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Emerald Group Publishing Limited
Copyright © 2012, Emerald Group Publishing Limited
Bellows grippers provide safe and gentle means of handling fragile workpieces
Article Type: Mini features From: Assembly Automation, Volume 32, Issue 1
*Optional sensors detect “open” and “closed” state for control feedback.
Festo has launched a range of bellows grippers that provide an ideal pneumatic automation solution to the problem of transporting or holding fragile workpieces. The new DHEB series grippers employ flexible bellows which expand in diameter when actuated by an integral pneumatically driven piston, to securely grip the interior surface of a workpiece.
Festo’s DHEB bellows grippers are suitable for a diverse range of processing and manufacturing industries, spanning the food, pharmaceutical, general industrial and electrical sectors. Typical applications include general handling, sorting and packaging of fragile workpieces such as glasses, bottles, cups, beakers and ceramic containers. One major application is in the handling of polyethylene terephthalate (PET) bottles, which may be subjected to a variety of functions during the gripping process, such as leak testing, disinfecting or filling.
DHEB grippers are single-acting devices, which simplifies their control significantly. When their air supply port is pressurised, an internal piston optionally draws a wedge up or down within a rubber bellow, causing it to deform outwards to meet and conform with the interior contour of the workpiece, such as the neck of a bottle. This provides a secure but gentle air-tight grip, which is maintained until the air supply port is exhausted, whereupon the force of the bellows returning to their original shape pushes the piston back into its initial position.
There are two versions of the DHEB gripper, offering a choice of bellows activation directions to suit different handling machine actions; one uses an upward piston stroke to deform and expand the bellows, the other uses a downward stroke. Furthermore, the grippers are not limited to gripping the inside of workpieces – multiple units can be configured for grasping external surfaces, such as the edges of a printed circuit board.
For maximum applications flexibility, the compressed air supply for the gripper can either be applied via a side port on the body of the gripper, or via a connector at the top of the unit. Alternatively, a variant of the side port version provides a straight-through path from the top connector to a port on the underside of the piston. This can be used for a wide variety of purposes; air can be injected to cool or clean the workpiece, air or vacuum can be employed to check the workpiece for leaks, or fluid can be introduced for disinfection or filling purposes. Any of these actions can be performed during workpiece transportation, which helps users maximise machine throughput by overlapping or combining several process stages.
DHEB series grippers are available with 11 choices of contiguous bellow sizes, ranging from 8 to 63 mm. The 8 mm model is suitable for workpiece diameters from 8 to 11 mm, while the largest gripper can accommodate workpiece diameters from 66 to 85 mm. Depending on various factors such as friction and the shape of workpiece, the largest grippers can accommodate workpiece weights as high as 52 kg. All models are designed for use with a standard supply of filtered compressed air, with a nominal operating pressure of four to eight bar. To reset the bellows to their unexpanded form, the pressure to the unit’s air supply port simply needs to be reduced to one bar or less. Opening and closing times depend on the size of the bellows – the 8 mm model, for example, opens in 10 ms and closes in 30 ms.
Festo also offers a choice of bellows material to best suit the application. Silicone bellows have a very long service life – in the order of one million cycles – and an extended operating temperature range of −70 to 200°C. The material is also approved by the FDA (US Food and Drug Administration) for use in food processing applications. For applications demanding a higher coefficient of friction, such as internally gripping heavy straight-sided workpieces, or when the workpiece might subsequently be painted or coated, bellows manufactured from ethylene propylene synthetic rubber (EPDM) are available. These have a service life of about 0.5 million cycles, and a temperature range of −50 to 140°C. Both types can easily be replaced by the user, which helps to minimise maintenance time and costs.
Uniquely, any Festo DHEB gripper can optionally be equipped with magneto-resistive or reed switches to sense the upper and lower positions of the internal piston, and provide control feedback on the state of the gripper bellows. The switches are self-aligning and mounted on an external sensor rail secured to the body of the unit.
For more information about Festo’s DHEB grippers, please visit the web site: www.festo.com/cms/en-gb_gb/13668.htm. This web site features animated presentations, showing the action of the gripper’s internal pneumatically driven piston and the manner in which the bellows deform.