Valve deadband and backlash
Backlash is lost motion: the signal changes, and nothing happens at the stem until the play in the linkage has been taken up. It is closely related to stiction and often confused with it, but the signatures differ in a way that is easy to see once you know what to look for.
Stiction resists movement in any direction and produces stick-slip. Backlash only bites on reversals — a valve moving steadily in one direction shows no backlash at all.
The signature: flat spots at reversals
On a trend, backlash appears as a flat section on the process variable occurring exactly where the controller output changes direction. The output reverses, the process variable holds still for a while, and then resumes following once the play has been taken up.
The width of that flat spot, expressed as a percentage of output travel, is the size of the deadband. That gives you a number to take to whoever will be doing the mechanical work.
How to measure it deliberately
In manual, move the output in one direction until the process variable is responding cleanly. Then reverse and make small steps in the opposite direction, noting the total output change before the process variable starts to move.
That total is the deadband. Repeat it in the other direction — the two are often different, which itself is diagnostic.
What causes it, most likely first
Worn or loose stem connector
most common tuning cannot fix this
Play at the coupling between actuator and stem.
- How to confirm it
- Physical inspection, or the deadband measurement above.
- What to do
- Tighten or replace the connector.
Positioner feedback linkage play
common tuning cannot fix this
Wear in the linkage that tells the positioner where the stem actually is.
- How to confirm it
- Deadband present with the positioner in service and different with it bypassed.
- What to do
- Inspect and adjust the feedback linkage.
Worn rotary valve shaft or coupling
common tuning cannot fix this
Ball and butterfly valves drive through a shaft and coupling that wear, producing rotational play that shows up as deadband.
- How to confirm it
- More common on rotary valves than on globe valves; measure the deadband as above.
- What to do
- Mechanical repair. This does not improve on its own.
Questions that come up
How is backlash different from stiction on a trend?
Stiction produces stick-slip in both directions and a sustained limit cycle even when the output is moving steadily one way. Backlash only appears at reversals — flat spots on the process variable exactly where the output changes direction, and nothing unusual while the output moves steadily.
Related
Last reviewed 2026-08-01.