limitations with starting torque and at low speeds due to the higher percentage of slippage or internal leakage relative to the lower total fluid flow. Vane motors are also limited by their tolerance to high pressure systems.

Piston motors, either radial or axial in design, are generally more expensive but are far more adaptable to high-torque, low-speed operation as well as higher system pressure applications.

Gear motors are the least expensive of the hydraulic motor families but are also the noisiest. They have the ability to operate at fairly high speeds but, as with the vane motor, their performance falls off at low speeds.

Gerotor hydraulic motors are gear motors with internal meshing gears. They are excellent high-torque, low-speed motors because of their inherent gear reduced operation.

Pneumatic motors

Pneumatic motors behave quite differently than their hydraulic relatives. The biggest contributing factors are the compressibility of the air and a much lower operating pressure range.

By far the most popular type of air motor is the vane motor. The smaller vane motors usually operate at speeds exceeding 20,000 rpm and are geared down to usable speeds with planetary gearing. This maintains their very small diameter and high power to weight ratio, making them excellent for use as hand tools. Piston motors, although not as popular, are excellent for low-speed operation where high starting torque is absolutely necessary.

The final operating speed of an air motor is very unpredictable. It usually results from a balance between the resisting torque (the load), the working pressure realized inside the motor and, of course the available unrestricted compressed air flow. Air motors should be sized to operate at the maximum power peak, which is usually about 60 per cent of the free running speed.

Air motors, however, have a big efficiency problem. When compared to the electric motor power used to drive the compressor, most air motors can only develop about 10 per cent efficiency. Luckily, most viable applications for air motors are intermittent in nature or spend a good deal of their time in a stalled state. In either case, the average airflow through the motor is relatively low and the low efficiency factor is meaningless.

While it is extremely doubtful that fluid power motors will ever threaten electric motors or fuel driven engines for a dominant place in the rotational power market, they will always provide a convenient and competitive solution in applications involving infinite speed control, stalling under full torque, small size, high power-to-weight ratio, and hostile operating environments.

Ted Grove, corporate training manager for Wainbee Limited of Mississauga, Ont., is an experienced fluid power trainer. This article is the seventh in a series. Visit www.mro-esource.com on and click on the Past Issues button to view Practical Automation columns from previous issues of Machinery & Equipment MRO.