Safeguarding Against Machine Hazards

Sept. 1, 2009
The repercussions of inadequate machine guarding and safety can be dire, ranging from fatal accidents to a wide range of injuries as well as costly and time-consuming damage to production equipment.

As today's manufacturing industry increasingly relies on efficient yet potentially dangerous equipment, worker safety has become a predominant issue to all those involved with production processes. Employees must be protected against machine-related injuries that could result in disfigurement, amputation or even death.

To protect employees and safeguard against machine hazards, appropriate machine safety devices and safeguards should be used. Safeguarding refers to the requirements, methods and solutions put into place to protect people who operate or come in contact with potentially dangerous machines from mostly preventable occupational injuries.

Safeguarding benefits both workers and manufacturers by reducing lost work days due to injury. In a report on nonfatal injuries and illnesses for 2006, the Department of Labor reported that the manufacturing sector had 200,970 instances that resulted in days away from work. Within that sector, installation, maintenance and repair occupations had 13,620 reported cases, and production occupations had 126,670, in which the average (median) number of absentee days per case was seven to eight days. Of those two occupational groups, nearly 25% of all employees were absent more than 31 days after the injury. In addition to minimizing injury-related expenses, safeguarding can improve overall productivity and worker morale, lower liability for employers and exhibit employers' compliance with safety standards to appropriate regulatory agencies.

Safety Strategy and Design

In developing a strategy to improve machine safety, the entire machine, manufacturing process and even the habits of workers must be taken into account to develop appropriate safety design and solutions. Machine safety can be thought of as an umbrella under which many aspects fall. These aspects of safety can include everything from machine design (eliminating hazards when possible), safeguarding hazards, warning signage and signals, personal protection equipment (PPE), supervision and training and installation and maintenance. A risk assessment is a powerful tool to gather all the items under the umbrella, and when referencing machine safety standards and regulations, can achieve the goal of creating a safe working environment.

One aspect under the safety umbrella is safeguarding. As defined by ANSI B11.19, “Performance Criteria for Safeguarding,” safeguarding is the “protection of personnel from hazards by the use of guards, safeguarding devices, awareness devices, safeguarding methods or safe work procedures.”

The most commonly used of the above solutions are guards. Guards are physical barriers that prevent exposure to an identified hazard and include fixed or hard guards and adjustable and interlocked guards. The next most frequently used solution is safeguarding devices, which detect or prevent inadvertent or intentional access to a hazard. Safeguarding devices vary widely and can be application specific but include moveable barriers, pull-backs or restraints, presence-sensing safeguarding devices (e.g., safety light screens, area scanning systems, optical systems and more), two-hand control devices, safety mats and edges, probe detection and single control.

Interlocking Switches

For applications that require a simple and permanent type of safeguarding used around the hazard as a barrier to prevent access or contain a dangerous hazard, a fixed guard can provide an ideal fit. However, since fixed guards don't move, they may need to be disassembled for maintenance and repair issues, which can result in the guard not being reinstalled. Fixed guards shouldn't be used if personnel require moderate to frequent access to whatever is behind the guard. For these applications, an interlocking guard can be employed.

Interlocking is a safeguarding solution used to monitor the position of a guard or gate. The interlocking device can be used to de-energize circuitry or actuators, control personnel access and prevent the machine from starting when the guard is open or not in place.

The means of interlocking include mechanical, pneumatic and hydraulic systems, but the most frequent is probably an electro-mechanical switch with “positive-opening” operation. “Positive opening” refers to the design of the switch in which operation (i.e., moving or opening the guard) results in a direct physical force opening the normally closed electrical contacts.

Light Screens/Curtains

Optical-based safeguarding devices typically are comprised of two basic components of optical technology: an LED and phototransistor. The most common optical safety system is the safety light screen (also referred to as a safety light curtain), an optoelectronic device used to guard a machine hazard. Safety light screens contain an LED emitter array and corresponding phototransistor receiver that together create a sensing field with a specified detection capability. If all the phototransistors detect light from their corresponding LEDs, the light screen is “complete,” and the safety outputs allow the machine to continue operation. When an opaque object, such as a hand, interrupts a light beam, the light screen sends a stop signal to the safety-related controls of the machine, which react immediately to stop the hazardous motion prior to the hand reaching the hazard.

Safety light screens often are used to guard high-access applications, such as a hand-fed process or an application where regular maintenance is required, since they allow frequent access to an area and can be positioned in relatively close proximity to the hazard. Additionally, safety light screens can be less expensive to install and maintain than hard guards, and they still allow for protection of multiple individuals while also permitting good visibility of the hazard.

The International Electrotechnical Commission IEC 61496-1/-2 is the international standard for design and construction of light curtains, and specifies two types: Type 2 and Type 4. Type 4 light curtains typically are used for high-risk applications and comply with OSHA and ANSI requirements for control reliability because of the redundancy and automatic self-checking circuitry required of this type of device.

Safety light screens offer numerous features and should be selected according to application requirements. For example, light screens vary greatly in detection capability (also known as resolution or minimum object sensitivity), which determines the ability of an optical system to reliably detect an object. Safety light screens are grouped based on this sensing potential. High-resolution light screens can detect a finger or hand, while medium-resolution light screens can detect an arm or ankle, and low-resolution light screens can be used to detect a torso.

When selecting a safety light screen, also consider the size and mounting options; the cascading capabilities to safeguard multiple areas or sides with a single system; the sensing range; reduced resolution and fixed blanking to ignore objects such as tooling; selectable automatic or manual reset; and the capability for external device monitoring.

Two-Hand Controls

Another common safeguarding device is a two-hand control, comprised of two actuating buttons (or hand controls) and specific logic used to start and control a machine cycle when an individual concurrently actuates both buttons within 0.5 seconds. In doing so, two-hand controls provide a safety function for the machine operator by physically occupying both of the operator's hands and keeping them out of the hazardous area when the machine is activated. Since two-hand controls only protect the individual who is operating the buttons, additional two-hand control devices or other safeguarding is required for other individuals.

A two-hand control safeguarding system is comprised of two basic elements. The hand controls typically are mechanical palm buttons, touch-activated photoelectric devices or capacitive touch switches. Ergonomically designed touch buttons requiring little or no physical pressure to operate are gaining popularity to eliminate hand, wrist and arm stresses associated with old-style mechanical pushbuttons.

The control logic should be provided by a safety module, safety controller or safety PLC. Many accidents have been reported that resulted from a standard PLC monitoring the palm buttons.

Emergency Stops

It should be noted that an emergency stop device is not considered a safeguarding device, as described above but fulfills a safety function as a safety device under the heading of complementary protective equipment. This is an important distinction because an emergency stop device, such as a button or rope/cable pull, should not be used in place or as an alternative for proper safeguarding of a hazard. As described by ANSI B11.19, “a safeguarding device detects or prevents inadvertent access to a hazard, typically without overt action by the individual or others. Since an individual must manually actuate an emergency stop device to issue the stop command, usually in reaction to an event or hazardous situation, it neither detects nor prevents exposure to a hazard.”

Among the requirements, an emergency stop device must have a red-colored actuator and, when possible, a yellow background, such as in the case of a palm button. The device also must have a positive-opening design (similar to safety interlocking switches). The emergency stop function must be initiated by single human action, override all other functions and operations and remove power to the machine actuators as quickly as possible without creating additional hazards. Resetting the emergency stop device (or function) must not restart or otherwise initiate a hazardous motion or situation.

Modules and Controllers

Another family of devices that fall under the heading of complementary protective equipment is safety modules, safety controllers and safety PLCs. Safety modules interface safety-related devices, such as the safety solutions described above, directly to the machine control, providing an increased level of safety functionality with self-checking logic. They can identify faults within a safety-related device or circuit, as well as monitor their own circuitry and outputs, simplifying the task of machine design, while also increasing the level of safety. Safety modules tend to monitor one or two safety or safeguarding devices.

For incorporating numerous input devices and managing multiple safety-related functions, a configurable safety controller can be used. Generally, easy-to-configure safety controllers can replace multiple dedicated safety modules to integrate E-stop buttons, interlocking switches, safety light screens, two-hand controls and many other devices, providing a cost-effective safety solution that protects against costly damage to machines as well as injuries to personnel.

Machine safety is everyone's responsibility, from the machine designer, to the installer and integrator, to the operator and maintenance personnel and, ultimately, the employer. There are a number of solutions for every problem and many resources to help achieve the goal of a safe working environment. Rely on trusted sources, such as OSHA, the ANSI standards that cover your particular situation and the many well known suppliers of safety and safeguarding equipment.

Mike Carlson has been employed in the machine safeguarding field for 20 years. He has worked with Minneapolis-based Banner Engineering for the past 12 years, where he develops and promotes safety products and the education of safeguarding solutions, safety product literature, sales brochures and Web-based information. Carlson is involved in the development of several safety standards recognized by ANSI.

This article originally appeared in MHM's sister publication, EHS Today.

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