Designing a complete vacuum-handling system involves many variables. These include the obvious, such as the size, weight, and type of material to be handled, how fast it must move, and the system operating pressure. Other factors are not so obvious, such as the type of vacuum cup that best suits the application as well as acceleration forces due to high-speed motion. In the end, the goal is the most cost-effective system that performs as intended, with minimal downtime and maintenance.

To simplify the design procedure, here’s a step-by-step process based on a typical example. Consider the job of moving smooth, flat, and dry steel sheets, stacked on a pallet. The sheets are 2500 mm long, 1250 mm wide, and 2.5 mm thick. The handling system is a portal transfer unit with an 8-bar compressed air supply and control voltage of 24 Vdc. The transfer procedure is horizontal to horizontal; that is, flat sheets lying on a pallet will be lifted, then moved laterally without changing their orientation. Maximum acceleration values for X, Y, and Z axes are 5 m/sec2 and cycle time is 30 sec, including less than 1 sec each for picking up and releasing the sheet.

Basic calculations

To determine the necessary holding forces, we need to know the work piece mass. To calculate mass, multiply the dimensions of the work piece (l × w × h) by the material density, ρ (in this case, 7850 kg/m3):
m = 2.5 × 1.25 × 0.0025 × 7850 = 61.33 kg. The suction pads must also handle acceleration forces which, in fully automatic systems, are by no means negligible. To simplify calculations, let’s look at the three most important and frequent load cases.

1. Horizontal suction pads, vertical force.  In this case, suction pads are placed on a horizontal workpiece which is to be lifted vertically. Calculate the theoretical holding force FT:
FT  = m × (G+a) × S,
where m = mass (kg),
G = acceleration due to gravity (9.81 m/s2),
a = system acceleration (m/s2), and
S = safety factor.  (For the safety factor use a minimum value of 1.5; for critical applications, inhomogeneous and porous materials, or rough surfaces, use 2.0 or higher.)
Here, FT  = 61.33 × (9.81 + 5) × 1.5
               = 1363 N.

2. Horizontal suction pads, horizontal force. The suction pads are placed on a horizontal workpiece which must move laterally.
FT  = m × (G+a/µ) × S
where µ  is the coefficient of friction. Typical values for µ are:
• 0.1 for oily surfaces.
• 0.2 to 0.3 for wet surfaces.
• 0.5 for wood, metal, glass, and stone.
• 0.6 for rough surfaces.
For this example:
FT  =  61.33 × (9.81 + 5/0.5) × 1.5              
      = 1822 N.

3. Vertical suction pads, vertical force. The suction pads move a vertical workpiece, or shift a horizontal workpiece to another orientation.
FT  = m/µ  × (g + a) × S, or  
FT  =  (61.33/0.5) × (9.81 + 5) × 2
     = 3633 N.

Always use the worst case with the highest theoretical holding force that applies to the application. For the example, we can ignore the last load case because workpieces are only handled in a horizontal orientation. Comparing the first two load cases, the second value for FT = 1822 N is greater. Therefore, use this for subsequent design calculations.

Select suction pads

Desgners normally select suction pads based on the following criteria:

Operating conditions — Single or multiple-shift operations, expected life, aggressive surroundings, temperature, and other environmental factors are all considerations when selecting suction pads.

Material — Suction pads come in a wide range of materials to meet specific application requirements. Some materials, for example, are particularly suited for smooth, rough, or oily surfaces, or easily damaged workpieces. There are also special antistatic suction pads for handling electronic components and suction pads which cause little marking of plastic parts.

Surface — The surface of the workpiece to be handled may make certain suction-pad versions more suitable than others. The product range includes flat and bellows suction pads with many types of sealing lips and sealing edges, which are available in a wide range of sizes and shapes.

Obviously, a lot goes into selection of a seemingly simple suction pad. Leading manufacturers offer extensive data on various types of pads and the advantages of each, along with load capacity, temperature limits, chemical compatibility, and so on. Schmalz PFYN/PFG flat suction pads, made of Perbunan NBR, are an economical choice for handling smooth, flat steel sheets. Medium-sized (2500 × 1250 mm) steel sheets normally require six or eight suction pads. The most important criterion for the number of pads is how much the steel sheet flexes during transport. In this example, six PFYN 95 NBR suction pads are sufficient and help keep costs down.

Suction force, FS, is calculated as FS = FT /n, where n is the number of suction pads. Thus, FS = 1822/6 = 304 N. According to the manufacturer’s data, six PFYN/PFG 95 NBR suction pads, each capable of a suction force of 350 N, will handle the task. The load-carrying capacity of the suction pad must always exceed the calculated suction force.