Safety devices in mechanical engineering

Work accidents when handling machines can never be completely ruled out – but can be significantly reduced by appropriate measures. Guards on machines and equipment prevent access to hazardous motion or areas. Not only do they protect against direct hazards, but they also make manipulation difficult and thus increase operational safety. The Machinery Directive 2006/42/EC as well as standards such as DIN EN ISO 12100 and DIN EN ISO 14119 recommend the use of such safety devices already in the design phase. In this article, you will get a compact overview of different types of protective devices and learn which solutions are suitable for your applications.

Safety devices on machines

The ultimate goal of protective devices is to protect people from hazards. Safety and security devices play a central role, especially in industry, in order to prevent workplace accidents and comply with legal requirements. The Machinery Directive 2006/42/EC requires manufacturers to design machines to minimize risk and integrate required safeguards. You can also find more information on this in our article on safety standards in mechanical engineering.

Protective devices are not to be understood as an option, but as an integral part of the machine design concept. They must be designed according to the principle of integrated safety, i.e. technical protective measures are preferable to subsequent risk reduction by means of control interventions, hazard warnings, personal protective equipment or other measures to be taken by the operator.

Types of Safety Devices

Basically, a distinction is made between separating and non-separating guards. The selection of the appropriate type of protection depends on the hazard analysis, the frequency of the intervention and the ergonomic requirements for operation.

Separating Guards

According to the Machinery Directive, a separating guard is a machine component that provides protection through a physical barrier. It prevents personnel from entering the hazardous area of a machine or equipment.

Separating guards are divided into:

• Fixed guards
• Movable guards

A fixed separating guard is, for example, a machine guard grid or safety fence in industry. Fixed means that the guard is securely connected to the machine. It is important that the protective device does not interfere with the machine's operation, as it remains a fixed part of the machine even during production. To visually delineate the protective device, it is usually given a colored coating. In addition, the protective device must be made of a robust material.

There is no access through a fixed protective device, so it must be carefully considered whether access to the hazardous area is necessary and how often. If access does not have to occur regularly, a fixed separating protective device may be used in accordance with DIN 14120. To prevent manipulation, this may only be removable with special tools during production downtime.

Movable separating guards are also permanently fixed to the machine, but allow regular access to the hazardous area. For this purpose, they are also recommended by DIN 14120. A special tool is not necessary for opening movable guards. They are usually implemented as flaps or doors.

Non-separating guards

According to the Machinery Directive, any device without a separating function is considered a non-separating protective device, which either reduces the risk on its own or, in conjunction with a separating protective device, ensures that persons do not gain access to the hazardous area of a machine or system.

Non-separating safeguards are divided into:

• Non-contact protective devices (BWS)
• Contact-actuated or pressure-sensitive protective devices
• Protective devices with location dependency

The non-contact protective device uses sensors to detect when a person or body part enters the defined protective area. This does not require direct mechanical contact. Their mode of action is based on different physical principles. Acoustically, for example via ultrasound, it is determined whether people are moving in the hazardous area. The optical principle uses light curtains, digital cameras, etc. Inductive proximity switches are suitable for electrically conductive objects and a capacitive spacer sensor is recommended at relatively short distances.

All non-contact protective devices have in common that they consist of a sensor, a control or monitoring function and output switching elements. These elements cause the safety function to trigger when the control or monitoring function responds. Requirements for non-contact protective devices are specified in DIN EN IEC 61496-1. If you would like to learn more about sensors and their importance in automation technology, please also read our article Sensor technology - selection and importance for automation technology.

Pressure-sensitive or contact-actuated protective devices trigger the safety function due to elastic deformation. This is realized, for example, by means of switch mats or switch strips. When a person steps onto the mat, internal signal transmitters detect deformation in a hollow body, resulting in tripping. It should be noted that the corresponding standards only include persons from a body weight of 20 kg upward.

Location-dependent protective devices must in most cases be installed outside the hazardous area. This type of protective device is triggered by hand or foot, e.g. two-hand pushbuttons, foot switches or two-hand controls. The operator is also outside the hazardous area due to the positioning of the protective device. In the case of two-hand switching, the hazardous function is only triggered, for example, if the operator initiates it by actuating two control devices. At this point, however, it is important to note that a two-hand control only protects one person. If several people are working on a machine, a two-hand function must also be set up for each of these people. The following principles apply to two-hand controls:

• Both hands are used.
• When a control unit is released, the hazardous function also stops.
• Accidental actuation and bypassing are excluded.
• The two-hand control is permanently out of the hazardous area.

Supplementary protective measures

In the event that an emergency situation does occur, machines and systems with a risk source have an emergency stop or an emergency off function. Actuating the emergency stop stops the machine functions, and actuating the emergency off completely disconnects the machine from the electrical energy source. However, emergency stop and emergency off are by no means a replacement for protective devices, but are always only to be understood as supplementary measures.

MISUMI offers a variety of switches, switch covers and related tools to help design your safety device.

Safety Devices and Standards

Protective devices are comprehensively standardized, including in the Machinery Directive (Directive 2006/42/EC). It regulates, for example, that fixed separating protective devices should only be loosened and removed with tools or that non-separating protective devices must be designed in such a way that they cannot start moving parts.

In general, the following requirements can be derived for safety devices:

• Rugged construction to withstand mechanical stresses.
• Secure attachment to prevent tampering.
• Do not create additional hazards yourself.
• Ergonomic design, no restriction of machine operation.
• Easy maintenance without the need for disassembly.
• Unrestricted visibility of the work area.

Safety distances

For safety devices, defined safety distances apply, as defined in standards such as DIN EN ISO 13854 and DIN EN ISO 13855. Particularly with separating guards that have openings, ensure that the distance to the hazardous area is large enough to prevent reaching in. Height also plays a crucial role – it must be chosen in such a way that crawling underneath is prevented. In many cases, a floor distance of up to 180 mm between the floor and the lower edge of the protective device is sufficient. In the case of fixed-location protective devices – such as those with a flexible cable connection – additional cable length limits must be observed to avoid unintentional pulling into the hazardous area.

The required safety distance can be calculated according to DIN EN ISO 13855:2010 according to the following formula:

Challenges and manipulation of safety equipment

Guards must be designed to be effective, tamper-proof, and user-friendly, according to the Machinery Directive. However, they are sometimes perceived as a hindrance in everyday work – for example, to make work easier, to increase production speed, or to gain better insight into the process. In such cases, there is a risk that protective devices may be bypassed or overridden. For prevention, tamper-proof screws and nuts can be used that can only be loosened with special tools. You can also read our article Safety screws - Tamper-proof screws.

Additional measures, such as concealed interlock systems or coded safety switches, also increase safety by making tampering physically more difficult. The use of non-resettable safety seals as well as the logging of safety door openings via the control system (e.g. via a PLC) are also proven means to detect and prevent unauthorized intervention.

In general, the following applies: Careful integration of the protective devices into the work process – such as ergonomic design or intelligent sensor technology – makes a decisive contribution to acceptance and the prevention of tampering.