Load bearing capacities of the suction cups
The load bearing capacity of the suction cups depends on the following factors:
- Size of the suction cups
- Pressure difference between the level of vacuum in the suction cup and the ambient pressure
- Load direction (vertical, parallel or sloped to the suction cup surface)
- Surface properties and porosity of the suctioned material
- Possible separation layers (dampness, dust, dirt) on the suction cups or surface to be suctioned
- Inherent rigidity of the material to be transported
Other factors like
- Weather conditions and ambient temperature
- Temperature of the surface to be suctioned
- G force from the lifting equipment
(jerky lifting or braking of lifted load)
should also be taken into consideration.
Effects of ambient pressure
The reason why vacuum suction devices can raise loads is down to the difference in pressure between the (partially extracted) vacuum suction cup and the ambient air pressure.
The air pressure in the vacuum suction cups is determined, for the most part, by the vacuum producer used (e.g. vacuum pump) and the tightness of the sealing lip.
The ambient air pressure depends on the place, weather and temperature. An important factor is, above all, the height above sea level where the vacuum suction cup is used.
The influence of the height above sea level on the ambient pressure can be calculated accurately enough using the so-called international height formula:
ph = 1013,25 hPa * (1-(6,5*h / 288000 m)5,255
h: height above sea level in m
ph: pressure at the height h in hPa
From the graph above is can clearly be seen how the ambient air pressure decreases as the height increases. This means for the load bearing capacity of the vacuum suction cup, that the load bearing capacity of the vacuum lifting device decreases in the same magnitude! Simplified, there is approx. 10% decrease in load bearing capacity for every 1000 m increase in height in areas relevant to vacuum lifting devices used for construction.
The load bearing capacity of the Wirth vacuum lifters is calculated for a working range from 0 to 100 m above sea level.
If you intend to use your OKTOPUS® vacuum lifter at other heights, please contact us.
Effects of the load direction
If the load to be lifted is positioned horizontally, only the effectively functioning suction area and the actual pressure difference between the vacuum level in the suction cup and the ambient pressure, is factored in when calculating the theoretical holding force. For safety reasons, DIN EN 13155:2003, however, requires the presumption of a load slope of at least 6°, even in this case, and to include the resulting horizontal load components, which can only be activated through friction, in the calculation.
When factoring in the safety coefficient, the theoretical load bearing capacity of a suction cup is calculated according to the following formula when the value
p = difference between ambient pressure
and pressure in the suction cup
A = the effective suction area
S = safety coefficient
F = (p x A ) / S
For loads which are completely vertical, the total necessary load bearing capacity must be created through the friction between the pressurised suction cup and the surface of the material that is to be lifted.
The level of the coefficient of friction µ is significant for the load bearing capacity.
When lifting sandwich panels and glass plates, the coefficient of friction is usually µ = 0,5.
F = (p x A x µ) / S
Using the formulas indicated, a safety coefficient of S = 2.0 in accordance with DIN EN 13155:2003, a coefficient of µ = 0.5, a negative pressure in the suction cup of -0.6 bars and applying the device at sea level, load bearing capacities for vertical and horizontal load directions are produced depending on the diameter of the suction cup, as shown in the following diagram.