Two hand control: types, timing and why it is often the wrong answer

Two hand control has an intuitive appeal that survives longer than it should: if both hands are on buttons, neither hand is in the press.

That is true, and it is the entire extent of what the device guarantees.

The requirements

ISO 13851 sets four behaviours, and all four are needed for the arrangement to mean anything.

• Both actuators. Both must be operated for the machine to move.
• Synchronously. Within about half a second of each other. Outside that window the cycle is refused.
• Continuously. Releasing either one stops the hazardous movement immediately.
• Released and re-pressed. Both must be released before the next cycle can be initiated, so holding one down between cycles achieves nothing.

The half second rule is the anti-defeat measure. Without it, one actuator can be taped, wedged or held by a knee, and the arrangement degrades to a single button.

The types

Type III is the one used on machinery, and it comes in three grades that differ in how faults are handled.

• IIIA. Synchronous actuation, continuous holding, basic monitoring.
• IIIB. As IIIA, with the fault detection expected of Category 3.
• IIIC. As IIIB, at Category 4, with faults detected at or before the next demand.

The type is not a preference. It comes from the required Performance Level, which comes from the risk assessment.

The physical requirements

The actuators have to be far enough apart that one hand cannot reach both, and shrouded so that a palm or a forearm cannot span them. The standard gives minimum separations.

They also have to be far enough from the hazard that the machine can stop before a released hand reaches it, which is the same safety distance calculation used for a light curtain, based on the overall stopping performance.

The limitation that matters

Two hand control protects the operator holding the buttons. It says nothing whatever about anybody else.

On a press where a second person loads blanks, or where a supervisor reaches in, or where the back of the machine is open to a walkway, two hand control is not a safeguard. It is a hand protection device for one person, and the machine still needs guarding for everyone else.

When it is genuinely the right choice

Small bench mounted presses and assembly fixtures, worked by one person, where the hazard zone is small enough to be visible in its entirety from the operating position, and where the loading action genuinely needs both hands free at the moment the machine moves.

Outside that, a light curtain with a proper safety distance usually gives more protection, to more people, with less to defeat.