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Analysis and Treatment of Common Faults in Hydraulic System

Working principle

The hydraulic pump converts the mechanical energy of the electric motor (or other prime mover) into the pressure energy of the liquid, and then performs hydraulic transmission by hydraulic pressure. The cylinder (or hydraulic motor) converts the pressure energy of the liquid into mechanical energy to drive the load.

hydraulic system of profile bending machine
hydraulic system of profile bending machine

Five parts

The hydraulic system usually consists of the following five parts.

Power plant part. Its function is to convert the mechanical energy provided by the motor (or other prime mover) into the pressure energy of the liquid. Simply put, it is a device that provides pressure oil to the system. Such as various hydraulic pumps.

Part of the control and adjustment device. Including pressure, flow, and direction control valves, which are used to control and adjust the pressure, flow and direction of the liquid flow in the hydraulic system to meet the force (or torque), speed (or speed) and direction of movement (or Movement cycle) requirements.

Part of the implementing agency. Its function is to convert the pressure energy of the liquid into mechanical energy to drive the working parts to move. Including hydraulic cylinders and hydraulic motors.

Automatic control part. Mainly refers to electrical control devices.

The auxiliary device part. In addition to the above four major parts, fuel tanks, fuel pipes, integrated blocks, oil filters, accumulators, pressure gauges, heaters, coolers, etc. They are indispensable for ensuring the reliability and stability of the hydraulic system and play an important role.

Hydraulic cylinder

The hydraulic cylinder is an executive element that converts hydraulic energy into mechanical energy. Common failures of hydraulic cylinders include: hydraulic cylinder crawling, hydraulic leakage, hydraulic cylinder mechanical strength, hydraulic cylinder air intake, hydraulic cylinder shock, etc.

Analysis and Treatment of Crawling Failure of Hydraulic Cylinder


Crawling failure of hydraulic cylinder: Crawling phenomenon refers to the state of jumping and stopping and walking when the hydraulic cylinder is moving. This phenomenon is prone to occur especially when moving at low speed. This is one of the most important failures of the hydraulic cylinder.

   (1). There is air in the cylinder or pipeline. The treatment method: set up an exhaust device; if there is no exhaust device, the hydraulic system can be operated to reciprocate several times at the maximum stroke to force the air to be exhausted; the system and the pipeline are sealed.

   (2). Negative pressure is formed somewhere in the cylinder. Treatment method: find out where the negative pressure is formed in the hydraulic cylinder and seal it; and exhaust. The gas-liquid pressurized cylinder manufacturer, in order to avoid the excessive pressure of the cylinder during pressurization and damage your mold or workpiece, please install a pressure reducing valve at the inlet of the pressurizing section to effectively protect your products.

  (3). The sealing ring is pressed too tightly. Treatment method: adjust the sealing ring so that it is not loose or tight, and ensure that the piston rod can be moved back and forth by hand.

   (4). The piston and the piston rod are not in the same axis. The treatment method: install the two together and place them on the V-shaped block to correct the same degree error within 0.04mm; replace the piston.

  (5). The piston rod is not straight (bends), the treatment method: single or with the piston on the V-shaped block, use a press to control the straightness and use a dial indicator to correct and straighten.

(6) The guide rail or slide block is clamped too tightly or not parallel to the hydraulic cylinder. Treatment method: adjust the tightness of the pressure (bar) of the guide rail or slide block to ensure the accuracy of the moving parts and ensure that the sliding resistance is small ; If the adjustment is invalid, check the parallelism of the cylinder and the guide rail, and repair the contact surface to correct it.

   (7). The nuts at both ends of the two piston rods are twisted too tightly. Solution: Adjust the tightness to keep the piston rod in a natural state.

  (8), the inner wall of the cylinder or the surface of the piston is strained, local wear (a basic type of component failure) is severe or corroded, etc. Treatment methods: boring the cylinder bore and refitting the piston.

Analysis and treatment of impact failure of hydraulic cylinder  

(1). No buffering device is provided. Treatment method: adjust the reversing time (>0.2 seconds), reduce the power speed of the hydraulic cylinder; add a buffering device.   

(2). The gap between the plunger and the hole in the buffer device is too large. Treatment method: replace the slow plunger or insert a sleeve in the hole, and the gap will meet the specified requirements; check the throttle valve.   

(3) The one-way valve with a buffer at the end is reversed. Treatment method: repair, research and match the one-way valve and valve seat or replace it. The gas-liquid pressurized cylinder has fast action speed, stable transmission, simple cylinder device, easy output adjustment, low energy consumption, soft landing without damaging the mold, easy installation, small space occupation, less failure, long life, and low noise.

Analysis and treatment of external leakage failure of hydraulic cylinder  

  (1). Piston rod surface damage, treatment method: repair the piston rod damage.   

(2), the edge of the sealing ring is damaged or aging, the treatment method: replace the sealing ring.   

(3) The pipe joint is not tightly sealed, and the treatment method: check the sealing ring and contact surface for scratches, repair or replace it.

(4) The cylinder head is not tightly sealed, and the treatment method: inspect the machining accuracy of the contact surface and the aging of the sealing ring, and replace or repair it in time.   

(5). Due to poor exhaust, the adiabatic compression of the gas causes local high temperature and damages the sealing ring. Treatment method: check the exhaust device, or add an exhaust device; exhaust in time.   

(6) The sealing of the buffer device is not tight, and the treatment method: the machining accuracy (precision) of the contact surface of the cylinder rod and the aging condition of the sealing ring should be replaced or repaired in time.

Analysis and treatment of internal leakage failure of hydraulic cylinder

(1) The fit gap between the cylinder bore and the piston is increased due to wear and out of tolerance. Treatment method: When the piston is worn seriously, the cylinder bore should be bored, the piston should be small, and the several grooves of the car should be equipped with seals or new pistons. .

(2). The seal ring on the piston is worn or aged. Treatment method: the seal ring should be replaced in time if it is worn or aged.

(3). Piston and cylinder barrel are installed out of concentricity or bear eccentric load to make the piston inclined or eccentrically worn. Treatment method: check the concentricity of cylinder barrel and piston and cylinder head piston rod hole, and trim the center.

(4). Poor straightness of the cylinder bore diameter or local wear caused by local waist drum boring cylinder bore. Treatment method: refit the piston.

Sound and noise

The air in the lower part of the piston is adiabaticly compressed. Processing method: move the piston slowly and reciprocate several times, each time to the top to remove the gas in the cylinder, this serious noise can be eliminated and the seal ring can be prevented from being burned.

Hydraulic control valve

The hydraulic control valve is the control and adjustment element in the hydraulic transmission system. It controls the direction, pressure and flow of the oil flow to meet the pressure, direction and speed requirements of the actuator, so that the actuator can drive the load to achieve a predetermined action . The speed is faster, the output is stable, the device is simple, the adjustment is easy, the maintenance is convenient, the output is large, and the output can reach the high output of oil pressure, and the power source is convenient. Low operating noise, no disturbance of oil pressure system heating up.

Performance requirements of hydraulic valves


In the hydraulic system, the performance requirements of the hydraulic valve mainly have the following points.

(1), the action is sensitive, reliable in use, the impact and vibration during work should be small, and the service life should be long.

(2). When the oil flows through the hydraulic pressure, the pressure loss should be small, the sealing performance should be good, the internal leakage should be small, and no external leakage.

(3), compact structure, convenient installation, maintenance and adjustment, and good versatility.

Hydraulic valve classification

  (1), pressure control valve (referred to as pressure valve). It is used to control the pressure in the hydraulic system to meet the requirements of the force (or torque) required by the actuator. Including overflow valve, pressure reducing valve, sequence valve, pressure relay, etc.

  (2), directional control valve (referred to as directional valve). It is used to control the direction of oil in the hydraulic system to meet the requirements of the movement direction of the actuator. Including one-way valves, reversing valves, etc.

   (3), flow control valve (referred to as flow valve). It is used to control the flow in the hydraulic system to meet the requirements of the movement speed of the actuator. Including throttle valve, speed control valve, etc.

   (4), compound valve (also called multi-element valve). It is used to control two or all of the three parameters of direction, pressure, and flow in the hydraulic system. For example, a one-way pressure reducing valve can not only control the direction of the oil, but also control the pressure of the oil.

Analysis and Treatment of Common Faults of Overflow Valves

The hydraulic valve takes the overflow valve as an example, a brief introduction.

(1) There is a problem with the regulating device of the overflow valve. Treatment method: disassemble and inspect, and replace with a suitable pressure regulating spring.

(2) The spool of the solenoid directional valve is stuck. After energizing, the electromagnetic force cannot make the spool switch in place, so that the valve port has a slight opening. Treatment method: disassemble, inspect, clean, and grind the solenoid valve core.

(3), the poppet spool of the pilot valve of the relief valve is defective.

Approach:

   A. Disassemble and inspect, and grind the sealing surface of the cone valve core. When the damage is serious, replace with new qualified parts.
   B. Disassemble, clean and check the degree of contamination of the hydraulic oil, filter the oil or change the oil according to the situation.

(4). The valve core of the electromagnetic valve in the relief valve is severely worn and internal leakage is aggravated. Treatment method: disassemble and inspect, clean hydraulic components and systems, eliminate factors that cause abnormal wear of hydraulic components, and replace with new reversing valves.

(5) The spool of the solenoid directional valve in the relief valve is seriously worn and internal leakage is aggravated. Treatment method: disassemble and inspect, clean hydraulic components and systems, eliminate factors that cause abnormal wear of hydraulic components, and then replace with new reversing valves.

(6), the main valve is faulty. Treatment method: disassemble and inspect, clean and trim the valve core and valve hole. If necessary, check the oil contamination, filter or replace the oil, and pay attention to maintaining the concentricity of the valve core and the valve hole during assembly.

Hydraulic pump

The hydraulic pump is a hydraulic power element, which is an energy conversion device that converts the mechanical energy input by the electric motor (or other prime mover) into hydraulic energy. Its role is to provide pressure oil to the hydraulic system.

Classification of hydraulic pumps:

  (1), gear pump (external gear pump and internal gear pump)

  (2), vane pump (single-acting vane pump and double-acting vane pump)

  (3), plunger pump (axial plunger pump and radial plunger pump)

Analysis and Treatment of Common Faults of Gear Pump

Gear pumps are hydraulic pumps commonly found in production lines. Here, only the gear pump fault gaps and treatment methods are introduced as follows:

(1). The clearance between the gear of the oil pump and the pump casing exceeds the specified limit. Treatment method: replace the pump casing or repair it by inserting the sleeve method to ensure that the clearance between the gear tooth top and the casing of the oil pump is within the specified range.

(2) Excessive wear of gear bushings and gear end faces makes the pressure relief seal ring insufficiently pre-compressed and loses the sealing effect, causing the high-pressure oil chamber and the low-pressure oil chamber of the oil pump to collude with each other, causing serious internal leakage. Treatment method: later Add a compensation gasket under the sleeve (the thickness of the compensation gasket should generally not exceed 2mm) to ensure the compression of the sealing ring.

(3) When disassembling and assembling the oil pump, install the guide wire in the wrong direction at the joint surface of the two shaft sleeves (the shaft sleeve of the spiral oil groove). Treatment method: ensure that the guide wire can rotate the two shaft sleeves according to the passive gear at the same time The direction is deflected by an angle to make the two shaft sleeve planes fit tightly.

(4) When disassembling the oil pump, the pressure isolating sealing ring is aging and damaged, and the pressure relief piece sealing rubber ring is installed incorrectly. Solution: If the pressure isolating sealing ring is aging, replace with a new one: the pressure relief piece sealing rubber ring should be installed On the side of the suction cavity (port) (low pressure cavity), and ensure a certain pre-tightening pressure. If it is installed on the side of the oil pressure chamber, the sealing rubber ring will be damaged quickly, causing the high pressure chamber to communicate with the low pressure chamber, and the oil pump will lose its working ability.

Further reading:Section bending machine hydraulic system use and maintenance