Set screws are used to clamp one tube inside another by pressing into the side of the inner tubing material. Set screws can be used in steel or aluminum dock parts. We find that zinc set screws work well in galvanized dock parts while stainless steel set screws work in aluminum dock parts. Marine grease is applied to the set screws in both scenarios. Over time you will see the zinc set screws get rusty and after about 10 years, it is time for replacement. This one shown below is on a dock that never gets removed and the zinc set screws are neglected and missing.
The difference between using a set screw vs a regular bolt is very important. The regular bolt is flat on the tip while the set screws we use have a cup shape that bites into the outer wall of the inside tubing material and that is what makes it work in Steel or Aluminum. Set screws allow you to get a dock perfectly level far better than a dock leg that has predetermined selections of pinning holes with sloppy tolerance every couple inches. Legs with set screws are held firm at the exact desired height anywhere along the dock pipe leg.
With aluminum dock parts, the threaded hole for the set screw is simply drilled and tapped out. With steel dock parts a steel nut is welded to the steel pipe over the drilled hole and not always galvanized afterward for extended life.
The galvanized holes then need to be tapped out after galvanizing. Working with steel parts is more labor and more weight so Great Northern Docks has switched to all aluminum with stainless set screws. Great Northern Docks aluminum tapped holes each have a holding force of 500 lbs.
You’ll find stainless set screws in Great Northern Docks aluminum stationary docks for securing the galvanized pipe legs at the proper level above the ground.
The docks can be raised or lowered with winches in place before loosening set screws. The set screws are used in the pipe feet as well as the DIY stationary dock pipe brackets. There are two set screws in the dock frame sockets for better stability and extra weight capacity.
A pipe held by a single set screw will shake more and pivot on that single set screw so both set screws must be installed for a proper installation.
Set screws bite into steel pipe better than aluminum pipe. The aluminum chips away from the stainless steel set screw and requires more retorquing. The steel is malleable and set screws stay snug for a better longer hold.
Smaller stainless steel set screws are used in the mooring whip brackets to secure the fiberglass mooring whips into the metal base. I’ve seen plastic set screws used in PVC fittings but they would not have the same holding strength as the set screws used on Great Northern Docks.