Turnbuckle

The essentials

A turnbuckle (also called a rigging screw or straining screw) connects two rods, wires or cables placed in line and allows their tension to be adjusted. Its central body, often a rectangular or hexagonal frame or a tube, is tapped at one end with a right-hand thread and at the other end with a left-hand thread. The two tie rods are threaded to match. When the body is turned in one direction, both rods are pulled in together; turned the other way, they move apart. The rods themselves do not need to rotate.

Key points

• Parts: body (central frame), two tie rods (one right-hand and one left-hand threaded), and two lock nuts.
• Change in length per turn of the body = 2 × pitch (for single-start threads), because both rods move.
• Example: with pitch 3 mm, 5 turns of the body change the length by 5 × 2 × 3 = 30 mm.
• Lock nuts tightened against the ends of the body keep it from turning under vibration.
• Uses: tightening tie rods of roof trusses, stay wires and cables, and similar members.
• The body is often given flats or a hexagonal section so that it can be turned with a spanner or a rod.

Exam tips

• In the drawing, indicate which end carries the left-hand thread, e.g. by a note.
• Show the lock nuts and the threaded rods entering the body from both ends.

Common mistakes

• Thinking both ends have right-hand threads; then turning the body would not change the length.
• Using only one pitch per turn in calculations.