Design of the Suction Cup
The design of the suction cup always depends on the actual application. For this reason, various physical values must be calculated and determined before the correct suction cup can be selected. Later, the design of a vacuum system is described in more detail based on a calculation example.
Suction rate or required volume flow
The volume flow that generates the vacuum is important for the suction force. The workpiece material is the principal factor for the required volume flow. The table shows typical values for the volume flow or suction rate depending on the diameter of the suction cup with smooth and air-tight surface.
Typical value (with smooth, air-tight surface)
| Suction cup Ø | Suction area A | Volume flow Vͦ | |
|---|---|---|---|
| [cm2] | [m3/h] | [l/min] | |
| up to 60 mm | 28 | 0.5 | 8.3 |
| up to 120 mm | 113 | 1.0 | 16.6 |
| up to 215 mm | 363 | 2.0 | 33.3 |
| up to 450 mm | 1,540 | 4.0 | 66.6 |
Important
Conduct suction trials for porous parts!
Friction Coefficient
The friction coefficient “µ” describes the relationship between friction force and normal force. It is not possible to specify generally valid values of the friction coefficient between the suction cup and the workpiece. It has to be determined correctly through trials with the condition of the workpiece surface (rough/dry/moist/oily) or the properties of the suction cup (shape/sealing lip/sealing edge/suction cup material/Shore hardness) having a major influence.
Calculation of the holding forces
The calculation of holding forces can only be about theoretical values. In practical applications, many factors, such as the size and shape of the suction cup, the surface finish and the rigidity of the workpiece (deformation) play a decisive role. That is the reason why we recommend a safety factor (S) of at least 2. The German accident prevention regulation, UVV, prescribes a binding safety factor of 1.5. When swiveling workpieces during the handling task, a safety factor of 2.5 or higher has to be used, in order to cope with the resulting turning forces.
The holding force of a suction cup is the product of:
F = Δp x A
F = Holding force (without safety factor, purely static)
Δp = Difference between ambient pressure and pressure of the system
A = Effective suction area (the effective area of a suction cup under vacuum)
Diameter of the suction cup
The holding force of a suction cup depends on its effective diameter. The condition of the workpiece and the number of suction cups are also crucial for the holding force that a vacuum system can generate.
The required diameter can be determined with the aid of the following formula:
For horizontal pick-up:
d = 1.12 × √ (m × S) ⁄ (PU × n)
For vertical pick-up:
d = 1.12 × √ (m × S) ⁄ (PU× n x µ)
d = Suction cup diameter in cm (with double lip ≈ internal diameter, with bellows suction cup = inner diameter of sealing lip)
m = Weight of the workpiece in kg
PU= Vacuum in bar
n = Number of suction cups
µ = Friction coefficient
S = Safety factor
Calculation example for horizontal pick-up
d=1.12 × √(50kgx2)÷(0.4barx4)
d= 8.85 cm
Plastic sheet: m = 50 kg
Vacuum: PU = -0.4 bar
Number of suction cups: n = 4
Measured friction coefficient: µ = 0.5
Safety factor: S = 2
A sensible selection is the suction cup PFYN 95 with a nominal diameter of 95 mm.
Calculation example for vertical pick-up
d=1.12 × √(50kgx2)÷(0.4barx4x0.5)
d= 12.5 cm
Plastic sheet: m = 50 kg
Vacuum: PU = -0.4 bar
Number of suction cups: n = 4
Measured friction coefficient: µ = 0.5
Safety factor: S = 2
A sensible selection is the suction cup PFYN 150 with a nominal diameter of 150 mm.
