What are backlash and lost motion?
Lost motion is the difference between the motion you commanded and the motion you got, sometimes called apparent backlash. It has several parts: compressibility and elastic deflection of parts, conventional backlash, sliding friction and thermal expansion. Backlash is the largest part in many machines, and comes from the clearance between moving parts, also called play.
Backlash is a system-wide effect, not a feature of one component: nearly every joint in the chain adds to it, and a softer part shows more lost motion than a stiffer one. In a linkage the contributors are the pins, the bearings, the threads, and the rods and mounts themselves.
How do you measure it?
With a dial indicator.
- Fix a dial indicator on a stationary part, with its tip on the output point of the linkage.
- Move the input one way until the output follows, and set the dial to zero.
- Move the input a known distance, then bring it back to the same input position.
- Read the dial. The difference from zero is the lost motion.
To see the play alone, push the output by hand in both directions, with the input held still, and read the movement. The usual rules apply: do not assume where the error comes from, and measure the effect of every adjustment you make.
Where does the play come from?
Work along the linkage and test each joint.
- Rod end or ball joint clearance. Internal clearance sets how far a linkage can move before it responds. A small amount may be fine in a manually adjusted mechanism but gives inconsistent travel in a positioning system, and a spherical plain bearing is never entirely free of play. See K02-Q1-07 for the clearance groups.
- Loose pins. If the ball is not clamped, it turns on the bolt and the motion takes place on the bore, so the clearance between bolt and bore adds to the play. The inner race must be clamped to the fitting, or the rod end can rotate around the bolt. Check the pin fit and retention (K03-Q1-14).
- Loose thread or lock nut. A jam nut that has come loose, or too little thread engaged, allows movement (Q1-04, Q1-05).
- Flex. A long, slender rod, or a flexible bracket, gives a little under load. This shows as lost motion, not as play.
To find the loose joint, push and pull each joint in turn by hand and look for movement at that joint.
How do you reduce it?
- Fix the biggest source first, then measure again.
- Take up the play in the worst joint. Replace worn bearings or pins, tighten loose clamps, relock the nuts.
- Choose tighter parts where the job needs them. A tighter rod end may improve positioning but needs more force to move. Tie "precision" to a number you can measure: clearance, turning torque or positioning accuracy.
- Stiffen what flexes. A larger diameter or a shorter rod (Q1-03).
- Approach the target from one side. Always arrive at a position from the same direction, so that the play is already taken up. It works only where the process allows an extra move.
Why does play matter more in a steering link?
In a steering link, play is an angle at the wheel. With an illustrative 0.3 mm of total play at a tie rod end acting on a 120 mm steering arm, the wheel can move through 0.3 ÷ 120 = 0.143° before the link responds, which is about 1.5 mm across a 600 mm tyre. Each time the load on the wheel changes direction, that play is taken up in the other direction, so toe shifts under braking, cornering or a bump even though the static setting looks right. Play at the suspension links lets the axle move in the same way. Measure it as above, on the wheel rim with a dial indicator, and find the loose joint before adjusting anything (Q1-12). The tie rod end buyer's guide covers the parts, and the allowable play comes from the vehicle maker.