Quick Couplings: What are they and what do they do?
A Quick Coupling is a mechanical device - a fast, easy sure way to repeatedly connect and disconnect almost any fluid or pneumatic line. This can be accomplished without the use of tools or special devices. A Quick Coupling consists of a female half (Socket), and a male half (Plug). See Figure 1 below.
Figure 1
Socket Plug
They are found everywhere today. In distribution systems for air, water, steam, hydraulic fluids, vacuum, oil, grease, paint, liquid food - in fact, almost anything that flows through a hose or tube. If a hose or tube is going to be connected or disconnected more than once a week, a Quick Coupling will save money and labor by speeding up the job. Although they are simple in concept, Quick Couplings are precisely engineered for specific applications. The more one understands about their design, construction and principles of operation, the more valuable they become in solving fluid and pneumatic line problems.
Types Of Quick Couplings
Single shut-off couplings: These are also referred to as One-Way shut-off or Pneumatic couplings. As the term implies, single shut-off couplings have a check valve in one half, usually the socket half and no valve in the mating half referred to as a plug (Figure 2). This design is normally used in systems where spillage of fluid on the down-stream side is unimportant. Single shut-off couplings are normally installed with the valved half on the pressure side of the circuit to provide automatic shut-off flow when the coupling is disconnected.
Figure 2
This type of coupling is commonly associated with compressed air systems. However, the coupling is used on many other applications including lubrication, paint spray and carpet cleaning equipment.
Single shut-offs normally have low working pressure capabilities ranging from 100 to 300 PSI and are commonly made from brass or steel.
Double shut-off couplings: These are also referred to as Two-Way shut-off or Hydraulic Couplings. Double shut-off couplings have a check valve in both the socket and plug (Figure 3). They are used in systems where downstream spillage is undesirable. Double shut-off couplings are commonly associated with use in hydraulic systems. However, many different applications exist for this type of coupling. They are found in systems handling steam, solvents, cooling water, oil and a host of other media.
Figure 3
Double shut-off couplings are normally capable of much higher pressures then single shut-off couplings, supporting pressures up to 10,000 PSI, or more, are common. Some special hydraulic coupling designs can reach 20,000 PSI, all depending upon design and material.
Double shut-off designs are quite versatile, allowing them to be converted to single shut-off designs or straight-through designs. They are also available in a wide range of materials with the most common being steel, brass and stainless steel.
Flat Face couplings: This special type of Two-Way shut off coupling is designed for applications where hydraulic spillage is a concern (Figure 4). The valve configuration prevents any loss of fluid on disconnection, plus minimizes any entry of air when connecting. This coupling is recommended for all applications where there is a risk of contamination of hydraulic circuits and to meet safety, environmental protection and cleanliness of the work area. Some typical applications include public utilities, construction equipment, agricultural equipment, mining equipment and railway maintenance. This design is also well suited for specialized applications in the chemical, pharmaceutical and food processing industries.
Flat face couplings usually have operating pressures up to 5000 PSI depending on the type of construction. Common materials include steel and stainless steel.
Figure 4
Straight-through couplings: Straight-through couplings are quick couplings that do not have any internal valves in either the socket or plug (Figure 5). They are normally used in conjunction with some form of manual valving to shut off fluid flow before disconnect. Straight-through couplings offer minimum pressure drop due to the absence of internal flow obstructions.
Straight-through couplings normally have operating pressures in the moderate range of 1000-5000 PSI depending on the type of material. Common materials are brass and stainless steel since these fittings are often associated with corrosive fluids found in fluid transfer applications.
Figure 5
 
Why Are Quick Couplings Used?
Direct Labor: Couplings reduce the amount of direct labor employed in making a fluid or pneumatic line connection. This can be measured in man-hours saved and alternative use of saved man-hours.
Equipment Productivity: Equipment can be more rapidly connected thereby reducing downtime and increasing output.
Reduced Component Requirements: Quick couplings with internal valves can eliminate costly manual valves and in line shut-off valves in the system. This saves on initial installation costs and maintenance of the additional components.
Conservation: Valved quick couplings eliminate costly loss of expensive fluids such as hydraulic fluid.
Safety: Quick couplings with automatic check valve(s) eliminate dangerous fluid spills contributing to a cleaner and safer working environment. Some designs prevent dangerous hose whip upon disconnection of the mating halves.
Where Are Quick Couplings Used?
Here are just a few of the many applications for quick couplings:
Manufacturing: Quick Couplings are used extensively to connect air, lubrication and control lines to machine tools and other production machinery.
Research and Development: Flexibility, reliability, precision and consistency are important to accurate results at the lowest possible cost in the R&D lab. Quick couplings provide these both in fluid flow and gas handling for either test lab or pre-production line operation. Almost any of those tentative, “experimental” hookups in laboratories are handled faster and more efficiently with Quick Couplings.
Testing: Quick Couplings provide speed and accuracy for random or continuous production testing for air or liquid leakage control devices, performance and accuracy of product quality – without disrupting production schedules or shutting down assembly lines.
Service and Repair: Provides flexibility of moving service equipment right to the job. Required compressed air and liquid lines are quickly and easily set up for fast servicing and repair – and just as easily removed when the job is finished.
Ground Transportation: Quick Couplings are used in a variety of ground transportation systems. For example, truck-tractor airbrake lines that are quick-connect to the trailer and railroad cars are hooked up swiftly and efficiently.
Marine Transportation: At sea or dockside, liquid and airlines are connected or disconnected quickly, conveniently and safely. The self-sealing ability of Quick Couplings prevents dangerous leaks or spillage.
Aircraft and Aerospace: The Quick Coupling principle is widely used for aerospace applications where space limitations and safety factors dictate component use. Every system – primary or redundant – is placed in service or taken out of service quickly and conveniently with minimum loss of gas or liquid.
How To Select Quick Couplings
Proper selection depends on the application and these key factors:
- Working pressure
- Media being conveyed & flow rate
- Size of system
- Temperature range
- Valving: one way, two way, flat face, straight through
- Seal material
See your Tuthill Coupling Group distributor for expert help on your next project.
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