The hydraulic cylinder is a hydraulic actuator that converts hydraulic energy into mechanical energy and makes linear reciprocating motion (or swing motion). It is simple in structure and reliable in operation. When it is used to realize reciprocating motion, the reducer can be avoided, there is no transmission clearance, and the motion is stable, so it is widely used in various mechanical hydraulic systems. Output force of the hydraulic cylinder is proportional to effective area of the piston and pressure difference on both sides; The hydraulic cylinder is basically composed of cylinder barrel and cylinder head, piston and piston rod, sealing device, buffer device and exhaust device. The buffer device and exhaust device depend on the specific application, and other devices are indispensable.

Chinese name Hydraulic cylinder Foreign name Hydraulic cylinder Energy conversion Hydraulic energy conversion into mechanical energy Classification Cylinder barrel, cylinder head, piston and other disciplines Mechanical engineering advantages Simple structure, reliable work

catalogue

1 Introduction

2 Classification

▪ Piston type

▪ Plunger type

▪ Telescopic

▪ Swinging type

3 Buffer device

4 Cylinder barrel processing

5 Fault diagnosis

Introduction, editing and broadcasting

The hydraulic cylinder is the actuating element in the hydraulic transmission system. It is an energy conversion device that converts hydraulic energy into mechanical energy. The hydraulic motor realizes continuous rotary motion, while the hydraulic cylinder realizes reciprocating motion. The structural types of hydraulic cylinders include piston cylinder, plunger cylinder and swing cylinder. The piston cylinder and plunger cylinder realize reciprocating linear motion, output speed and thrust, swing cylinder realizes reciprocating swing, and output angular speed (speed) and torque. In addition to single use, hydraulic cylinders can also be combined with two or more or with other mechanisms. To perform special functions. The hydraulic cylinder is simple in structure and reliable in operation, and has been widely used in the hydraulic system of machine tools. [1]

Classified editing and broadcasting

Hydraulic cylinders have various structural forms, and their classification methods are also various: according to the motion mode, they can be divided into linear reciprocating motion type and rotary swing type; According to the action of hydraulic pressure, it can be divided into single action type and double action type; According to the structure, it can be divided into piston type, plunger type, multi-stage telescopic sleeve type, gear rack type, etc; According to the installation form, it can be divided into pull rod, ear ring, foot, hinge shaft, etc; According to the pressure grade, it can be divided into 16Mpa, 25Mpa, 31.5Mpa, etc.

Piston type

The single piston rod hydraulic cylinder only has a piston rod at one end. As shown in Figure 1, it is a single piston hydraulic cylinder. The inlet and outlet oil ports A and B at both ends can be connected with pressure oil or return oil to realize two-way movement, so they are called double acting cylinders.

Figure 1

Figure 1

The piston can only move in one direction, and its reverse movement needs to be completed by external force. However, its stroke is generally larger than that of the piston type hydraulic cylinder.

Piston type hydraulic cylinder can be divided into single rod type and double rod type structures. Its fixing methods are cylinder block fixation and piston rod fixation. According to the action of hydraulic pressure, there are single acting type and double acting type. In the single acting hydraulic cylinder, the pressure oil is only used for one chamber of the hydraulic cylinder. The hydraulic pressure enables the cylinder to move in one direction, while the reverse movement is achieved by external forces (such as spring force, dead weight or external load); The movement of the piston in both directions of the double acting hydraulic cylinder is completed by the action of hydraulic pressure by alternately feeding oil into the two chambers.

Fig. 2 shows the schematic diagram of single rod double acting piston hydraulic cylinder. The piston rod is only arranged on one side of the piston, so the effective area of the two chambers is different. When the oil supply is the same, the piston moves at different speeds when the oil is fed into different chambers; When the load force to be overcome is the same, the oil supply pressure required by different cavities is different, or after the system pressure is set, the load force that can be overcome by the hydraulic cylinder of the sanitation garbage truck moving in two directions is different.

Figure 2

Figure 2

Plunger type

(1) The plunger type hydraulic cylinder is a single acting hydraulic cylinder, which can only achieve movement in one direction by hydraulic pressure, and the return stroke of the plunger depends on other external forces or the weight of the plunger;

(2) The plunger is only supported by the cylinder liner and does not contact with the cylinder liner, so the cylinder liner is very easy to process, so it is suitable for making a long stroke hydraulic cylinder;

(3) The plunger is always under pressure during operation, so it must have sufficient stiffness;

(4) The weight of plunger is often large, and it is easy to sag due to its own weight when it is placed horizontally, causing unilateral wear of seals and guides, so its vertical use is more favorable.

Telescopic

The telescopic hydraulic cylinder has two-stage or multi-stage pistons. The sequence of piston extension in the telescopic hydraulic cylinder is from large to small, while the sequence of no-load retraction is generally from small to large. The telescopic cylinder can achieve a long stroke, while the retracted length is short and the structure is compact. This kind of hydraulic cylinder is commonly used in construction machinery and agricultural machinery. There are multiple pistons moving at one time. When each piston moves step by step, its output speed and output force change.

Double acting single piston rod hydraulic cylinder

Double acting single piston rod hydraulic cylinder

Swinging type

Swing type hydraulic cylinder is an actuator that outputs torque and realizes reciprocating motion. It has several forms, such as single blade, double blade, spiral swing, etc. Blade type: the stator block is fixed on the cylinder block, while the blade is connected with the rotor. According to the oil inlet direction, the blade will drive the rotor to and fro swing. The spiral swing type can also be divided into single spiral swing and double spiral swing. Now double spiral is more commonly used. The linear motion of the piston in the hydraulic cylinder of the two screw pairs is converted into the combined motion of linear motion and rotation motion to achieve the swing motion.

Buffer device editing broadcast

In the hydraulic system, the hydraulic cylinder is used to drive the mechanism with a certain quality. When the hydraulic cylinder moves to the end of the stroke, it has greater kinetic energy. If no deceleration treatment is taken, the piston of the hydraulic cylinder will collide with the cylinder head mechanically, causing impact, noise, and destructive. In order to mitigate and prevent the occurrence of such hazards, a reducer can be set in the hydraulic circuit or a buffer device can be set in the cylinder body.

Cylinder barrel processing, editing and broadcasting

As the main parts of hydraulic cylinder, mining single prop, hydraulic support, gun barrel and other products, the processing quality of the cylinder directly affects the life and reliability of the entire product. The cylinder barrel has high processing requirements, and its internal surface roughness is required to be Ra0.4~0.8&um, with strict requirements on coaxiality and wear resistance. The basic feature of cylinder barrel is deep hole machining, which has been puzzling the processing personnel.

Rolling processing is adopted. Because the surface layer has residual compressive stress, it is helpful to close the surface micro cracks and hinder the expansion of erosion. Therefore, the corrosion resistance of the surface can be improved, and the generation or expansion of fatigue cracks can be delayed, thus improving the fatigue strength of the cylinder barrel. Through rolling forming, a layer of cold work hardening layer is formed on the rolling surface, which reduces the elastic and plastic deformation of the contact surface of the grinding pair, thus improving the wear resistance of the inner wall of the cylinder barrel, and avoiding the burn caused by grinding. After rolling, the reduction of surface roughness value can improve the fit property.

The oil cylinder is the most important part of construction machinery. The traditional processing method is: broaching the cylinder block - fine boring the cylinder block - grinding the cylinder block. The rolling method is: broaching the cylinder block - fine boring the cylinder block - rolling the cylinder block. The process is three parts, but the time comparison: grinding the cylinder block for 1 m is about 1-2 days, and rolling the cylinder block for 1 m is about 10-30 minutes. Investment comparison: grinder or quilting mill (tens of thousands to millions), rolling knife (tens of thousands to tens of thousands). After rolling, the surface roughness of the hole is reduced from Ra3.2~6.3um to Ra0.4~0.8um, the surface hardness of the hole is increased by about 30%, and the fatigue strength of the inner surface of the cylinder barrel is increased by 25%. If only the influence of cylinder barrel is considered, the service life of oil cylinder will be increased by 2-3 times, and the efficiency of boring and rolling process will be increased by about 3 times compared with that of grinding process. The above data shows that the rolling process is efficient and can greatly improve the surface quality of the cylinder barrel.

After the cylinder is rolled, there is no sharp and small edge on the surface, and the seal ring or sealing element will not be damaged by long-time movement friction, which is particularly important in the hydraulic industry.

Fault diagnosis editing broadcast

The hydraulic cylinder is the actuating element that converts hydraulic energy into mechanical energy in the hydraulic system. Its faults can be basically summarized as hydraulic cylinder misoperation, inability to push load, piston sliding or creeping, etc. It is not uncommon for the hydraulic cylinder to stop due to its failure. Therefore, attention should be paid to the fault diagnosis, use and maintenance of the hydraulic cylinder.

1、 Fault diagnosis and treatment

1. Misoperation or malfunction

The causes and treatment methods are as follows:

(1) The valve element is stuck or the valve hole is blocked. When the flow valve or direction valve core is stuck or the valve hole is blocked, the hydraulic cylinder is prone to misoperation or malfunction. At this time, check the oil contamination; Check whether the valve element is stuck or the valve hole is blocked by dirt or colloidal sediment; Check the wear of the valve body, clean and replace the system filter, clean the oil tank and replace the hydraulic medium.

(2) The piston rod and cylinder barrel are stuck or the hydraulic cylinder is blocked. At this time, no matter how it is operated, the hydraulic cylinder does not act or acts very little. At this time, it should be checked whether the piston and piston rod seals are too tight, whether dirt and colloidal deposits enter: whether the axis line of the piston rod and cylinder barrel is aligned, whether the wearing parts and seals are invalid, and whether the load is too large.

(3) The hydraulic system control pressure is too low. The throttling resistance in the control pipeline 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, check the control pressure source to ensure that the pressure is adjusted to the specified value of the system.

(4) Air enters the hydraulic system. This is mainly because there is leakage in the system. At this time, check the liquid level of the hydraulic oil tank, the seals and pipe joints on the suction side of the hydraulic pump, and the oil suction strainer for dirt. If so, the hydraulic oil shall be replenished, the seals and pipe joints shall be treated, and the primary filter element shall be cleaned or replaced.

(5) The initial action of the hydraulic cylinder is slow. In the case of low temperature, the hydraulic oil has high viscosity and poor fluidity, which leads to slow action of the hydraulic cylinder. The improvement method is to replace the hydraulic oil with good viscosity temperature performance. Under low temperature, the oil temperature during startup can be increased by means of a heater or the machine itself. The oil temperature of the system under normal operation should be kept at about 40 ℃.

2. Load cannot be driven during operation

It mainly shows that the piston rod stops incorrectly, the thrust is insufficient, the speed drops, and the operation is unstable. The reasons are:

(1) Internal leakage of hydraulic cylinder. The internal leakage of hydraulic cylinder includes the leakage caused by excessive wear of hydraulic cylinder seal, piston rod and seal cover seal and piston seal.

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

The main reasons for excessive wear of piston seals are improper adjustment of speed control valve, excessive back pressure, improper installation of seals or contamination of hydraulic oil. Secondly, foreign matters entered during assembly and the quality of sealing materials was poor. The consequence is that the action is slow and weak, and in serious cases, the piston and cylinder barrel will be damaged, resulting in "cylinder pulling". The treatment method is to adjust the speed control valve, and make necessary operation and improvement according to the installation instructions.

(2) The hydraulic circuit is leaking. Including leakage of valve and hydraulic pipeline. The maintenance method is to check and eliminate the leakage of the hydraulic connecting pipeline by operating the reversing valve.

(3) The hydraulic oil is bypassed to the oil tank through the relief valve. If the overflow valve gets dirty and blocks the valve core, making the overflow valve normally open, the hydraulic oil will directly flow back to the oil tank through the bypass of the overflow valve, resulting in no oil entering 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 act. If the regulating pressure is low, the vertebral force required for still load cannot be reached due to insufficient pressure, which is manifested as insufficient thrust. At this time, check and adjust the relief valve.

3. Piston slipping or creeping

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

(1) The hydraulic cylinder is sluggish inside. The internal parts of the hydraulic cylinder are improperly assembled, deformed, worn or the shape and position tolerance exceeds the limit, and the action resistance is too large, which makes the piston speed of the hydraulic cylinder change with the different stroke positions, and slip or crawl occurs. Most of the reasons are due to the poor assembly quality of the parts, scars on the surface or iron chips generated by sintering, which increase the resistance and decrease 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 is offset from the guide rail, and the sealing ring is too tight or loose. The solution is to repair or adjust again, replace damaged parts and remove iron filings.

(2) Poor lubrication or out of tolerance machining of hydraulic cylinder bore. Because the piston and cylinder barrel, guide rail and piston rod all have relative motion, if the lubrication is poor or the hydraulic cylinder bore diameter is out of tolerance, the wear will be aggravated, and the straightness of the cylinder barrel centerline will be reduced. In this way, when the piston works in the hydraulic cylinder, the friction resistance will increase and decrease, resulting in sliding or creeping. The elimination method is to first repair and grind the hydraulic cylinder, then prepare the piston according to the matching requirements, repair and grind the piston rod, and configure the guide sleeve.

(3) Air enters the hydraulic pump or cylinder. Air compression or expansion will cause the piston to slip or crawl. The elimination measures are to check the hydraulic pump, set up a special exhaust device, and quickly operate the full stroke to and fro exhaust several times.

(4) The quality of seals is directly related to slip or creep. When the O-ring is used under low pressure, compared with the U-ring, it is easy to slip or crawl due to the high surface pressure and the large difference between dynamic and static friction resistance; The surface pressure of the U-shaped seal ring increases with the increase of the pressure. Although the sealing effect also increases correspondingly, the difference between dynamic and static friction resistance also increases, and the internal pressure increases, affecting the rubber elasticity. As the contact resistance of the lip increases, the seal ring will tip over and the lip will stretch, and it is also easy to lead to sliding or crawling. In order to prevent its tipping, the support environment can be used to keep it stable. [2]

4. Adverse consequences of scratches on the inner hole surface of the hydraulic cylinder block and quick repair methods

① The material scraps extruded from the scratch groove will be embedded into the seal, which may cause new scratches while damaging the working part of the seal during operation.

② Deteriorate the surface roughness of the inner wall of the cylinder barrel, increase the friction, and easily lead to creeping.

③ The internal leakage of the hydraulic cylinder is increased to reduce the working efficiency of the hydraulic cylinder. The main causes of scratches on the cylinder block bore surface are as follows.

(1) Scars caused when assembling the hydraulic cylinder

① Marks caused by foreign matters during assembly Before the general assembly of the hydraulic cylinder, all parts must be fully deburred and cleaned. During installation with burrs or dirt on the parts, foreign matters are likely to be embedded into the cylinder wall surface due to "stiffness" and the self weight of the parts, causing scars.

② When installing the hydraulic cylinder, the piston, cylinder head and other parts have large mass, size and inertia. Even with the assistance of lifting equipment, the specified fit clearance is small, so the hydraulic cylinder will not be put into operation in any way. Therefore, when the end of the piston or the boss of the cylinder head collides with the inner surface of the cylinder wall, it is very easy to cause scars. The method to solve this problem: for small products with large quantity and large batch, the special assembly guide tool shall be used during installation; Only careful and careful operation can avoid heavy, thick and large hydraulic cylinders.

③ The scar caused by the contact of the measuring instrument is usually measured by the inner diameter dial indicator when measuring the inner diameter of the cylinder block. The measuring contact is inserted into the hole wall of the cylinder block while rubbing. The measuring contact is mostly made of wear-resistant hard alloy with high hardness. Generally speaking, thin and long scratches with small depth caused during measurement are slight, which will not affect the operation accuracy. However, if the size of the measuring rod head is adjusted improperly, the measuring contact will be hard embedded, which will cause relatively severe scars. To solve this problem, first measure the length of the adjusted measuring head. In addition, stick a piece of paper tape with holes only in the measuring position on the inner surface of the cylinder wall, which will not produce scratches of the above shape. Slight scratches caused by measurement can be removed by using the reverse side of old emery cloth or toilet paper.