Gentle handling can boost productivity

For many products, gentle handling during manufacture and packing is essential if damage is to be avoided. Rexroth’s Stuart Cheyne looks at the options.

Many conventional product- handling systems make use of mechanical grippers that are usually operated pneumatically, although solenoid-driven versions are also available. It would be easy to assume that gripper-based systems would be unsuitable for handling delicate products, but this is not necessarily so.

Modern gripper products, such as those in the Rexroth GSP Series, are precision engineered, which means that closing of the gripping fingers is very accurately controlled. Repeat accuracies of 0.05mm or better are easily achieved, so, for regularly shaped products, the risk of crushing in a properly designed system is minimal.

The grippers can be supplied in versions with two, three or four fingers, as required by the application, and, if necessary, the gripper fingers can be faced with resilient material to minimise the shock loading when a product is picked up.

These features mean that it is well worth considering conventional grippers for gentle handling, especially as they are available in a variety of shapes and sizes, and are cost effective. There are, however, many applications, especially those involving the transport of items, that are not only delicate but also irregularly shaped, where no form of gripper system is likely to prove satisfactory.

A good example is a machine that is being used to transport toothbrushes that were in transparent bubble packs with a card backing. The shape is very irregular, making it difficult to decide on a point where a gripper could be used reliably, and the packs are susceptible to crushing and creasing.

In instances such as this, a different approach is needed and this is often conveniently provided by vacuum pick-ups. These devices, also available in a wide range of sizes and formats, effectively use suction to pick up and support the product. This does, however, mean that they can usually only be used with non-porous products, where a reasonable degree of sealing can be achieved between the product and the pick-up.

It might be supposed that the need for a vacuum generator would make these devices expensive and possibly unattractive to use. A simple and inexpensive solution is, however, available in the form of a

venturi-based vacuum generator. These devices, which have no moving parts, produce a vacuum simply by passing a fast moving stream of compressed air over an orifice. In effect, the airflow draws air in from the orifice, creating a vacuum. Since most manufacturing plants of any size have an existing compressed air supply, this is a very convenient solution.

Venturi-based vacuum generators of this type are readily available — in the Rexroth EBS range, for example, there are many models.

For particularly delicate items that must not even be touched, such as solar cells, which are very susceptible to contamination, the answer is a novel type of non-contact lifting and transport device that has recently been developed by Rexroth. Designated the NCT, or Non-Contact Transfer, Rexroth’s innovative lift is based on the so-called Bernoulli principle and, like the vacuum generator, it operates from a compressed air supply. Because of the geometry of the Rexroth NCT, airflow under the device generates

a differential pressure that creates a lifting force.

Although this may seem superficially similar to a vacuum pick-up, there is a very important difference — with Rexroth’s NCT, the lifting force generated is sufficient to be useful without a seal between the device and the item that is to be transported.

This feature means that the lift can transport porous items, such as biscuits and items with rough surfaces that would prevent a conventional vacuum pick-up from functioning reliably. Interestingly, the Rexroth NCT can even transport items through liquids — a benefit that is being exploited for transporting solar cells through baths of cleaning solutions. Since none of the surfaces of the cell are in contact with the pick-up, all of the surfaces can be cleaned simultaneously and efficiently.

In case it might be thought that such a device would be suitable for use only with tiny lightweight objects, it is worth noting that the largest version currently available produces a lifting force of 0.9kg. Another point is that as no vacuum generator is needed with the Rexroth NCT devices, they can be easily retrofitted to existing machines in place of conventional mechanical grippers.