By what should happen when the operator lets go, which is usually a safety question rather than a preference.

A detented valve stays in the position it was moved to. That is what you want for a clamp, a jig lock or a selector - the operator sets it and then uses both hands for the work.

A spring-return valve goes back to rest as soon as the control is released. That is what you want when the motion itself is the hazard: the operator's continued hold is what permits the movement, and releasing is what stops it.

So the question to ask is: if the operator lets go halfway through, or falls, or is distracted, what should the machine do? If the answer is 'stop and stay safe', use spring return.

Detented valves on hazardous motions are a recognised bad practice, because the machine continues regardless of the operator. Where both a held motion and a maintained clamp are needed, use different valves for each rather than compromising.

Only with a device designed and certified for it - two ordinary lever valves are not a two-hand control.

A genuine two-hand control must require both hands simultaneously, within a defined time window, and must prevent the machine operating if either control is blocked, tied down or operated alone. It must also require both to be released before another cycle can be initiated, so an operator cannot latch one and work single-handed.

Achieving all of that reliably needs a purpose-built two-hand control valve or module, assessed against the machinery safety standards and integrated to the required performance level.

Plumbing two hand valves in series looks like it does the job, and it does not: nothing prevents one being taped down, and there is no timing requirement or release logic.

Where two-hand control is required by a risk assessment, specify a certified device. It is one of the areas where improvisation is both common and clearly unacceptable.

Force, travel, direction of motion and the clarity of the position indication - all of which affect whether the control is used correctly.

The force to operate should be comfortable for repeated use through a shift. A stiff valve is tiring and encourages operators to use a tool or their body weight, which damages it.

The direction of movement should correspond intuitively to what the machine does - a lever pushed forward producing a forward motion. Controls that behave counter-intuitively cause errors under pressure.

Position should be visible at a glance, so an operator can see the state of the machine without testing it. Detented valves with ambiguous lever positions are a frequent cause of confusion.

And the control should be positioned where it cannot be operated accidentally - by a passing person, by material being handled, or by leaning on it. Guarded, shrouded and recessed actuators are available for that reason.

Usually panel-mounted through a hole in the console, with the actuator on the operator's side and the ports behind.

A threaded bush on the valve passes through a clearance hole in the panel and is secured with a nut, leaving the lever, toggle or knob presented to the operator and all the tubing behind the panel out of the way.

That gives a tidy console and protects the connections from damage, and it lets several valves be arranged in a logical group.

Body or bracket mounting is used where the valve is placed at the machine rather than on a console - for a local control at a station, or where the operator works at the point of action.

Sub-base mounting is worth considering where valves may need replacing, since the tubing stays connected to the base.

When laying out a panel, leave room behind for the fittings and the tube bend radius - a common and irritating oversight that makes assembly difficult and maintenance worse.

The usual range, with three-port and five-port two-position types covering most manual control, and three-position versions where a neutral is needed.

A three-port valve drives a single-acting cylinder or generates a pilot signal - it connects the outlet either to supply or to exhaust.

A five-port valve drives a double-acting cylinder, pressurising one side while exhausting the other, and reversing on the other position.

Three-position versions add a centre condition, and its type matters: a closed centre traps air and holds the cylinder, an exhausted centre lets it move freely by hand, and a pressured centre applies pressure both sides.

On manual controls the centre condition is often the reason for choosing three positions at all - a lever with a spring-centred neutral that stops the motion when released, used on mobile equipment and hoists.

Decide the centre condition from what the machine should do when the lever is released mid-motion.

Frequently - as manual overrides, setup controls and local operation for maintenance.

A machine controlled automatically still needs a way to move things by hand: to set it up, to clear a jam, to position a mechanism for maintenance, or to operate it when the control system is not running.

Hand valves fitted for that purpose give direct control without depending on the PLC, and they work when the electrics are isolated - which is precisely when they are most needed.

The design requirement is that manual and automatic control must not conflict. Isolating or interlocking arrangements ensure that when the manual controls are in use, automatic operation cannot occur, and that the machine cannot be left in a state where a hand valve silently overrides the control system.

A hand valve left in an override position is a well-known source of baffling faults, so clear labelling and position indication matter as much on these as on the operator's controls.

Very little mechanically, but the actuator and the seals are the wearing items - and operator misuse is the commonest cause of failure.

The valve internals are simple and long-lived on clean, correctly lubricated air. What wears is the actuating mechanism: detent balls and springs, lever pivots and the return spring.

A detent that has worn becomes vague, so the lever no longer holds its position positively - which is a safety concern on a clamp control as well as a nuisance.

Misuse accelerates everything. A stiff valve gets levered with a tool, an operator leans on a control, or a lever is used as a handhold. Guarding the actuator and keeping the operating force comfortable prevent most of it.

Keep the air clean and dry, and confirm whether the valve requires lubricated air - running a lubricated valve dry hardens the seals and increases the operating force, which starts the cycle of misuse.