Some factors to consider when selecting and using pneumatic linear actuators – air cylinders.
Mode of action
Three basic types:-
Single-acting, spring return
Movement and force by air pressure in one direction, return movement by internal spring force – available as spring to instroke and spring to outstroke.
Double-acting
Air pressure required to produce force and movement in both directions of travel.
Double-acting, through rod
Double-ended piston rod which acts as a normal double-acting cylinder but mechanical connections can be made to both ends of through rod.
Rodless cylinders
Where space is at a premium and there are potential loading and alignment problems, a variety of rodless cylinder designs are available. The range of available bore sizes is limited e.g. 16-100 mm.
Quality classes
Basically three qualities of unit available:-
Light duty and compact
Limited range of bore sizes, up to 100 mm. Not cushioned at stroke ends, or cushion pads only. Check manufacturer’s data sheets for serviceability and susceptibility to corrosion.
Medium duty/standard
For normal factory environments. Some degree of corrosion resistance. Serviceable. Cushioned at both ends. Usually double-acting. Bore size range, 32 mm and greater.
Heavy duty
Rugged construction. Serviceable. Non-corrodible materials. Superior cushioning – thicker piston rods and heavy duty mountings. Bore size range, 32 mm and greater.
For interchangeability and standard mounting dimension, see BS ISO 6432 – 8 to 25 mm bore, BS ISO 6431 – 32 to 320 mm bore – standard duty, double-acting, metric dimensions, BS ISO 15552 32 mm to 320 mm – Basic and mounting dimensions and accessories and BS ISO 21287 1000 KPa (10 bar) series – Bores from 20 mm to 100 mm.
Standard bore sizes
Double-acting
8, 10, 12, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125, 160, 200, 250 and 320 mm.
Standard, stocked strokes
Double-acting
25, 50, 80, 100, 125, 160, 200, 250 and 320 mm.
Cylinder thrust
To calculate the theoretical thrust of the double-acting cylinder, use the formula:-
Thrust= [add calculation] newton
Where:-
D = diameter of piston (mm).
P = Gauge pressure (bar).
Pull will be less, due to area of piston rod.
Pull = [add calculation] newton
Where:
d = Diameter of piston rod (mm).
For static loading, i.e. where full thrust is only required when the cylinder comes to rest, e.g. clamping – use the above calculation.
For dynamic loading, i.e. where thrust is required throughout the piston travel, allowance has to be made for the exhaust back-pressure, friction, changes in driving pressure, etc. – add 30% to the thrust figure required for normal speeds. For higher speeds add as much as 100%.
Cylinder speeds
With normal loading, valve and pressure – 5-7 bar, the important factor is the relationship between the bore area of the cylinder and the actual bore area of the cylinder inlet ports. Conventionally this is in the order of 100:1 and would result in speeds of 0.3-0.5 metres per second. For normal speeds use a directional control valve and piping of the same size as the cylinder ports. For higher speeds user cylinder of larger bore size than necessary plus larger valve and pipework but be careful of cushioning problems.
Stroke lengths
For static loading use the convenient standard, stocked stroke length as cushioning is not important. For dynamic loading, order the correct required stroke length as the use of external stops effects cushioning potential.
With long stroke lengths, i.e. more than 15 x bore diameter, care must be taken to avoid side-loading on bearing, etc. – use pivot-type mountings. Check diameter of piston rod to avoid buckling under load. If necessary, use a larger bore size cylinder the normal as this will probably have a longer bearing and a thicker piston rod.
More information
The BFPA publication ‘Fluid Power Engineer’s Data Book’ is available by contacting the Association at info@bfpa.co.uk, or Tel: 01608 647900.
