Starting Information Assume that a perpendicular light curtain (i.e. vertical) is to be used for point of operation guarding of a circuit board assembly machine. See figure 1 below. The machine runs on a single cycle . For speed of production it is essential that the attendant must able to reach into the machine to insert the board and some parts. A forming platen is then lowered by machine power. The objective is to minimize the downtime losses through allowing rapid but safe access into the workpiece.
There is of course a hazard of shearing and crushing to the hands and arms of the attendant when reaching into the machine. The machine movements must therefore stop when a hand approaches the hazardous point and the light beam detection must be arranged at a safety distance sufficient to ensure all motion is stopped before the hand and arm can reach the hazard. The following data applies: Response time of the light curtain detection and interlock trip action: 220 milliseconds worst case Machine stopping time 150 milliseconds. Minimum object sensitivity of the light curtain: 25 mm Height of the Hazard point above ground: 1100 mm
Figure 1: Representation of the assembly machine with proposed point of operation guarding assembly machine. Danger zone and proposed light curtain are shown.
Step 1: Calculate the minimum necessary safety distance from the vertical centre line of the light curtain to the nearest point of hazard allowing for a safety margin of 20 % on the machine stopping time. Step 2: Then check whether or not this distance will allow a person to be undetected between the light curtain and the machine. In the event of this being possible what additional protection measure could be taken to ensure safety? Step 3: Now that you have found the safety distance in step 1 use table 1 and fig 2 overleaf to calculate the height of the top beam of the safety curtain. Assume that the lowest beam is to be 300 mm above ground note the total height of the light curtain Step 4: Suppose that a horizontal light curtain is to be considered as an alternative to the perpendicular version. Calculate the safety distance for the outermost light beam if the horizontal curtain is mounted at 500 mm above ground. Compare with step 3 and decide which beam arrangement will require the least number of beams. Step 5: Determine the required performance level that should be specified for the light curtain safety system based on the assumption that the attendant has to use the machine for several hours per day with frequent cycles. The expected injury level will be damage to hand that should not be permanent.. Your decision may be based on the risk chart published for guidance in ISO 13849-1 and included in the session 3 slides.
The diagram and table shown below are based on values proposed for ISO 13855 for the height of the top of a guard (b) based on the height of the danger zone (a) and the safety distance (c).
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