TechArchive

Original article date: October 1998

Norgren has produced an objective detailed and useful gide for OEMs and end users to explain the different types of pneumatic actuator and specify the right kind of actuator for each specific job.

For many applications cylinders can be allowed to run at their own natural maximum speed. This results in rapid mechanism movement and quick overall machine cycle times. However there will be applications where uncontrolled cylinder speed can give rise to shock fatigue noise and extra wear and tear to the machine components. This natural speed is determined by cylinder size port size inlet and exhaust valve flow air pressure bore and length of the hoses and the load against which the cylinder is working.

From this natural speed it is possible to either increase speed or as is more than often the requirement reduce it. The natural speed can be changed by valve selection: generally the smaller the selected valve the slower the cylinder movement. When selecting for a higher speed however the limiting factor will be the aperture in the cylinder ports. Valves with flow in excess of this limitation will give little or no improvement in cylinder speed.

The aperture in the cylinder ports is determined by the design. Robustly constructed cylinders will often be designed with full bore ports. This means that the most restrictive part of the flow path will be the pipe fitting. These cylinders are the type to specify for fast speed applications and would be used with a valve having at least the same size ports as the cylinder. Lighter duty designs particularly small bore sizes will have the port aperture much smaller than the port’s nominal thread size. This has the desired effect of limiting the speed of the cylinder to prevent it from self-destructing through repeated high velocity stroking. The maximum natural speed of these cylinders can often be achieved with a valve that is one or two sizes down from the cylinder port size.

Large bore cylinders are designed with port sizes large enough to allow fast maximum speeds. In many applications however they are required to operate at relatively low speeds. For an application like this a cylinder can be driven from a valve with smaller sized ports than those of the cylinder.

Once a cylinder/valve combination has been chosen and the load is known the natural maximum speed will be dependent on pressure. For an installed cylinder and load an experiment can be carried out. Connect a control valve that will cause the cylinder to self-reciprocate. Then start the system running at a low pressure and gradually increase it. The cylinder will cycle faster and faster until a limiting speed is reached. This is the optimum pressure for that application. Increase the pressure further and the cylinder starts to slow down. This is caused by too much air entering the cylinder entering on each stroke. More time is therefore taken to exhaust it and results in a slower cylinder speed.

With any fixed combination of valve cylinder pressure and load it is usually necessary to have adjustable control over the cylinder speed. This is effected with flow regulators and allows speed to be tuned to the application.

For the majority of applications best controllability results from unidirectional flow regulators fitted to restrict the flow our of the cylinder and allow free flow in. The regulator fitted to the front port controls the outstroke speed. Speed is regulated by controlling the flow of air to exhaust which maintains a higher back pressure. The higher the back pressure the more constant the velocity against variations in load friction and driving force. On the other side of the piston full power driving pressure is quickly reached. Many flow regulators are designed specifically for this convention.

• IMI Norgren
• Tel: 01543 414333

October 1998