How do you measure the length of a linkage?
From the centre of one ball to the centre of the other. This is the centre-to-centre length, written C–C. SYZ's linkage rods page gives the same instruction for rod ends: measure centre to centre with the assembly adjusted to its mid-adjustment position. The full C–C formula and the adjustment range are in K06-Q1-04.
To make the rod itself, you need to know how far the ball centre sits from the socket's end face. Maker tables list this as "a" (centre to shank end). A second column, "e min.", is the length of the female thread inside the socket. The rod length is:
rod length = C–C − a₁ − a₂ + insertion depth₁ + insertion depth₂
Example, with two joints that have a = 22 mm and e min. = 10.2 mm (example values; read yours from the maker's table) and a required C–C of 200 mm. The gap between the two socket end faces is 200 − 22 − 22 = 156 mm. With 10 mm screwed into each socket, the rod length is 156 + 10 + 10 = 176 mm.
Keep each insertion depth below e min. (10.2 mm here) and above the engagement your design needs. See Q1-23.
Which thread goes with which ball?
On DIN 71802 joints, each ball size comes with a set thread size. The standard assigns a corresponding thread size to each size of component. The table pairs the sizes listed in the Ganter / JW Winco catalogue with the length change per turn.
| Ball diameter | Thread | Length change per turn, one end | Per turn, RH/LH rod |
|---|---|---|---|
| 8 mm | M5×0.8 | 0.8 mm | 1.6 mm |
| 10 mm | M6×1 | 1.0 mm | 2.0 mm |
| 13 mm | M8×1.25 | 1.25 mm | 2.5 mm |
| 16 mm | M10×1.5 | 1.5 mm | 3.0 mm |
| 16 mm | M12×1.75 | 1.75 mm | 3.5 mm |
| 19 mm | M14×1.5 | 1.5 mm | 3.0 mm |
The length change per turn equals the thread pitch (K06-Q1-04 works this through).
Note that M10 and M12 share the 16 mm ball. A bigger thread makes the stud and the rod stronger. It does not make the socket hold the ball any harder, because the catalogue pull-off figure is the same for both (Q1-05).
For inch threads, the same rule applies. A rod with one right-hand and one left-hand end changes length by two pitches per turn. A pitch is 1/32 in on 10-32, 1/28 in on 1/4-28, 1/24 in on 3/8-24 and 1/20 in on 1/2-20.
Why use one right-hand and one left-hand thread?
Because turning the rod then moves both ends at once, and neither end has to be taken off. Rotating the central body moves both ends inward or outward at the same time, and the rod adjusts steplessly within its range (K06-Q1-01 explains how a turnbuckle, a jack screw and a linkage rod differ).
With two right-hand threads, the adjustment is awkward. With RH/RH the centres can only be set to within one thread pitch: one end has to be unscrewed and turned, so length changes only in whole-pitch steps. A mismatched hand is easy to miss and stops the adjustment working, so check the hand on each end before you assemble.
How do left-hand threads work on an angle joint?
On a DIN 71802 joint, the left-hand thread is only in the socket. The stud thread stays right-handed.
Left-hand sockets carry an L after the thread (M5L to M12L); for M14, F and FL mark fine and left-hand fine threads. Check the marking on your part drawing.
Fine or coarse thread?
Fine gives more precise adjustment. Coarse is easier to fit and tolerates dirt better. M14 comes in both M14×1.5 and M14×2.0 in some catalogues.
What can the middle of the linkage be?
Three common forms. A solid threaded bar, cut to length and usually thread-rolled. A double-ended adjuster with a thread of opposite hand at each end, such as a jack screw. And a tube with threaded inserts welded in, for longer links (K07-Q1-01). Do not weld a ball socket itself, because heat ruins the grease and the snap ring.
Which is best depends on length and load. The linkage rod and turnbuckle articles cover these in more depth (K06-Q1-01).
How do you assemble and lock it?
Screw both ends in to the same depth, with room to spare to adjust. Measure C–C, and turn the rod to correct it. Turn each socket to the middle of its tilt range (Q1-09). Then tighten the jam nuts; a left-hand thread needs its own left-hand nut (Q1-23). Move the linkage through its full stroke and re-check.
Tolerances are worth stating on the drawing. As an example of general tolerances, one maker quotes ±0.8 mm for formed rods and linkage assemblies and ±0.25 mm for machined parts. Other suppliers will differ.
How closely must a throttle or shift rod be set on a vehicle?
Throttle and shift rods are the usual adjustable links on a vehicle, and a length error shows up as an angle at the lever. An illustration with assumed numbers: a lever arm of 50 mm and a right-hand/left-hand rod with M6×1 ends, which changes length by 2.0 mm per turn. One turn moves the lever by 2.0 ÷ 50 rad, about 2.3°, and a quarter turn by about 0.6°. So the length is best set on the vehicle, with the lever at both of its stops, and not from a drawing alone.
Vibration is the other difference. A thread that is locked on a bench can still back off on an engine-mounted linkage, so lock both ends, and use a left-hand nut on the left-hand end (Q1-23). Check also that the rod is screwed into each socket past the engagement your design needs and no deeper than the socket's e min. length, as above.
Whether a particular rod length, thread or joint is acceptable on a throttle, shift or steering link is for the vehicle maker to decide. For a rod built from tube and rod ends, see SYZ's linkage rod buyers guide.