What is a Hydraulic Cylinder?

Hydraulic cylinder is a hydraulic actuator that converts hydraulic energy into mechanical energy and performs linear reciprocating motion (or swing motion). It has a simple structure and reliable operation. When it is used to realize reciprocating motion, the reduction device can be eliminated, and there is no transmission gap, and the movement is stable, so it is widely used in hydraulic systems of various machines. The output force of the hydraulic cylinder is directly proportional to the effective area of the piston and the pressure difference on both sides; the hydraulic cylinder is basically composed of a cylinder barrel and a cylinder head, a piston and a piston rod, a sealing device, a buffer device and an exhaust device. The buffer device and exhaust device depend on the specific application, and other devices are necessary.

Hydraulic cylinder is the actuator in the hydraulic transmission system. It is an energy conversion device that converts hydraulic energy into mechanical energy. The hydraulic motor implements continuous rotary motion, while the hydraulic cylinder implements reciprocating motion. There are three types of hydraulic cylinders: piston cylinder, plunger cylinder, and swing cylinder. The piston cylinder and plunger cylinder realize reciprocating linear motion and output speed and thrust. The swing cylinder realizes reciprocating swing, output angular velocity (rotational speed) and torque. In addition to being used individually, hydraulic cylinders can also be used in combination of two or more or in combination with other mechanisms. To perform special functions. The hydraulic cylinder has a simple structure and reliable work, and has been widely used in the hydraulic system of machine tools. ^[1]

Hydraulic cylinders have various structural forms and classification methods: they can be divided into linear reciprocating and rotary swinging according to the movement mode; they can be divided into single-acting and double-acting according to the action of the hydraulic pressure; according to the structure The form can be divided into piston type, plunger type, multi-stage telescopic sleeve type, rack and pinion type, etc .; according to the installation form, it can be divided into pull rod, earrings, feet, hinge shaft, etc .; according to the pressure level, it can be divided into 16Mpa, 25Mpa. , 31.5Mpa and so on.

In the hydraulic system, the hydraulic cylinder is used to drive a mechanism with a certain quality. When the hydraulic cylinder moves to the end of the stroke, it has a large kinetic energy. Without deceleration treatment, the hydraulic cylinder piston and the cylinder head will mechanically collide, resulting in impact and noise. Destructive. In order to mitigate and prevent such a hazard, a reduction gear can be provided in the hydraulic circuit or a buffer device can be provided in the cylinder.

The cylinder is used as a hydraulic cylinder, a single pillar for mining,

Hydraulic cylinder is an actuator that converts hydraulic energy into mechanical energy in a hydraulic system. The faults can be basically summarized as malfunction of the hydraulic cylinder, inability to push the load, and sliding or crawling of the piston. The failure of hydraulic cylinders to cause equipment shutdown is not uncommon, so attention should be paid to fault diagnosis and maintenance of hydraulic cylinders.

I. Fault diagnosis and treatment

1. Malfunction or malfunction

There are several reasons and solutions:

(1) The spool is blocked or the valve hole is blocked. When the flow valve or directional valve spool is blocked or the valve hole is blocked, the hydraulic cylinder is prone to malfunction or malfunction. At this time, check the oil pollution; check whether the dirt or colloidal deposits are stuck in the valve core or block the valve hole; check the valve body for wear, clean and replace the system filter, clean the fuel tank, and replace the hydraulic medium.

(2) The piston rod is stuck with the cylinder or the hydraulic cylinder is blocked. Regardless of how it is manipulated at this time, the hydraulic cylinder does not move or moves very little. At this time, you should check whether the seals of the piston and the piston rod are too tight, whether they have entered the dirt and colloidal deposits: whether the axis of the piston rod and the cylinder are aligned, whether the wearing parts and seals have failed, and whether the load is too heavy. Big.

(3) The control pressure of the hydraulic system is too low. Throttling resistance in the control line may be too large, the flow valve may be improperly adjusted, the control pressure may be inappropriate, and the pressure source may be disturbed. At this time, the control pressure source should be checked to ensure that the pressure is adjusted to the specified value of the system.

(4) Air enters the hydraulic system. Mainly because of a leak in the system. At this time, the liquid level of the hydraulic oil tank, the seals and pipe joints on the suction side of the hydraulic pump, and the primary suction strainer should be checked for dirt. If so, replenish hydraulic oil, handle seals and pipe joints, and clean or replace the coarse filter element.

(5) The initial movement of the hydraulic cylinder is slow. When the temperature is low, the hydraulic oil has a large viscosity and poor fluidity, which causes the hydraulic cylinder to move slowly. The improvement method is to replace the hydraulic oil with better viscosity and temperature performance. At low temperature, you can use a heater or the machine to heat it to increase the oil temperature at startup. The normal working oil temperature of the system should be maintained at about 40 ° C.

2.Can't drive load when working

The main manifestations are inaccurate stopping of the piston rod, insufficient thrust, reduced speed, unstable work, etc. The reasons are:

(1) Internal leakage of hydraulic cylinder. Internal leaks in hydraulic cylinders include leaks caused by hydraulic cylinder block seals, excessive wear on piston rod and seal cap seals, and piston seals.

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

The main cause of excessive wear on the piston seal is improper adjustment of the speed control valve, resulting in excessive back pressure and improper seal installation or hydraulic oil contamination. Followed by foreign matter entering the assembly and poor quality of the sealing material. As a result, the action is slow and weak. In severe cases, the piston and the cylinder can be damaged, and a "pulling cylinder" phenomenon occurs. The processing method is to adjust the speed control valve, and the necessary operation and improvement should be done according to the installation instructions.

(2) The hydraulic circuit is leaking. Including leaks in valves and hydraulic lines. The maintenance method is to check and eliminate the leakage of the hydraulic connection pipeline by operating the directional valve.

(3) The hydraulic oil is bypassed back to the tank through the relief valve. If the overflow valve enters the dirt and jams the spool, so that the overflow valve is always open, the hydraulic oil will bypass the overflow valve and flow directly back to the tank, resulting in no oil in 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 get 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 reached due to insufficient pressure, and the thrust is insufficient. Check and adjust the relief valve at this time.

3.Piston slips or crawls

Sliding or crawling of the piston of the hydraulic cylinder will make the hydraulic cylinder unstable. The main reasons are as follows:

(1) The inside of the hydraulic cylinder is sluggish. The internal parts of the hydraulic cylinder are improperly assembled, the parts are deformed, worn or the shape and position tolerances are exceeded, and the action resistance is too large, so that the piston speed of the hydraulic cylinder changes with different stroke positions, and slippage or crawling occurs. Most of the reasons are due to the poor assembly quality of the parts, the surface has scratches or iron chips generated by sintering, which increase the resistance and reduce the speed. For example: the piston is not concentric with the piston rod or the piston rod is bent, the installation position of the hydraulic cylinder or piston rod to the guide rail is offset, the sealing ring is too tight or too loose, etc. The solution is to repair or adjust again, replace damaged parts and remove iron filings.

(2) Poor lubrication or poor machining of hydraulic cylinder bore. Because the piston and the cylinder tube, the guide rail and the piston rod have relative movements, if the lubrication is poor or the hydraulic cylinder bore diameter is too poor, it will aggravate the wear and reduce the linearity of the cylinder centerline. In this way, when the piston works in the hydraulic cylinder, the frictional resistance will increase and decrease, causing slippage or crawling. The elimination method is to repair the hydraulic cylinder first, then prepare the piston according to the matching requirements, repair the piston rod, and configure the guide sleeve.

(3) Hydraulic pump or cylinder enters the air. Compression or expansion of the air can cause the piston to slip or crawl. The exclusion measure is to check the hydraulic pump, set up a special exhaust device, and quickly operate the full stroke to exhaust several times.

(4) Seal quality is directly related to slippage or crawling. When O-rings are used under low pressure, compared with U-shaped seals, due to higher surface pressure and larger difference between dynamic and static friction resistance, slippage or crawling are easy to occur; the surface pressure of U-shaped seals increases with pressure When it is increased, although the sealing effect is correspondingly improved, the difference between dynamic and static friction resistance is also increased, and the internal pressure is increased, which affects the elasticity of the rubber. Due to the increased contact resistance of the lip, the sealing ring will tip over 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. ^[2]

4. The bad consequences of scratching the surface of the bore in the cylinder of the hydraulic cylinder and the quick repair method

Scraps of material extruded from the scratch groove will be embedded in the seal. When the seal's working part is damaged during operation, it may cause new scratches in the scratched area.

The surface roughness of the inner wall of the cylinder is deteriorated, the friction is increased, and the creep phenomenon is easy to occur.

The internal leakage of the hydraulic cylinder is aggravated, which reduces the working efficiency of the hydraulic cylinder. The main causes of scratches on the bore surface of the cylinder are as follows.

(1) Scars caused when assembling hydraulic cylinders

Injuries caused by foreign matter during assembly Before the overall assembly of the hydraulic cylinder, all parts must be fully deburred and cleaned. When the parts are installed with burrs or dirt, they are easy to be embedded in the cylinder due to the "other strength" and the weight of the parts Wall surface, causing scars.

Damages in the installation parts When the hydraulic cylinder is installed, the parts such as pistons and cylinder heads are of high quality, large size and large inertia. Even with the auxiliary installation of lifting equipment, due to the small specified clearance, they will invest in any way Therefore, when the end of the piston or the boss of the cylinder head bumps against the inner surface of the cylinder wall, it is easy to cause injuries. The solution to this problem: For small-scale products with large quantities and high batch sizes, special assembly guide tools are used for installation; counterweight, coarse, large and medium-sized hydraulic cylinders can only be avoided by careful and careful operation.

Injuries caused by measuring instrument contacts are usually measured by the inside diameter dial gauge. The measuring contact is inserted into the wall of the bore of the cylinder while being rubbed. The measuring contacts are mostly made of wear-resistant hard alloy with high hardness. Generally speaking, slender scratches with small depth during measurement are slight and do not affect the running accuracy. However, if the measurement head size is improperly adjusted, the measurement contact will be hardly embedded, which will cause more serious injuries. The countermeasures to solve this problem are to first measure the length of the adjusted measuring head. In addition, use a piece of paper tape that is only open at the measurement position and stick it to the inner surface of the cylinder wall. . Slight scratches caused by measurement can usually be wiped off with the opposite side of an old emery cloth or horse dung paper.

(2) No serious running wear marks

Transfer of scars on the sliding surface of the piston Before the piston is installed, there are scars on the sliding surface. The pistons are left untreated and installed intact. These scars will in turn scratch the inner surface of the cylinder wall. Therefore, before installation, these scars must be fully repaired.

The sintering phenomenon caused by excessive pressure on the sliding surface of the piston causes the piston to tilt due to the weight of the piston rod, which causes other phenomena, or the pressure on the sliding surface of the piston rises due to the effect of lateral load, etc., which will cause sintering. In the design of the hydraulic cylinder, its working conditions must be studied, and the dimensions of the length of the piston and the sleeve and the clearance must be fully paid attention to.

The peeling of the hard chrome plating layer on the inner surface of the cylinder body is generally believed that the reason for the peeling of the hard chromium plating layer is as follows.

a. The plating layer does not adhere well. The main reason for the poor adhesion of the electroplated layer is that before the electroplating, the parts are not adequately degreased and degreased; the surface activation of the parts is not completely performed, and the oxide film layer is not removed.

b. Hard palate wear. The abrasion of the electroplated hard chrome layer is mostly caused by the abrasive action of the friction iron powder of the piston. When there is moisture in the middle, the wear is faster. Corrosion caused by the metal's contact potential difference occurs only at the parts where the piston touches, and the corrosion occurs in spots. In the same way as above, corrosion is promoted when moisture is interposed. Compared with castings, the contact potential difference of copper alloy is higher, so the corrosion degree of copper alloy is more serious.

c. Corrosion due to contact potential difference. Contact potential difference corrosion is not easy for long-running hydraulic cylinders; it is a common failure for long-term unused hydraulic cylinders.

Damage of the piston ring The piston ring is damaged during operation, and its fragments are caught in the sliding part of the piston, causing scratches.

The material of the piston sliding part is sintered and cast the piston, which will cause sintering when subjected to a large lateral load. In this case, the sliding part of the piston should use copper alloy or weld such materials.

(3) Foreign matter is mixed in the cylinder

Among the failures of hydraulic cylinders, the most problematic is that it is difficult to judge when foreign objects enter the hydraulic cylinders. After a foreign object enters, if a seal with a lip is installed on the outside of the sliding surface of the piston, the lip of the seal can scratch the foreign object during work, which is beneficial to avoid scratching. However, the piston with 0-shaped sealing ring has sliding surfaces at both ends, and foreign objects are sandwiched between the sliding surfaces, which is easy to form scars.

There are several ways for foreign objects to enter the tank.

Foreign matter entering the tank

a. Because care is not taken to keep the oil port open, conditions for receiving foreign objects at all times will be created, which is absolutely not allowed. It must be filled with antirust oil or working oil during storage and plugged.

b. Foreign matter enters the cylinder when it is installed. The place where the installation operation is performed is not in good condition, and foreign bodies can enter unconsciously. Therefore, the surroundings of the installation site must be cleaned up, especially the parts where the parts are placed must be cleaned up so that there is no dirt.

c. There are "burrs" on the parts, or insufficient scrubbing. The oil port on the cylinder head or the buffer device often has burrs left during drilling, which should be paid attention to, and then installed after the sand grinding is removed.

Foreign matter generated during operation

a. Friction iron powder or iron filings formed due to the buffer plunger's strength. The mating clearance of the buffer device is small, and when the lateral load on the piston rod is large, sintering may be caused. These rubbing iron powder or sintered metal fragments will remain in the cylinder.

b. Scars on the inner surface of the cylinder wall. The high pressure on the sliding surface of the piston causes sintering. Therefore, the surface of the cylinder body is crushed, and the crushed metal falls off and remains in the cylinder, which will cause scars.

There are many cases of foreign matter entering from the pipeline.

a. Do not pay attention when cleaning. When the pipeline is installed for cleaning, it should not pass through the cylinder block, and a bypass line must be installed in front of the cylinder port. this point is very important. Otherwise, foreign matter in the pipeline will enter the cylinder. Once it enters, it will be difficult to exclude it from the outside. Instead, it will become a foreign body into the cylinder. In addition, the cleaning method should be taken into account when removing foreign objects from the installation pipeline operation. In addition, the corrosion in the pipe should be carried out before the pipeline is installed, such as pickling, and the rust must be completely removed.

b. Chips formed during pipe processing. After the pipe has been cut to length, there should be no residue when doing deburring at both ends. Furthermore, placing steel pipes near the site where the welding pipeline is being operated is the cause of the mixing of welding foreign objects. The nozzles of pipes placed near the welding operation site must be sealed. It must also be noted that the pipe material should be fully prepared on a dust-free workbench.

c. The sealing tape enters the cylinder. As a simple sealing material, polytetrafluoroethylene plastic sealing tape is often used in the installation and inspection. If the winding method of the linear or tape-shaped sealing material is incorrect, the sealing tape will be cut off and enter the cylinder. The line-shaped seal does not affect the winding of the sliding part, but it will cause the check valve of the cylinder to malfunction or the buffer adjustment valve to be unable to be adjusted to the end. For the circuit, it may cause the diverter valve and overflow. Valve and pressure reducing valve malfunction.

The traditional repair method is to repair the damaged parts by external repair, or perform brush plating or overall surface scraping. The repair cycle of the hydraulic cylinder is long and the repair cost is high.

Repair process:

1. Rotate the scratched area with oxygen-acetylene flame (control the temperature and avoid surface annealing), and bake the oil that permeates the metal surface all year round.

2. Scratch the surface with an angle grinder, grind it to a depth of more than 1 mm, and grind a groove along the guide rail, preferably a dovetail groove. Scratch the holes at both ends to deepen and change the force situation.

3. Clean the surface with absorbent cotton dipped in acetone or absolute ethanol.

4. The metal repair material is applied to the scratched surface; the first layer should be thin, and the scratched surface should be evenly and completely covered to ensure the best adhesion of the material to the metal surface. Then apply the material to the entire repaired area and press repeatedly Make sure the material is filled and reaches the required thickness, making it slightly higher than the surface of the rail.

5. It takes 24 hours for the material to fully reach various performances at 24 ° C. In order to save time, the temperature can be increased by using a tungsten halogen lamp. Each time the temperature is increased by 11 ° C, the curing time will be reduced by half.

6. After the material is cured, use a fine grindstone or scraper to repair and level the material higher than the surface of the guide rail, and the construction is completed.