TechArchive

Original article date: October 1998

MARTIN EADON explains the options available to dissipate the energy within a pneumatic actuator and why the RHC cylinder from SMC Pneumatics is a major advance.

The force generated by a pneumatic cylinder is determined by the air pressure applied to the cylinder and the effective area of the piston. As pressure and bore size increase so will the force.

Newton’s second law tells us that the acceleration of a body from rest is directly proportional to the force applied to it and inversely proportional to the mass to be moved. Force is generated by the cylinder and moves the load. Kinetic energy is therefore created within the system which must be dissipated in the cushioning.

The kinetic energy carried by a cylinder into cushioning is proportional to two things the moving mass and the square of the velocity. Moving mass is the load weight plus all linkages and the moving parts of the cylinder. The velocity is the speed at the end of stroke or at the start of deceleration.

Kinetic energy cannot be destroyed so it must be converted into a form which will not cause damage or danger. It is common for actuator ports to have a restricted orifice which by restricting the airflow serves to limit piston speed to protect against high impact loads at end of stroke. These measures prevent the actuator destroying itself.

An essential but often overlooked part of sizing a pneumatic system is to confirm that the expected kinetic energy can be absorbed by the actuator in the system. If it cannot precautionary measures must be taken.

The cylinder is also adding energy which must be absorbed in cushioning at the end of stroke because usually the air pressure is still driving the load right through the full stroke.

Kinetic energy can be absorbed or dissipated in several ways for example by being transmitted through the cylinder mounts into the machine frame as concussive vibration as noise. or by forcing air or oil through a constriction in a shock absorber.

A simple option is to have no cushioning and allow a metal to metal contact between the piston face and the cylinder end cover. This obviously saves money and well as reducing the size and weight of the cylinder to an absolute minimum. The disadvantage is that even slow speeds and low masses will produce noise and vibration. If this is sustained damage will occur to the actuator or the machine

External shock absorbers or dampers can be fitted. Some actuator designs lend themselves readily to this. In slide tables and guided units a hydraulic damper can be integrated into the actuator body. Alternatively dampers or bumper blocks can be incorporated into a machine design when a conventional cylinder is used. If this is not possible then the cylinder itself must provide the cushioning for the load.

Two types of internal cushioning are found inside pneumatic cylinders. The simple kind is solid bumpers where rubber or urethane damper rings are mounted on the piston or inside the cylinder end covers. This absorbs some energy by elastic deflection of the damper and avoids the metal to metal contact. The other type is air cushioning also called pneumatic damping.

Air cushioning uses the air being expelled on the exhausting side of the cylinder to arrest the load. As the cylinder nears the end of stroke a cushioning bush on the piston engages in a seal and traps the air in the exhausting end of the cylinder. The only means of escape for this air is through an adjustable restriction to reach the exhaust port. The effect of this is to produce a sharp rise in the back pressure within the cylinder which acts on the piston to slow the load. The kinetic energy is absorbed by forcing the exhausting air through the restriction.

The amount of kinetic energy that can be absorbed in the air cushioning is determined by the length of the cushioning and the setting of the adjustable airflow restriction. Constricting the flow increases the back pressure which slows the cylinder. Unfortunately if the back pressure is increased a point is reached where the cylinder bounces off the cushioning before the end of stroke is reached. If there is insufficient back pressure to stop the load the cylinder will reach the end of stroke with some velocity remaining and transfer the energy as an impact.

The RHC High Power Cylinder increases the energy absorption by increasing the length of the cushioning. The load is therefore brought to a stop over a greater distance. The cushion bushes on the piston are longer so engage in the seals earlier as the cylinder approaches the end of stroke. Careful attention is also paid to the cushion seal and orifice design for maximum efficiency.

The Kinetic energy that can be absorbed is therefore dramatically increased compared to standard cylinder designs. The following figures give an indication of the improved energy control capability gained from these changes in this case by a factor of 18.

40mm bore CG1 cylinder rubber cushioned. 1.2 Joules

40mm bore CG1 cylinder air cushioned. 1.8 Joules

40mm bore RHC High power cylinder. 33 Joules

• SMC Pneumatics
• Martin Eadon
• 01908 563888

October 1998