The jaws are interchangeable and can be fitted with special friction linings to increase static friction. This can reduce the gripping force and increase safety. But first, here are some basic rules on how to use grippers correctly. The terms gripper jaw and gripper finger are often used synonymously and are not used consistently.

Gripping should be planned so that the center of mass of the gripped object is located between the gripping elements in order to avoid unnecessary tilting moments, especially during acceleration phases of the robot's arm movement. The gripping point should be as close as possible to the gripper housing. This results from the requirement for fingers that are as short as possible. Holding workpieces in pairs is preferable. It is safer and can often be realized with much less gripping force (Fig. 1). This considerably reduces the load on the workpiece.

Figure 2: Gripper jaws must be low-mass! 1 Sensor, 2 Base jaw, 3 Gripper jaw, 4 Gripper object

Naturally, gripper jaws should also be low in mass. This helps to improve the dynamics of the handling process and mechanically relieve the gripper. Figure 2 shows an example of this. This is a parallel jaw gripper. Unnecessary accumulations of material have been eliminated. The two gripper jaws should also not be of different weights, because when opening and closing quickly, the gripper mechanism can start to vibrate undesirably due to unequal loads and fail prematurely.

Incidentally, fingers with a wide contact surface have no advantage over jaws with a small contact surface in terms of the frictional force generated during clamping. However, the latter generate greater surface pressure. This can lead to surface damage on the workpiece, such as clamping marks or dents in thin-walled hollow parts. In these cases, the surface pressure should be recalculated.

Figure 3: Gripper jaw with simple gripping width adjustment. 1 base jaw, 2 gripper jaw

If an additional mechanical interface is provided between the gripper base jaw and the gripper jaw, as shown in Figure 3, it is possible to adjust the gripper to different workpiece widths and also to change to differently shaped gripper jaws.

Instead of solid jaws, flat forks can also be used as gripping elements. Figure 4 shows a gripper for cardboard boxes. The object is forked under at the side and pressed on with pressure elements before the planned movement begins. The fork is advanced in a slight arc and is also spring-loaded. The gripper is connected to a rotary axis (manipulator, industrial robot).

Figure 4: Carton gripper. 1 Compensation rod, 2 Pick-up fork, 3 Pneumatic cylinder, 4 Pressure plate, 5 Gripping object, 6 Arm of the handling device, 7 Supply lines, 8 Tensioner for lines, 9 Swivel joint axis

Today, gripper jaws with complicated geometries can also be designed on the computer and produced quickly and efficiently from plastic using laser sintering. When using such gripping elements in the food sector or in medicine, the existing hygiene regulations must be observed.

The requirements regarding material, surface coating and design (easy-to-clean design, rounded edges, no open holes, etc.) must be taken into account.

Gripper jaws are also increasingly being equipped with sensors. This allows additional information to be obtained about the gripping process or the gripped object. For example, when separating sheet metal blanks by gripping, a double sheet check can be carried out; this ensures greater process reliability.

Stefan Hesse

Literature: Stefan Hesse, Greifertechnik - Effektoren für Roboter und Automaten. Carl Hanser Verlag, Munich 2011