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As a product category, hydraulic cylinders are as ubiquitous as pumps and motors combined.
Therefore, if a facility operates a large volume of hydraulic equipment, cylinder repair costs are likely to represent a significant portion of total maintenance costs.
It’s generally believed that up to 25% of mechanical equipment failures are design-related. For hydraulic cylinders, this means that up to a quarter are not adequately designed for the applications they operate in. This doesn’t mean the cylinder can’t perform the required work; it can—but its service life won’t be acceptable. If a particular cylinder requires frequent repairs, one or more of the following design-related issues needs to be addressed.
Bent Piston Rod
Bent piston rods can be caused by insufficient rod diameter, insufficient material strength, improper cylinder barrel tube installation, or a combination of all three.
A bent piston rod places excessive load on the piston rod seal, leading to premature seal failure.
When servicing a hydraulic cylinder, always check the straightness of the piston rod. To test straightness, place the piston rod on rollers and measure the radial runout with a dial indicator. Position the piston rod so that the distance (L) between the rollers is as large as possible, then measure the runout at the midpoint between the rollers (L/2).
The piston rod should be as straight as possible, but a runout of 0.5 mm per meter of piston rod length is generally considered acceptable (for standard cylinders, not servo cylinders). To calculate the maximum allowable runout (measured at L/2), use the following formula:
Maximum runout (mm) = 0.5 x L ÷ 1000
Where L is the distance between the rollers in millimeters.
For example, if the distance between the rollers is 1.2 meters, the maximum allowable runout measured at L/2 is 0.5 x 1200 ÷ 1000 = 0.6 mm.
If the piston rod is bent, the actual piston rod load should be compared with the allowable piston rod load based on the cylinder barrel installation method and the tensile strength of the piston rod material. If the actual piston rod load exceeds the allowable load, a new piston rod should be manufactured from a higher tensile strength material and/or the piston rod diameter should be increased to prevent bending during use.
Cylinder barrel tube bloat
Cylinder barrel tube bloat is typically caused by insufficient cylinder wall thickness and/or material strength to withstand the operating pressure. Once barrel tube bloat occurs, the proper tolerance between the piston seal and the cylinder barrel tube wall is lost, allowing high-pressure fluid to bypass the seal. This high-velocity fluid erodes the seal, and the localized heating caused by the pressure drop across the piston reduces seal life.
Testing Cylinder barrel tube integrity
The traditional method for testing piston seal integrity in double-acting cylinders is to pressurize the cylinder barrel tube at the end of stroke and measure any leakage past the seal. This is often referred to as an end-of-stroke bypass test.
A major limitation of the end-of-stroke bypass test is that it often fails to reveal cylinder barrel tube bloat caused by hoop stresses due to underdesigned cylinder barrel tube wall thickness or excessive honing resulting in wall thinning. The ideal method for testing cylinder barrel tube bloat is to perform a piston seal bypass test at midstroke. A disadvantage of this procedure is that the forces generated by the cylinder barrel tube must be mechanically resisted, which is impractical for large-diameter, high-pressure cylinder barrel tube.
However, a mid-stroke bypass test can be performed hydrostatically, utilizing the pressurization effect. The necessary circuit is shown in Figure 2.
Pressure Multiplication
The force generated by a hydraulic cylinder is the product of pressure and area (F = p x A).
In a traditional double-acting cylinder, the effective area, and therefore the force, on the piston and rod sides are unequal. Therefore, if the effective area on the rod side of the cylinder is half that on the piston side, it will also generate half the force at the same pressure.
The equation F = pxA can be converted to p = F/A: that is, pressure equals force divided by area. For the rod side of the cylinder to resist the force generated on the piston side with only half the area, the pressure must be doubled. This means that if the piston side is pressurized to 3,000 PSI, the rod side requires 6,000 PSI to generate the same force, which explains the pressure multiplication that occurs in double-acting cylinders.
If, for any reason, the piston side of a double-acting cylinder is pressurized while fluid flow is blocked from the rod side, pressure on the rod side of the cylinder barrel will increase until the forces reach equilibrium or the cylinder fails catastrophically. Pressure multiplication in a double-acting cylinder is a dangerous phenomenon and must be thoroughly understood when testing a hydraulic cylinder using the following procedure:
1. Position the cylinder barrel tube with the service port facing upward.
2. Fill both sides of the cylinder barrel tube with clean hydraulic oil through the service port.
3. Connect ball valves (1) and (2), pressure gauges (3) and (4), relief valve (5), and directional control valve (6) as shown in Figure 2.
4. With ball valves (1) and (2) open, use the directional control valve (6) to stroke the cylinder several times to purge any trapped air from both sides of the cylinder barrel tube—be careful to avoid the “diesel effect” (knocking) of the cylinder barrel tube.
5. Position the piston rod at mid-stroke and close the ball valve (2).
6. Loosen the adjustment knob on the relief valve (5) to direct flow to the rod side of the cylinder barrel tube.
7. Increase the setting on the relief valve (5) until the pressure gauge (3) indicates the rated pressure of the cylinder barrel tube.
8. Close the ball valve (1) and return the directional control valve (6) to the center position. Note: It is assumed that the hydraulic power unit used for the test has its own overpressure protection.
9. Record the pressure readings on each of the pressure gauges (3) and (4) and monitor any changes over time.
If the effective area ratio of the piston side to the rod side of the cylinder barrel tube is 2:1, and if the rod side is pressurized to 3,000 PSI, the piston side pressure gauge (4) should show 1,500 PSI. If the pressure differential across the piston seal is not maintained, this indicates a problem with the piston seal and/or the cylinder barrel tube.
Never direct flow to the piston side of the cylinder barrel tube with the ball valve (1) closed, as this could result in cylinder barrel tube failure and personal injury. Always wear appropriate personal protective equipment when performing this or any other hydrostatic (pressure) test.
Insufficient Bearing Area
If the internal bearing area inside the gland and at the piston is insufficient to withstand the torque loads transmitted to the cylinder barrel, excessive loads will be placed on the piston rod and piston seal. This can cause seal deformation and ultimately premature failure.
Rod Surface Treatment
The surface treatment of the cylinder barrel tube and piston rod has a significant impact on the life of the piston rod seal. If the surface roughness is too low, seal life will be shortened due to insufficient lubrication. If the surface roughness is too high, the potential for contaminant ingress increases and unacceptable leakage levels may occur.
In the context of extending cylinder barrel tube life, the piston rod surface should be considered a lubricated wear surface and treated accordingly. In some applications, using alternative piston rod surface treatments with superior mechanical properties to traditional hard chrome plating, such as black nitriding or high-velocity oxygen fuel (HVOF) metal spraying, can extend the life of the piston rod and its seal. Installing a protective shield to protect the piston rod surface and seal from impact damage and contaminants can also provide similar life-extending benefits.
Repair or Redesign?
Not all hydraulic cylinders are created equal. Therefore, if a cylinder repeatedly fails, a design revision is likely necessary to break the cycle of failure and repair.
