Pallet gripper: Lightweight, cost-effective and tough

Profile

• What was needed: drylin R linear plain bearings
• Requirements: First and foremost, particularly lightweight and functionally reliable technology was needed that would also function without failure.
• Industry: Packaging technology
• Success for the customer: The drylin plastic bearings are light, cost-effective and yet very tough. They withstand shocks and dirt as well as strong vibrations or extreme weather conditions. Krones has not received any complaints so far.

The butterfly gripper is a modular system of its own, which is assembled from small modules depending on the gripping task.

Problem

Krones AG has developed special packaging grippers for the beverage industry for its Rosenheim plant. They do not grip containers, but the intermediate layers of the pallet, and also, for example, pallets themselves, cover frames or slip-on covers. The grippers that use the Fin-Ray effect visually resemble the flapping of butterfly wings when gripping an interlayer, hence the name "butterfly-grippers".
Since the cardboard boxes are delivered in stacks, the individual layers are tightly packed together. This makes it difficult to grip. In the past, the intermediate layers were therefore neatly separated by a brush magazine. They prevent the tightly pressed intermediate layers underneath from moving when a layer is lifted. However, such brush magazines cost a lot of money.
The gripper head accomplishes the task without the brush magazine: It lifts the sides upwards and the vacuum underneath is released.
But how does the gripper work technically? The entire assembly is pushed to the side using a tube as a guide. In a type of pulling action, the butterfly function is created via the movement of plain bearings. Pneumatic cylinders move together and the distance between the individual links is reduced.
The technology will be further improved in the future. It should become lighter and have an improved structure by using more plain bearings.
The gripper technology for packaging aids must function perfectly, otherwise the system stops. The requirement is to build in a functionally reliable and yet inexpensive way.

Solution

Today, plastic plain bearings play an important role for the movements of the grippers. Specifically, drylin R linear bushings of type RJMP-01-25 are used here. They were favoured by Krones because they are made of plastic and are therefore lightweight, cost-effective and yet very tough. This has been very well demonstrated in practice: Krones has not yet had any complaints about the resistant bearings. They are robust against shocks or dirt and also withstand strong vibrations or extreme weather conditions. Here again, it is the small refinements that count, such as the special segmented design of the bearings. This special feature proves its worth in dusty environments. This is where the compressed dust molecules normally accumulate on the guide shaft. As the dust layer increases day by day, the diameter of the shaft increases. As a result, the traversing movement becomes increasingly sluggish due to the strong friction.
Everything runs better in the plain bearing with the slots. There, the dust does not settle in the bearing, but is literally carried through the bearing without any significant changes in friction and wear behaviour.
The Krones gripper specialists, on the other hand, have had different experiences with the previously used recirculating ball bearings. Even a collision could destroy such bearings. The balls are often pressed into the shaft.
But 'conventional' packaging grippers from Krones also benefit from the plain bearings during operation. Because the plastic plain bearings fit perfectly into Krones' modular gripper system.

"The dirt is guided by the groove structure through the igus linear plain bearing RJMP-01-25," explains Peter Kirschner, the person in charge of the pallet gripping systems at Krones in Rosenheim.

Depending on the customer's product, a suitable gripper head can be selected from Krones' range. The modular system allows for customer-specific adaptation. There is no need for design work.