Understanding the Safe Operating Speed of a Machine: Beyond Simple Limits

Is the safe operating speed of a machine a real solution or just a convenient simplification? Many engineers and machine operators use this term during service modes, retooling, or manual control. Speed limitation is associated with risk reduction, but is it enough?

Consider an industrial robot in teach mode moving at 250 mm/s. Standards suggest this speed is low enough to be considered ‘safe.’ But what about a massive hydraulic press slide moving even slower at 10 mm/s? Does a lower speed always equate to reduced danger?

In practice, machine safety involves more than just speed; it encompasses energy, mass, force, and—most importantly—risk analysis. This article will demonstrate why speed alone is insufficient and how technical standards help understand what truly impacts operator safety.

What Do Standards Say About the Safe Operating Speed of a Machine?

Although the term ‘safe operating speed of a machine’ is not directly defined in harmonized standards, many provide guidelines for limiting machine speed under specific conditions. Standards like ISO 10218-1, ISO 14120, and EN 13418 specify speed values that help minimize risk during machine operation.

• Industrial Robots in Teach Mode
  According to ISO 10218-1, robots in teach mode must not exceed a speed of 250 mm/s. This limit is based on the assumption that at this speed, even if an operator comes into contact with the moving arm, the risk of serious injury is minimal.
• Mechanical and Hydraulic Presses
  For presses, standards require that in setup mode, the slide speed be limited to a maximum of 10 mm/s. This low speed allows the operator full control over the process, and potential hazards can be easily identified and neutralized.
• Roller Conveyors in Rubber and Plastic Processing
  According to EN 13418, rollers can move at a maximum speed of 15 m/min (250 mm/s), while other machine elements are limited to 5 m/min (83 mm/s). These limits are based on kinetic energy analysis and the potential effects of machine contact.
• Moving Guards
  The ISO 14120 standard indicates that moving machine guards must be designed so that their kinetic energy during closing does not exceed 10 J (for guards with additional safeguards) or 4 J (for guards without such safeguards).

All these values relate to specific cases and serve as guidelines in the risk analysis process. Furthermore, standards like ISO 13855 emphasize that besides speed, the system and operator’s reaction time, as well as the stopping distance required for the machine, are crucial.

Industrial Robots in Teach Mode

According to ISO 10218-1, robots in teach mode must not exceed a speed of 250 mm/s. However, speed alone is only part of the safety measure. In this mode, the operator uses a teach pendant—a control device equipped with a three-position safety switch. This switch allows for controlling the robot’s operation:

• Neutral position (middle): the robot operates in the designated mode.
• Extreme position (fully pressed or released): immediate stop of the robot’s movements.

Additionally, standards require that in teach mode, the SLS (Safely Limited Speed) function continuously monitors the robot’s speed. If the set speed limit is exceeded, the system automatically stops the machine, preventing dangerous situations.

Conclusion: The speed of 250 mm/s is not ‘safe’ by itself—its effectiveness results from combining speed limits, safety functions, and protective devices like the teach pendant.

Energy and Force vs. Speed: Why Risk Analysis is the Foundation of Safety

Speed alone rarely poses the primary threat. Kinetic energy—the result of mass and speed—and the force of impact determine whether machine movement is safe. An example? A 5-gram sheet of paper moving at 1 m/s has a kinetic energy of only 0.0025 J and poses no threat. However, a small metal chip weighing 1 gram, striking an operator’s eye at a similar speed, can cause serious injury.

Now consider larger masses:

• A steel object weighing 1 kg, moving at 1 m/s, has a kinetic energy of 0.5 J. If it hits an operator and stops, the energy dissipates upon contact, potentially causing injury but limited to that moment.
• The same object, moving at 0.1 m/s, has a kinetic energy of only 0.005 J, but what if the machine does not stop after a collision and begins to compress the operator? In such cases, the force of impact becomes significant, not just the kinetic energy.

Standards like ISO 14120 consider both parameters when designing moving guards:

1. Kinetic energy – limited to:
   • 10 J, if the guard has additional protective devices (e.g., pressure-sensitive strips).
   • 4 J, if such devices are absent.
2. Contact force – dependent on the presence of protective devices:
   • 150 N for guards with protective devices.
   • 75 N for guards without additional safeguards.

These requirements consider both the risk from impact and situations where the machine continues to press on the operator after a collision. Simply reducing speed often isn’t enough—it’s also crucial that the safety system stops the machine’s movement at the right moment to prevent further danger.

Conclusion:
The safe operating speed of a machine does not exist in isolation from other parameters. Risk analysis must consider kinetic energy, contact force, and the system’s behavior after a potential collision. Technical standards like ISO 14120 provide clear guidelines on designing systems that mitigate risk from both impacts and continued pressure.

Tools and Functions Supporting Machine Speed Limitation

Safe speed is not just about setting the right limit—tools and functions that enable control over machine movement and response in emergencies are equally important. Many technical standards suggest solutions that effectively and safely limit speed.

Safety Functions in Control Systems

Compliance with the Machinery Directive 2006/42/EC requires that machine design considers safety not only in normal operating conditions but also in service, retooling, or maintenance modes. In these situations, the risk of operator contact with the machine is greatest, necessitating appropriate safety functions.

• Functional Safety: A key aspect of design is anticipating emergency scenarios and implementing control systems that allow for risk management at every stage of machine operation. Functions like STO, SLS, or SS1 are mandatory in this context, especially for machines requiring direct operator interaction.
• Service and Manual Modes: In modes where the operator is in the direct work area of the machine, limiting the speed of working movements (e.g., to 10 mm/s for presses or 250 mm/s in robot teach mode) is essential. The SLS function not only monitors speed but also actively prevents exceeding these limits, ensuring compliance with functional safety requirements.
• Design in Accordance with Harmonized Standards: Standards like ISO 13849-1 and IEC 62061 specify safety integrity levels (PL or SIL) that must be met depending on risk analysis. For example, high-risk machines require higher reliability levels for safety functions.

• SLS (Safely Limited Speed): A real-time speed limitation function. It monitors machine speed and automatically stops movement if set limits are exceeded. Used in industrial robots during teach mode.
• STO (Safe Torque Off): A function that disables motor torque, allowing for immediate machine movement stoppage. Particularly useful in emergencies.
• SS1 and SS2 (Safe Stop 1 and 2): Controlled braking—SS1 stops movement and the machine transitions to STO, while SS2 stops the machine but maintains torque, allowing for position holding.

Operator Support Devices

• Enabling grip switches: Three-position buttons used in teach pendants. They enable safe machine control—if the operator presses the button too hard or releases it, the machine automatically stops.
• Pressure-sensitive strips and safety mats: React to human contact, stopping the machine upon detecting pressure. Particularly useful in moving guards.
• Speed and position sensors: Used in the SLS function, they allow precise monitoring of movement speed and detection of limit exceedance.

Conclusion:
Safe speed results from the interaction of speed limits, appropriate safety functions, and tools supporting operator work. Solutions like SLS, STO, or enabling devices minimize risk and enhance safety when working with machines, regardless of their type and application.

Example of a Production Line for Processing Flexible Material – Service Mode

Description of the Situation

On a production line, a machine processes flexible material (e.g., foil, rubber, or thin fabrics). To start production, the material must be manually fed between pressing rollers. The material’s characteristics prevent full automation of this process—the operator must manually set the initial position and then start the machine.

If a hand is placed between the rollers, crushing or serious injury may occur. The risk increases because, in service mode, the machine operates at reduced speed, but this does not eliminate the danger from pressure.

Scenarios for Safe Solutions:

Functionality Based on SLS and STR – Limiting Speed and Torque

Mechanism of Action:

• In service mode, the SLS (Safely Limited Speed) function limits roller speed to a level allowing immediate operator response (e.g., to 5 m/min – 83mm/s).
• Additionally, when setting the material, the only way to rotate the shafts is by using maximum torque control, the STR (Safe Torque Range) function. If we limit the force resulting from the roller diameter to, for example, 20 N (which does no harm), we have a safe service mode solution.

Additional Safeguard:
Starting the machine requires pressing a three-position enabling button. If the operator presses too hard or releases the button, the system automatically stops the rollers.

Advantages:

• Full operator control over roller speed and movement.
• Quick response in case of danger detection.
• Minimal injury risk due to torque limitation.

FAQ: Safe Operating Speed of a Machine