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A quick coupling is a special quick-connecting valve component, which internally consists of a valve sleeve, valve stem, dynamic/static seals, springs and other components. Compared with other hydraulic parts, its biggest characteristic is - the docking surface needs to be frequently switched between an open environment (disconnected state) and a sealed system (docked state). There is no other component that has similar working conditions (there are similar conditions when hydraulic components are repaired), and this is the biggest challenge for quick release couplings. Therefore, the quick coupling has become a closed hydraulic system “opening”, the correct selection and use can not only improve the service life of the quick coupling, but also for the normal operation of the entire hydraulic system to provide protection.
Here is a list of some common mistakes in the selection and use of quick couplings, which can help you select the most suitable product for your system and guide you to the correct use of quick couplings.
In general, quick couplings are recommended to be used after docking, and the working pressure in docking state is the pressure that almost all people consider, and there is no problem in general.
Here must be emphasized is the disconnected state of the working pressure, but also most of the selection stage does not consider the problem. It is generally not recommended that quick couplings be used as plugs, especially in the commissioning and testing stage of new machines, mainly because:
① Some quick couplings have different maximum working pressures in the docking state and disconnected state (the pressure-bearing parts of the two states are different), and the working pressures in the disconnected state and docking state must be verified at the selection stage.
② The sealing principle of female couplings with tapered valve or flat valve structure is that the O-ring is pressed on two metal taper surfaces, and when this sealing structure is subjected to high pressure for a long period of time or when there is a hydraulic pulse, the sealing ring will have the risk of collapsing. When the quick coupling must be used as a plug condition, it is recommended to use the male coupling with flat valve structure (the spool seal is radial seal in the disconnected state).
③ It is not recommended to use the quick coupling as a plug during the commissioning stage of a new machine. At this time, the quick coupling becomes the “terminal” of the system, rather than the circulatory system, any “pollutants” that may be generated by the entire system will be washed to the “terminal” of the quick coupling, causing damage to the internal sealing of the quick coupling. Any “contaminants” that may be generated by the entire system will be washed into the “end” quick coupling area, causing damage to the internal seal of the quick coupling. In addition, because it is not a circulating system, the “terminal” quick coupling cannot be filled with hydraulic fluid, and the high pressure and mixed air may cause “cavitation” and damage to the quick coupling.
Residual pressure is the pressure that remains in the line after the equipment is shut down. Residual pressure is what quick-release couplings refer to when they are operated under pressure.
The formation of residual pressure generally includes two aspects: one is the design of the system can not drain off (or quickly drain off) pressure, such as O-valve spool function, etc.; the other is a closed system in the temperature change caused by the pressure rise, the study shows that the closed, no air mixing system, the temperature rises 1 ° C, the pressure rises about 8 bar.
It must be emphasized here is the pressure change caused by temperature, especially docking state no residual pressure, but the disconnected state has one end is a closed circuit conditions. For example, if the spool of a device is designed as an H-type function, there is theoretically no residual pressure in the pipeline after shutdown, and the disconnection operation is a pressureless operation. After disconnection, the quick coupling at the equipment end benefits from the H-type spool function and remains pressureless. When the other end is disconnected, it becomes a closed system. Temperature changes will cause pressure changes, and if the quick coupling is docked at this time, it will become an operation with residual pressure.
It is strongly recommended that only products that support residual pressure operation be used for residual pressure operation.
Operation with residual pressure must be divided into three conditions: disconnection with residual pressure, single-sided docking with residual pressure, and double docking with residual pressure.
① Disconnection with residual pressure: Theoretically, any steel ball locking (including flat and tapered all products) products do not support disconnection with pressure, which is related to the locking mechanism, with pressure conditions, the locking ball is pressed in the female body, sheath, male groove, theoretically difficult to push the sheath to realize the disconnecting operation (prohibit the use of tools to knock the sheath to disconnect the quick release coupling, it may damage the quick release couplings or personnel injury). This condition can only be achieved by seeking to relieve pressure. In this case, you can only seek pressure relief, or use thread locking, combination plate and other products that support disconnection under pressure.
② unilateral docking with residual pressure: to realize the docking with residual pressure must solve two problems: First, we must overcome the internal pressure acting on the spool; Second, the spool is opened at the moment of high media flow rate, we must ensure that the seal of the spool is not washed away or damaged (not all of the threaded locking products support the operation of the residual pressure, threading is only a solution to the force of the problem, can not solve the sealing risk of damage). The products on the market that only support single side docking with residual pressure generally require the other side of the return oil, products that support double side with residual pressure can also be used in this condition.
③ Dual-side docking with residual pressure: It is the most stringent test for both operating force (torque) and seals. Some products that do not support double-sided operation with residual pressure can be used in this condition, the docking process may not be a problem, but after disconnection, oil leakage may occur, because the spool seal has been broken during the docking process.
The “Brinell effect” occurs mainly in products with ball locking. Under high impulse conditions, the locking steel ball is like measuring the Brinell hardness, constantly squeezing the locking groove of the male connector, after a period of time there may be a steel ball “embedded” in the male connector valve body, resulting in the inability to disconnect. For high pulse conditions, it is recommended to choose a threaded locking product.
The actual diameter of the quick coupling and the size of the connection port are not necessarily the same, for example, 12.5mm diameter quick coupling can be equipped with G1/2, G3/4 and other connection ports. At the selection stage, it is recommended to use the pressure loss as a reference value to choose the diameter of the quick coupling, taking into account the efficiency and price. The most common mistake is to take the quick coupling connection thread size as a reference, resulting in selection errors.
