Common faults and troubleshooting methods of universal material testing machines

The sensitivity of the dial pointer is poor, and there is a phenomenon of stopping midway after unloading, or the zero position often changes.

 

The causes of this phenomenon are multifaceted.

1. The pulley and its track on the toothed rod have excessive dust and rust. At this point, it is necessary to remove and clean it thoroughly, and then add a little clock oil.

2. The pointer rotation axis is dirty and should be cleaned.

3. There is contact between the toothed rod pressing plate and the toothed rod, which should be loosened.

4. The oil return of the buffer is poor and should be adjusted or cleaned. (Refers to a blocked bend)

5. The belt on the force measuring piston falls off or is too loose (causing the piston to not run at a uniform speed).

6. If the friction force of the force measuring piston increases or gets stuck in the oil cylinder, apply chromium oxide grinding paste to grind it until it is normal.

7. The pendulum has obstacles during the lifting process, or the pendulum shaft is too dirty or rusted, causing significant changes in the pointer to return to zero.

 

When loading, sometimes the * large load cannot be applied or unloaded during the loading process.

 

This type of malfunction often occurs and has multiple causes.

1. The viscosity of the oil is too low (there is oil flowing out of the return pipe of the oil supply valve during loading). Generally, medium viscosity mineral oil is used, which does not contain water, acid, or other mixtures. It does not decompose at room temperature and does not become thick. If necessary, a viscometer can be used to measure it. If oil is used improperly, it can cause valve and oil circuit blockage, and may cause vibration or pipeline leakage* Good oil should be used according to the instructions in the manual.

2. Severe oil leakage in the oil pressure system. Oil leakage in the oil circuit system usually occurs at the connections of the buffer valve, return valve, supply valve, oil pump, and pipeline. After adding padding, tighten the screws. If the sealing gasket is damaged, it must be opened here and replaced with a new one. The gasket should be made of copper or aluminum alloy.

3. Insufficient fuel in the fuel tank. When loaded to a certain load, there is a "pu" or "pu" sound in the return pipe of the oil supply valve, and there is less oil output and bubbles. When the load cannot rise again, there is no oil flowing out of the return pipe. Exclusion method: Add oil to the fuel tank of the testing machine until the oil level reaches half of the sight glass.

4. The oil collector of the oil pump is not tightly pressed onto the pump body. In this way, each load can only reach a certain load, and the load cannot rise again, and there is no oil flowing out of the return pipe of the oil supply valve. Troubleshooting method: Use a wrench to tighten the fixing screw at the rear end of the oil pump. If there are still problems, remove the oil collector and add a gasket between the oil collector and the pump body.

5. The clearance between the working piston and the oil cylinder is too large, and there is too much oil leakage in the clearance, which can be seen from the oil overflow outlet. The solution is to thicken the viscosity of the oil. If the problem cannot be solved, after precise measurement, the piston and oil cylinder should be replaced at the production plant.

6. There is a large gap between the force measuring piston and the force measuring cylinder, which can be seen from the lower part of the force measuring cylinder when it reaches a high tonnage. Under normal circumstances, there is no visible oil dripping or intermittent oil dripping at high tonnage. If it flows out in a linear pattern, it indicates that the force measuring piston should be replaced. When observing oil leakage, attention should be paid to the difference between joint leakage and cylinder plug leakage, and incorrect judgments should not be made due to misjudgment.

7. The piston and piston sleeve in the oil delivery valve are damaged or the spring becomes soft. In this way, the load that can be achieved each time is basically unchanged, and the noise of the oil supply valve is loud. When the load cannot rise again, the oil output of the return pipe of the oil supply valve is large. The piston and piston sleeve should be removed and ground, and if severely damaged, the spring should be replaced.

8. The stiffness of the pressure stabilizing spring of the oil delivery valve is low or the throttle needle cavity is blocked. Main performance: Each load can only reach a certain load, and when the load cannot increase further, the oil output from the return pipe is large. Troubleshooting method: Replace the stabilizer spring with a suitable stiffness or clean any dirt.

9. The gap between the damping needle of the oil delivery valve and the damping hole is too small. Main performance: Each load can only reach a certain load, and when the load cannot rise again, the oil output from the return pipe of the oil supply valve is large. Exclusion method: Take out the damping needle and file a portion along the axial direction of the damping needle with a file.

10. The belt that drives the oil pump is loose and there is slipping. After being loaded to a certain load, it cannot rise again, there is no oil flowing out of the return pipe of the oil supply valve, and there is no working sound from the oil pump. Troubleshooting method: Loosen the fixing screws of the motor, adjust the position, tighten the belt, and then tighten the fixing screws of the motor. If the belt deforms too much, it needs to be replaced with a new belt.

11. The oil pump plunger is corroded. Main performance: During the loading process, the oil output from the return pipe of the oil supply valve is discontinuous or fluctuates. When a certain load cannot rise again, there is no oil flowing out of the return pipe. Troubleshooting method: Remove the plunger and re polish it.

12. A testing machine with a safety valve can automatically open when the heavy load exceeds 3% to 5%, ensuring that the testing machine is not overloaded and damaged. When the safety valve is not adjusted properly or the valve tip and hole are not tightly closed, the pressure will not be high. The adjustment method is to disassemble the safety valve adjustment screw, check the damage of the parts, and replace it. If there is too much overload, rotate the screw downwards, and vice versa. Common faults: During the loading process of the testing machine, the body vibrates, the pointer rotates unevenly, and the numerical value cannot be accurately read. There are many situations where this reason arises. Firstly, eliminate any vibration sources that may cause resonance around the testing machine, and then observe whether the installation foundation of the machine is firm and check if the motor is faulty. After solving these problems, if there is still unstable pointer phenomenon, then it is necessary to consider whether there is air in the oil circuit system. The method is to start the oil pump, close the return valve, raise the main piston by a certain height, and then open the return valve to allow the oil to flow back to the tank from the main oil cylinder of the oil pump through the return valve. After circulating for a period of time, the air in the main oil cylinder can be completely discharged. Unscrew the screw of the exhaust hole in the oil collection tank again, and tighten it again when the emerging oil does not contain any bubbles. In addition, the high-pressure oil pipe of the oil pump can also be removed and the motor can be started to completely eliminate the air inside the oil pump. For testing machines that are used for a long time, oil should be changed regularly and the gear rods, gears, pointers, and pulleys of the force measuring section should be cleaned. (It should be noted that each time the wire wheel is cleaned, the diameter of the replaced chord wire should meet the requirements, and the direction and number of turns around the wire wheel should be appropriate.) If the oil is too dirty, sometimes the pressure balance of the oil delivery valve can be changed due to debris blockage, resulting in uneven oil circulation. The length of the two V-belts connecting the oil pump is inconsistent, and the plunger in the oil pump is not.

 

When conducting a tensile test, the fracture surface of the specimen always breaks on both sides.

 

The reason for this phenomenon should first consider whether the installation of the main body of the testing machine is vertical. If this reason is excluded, it can be ruled out from the following three aspects.

1. If the jaws are not properly clamped, the specimen should be clamped symmetrically according to the requirements.

2. Poor quality of the jaws and damaged teeth not only affect the different centers of the jaws, but also cause the sample to slip during the experiment, making it difficult to identify the yield point. At this point, the jaws should be replaced.

3. The adjustment of the lifting guide wheel is not correct, causing the upper and lower jaws to be not concentric. A inspection rod should be processed, and after tightening the upper and lower jaws, two force columns should be used as the basis for measuring with a dial gauge until the adjustment is qualified.

 

The position of the pendulum is abnormal, and the pendulum rod is not aligned properly with the vertical mark.

 

The main reason is that the viscosity of the oil is too high or the oil is too dirty.

Troubleshooting method: Replace with oil with appropriate viscosity. The force measuring piston rotates with significant friction or does not rotate. Troubleshooting method: Check if the testing machine is level and troubleshoot the force measuring piston. There is unstable friction. Exclusion method: Clean the swing shaft bearings, gear rods, pointers, and pulleys, adjust the level of the testing machine, and reduce the existing unstable friction. Common faults: The hammer returns abnormally, sometimes fast and sometimes slow. Usually, simply rotate the buffer to the appropriate position. However, if the oil hole of the buffer is blocked, or there is dirt or excessive clearance between the steel ball of the buffer valve and the inlet, it will cause the buffer to malfunction. The solution is to clean the buffer valve and adjust the gap between the steel ball and the valve seat to about 0.5mm. Place the oil needle knob to reduce the gap between the oil needle and the valve body, and re list the A.B.C weight markings. Also, when the temperature has a significant impact, the oil can be replaced appropriately. Common fault: The passive needle cannot stop well in any position and does not coincide with the main needle. The elimination method is to remove the dial glass, adjust the passive needle pressure spring screw, and adjust the pointer to coincide with the main needle.

 

During loading, the oil circuit system experiences severe oil leakage or oil pipe rupture.

 

Firstly, check whether the joints of the oil circuit system are tightened. If washers need to be replaced, they should be replaced in a timely manner.

If the oil pipe is broken, it is necessary to replace it with a stronger one. In addition, it is also necessary to observe whether the oil delivery valve and relief valve pistons are stuck or installed upside down.

 

After the sample fractures, the pendulum quickly falls back, causing an impact. Mainly due to buffer failure, in addition to regular adjustments, consideration should also be given to whether the oil used is too thin or dirty.

 

In short, it is important to regularly pay attention to the operating conditions of the testing machine, and to prevent eccentricity and friction from the main engine; For the force measurement section, prevent impact and dust from entering. Regularly inspect and calibrate the testing machine to ensure its normal use.