Impact cylinders
Original article date: April 1995
Very rapid acceleration can be obtained from pneumatic impact cylinders. IMI NORGREN explains how they can not only be used in place of conventional press machinery but can even improve on the results obtained.
For a fraction of the capital outlay the impact cylinder provides in itself a power unit capable of activating a variety of press work applications conventionally carried out on machinery such as fly presses kick presses drop stamps and crank presses. Many applications can benefit from impact working as the high tool velocity and rate of energy application can produce results such as cleaner edges during cutting operations and an improved grain structure when forging.
Coupled to one of the simple control circuits an impact cylinder will operate from a normal air line and can be mounted on a frame to form a self-contained press or form part of a special purpose machine pneumatically interlocked with automatic feed and eject mechanisms.
The energy in an impact cylinder is realised by very rapidly accelerating the piston rod and tool assembly over a free stroke of approximately 65mm before it contacts the workpiece or target. The only moving part is the piston and assembly. The very rapid acceleration is produced by a differential area arrangement which suddenly exposes the full area of the piston to high pressure air built up in the reservoir in the rear end of the cylinder. The value of this pressure normally from a 5 to 7 bar air line can be adjusted with a pressure regulator to control the level of the energy output.
Wherever possible the moving tool assembly should be attached to the piston rod of the impact cylinder and accelerated with it for considerable losses in efficiency can result from impacting onto a tool set as well as creating unnecessary noise and damage from shock fatigue.
Although an impact cylinder can be used in any attitude generally fewer problems are involved when it is used to impact vertically up or vertically down. At the point of impact the reaction between the top and bottom plates of a frame will be only the thrust developed over the piston at a reduced air pressure. However the frame must be stiff enough to take the recoil force generated in the body of the cylinder at the instant of firing which can be enough to just lift a light frame. Any butt joints in the frame should be in compression and not in shear. Light cast frames must be used as there is a danger of shock fatigue failure.
For cylinders being used vertically up or down front mounting only is quite sufficient and allows for easier servicing. For a cylinder being used in any other attitude it must be located both at the front end and at the rear to ensure that is properly aligned. Any pull on the cylinder body relative to its front end will produce a side load on the bearing which could seriously reduce the output.
Relatively heavy tools can be added to the piston rod without seriously reducing its useful output provided the weight is reasonably distributed. For applications working vertically up greater weight can be carried than for cylinders working vertically down but a modification in the control circuit may be needed.
In either event heavy tools should not be allowed to return too quickly. It is advisable to flow-regulate the exhaust from the rear end. The stripping force available from the cylinder will be reduced and this should be taken into account when selecting the equipment. Heavy tools should not be added to horizontally-mounted impact cylinders unless they are supported by additional symmetrical guides.
For special purposes applications or to double up on energy output two impact cylinders can be used vertically in opposition and operated together. With this arrangement the reaction in the framework is neutralised and the closing tool/component velocity is doubled.
Twin moving tools are used on opposed impact cylinder installations and ideally the component is loaded onto the one working vertically up and accelerated with it. Components can be suspended in a mid-position but must be able to float up or down to compensate for slight changes that may occur in the meeting point of the tools. Unless open tolerance concentric tools are used they will need to be guided relative to one another.
- IMI Norgren
- Tel: 01543 414333
- Fax: 01543 268052
- www.norgren.com
April 1995