Suction cups lift almost anything

Oct. 6, 2011
Suction pads come in a vast range of sizes, shapes, and designs to safely handle most any product

Authored by:
Justin Katz
Applications Engineer
Schmalz Inc.
Raleigh, N. C.
Edited by Kenneth J. Korane
[email protected]
Key points:
• Flat pads, for flat or slightly curved parts, offer quick evacuation and good stability.
• Bellows pads adapt well to curved or uneven workpieces.
• Special-duty pads are designed for specific applications, such as handling wood, sheet metal, or candy.
Schmalz Inc.

Designing a vacuum-handling system involves many factors. On one hand, engineers must contend with the size, weight, and type of material to be handled, and how fast and precisely it must be moved. On the other, there’s the size and type of vacuum generator, system operating pressure, as well as components such as valves, hoses, and connectors.

But it all comes together at the suction cup or pad. This seemingly simple device must firmly and safely support the load, resist gravitational and acceleration forces, minimize air consumption, and not mar or damage the workpiece. And, of course, it must resist fatigue and abrasion, withstand dirt, contaminants, and temperature extremes, and provide long, economical life with little or no maintenance.

Selection criteria
Obviously, a lot goes into selecting a 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. Engineers normally select suction pads based on the following criteria:

Operating conditions. Cycle rates, 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. Common materials include nitrile, silicone, and natural rubbers, fluoroelastomers, and polyurethanes. 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 pads which won’t mark plastic parts. Environmental conditions can affect the material choice when the pads must resist ozone, chemicals, extreme temperatures, or operate silicone-free.
Surface. The workpiece surface may make certain types of suction pads more suitable than others. The wide range of available products includes flat and bellows suction pads in many sizes and shapes with various types of sealing lips and sealing edges.

Physical factors
Engineers 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 or wet surfaces: µ = 0.2 to 0.4.
• Glass, stone, plastic (dry): µ = 0.5.
• Wood and metal: µ = 0.5.
• Rough surfaces: µ = 0.6.
• Sandpaper (dry): µ = 1.1.

Holding forces. Calculated holding forces can never exceed the theoretical maximums. In practice, many factors, such as the size and shape of the suction pad and the surface finish and rigidity (deformation) of the workpiece play an important role. For this reason, we recommend that engineers include a safety factor, S, of at least 2. The German accident-prevention regulations demand a minimum safety factor of 1.5. In operations which swivel or turn over the workpiece, use a safety factor of 2.5 or higher to cope with the resulting forces.

Suction pad diameter. The absolute holding force depends on the suction-pad diameter and the workpiece surface finish. Determine the required diameter with the following equations.

With the force applied horizontally:

With the force applied vertically:

Here, d = suction diameter, cm. (For suction pads with a double lip, d = the internal diameter; for bellows suction pads, it’s the minimum internal diameter of the sealing lip.) Also, m = mass of the workpiece, kg; PU = vacuum, bar; n = number of suction pads; S = safety factor; and μ = coefficient of friction.

Suction capacity. The required vacuum level and circuit volume are critical for calculating the necessary suction capacity. However, keep in mind that the workpiece material is the decisive factor which determines suction capacity. For porous parts, always carry out suction tests to ascertain actual capacity requirements.

Additional factors. In addition to the factors mentioned here, vacuum-system manufacturers often list other data for individual suction pads to aid in the selection process. For example, data for Schmalz suction pads include:

Theoretical suction force. The theoretical holding force of a suction 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. This is used when calculating the total volume of the gripper system and, thus, total evacuation time and equipment cycle rate.

Minimum radius of curvature. This specifies the minimum radius which a suction pad can securely grip, important for curved workpieces.

Suction-pad stroke. This is the lifting effect during evacuation of a suction pad. It typically applies to bellows-type pads. It also allows for height compensation and gentle cup placement.

Manufacturers generally classify suction pads as universal or special purpose. Universal pads cover a wide range of requirements, while specific-duty pads are designed to meet special requirements such as handling thin automotive bodywork parts or porous workpieces in the woodworking industry. The two most-common types of universal products are flat and bellows suction pads.

Flat suction pads are particularly suited for handling objects with flat or slightly curved surfaces. They come in a wide range of materials, sizes, and shapes, including round and oval, with steep or flat sealing lips.

A low overall height and small internal volume means short evacuation times, letting them grip workpieces quickly. The flat shape ensures good stability when attached to a load and they withstand high lateral forces during fast movements.

Flat suction pads are typically used for handling smooth or slightly rough workpieces such as metal, glass, or wood sheets, cardboard boxes, and plastic parts. Wear-resistant versions can handle higher loads and give up to 10 times the life of standard NBR (nitrile-rubber) units. The pads are recommended for demanding applications with quick cycle times, such as handling oily metal sheets in automobile production or extremely abrasive workpieces, like rough cardboard.

Suction plates are a variation of flat pads. One type, for instance, has a flexible sealing ring attached to a metal base plate. Like suction pads, they come in a wide range of sizes and materials and have low internal volume for fast cycling. They are designed for handling rough or structured surfaces, such as ornamental glass, checker plate, and broken natural stone. Users can quickly change the sealing ring without tools.

Flat, oval suction pads produce high forces despite relatively small dimensions. In fact, they can generate considerably higher forces than round, flat pads when handling narrow workpieces. They’re best suited for thin and curved objects, such as extrusions, pipes, and sections of door and window frames.

Bellows suction pads compensate for varying workpiece heights and can handle parts with uneven surfaces or that easily damage. Like flat pads, they come in many shapes, sizes, and materials.

ellows suction pads adapt well to curved or irregular workpiece surfaces and can lift products when vacuum is applied. Typical versions have bellows with 1.5, 2.5, or 3.5 folds. More folds are generally better for handling workpieces with extremely uneven surfaces, or when a longer suction-pad lifting stroke is necessary.

The upper bellows are often stiffer for stability during horizontal acceleration, while the lower bellows and sealing lips are softer and more flexible to conform to curved and uneven surfaces and give good sealing, even on nonrigid workpieces. The bellows also provides damping when it contacts the workpiece, beneficial when handling fragile parts.

Bellows pads are typically used on curved or uneven parts such as car-body components and pipes; cardboard boxes and blister-packs; and easily damaged items such as electronic components and injection-molded plastic parts. They are also used to move cardboard, sheet metal, wood, glass, and ceramics.

Recent innovations
Manufacturers also offer many suction pads designed for specific applications. These include flat and bellows pads for handling sheet metal and metal tubing; wood products ranging from furniture parts and parquet flooring to laminated chipboard and rough-sawn lumber; packaged goods such as film-packed and blister-packed products, cardboard boxes, and soft bags; plastic film and paper; and CDs and DVDs. Here’s a look at some recent innovations from Schmalz.

Sheet metal. Metal-stamping and automotive assembly operations are growing more complex as automation processes deal with quicker cycle times and a wider variety of part shapes and sizes. A new type of round, bell-shaped suction pad (called the SAXM) with a flexible sealing lip, a reinforced structure, and special oil grooves is built for high-speed operations.

The shape offers a longer suction-pad stroke, letting it deftly adjust to contoured workpieces. And an inner support prevents deep-drawing of thin metal sheets. It evacuates quickly, produces high holding forces, and withstands extreme lateral acceleration even on oily surfaces. And it’s made of wear-resistant material that gives triple the life of standard NBR pads. The pads are designed for handling sheet-metal and car-body parts, loading CNC and laser-cutting machines, and handling blanks at destackers.

Structural shapes. SAOK suction pads grip structural sheet-metal components with round edges and small radii, including pipes, stiffening plates, and frame parts. A soft sealing lip ensures a good fit to securely hold the workpiece in fast-cycling machines. Edge-gripper suction pads are available in NBR for smooth metal parts, and in a high-temperature material for handling hot-formed components and hot sheet metal up to 250°C.

Chocolates. The SPG suction pad has an extremely thin and supple sealing lip that seals against smooth, glazed chocolates. The tapered suction pad completely encloses and securely holds without breaking round and square chocolates moving at high process speeds. High nominal flow ensures that the pad reaches required operating vacuum quickly and compensates for any leaks. The SPG is manufactured from FDA-approved silicone for direct contact with food. It can also be steam sterilized and washed using standard cleaning agents. It handles round chocolates up to 35 mm diameter and oval and square chocolates with a maximum diagonal measurement of 30 mm.

Soft packaging. Automated handling of flexible-form packaging is one of the biggest challenges in handling technology. This is due, in part, to the broad spectrum of different bag designs and materials. But it is also due to the non-rigid nature of the packaging and loose contents, such as powders or liquids, that must be picked at high speeds.

A bellows suction pad with 4.5 corrugations, the new SPB4 Series, reliably handles such packages at process speeds up to 100 cycles/min. The long, stable bellows has a thin and flexible sealing lip with integrated flow vanes. This lets the pad adjust to the bag and hold it with maximum surface coverage. Large nominal diameters ensure high-volume flows and accommodate any air leakage between the suction pads and the product. And an integrated joint in the pad compensates for any deformation from the contents shifting during movements.

A roughened surface and raised portion on the bellows corrugations prevents an “adhesive effect” where the folds stick to each other, providing additional process reliability even when the bellows are fully compressed. The series uses food-safe silicone and conforms to FDA guidelines, and can be used in toploader, multipack, blister packaging, and form-and-seal equipment.

Package handling. A new series of suction pads is for heavy-duty applications handling cardboard and plastic boxes and high-gloss printed packages. Made of proprietary Elastodur material, it gives triple the life of NBR. They include flat suction pads (SPF) and round (SPB1) and oval (SPOB1) bellows suction pads with 1.5 corrugations. The wear-resistant pads have adaptable lips for good sealing on porous surfaces. Support ribs on the suction surface ensure stability and high gripping force, letting them withstand the high lateral loads of fast-moving cardboard-box aligners. An optional prefilter integrated into the suction pads prevents contamination of the vacuum system.

High temperature. Pads made with fluorocarbon-based materials resist temperatures up to 400°C for short periods with little marking of the workpieces — making them well suited for the glass industry. The suction pad attaches to a large metal base for good heat dissipation and quick cooling. They are for handling hot workpieces, such as removing glass from molds and gripping hot metal parts.

Schmalz makes a special version (SPL-HT) with a stainless-steel body and textile seal that withstands temperatures to 600°C. It’s for flat workpieces, such as in float-glass manufacturing, glass-tempering processes, and hot forming in the metal industry.

Modular end-effectors. A new modular system for quickly configuring vacuum end effectors (VEE) is for use in pick-and-place processes, case packers, carton and tray erecting machines, and other packaging machines with frequent format changes.

The VEE elements give system integrators and machine and robot manufacturers a new way to set up or reconfigure flexible end effectors, including a wide selection of vacuum feeds, connection elements, and suction pads. CAD models of all the modular elements can be downloaded from the company’s Web site and quickly combined into the required configuration.

Users can construct a system with one to 12 suction pads, including flat and bellows pads for handling fragile foil and blister packaging, cardboard boxes, and filled bags or flow-wrap packaging. The VEE handles loads up to 2 kg and process accelerations up to 10 g, for fast pick-and-place processes typical for the packaging industry. All VEE elements are made of FDA-approved polysulfone. They resist alkaline and acidic cleaning agents and can be steam sterilized, making them particularly suitable for use in food processing.

Calculating g forces
To determine the necessary holding forces for a suction pad, consider more than just the workpiece mass. 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 lifted vertically. Calculate the theoretical holding force FT (N) from:

FT = m × (g + a) × S

where m = mass (kg), g = acceleration due to gravity (9.81 m/sec2), a = system acceleration (m/sec2), and S = safety factor.

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 = coefficient of friction. Typical values for µ are listed under “Coefficient of friction” in the main article.

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.

Always use the worst case with the highest theoretical holding force that applies to the application.

© 2011 Penton Media, Inc.

About the Author

Kenneth Korane

Ken Korane holds a B.S. Mechanical Engineering from The Ohio State University. In addition to serving as an editor at Machine Design until August 2015, his prior work experience includes product engineer at Parker Hannifin Corp. and mechanical design engineer at Euclid Inc. 

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