Suction cup

A suction cup has two jobs: it seals against the surface, and it transmits the force. Both depend on the sealing lip. If it does not seat all the way round, air leaks in and the vacuum collapses – however large the vacuum generator is.

For the calculation, the effective area inside the seated lip counts. It is smaller than the area from the nominal diameter because the lip deforms under load. The manufacturer’s holding force figure is therefore more reliable than one’s own calculation from the outside diameter.

Flat cups have a small internal volume, build up vacuum quickly and support the workpiece well against shear forces – the choice for flat, smooth surfaces. Bellows cups with one or more convolutions compensate for height differences and angles and conform to curved surfaces; in return they are softer sideways and produce a stroke when suction is applied.

Oval cups make use of narrow faces such as profiles or webs. For rough and textured surfaces there are cups with a soft foam or sponge rubber seal that presses into the texture.

The material follows from medium, temperature and abrasion. The manufacturer states the limits of use; they differ from compound to compound.

When the load is tilted or accelerated, the cup has to transmit shear forces by friction. Hard lips and cups with internal support – cleats or ribs on which the workpiece rests – have the advantage here. The support also prevents thin sheet from being drawn into the cup and permanently deformed.

On delicate visible surfaces a lip can leave marks. Low-marking materials help, as does a trial on the real part before the choice is fixed.

Sealing lips are wear parts. They harden, tear, fray at the edge or lose their shape. A cup with a damaged lip may still hold at rest – but on loss of energy it shortens the holding time drastically. A visual check of the lips therefore belongs to the pre-use check, and spare cups belong in stock.