by Frederike Berg
Why a 1 ½ inch scaffold tube does not have a 1 ½ inch diameter
Anyone who works in scaffolding cannot avoid the number 48.3. No matter where you go, a scaffolding tube will always have a standard diameter of 48.3 mm. This number is still derived from its old unit of measurement of 1 ½ inches. And the fact that we do not continue to measure scaffold tubes, threaded tubes or similar in inches today also has a very specific purpose: uniformity. Errors and confusion can easily be avoided by using a uniform system of measurements. And it is precisely for this reason that the measurement of a scaffold tube in inches had to give way to the metric system in Europe and large parts of the world. Read more about the introduction of the metric system here.
A scaffold tube does not even have 1 ½ inch diameter
But wait a minute. You don’t even have to measure properly to realise that 1 ½ inches are 38.1 mm and not 48.3 mm. However, no one has made a mistake in the conversion from inches to metric, but the whole thing has historical reasons.
1 ½ inch scaffold tube diameter originally refers to the inner diameter. In order to produce a stable steel tube in the past, more material was needed due to the poorer quality of steel and production possibilities compared to today. Since more steel was needed for a stable tube, the tube walls were consequently thicker and the inner diameter was therefore smaller.
Today, we can get by with less steel to produce a stable tube. Material can be saved and the tube walls can be thinner. Consequently, the inner diameter is also larger, while the outer diameter remains the same. And that is important because, as the craftsman knows, it is the outer diameter that counts. After all, couplers and other scaffolding parts should fit reliably and always. So it's no surprise that when specifying the diameter of a scaffolding tube today, the decisive outer diameter is meant and not, as in the past, the inner diameter. After all, this should always remain the same, regardless of the steel quality and wall thickness. In the course of time, the tube did not lose its external diameter, but only gained internal diameter in order to maintain the old standards.
EN 39 ensures uniform outer diameters of scaffold tubes
The fact that 1 ½ inch is not 48.3 mm can therefore be attributed to the change in the designation of the inner diameter to the outer diameter. The only thing that has remained is the occasional designation as 1 ½ inch scaffold tube.
You can rely on the fact that a 1 ½ inch or 48.3 mm scaffold tube really has an outer diameter of 48.3 mm. This is ensured by European norm EN 39, which stipulates that all system-free scaffold tubes must have an outer diameter of 48.3 mm and a nominal wall thickness of at least 3.2 mm. The inner diameter can vary depending on the steel quality, but the outer diameter does not.
Some things in scaffolding have always been like that - but why? Maybe you've asked yourself, for example, why scaffolding is available in 07 dimensions? At first glance, the metric measurement seems much more logical. Read our blog post to find out.
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