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Sat, Aug 23, 2014

Why should I measure Stopping Time and Distance?

Why Measure Stopping Time?

Where the safety distance is small, one can for example sit close to the machine and work, as in the picture on the left. If the safety distance is greater, it may be necessary to approach the machine to intervene, and also perhaps use additional protection to prevent starting when someone is within the protected distance, as in the picture on the right.

...to find out which safety arrangements can be used in a certain area around a machine and where they should be located!

Stopping Time

The safety distance (how far away from the risk area a safety component must be placed) is based upon the machines stopping time. The basic idea is that a safety component should be placed so far from the risk area that it is not possible to enter the area before the machine has stopped.

The stopping time for manually operated machines is especially important when light beams and light curtains are used as safety components. By reflex action, the operator tries to grab or adjust if something has gone wrong in the machine tool, even if the machine has started. It is then imperative that the machine stops before the hand reaches the risk area. A short stopping distance is also of importance for achieving ergonomics.

Grabbing or adjusting is also common when using automatic machines. Usually this is done to prevent production downtime by quickly adjusting a work piece. The stopping time is also of great importance if someone trips and falls into the machine.

Stopping time, walking speed (1.6 m/s) and hand speed (2.0 m/s) is used for the calculation of safety distances. Sometimes a fixed minimum distance is also used. See the standard EN ISO 13855 for more details on the calculation of safety distances.

Example: How the stopping time affects the choice of protective equipment...

There was a case where we measured the stopping time of the rollers in a textile industry company. The company had planned to place light beams or a light curtain in front of the rollers to prevent the operators from being caught in the material and dragged in. The stopping time measurement showed that it took over one second for the rollers to stop. During this time the material was pulled in by almost two meters.

In order to obtain sufficient protection distance, the light beams would have needed to be positioned almost three meters from the machinery, and a light curtain about two meters away. The factory did not have that much space, nor was it realistic. Instead, the solution became vertical sliding safety barriers.

Stopping Distance

For safety contact strips it is extra important that the stopping distance is monitored. An incorrect stopping distance could result in very high risks. The stopping distance is also needed during area limiting—e.g. for robots when dividing the working area into sectors.

For door sensitive edges, it is important that the stopping distance is shorter than the soft part of the sensitive edge.

Annual Checks

Wear in a machine is something that can affect braking and motors, which means that the stopping time of a machine can change with time. Certain other changes in a machine, such as changing the weight of a workpiece or alterations in pneumatic pressure, can also affect the stopping time. For these and other reasons it is important to perform an annual check on the stopping time.

Regulations and Standards

It is also important to measure the stopping time to meet the requirements set by the machinery standards, directives and regulations. Here we can help, with our long experience in the practical application of regulations and standards, from the viewpoints of both the authorities and production. In addition, we collaborate with the standardization committees responsible for producing these standards. One example is EN ISO 13855, which deals with the placingt of safety devices around a machine based on its stopping time. The standard is general for all types of machinery, although for some, where there is a harmonized C standard, the requirements for minimum distance and stopping time measurement will apply. For example, in the case of mechanical press tools, there is also a requirement in EN 692 for how stopping time measurements are to be performed. Iin the case of hydraulic press tools, the requirement is in EN 693.