Whenever a moving part passes the valve in both directions but a signal is only wanted in one.

A standard roller valve is actuated by anything that depresses it, so a carriage travelling out and back trips it twice - once on each pass. In a sequential circuit that second, unwanted signal can advance the sequence at the wrong moment or hold a valve piloted when it should have released.

A one-way roller, also called an idle-return roller, is hinged so that it actuates the valve when driven from one direction and simply folds out of the way when driven from the other. The part passes without any signal on the return.

That allows the circuit to know the difference between advancing and retracting, which is fundamental to most sequences.

When fitting one, check the orientation carefully - it is easy to mount a one-way roller backwards, and the symptom is a sequence that works in reverse or misses a step, which is often diagnosed as a circuit fault.

So the movement carries the actuator fully through its switching travel and stops short of its maximum - and the margin matters at both ends.

The valve does not switch until the actuator has moved a specified distance, so a part that only just touches it produces an unreliable signal that comes and goes with wear and vibration.

At the other extreme, the actuator has a maximum travel. Drive it beyond that and the plunger bends or the mechanism breaks, and this is the commoner failure - because when a valve switches intermittently the instinct is to move it closer.

The correct setting takes the actuator comfortably past its switching point with clearance to its limit. Where the part must continue travelling past the valve, an overtravel roller lever accommodates that by swinging aside.

Re-check the setting after any mechanical work. These valves are mounted on brackets near moving parts and are the first thing to be knocked out of position.

Fast enough for most machinery, with the limit set by the actuator's mechanical movement and the length of the signal line rather than by the valve.

The valve element itself shifts quickly. What takes time is the actuator being pushed through its travel by the machine, which depends on how fast the part is moving, and then the pressure signal propagating down the pilot tube to whatever it controls.

On a long, small-bore pilot line that propagation delay is measurable, and on a fast machine it can matter. Keeping signal lines short and using an appropriate bore is the usual remedy.

There is also a minimum dwell: a part passing very quickly may depress the actuator for too short a time to generate a usable signal, particularly if the downstream volume is large.

Where speed is genuinely marginal, an electrical sensor with a fast PLC input is the more suitable technology - pneumatic sensing is robust and simple but not fast.

A firm push, considerably more than a whisker valve requires, which is both a strength and a constraint.

A plunger or roller valve must overcome its internal seal friction and return spring, so it needs a definite force from the machine member - which a cylinder rod or a driven carriage provides easily.

That relatively high force is a virtue in a machine environment, because it means vibration, air movement and incidental contact will not produce false signals.

It becomes a constraint when the part doing the actuating is light, fragile or moving under its own momentum - a small component on a conveyor may not have the mass to operate the valve, and forcing it to do so may mark or deflect it.

For those cases a whisker or trip valve, designed for very low actuating force, is the correct product. Check the published actuating force against what the application can genuinely apply rather than assuming a moving part will operate any valve.

Where the circuit is already pneumatic, where there is no electrical supply, or where the environment makes electrical sensing awkward.

On a machine controlled entirely by air, adding an electrical sensor means adding a power supply, wiring and a means of converting the electrical signal back into a pneumatic one - which is a great deal of apparatus to detect a position. A roller valve does it with one component and a length of tube.

In hazardous areas the advantage is stronger: no electrical energy is present, so nothing needs certifying.

Washdown, wet and dirty environments suit them, as a mechanical valve tolerates water and grime that would require a protected sensor.

Against that, electrical sensing is faster, easier to diagnose, has no moving parts to wear, and integrates directly with a PLC. On any machine that already has a control system, an electrical sensor is usually the better choice unless the environment argues otherwise.

Wear of the roller and its bearing, contamination stiffening the mechanism, and physical damage from over-travel or impact.

The roller is contacted repeatedly at speed and its bearing wears, so the roller develops play and eventually the switching point drifts. A worn roller can also seize and be dragged rather than rolling, which wears a flat on it and damages the part passing.

Contamination - swarf, coolant, dust, paint overspray - works into the actuator and stiffens it, so the return becomes sluggish and the valve holds its signal after the part has passed.

Physical damage comes from a part driving the actuator beyond its limit, from the machine moving on its mountings so geometry changes, or from an operator or fitter knocking the valve while working.

Because these are inputs to the circuit, a failure presents as a machine that stops mid-sequence or behaves erratically - so checking the position-sensing valves is a good first step when diagnosing a sequence fault.

Small ones generally should not - they are signal devices, and driving a cylinder directly through one is usually a mistake.

Most plunger and roller valves are small-ported devices intended to generate a pilot signal. Their flow capacity is modest, so driving a cylinder through one gives slow, weak movement, and it exposes the small valve to the full working flow and its contamination.

The normal arrangement is that the roller valve pilots a larger main valve, which handles the cylinder's air. That keeps the signal path small and fast and the power path generously sized.

Larger mechanically operated valves do exist and can drive small cylinders directly, which is legitimate on a simple jig where the whole circuit is two components.

Check the flow capacity against the actuator's requirement before connecting one directly. A cylinder that moves sluggishly on an otherwise healthy system is often being fed through a valve intended only to signal.