In fluid power systems where hoses or tubes will be required to be connected and disconnected frequently, such as once a week, quick connect couplings have the potential to pay for themselves quickly through increased productivity. Originally designed for specific fluid applications, quick connect couplings have evolved over the years as hydraulic systems have grown in many industries and their widespread use has produced a wide variety of designs that serve not only specific but general industrial applications.
All quick-release couplings have some elements in common. All have two parts: a plug and a socket. The plug is the male part and the socket is the female part. When properly connected, these parts effectively seal and lock the fitting to control internal pressure and resist any pulling forces that would tend to pull the fitting apart. By disengaging the locking mechanism and separating the parts, these parts are easy to disconnect without tools.
Where are they used?
The more often hoses must be connected and disconnected, the more valuable quick-release couplings become. They also become more important as machine productivity increases.
A common application is hydraulic testing. If you need to connect a hydraulic test to a product via threads, which would take a lot of time to connect and disconnect, with quick couplings, the component is ready for testing with just a quick push/pull.
Valve arrangements
Among the many different designs of quick-release couplings, there are two types available for certain applications. The non-valved type has the advantage of low-pressure loss through the coupling, but there is no provision to prevent leakage of fluid after the coupling is disconnected. However, if the pressure drop in the system must be kept to a minimum and fluid flow from the disconnected hose is tolerated, then the valveless coupling may be the designer’s first choice.
Obviously, in all applications, all other factors being equal, a coupling that will not leak on disconnection would be preferred. By incorporating a shut-off valve in one or both halves of the coupling, fluid will only flow through the coupling if both halves are connected. When the coupling is disconnected, the mechanical connection between the two coupling halves is broken, causing the valve to close, and blocking the fluid.
When only one-half of the coupling is valved, it is usually located at the source (upstream) end of the quick coupling. Pneumatic systems typically use this setup: the coupling half with a valve prevents air from being lost from the system when the connection is disconnected, and the coupling half without a valve allows air to escape downstream.
In hydraulic applications, both halves of the quick-release coupling are usually valved. This practice not only minimizes fluid leakage but also limits the amount of air, dirt, and water that can enter the system. When the coupling is disconnected, air can become trapped between the valves and enter the system when the coupling is reconnected. As a result, special provisions may be needed to exclude air if the hydraulic system cannot tolerate air entrapment. To address these problems, many manufacturers now offer flat-face couplings that reduce fluid spillage to a drop or less each time the coupling is disconnected. In addition, when the coupling is disconnected, the mating surfaces of each quick coupling are flush. This minimizes the ingress of air and the need to wipe the mating surfaces before reconnection.
Valve considerations
While these valved designs offer the convenience of controlling fluid losses, there are tradeoffs. First, valved couplings produce a much higher pressure drop than non-valved designs. The magnitude of this loss depends on the size and design of the coupling. By increasing the size of the coupling, the pressure drop can be reduced to some extent. Different coupling designs may also have some deviations in pressure drop. If pressure drop is an issue, be sure to check the manufacturer’s literature for proper data.
Other disadvantages of valved couplings include larger size and higher cost. Cost differences will vary depending on size and individual design. In general, couplings designed for low pressure drop, no fluid leakage and no air entrapment are more expensive. However, manufacturers point out that the price difference is offset by the higher productivity gained by not having to clean up fluid spills.
Coupling types
There are more than a dozen common quick release coupling designs. This article describes six of the most popular locking mechanisms used in fluid power applications.
Figure 1. The ball lock coupling is the most popular quick coupling available today from many manufacturers.
Ball Lock Quick Couplings
The ball lock, Figure 1, is the most common design and has the widest range of applications. A set of balls is placed in holes located around the inner diameter of the socket. These holes are usually tapered or stepped to reduce their diameter at the socket body diameter so that the balls do not fall into the cavity vacated by the plug when the coupling is disconnected.
A spring-loaded sleeve around the outer diameter of the socket body forces the ball to move toward the inner diameter of the socket. When the plug is attached, the sleeve is pushed back, thus opening the gap and allowing the ball to move freely outward. Once the plug is in place, releasing the sleeve forces the ball inward against the locking groove on the outer diameter of the plug. To disconnect, the sleeve is pushed back, providing clearance for the ball to move outward, allowing the plug to be removed.
Figure 2. Roller-lock coupling design positions rollers circumferentially around the ID of the socket to grip the plug.Roller-lock couplings, Figure 2, use locking rollers or pins spaced end-to-end in grooves or slots around the socket’s ID. As the plug is inserted, a ramp on the plug OD pushes the rollers outward. Once the plug is inserted the prescribed distance, the rollers slip into a retention groove on the plug’s OD. Retracting the locking sleeve, which allows the ramp on the plug OD to move the rollers outward, releases the plug.
Figure 3. Pin-lock couplings use pins arranged in a truncated-cone formation to grip and hold the plug in the socket.Pin-lock couplings, Figure 3, allow push-to-connect joining using only one hand because the outer sleeve does not need to be retracted to make a connection. In this design, pins are mounted around the socket body ID in a truncated-cone-shaped formation. Pushing the plug into the socket moves pins back and outward, due to a ramp on the plug. Shear across pins locks the plug into the socket. Retracting the springloaded sleeve, which forces the pins back out of the locking groove, releases the plug from the socket.
Flat Face Quick Couplings
Figure 4. Flat Face couplings can virtually eliminate spillage by limiting leakage to a drop or less of fluid when disconnected. The flat mating surface is also easy to keep clean, thus preventing contamination of the hydraulic fluid when reconnecting. Flat Face quick couplings have a poppet type shutoff valve on each mating half. Most limit leakage during decoupling, have only an oil film on the coupling face, and prevent air from entering during coupling. They are also designed for minimal flow restriction, thus minimizing pressure drop when the unit is operating.
Figure 5. A twist of the sleeve secures the plug once it has been inserted into the socket of the bayonet-type coupling.Bayonet couplings, Figure 5, rely on the familiar twist locking arrangement and are widely used in a variety of applications, especially in plastic couplings for lighter-duty pneumatic equipment. To join the couplings halves, lugs on the OD of the plug engage slots in the socket sleeve as the plug is pushed into the socket. A quick turn locks the lugs into position. Turning the plug in the opposite direction allows the halves to be pulled apart.
Figure 6. Ring-lock couplings secure by pushing plug into socket; they disconnect by rotating the socket’s outer sleeve.
Ring-lock couplings, Figure 6, use a split ring seated in a groove and slot in 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. Rotating an external sleeve expands the ring, thus releasing it from the retention shoulder so the halves can be pulled apart. This design provides maximum flow in a small envelope for normal shop air applications. A variation of this design uses jaws instead of a split ring to lock the parts together.
Figure 7. Folding back the levers on the cam-lock coupling secures the socket to the plug.
Cam-lock couplings, Figure 7, lock the socket to the plug when two external levers are folded back against the sides of the socket. These are most common in larger sizes and generally require more spaces than comparable couplings of the same size. Moreover, the locking mechanism can wear if lines are connected or disconnected frequently, which can allow leakage.
Figure 8. Multi-tube connectors quickly connect many lines of tubing in a specific orientation.Multi-tube connectors, Figure 8, are the fluid equivalent to electrical Cannon-style connectors. They quickly and easily connect or disconnect several tubing lines, while maintaining a correct line orientation and discrete flow paths during reconnection.
Coupling selection
Before selecting a coupling, questions must be answered regarding its expected performance. These questions focus not only on the coupling, but the fluid medium as well. For example, what fluid will flow through the coupling? Characteristics of the fluid – viscosity, corrosivity, etc. – will influence the type of coupling that should be used.
Other questions concerning the fluid deal with temperature (high, low, or wide variation), pressure, and flow rate.
Knowing details on the fluid, questions must be answered about the coupling’s construction. How often will the coupling be connected and disconnected? What type and diameter of hose or tubing will be used to contain the fluid? Will the coupling or hose be subjected to abuse such as impact from falling objects, severe vibration, or contamination from the environment?
Once these questions have been answered, a preliminary selection of coupling type can be made: one, two, or no shutoff valves, and the type of connect/disconnect mechanism. Keep in mind that one style may offer the greatest convenience in service, but a different model’s lower pressure loss may be more desirable for the application.
Materials of construction are another consideration. A wide variety of O-ring and seal materials – elastomers, PTFE, etc. – are available to accommodate most any type of fluid at a wide range of temperatures. Material chosen for the plug and socket also is important. Steel, stainless steal, brass, and aluminum are common. In addition, many parts are made from carbon steel and plated with a corrosion-resistant metal to keep material costs down.
Plastic may be used for many applications if pressure, temperature, and chemical environment permit. Keep in mind that plastic couplings may contain internal metal components that could be corroded by certain types of hydraulic fluid.
Pressure rating relates to values that provide optimum service life and maximum pressure that can be tolerated without failure. Literature should include data for determining pressure drop through the coupling at expected flows and pressures. Many of these calculations are based on flow of water at 60° F.
Keep in mind that pressure drop for oil will be higher because of its higher viscosity. Calculations for air are more complex because a gas’s density varies widely with its pressure and temperature. A rule of thumb for air to estimate maximum air flow at 100 psig inlet and 5-psi pressure drop is to multiply flow coefficient of the coupling by 25. Often, literature contains more detailed data on maximum air flow at prescribed inlet pressures and pressure drop. Therefore, precise valves for pressure drop for specific couplings should be obtained from the manufacturer.
Also be aware that couplings may be subjected to pressures well above the maximum operating pressure. Sudden shifting of valves or abrupt application of heavy loads can cause system pressure to quickly rise and fall within milliseconds. These pressure spikes often go undetected in a system, but still can damage seals and locking elements of the coupling. Ultimately, then, the coupling would develop leaks, become difficult to disconnect or reconnect, or any combination of these. To prevent these problems from occurring, select a coupling with a pressure rating substantially higher than the anticipated maximum operating pressure.
Depending on the application, the coupling may be subjected to vibration or relative rotation between the mating halves while pressurized. In most cases, these conditions will shorten the expected life of the coupling by causing leakage or difficulties in connecting or disconnecting. Therefore, check with the manufacturer to determine if the coupling will tolerate these conditions.
