How shops can manage the complexities of machine safeguarding
October 9, 2012
Applied correctly, safeguarding can add to a company’s bottom line by preventing injury, improving the manufacturing process, and optimizing the overall shop environment.
Machine safeguarding is becoming an increasingly intricate proposition for today’s manufacturer. Complex manufacturing and safeguarding technologies along with numerous safety standards and regulations challenge shop operators who, above all, must squeeze out increased productivity in a thorny business climate. For these reasons, any machine safeguarding project must follow a few basic steps.
Successful safeguarding begins with assembling an effective project team that draws experience from various viewpoints and areas of expertise. At the minimum, this team should include a representative from upper management, the floor supervisor, a machine operator, and an experienced outside safety systems integrator. Each plays a specific and independent role in the overall safeguarding process.
Upper management must commit the proper emphasis and resources to the safeguarding project. Without buy-in from the top, any safeguarding program is doomed to fail from insufficient support and funding.
The floor supervisor knows how things actually work in the shop. Many safeguards, once implemented, impede workflow. That’s because the numerous operational variables—best understood by the supervisor—were not distilled into the safeguarding system design. The floor supervisor must voice his opinion about whether or not a safeguarding scheme will impede or enhance throughput.
More than anyone else, machine operators understand the intricacies of each machine and manufacturing process. Their input is invaluable. Ultimately, they are the ones who must use the safeguarding system successfully and not be compelled to circumvent it.
Finally, the safety systems integrator brings expertise in the applicable standards and regulations. This person also has the technology and product knowledge necessary to develop the best safeguarding for the applications at hand. Most safety integrators can also be relied upon to do a professional and correct installation and retrofit.
A risk assessment must systematically consider and document every task associated with a machine and its operation. The assessment team then translates this information into a detailed matrix that identifies each possible hazard exposure and assigns it to an ascending grading scale from tolerable to intolerable risk.
Tolerable risks typically occur infrequently and result in minor or negligible severity of harm. Most tolerable risks can be safeguarded by vigilant training and detailed written procedures.
Intolerable risks are deemed as such for various reasons. For instance, a risk’s severity of harm may be small, but its potential frequency of its occurrence is high, thus creating a degree of intolerable risk. Conversely, another risk may have a low potential for occurrence, but its potential harm could be catastrophic. In general, intolerable risks can cause lost-time incidents, which can be extremely expensive, both in direct and indirect costs. These incidents include amputation, dismemberment, and sometimes death.
It is the intolerable risks that keep company owners up at night, and a successful safeguarding program can mitigate these risks. After the risk assessment, the team collaborates to translate the risk matrix for each machine into an actual safeguarding roadmap.
The following are 10 critical questions that can help the risk assessment team construct an effective, functional safeguarding program that maximizes operator safety, machine productivity, and the dollars invested.
1. Is the application best safeguarded by area or point-of-operation guarding?
This is first on the list because it is a fundamental element of effective safeguarding. The team must decide if they are trying to keep personnel away from a hazardous area, or protect an operator that must physically work within a hazardous space.
Area safeguarding can apply to various work areas, including robotic cells. In the operation shown in Figure 1, the operator loads the part, steps out of the working area, and activates the machine. There is no need for him to be near the hazardous movement of the operation. An area safeguarding device surrounding the work envelope prevents the machine from activating when there is an intrusion in the working area. In other words, area safeguards ensure the operator is out of harm’s way.
The press brake is a classic example of point-of-operation safeguarding. An operator typically must hold the part in the working area to support it while the press brake is cycled. The press brake’s very nature requires the operator to be close to the machine’s hazardous motion. While the ram is open and creates a hazard, the point-of-operation safeguard prohibits machine movement when the operator intrudes near the working area. When the ram is closed and no longer presents a hazard, it allows the operator to access the working area.
2. How many operators come in contact with intolerable risks?
A risk assessment must identify who works with the machine and how they interact with it. Some people, such as passersby or personnel delivering material, fall into the tolerable risk category and usually are not in the group of people that the safeguarding scheme is designed to protect.
The machine operator, on the other hand, falls squarely in the group that must be protected. In many operations, an operator needs a helper to manipulate the part through the process. That helper needs to be protected as well. The assessment team must observe every action people make with the machine to determine how the safeguard will affect each task and whether or not everyone will be protected.
For example, a dual-palm-button operating station can be the primary point-of-operation safeguard on a stamping press when run by a single person. However, if a large part requires two people to place it into and remove it from the die, then two dual-palm operating stations are required—one for the operator and another for the helper.
3. What are the characteristics of the parts fabricated by the operation?
Are they small parts, large parts, floppy parts, curved parts, heavy parts, complicated parts, simple parts? Just as it did with the individuals who interact with the machine, the assessment team needs to consider the safeguard’s effect on the part families the machine will process.
Again, a good safeguarding system should be easier to work with rather than around. Consider a press brake operation. If the operator bends small parts, he works right up against the tooling, coming in direct contact with the point of operation. When making larger parts, the operator can be farther away from the hazardous area. Each part creates a different safeguarding challenge.
If a light curtain protects workers on a press brake that processes both large and small parts, the safeguarding must incorporate two different features. First, to make the small parts, the ram must stop at 0.25 in. or less above the part, at which point the light curtain mutes to allow the operator to safely manipulate the part within the guarded area. Second, the safeguarding may require beam blanking for large parts, because the workpiece may interrupt the light curtain as it extends out from the point of operation. In this instance, the safeguard must have both features to ensure safe and productive operation. If either feature is left out, the operator in all likelihood will work around the safeguard.
Quite often an operator bypassing a safeguard is a telltale sign that team members did not consider the part being made when they designed the safeguarding scheme. Incidents happen when safeguards are bypassed.
4. What are the tooling setup characteristics for the parts being made?
The risk assessment team should consider all possible tooling setups. Behind part configuration, poor tooling setup is the second-most prominent reason operators bypass a safeguarding system. If a tool can’t be used unless the guard is removed, the assessment team is to blame.
When calculating safe-distance requirements, the team needs to measure from the pinch point closest to the operator. For example, if a stamping press die set hangs outside the bolster, the closest pinch point may also be outside the bolster. If the safe distance were measured from the machine frame without this tool in place, the assessment team would not have calculated the correct safe distance. This creates a more hazardous situation than an unguarded machine. That’s because the operator trusts a system that in fact won’t stop the machine in time to prevent an injury.
5. How many tool changes are made per shift?
The safeguard system should not impede or prolong the tool-change process. The principles of “lean equals safe” are no more apparent than when handling tooling. Less handling of a tool during changeover directly reduces exposure to hazards. A setup reduction effort for operations with frequent tool changes results in a safer operation. It follows then that safeguarding for a machine with permanently installed tooling is much simpler than for a machine in which tool changes occur 10 times or more per shift.
Multiple parts and tooling setups are the main reasons a machine safeguarding system must be customized (see Figure 2). An all-purpose, off-the-shelf, generic safeguard typically cannot provide for all variables created by multiple parts and tools. Ultimately, a correctly designed safeguarding system must account for actual machine usage. Specifically, it must allow operators to maintain their productivity while eliminating any need to bypass the safeguard.
6. What is unique about the manufacturing process?
The assessment team should look at the bigger manufacturing picture. The team should understand why an operator does certain activities and what unique circumstances make these activities special. Not only can a machine’s process be made safer, it can be made more efficient. A leaner process is a safer process.
7. What changes can be made to the manufacturing process to make it safe?
Adding a safeguarding system always changes an operation, but there is no reason those changes should not make the process more efficient. Does the operator perform actions that increase his exposure to hazards, and are those actions caused by a deficient process at a previous machine? Can certain changes upstream help create a safer operation?
Consider an operator of a stamping press outfitted with an automatic feeder. The operator must reach into the working area continually to shovel slugs into the scrap bin. Here, an improved scrap removal system not only eliminates the need for this hazardous activity, it lowers the labor cost associated with this process (see Figure 3). The operator then can spend time doing something else more productive.
8. What is unique about the machine?
A machine’s power source, control system, stored energy, and stopping speed are of particular interest when integrating a safeguarding system. Quite frequently a machine’s optional features require special safeguarding configurations. Even machines of the same make and model can be different.
While there may be many different safeguarding methods for the machine being evaluated, its distinctive characteristics may limit the safeguarding alternatives. Application knowledge and understanding the requirements to correctly interface the safeguard in a control-reliable fashion are vital. They are keys for developing a safeguard that is easier to work with, rather than work around.
9. What happens when a stop signal is given to the machine?
Most important in defining which machine safeguarding method to use is determining what it takes to stop hazardous motion. The assessment team must perform a stop-time test to determine the machine’s reaction time after receiving a stop signal (see lead image).
The longer a machine takes to stop, the less likely electronic safeguards such as light curtains can be used. A machine that stops in 100 milliseconds results in a safe-distance factor of 6.3 in. (without adding an object’s depth of penetration through the light beams); a machine that stops in 500 ms results in a safe distance beginning at 31.50 in. from the point of operation.
Consider a mechanical fourslide machine application in Figure 4. Pressing the e-stop button removes all electrical and pneumatic power. The stored energy causes the machine to coast to a stop in approximately 2 seconds. To safeguard with light curtains would require extensive, prohibitively expensive modifications. So instead, the team opts for an enclosure that has access doors interlocked with solenoid latching switches. These switches keep the doors locked until the machine motion stops.
10. Would it be better to replace the machine rather than upgrade it?
Like any other business expenditure, all safeguarding must be justified financially. If adding a machine safeguarding system costs more than buying a newer, more user-friendly, and easier-to-safeguard machine, the answer is simple. Unfortunately, that is rarely the case. Usually the machine is a shop workhorse that is nowhere near the end of its usefulness. This can make a properly designed and installed safeguarding retrofit very cost-effective.