Bellows grippers can be used to transport or hold fragile workpieces
19 August 2011
Festo has launched a range of bellows grippers for transporting or holding fragile workpieces. The new DHEB series employs flexible bellows that expand in diameter when actuated by an integral pneumatically-driven piston, to securely grip the interior surface of a workpiece. 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.
DHEB series grippers are available with eleven choices of contiguous bellow sizes, ranging from 8 to 63mm. The 8mm model is suitable for workpiece diameters from 8 to 11mm, while the largest gripper can accommodate workpiece diameters from 66 to 85mm. Depending on various factors such as friction and the shape of workpiece, the largest grippers can accommodate workpiece weights as high as 52kg.
All models are designed for use with a standard supply of filtered compressed air, with a nominal operating pressure of 4 to 8 bar. To reset the bellows to their unexpanded form, the pressure to the unit's air supply port simply needs to be reduced to 1 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 1 million cycles - and an extended operating temperature range of -70 to 200 degrees Celsius. 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 degrees Celsius. Both types can easily be replaced by the user, which helps to minimise maintenance time and costs.
Any Festo DHEB gripper can, as an option, 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.
Click here for animated presentations, showing the action of the gripper's internal pneumatically-driven piston and the manner in which the bellows deform.
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