Designing a safe, highly productive system

Tips for workplace wellness

May 30, 2002
By: Steve Freedman

The best way to make sure that machine operators can do their jobs effectively with proper safeguarding is to desgin machine systems around those safety devices in the first place.

Here's a good rule of thumb for shop system design: Start thinking about safety early.

One of the most frequent mistakes system designers make during the design stage is to think only of the production process and ignore that the system eventually must be safeguarded.

The result is that operators often find that safeguards inhibit their ability to perform their jobs efficiently, resulting in reduced productivity. Many operators therefore bypass the safeguards to keep a steady production pace or to troubleshoot equipment. This literal go-around has become one of the most frequent causes of machine-related accidents in automated production today.

Training operators in proper machine use is critical to preventing this; however, the safest systems are those for which the required safeguards are considered simultaneously with the production system design. Such systems are allow personnel to perform their jobs safely and easily with no need to bypass safeguards. As a side benefit, productivity often can be boosted dramatically while helping to eliminate injuries.

Proper System Design

Assessing Risk. Properly designing a safeguarding system is not a simple task. It requires a clear understanding of the system's inherent hazards. This is commonly gained through a formal risk assessment process that identifies and documents all production and non production tasks and the hazards associated with them. The hazards then are classified according to criteria such as severity of the potential injury, frequency of access to the hazard, and the possibility of avoidance.

Risk assessments should be performed during the design phase and before commissioning to ensure that no new hazards have arisen in the integration process.

Evaluating Safeguards. Once you have identified and classified the hazards, getting up-to-date knowledge of current safeguarding technologies is crucial.

Access. The first thing you should consider is whether or not access to the equipment is required for production, maintenance, or any other reason. If access is needed infrequently, or if there is a need to contain loose or flying parts, then fixed barriers with safety interlocks on the access gates should be used.

If there is no risk of loose or flying parts—or if access is required more frequently—then using presence-sensing safeguard devices may be better. These devices include safety area scanning devices; safety mat systems; and light grids that can detect finger, hands, and other body parts.

Blanking. Many of today's modern safeguarding devices have features designed to enhance productivity safely. Features such as blanking allow material to flow through a portion of the protected field while guarding the rest of the field. This affects the proper safe mounting distance of the device from the hazard but can be a flexible way of safeguarding. You even can configure tolerances for certain object sizes or positions.

Muting. Muting a safeguard in a control-reliable way can be another important productivity enhancement.

Most safeguarding equipment manufacturers offer ways for operators to detect materials and allow them to pass through the entire safeguard field while preventing operator access to the hazard being guarded. This can be difficult for users of safety equipment to accomplish on their own using relay logic alone.

Multiple Devices. It is important to note that the best and safest solutions often combine multiple types of safeguarding devices. Further consideration also should be given to proper training and awareness methods, such as warning signs, alarms, and flashing lights, to keep personnel out of hazardous areas. Training and awareness devices should not be the only safeguarding methods, but rather should supplement automatic detection that does not depend on an operator's deliberate action.

Many advances have been made in safeguarding technology that allow for greater flexibility in safety system design and integration. Designers should be well-versed in the productivity-enhancing aspects of safeguarding. Functionality—such as presence-sensing device initiation (PSDI), which allows the safeguard to perform double duty by protecting operators and initiating the machine cycle—has improved productivity for many companies significantly while reducing the ergonomic repetitive stress associated with a separate cycle-start button.

It always is advisable to select safety components and interfaces that have been third party-certified to local and world standards by an accredited agency to ensure their intended functionality.

Applying Safeguards. Once you have decided on a particular product, you must know how to apply the safeguards safely.

For instance, you must understand safety distance, or the distance from a hazard that the safeguard must be mounted to ensure that the hazard ceases before somebody can reach it. Safe-distance calculations should be made at the initial commissioning of the system and periodically thereafter to ensure that mechanical wear on the system over time has not caused the hazard's stopping time to increase, thereby increasing the required safety distance.

You also must be very careful to interface safeguards with the machine control and e-stop circuitry properly.

Depending on the risk assessment or the standards for the type of machine being safeguarded, safety circuits may have to be designed in such a way that component failures will not prevent hazardous motion from stopping and that restart will be prevented until the fault is corrected.

This obviously requires a complex circuit design. Using prepackaged safety relays (DIN rail-mountable, prewired circuits) is an easy way to reduce safety circuit design time and wiring errors. Safe programmable logic controllers and safe bus networks also are becoming popular ways to reduce safety wiring and troubleshooting costs for companies on the forefront of safety technology.

So how should a company handle these complex issues surrounding the safety of their personnel and ensure that their systems are designed properly? Many companies develop this expertise internally, while others turn to outside safety consultants and manufacturers of safeguarding systems for their expertise rather than trying to keep up with the rapidly changing technologies and industry standards.

Either way, the goals of providing a safe environment and enhancing productivity are better achieved when considered together.

Steve Freedman

Contributing Writer
6900 West 110th St.
Minneapolis, MN 55438
Phone: 952-941-6780
Fax: 952-941-9287
Safety Systems Division manager at SICK Inc., a supplier of safeguarding systems. Freedman has helped many global corporations develop safeguarding strategies and is a voting member of the ANSI/RIA R15.06 Subcommittee on Safety for Industrial Robots and Robot Systems.

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