Will Not Cycle

Air pressure to the drive does not cause the pump to start cycling.

The air drive pressure is used for driving the main air piston and for shifting the cycling valve by pressurizing and venting the pilot system cavity. Because of the large forces involved with the air drive piston, it is unlikely to be the cause of the inability to cycle. The cycling spool, using relatively low pressure and small surface areas, has very low forces driving it. Therefore it does not take much restraining force to stop it.

The most common causes of inability to cycle are as follows:
a. Drying out of lubrication on the spool "O" rings due to an extremely "dry" air supply.
b. Washing out of the lubrication on the spool "O" rings due to excessive water in the air supply.
c. Contamination in the air cycling valve sleeve causing the spool to be "jammed" by particulate interference.
d. Swelling of "O" rings due to presence of "incompatible" fluids. (i.e. - compressor oils, pumped fluids leaking into the air drive chamber, etc.)
e. In the case of extremely incompatible pumped fluids, the air piston drive "O" ring could swell enough to actually resist the driving force on the piston. This is the reason for using a 'distance piece" pump when pumping fluids that are not compatible with the air drive "O" rings.
f. If the pilot valve stems are too short and do not get enough travel to open properly, it can also cause an inability to cycle. This condition usually occurs after an overhaul, where there are two different length pilot stems that have accidentally been switched.
g. Lack of a separate pilot pressure source for pumps equipped with the "remote pilot modification". It can also happen if the unit is equipped with an air pilot switch that is "normally closed" and set to open at some higher pressure.

For items a and b, remove retaining cap and cycling spool and inspect "O" rings for wear or damage. Replace "O" rings as required. Re-lubricate with Haskel lubricant (from seals kit), and re-assemble.

For item c, remove retaining cap, spool and sleeve, and clean thoroughly with solvent. Inspect for "O" ring damage. Replace seals as required. Re-assemble with Haskel lubricant.

For items d and e on a non-distance piece pump, determine if "O" ring seals have been swelled by compressor air or by liquid section seal leakage into the air drive chamber. For distance piece pumps, only the compressor air can be a factor. Determine what is the proper "O" ring compound that will satisfactorily resist these fluids, and replace seals, lubricate and re-assemble. NOTE: if the swelling is caused by the leakage of the pumped fluid (non-distance piece pump), then check plunger seal package to correct leakage. In that case, a distance piece pump should have been specified originally.

For item f, check assembly drawing to verify proper location of stems. Switch stems, if necessary, to verify proper placement.

For item g, verify whether a "remote pilot" modification has been installed in the pump. Usually, this deletes the 1/8" plug located just adjacent to the 1/2" pipe plug at the end of the cycling valve. If that port is open, it most likely needs a separate pilot air pressure supply to match the drive pressure (or 40 psig minimum with the "low pressure" modification).