Designers should determine certain physical parameters as part of the selection process.
Coefficient of friction — It is usually not wise to assign a ballpark value for the coefficient of friction, μ, between the suction pad and workpiece. This means designers must determine μ beforehand through testing. However, as a general guide, approximate values for various workpiece surfaces include:
- oily surfaces: μ = 0.1,
- moist, wet surfaces: μ = 0.2 to 0.4,
- glass, stone, plastic (dry), wood, and metal: μ = 0.5,
- rough surfaces: μ = 0.6, and
- sand paper (dry): μ = 1.1.
Holding forces — Many factors, such as the size and shape of the vacuum pad, its surface finish, plus rigidity (deformation) of the workpiece, are important when calculating holding forces. For this reason, we recommend that engineers include a safety factor of at least two. German accident-prevention regulations demand a minimum safety factor of 1.5. Operations that swivel or turn over the workpiece use a safety factor of 2.5 or higher to cope with the resulting forces.
Vacuum pad diameter — Absolute holding force depends on the vacuum pad diameter and the workpiece surface finish. Determine the required diameter with the following equations.
For forces applied horizontally:
d = 1.12 [(m S) (PU n)]½
For forces applied vertically:
d = 1.12 [(m S) (PU n μ)]½
where, d = internal diameter of the sealing lip, cm, m = mass of the workpiece, kg, PU = vacuum, bar, n = number of suction pads, S = safety factor, and μ = coefficient of friction.
Vacuum capacity — The level of vacuum required and circuit volume are critical for calculating the necessary vacuum capacity. However, keep in mind that the workpiece material is the decisive factor that determines vacuum capacity. For porous parts, always carry out vacuum tests to ascertain actual capacity requirements.
Additional factors — In addition to the factors mentioned here, vacuum system manufacturers often list other data for individual vacuum pads to aid in the selection process. For example, data for Schmalz suction pads include:
- Theoretical suction force of a vacuum pad is calculated from the pressure differential and the surface area: F = ΔpA. Schmaltz bases this value on a –0.6 bar vacuum (at sea level) with a smooth, dry workpiece surface. Depending on operating conditions, this value may have to be reduced to account for the safety factor, friction losses, or a lower vacuum level — for example, due to a porous workpiece.
- Internal volume is used when calculating the total volume of the gripper system and, thus, total evacuation time and equipment cycle rate.
- Minimum radius of curvature specifies the minimum radius that a suction pad can securely grip, which is important for curved workpieces.
- Vacuum pad stroke is the lifting effect during evacuation of a vacuum pad. It typically applies to bellows-type pads. It also allows for height compensation and gentle cup placement.