Main Catalogue - VUOTOTECNICA - #19

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18 Developing the previously described formula: ÄP = Patm – P1 and always considering a vacuum level equal to 250 mbar abs. (-75 KPa) inside the cup, the force exerted by the atmospheric pressure on the cup at 1000 m above sea level will be: ÄP = 894.6 – 250 = 644.6 mbar equal to: 644.6 mbar x 1.02 x 10-3 = 0.657 Kg/cm2 As we can notice, we have gone from a force of 0.778 Kg/cm2 at the sea level, to a force of 0.657 Kg/cm2 at 1000 m above the sea level; all this confirms that, as the atmospheric pressure decreases, the force also decreases. Therefore, during the design stage, the altitude of the place in which the lifting system will be installed must always be taken into account. With reference to the example described above, with a steel sheet of 140 Kg, considering the use of 6 cups with a vacuum level of -60 KPa and safety factor 3, we’ll have: D = 113 x 140 x 3 = 122 mm 60 x 6 Therefore, to solve this problem, 6 cups with a diameter equal to or larger than 122 mm will be necessary. LIFTING WITH VACUUM CUPS ABOVE THE SEA LEVEL With altitude, the atmospheric pressure decreases according to the values shown in the table below. A vacuum gauge, for measuring the vacuum level, is normally calibrated to the atmospheric pressure atmospheric pressure at sea level, i.e., 1013.25 mbar. BAROMETRIC PRESSURE VACUUM GAUGE VALUES AT 1013.25 mbar mmHg mbar Altitude m -60 KPa -75 KPa -85 KPa -90 KPa -99 KPa 760 1013.25 0 60.0 75.0 85.0 90.0 99.0 750 999.9 111 58.7 73.7 83.7 88.7 97.7 740 986.6 200 57.3 72.3 82.3 87.3 96.3 730 973.3 275 56.0 71.0 81.0 86.0 95.0 720 959.9 467 54.7 69.7 79.7 84.7 93.7 710 946.6 545 53.3 68.3 78.3 83.3 92.3 700 933.3 655 52.0 67.0 77.0 82.0 91.0 690 919.9 778 50.7 65.7 75.7 80.7 89.7 671 894.6 1000 48.1 63.1 73.1 78.1 87.1 593 790.6 2000 37.7 52.7 62.7 67.7 76.7