Hydraulic Cylinders Troubleshooting and Repair

A hydraulic cylinder is an executive element that converts hydraulic energy into mechanical energy in a hydraulic system. The faults can be basically summarized as the malfunction of the hydraulic cylinder, the inability to push the load, and the sliding or crawling of the piston. It is not uncommon for the equipment to stop due to the failure of the hydraulic cylinder. Therefore, we should pay more attention to the failure diagnosis and use and hydraulic cylinders repair.

1. Malfunction

The valve core is stuck or the valve hole is blocked.

When the flow valve or directional valve spool is stuck or the valve hole is blocked, the hydraulic cylinder is prone to malfunction. At this time, check the contamination of the oil, to see whether dirt or colloidal deposits are stuck in the valve core or block the valve hole. Then check the wear of the valve body, clean and replace the system filter, clean the oil tank, and replace the hydraulic medium.

The piston rod and the cylinder are stuck or the hydraulic cylinder is blocked.

At this time, no matter how you manipulate it, the hydraulic cylinder does not move or moves slightly. Here we should check whether the piston and piston rod seals are too tight, whether dirt and colloidal deposits have entered, whether the axis line of the piston rod and the cylinder barrel is aligned, whether the wearing parts and seals are invalid, and whether the load is too large.

The control pressure of the hydraulic system is too low.

The throttling resistance in the control pipeline may be too large, or the flow valve is improperly adjusted, or the control pressure is inappropriate, or the pressure source is disturbed. Then we should check the control pressure source to ensure that the pressure is adjusted to the specified value of the system.

Air enters the hydraulic system.

Mainly because there are leaks in the system. At that time, check the liquid level of the hydraulic oil tank, the seals and pipe joints on the suction side of the hydraulic pump, and whether the oil suction strainer is too dirty. If so, we should add hydraulic oil,  treat seals and pipe joints, then clean and replace the coarse filter element.

The initial movement of the hydraulic cylinder is slow.

In the case of low temperature, the hydraulic oil has high viscosity and poor fluidity, which causes the hydraulic cylinder to move slowly. The improvement method is to replace hydraulic oil with better viscosity and temperature performance. At low temperatures, use a heater or use the machine to heat it to increase the oil temperature at startup. The normal operating oil temperature of the system should be maintained at about 40°C.

2. The load cannot be driven during work.

The main manifestations are inaccurate stop of the piston rod, insufficient thrust, reduced speed, unstable work, etc. The reasons are as follows.

Leakage inside the hydraulic cylinder.

The internal leakage of the hydraulic cylinder includes the leakage caused by the hydraulic cylinder body seal, the seal of the piston rod and the sealing cover, and the excessive wear of the piston seal.

The reason for the leakage of the seal between the piston rod and the seal cover is that the seal is wrinkled, squeezed, torn, worn, aging, deteriorated, deformed, etc. At this time, a new seal should be replaced.

The main reason for excessive wear of the piston seal is improper adjustment of the speed control valve, resulting in excessive back pressure and improper installation of the seal or contamination of the hydraulic oil. The second is that foreign matter enters during assembly and the quality of the sealing material is not good. The consequence is slow movement and weakness. In severe cases, it will cause damage to the piston and cylinder, causing the phenomenon of “cylinder pulling”. The treatment method is to adjust the speed control valve, and make necessary operations and improvements according to the installation instructions.

Leakage of hydraulic circuit.

Including leakage of valves and hydraulic lines. The hydraulic cylinders repair method is to check and eliminate the leakage of the hydraulic connection pipeline by operating the reversing valve.

The hydraulic oil is bypassed back to the oil tank through the overflow valve.

If dirt enters the overflow valve and jams the spool, making the overflow valve normally open, the hydraulic oil will bypass the overflow valve and directly flow back to the oil tank, causing no oil to enter the hydraulic cylinder. If the load is too large, although the regulating pressure of the relief valve has reached the maximum rated value, the hydraulic cylinder still cannot obtain the thrust required for continuous action and does not move. If the adjustment pressure is low, the vertebral force required for still loading cannot be achieved due to insufficient pressure, which is manifested as insufficient thrust. At this time, check and adjust the overflow valve.

3. Piston slips or crawls.

The sliding or crawling of the hydraulic cylinder piston will make the hydraulic cylinder work unstable. The main reasons and hydraulic cylinders repair methods are as follows:

The internal sluggishness of the hydraulic cylinder.

The internal parts of the hydraulic cylinder are improperly assembled, the parts are deformed, worn, or the geometric tolerance exceeds the limit, and the action resistance is too large. The speed of the piston of the hydraulic cylinder varies with the stroke position and slips or crawls. The reason is mostly due to the poor assembly quality of the parts, the surface scars or the iron filings produced by sintering, which increase the resistance and reduce the speed. For example, the piston and the piston rod are not concentric or the piston rod is bent, the installation position of the hydraulic cylinder or the piston rod on the guide rail is offset, the sealing ring is installed too tightly or too loosely, etc. The solution is to re-repair or adjust, replace damaged parts and remove iron filings.

Poor lubrication or out-of-poor machining of the bore diameter of the hydraulic cylinder.

Because the piston and the cylinder, the guide rail and the piston rod all have relative motion, if the lubrication is poor or the hydraulic cylinder bore is too poor, it will aggravate the wear and reduce the linearity of the cylinder center line. In this way, when the piston is working in the hydraulic cylinder, the frictional resistance will be large and sometimes small, causing slippage or crawling. The elimination method is to grind the hydraulic cylinder first, then prepare the piston according to the matching requirements, grind the piston rod, and configure the guide sleeve.

The hydraulic pump or hydraulic cylinder enters the air.

Compression or expansion of air will cause the piston to slip or crawl. The troubleshooting measures are to check the hydraulic pump, set up a special exhaust device, and quickly operate the exhaust for several times throughout the entire stroke.

The quality of the seal is directly related to slippage or crawling.

When the O-ring seal is used under low pressure, compared with the U-shaped seal, due to the higher surface pressure and the larger difference in dynamic and static friction resistance, it is easy to slip or crawl. The surface pressure of the U-shaped seal increases with the pressure. Although the sealing effect is correspondingly improved, the difference between dynamic and static friction resistance also increases, and the internal pressure increases, which affects the elasticity of the rubber. Due to the increase in the contact resistance of the lip, the sealing ring will tilt and the lip will extend. It is also easy to cause slippage or crawling. To prevent it from tipping over, a support ring can be used to keep it stable.

It becomes crutial to know more information of hydraulic cylinders repair when in application. In the next article, we’ll discuss the bad consequences of the scratches on the surface of the hydraulic cylinder bore and the quick repair method.

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