Failure analysis of hydraulic pumps unit and oil cylinder

Ⅰ. Failure analysis of Telescoping Hydraulic System and Hydraulic Pump Station and its diagnose in use

1. Failure: While running, hydraulic pump system and hydraulic elements lose regular function.

2. Diagnose in use:

(1) Inquiry: Whether it is a sudden failure, gradual failure or after reorganization failure.

(2) Vision diagnosis: Whether hydraulic oil in tank has bubble or color changing; observe the sealing section and find out whether it has oil leaking in pipe joint and other joint surface; observe whether the gauge hand has the failure of wobble, drop-off and pressure regulating fails to come up. 

(3) Acoustic diagnose: High-pitched shrill whistlers means air intake, while, the cavitations’ noise could be oil filter blocking, low oil level or too much water in hydraulic oil.

(4) Olfactory diagnose: Abnormal smell can judge whether the electronic elements has Insulation broken circuit, pay attention to emulsification bad smell of hydraulic oil.

(5) Tactile diagnose : To take advantage of sensitive finger touching, and check whether the shaking and shock occurs and whether the oil temperature is in a normal range.

Ⅱ. Failure analysis of solution of hydraulic pump unit and oil cylinder

1. Exhaust-gas Disposal must be carried out before installation and usage of standing tower

Since there is no oil in oil tank before the usage of oil tank, while the hydraulic oil refill in the tank, part of air is filled with hydraulic oil, other part forms bubble, floating in the surface of hydraulic oil. Hydraulic oil can produce obvious spring due to the Compressibility of air, and stop at the top of tower, sneak in the cylinder at a distance because of the loading weight of cylinder, which comes some difficult in work.

Exhaust-gas solution:

(1) When there is no loading of cylinder, oil takes in from large cavity reaching into the cylinder, and piston open the venting screw (each one in large and small cavity) to a quarter turn counterclockwise when the piston near limit, then let the piston reach limit; operate the selector valve and build the pressure for 3 to 4 times in a short time, shut off two air vent screw when there is bubble and hydraulic oil leaking at the screw hole.

(2) When there is no loading in cylinder, oil takes in from small cavity (cylinder shrinking), and piston open the venting screw (each one in large and small cavity) to a quarter turn counterclockwise when the piston near limit, then let the piston reach limit; operate the selector valve and build the pressure for 3 to 4 times in a short time, shut off two air vent screw when there is bubble and hydraulic oil leaking at the screw hole.

(3) Operate the cylinder back and forth for 3-4 times by the method above, and air in cylinder can be exhausted basically, and the telescoping operation can be carried out.

2. Reach cylinder in low speed or without moving

Failure analysis:

(1) Insufficient pressure in relief valve or position selector valve is not in place;

(2) When position selector valve is not in the exact place, pressure loss of valve hole will increase greatly, which will influence the operating speed of cylinder;

(3) When balance valve of oil channel in small cavity has a higher pressure, that is 2.5Mpa, the oil resistance will increase while reaching the cylinder, which is caused by flow resistance.

Solutions:

(1) Hit off the pin of piston rod from the support beam, leaving piston rod in no-load condition; operate the selector valve to let piston rod reach into limit and then build pressure; observing whether the gauge hand points in nominal pressure range, if the pressure is not enough, rotate clockwise the handles of overflow valve in the panel. If there is still no change in pressure, remove the coupling screw, which connect the panel and oil tank; and then lift the motor and panel with flow-combining valve unit coming out from oil cylinder and oil filter dissolving in the oil; operate motor to build pressure, observe oil flow in which flow-combining valve, rotate clockwise pressure adjusting screw to nominal pressure range; if the pressure is still at low level, repair or change the flow-combining valve.

(2) Adjust the selector valve to neutral position, and solid it with the panel.

(3) Readjust the balance valve in regular pressure 2.5Mpa; connect the two oil hole of selector valve with one high pressure pipe; operate the reaching position of selector valve, at the same time, observe whether the gauge pressure is in nominal pressure.

3. Cylinder shrinking in low speed or without moving

Regular failure results from the insufficient pressure at cylinder shrinking (oil takes in from small cavity) or check valve’s failure to operation caused by spring plate locking, and its reason mainly comes as following:

(1) If the spring plate is locked, the check valve cannot open entirely, causing the increasing of flowing blocking, the great loss of pressure, and the oil with pressure cannot flow the cylinder smoothly, which cause the failure open of hydraulic lock and the motionless of cylinder.

(2) Since the oil cylinder shrinking of itself while the reaching pressure in cylinder is 6.5Mpa with the open of hydraulic lock, the pressure will be over low, insufficient to open the hydraulic lock, which make the failure of oil flow back in the large cavity without the flow of oil, even the entrap in large cavity and the motionless of oil cylinder.

Solutions:

Make sure the position of gauge hand at 6.5Mpa at the time of cylinder shrinking and building pressure. If the pressure is still insufficient to work normally, disassemble and check the check valve and find out whether it is locked. Cylinder stops working, if the check valve can be open normally, shut the cylinder and build pressure; if the pressure is not enough, release the screw which connected the panel and oil tank, lift the panel and let the connecting valve unit come out with oil filter dissolving into the oil, start the motor with the operation of selector valve at the time of shutting cylinder and building pressure, rotate the pressure adjusting screw clockwise, and adjust the pressure to 6.5Mpa. If the pressure cannot come up, repair or change the flow-combining valve.

4. Shaking on cylinder shrinking

Shaking phenomena is caused by large flowing hole in adjusting valve (needle valve) in balance valve of cylinder. Due to the small resistance and low backpressure on oil flow back in large cavity, the capable ability to bear loading cannot be formed. While the oil takes in from small cavity with the open of hydraulic lock, bearing loading of oil cylinder will fall down of its weight. Since the rapid speed of oil back flowing in large cavity, oil pump does not have enough time to fill the cylinder with oil and at this time the pressure in small cavity decreases with hydraulic lock closed and stopping working of oil cylinder. While the small cavity is filled with oil by oil pump, the pressure will rise again with the reopen of hydraulic lock and the oil cylinder will fall down rapidly in the case of loading weight with the shaking phenomena producing now and then in cycles. The increasing of back pressure prevents the oil cylinder from shaking with good stability in motion. If too much air is dissolved in the oil, pressure crawling will occur with the appearance of shaking phenomena.

Solution: release the locking nut, rotate the needle valve clockwise to orifice and shut all of them; rotate the needle valve counterclockwise slightly to let the oil cylinder not shake with normal operation.

5. Telescoping in motionless or slow motion

Reasons:

(1) Volumetric efficiency loss caused by pump leakage with insufficient flow comes into slow telescoping motionless.

(2) Sludge or poor cleanliness is formed by oxidative deterioration of oil, and some other dirt causes the blocking of oil filter, bad pump suction, and insufficient flow.

(3) Climate changing elements, especially in north part of china, with the temperature of -10 degree, the low oil temperature result into the high viscosity in hydraulic oil, which causes the increasing of resistance, poor suction of pump and even the burning of pump.

(4) Whether the motor rotates in right direction or the wrong direction of pump suction and oil venting will cause the failure of oil discharge or oil burning.

Solutions:

(1)To remove the screw connecting panel and oil cylinder, and lift the motor and panel for checking to make sure whether there is impurity and emulsification in the oil. To check whether there is blocking in oil filter, if there is any blocking, flush and reinstall the filter. 

(2)To check whether there is failure in oil pump, and insert the filter into the oil; start the motor with selector valve coming up to reach cylinder to limit and then build pressure. Observe two overflow valves, if the overflow valve is not started and the gauge shows that the pressure is insufficient with the normal leakage on oil returning pipe of selector valve, it shows that great leakage inside the oil cylinder, and it should be changed.

(3) Heat the oil by heater before using if the temperature is low and the viscosity is high.

Ⅶ. Cylinder sneaking: cylinder will fall down by itself based on self weight after telescoping is stopped; the lug of piston rod is used to support beam while installation and the piston rod still reach out based on self weight after selector valve comes back to neutral position.

(1) After the selector valve comes back to its position, telescoping needs to stop, however, the cylinder still falls down based on self weight, which may be caused by internal leakage of hydraulic lock and selector valve’s failure to neutral position.

(2) Piston rod reaches out because of self weight, and the unsealing of check valve in small cavity or the too low pressure of balance valve leads to the failure of support beam while the balance valve started and the piston rod reaching out because of self weight.

(3) The harm of piston seal cause oil intermingle both in large and small cavity, which leads to cylinder sneaking.

Solutions:

(1) After the telescoping is stopped, close the motor, and observe whether the frame falls down. If the frame has not fallen down, it shows that the selector valve is not in the neutral position and it needs reinstall. However, when it still falls down after that, rotate the throttle valve (needle valve) clockwise to the bottom normally; observe whether the cylinder sneaks or not, if not, it is evident that internal leakage of hydraulic lock leads to cylinder sneaking. Besides, whether the balance valve with the pressure of 2.5Mpa is still too high in pressure needs to check. After the sudden stopping of telescoping, frame units fall down by itself based on self weight, piston controlled by hydraulic lock cannot go back to the original position immediately, which causes the check valve inside hydraulic lock are still in start condition. Oil of large cavity flows from neutral position of selector valve to oil tank, leading to frame units fall down and cylinder sneaking.

(2) If piston rod still reaches out after the motor shuts off, check whether the check valve in small cavity has foreign material fill in the hole. If there is some, remove it; If there is no, check whether the balance valve with the pressure of 2.5Mpa has too low pressure, and if so, turn it to 2.5Mpa.

(3) Let the piston rod reach to its limit, and remove the oil pipe in small cavity. Feed oil to large cavity and build pressure to check whether oil overflows from small cavity. If the oil flows without stopping, it shows that piston seal has been damaged. If there is no oil flowing, it proves that the piston seal is in good condition.

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