The human hand as a model

GMG develops new robot grippers with plastic plain bearings from igus

Mohsen Saadat has always been inventive and technically gifted: as a child, at the age of five, the Iranian invented his own toys, building small cars out of wire, for example - without any tools at all. A passion for technology that never left the now 75-year-old. At 19, he moved to Germany, studied engineering, and taught as a professor. In 1991, he finally founded GMG - Gesellschaft für modulare Greifersysteme in Soest, North Rhine-Westphalia. He reports that the aim was to combine scientific methods of higher design theory with state-of-the-art manufacturing processes."Within a few years, we succeeded in developing lightweight and flexible gripper systems for robots and automation systems based on the human hand," says Saadat proudly. He convinced numerous industries of his technology. The grippers, which have up to six individually movable fingers and in some cases over 100 joints, are used to handle car wheels in the automotive industry, to hold up sacks when filling them with bulk material in filling plants, or to package sausage and cheese in the food industry.

Problem:

One of the challenges in the development process was to find suitable bearings for the moving metal components of the gripper, such as the finger joints. This was challenging, as the components have to be all-rounders, low-friction, abrasion-resistant, maintenance and lubrication-free, and easy to install.The use of classic rolling bearings was ruled out. "Rolling bearings are suitable for sustained rotational motion. They require the manufacture of more accurate fit, involve complex assembly and disassembly, and result in metallic noise." In addition, very space-saving plain bearings were necessary, as otherwise the compact design of the grippers could not be implemented. 

Solution:

Plain bearings made of high-performance plastics from igus provided the solution. They are tribologically optimised, i.e. improved in terms of friction and wear. "We have been relying on plain bearings from igus since 1991. It was therefore clear from the outset that the polymer bearings would also be used in the multifunctional long arm gripper. There is virtually no alternative to these bearings for us in automation technology, where there are constant back and forth movements," says Saadat.

Another advantage is that, compared to many classic metal bearings, the polymer bearings are light, corrosion-free and maintenance-free. They allow dry operation without a single drop of lubricating oil thanks to integrated solid lubricants. "That's a very important specification. Because we couldn't expect our customers to have to lubricate the gripper on a regular basis."

Virtually no signs of wear after 200,000 cycles

The iglidur polymer bearings are so robust that the bearings ideally last a gripper life of around 30 years. "An advantage for our multifunctional gripper, as the maintenance effort is reduced and productivity increases." A test in igus' in-house laboratory, which is the largest in the industry, also proves how wear-resistant the bearings are. Here the polymer bearings were competing against classic metal bearings. Both bearing types pivoted on a gas-nitrided St52 steel shaft - with a load of 30MPa and a speed of 0.01 metre per second. "In the case of the metal bearings, the gliding layer was already worn after 60,000 cycles," says Stefan Loockmann-Rittich, Head of Business Unit iglidur Plain Bearings.

"The iglidur G series plain bearings, on the other hand, showed almost no signs of wear even after 200,000 cycles. They are therefore ideally suited for reliable and maintenance-free use over many years." Specifications that, by the way, also apply to another type of bearing found in GMG's multifunctional gripper: the linear bearings of the drylin R series (RJM-01-16), which enable a controlled and safe linear movement of the gripper linkage. "The igus bearings convince us time and again, so we will continue to rely on the iglidur polymers in the future," sums up Saadat.