Tie rods and why we need adjustable joints
A tie rod is a rod that carries a pull (tension). It ties two parts together so they do not move apart: roof trusses, gates, bridges, cranes, wire fences.
Rods stretch a little over time, or the parts settle, so the tie becomes slack. We need a joint that can change the length of the tie by small amounts and lock it. The turnbuckle (also called a rigging screw or straining screw) does this.
Turnbuckle: parts and working
- Body: a long nut, often an open frame or a closed tube, with a hexagon or flat middle so a spanner can turn it.
- Two rods: one has a right-hand (RH) thread, the other a left-hand (LH) thread. The body has an RH thread at one end and an LH thread at the other.
- Lock nuts: after tightening, a nut on each rod is turned against the body so it cannot loosen.
Why opposite threads?
If both threads were the same hand, turning the body would push one rod in and the other out: no change in length. With opposite hands, both rods move in (or both out) together, and the rods themselves do not need to turn.
Change in length
One full turn moves each rod by one pitch p, so the total length changes by 2p per turn. With n turns: change = 2 × p × n.
Flanged pipe joint
Pipe joints must be leak-proof, strong enough for the pressure, and easy to take apart for cleaning or repair. The flanged joint is the most common joint for large pipes.
- Flange: a flat circular ring at each pipe end. It may be cast with the pipe, screwed on or welded on.
- Gasket: a thin soft ring (rubber, fibre or soft metal) placed between the two flanges. When squeezed it fills tiny gaps and stops leaks.
- Bolts and nuts: placed on a circle called the pitch circle. Their number is always even (4, 6, 8…) so they can be tightened in opposite pairs and the gasket is pressed evenly.
- For high pressure, flanges may have a spigot and socket (a raised ring fitting into a recess) to keep the pipes in line.
Typical proportions (D = pipe bore)
Flange thickness ≈ 1.5 × wall thickness; pitch circle diameter ≈ D + 2.5 × bolt diameter + 2 × wall thickness. Follow the sizes given in the question.
Drawing tips
- Turnbuckle: draw the centre line, the body with its hexagon middle, the two rods, then mark "RH" and "LH" on the threads. Show threads with thin lines (conventional representation).
- Flanged joint: draw the front view with the upper half in section: hatch the flanges in opposite directions, show the gasket as a thick black line, and show bolts unsectioned.
- In the side view, draw the bolt circle as a chain line and place the bolt holes evenly (360° ÷ number of bolts apart).
Key formulas and definitions
- Turnbuckle: change in length = 2 × pitch × number of turns
- Angle between bolts = 360° ÷ number of bolts
- Number of flange bolts = even number (4, 6, 8 …)
- Joint parts: flange + gasket + flange + bolts and nuts
Worked examples
1. A turnbuckle has threads of pitch 2.5 mm. How much does the length change in 4 full turns?
Change = 2 × 2.5 × 4 = 20 mm.
2. A tie rod is 12 mm too long. The turnbuckle pitch is 2 mm. How many turns are needed?
Each turn = 2 × 2 = 4 mm. Turns = 12 ÷ 4 = 3 turns.
3. A flange has 6 bolts. What is the angle between neighbouring bolts?
360° ÷ 6 = 60°.
4. Why does a flanged joint need a gasket if the flange faces are machined flat?
Even machined faces have tiny bumps and scratches. The soft gasket squeezes into them and blocks every leak path.
Common mistakes
- Thinking both ends of a turnbuckle have the same thread: one is right-hand and the other left-hand.
- Using 2 × pitch only once: the change is 2 × pitch for EACH turn.
- Drawing an odd number of flange bolts: use an even number so pairs can be tightened opposite each other.
- Leaving out the gasket in the drawing: show it as a thick line between the flanges.