Rexroth Offers Devices For Handling Delicate Items

Rexroth offers a number of solutions for handling delicate items during the manufacture and packing of a range of products, including food items and electronic devices. Stuart Cheyne, industry segment manager for automation with the pneumatics division of Rexroth, looks at some of the options now available for handling delicate items. Many conventional product handling systems make use of mechanical grippers that are usually operated pneumatically, although solenoid driven versions are also available, he said.

According to Cheyne, it is often assumed that gripper-based systems are 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 the closing of the gripping fingers is very accurately controlled. Repeat accuracies of 0.05mm or better can be achieved, so, for regularly shaped products, the risk of crushing in a properly designed system is minimal.

In addition, 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. Cheyne claims that, considering these features, conventional grippers may be suitable for gentle handling, especially as they are available in a variety of shapes and sizes and are very cost effective.

Nevertheless, there are 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. An 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 provided by vacuum pick-ups. These devices, which are available in a 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 assumed that the need for a vacuum generator would make these devices expensive and possibly unattractive to use. A solution is available in the form of a Venturi-based vacuum generator, according to Cheyne.

These devices, which have no moving parts, produce a vacuum 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 said to be a very convenient solution. Venturi-based vacuum generators of this type are available in the Rexroth EBS range, for example. Examples of ultra-delicate items that cannot be touched, let alone gripped, are solar cells, which are just a few millimetres thick and are very susceptible to contamination and handling damage.

Another application would be potato snacks or biscuits being placed into small, individual packs - here, the same problems occur, as well as issues with placement accuracy and repeatability. These products can be picked up and transported reliably using a recently developed non-contact lifting and transport device from Rexroth. Designated the NCT (non-contact transfer), Rexroth's lift is based on the so-called Bernoulli principle and, like the vacuum generator, it operates from a compressed-air supply. As a result of the geometry of the Rexroth NCT, airflow under the device generates a differential pressure that creates a lifting force.

With the 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. 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.

The largest version currently available from Rexroth produces a lifting force of 0.9kg. As the Rexroth NCT devices do not require vacuum generators, they can be easily retrofitted to existing machines in place of conventional mechanical grippers. The company also develops custom lift subassemblies, ready to install as single components, helping machine builders to lower costs, streamline their supply chain and reduce engineering expenses.