There are many causes of rod seal leakage; defective rod bearing, bent rod, contaminated fluid, high temperatures, chemicals AND the plant knob turner! Every plant has one or more knob turners that, when a hydraulic issue occurs, the first thing they do is adjust the pressure. Have you noticed that their wrist only works in a clockwise direction, which increases the pressure? Many times cylinder rod seals can fail even though the gauge at the pump indicates that the pressure is below the rating of the cylinder. How can this be? In the illustration, an 8″ cylinder with a 5″ rod is used to extend out and move a 50,000 lb. load. On the full piston side of the cylinder there is approximately 50 square inches of area for the pressure act upon when extending. The rod side of the cylinder has approximately 30 square inches of area. The cylinder has a maximum pressure rating of 3000 PSI.

If the flow control in the rod side line is fully open, the low pressures will be indicated on the “A” and “B” gauges as the cylinder extends. Once the cylinder rod contacts the part, the pressure will only build high enough to move the 50,000 lb. load. We can determine the pressure in the full piston side by the following formula;

Force = Pressure X Square Inches of Area
50,000 lbs. = PSI X 50 square inches
50,000 divided by 50 = PSI
1000 = PSI

Since 1000 PSI is required to move the load, the maximum pressure limiting device should be set at 1200 PSI. The maximum pressure in a fixed displacement pump circuit is limited by the relief valve. The compensator setting determines the maximum pressure in pressure compensating pump circuits.

If a 30 GPM pump is used and the flow control is set to allow 20 GPM out of the rod side of the cylinder, pressure will build up in the full piston side to the setting of the maximum pressure device, 1200 PSI. We can see how much force is exerted in the full piston side as the cylinder extends:

Force = 1200 PSI X 50 square inches of area
Force = 60,000 lbs of force.

Since the load has not been contacted, the 60,000 lbs of force is exerted on the fluid in the rod side of the cylinder.

60,000 lbs. = PSI X 30 square inches of area
60,000 divided by 30 sq. inches = PSI
2000 = PSI

This pressure is well below the maximum pressure rating of the cylinder (3000 PSI). Once the 50,000 lb. load is contacted the pressure in the rod side will drop because 50,000 lbs. of the force developed on the full piston side will be used to move the load. The other 10,000 lbs. of force is exerted on the rod side of the cylinder. The pressure in the rod side will be:

10,000 lbs. = PSI X 30 square inches of area
10,000 divided by 30 = PSI
333.33 = PSI

If the local knob turner turns the pressure up to 2500 PSI, the force developed in the full piston side is:

Force = 2500 X 50 square inches
Force = 125,000 lbs.

Prior to the cylinder contacting the load, the pressure in the rod side is:

125,000 = PSI X 30 sq.inches
125,00 divided by 30 = PSI
4,166 = PSI

This pressure is now above the rating of the cylinder which results in failure of the rod seal! If you’re experiencing rod seal failures in systems with varying load conditions and flow controls, check the rod side pressure to verify that maximum pressure of the cylinder is not exceeded.