Stand on guard: Selection and use of safeguarding devices
November 16, 2007
By Ian Brough
Machine guarding is an important task, but it can also be a daunting one because of the many standards and processes involved. Selecting the proper type of guard for your application must begin with the basic understanding of the purpose of a safeguard. Simply put, a safeguard is designed to prevent a person, operator or automated equipment from reaching a pinch point or hazard on a machine that could cause injury.
TYPES OF SAFEGUARDS
Barrier guards or fencing
Barrier guards or fencing are mechanical barriers that are either fixed or moveable. These types of guards can be inexpensive and provide protection against flying parts. Mechanical barriers or guards are typically made of sheets of wire mesh, metal or see-through plastic. Plastic, however, is prone to fading over time and may need to be removed or dismantled. Barrier guarding usually requires a minimum amount of maintenance and is best suited where repetitive access to the machine is not required.
If the barrier (i.e. a gate) slides open, a method of signalling that the gate is open is required. You can use a safety-rated switch, commonly referred to as an interlocking switch, to provide a stop signal if the gate is opened. Interlocking switches can consist of Type 1 devices that have a lever arm or plunger built in. In a Type 2 device, the actuator or key is a separate piece that fits into the switch to actuate a cam that will mechanically pull apart the contacts inside if the key is pulled out. This guarantees the contacts of the switch will not stay welded shut.
Another type of interlock or gate switch is the locking type. This has mechanical or electrical locking mechanisms to prevent the gate from opening until it is safe to enter the guarded area. One example is where a machine cannot come to an immediate stop once signalled to do so, such as a saw blade or a motor with no braking mechanism.
Fencing can be inexpensive to implement and provides one of the best methods of protection; however, it can prove to be a hindrance when access during the cycle of the machine is needed. A gate gives the operator the ability to bypass the fence and access the hazardous area, which can be dangerous. In this case, a light curtain can provide easier, safer access to the machine.
Commonly referred to as light curtains, optical guards can be divided into three groups–point-of-operation (light curtains), perimeter guards (only a few beams) and area guards (continuous detection in a given area).
The defining characteristics of these types of solutions are resolution and range. Typically, the standards call for these devices to be separated into two categories: devices with a range less than 64 mm or devices with a range greater than 64 mm. Resolution refers to the smallest object the optical guard can reliably detect and affect the depth of penetration (Dpf)–how far you can get your fingers or another object onto the light curtain before it is detected. For example, a 14-mm resolution device has a Dpf of around 19 mm, whereas a 30-mm resolution gives you a Dpf of 70 mm. Range is affected by resolution; the smaller the resolution the shorter the range. Typically, a 14-mm resolution light curtain will have a distance of up to six metres between the sender and the receiver. Resolutions of 20 mm and up can have a distance of up to 19 metres between the sender and receiver.
• Point-of-operation protection: Typically, a light curtain will have a resolution that is guaranteed to detect the entry of an object of a given size. Common applications for point-of-operation protection are repetitive access points where operators’ hands are constantly removing parts or inserting parts before the next machine cycle is initiated.
• Perimeter guards: Perimeter guards are ideal for enclosing large robot work cells such as laser turret machines, as well as guarding the entrance or exit to a machine or work cell. A perimeter guard’s main purpose is to prevent access of the whole person, not just a hand or limb. It is similar to a light curtain but has anywhere from two to 12 beams, which are spaced 100 or more millimetres apart. Since the major role of a perimeter guard is to provide an “optical fence” around an area to be guarded, this usually covers a long distance–70 m or more between the sender and the receiver. Depth of penetration for a perimeter guard is calculated from the maximum distance a person could reach his/her arm in between the beams or over the top beam. The minimum distance is one metre.
• Area guarding: Area guarding is used when continuous detection within the work cell or hazardous area is required. An area scanner is a laser-based scanning device that will detect the presence of an object if it is within the scannerÃs pre-configured area.
Scanners work horizontally or vertically and can cover an area up to 270-degrees with a range up to a seven-metre radius. Using the software for these devices, the area of coverage can be configured to any shape. Historically, this method of detection was done with safety mats. Since safety mats have a slower response time than an area scanner, the Dpf factor is calculated from how far a person, standing at the edge of the detection area, could reach over the guarding field with his/her arm. This is typically one or 1.2 metres.
Ian Brough is a safety specialist with the SICK Safety Systems Division in Dundas, Ont. He is a regular attendee of the standards committee held by the Robotics Industry Association, and is a frequent speaker about machine guarding and basic safety circuit design at conferences in Canada and the United States.