Machine Security Guards

With the advancement of technology for manufacturing and processing, we must not forget the risks derived from the installation, operation and repair of machines towards people.

Hazards appear when parts of the machine move or have the potential to move. The movements can be rotational, displacement, transverse or alternative (eg up and down or back and forth). The different hazards are described below.

Abrasion (or Friction)
It is caused by friction between the skin and a moving part. The risk increases with movement speed and surface roughness.

Cuts
A form of extreme (and deeper) abrasion. The risk increases with the sharpness of the edge and with the inertia (velocity and mass) of the moving part.

Impact
Caused when a body part is struck (but not penetrated) by a moving part. The risk increases with the inertia of the moving part; it can also be influenced by the shape of the element.

Nailing (or Punching)
An extreme (and deeper) form of impact. Caused when a part of the body is punctured by the moving part. The risk increases with the sharpness and inertia of the machine part.

Squeezing (or pinching)
This is caused by two machine parts moving towards each other and a body part getting caught between them. Sometimes it occurs between a moving part and a fixed part (eg wall or structure) or between a moving part and the material being manufactured.

Shearing
Caused when a body part is caught between two intersecting machine parts. Sometimes it occurs between a part of the machine and the material being manufactured. The risk increases with the sharpness of the moving parts and their inertia.

Caught By or Caught Between
Caused when a body part becomes caught by or between moving machine parts. It can occur due to entrapment:

By rotating solid surfaces (eg spindles, axes). The risk is greater if the surface is rough or has projections, eg. screws, bolts; between rotating parts; between moving parts (towards pinch points) parts rotating against each other; parts that rotate and parts that move tangentially; rotating parts and fixed parts.

Damage can occur directly from entrapment or because the body part is drawn into the danger zone where other damage can occur. The risk is significantly increased for people who wear loose clothing, gloves, a tie, or jewelry; with long hair; using cloths to clean.

Other dangers

In addition to those described above, other hazards may be present and must be considered from the perspective of machine guards:

Electrocution Hazard
Any exposed electrical conductor operating at voltages greater than 50V presents an electrocution hazard;

High Temperatures
Hot surfaces or materials may present a burn hazard to exposed skin. This includes surfaces with temperatures >65°C (for metal parts) and >80°C (for glass/ceramic parts), liquids >50°C and vapors at any temperature;

Sharp Edges Sharp
points or edges can cause cuts during operation, cleaning, etc;

Projected materials Projected
materials or parts of the machine can cause damage, particularly to the eyes (if they are not protected). Parts of the machine can be projected due to breakage or because they are not well secured; maintenance tools can also be projected if they have not been removed from the machine before it is put into operation. A material can be projected if its container, pipes or connections fail. Sometimes materials are sprayed as part of the normal operation of the machine.

Stored energy

Even when all parts of the machine are stationary, the machine can still present a risk from stored (potential) energy. Stored energy can take many forms:

Stored mechanical energy (eg due to a jam in a filling or packaging line)

Stored gravitational energy (eg due to a container raised above the floor);

Energy stored by fluids (eg due to pneumatic or hydraulic systems under pressure, or due to vessels under pressure or vacuum);

Stored electrical energy (eg due to electrical charges accumulated in the system).

INTRINSIC SAFETY (BY DESIGN)

Wherever possible, hazards should be eliminated during the design stage, thus avoiding the use of machine guards or reducing their number. Where this is not possible, other modifications may reduce the hazard or access to it.

Avoid or Reduce Hazard

Some examples of this proposal:

Reduce the mass, speed or travel of moving parts; ensure that the rotating parts do not have projections and their surfaces are smoothly polished; reduce the sharpness and increase the radius of curvature of the machine parts; increase the separation between moving parts (or between moving and fixed parts) to avoid risks of trapping or shearing; fill or reduce the size of openings to prevent access to pinch or shear hazards; reduce working pressures and temperatures; reduce voltages.
Reduce Access to Hazards
Some examples of this proposal:
Allow remote adjustments or lubrication so that it can be done without removing machine covers; separate low-voltage control circuits from high-voltage power circuits so that work on the control circuits does not present an electrocution hazard; provide automatic transfer systems for products and components at the entrance and exit of the machine; use transparent materials to allow observation without the need to remove the guard.
Reducing risks through intrinsic safe design principles is always preferable to installing guards, which can fail or be bypassed. However, once the machine is built, opportunities are often limited, which is why special attention must be paid to machine safety during the design stage.

Basic types of machine guards
They come in many forms, including:

Guards , physical barriers designed as part of the machine, or added to the machine, for example wrap-around guards (fixed or mobile), covers, shields, etc;
Presence Detection Devices do not prevent access but stop the movement of dangerous parts before contact with them is achieved, for example light curtains, area scanners and pressure mats;
Protection Devices , do not prevent access or stop the machine but allow a safer operation. For example, “dead man” or “double button” devices;
Alarms and Warnings warn personnel of hazards. They can be permanent (eg warning signs) or activated when hazards appear;
Emergency Stops do not prevent access to dangerous parts or risk, but if activated during an emergency, they can prevent incidents or significantly reduce them.
Before selecting the type of guard, a risk assessment of the machine must be carried out.

Types of Machine Guards

The different types of machine guards are described and illustrated below.

Fixed Guards

They are guards that are kept in place permanently, either welded or riveted or using fasteners such as screws, nuts, etc. They are the preferred type of guards when frequent access is not required. There are two basic types: encircling guards and distance guards.

Enveloping Guards

They are guards that prevent access to dangerous areas from all sides. Wraparound guards are usually fixed but, if access is frequently required, access sectors can be created using appropriate interlocks (see section on interlock systems). A wraparound guard provides more protection than a ranged guard. If protection from noise, dust, or flying materials is required, a solid wrap-around guard would be the best option.

These guards do not completely enclose the danger zone, but due to their dimensions and distance from it, they reduce access. There are two basic types: tunnel and perimeter guards.
Guards by Distance

Tunnel Guards They can be fixed or interlocked. Your openings should be as small as possible to prevent access.

Perimeter Guards They are made up of fixed fences with one or more access doors with interlocks. The space between the machine and the fence should be as small as possible to prevent the doors from closing with personnel inside. If the machine can be started while a person is inside, then additional guards should be considered, for example:

Transparent material for fences and commissioning boards near danger zones to easily verify that no one is inside; One guard with retained key interlock (see below);

Light curtains, area scanners or pressure mats to detect the presence of personnel inside the fence and prevent start-up.

Mobile Guards

They are guards that can be moved, but remain attached to the machine through hinges and brackets. There are three basic types: adjustable guards, self-adjusting guards, and guards with interlocking devices.

Adjustable Guards

They are mobile guards that can be adjusted or that incorporate adjustable parts, in general simple adjustment elements are used, for example wing nuts. Adjustment depends on the operation and is done by the operator. Although they are commonly used on some machines (eg circular saw, drill press) they depend on the operator keeping them correctly adjusted. For this reason, adjustable guards are highly dependent on operator behavior and should only be used where a fixed or interlocking guard is not possible.

They are mobile guards operated by an element of the machine. They usually use a spring or the force of gravity to return the guard to its normal position.
Self-adjusting guards

They are more reliable than the adjustable ones but they can also fail and be defeated easily. As with adjustable guards, they should only be used where a fixed or interlocking guard is not possible.

They are mobile guards that are used together with one or more interlocks. These devices activate when the guard is opened and stop the machine before anyone can reach the danger zone.
Guards with interlocking devices

They are used where access for maintenance or operational reasons must be fast or frequent. The closing of the guard must not restart the operation of the machine, for this a separate control must be activated.

Before installing an interlocking guard, careful analysis should be performed to determine which components must be isolated from what type of energy when the device is activated.

In some machines, moving parts may have very high inertia and take a significant time to stop, or may be designed to complete one cycle before stopping when a stop command is given. In these cases, a normal interlocking guard may not provide sufficient protection and a guard locking device will be required, that is: the interlocking guard has a latching device that prevents the guard from opening before the bolt opens.
Guards with interlock and guard lock

Retained key interlock guards

They are guards with interlocking and guard locking that use a single key to unlock the guard (or access door) and to activate the start-up. The key cannot be removed from the guard until it is closed and locked; similarly, it cannot be removed from start control until the machine has been stopped.

Guards with interlock control

They are interlocking guards that, when closed, start the machine operation. These types of guards are not recommended for general use and should be used only when there is no chance of an operator (or part of their body) being within the danger zone when the guard is closed. In addition, the interlock must have a high level of reliability to avoid sudden starts.

GENERAL CONSIDERATIONS FOR THE DESIGN OF MACHINE GUARDS

Access

Machine guards must be carefully designed to avoid the need to open them for adjustments, lubrication, cleaning or maintenance. In some cases, it may be necessary to modify the machine itself. The following design aspects may be helpful:

Install small guards that prevent access to dangerous parts but allow access to safe parts;
Build the guards of transparent materials (eg metal mesh, polycarbonate), or incorporate vision panels in the guards.
Incorporate small openings for screwdrivers, bits, etc;
Paint the metal meshes used as guards in matt black to allow a clearer view of the components.

Line Cleaning and Clearance

To improve product quality, machine guards must be designed and constructed to minimize the buildup of potential contaminants, prevent spillage, and provide safe and easy cleaning. To minimize dust accumulation, edges and horizontal surfaces should be kept to a minimum; this also provides the security that the guards cannot be climbed or used to sit on.

Containment

If there is a risk of projection of dangerous materials or substances (eg fluids, steam, gases, dust) the machine guards must be designed to contain them. It must be an enclosing guard, built with appropriate materials and with sufficient mechanical strength. Depending on the substances treated, it will be necessary to choose between materials with the appropriate chemical resistance.

Noise

When designing guards for noisy equipment, there is an opportunity to reduce noise emissions. In general, it is best to use wrap-around guards with good seals between the guard and the machine. Where possible, long metal panels that can vibrate or reflect sound should be avoided.

Alternative materials of construction, along with detailed design, must be carefully selected.

Ergonomics

Machine guards must be designed in such a way that they do not impede the work of personnel. To facilitate this, both operators and technicians must be included in the design of the guard. However, it is often difficult to assess the impact of a new guard before it is placed, and it is then necessary to modify the guard once it is installed. Sometimes, as part of the design process, mock-ups of the guards can be built and placed in position for operators and technicians to assess whether or not they find it comfortable to work with. As mentioned before, designing guards that minimize the need to remove them will go a long way.

All guards must, at some point, be moved or removed, therefore, they must be prepared so that these tasks can be carried out manually and easily, considering their weight, size, etc. Handles should be properly shaped, free of sharp edges, and positioned in locations that facilitate handling. If sliding doors or guards are available, they should be easy to open and should not require excessive effort. Additional assistance such as counterweights or springs can be installed, these will require proper preventive maintenance. Where guards are too heavy to move manually, a lifting system should be provided; In this case, the weight and method of handling must be identified on the guard.

bras

Fasteners shall be used to prevent the guards from loosening or moving due to vibrations, using, for example, springs or nuts. To remove a guard, special tools must be used, which will not be manipulated by the operators, therefore “butterfly” nuts, etc., must not be used. Where possible, guards should be designed so that they cannot stay in place without proper fasteners, making it difficult to replace the guard without properly securing it.

machine guard dimensions

If distance guards are used instead of enclosures, it is extremely important that they are sized correctly to prevent access. Below is a summary of the recommendations.

Perimeter Guards

The height of a perimeter guard (b) should be up to 2.5m but should reflect the height of the danger zone (a) and the horizontal distance to the danger zone (c). In all cases, the perimeter guards must be at least 1m high to prevent personnel from “climbing” them. However, if ladders are available in the area, then the guard must be extended, as well as if there are plant elements or equipment that could be used to “climb” the guards. If increasing the height of the guard is too difficult, then a wrap-around guard would be more convenient.

Above Usual Working HeightA small opening may be left for cleaning between the lower parts of the guard and the floor; however, it should be as low as possible and not allow access to danger zones; in any case, this opening must be less than 200 mm.

If the danger zone is 2.7m above the reference plane (floor) then it will not require a guard. However, if there is access to stairs or steps nearby, guards should be placed on the machine or installed higher. In the same way, if there are elements of plant or equipment that can be used to reach the danger zone, then the 2.7m safety distance will no longer provide sufficient protection.

Relationship: size of opening / distance to the point of danger

Holes, slots, and other openings in machine covers or guards are permissible as long as they are small enough (and at a considerable distance) to prevent access to hazardous areas. The closer you are to the danger zone, the smaller the opening must be to prevent access.

DESIGN OF INTERLOCK SYSTEMS

There are two basic types of interlocks:

Mechanical activation devices :
Cam position detector, with rotating or movable guards; Reed Operated Switch; Direct interlock between guard and start-up control; key retained devices; Locked devices.
Non-mechanical activation devices (electric, magnetic):
Magnetic – proximity sensors
Electrical – plug and socket actuator

Why do interlocks fail?

The placement of interlocking guards is an effective way to prevent access to dangerous parts, but incidents often occur on machines with these types of guards. The reasons for this to happen fall into two categories:

Malfunction of interlock systems;
Deliberate cancellation, by the personnel, of the interlocking systems.

Interlock
malfunctions Some common failures of interlock systems are:

Loose or misaligned parts; adhesion of moving parts; poor electrical connections; short circuit of electrical components; spring breakage; corrosion.

The main causes of these failures are vibrations (due to use), deterioration, corrosion, accumulation of dust, liquids, etc. that have not been adequately considered during design or routine maintenance. The following actions can decrease the probability that an interlock will fail:

Use good quality components; Install interlocks securely so they do not loosen or move during use; Ensuring that there is no electronic software in the security circuit (unless it is a security-related system designed for use in security systems); Protect components and wiring from physical damage; Keep the actuator design as simple as possible; Test the operation of the interlocks after their installation; Check the condition and operation of the interlocks as part of a planned maintenance routine and at the beginning of each shift.

Bypassing Interlocks
Deliberate bypassing of interlocks must be strictly prohibited. However, they are occasionally overridden by the staff. In these cases, a robust investigation should be carried out with a view to possible disciplinary action. The interlocks must be designed and installed to minimize their chances of cancellation and, therefore, minimize the risk, for this the following must be done:

Install interlocks using permanent fastening techniques (eg welding, riveting) or fasteners that cannot be removed with common tools;
Place interlocking devices behind covers, guards, etc. So that they cannot be easily accessed;

Types of protection devices

General Approach

There may be rare circumstances where certain tasks on a machine require intervention with interlocked guards temporarily suspended. These types of interventions are extremely dangerous and should be avoided whenever possible. If suspension of interlocks cannot be avoided, then additional controls will be necessary, usually including the use of protective equipment. It should be noted that permanent removal of interlock devices is not acceptable.

Protective equipment allows machine parts to move but reduces risk to the operator. There are two basic ways:

Reduce access to the danger zone: It is achieved by using devices with “Double button ” and/or “Dead man ” devices . These devices allow machine parts to work but only while a control is being operated by someone at a safe distance from the danger zone;

Reduce irrigation in the danger zone: It is achieved by using speed and/or movement limiting devices. These devices allow machine parts to move but only at low speeds or only for a short distance (eg step by step) respectively. They do not prevent access to dangerous areas but reduce risks.

Depending on the circumstances, it will probably be necessary to apply a combination of the above elements in a single protection device. Whichever functionality is selected:

Devices with “Double Button” and “Dead Man”

The devices that use these systems apply the concept of security by distance. They force the operator out of the danger zone in order to operate the machine from a safe location. The device can be fixed to the machine or of the mobile type, attached to the machine by a connection cable. In both cases, they must be carefully designed to prevent the operator from disabling them, for example with actuators that use other parts of the body.

Devices with “double buttons” and/or “dead man” devices provide a measure of protection only to the person who is using them, therefore, additional precautions must be taken to ensure that no one can enter the danger zone, placing for example barriers or warning signs.

“Double Button” Device “Double Button”
devices require the operator to use both hands simultaneously to actuate two separate controls and start the machine. This method is widely used where the operator must be very close to the machine and could, if it were a one-handed control, place their other hand in the danger zone.

If the protection system depends only on the functionality of the “double button” device, the duration of a machine cycle must be so short – and the controls placed at a sufficient distance from the danger zone – so that the operator You cannot take your hands off the controls and access moving parts before they have come to a complete stop. If this is not the case, the controls must be removed from their place and placed at a greater distance from the danger zone or additional protections must be applied to stop the parts faster (eg limit switches, brakes).

“Dead Man” Devices “Dead Man”
devices require the operator to continuously press and hold a control to start and keep a machine running. The machine should stop when the control is released. If the protection system depends only on the functionality of the “dead man” device, then:

The control must be placed at such a distance that the operator cannot reach the danger zone with one hand while holding the control down with the other; Y
The control must be positioned at such a distance – and the machine must stop so quickly – that the operator cannot remove his hands from the control and access moving parts before they come to a complete stop.

If either of these two conditions is not valid, then the controls must be removed from their place and placed at a greater distance from the danger zone or additional protections must be applied to stop the parts more quickly (for example limiters of movement, brakes ).

Guards for Hot Surfaces or Materials

Those equipments that contain exposed hot surfaces or materials must have guards to prevent contact with them. The first point to consider is risk reduction, for example by reducing the temperature or the area/volume of the hot material/surface. If the temperature can be reduced below the burn threshold then no additional protection is required. However, if the temperature remains elevated, then other optional guards should be considered, for example:

Enclose hot surfaces/materials in a separate part of the machine; Apply an insulating blanket (or thick covering of insulation) on exposed surfaces; Install a physical barrier/guard to prevent contact with surfaces/materials.

It must be remembered that it takes a significant period of time for hot surfaces and materials to cool down. For this reason, a simple interlocked guard is not appropriate since the guard can be opened while the risk of contact with the hot surface/material continues.
In all these cases, hot surface/material warnings will be placed on all relevant parts of the machine to alert personnel to potential hazards. These warnings can become very important since the presence of hot surfaces/materials may not be obvious, and appropriate Personal Protection Items (PPE) should be provided.

Alerts and Warnings

Warning Signs
Can be a helpful reminder to personnel of machine hazards and relevant safe procedures. However, they should never be taken as the only control measure, they should be used to complement other control systems such as machine guards, training and supervision. If warning signs are used, they should be in the local language and use standard formats to facilitate understanding; in many countries, these formats are established in legislation or industry codes.

Visual/Audible
Alerts Audible and/or visual alert systems are used to warn personnel:
About a present or imminent risk; or that the equipment is about to be put into operation; or that the machine guards are suspended.
Although some new machines already have an alert system; It can increase safety in old machines or a set of interconnected machines in some situations, for example:
Start-up of a machine with more than one operator; automatic start-up of a machine where personnel may be in the danger zone; suspension of equipment with interlocking in filling and packaging equipment.

EMERGENCY STOPS

Emergency stops must be placed at each operating station and within reach of both hands during any dangerous situation that arises in operation and maintenance activities. The emergency stop circuit must be available at all times, regardless of the mode of operation; remove the hazard as quickly as possible and not create new hazards; override all other functions and operations in all modes; use positive security; include a latching device so that the emergency stop command is maintained until it is formatted by a manual actuation of the device. Formatting the emergency stop should allow – but not start – the machine start-up; be connected to a reliable control system; be clearly identified and easily accessible.

Actuator Design

Emergency stop actuators must be located at each operating station and at any other location where an emergency stop may be required. They must also be designed for ease of operation. Some types of actuators are:

mushroom buttons

Cables It must act under two conditions, when the cable is pulled or when the cable loses tension. The format control must be positioned so that the entire length of the cable can be seen from the control;
pedals . They are not the most recommended, but they may be appropriate in certain applications.

Whatever the type of actuator, it must be red, preferably with a yellow background and the words “Emergency Stop”.

References
Dec. 351/79, Law 19587 – Hygiene and Safety at Work
ISO Standard 12100:2003 “Safety of Machinery – Basic Concepts, general principles for design”
IEC 60204-1 “Safety of machinery – Electrical equipment of machines” Part 1 : General requirements
IRAM 3578 “Safety protections in machinery”

Guards with interlock and guard lock