Drop-lock pin and spring clip
Small locking pins that make stacked scaffold joints resist tension and stop push-on parts such as guard rail posts from being lifted out.
Also called: gravity lock pin, drop pin, frame pin, pigtail pin, spring clip, tube lock pin
A system scaffold is normally just stacked: the frame or standard of the next lift sits on the spigot of the one below. Under compression that is perfectly adequate. As soon as tension appears, however, the parts could pull apart: wind getting under a flat roof, an internal building corner, untied top lifts or a scaffold section hanging from a crane hook. The drop-lock pin prevents this. Pushed through the hole in the joint, it makes the connection tension-proof. It also stops parts that are only pushed on from being pulled off, such as guard rail posts, roof screen posts and deck locks.
Variants
- Drop-lock pin: a steel pin with a hinged toggle at the end that drops under its own weight once pushed through and stops the pin from sliding back. It can only be pulled out by deliberately lifting the toggle.
- Pigtail pin: named after its coiled shape; it is twisted through the hole and held by spring tension.
- Spring clip (tube lock pin): a pin with a hinged spring bow that is folded around the tube and snaps into place, often 12 mm in diameter.
- Frame pins: 8 or 12 mm in diameter depending on the manufacturer, matched to the holes of the system.
All are galvanised steel, many are colour-marked so they are easy to spot on the scaffold. Pins where the bolt and its retainer are separate parts are described under pins, bolts and clips.
Where and how they are fitted
The instructions state where tension-proof joints are required, for example under roofs pitched at less than 20°, at internal corners, on lifts above the last tie and when a scaffold is moved by crane. Align the holes, push the pin all the way through, let the toggle drop or close the bow, then give it a short tug. When dismantling, pull the pin only once the upper part is held by hand and carries no load.
On a frame scaffold the hole sits in the spigot at the frame head, so frame is locked to frame. On a modular scaffold the standards are joined by pressed-in or bolted spigots; where tension resistance is needed there, tube lock pins or the bolts specified by the manufacturer are used.
Typical faults
- Forgotten pins on top-lift posts, often only noticed during the scaffold inspection.
- Nails, wire or screws without nuts as a stand-in. The pin must match the hole diameter.
- Toggles jammed by rust or mortar, and bent pins that crews then leave out.
Pins are wear items and go missing on every job. When buying used, look for straight pins, intact spring bows and toggles that move freely, and keep a good supply in a mesh box. Small parts are listed under accessories.