Introduction to the vacuum

The term “vacuum” refers to the physical situation that occurs in an environment where the gaseous pressure is lower than atmospheric pressure.

Positive pressures are defined as all pressures with values higher than atmospheric pressure; all those with lower values are negative.

P atm = Atmospheric pressure

P o = Zero pressure, absolute vacuum

P+ = Positive relative pressure

P ass = Absolute pressure

P - = Negative relative pressure

Pressure dimensionally represents a force per unit of surface, its unit of measurement is the pascal (Pa symbol) which represents the pressure obtained when a force of 1 N (newton) is exerted on the unit surface of 1 m2.

Therefore, the result is:

pascal = newton/metro² ==> Pa = N/m²

It is common for some applications to use alternative units of measurement such as the millibar (mbar) equal to 100 Pa and the Torr or mmHg, which is equal to 133.322 Pa; the latter is mainly used for measurements in the medical field (blood pressure) although it is not used as a unit of measurement in the International System (IS)

Atmospheric pressure, measured in mbar, decreases as altitude varies (measured in m), as better specified in the diagram below.

Comparison between atmospheric pressure values measured at sea level with the same values measured at different altitudes.

Lifting force

One of the reference parameters for selecting vacuum cups and reported in the tables is the lifting force (expressed in kg), unique for each type of vacuum cup and calculated with the following formula:

Where:

F = Lifting force expressed in Kg;

S = Gripping surface of the vacuum cup, expressed in cm²;

P = Force exerted by atmospheric pressure, depending on the degree of vacuum, expressed in Kg/cm²;

η = Safety coefficient.

Safety coefficient

The vacuum cups in the catalogue have been designed to theoretically keep a load suspended equal to three times the value of the force indicated in the tables.

The values shown have been obtained taking into consideration:

P = 0.75 Kg/cm²: value developed by a vacuum level of approximately 250 mbar abs. (-75 KPa).

η = 3: safety coefficient, valid when the gripping surface of the vacuum cups is horizontal, the surface is smooth and waterproof and the acceleration or deceleration of the moving load is less than 10 m/s².

Volume

The internal geometric volume of the vacuum cup and represents the volume to be added to the entire distribution circuit for the calculation of the evacuation time, especially if multiple vacuum cups are used.

In practice this is the quantity of air to be "emptied" and which must be added to the volume of the entire vacuum distribution circuit to calculate the evacuation time; a parameter used to determine the suction flow capacity of the vacuum generator

This parameter takes on particular relevance when the sizing of the generator is related to the handling of products that generate a perfect seal with the lower surface of the vacuum cups, for example metal sheets or glass sheets.

In all situations in which the material to be handled is potentially "breathable", it is recommended to carry out grip tests with the single vacuum cup, in order to determine the correct suction flow rate.

Parameters for the choice of vacuum cups

To select the type of compound most suitable for each application and type of product, the following parameters should be considered: