Bonding a swing chair suspension

A relaxed adhesive engineer enjoying a cup of good coffee in a swing chair on his garden terrace. How idyllic is that! But why do we show you this picture? Since our colleague Martin is an adhesive engineer and works with our adhesives every day, it goes without saying that no screws were used in the hanging of the chair – it was completely glued.

Would you trust this bond and enjoy a lazy afternoon as laid-back as Martin does? If the bonding failed, not only would you bruise your tailbone, but also suffer a huge headache.

I’ll explain to you why our colleague Martin is so relaxed while swinging in his chair.

First, we’ll take a look at what exactly has been bonded here: a stainless steel suspension to a hot-dip galvanized steel plate.

A galvanized surface is quite difficult to bond. Yet the zinc layer can be removed pretty easily with a piece of sandpaper. Once this layer is removed, it is just a connection between steel and stainless steel, two surfaces that are much easier to bond.

To obtain an even higher bonding strength, the stainless steel surface was roughened additionally – this increases the surface area for bonding.

After this pre-treatment, just clean the surfaces thoroughly with WEICON Surface Cleaner and they are ready to bond (or to be bonded, more precisely). How important cleaning before gluing is, can be read here.

For the bonding, Martin used our adhesive Easy-Mix RK-7100.

Okay then – let’s calculate the bonding strength. In order to do so, we’ll take a look at the technical data sheet of the adhesive Easy-Mix RK-7100 first.

According to this, the adhesive has a strength of 23N/mm² on steel and 22N/mm² on stainless steel, tested in accordance with the DIN 1465 standard. This only helps if you know what the DIN 1465 standard actually says.

That’s why here is a picture of the test specimens as shown in the standard:

The specimens are made for a so-called tensile shear test. In this test, the bonded parts are exposed to tensile, shear and peeling forces. Peeling forces occur as the test sample is asymmetrical, which results in a torque.

All in all, these are complex forces, which have little in common with the suspension of our swing chair that is mainly exposed to just tensile loads.

Nevertheless, we can use the value from the tensile shear test for our calculation, as with a pure tensile load, an adhesive is capable of transferring higher forces than when exposed to a tensile shear load. So in our case, we are on the safe side.

For the sake of simplicity, we assume that the adhesive holds 20N/mm². Now, we have to figure out the size of our bonding area.

The diamond-shaped suspension has a size of 80 x 40 mm, so it has an area of 1,600 mm². From this, we subtract the holes, so an area of 1,480 mm² remains.

Now, we multiply the strength per square millimeter (20N/mm²) by the bonding area (1,480 mm²) to obtain the maximum load capacity of the adhesive joint = 29,740 N.

If we divide this value by the acceleration of gravity (9.81 m/s²), the result is a maximum weight of 3,031 kg.

Yes, that is a weight of more than three tonnes you’d have to attach to the carabiner, before the adhesive bond would break.

The swing chair weighs roughly 5 kg, our colleague about 90 kg and his child, who sometimes joins him in the chair, weighs 20 kg. All in all, that’s not more than 115 kg.

At this point, our colleague Martin leans back completely relaxed, enjoys his coffee and doesn’t worry about ageing of the adhesive or potential bruises or headaches.

Since the protective zinc layer was removed from the steel before bonding, a protective coating should be applied after the adhesive is fully cured. Our metal sprays or our Zinc Paint, for example, are a perfect protection against corrosion.

Fun fact: Four M5 V2A-50 screws have a yield strength of 1,216 kg.

So lean back and enjoy your coffee!