Estimating cylinder speed is extremely difficult because of the flow losses within the piping, fittings, and porting through the valves. When compressed air flows through a system, there are pressure losses due to friction against the tube wall, flow around bends, and restrictions in valves and fittings (to name a few issues). Higher speeds result in greater pressure loss as the air must flow faster through the valves, tubing, and ports. Attaining higher speeds also requires that the cylinder deliver more force in a shorter amount of time. A force that exceeds the load by 50% or more may be required to reliably move a load at high speeds. For example, a typical air compressor might supply air to a system at 100 psi. In an application with a slow-moving load, the actual pressure available at the piston might be reduced to no less than 90 psi. With that same load moving at a much faster rate, the available pressure could drop as low as 70 psi.
To be able to determine the maximum speed of the cylinder, the sum of all flow losses, pressure required for the force output and the available inlet pressure must be known. Circuit losses are nearly impossible to determine or calculate accurately. Rules of thumb are generally relied upon to determine an approximation of air cylinder speed.
The first general rule of thumb is choose a cylinder which will allow for at least 25% more force than what is required. For extremely fast operations, chose a cylinder which will allow for 50% more force than what is required. This will leave 25% or 50% of inlet pressure to satisfy system losses.
The second rule of thumb is to select a directional control valve which has the same port size as the cylinder which it will be operating. For large valves, internal flow capacity is typically the same or greater than the capacity of the intended connection fittings. On smaller valves, the internal flow capacity is typically much less than that of the connection fittings. But always be sure to check the valves flow rate, and do not rely solely on the port size.
Lastly: use large diameter tubing (relative to cylinder bore), keep the tubing length as short as possible and use as few fittings as possible.
If a specific cylinder speed is desired, it's advisable to size the valve, tubing, fittings, and cylinder for a faster stroke. And then use flow control valves to slow the cylinder down to the desired speed. If extremely fast action is desired, some trial and error testing will probably be required. Consider the use of cushioned cylinders to prolong the life of the components in high speed systems.
|Theoretical time (in seconds) required to make a inch stroke|
|Cylinder Bore Sizes (in inches)||Cv of Valve|
NS = Not Suitable. Valve/cyllinder combinations that result in low theoretical speeds will likely result in jerky or lurching motion.
VF = Very Fast. Predicting theoretical speeds is not practical beyond certain limits (see note below). Other "fast" speeds that ARE shown should be taken with a grain of salt (see note below).
¹ The pressure drop shown is the DROP ACROSS THE VALVE ONLY, see note below. In practice, you can not choose ΔP (pressure drop) - the pressure drop of a given system is dependent on many things.
NOTE: This chart does not take into account tubing diameter or length, number or size of fittings and orifices, size or weight of the load, etc. You may need to factor in these (and other) values to determine the actual duration of a stroke. There are also practical limits to how fast a cylinder can physically operate.
Here is an interactive timing chart showing actual CV values for Nitra® valves and bore sizes for Nitra® cylinders.
Information about cylinder speed based on valve CV is here.