A coupler is a small forged or pressed fitting that joins two scaffold tubes, and it is the most quietly incompatible item in the whole trade. Buyers regularly order fittings that look right in a photograph and then find they do not suit the tube on site. The confusion is worth a few minutes, because it is almost always about two things: the tube the fitting is designed around, and the regional pattern it follows.
What EN 74 covers
EN 74 is the European standard for couplers, spigot pins and baseplates used in working scaffolds and falsework made from steel tubes. It sets out the classes, the materials and the testing. When a coupler is described as EN 74 with a class, that class is what tells you the performance category, in the same way that the class rather than the standard number carries the meaning for props.
KEAA manufactures couplers to EN 74-1 classes B and BB, with Ü-mark conformity assessed by Sigma Karlsruhe. As with props, the third-party assessment is the part a European buyer will ask to see.
Where the patterns diverge
The practical difference between British and European fittings is the tube they are built around and the fixing detail. British practice and European practice grew up around different tube dimensions, so a fitting sized for one does not grip the other correctly. A coupler that is slightly oversized will not develop its designed slip resistance, and one that is undersized simply will not close.
Within each pattern there are further variations: the fixing may be a bolt and nut or a T-bolt with a captive wing nut, and the jaw may be pressed or forged. These affect handling and durability rather than compatibility, but they matter to a crew doing it all day.
The types you will actually order
Right angle couplers join tubes at ninety degrees and are the load-bearing workhorse. Swivel couplers join tubes at any angle and are used for bracing. Sleeve and joint pin connectors extend a tube in line, and they behave differently from each other: a sleeve couples outside the tube, a joint pin inside. Putty and check couplers, baseplates and beam clamps fill in around them.
The common error is substituting a swivel where a right angle belongs. A swivel is designed for the bracing role and should not be treated as a general-purpose alternative.
How to specify without guessing
State the tube outside diameter and wall thickness, not just "scaffold tube". State the pattern, British, American or European. State the standard and class you need evidence for. If you are adding to existing stock, send a photograph of the fitting you already use alongside the dimensions, because a photograph resolves pattern questions faster than a description does.
Finish matters here too. Couplers are dropped, kicked and left in the weather, and hot dip galvanizing to DIN EN 1461 is what keeps the thread turning after a few seasons.
Why a mixed yard is the usual situation
In principle a scaffold is built from one pattern of fitting. In practice yards accumulate. Stock arrives with a hired scaffold, a job in an export market comes back with fittings that stay, and within a few years a rack holds both British and European couplers that look broadly alike at arm's length.\n\nThat matters because the two are not interchangeable in the way their appearance suggests. They are built to different standards, BS 1139 and EN 74, with different dimensions and different test regimes behind them. A fitting that grips a tube is not the same as a fitting that has been shown to hold the load that standard requires at that grip, and the difference is invisible once both are galvanized.\n\nThe practical control is not a policy nobody follows, it is making the two visually distinguishable in the rack and buying replacements to one pattern deliberately. If your yard is already mixed, the useful question at the point of reorder is which pattern the majority of your working stock is, and standardising future purchases on that rather than on whichever is cheaper that month.
Common questions
A half coupler is the adapter between a scaffold tube and something that is not a tube: a brace, a rod, a strip or a lock. Each welded attachment suits a different connection, which is why the range runs to welded rod, welded L rod, welded tube, welded V-strip, welded strip and L strip in long and short, rather than one generic part. Ordering the wrong variant is the usual cause of a box of fittings that cannot be used on the job they were bought for, so it is worth naming the attachment as well as the coupler type when you enquire.
Yes, and it is worth doing. A coupler grips a specific tube diameter and wall thickness, so its performance is partly a function of what it is clamping. Galvanized tube is catalogued alongside the fittings, and if you are buying couplers for tube you already hold, send the tube dimensions with the enquiry.
Not reliably. They are built around different tube dimensions, so a fitting for one pattern will not grip the other correctly. Specify the pattern and the tube dimensions together.
A sleeve coupler joins two tubes end to end from the outside; a joint pin does it from the inside. They behave differently under load, and which is appropriate is a design decision rather than a preference.