Selecting Quick-Connect Couplings For Compressed Air Service

Most manufacturing plants using compressed air apply quick-connect couplings. Proper selection and sizing of these inexpensive, ubiquitous devices are usually not given the consideration they deserve.


Most manufacturing plants using compressed air apply quick-connect couplings. Proper selection and sizing of these inexpensive, ubiquitous devices are usually not given the consideration they deserve. Although not a major maintenance expense, the effect of poorly chosen couplings can create unexpected problems with productivity, energy conservation, and safety.

Couplings for compressed air service are available in several connection configurations and materials of construction. The choice depends on service requirements. Industrial interchange couplings (Mil~-C-4109 & ISO 6150 Series B) are the most common for industrial applications. They are offered in the widest size and configuration selection.

Selection factors

The first decision in the proper selection of a quick-connect coupling is choosing an appropriate style for the application involved. There are four important factors to consider: environment, frequency of use, location, and safety.


Couplings used outdoors must be resistant to weather. Those made from copper alloys or malleable iron last the longest; plated steel rusts. If the application involves physical abuse or a dirty environment, such as a foundry, the proper choice would be quick couplers with no moving parts, because they are not affected by dirt as much as couplers with movable sleeves.

Frequency of use

How frequently a quick-connect coupling is used has a direct bearing on the type selected. Axial connect couplings are usually valved and can simply be disconnected. A rotary connect coupling has no valving mechanism. When the need for disconnect arises, the air must be shut off at the source.

The most common wear points on valved quick-connect couplings are the valve, springs, and seals. For heavy-duty use, couplings without spring-loaded, poppet-type valves and with O-ring seals instead of flat washers should be considered.


Position or location of the coupling affects the selection process. If the coupling is at the end of a loose hose, most styles will do. When a coupling is located on a pipe or other fixed location, an automatic latch or zero-pressure type is convenient to use, because only one hand is required to operate it. When a coupling is located at the end of an overhead hose drop, a zero-pressure type is best because of the lack of engagement effort and ease of disengagement without hose whip.


Since air is compressible, release of pressure upon disconnect is accompanied by a rapid expansion of air from the unvalved half of the coupling. This release, similar to the action of a rocket engine, causes the hose, if unrestrained, to flail about with the potential for injury. The larger the air volume contained in the hose, the more violent and sustained the action.

There are two ways to eliminate the problem of hose whip. The downstream coupling half -- the plug -- can be equipped with a ball check valve. When air is released at disconnect, the ball prevents the air from escaping rapidly. A disadvantage of this design is restricted airflow during use.

An alternative is to select a zero-pressure, quick-connect coupling. This design automatically depressurizes the downstream air volume as it shuts off the upstream supply. By the time the coupling is disconnected, there is no residual air pressure remaining to cause hose whip.


Quick-connect couplings are sized according to two factors; connection size determined by the hose or pipe fitting to which the coupling is connected, and body size determined by the required airflow.

When using couplings with cleaning or dusting devices, the flow rate is not critical. However, the majority of couplings are used with air tools that require 90 psi at the inlet. Since typical plant compressor output is between 100 and 125 psi, the amount of pressure drop allowed at the coupling is usually quite small.

The following rule-of-thumb, based on 90-psi inlet and 2-psi pressure drop, helps determine coupling body size based on flow requirements. (Refer to the manufacturer's catalog to determine actual pressure drop.)

Pipe size, in. Consumption, scfm

1/4 15

3/8 30

1/2 50


Minimize the effect of vibration or shock from tools which can deform the plug by impacting the latching balls or pins or elongate ball bodies in sockets. This action can be done by using a 2-ft whip hose between the tool and coupling.

Check couplings periodically for leaks in the coupled and uncoupled position. Compressed air leaks are expensive in terms of energy cost and excessive compressor capacity. Selection of O-ring seals instead of washer seals provides a product that lasts longer and is leak free.

Replace couplings that are not performing properly. Although repairable, their initial cost does warrant the cost of labor to repair.

-- Edited by Joseph L. Foszcz, Senior Editor, 847-390-2699,

More info

The author is available to answer questions regarding the selection of quick connect compressed air couplings. He can be reached at 314-629-3700.

Key concepts

Rotary couplers are better suited to harsh environments than axial types.

Frequently used couplings should have a built-in shutoff valve.

Airflow determines the coupling body size.

Types of couplings

Rotary Axial

Glad hand Ball latch

Universal claw Pin latch

Quick lock Bar latch

No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2013 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
Sister act: Building on their father's legacy, a new generation moves Bales Metal Surface Solutions forward; Meet the 2015 Engineering Leaders Under 40
2015 Mid-Year Report: Manufacturing's newest tool: In a digital age, digits will play a key role in the plant of the future; Ethernet certification; Mitigate harmonics; World class maintenance
2015 Lubrication Guide: Green and gold in lubrication: Environmentally friendly fluids and sealing systems offer a new perspective
Drilling for Big Data: Managing the flow of information; Big data drilldown series: Challenge and opportunity; OT to IT: Creating a circle of improvement; Industry loses best workers, again
Pipeline vulnerabilities? Securing hydrocarbon transit; Predictive analytics hit the mainstream; Dirty pipelines decrease flow, production—pig your line; Ensuring pipeline physical and cyber security
Cyber security attack: The threat is real; Hacking O&G control systems: Understanding the cyber risk; The active cyber defense cycle
Designing positive-energy buildings; Ensuring power quality; Complying with NFPA 110; Minimizing arc flash hazards
Building high availability into industrial computers; Of key metrics and myth busting; The truth about five common VFD myths
New industrial buildings: Greener, cleaner, leaner; New building designs for industry; Take a new look at absorption cooling; Offshored jobs start to come back

Annual Salary Survey

After almost a decade of uncertainty, the confidence of plant floor managers is soaring. Even with a number of challenges and while implementing new technologies, there is a renewed sense of optimism among plant managers about their business and their future.

The respondents to the 2014 Plant Engineering Salary Survey come from throughout the U.S. and serve a variety of industries, but they are uniform in their optimism about manufacturing. This year’s survey found 79% consider manufacturing a secure career. That’s up from 75% in 2013 and significantly higher than the 63% figure when Plant Engineering first started asking that question a decade ago.

Read more: 2014 Salary Survey: Confidence rises amid the challenges

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.