Heavy Duty Die Cast Corner Bracket for Aluminium Profile 60 80 90 100 mm Series

On a large frame, the joint is the weak point

It is natural to assume that a big frame fails at the profile, because the profile is the part that visibly bends. In practice it usually fails at the joint. A 100 x 100 profile has a second moment of area of roughly 250 times that of a 20 x 20 section, so a large frame member is enormously stiff in bending. The corner is joined by a bracket with two to four bolts, and no amount of profile stiffness helps if the bracket lets the corner rotate.

That is why the bracket specification matters more on a large frame than on a small one. Doubling the profile size doubles the section depth and multiplies the stiffness by roughly sixteen, while the bracket usually goes up by one size. The gap between the two has to be closed by adding bolts, adding a brace, or both.

Why two bolts per flange is a step change

A single bolt per flange resists the corner opening almost entirely by friction. The bolt preload presses the flange against the profile face, and the friction force resists sliding. That resistance is proportional to the preload and to the friction coefficient, and it acts at a very short radius, so the moment it can resist is small.

Two bolts change the mechanism. The pair forms a couple with a lever arm equal to the distance between them, which on a 100 mm flange is around 60 mm. Taking a typical M8 preload of about 10 kN and a friction coefficient of about 0.15, a single bolt flange resists roughly 30 Nm of corner moment. The same preload acting as a couple over a 60 mm arm resists roughly 90 Nm. The bolt size did not change; the arrangement did.

When the corner needs a brace as well

A bracket resists the corner rotating. A brace resists the frame racking, which is a different and usually larger effect. The two are not interchangeable, and on a tall frame the brace is the one that matters.

A useful working rule is to look at the free height of the frame above the point where it is restrained, and compare it with the section width. Where the free height is more than about fifteen times the section width, the corner moment from an ordinary horizontal load exceeds what a bracket alone will hold reliably, and a diagonal brace should be added. Below that ratio the bracket is usually enough.

The rule is a starting point, not a substitute for a check. A frame carrying a concentrated load at the top, or one that sees vibration, needs a brace at a much lower height than the table suggests. Send us the frame dimensions and the load and we will confirm whether a bracket alone is adequate.

Fitting a heavy duty bracket

1. Load the nuts before the frame is standing Four bolts per corner means eight nuts on a typical two-member joint. Loading them into the groove while the member is on the bench is far easier than trying to feed them in once the frame is upright.
2. Start all four bolts before tightening any Start the bolts by hand on both flanges first. Tightening one flange before the other is started pulls the bracket off position and the remaining bolts will not line up with the slots.
3. Square the frame using the slots Measure the diagonals of the frame and move the brackets in their slots until the diagonals are equal. On a large frame this is the only practical way to square it, because the cut lengths will not be exact over several metres.
4. Tighten in a diagonal sequence Tighten the four bolts in a diagonal pattern and in two stages, half torque then full torque. Tightening one flange fully before the other bows the flange and leaves the bracket sitting on one edge.
5. Re-check the diagonals and the bolt torque Measure the diagonals again after tightening and re-check the torque after the frame has been standing for a day, because a large bracket settles into the profile face.

Frequently asked questions