The workpiece shapes have the greatest influence on the design, especially the fingers. However, grippers have also been designed as assembly devices. The gripper picks up the components one by one, brings them together as they move on and places them at the destination as a joined assembly. The gripper must have a corresponding number of individually controllable gripping elements. It is then a special gripper and is not often available in this design. A first example is shown in Figure 1. The joining forces are small. However, powerful pressing operations would only be possible with specially equipped grippers, which do exist.

The task is technically quite simple, the volume output is limited and whether the solution makes sense at all depends on a holistic view of the assembly task, the expected volume output and the planned environment. But: It can be done. The size and mass of a gripper upgraded in this way should not be underestimated. This can lead to restrictions in the fast handling process.

Figure 2: Assembly gripper for joining a switch shaft in the gripper. 1 Holding jaw, 2 Pin feeding device, 3 Switch shaft, 4 Sheet metal bracket, 5 Parallel pin, 6 Finished assembly

Now to another example: In Figure 2, the gripper and pin device merge to form an assembly unit. Here too, the gripper moves to several positions to pick up assembly parts. The jaws are specialized. First, the shaft is gripped and clamped. This is done directly at the drilling device to ensure that the orientation of the hole is sufficiently accurate. The sheet metal bracket is picked up in such a way that it is already precisely positioned on the shaft. Clamping jaws hold it in this position. Finally, a pin is pressed into the shaft and the three-part assembly is deposited. The integrated pinning device with automatic feeding of the pins is only indicated in the picture. Pinning is not considered to be automation-friendly. Perhaps the design of the assembly can be made more assembly-friendly before automation is introduced.

Figure 3: Clamping gripper for open rings. 1 Pull rope, 2 Gripper base body, 3 Spring band steel, 4 Gripper segment as rubber molded part, 5 Workpiece

Figure 3 shows a special gripper that grips a finely machined open ring with its segmented gripper jaws. Parts with atypical outer contours can also be considered as gripping objects. It is not a special gripper, but a mechanical clamping gripper. The gripping segments apply themselves to the gripping object with the same clamping force. Release is achieved by means of compression springs between the segments and the spring band as the base. Two independent actuators are required. These can be pneumatic cylinders, for example. However, cable pulls, small springs and deflection pulleys do not indicate that the gripper is very robust.

Figure 4: Principle of a four-finger gripper. 1 gripper finger, 2 pull rope, 3 deflection pulley, 4 workpiece

Figure 4 shows the principle of a gripper for different object shapes. It is a four-point gripper. Two fingers are fixed, the other two move towards the object to be gripped. It is driven by a single traction cable and reset by springs (not shown) that act on the moving fingers. A working cylinder can serve as an actuator. Apples, for example, can be used as gripping objects because larger deviations from the shape and size of the object can be handled well. This is actually the biggest advantage of this four-point mechanism. This is also shown by the other gripping examples. However, special accuracy requirements should not be made.

Stefan Hesse