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Robotics Tech Papers

Suction Cups Enable the Vacuum to Grip


Vacuum systems designed to lift, hold and move objects need to be fitted with suction cups. These come in all shapes, forms and sizes, so it can be hard to choose one that fits a certain application. First, you need to know how they work.

Suction cups are essential components in any vacuum system designed for materials handling. They provide a simple and low-cost, yet reliable and proven solution. Suction cups can lift, hold and move objects weighing anything from a few grams to hundreds of kilos.

Usually made of pliable materials such as rubber, suction cups provide a tight seal against the handled object, ensuring a firm grip. So, how is this possible?

Low pressure under the cup
A suction cup adheres to the surface of the handled object when the pressure under the cup is lowered. This causes the higher atmospheric pressure outside the cup to press the cup down onto the surface. The low pressure under the cup is, of course, a result of the suction cup being connected to a vacuum pump that pumps away the air from under the cup.

The atmosphere surrounding the cup will exert pressure on all sides of the suction cup, making sure that the cup is firmly held against the handled object.

Use a big cup and a low vacuum
If the vacuum level is too high, the suction cup will work harder and wear out quicker. Higher vacuum levels also use more energy. Hence, it is usually preferable to maintain a lower vacuum and instead increase the area of the suction cup, as the lifting or holding capacity of a suction cup is determined by:

Lifting force = pressure x area

A simplified formula can be used to roughly estimate the cup size needed for a certain task. Typically, the diameter (D) of the suction cup is unknown, whereas the weight of the object being handled (m) and the vacuum level (U) are known. A safety factor also needs to be included (n) as well as the number of suction cups used in the system (s). The equation for working out the required suction cup diameter will end up like this:

More than size matters
Although the size plays an important part, there are other equally important considerations when selecting an appropriate suction cup. The dimensions of the handled object as well as the material it is made of will also affect the choice.

The texture of the object’s surface is important. Seemingly flat surfaces may have small imperfections or be grainy, which can result in leakages. Properly adjusted suction cups can solve such issues. The surface temperature is another property worth taking into account. In the case of extreme surface temperatures, suction cups made of a silicon material are recommended.

The porosity of the material must also be considered. Porosity is defined as the amount of outside atmospheric air that passes through an object under vacuum pressure. Solid sheets of metal or glass are non-porous, but boxes of corrugated cardboard or sheets of paper are porous and will let air through.

Tailored to fit
Although most of these considerations can be addressed by a few basic suction-cup shapes, it is important to remember that suction cups need to be tailored to suit specific purposes.

Flat objects are best handled by flat style suction cups. Such cups can be fitted with moulded rubber cleats inside that spread the vacuum force over the entire cup area to limit cup movement on the handled object.

The bellows style shape is very popular as it allows the cup a degree of level adjustment, meaning that it can compensate for irregularities in the surface and varying height of handled objects. An oval style cup can be used in tight areas where a round shape simply would not fit, and a deep style cup is suitable for lifting around corners or edges.

Soft lips handle with care
The cup’s durability, wear and oil resistance are important factors to bear in mind. As the friction between the suction cup and the surface is of vital importance, suction cups with high capability to handle shear forces resulting from rapid acceleration and retardation should be used.

Polyurethane has better abrasion and tear resistance than rubber. In addition, it has higher load bearing capacity and exceptional elastic memory.

Suction cups in polyurethane with a soft, high friction, flexible lip and a stable body are a good choice for many applications. For instance, these are able to keep a tight hold on bags, and yet not damage them. The latest soft and durable lips have excellent sealing capacity and are designed to control how the bags wrinkle.

When safety is a priority
Extra-large flat suction cups are ideal for the handling of for instance large panes of glass with either flat or slightly concave surfaces. The cups can be equipped with a specially designed friction pattern that is tailor-made for the vertical handling of glass or metal sheets. The friction pattern stops sheets from slipping, even if they are slightly wet or greasy.

A double lip design can act as an extra safety barrier. If the cup’s outer lip is accidentally torn or cut, the inner lip will make sure that such damage does not have fatal consequences. It also safeguards against the effects of overloading.

Finish off with accessories
The right accessories will enhance the performance of the materials handling system. If the pick-up area relative to the horizontal or vertical planes changes, or if the alignment of objects or the pick-up height vary from time to time, there are accessories that will compensate for these variations.

For instance, a level compensator or shock-absorber accessory will automatically adjust to different heights. A ball-joint connector can provide angle deviation to address problems relating to objects that are not completely vertical or horizontal. This is particularly useful on curved surfaces.

Do you want to learn more about suction cups?
On our website we offer self-training modules on this and other subjects, and of course you are always welcome to contact us for help.

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