Henry Fleischer
Vice President
Research and Development
Numatics Inc.
Highland, Mich.
Many engineers unfortunately spend pitifully little time designing pneumatic circuits, properly sizing components, and selecting optimum pressures for pneumatic systems. Taking the time to satisfy all system requirements is critical in pneumatics. The potential savings from using smaller components, increasing productivity, and recovering energy easily make up for extra design time. Although actuators, or air cylinders, are the last link in pneumatic circuits, the design of a pneumatic system typically begins by selecting actuators. They are a good starting point for this discussion because actuators are often improperly sized. Furthermore, in many plants actuators consume more compressed air than any other component.
One problem with choosing the right actuator is that there are no foolproof formulas for actuator selection. Because of this, engineers simply use excessive safety factors and wind up with increased operating costs, hampered productivity, and parts that are more expensive than necessary. A database from Numatics Inc., Highland, Mich., addresses these problems by compiling results from several hundred thousand tests on pneumatic systems. The database lets engineers specify system requirements and parameters to help them choose the best components and pressures for their designs.
Incomplete loads
A few critical mistakes can impede performance and often crop up when designing pneumatic systems. One common mistake is failing to determine all loads that actuators must overcome. This is a vital step in determining appropriate bore sizes. Engineers often select actuators by focusing only on available pressure, mechanical loads, and one of three multipliers, 1.25, 1.5, and 2.0, for piston velocity. Although multipliers are a step in the right direction (because velocity is a major load that actuators must overcome), most designs are still inadequate when they’re limited to mechanical loads, multipliers, and available pressure.
For more reliable designs, include several additional factors. Calculating actual mechanical loads is typically the starting point for cylinder design. This value includes friction components for extend and retract strokes and the application’s angle of inclination. When inclined angles are large enough, take advantage of gravity-assisted forces to reduce the actuator’s required downward force. This, in turn, reduces pressure requirements for downward motion.
The force required to generate velocity is another load constraint actuators must overcome and should be folded into total-load equations. Increasing velocity of the mass requires higher forces. For high-speed applications, the force demanded of actuators increases dramatically to the point where the F = m a portion of the total load exceeds mechanical loads.
Actuator motion involves either extension or retraction. Actuators extend by moving a rod out of a cylinder housing and retract by moving the rod into the housing. The rod connects to a piston inside the housing. The piston effectively divides the housing into two separate “actuator ends,” each of which has pressure independent of the other. This pressure differential moves the piston, rod, and load. Interestingly, the two loads for an actuator’s extend and retract strokes are rarely equal. Instead there is a major load for the direction that requires work and a minor load required to return the tool. Although many designers simply use a single high-pressure source to move both loads, a better approach uses two different pressures to move each load. This often shortens cycle periods, saves energy, and increases productivity.
The quantity and rapidity of exhausting air from actuators and conductors also impacts actuator performance. This happens because pistons remain stationary until sufficient air is evacuated to commence motion. In some applications this delay exceeds piston-motion time. However, if the actuator end being exhausted carries a minor load, the actuator only requires a low pressure, and, therefore, less air than if the actuator were carrying major loads. This shortens delay time and overall stroke time. Matching pressures to loads reduces air consumption and simultaneously increases productivity.
Reducing initial charge volumes is another way to rapidly evacuate air. This shortens performance time, which consists of delay time plus piston-motion time (stroke time). All cylinders, regardless of load or pressure, experience some delay in stroke time. Moving pistons from rest requires a pressure differential between the two actuator ends. The air being charged on the end requiring motion fills quickly because the volume only consists of the conductor. The air discharging at the opposing end, however, evacuates via gravity only because the moving piston does not assist evacuation until the actuator reaches the required differential pressure.
Optimizing performance
Pneumatic components are often measured in terms of their conductance, Cv, which is the capability of pneumatic devices to move air under a differential pressure. In other words, the greater the Cv, the better the flow. System conductance refers to the total conductance of all the various pneumatic-circuit components. To achieve ideal performance, system conductance must exhaust all stored air within an allotted time.
Although increasing conductor sizes between the valve and actuator exhausts stored air faster and improves conductance, it also increases the exhaust volume. Therefore, increasing conductor size has both beneficial and detrimental effects on actuator performance times.
The object is to select the optimum conductor size to maximize productivity. Fortunately, for every conductor length there is one optimum size that satisfies peak productivity. If, however, the object is to minimize component sizes, the optimum conductor will likely be different than the optimum conductor for high productivity. Similarly, if energy conservation is the goal, the optimum conductor will again be different.
Actuator designs behave similarly to those of conductors. Increasing bore size, for instance, generates higher forces to move loads and, in turn, increases speed. However, doing so also impacts the volume that must be evacuated and thus impedes performance time. Similarly, minimizing component sizes or reducing energy consumption each requires a different bore size. Although there is one bore size that optimizes productivity, there traditionally is no rule of thumb for selecting bore sizes based on loads. In many cases, however, a 50% formula works.
The 50% formula states that a cylinder-bore propelling force with approximately twice the total load generates the greatest speed. For example, to move a 100-lb load at the fastest possible speed, use a cylinder with a 200-lb propelling force. Propelling force equals pressure times face area. Often, however, designers are limited to off-the-shelf cylinder bores that do not match the calculated result. Therefore, the 50% value may vary slightly one way or the other.
Reaching design goals
The goal of any pneumatic design is to optimize components and fulfill several objectives. We have concluded earlier, for example, that there is no best component for a circuit unless accompanied by an objective. For instance, minimum-size components save initial capital expenses and require specific cylinder and valve sizes and specific pressures. Maximizing productivity requires larger valves with different pressures. Energy conservation demands a third valve size and, perhaps, a cylinder with different pressures. Other objectives, such as energy optimization, carry distinct component sizes and pressures. It is evident that there is no ideal set of components or pressures capable of satisfying all objectives.
Numatics has addressed these and other issues with its Numasizing program. It is based on observations of over 250,000 test firings. The tests primarily used double-acting air cylinders with bore sizes ranging from 3⁄8 to 14 in. The cylinders carried various loads and pressures in combination with different valves, conductors, and fittings.
The database lets users confidently predict performance data, such as actuator response times and terminal velocity, circuit pressure drops, horsepower requirements, flows, and air costs. It also lets designers select optimum components, port sizes, and pressures. The program accounts for all customary physical parameters of pneumatic networks, such as conductor length and diameter, available pressure, cylinder loads, and response times. The program also assesses various design goals, such as cycle time, energy consumption, and component sizes. Essentially engineers can design tailor-made circuits that fit their physical and objective needs.