This page provides engineering guidance on machine interfaces and control design, including push buttons, switches, levers, knobs and handwheels. The data supports ergonomic design of control systems to help provide safe, efficient and reliable human interaction with machinery.
Control design should be considered alongside human strength and endurance data and anthropometric measurements to ensure usability across a suitable range of operators.
Machine controls form the physical interface between an operator and a machine. Their size, position, direction of movement, operating force and identification can have a significant effect on safety, usability and the likelihood of operator error.
Control design should take account of the intended user population, operating posture, reach, strength, frequency and duration of operation, environmental conditions and the consequences of incorrect operation.
The dimensions and operating forces on this page are provided as general engineering guidance. For detailed or safety-critical machinery design, the applicable current standards, legislation and industry requirements should be established and used.
The following standards have historically been referenced by RoyMech for machine interface and control design:
Standards are revised and replaced over time. Engineers should confirm the current applicable edition and requirements before using this page for detailed design.
A control should not be selected from its dimensions or operating force alone. The complete task, intended operator population and operating environment should be considered.
A control used repeatedly throughout a working shift may require a low operating force, comfortable posture and convenient reach. A control used only occasionally for adjustment or maintenance may permit different ergonomic criteria, subject to appropriate assessment and the applicable safety requirements.
Frequency should not be considered in isolation. Operating force, posture, reach, duration, precision, environmental conditions and the consequences of incorrect operation should also form part of the engineering assessment.
Controls associated with hazardous machine functions may require measures to prevent accidental operation, clear identification, suitable separation from adjacent controls or other protective arrangements.
Human factors should also be considered alongside other engineering requirements including machine safety, security, maintainability, reliability and operational needs. Mandatory requirements should be distinguished from recommended or preferred ergonomic criteria.
The following values provide suggested sizes for letters and symbols used on machine controls and displays in well illuminated areas.
Contrast between the characters and their background is important. Black characters on a light background generally provide good readability under normal illumination, while light characters on a dark background may be advantageous under relatively dark conditions.
| Distance from Eye (mm) | Height of Letters or Figures (mm) |
| Less than 500 | 2.5 |
| 501 – 900 | 5.0 |
| 901 – 1800 | 9.0 |
| 1801 – 3600 | 18.0 |
| 3601 – 6000 | 30.0 |
Where appropriate, control movement should follow established conventions and the operator's natural expectation of how the machine or displayed variable will respond.
For many rotary controls, clockwise rotation is conventionally associated with an increase and anticlockwise rotation with a decrease.
For linear controls, movement forwards, upwards or to the right may commonly represent an increase in the controlled variable, with movement towards the operator, downwards or to the left representing a decrease.
These conventions should not be applied blindly. The movement of a control should be compatible with the resulting movement or response of the machine where this provides a clearer and more intuitive relationship for the operator.
The following dimensions and operating forces are indicative values for common machine controls. They provide useful preliminary design guidance but should not be interpreted as mandatory limits.
The appropriate dimensions and operating forces depend on factors including the intended users, frequency and duration of operation, required precision, working posture, gloves or other personal protective equipment, environmental conditions and the safety significance of the control.
Push buttons may be designed for operation by a finger or by the hand. The following values provide suggested dimensions and operating forces.
| Parameter | Suggested Value |
| Diameter — finger operation | 12 – 15 mm |
| Diameter — emergency stop | 30 – 40 mm |
| Travel — finger operation | 3 – 10 mm |
| Resistance to motion — finger operation | 2.5 – 5.0 N |
| Diameter — hand operation | 60 mm |
| Travel — hand operation | 10 mm |
| Resistance to motion — hand operation | 10 N |
Toggle switches are commonly used for two-position functions such as on/off selection. Where appropriate, they may be positioned on a vertical panel with vertical toggle movement.
| Parameter | Suggested Value |
| Angle of movement | Approximately 45° |
| Toggle diameter | 3 – 25 mm |
| Toggle length | 12 – 50 mm |
| Resistance to movement | 2.5 – 15 N |
Large levers may be used where significant operating forces or relatively large control movements are required.
| Parameter | Suggested Value |
| Maximum forward and backward angular movement | 90° total |
| Maximum forward and backward linear movement | 350 mm total |
| Maximum lateral angular movement | 45° total |
| Maximum lateral linear movement | 150 mm total |
| Maximum operating force — forward/backward | 130 N |
| Maximum operating force — sideways | 90 N |
Rotating knobs provide a convenient method of controlling variables. They should fit the intended grip comfortably, be easy to rotate and should not unnecessarily obscure associated displays or markings.
Rotary controls may use discrete indexed positions or continuous movement. Indexed controls provide defined settings, while continuous controls allow progressive adjustment of a variable.
Where individual positions are indicated visually, successive positions should typically be separated sufficiently for the operator to identify the selected setting. Greater angular separation may be required where selection is primarily by feel.
| Parameter | Suggested Value |
| Diameter | 35 – 75 mm |
| Height | 20 – 50 mm |
| Continuous resistance | 0.15 Nm |
| Maximum torque | 0.32 Nm |
| Resistance at indexed positions | 12 – 18 N |
| Angle between indexed positions | 15° – 40° |
Continuous motion knobs allow progressive regulation of a variable and may be designed for fine finger adjustment or for operation using a larger hand grip.
| Parameter | Suggested Value |
| Diameter — finger operation | 10 – 30 mm |
| Diameter — whole-hand operation | 35 – 75 mm |
| Height — finger operation | 15 – 25 mm |
| Height — hand operation | 30 – 50 mm |
| Diameter — hand operation | 60 mm |
| Maximum turning torque — small knobs | 0.8 Nm |
| Maximum turning torque — large knobs | 3.2 Nm |
Arrow-shaped or pointed bar knobs provide a clear indication of the selected position and allow settings to be made quickly.
| Parameter | Suggested Value |
| Length | 25 mm minimum |
| Thickness | 25 mm maximum |
| Height | 12 – 70 mm |
Handwheels allow relatively large operating forces to be applied and may be used with gearing where slow or precise machine movement is required. Depending on the arrangement, operation may use one or both hands.
| Parameter | Suggested Value |
| Handwheel diameter | 180 – 530 mm |
| Rim diameter | 20 – 50 mm |
| Number of spokes | Minimise |
| Minimum resistance | 20 N |
| Maximum resistance — single handed | 135 N |
| Maximum resistance — two handed | 225 N |
Cranked handwheels allow machinery to be operated through relatively long movements using continuous rotation of the crank. Gearing between the handwheel and machine movement may provide coarse and fine adjustment, allowing rapid positioning followed by more precise control.
| Parameter | Suggested Value |
| Crank radius for low torque — up to 200 rpm | 60 – 120 mm |
| Crank radius for high torque — up to 160 rpm | 150 – 220 mm |
| Crank radius for quick setting | Up to 120 mm |
| Resistance torque — 120 mm radius crank | 0.5 – 2.5 Nm |
| Resistance torque — 240 mm radius crank | 3.0 – 4.0 Nm |
| Handle diameter | 25 – 30 mm |
| Handle length — one handed | 80 – 120 mm |
| Handle length — two handed | 190 – 250 mm |
The following sketches provide general guidance on display positioning and viewing arrangements. The illustrations are retained as useful engineering reference material but should not be treated as current dimensional requirements.
Good machine-interface design requires more than selecting a control from a dimensional table. The control, operator, machine response and working environment form a complete system.
Frequently operated controls should generally be positioned within comfortable reach and should not require unnecessarily high operating forces. Controls requiring precision should permit suitable fine movement and provide appropriate visual, tactile or other feedback.
Less frequently operated controls may permit different arrangements, but an infrequent task is not automatically an acceptable one. Excessive force, awkward posture, poor access or significant consequences of operator error may still require design changes or additional safeguards.
Where ergonomic recommendations conflict with safety, security, maintainability or operational requirements, the applicable mandatory requirements should first be established. The resulting design should consider the overall engineering risk rather than applying an individual human-factors recommendation in isolation.
For reach, clearance and workspace layout, see human access space requirements.
Machine interface controls are devices such as push buttons, switches, levers, knobs and handwheels that allow an operator to interact with and control machinery or equipment.
Ergonomic control design helps operators use equipment safely, efficiently and reliably. Appropriate control position, size, operating force and movement can reduce fatigue and the likelihood of operating errors.
The BS EN 894 series has historically provided ergonomic requirements for displays and control actuators associated with machinery. Engineers should confirm the current applicable standards and regulatory requirements for the equipment being designed.
Control dimensions and operating forces should be selected with regard to the intended user population, human strength, anthropometric data, reach, posture, task frequency and duration, required precision and the applicable ergonomic and machinery safety requirements.