Taking the slack out of an old control linkage
Six worn pivots in an old throttle and shift linkage give six to ten millimetres of slack at the lever. These are pivots with no sliding surfaces — they bend instead of turning — plus a shadow joint that takes over if a blade cracks, so gear control does not disappear alongside a stone quay.
The problem
Every pivot in a sixty-year-old Bowden or Morse linkage is a worn pin or ball. Six of them in series give six to ten millimetres of lost motion at the lever, so the gearbox misses engagement and the throttle will not hold a setting. Individual parts for these systems are long gone. The owner's only route today is to replace the whole control head with a modern unit — which works, and which removes the one fitting a classic buyer looks at first.
The fix
Each worn pivot is replaced with a cross-axis flexure printed in one piece in PA-CF: two 0.9 mm blades crossing at 90° on the pivot axis, blade length 16 mm. That gives ±28° of rotation, more than a throttle or shift linkage ever asks for, with no backlash, no lubrication and no wear, because nothing slides against anything. The blades print flat with the perimeters running along the blade so the fibres carry the bending. Then it is not wear that sets the life but fatigue: at ±28° peak blade strain is around 1.1 %, and the part is sold as something to be replaced, not as something permanent.
Losing gear alongside a stone quay is not acceptable, so every joint carries a shadow joint: a printed pin-and-slot in parallel, running 0.4 mm clear. It does nothing while the flexure is intact, and becomes an ordinary — sloppy, but entirely usable — pin joint the second a blade cracks. The point, and the difference from a normal hard stop, is that the pin is sized for the full working load and the full travel, not to limit strain. The original control head, levers, knobs and escutcheons are not touched. Each set is drawn against that particular head from photographs plus one worn joint sent in, and the fixings use the original pin diameters, so nothing on the boat is modified.
This is published openly on purpose. The cross-axis flexure itself is known and freely available — BYU has published a printable version — and the business here is the measuring, the fitting and the honesty about fatigue, not the part. And the honesty is not free: ten years is not a number I write down before a set of blades has been run in a cycling rig until it breaks.
More ideas
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