Consider every system component to ensure top performance with minimal losses.
Tabulated results are for the example circuit lifting a 10-lb mass 3 in. in 0.170 sec or less. Cylinder ports are 1/8 NPT, valve ports are 1/4 NPT, and there are no flow controls in the system. Note that the 11.96-psi cylinder fitting exhaust pressure drop is the limiting factor with the larger cylinder, and the 79.70-psia Check Sum pressure limits the small-cylinder circuit.
Optimizing circuit design
Here are a few suggestions for optimum circuit design:
- Make air-line lengths as short as possible.
- Make air lines between the valve and cylinder as straight as possible with minimal bends.
- Select cylinder bore sizes to handle the expected load plus a reasonable safety factor. Larger-than-necessary cylinders cost more money, waste energy, and add cycle time.
- Cylinder stroke should be no more than required. Longer-stroke cylinders cost more, waste energy, and add cycle time.
- A valve can be oversized without appreciably wasting energy. However, cycle time will increase if solenoid shift time increases.
- Overpressurizing a circuit beyond a certain point -- the maximum pressure drop -- does not increase cylinder speed but does waste air and can increase delay time and total cycle time.
- If the application calls for different loads or speeds for extend and retract motions, consider using different pressures or add flow controls.
- Consider using quick-exhaust dump valves. The dumped air bypasses the exhaust circuit, possibly reducing cycle time and increasing speed.
- Each application has an optimum air-line ID. Increasing the air-line diameter increases Cv but also increases the volume that must be filled and evacuated each cycle.
- Components with the smallest Cvs and largest pressure drops limit circuit performance. Increase these Cvs first to have the greatest impact on circuit performance.
- Components with the largest Cvs and smallest pressure drops are possibly oversized. Decreasing these Cvs could improve circuit performance.
Al = Air-line cross-sectional area, in.2
d = Cylinder diameter, in.
df = fitting ID, in.
dt = tubing ID, in.
fl = line-friction coefficient
l = line length, in.
Q = Flow rate at 1 atmosphere, 68°F, and 36% relative humidity, in scfm
Cv = Flow coefficient
G = Specific gravity of air at one atmosphere, 68°F, and 36% relative humidity. Usually, G = 1.
g = Acceleration of gravity
ks = Specific-heat ratio; ks = 1.4 for air.
n = Number of fittings
Pe = Exhaust pressure, psi
Ps = Supply pressure, psi
P1 = Upstream pressure at T1, psia
P2 = Downstream pressure, psia
qm = Mass flow rate
T1 = Upstream temperature, °R. Usually, T1 = 528°R.
Ve = Exhaust volume, in.3