The basic structure and working principle of the pressure limiting valve

The pressure limiting valve is generally composed of a valve body (diagonal shape, spherical shape), a valve seat, an adjustment spring, a pressure regulating screw, a lock nut, and a sealing gasket.

The working principle of the pressure limiting valve is: in most of the time, the pressure limiting valve is closed. Only when the pressure of the working medium exceeds a certain value, the pressure limiting valve is opened, and a part of the medium is drained from the bypass to reduce the pressure in the system. When the pressure of the system reaches the lower limit, the pressure limiting valve is closed and the pressure of the system rises again. This cycle is repeated so that the medium pressure in the system is often at a reasonable level to ensure that the fuel system, lubrication system, cooling system and hydraulic system are working properly.

The pressure limiting valve can limit the oil pressure in the circulating oil circuit, prevent excessive oil from entering the combustion chamber, increase the oil consumption, and the engine work is abnormal. At the same time, in order to avoid excessive oil pressure, the pipeline and the filter are blasted. The pressure limiting valve is a one-way valve that controls the oil pressure by the pressure of the spring. When the oil pressure in the oil passage is greater than the spring pressure, the valve ball is opened, and some of the oil flows back to the oil sump, so that the oil circuit is relieved. It is usually installed on the oil pump or main oil passage.

The conventional fuel tank cap works like a radiator cap with at least one pressure limiting valve, such as an air-vapor valve to balance the pressure inside and outside the fuel tank. As the fuel in the fuel tank is gradually consumed, the oil level is continuously decreased, and a negative pressure is formed above the oil surface. When the vacuum reaches a certain level, the air valve of the fuel tank cap (ie, the intake valve) is pushed away by the atmospheric pressure, and the outside air enters. In the fuel tank, the pressure inside and outside the tank is balanced. If the air valve cannot be opened for any reason, there is a significant negative pressure in the fuel tank, and the fuel pump may not be able to suck the oil, which will cause the engine to be unstable and even suck the fuel tank. When the temperature increases, the fuel vapor in the fuel tank is excessive, and when the oil pressure is higher than the atmospheric pressure, the steam valve of the fuel tank cap is opened, and the fuel vapor (HC) in the fuel tank escapes from the valve into the atmosphere, so that the fuel tank is inside and outside the tank. The pressure is balanced to prevent the fuel tank from bursting.

The common fault of the pressure limiting valve is abnormal leakage, or it cannot be opened normally. The judgment method is as follows.

For the pressure limiting valve of the fuel system, if the following conditions occur: the engine is difficult to start, the phenomenon of “lack of cylinder” appears after starting, and the load is turned off when the load is applied; when the oil is pumped by the hand pump, the resistance is small; the oil supply cannot keep up, it seems If there is air, check the pressure limiting valve of the fuel injection pump for leaks.

For the pressure limiting valve of the lubrication system, the more obvious fault phenomenon is that the engine "burns the tile", sometimes the oil, the bearing bush, the crankshaft are replaced and the lubricating oil passage is cleaned, and the load is loaded and burned. This phenomenon is often caused by the leakage of the oil pressure limiting valve, causing the oil pressure to be too low, and it is difficult to form an effective lubricating oil film in the friction pair, thus causing the burning of the tile.

For the pressure limiting valve of the hydraulic system, if the working device is lifted, the throttle will rise when the throttle is large, and the throttle will automatically settle when the throttle is lowered slightly. Sometimes the sound of the oil leakage can be heard, indicating that the pressure limiting valve of the hydraulic system is leaking. malfunction.

For the vacuum pressure reducing valve on the fuel tank (essentially the intake valve), the hand vacuum pump can be used for testing. The detection method is: first clean the vacuum pressure reducing valve body, then connect the hand vacuum pump to check the valve. The opening pressure and the degree of vacuum should be 15.3 to 20. 0 kPa, and the degree of vacuum should be 6.0 to 3. 3 kPa. If the above range is exceeded, the fuel tank cap assembly should be replaced.

Common fault phenomenon of pressure limiting valve

The valve body and seat are worn. The abnormal leakage of the pressure limiting valve is mostly caused by wear, which is often caused by the medium being unclean. The wear of the pressure limiting valve is characterized by wear grooves or rust, resulting in a tight seal of the pressure limiting valve. At this point, the valve seat needs to be removed from the casing by means of a special tool and then repaired. When the wear is slight, it can be placed on the platform and ground with fine abrasive sand. When the wear is severe, new parts should be replaced.

When the new valve body is installed in the valve seat, it is recommended to use a copper rod for hammering, so that the valve body and the valve seat are closely matched. When hammering, pay attention to the copper rod must be aligned with the center line of the valve seat, the hammer head should be flat and should not be deflected. Then grinding, and finally using kerosene penetration method to check whether the sealing performance meets the requirements, requiring at least 1 minute without leakage.

The pressure limiting valve is blocked, causing the pressure warning light to illuminate. At this point, the lower pressure valve should be removed for thorough cleaning, and the cemented material in the valve should be completely removed.

The pressure relief valve is stuck in the closed position, or the pressure limiting valve is padded by debris and stuck in the open position. In either case, the parts of the pressure limiting valve (including the valve seat) should be thoroughly cleaned and replaced with new ones if necessary.

The pressure regulating spring is deformed, broken or weakened. If the spring force of the pressure-regulating spring is weakened, as a temporary emergency measure, a washer with a thickness of about 1 mm can be placed in the spring seat for compensation to restore the pre-tightening force of the pressure-regulating spring; if the pressure-regulating spring is broken, it is necessary to replace the new one.