Fundamentals of Quick Connect Coupling

quick-coupling-applications-flat-face-1024x478

In fluid power systems, if hoses or pipes need to be connected and disconnected frequently, say once a week, then Quick Connect Coupling are likely to pay for themselves quickly through increased productivity. Initially, Quick Connect Couplings were designed specifically for specific fluid applications, and with the development of hydraulic systems in many industries, wide use over the years has resulted in a wide variety of designs that serve not only specific but general industrial applications, as well.

All quick couplers have some elements in common. They all have two parts: a plug and a socket. The plug is male half and the socket is female half. When connected correctly, these parts effectively seal and lock the connection to control internal pressure and resist any tension that might pull the connection apart. The section is easily disconnected, without tools, by unlocking the locking mechanism and separating the section.

Where are they used?

quick-couplings-on-hydraulic-testing-devices
quick-couplings-on-hydraulic-testing-devices

The more frequently hoses are connected and disconnected, the higher the value of quick connections. As machines become more productive, they become more important. A common application is hydraulic testing, if you need to connect a hydraulic test to a product’s thread, it will take a lot of time to connect and disconnect, use a Quick Connect Coupling, just a quick push/pull, and assembly ready for the test. Another application is to change skid steer attachments. The Bobcat skid Steer Machine is an example, all their machines use quick couplings to quickly connect and disconnect the hydraulic system to change the skid steer attachments.

Valve arrangements

In many different quick coupling designs, either of the two types is used for applications. The valveless type has the advantage of low-pressure loss through the coupling but does not protect fluid escape after the coupling is disconnected. However, if the pressure drop in the system must be kept to a minimum and fluid discharge from disconnected hoses can be tolerated, valveless couplings may be the designer’s first choice.
Coupling that does not leak when disconnected is probably the preferred choice for all applications – all other factors being equal. A shutoff valve is inserted in one or both halves of the coupling so that fluid can pass through the coupling only when the two halves are connected. When the coupling is disconnected, the mechanical connection between the two halves of the coupling is disconnected, causing the valve to close and blocking the flow.

When only half of the coupling is a valve, it is usually located at the source (upstream) end of the joint. Pneumatic systems usually adopt the following Settings: When the joint is disconnected, the valved half of the joint prevents air from escaping from the system, and the unvalved half allows downstream air to escape.

ISO 5675 Quick Coupler Valve
ISO 5675 Quick Coupler Valves both ends

In hydraulic applications, valves are usually used for both half couplings. This not only minimizes fluid leakage, but also limits the amount of air, dirt, and water that enters the system. When the coupling is disconnected, the air is trapped between the valves and enters the system when it is reconnected. Therefore, if the hydraulic system cannot tolerate air inclusion, special provisions for air removal may be required. To address these issues, many manufacturers now offer flat face couplings that reduce liquid leakage to one drop or less each time the coupling is disconnected. In addition, when the coupling is disconnected, the mating surface of each half coupling is flush. This minimizes air entry and wipes with surfaces before cleaning before reconnecting them.

Valve considerations

Although these valve designs provide the convenience of controlling fluid loss, there are some disadvantages. First, couplings with valves produce higher pressure drops than couplings without valves. The size of this loss depends on the size and design of the coupling. The pressure drop can be slightly reduced by increasing the size of the coupling. There may also be some variation in pressure drop from one coupling design to another. If you are concerned about pressure drops, be sure to check the manufacturer’s literature for appropriate data.
Other disadvantages of valve couplings include larger size and higher cost. Cost differences will vary according to size and individual design. In general, couplings designed for a low-pressure drop, no fluid leakage, and no air retention are more expensive. However, manufacturers point out that the price difference is offset by increased production efficiency as there is no need to clean up spills.

Coupling types

There are more than a dozen common designs of quick-acting couplings. This article introduces six of the most popular locking mechanisms in fluid power applications.
Ball-lock couplings are the most popular quick action couplings in use today and are offered by many manufacturers.

ISO 7241 A Ball Locked Quick Coupling
ISO 7241 A Ball Locked Quick Coupling

Ball-lock is the most common design and has the widest range of applications. A set of balls is positioned in holes located around the socket body ID. These holes are usually tapered or stepped to reduce their diameter at the socket body ID so that when the coupling is disconnected the ball does not fall into the cavity vacated by the plug.

The spring sleeve around the outer diameter of the sleeve body presses the ball towards the sleeve body ID. To connect the plug, push the sleeve back to open the gap so that the ball can move outward freely. Once the plug is in place, the release sleeve forces the ball inward to squeeze the locking groove on the plug’s outer diameter. When disconnected, the sleeve is pushed back to provide clearance for the ball to move outward and allow the plug to be removed.

The roller lock coupling is designed to position the roller around the inner diameter socket to grab the plug. A roller lock coupling uses the ID of the locking roller or pin end-to-end interval in or around the slot. When the plug is inserted, a ramp on the outer diameter of the plug pushes the roller outward. Once the plug is inserted at the required distance, the roller slides into the retention slot on the outer diameter of the plug. Retract the locking sleeve, allowing the ramp on the plug’s outer diameter to move the roller outward, releasing the plug.

The pin lock coupling uses pins arranged in the form of a truncated cone to grab and hold the plug in the socket. The pin lock coupling allows only one hand to push the connection because the external sleeve does not need to be retracted for connection. In this design, the pin is installed in a truncated cone form around the socket body ID. Insert the plug into the socket with the pins moving backward and outwards due to the ramp on the plug. Shear across pins locks the plug into the socket. Retract the spring-loaded sleeve to force the pinout of the locking slot and release the plug from the socket.

ISO 16028 Flat Face Quick Coupilng
ISO 16028 Flat Face Quick Coupling

When disconnecting, a flat face coupling can limit leakage to a drop or less liquid, which can almost eliminate leakage. The flat mating surface is also easy to keep clean, preventing contamination of hydraulic fluids during reconnection. Flat face, no-spill coupling with a lift shutoff valve on each matching half. Most limit leakage during separation only to the oil film on the coupling mating surface and prevent air from entering during the coupling. They are also designed for minimum flow limits, thus minimizing pressure drop during the operation of the equipment. A twist of the sleeve secures the plug once it has been inserted into the socket of the bayonet-type coupling.

Bayonet couplings, relying on common torsional locking devices, are widely used in a variety of applications, especially plastic couplings for light pneumatic equipment. When the plug is pushed into the socket, the lug on the outer diameter of the plug engages with the slot on the socket sleeve. A quick turn locked the stud. The two halves can be pulled apart by turning the plug-in in the opposite direction.

The ring lock coupling is safe by pushing the plug into the socket; They are disconnected by the coat of the rotating socket. The ring lock coupling, using an open ring, is mounted in the slot and slot of the socket. Pushing the plug into position causes a ramp on the plug to spread the ring apart at the split until the ring snaps closed behind a retention shoulder on the plug. Rotate the outer sleeve expansion ring to release it from the holding shoulder so that the two halves can be pulled apart. This design provides maximum flow in a small envelope for normal shop air applications. Another variant of this design uses jaws instead of split rings to lock parts together.

The CAM locking coupling locks the socket to the plug when the two external levers fold back to the side of the socket. This is the most common large size and usually requires more space than comparable couplings of the same size. In addition, if the lines are frequently connected or disconnected, the locking mechanism will wear out, which can lead to leaks.

Multi-tube connectors quickly connect to the tubing line in a specific direction. The multi-tube connector is the fluid equivalent of the electrical cannon-type connector. They can quickly and easily connect or disconnect multiple tubing while maintaining correct tubing orientation and discrete flow paths when reconnecting.

Coupling selection

Seal materials - Nitrile rubber
NBR Seal o-ring Materials

Before choosing a coupling, you must answer questions about its expected performance. These problems focus not only on coupling but also on fluid media. For example, what fluid will flow through the coupling? Characteristics of fluid – viscosity, corrosion, etc. – Will affect the type of coupling that should be used.

Other problems related to fluids involve temperature (high, low, or large variations), pressure, and flow rate.

Once fluid details are known, questions about coupling construction must be answered.What is the coupling connection and disconnection frequency? What type and diameter of hose or pipe will be used to hold the fluid?Are couplings or hoses subject to impacts such as falling objects, violent vibrations, or environmental contamination?

Once these questions have been answered, initial choices can be made about the type of coupling: one, two, or no shutoff valves, and the type of connection/disconnect mechanism. Keep in mind that one model may offer the greatest convenience in service, but the lower pressure loss of different models may be more suitable for applications.

Building materials are another consideration. A variety of O – ring and sealing materials – elastomer, ptfe, etc.- Suitable for most any type of fluid at all temperatures. The choice of material for plugs and sockets is also important. Steel, stainless steal, brass, and aluminum are common.In addition, many components are made of carbon steel and coated with corrosion-resistant metals to reduce material costs.

CPC-APC-quick-connect-fittings
Plastic Quick Couplers

Plastics can be used for many purposes if the pressure, temperature and chemical environment permit.Keep in mind that plastic couplings may contain internal metal parts that may be corroded by certain types of hydraulic fluids.

A pressure rating is a value that provides the best service life and maximum pressure that can be sustained without failure. Data on pressure drops determined by coupling at expected flow and pressure should be included in the document.Many of these calculations are based on water flowing at 60 degrees Fahrenheit.

Keep in mind that due to the higher viscosity of oil, its pressure drop will be higher. Calculations for air are more complicated because the density of gases varies greatly with pressure and temperature.The rule of thumb for estimating maximum flow at 100 psi inlet and 5 psi pressure drop is to multiply the coupling’s flow coefficient by 25.Usually, the literature contains more detailed data on the specified inlet pressure and pressure drop for the maximum air flow. Therefore, accurate pressure drop valves for specific couplings should be obtained from the manufacturer.

Also note that the coupling may be subjected to pressures well above the maximum operating pressure. Sudden switching of valves or sudden application of heavy load can cause system pressure to rise and fall rapidly within milliseconds. These pressure spikes are usually undetectable in the system, but can still damage the seals and locking elements of the coupling. Eventually, the coupling will leak and be difficult to disconnect or reconnect, or any combination of these problems. To prevent these problems, couplings with pressure levels significantly higher than the maximum expected working pressure should be selected.

Depending on the application, the coupling may be affected by vibration or relative rotation between mating parts when pressurized.In most cases, these conditions can lead to leakage or difficulty connecting or disconnecting, thereby shortening the life expectancy of the coupling.Therefore, check with the manufacturer to determine if the coupling will tolerate these conditions.

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