Air Cylinder

February 1, 2008

Because pneumatics are much simpler and less costly than hydraulics, the cake discharge cones of Vincent screw presses are almost always actuated by air cylinders. We also use air cylinders in CIP backflush systems and elevation adjustment mechanisms for our Fiber Filters. This gets us into air cylinder maintenance.

For small diameter, long strokes, we use Bimba air cylinders. These are throwaway units: when the piston or seals start leaking, they must be replaced. Where the application calls for a larger diameter unit, we generally use Parker air cylinders. There are rebuild kits available for these.

An air cylinder will leak either at the shaft seal where the rod moves in and out, or past the seals of the internal piston. It is easy to find a leaky shaft seal by putting some soap bubble solution on the shaft. To check for a leaky piston, disconnect one of the air hoses, apply air to the other end of the cylinder, and see if air comes out the open port.

A leaking piston usually means that the internal bore of the air cylinder has rusted. Sometimes a sleeve kit will fix this. But, it is probably time to replace the air cylinder.

A far more common problem, especially with new units, is a sticky air cylinder. Sometimes the internal tolerances are so tight that, especially at low air pressures like 10 psi, the cylinder will stay stuck or move in jerks.

The best cure for a sticky air cylinder is to insert some STP Oil Treatment into the cylinder. STP is not called "motor honey" for nothing: it is as hard to pour into a small hole as bee's honey. The clever way to do this is to cut a hole in one corner of a freezer Baggie. Insert a piece of air hose through this hole, into the bag, and seal the joint with duct tape. Then pour STP into the bag, and seal the bag. When you squeeze the bag, it acts as a pump, making it relatively easy to get the oil to flow into the cylinder.

To regulate air pressure, Vincent presses come with an air FRL (Filter, Regulator, Lubricator) set, which is followed by a 4-way valve. The FRL sets are generally the Parker or Watts brand, while the 4-way valve is made by Parker. Also, a bottle of light oil is supplied for filling the lubricator of the FRL set. The lubricator admits a few drops of oil along with the air, assuring lubrication and rust protection of the inside of the air cylinder.

The 4-way valve has four hose connections and a handle for two positions. In one position, compressed air from the regulator goes to one side of the air cylinder while the other end of the air cylinder is opened to atmosphere. In the other position, the air goes to the other end of the air cylinder while the opposite end is vented to atmosphere.

Moving the valve handle from one position to the other causes the air cylinder to push in the opposite direction, either opening or shutting the press discharge cone. Thus, there is a burst of air through the vent line whenever the valve handle is moved from one position to the other. If air bleeds continuously from the vent line, it is a sign that air is leaking past the piston seals of the air cylinder. Alternatively, the air leaking could come from a faulty seat in the Parker valve.

Issue 196