The hydraulic cylinder

   Hydraulic cylinder is a hydraulic actuator that converts hydraulic energy into mechanical energy and makes linear reciprocating motion (or swing motion). The utility model has the advantages of simple structure and reliable operation. When it is used to realize reciprocating motion, it can avoid the reduction device, and there is no transmission clearance, and the motion is stable. Therefore, it is widely used in various mechanical hydraulic systems. The output force of the hydraulic cylinder is directly proportional to the effective area of the piston and the pressure difference between its two sides; The hydraulic cylinder is basically composed of cylinder barrel and cylinder head, piston and piston rod, sealing device, buffer device and exhaust device. Buffer and exhaust devices depend on the specific application, and other devices are essential.
classification
There are various structural forms of hydraulic cylinders, and there are also many classification methods: according to the motion mode, it 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 structural form, it can be divided into piston type, plunger type, multistage telescopic sleeve type, gear rack type, etc; According to the installation form, it can be divided into pull rod, earring, foot, hinge shaft, etc; According to the pressure grade, it can be divided into 16MPa, 25MPa, 31.5Mpa, etc.
Hydraulic cylinder (engineering machinery) hydraulic cylinder (engineering machinery)
Folding piston type
A single piston rod hydraulic cylinder has a piston rod at only one end. As shown in the figure, 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 it is called double acting cylinder.
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 piston hydraulic cylinder.
Working principle of folding piston hydraulic cylinder
Piston hydraulic cylinder can be divided into single rod type and double rod type. Its fixing mode is fixed by cylinder block and piston rod. 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 supplied to one chamber of the hydraulic cylinder. The cylinder moves in one direction by hydraulic pressure, and the reverse movement is realized by external force (such as spring force, self 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 through the alternating oil inlet of the two chambers.
As shown in the figure, it is the schematic diagram of single rod double acting piston hydraulic cylinder. Only one side of the piston is provided with a piston rod, so the effective action areas of the two cavities are different. When the oil supply is the same, the movement speed of the piston is different in different cavities; 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 the hydraulic cylinder of the sanitation garbage truck can overcome in two directions is different.
Plunger type
(1) Plunger hydraulic cylinder is a single acting hydraulic cylinder, which can only move in one direction by hydraulic pressure, and the return stroke of plunger depends on other external forces or the self weight of plunger;
(2) The plunger is only supported by the cylinder liner without contact with the cylinder liner, so the cylinder liner is very easy to process, so it is suitable for making long stroke hydraulic cylinder;
(3) When working, the plunger is always under pressure, so it must have sufficient stiffness;
(4) The weight of the plunger is often large. When placed horizontally, it is easy to sag due to its own weight, resulting in the wear of one side of the seal and guide, 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, and the length is short when retracted, and the structure is compact. This kind of hydraulic cylinder is commonly used in engineering machinery and agricultural machinery. There are multiple pistons that move once. When each piston moves one by one, its output speed and output force change.
Double acting single piston rod hydraulic cylinder double acting single piston rod hydraulic cylinder
Swing type
Swing hydraulic cylinder is an actuator that outputs torque and realizes reciprocating motion. It has several forms, such as single blade, double blade, spiral swing and so on. Vane type: the stator block is fixed on the cylinder block, and the blades and rotor are connected together. According to the oil inlet direction, the blade will drive the rotor to swing back and forth. Spiral swing type is divided into single spiral swing and double spiral. Now double spiral is more commonly used. The linear motion of the piston in the hydraulic cylinder is transformed into the compound motion of linear motion and rotation motion by two spiral pairs, so as to realize the swing motion.
Buffer device
In the hydraulic system, the hydraulic cylinder is used to drive the mechanism with certain quality. When the hydraulic cylinder moves to the end of the stroke, it has large kinetic energy. If there is no deceleration treatment, the piston and cylinder head of the hydraulic cylinder will have mechanical collision, resulting in impact, noise and destruction. In order to mitigate and prevent this kind of harm, a deceleration device can be set in the hydraulic circuit or a buffer device can be set in the cylinder.
machining
As the main components of hydraulic cylinder, mining single pillar, hydraulic support, gun barrel and other products, the processing quality of cylinder barrel directly affects the service life and reliability of the whole product. The processing requirements of cylinder barrel are high, and the roughness of its inner surface is required to be Ra0 4 ~ 0.8-um, strict requirements on coaxiality and wear resistance. The basic feature of cylinder barrel is deep hole processing, which has always troubled the processing personnel.
Rolling processing is adopted, because the surface residual compressive stress is left on the surface layer, which is conducive to the closure of micro cracks on the surface and hinders the expansion of erosion. So as to improve the corrosion resistance of the surface, delay the generation or expansion of fatigue cracks, and improve the fatigue strength of the cylinder barrel. Through rolling forming, a 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, improves the wear resistance of the inner wall of the cylinder barrel, and avoids the burn caused by grinding. After rolling, the reduction of surface roughness can improve the matching properties.
Oil cylinder is the most important part of construction machinery. The traditional processing method is: broaching cylinder block - fine boring cylinder block - grinding cylinder block. The rolling method is: broaching cylinder block - fine boring cylinder block - rolling cylinder block. The process is three parts, but the time comparison: grinding cylinder block for 1m is about 1-2 days, and rolling cylinder block for 1m is about 10-30 minutes. Input comparison: grinding machine 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 changed from RA3 before rolling 2 ~ 6.3um is reduced to Ra0 4 ~ 0.8-um, 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 can be increased by 2 ~ 3 times, and the efficiency of boring rolling process is about 3 times higher than 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 oil cylinder is rolled, there is no sharp micro edge on the surface, and the sealing ring or seal will not be damaged by long-term motion friction, which is particularly important in the hydraulic industry

fault diagnosis
The energy of the hydraulic cylinder is converted into the mechanical energy of the hydraulic system. Its faults can be basically summarized as hydraulic cylinder misoperation, inability to push load, piston sliding or crawling, etc. Equipment shutdown caused by hydraulic cylinder failure is common. Therefore, attention should be paid to the fault diagnosis, use and maintenance of hydraulic cylinder.
1、 Fault diagnosis and treatment
1. Misoperation or action failure
The causes and treatment methods are as follows:
(1) The valve core is stuck or the valve hole is blocked. When the valve core of flow valve or direction valve is stuck or the valve hole is blocked, the hydraulic cylinder is prone to misoperation or action failure. At this time, check the oil pollution; Check whether dirt or colloidal sediment blocks the valve core or blocks the valve hole; 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 to operate, the hydraulic cylinder does not act or acts very little. At this time, check whether the sealing of piston and piston rod is too tight, whether dirt and colloidal sediment enter: whether the axis line of piston rod and cylinder barrel is aligned, whether vulnerable parts and seals fail, 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 is not adjusted properly, the control pressure is inappropriate, and the pressure source is 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. 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 oil suction side of the hydraulic pump, and whether the oil suction strainer is too dirty. If so, replenish hydraulic oil, deal with seals and pipe joints, and clean or replace the primary filter element.
(5) The initial action of the hydraulic cylinder is slow. When the temperature is low, the hydraulic oil has high viscosity and poor fluidity, resulting in slow action of the hydraulic cylinder. The improvement method is to replace the hydraulic oil with good viscosity temperature performance. At low temperature, the oil temperature during startup can be increased by means of heater or heating by the machine itself. The oil temperature of the system under normal operation should be maintained at about 40 ℃.
2. The load cannot be driven during operation
It is mainly manifested in inaccurate stop of piston rod, insufficient thrust, speed drop, unstable operation, etc. the reasons are as follows:
(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 seal is wrinkled, squeezed, torn, worn, aged, deteriorated, deformed, etc. at this time, the seal should be replaced with a new one.
The main reasons for excessive wear of piston seal are improper adjustment of speed control valve, resulting in excessive back pressure, improper installation of seals or hydraulic oil pollution. Secondly, foreign matters enter during assembly and the quality of sealing materials is poor. The consequence is that the action is slow and weak. In serious cases, it will also cause damage to the piston and cylinder barrel, resulting in the phenomenon of "cylinder pulling". The treatment method is to adjust the speed control valve, and make necessary operation and improvement according to the installation instructions.
(2) Hydraulic circuit leakage. Including leakage of valve and hydraulic pipeline. The maintenance method is to check and eliminate the leakage of hydraulic connecting pipeline by operating the reversing valve.
(3) The hydraulic oil is bypassed to the oil tank through the overflow valve. If the overflow valve enters the dirt and blocks the valve core, making the overflow valve normally open, the hydraulic oil will flow directly 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 overflow valve has reached the maximum rated value, the hydraulic cylinder still does not act because it cannot get the thrust required for continuous action. If the regulating pressure is low, the vertebral force required for still loading can not be reached due to insufficient pressure, which shows that the thrust is not enough. At this time, check and adjust the overflow valve.
3. Piston slipping or crawling
The 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. Improper assembly of internal parts of the hydraulic cylinder, deformation and wear of parts or out of tolerance of shape and position, and excessive action resistance make the piston speed of the hydraulic cylinder change with different stroke positions, resulting in sliding or crawling. Most of the reasons are due to the poor assembly quality of parts, scars on the surface or iron filings produced by sintering, which increases the resistance and reduces 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 installed too tightly or too loosely, etc. 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 moves relative to the cylinder barrel, the guide rail and the piston rod, if the lubrication is poor or the hole diameter of the hydraulic cylinder is out of tolerance, the wear will be intensified and the straightness of the center line of the cylinder barrel will be reduced. In this way, when the piston works in the hydraulic cylinder, the friction resistance will be large and small, resulting in sliding or crawling. The solution 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.
(3) Air enters the hydraulic pump or hydraulic cylinder. Air compression or expansion will cause the piston to slip or crawl. The elimination measure is to check the hydraulic pump, set up a special exhaust device, and quickly operate the full stroke to 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 pressure. Although the sealing effect also increases correspondingly, the difference between dynamic and static friction resistance also increases, and the internal pressure increases, which affects the rubber elasticity. Due to the increase of the contact resistance of the lip edge, the seal ring will overturn and the lip edge will extend, which is also easy to cause sliding or crawling. In order to prevent its overturning, support can be used to maintain its stability.
4. Adverse consequences of scratches on the inner hole surface of the hydraulic cylinder block and rapid repair methods: ① the material scraps extruded from the scratched groove will be embedded in the seal, which may cause new scratches in the scratched area while damaging the working part of the seal during operation.
② Worsen the surface roughness of the inner wall of the cylinder and increase the friction, which is easy to produce crawling phenomenon.
③ Increase the internal leakage of the hydraulic cylinder and reduce the working efficiency of the hydraulic cylinder. The main causes of scratches on the inner hole surface of the cylinder block are as follows.
(1) Scars caused when assembling the hydraulic cylinder
① During assembly, foreign matters are mixed into the hydraulic cylinder to cause scars. Before general assembly, all parts of the hydraulic cylinder must be fully deburred and cleaned. When the parts are installed with burrs or dirt, foreign matters are easy to be embedded into the surface of the cylinder wall and cause scars due to "other strength" and self weight of the parts.
② When installing the hydraulic cylinder, the piston, cylinder head and other parts have large mass, large size and large inertia. Even if there is lifting equipment for auxiliary installation, due to the small specified fit clearance, they will be put into force anyway. 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. Methods to solve this problem: for small products with large quantity and batch, autocratic assembly guiding tools shall be used during installation; For heavy, thick and large large large and medium-sized hydraulic cylinders, only careful and careful operation can be avoided.
③ When the inner diameter dial indicator is usually used to measure the inner diameter of the cylinder body, the measuring contact is inserted into the hole wall of the cylinder body while rubbing, and most of the measuring contacts are made of high hardness wear-resistant cemented carbide. Generally speaking, the slender scratch with small depth during measurement is slight and does not affect the operation accuracy. However, if the size of the measuring rod head is adjusted improperly and the measuring contact is hard embedded, it will cause more severe scars. The first solution to this problem is to measure the length of the adjusted measuring head. In addition, stick a paper tape with holes only in the measuring position on the inner surface of the cylinder wall, that is, there will be no scratches of the above shape. Slight scratches caused by measurement can be wiped off with the back of old emery cloth or toilet paper.
(2) Minor running wear marks
① The scars on the sliding surface of the piston are transferred. Before the installation of the piston, there are scars on the sliding surface. It is installed intact without treatment. These scars will in turn scratch the inner surface of the cylinder wall. Therefore, these scars must be fully repaired before installation.
② Sintering phenomenon caused by excessive pressure on the sliding surface of the piston. The piston tilts due to the self weight of the piston rod, resulting in other force phenomenon, or the pressure on the sliding surface of the piston rises due to the action of transverse load, which will cause sintering phenomenon. In the design of hydraulic cylinder, its working conditions must be studied, and full attention must be paid to the length and clearance of piston and bushing.
③ The hard chromium plating layer on the inner surface of the cylinder is stripped. It is generally believed that the reasons for the stripping of the hard chromium plating layer are as follows.
a. Poor adhesion of electrodeposited coating. The main reasons for poor adhesion of Electrodeposited coatings are: insufficient degreasing and degreasing treatment of parts before electroplating; The activation treatment of the part surface is not complete, and the oxidation film is not removed.
b. The hard layer is worn. The wear of electroplated hard chromium layer is mostly caused by the friction of piston and the grinding effect of iron powder. When there is water in the middle, the wear is faster. The corrosion caused by the contact potential difference of metal only occurs at the part contacted by the piston, and the corrosion occurs in spots. As mentioned above, when there is water in the middle, it will promote the development of corrosion. Compared with castings, the contact potential difference of copper alloy is higher, so the corrosion degree of copper alloy is more serious.
c. Corrosion caused by contact potential difference. Contact potential difference corrosion is not easy to occur for hydraulic cylinders operating for a long time; It is a common fault for hydraulic cylinders that have been stopped for a long time.
④ Damage of piston ring the piston ring is damaged during operation, and its fragments are clamped in the sliding part of the piston, causing scratches.
⑤ The material of piston sliding part is sintered cast piston, which will cause sintering phenomenon when bearing large transverse load. In this case, the sliding part of the piston shall be made of copper alloy or welded with such material.
(3) Foreign matters are mixed in the cylinder
Among the faults of the hydraulic cylinder, the most problematic is that it is difficult to judge when the foreign matter entered the hydraulic cylinder. After foreign matters enter, if a seal with lip is installed on the outside of the sliding surface of the piston, the lip of the seal can scrape the foreign matters during operation, which is beneficial to avoid scratching. However, for the piston equipped with 0-shaped sealing ring, its two ends are sliding surfaces, and foreign matters are sandwiched between the sliding surfaces, which is easy to form scars.
There are several ways for foreign matters to enter the cylinder.
① Foreign matter entering the cylinder
a. If the oil port is not kept open during storage, the condition of accepting foreign matters at all times will occur, which is absolutely not allowed. Antirust oil or working oil must be injected and plugged during storage.
b. Foreign matters enter the cylinder block during installation. In the place where the installation operation is carried out, the conditions are not good, and foreign matters can enter unconsciously. Therefore, the area around the installation site must be cleaned, especially the place where the parts are placed, so that there is no dirt.
c. There are "burrs" on parts or insufficient scrubbing. There are often burrs left during drilling in the oil port or buffer device on the cylinder head, which should be paid attention to and installed after sanding.
② Foreign matters generated during operation
a. Friction iron powder or iron filings formed due to the plug force of the buffer column. When the fit clearance of the buffer device is very small and the transverse load on the piston rod is very large, sintering may be caused. These friction iron powders or metal fragments that have fallen off due to sintering 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, so the surface of the cylinder body is cracked, and the cracked metal falls off and remains in the cylinder, which will cause scars.
③ There are many situations for foreign matters entering from the pipeline.
a. Do not pay attention when cleaning. When the pipeline is cleaned after installation, it should not pass through the cylinder block. A bypass pipeline must be installed in front of the oil port of the cylinder block. This is very important. Otherwise, the foreign matter in the pipeline will enter the cylinder. Once it enters, it is difficult to eliminate it, but it will become the transportation of foreign matter into the cylinder. In addition, the removal method of foreign matters in the installation pipeline operation shall be considered during cleaning. In addition, for the corrosion in the pipe, pickling and other procedures shall be carried out before the pipeline installation, and the corrosion must be completely removed.
b. Chips formed during pipe machining. After the pipe is machined to length, there shall be no left over during the deburring operation at both ends. Moreover, placing steel pipes near the site for welding pipeline operation is the reason for the mixing of welding foreign matters. The pipe orifice placed near the welding operation site shall be sealed. It must also be noted that the pipe fitting materials shall be fully prepared on the dust-free workbench.
c. The sealing belt enters the cylinder. As a simple sealing material, polytetrafluoroethylene plastic sealing belt is often used in installation and inspection. If the winding method of linear and belt sealing materials is wrong, the sealing belt will be cut off and enter the cylinder. The wire belt seal will not have any impact on the winding of the sliding part, but it will cause the one-way valve of the cylinder to act improperly or the buffer regulating valve can not be adjusted to the end; For the circuit, it may cause the action failure of directional valve, overflow valve and pressure reducing valve.
The traditional repair method is to repair the damaged parts by outsourcing after disassembly, or brush plating or overall scraping and grinding of the surface. The repair cycle is long and the repair cost is high.
Repair process:
1. Bake the scratched part with oxygen acetylene flame (master the temperature and avoid surface annealing), and bake the oil on the perennial metallized surface until there is no spark splashing.
2. The scratched part shall be treated with an angle grinder with a grinding depth of more than 1mm, and the groove shall be polished along the guide rail, preferably a dovetail groove. Drill holes at both ends of the scratch to deepen and change the stress condition.
3. Clean the surface with absorbent cotton dipped in acetone or absolute ethanol.
4. Apply metal repair materials to the scratched surface; The first layer should be thin, evenly and completely cover the scratched surface to ensure the best bonding between the material and the metal surface, and then apply the material to the whole repair part and press it repeatedly to ensure that the material is filled and reaches the required thickness, making it slightly higher than the surface of the guide rail.
5. It takes 24 hours for the material to fully achieve all properties at 24 ℃. In order to save time, the temperature can be increased by tungsten halogen lamp. The curing time will be shortened by half every 11 ℃ increase in temperature, and the optimal curing temperature is 70 ℃.
6. After the material is solidified, use a fine grinding stone or scraper to repair and level the material higher than the guide rail surface, and the construction is completed.
matters needing attention:
In order to ensure the normal operation of the equipment, it must be ensured that:
1. Strict and careful installation;
2. Clean the putty and impurities left in the equipment;
3. Replace the lubricating oil and improve the equipment lubrication system.
4. Replace the roller stopper to ensure effective cleaning of iron filings on the guide rail
Inspection and troubleshooting of common faults in forklift hydraulic system
The main hydraulic operation parts of forklift include lifting cylinder and tilt cylinder. The troubleshooting methods are listed as follows:
1. The forklift cannot lift and tilt when it is empty
Fault inspection 1: insufficient oil in the system
Troubleshooting: add oil to the oil mark.
Troubleshooting 2: the displacement of the gear pump is insufficient, and the backlash between the top gear and the side gear is too large
Fault inspection: replace the oil pump or parts
Fault inspection 3: the oil inlet of multi-way valve is blocked
Troubleshooting: dredge the oil circuit of multi-channel valve
2. The piston rod of lifting cylinder of forklift truck moves slowly, and the gantry is interrupted or impacted during lifting
Fault inspection 1: insufficient oil in the system
Troubleshooting: add oil to the oil mark
Fault inspection 2: oil leakage and oil pipe rupture caused by poor sealing and loose oil pipe connection
Troubleshooting: encrypt the seal, tighten the pipe joint and replace the oil pipe
Fault inspection 3: insufficient oil supply due to oil pump fault
Troubleshooting: repair or replace the oil pump
Fault inspection 4: improper adjustment or damage of safety valve
Troubleshooting: adjust or repair the safety valve
Fault inspection 5: hydraulic filter blocked
Troubleshooting: clean or replace the hydraulic oil filter
Fault inspection 6: there is air in the hydraulic system
Troubleshooting: remove the air
3. Forklift fork frame lowering speed is too fast
Fault inspection: the throttle valve is stuck in the open position or installed reversely
Troubleshooting: reinstall the throttle valve
4. The free sliding amount of forklift lifting cylinder is too large or the self inclination angle of tilt cylinder is too large
Fault inspection: the leakage in the valve rod of the directional valve is too large, and the piston sealing ring of the tilt cylinder is invalid
Troubleshooting: replace the valve rod or seal ring
5. Hydraulic cylinder piston rod with oil
Fault inspection: the oil seal is damaged
Troubleshooting: replace the oil seal
6. Hydraulic system noise (unstable cylinder movement)
Fault inspection 1: the oil inlet pipe is blocked
Troubleshooting: dredge the oil pipe and replace the oil
Fault check 2: there is air in the system
Troubleshooting: remove from the vent hole
Fault inspection 3: the bearing of gear pump is worn or the side plate of gear pump is worn
Troubleshooting: replace the gear pump bearing or side plate
Note: air exhaust method in forklift hydraulic system:
Lift the lifting cylinder, fill the hydraulic oil to the oil level specified by the scale, make the forklift load about 1 / 10 of the rated load, loosen the vent bolt on the lifting cylinder, operate the forklift vertical cylinder to lift and lower several times until there is no bubble at the vent plug, and then tighten the vent plug.