Sucking it up

Design Engineering, with help from Mannesmann Rexroth, takes a look at how to choose the right suction cup for a particular application

Designers building pick and place systems will be all too familiar with the use of the suction cup. Without it, systems would neither pick, nor place. Yet even in this relatively basic area of pneumatics, designers are faced with more than just a single option. And the choice of the right suction cup can lead to success or failure of a pneumatics system.

The suction cup is the gripper in a vacuum system. The suction cup performs the holding function, by being pressed against the object by the atmosphere pressure surrounding it. Indeed, it is the difference in pressure between the atmospheric pressure and the pressure in the suction cup that creates a holding force between the cup and the object.

When choosing a suction cup, it is important to consider the shape and material of the object to be handled as well as whether the direction of handling is horizontal or vertical.

There are seven variants of suction cup that the designer can consider. First off, there is the Flat and Simple suction cup. This suction cup is mounted directly on the end of a pipe. It is suitable for horizontal handling of light objects such as sheets of paper.

The Flat suction cup, on the other hand, is stable and flat and has three sealing lips. The two inner lips are short ensuring stability. The outer lip is long and soft, so that the suction cup makes gentle contact. It also provides additional safety if leakage should occur over the inner lips. This type is suitable for both horizontal and vertical handling of objects with a flat or slightly curved surface that are made from materials such as metal or glass.

Flat and Plain variants are suitable for horizontal and vertical handling of smaller objects and feature a screw connection. However, if you need to lift long narrow objects with a flat surface, then consider using Flat and Oval suction cups. If your design calls for handling of flat objects with uneven surfaces, such as wooden planks, then you should choose a Flat Suction cup with a soft and replaceable lip.

Bellows suction cups come in two variants, those with one and a half bellows and those with two and a half bellows. With one and a half bellows designs, you can design systems that can handle objects with a curved surface such as eggs and corrugated sheet metal. This type of suction cup can also be used for shorter lifts as well as to compensate for minor level differences. Two and a half bellows designs can be used for higher lifts and to compensate for greater level differences.

Suction cups are manufactured from a number of different materials, such as chloroprene, nitrile, terban and silicone. These sport different resistances to oil, different temperature ranges and wear resistance. Table 1 shows the different characteristics of the materials and their suitability for a specific application.

To ensure a successful design, suction cups should be chosen according to type, material and size, and in that order. Designers who choose wisely should ensure that if their pneumatics system design does run into some problems, the suction cup is not the likely culprit.

{{TABLE 1 Chloroprene Nitrile

Oil resistance ** ****Wear resistance *** **Temperature range -10deg C to +70deg C -10deg C to +70deg CFoodstuffs application * *

**** Very good/resistant *** Good/resistant** Fairly good/resisent * Less good/resistant

Terban Silicone

Oil resistance *** *Wear resistance **** *Temperature range -20deg C to +130deg C -30deg C to +170deg CFoodstuffs application * ****

Some materials are better than others. Make sure you are aware ofthe differences before you make a choice.}}