For over 20 years now, the Treif Maschinenbau GmbH from Westerwald has been relying on plastic plain bearings in their machines, plants and systems for slicing food items such as bone-in and boneless meat parts, cheese, or bread. Installed in 1993 for the first time in a multi-tooth gripper, today their machines use lubrication-free and maintenance free products from polymer specialist Igus. The technical demands are rising steadily.

"Our goal is to involve our suppliers in product development at a very early stage," explains diplom-ingenieur Rainer Hebisch from the slicing division of Treif Maschinenbau GmbH in Oberlahr. "This is how we gain valuable time in constructing our systems."

This was also the case with the high-performance slicer Divider 880 for the food industry, which has a high-speed depositing system as one of its technical features. "It makes a difference whether meat or hard cured sausage is sliced," clarifies Iris Henrich, who is responsible for press and public relations. "For this reason, each machine is tailor-made to customer requirements."

NO ConTAMINATION BY LUBRICANTS

The slicing machine uses corrosion-resistant linear bearings and matching precision shafts from Igus. In total, the user gets a long-lasting linear motion solution for a specific application, which allows, among other things, high speeds and accelerations. Due to their wear and friction properties, the maintenance-free and lubrication-free linear sliding elements are suitable for most linear applications.

Designed for dry operation, their applications are protected from contamination by grease and oils, i.e. active lubrication. In addition, the plain bearings display their strength in the high-performance slicing machines.

SLICING WITHOUT IDLE CUTS

The task was to move the sliced goods out of the cutting area and feed them to the packaging machine without interrupting the product flow.

The handling works via the depositing system, a type of fork that is propelled into the product flow. The slicing process requires 50 to 75 percent of the available time. The time reserved for non-productive time, such as moving the fork, is reduced to a few milliseconds.

"If the fork is propelled into the product flow, enormous accelerations occur," explains Mr Hebisch. While ready-sliced packages are removed directly from below the knife and safely transported, the slicing continues. This allows a continuous product flow without idle cuts, thus there are no blade rotations in which the product is not cut. Up to six rows of products can be simultaneously sealed.

All in all, the advantages are obvious: fewer blade rotations necessary, the product is not affected, work steps are eliminated and it is possible to slice at higher product temperatures. Also, the cost of product cooling can be significantly reduced. The product quality increases because the goods can change in taste, look and sensory factors through light freezing.

ACCOMMODATING TO ALL NEEDS

When the machine was designed, the task was quickly defined in advance: no blank cuts. Blank cuts (knife rotations, wher no product is cut) require output or increase the required speed. They bring on high dynamic loads in the product. The product must be stopped and then again accelerated.

The acceleration especially of softer/hotter sliced products has natural limits: to increase the strength of the slicing material, they would have to be cooled; the blade performance and a chamber size for inserting the products to be sliced result in physical requirements; and the performance data must be compulsorily met by the machine elements.

At the same time, the slices must at all times be clean and absolutely uniform. "Due to the complex requirements profile of the continuous fast movements, we came up with a design. We then tested the suitable plain bearings ourselves for months," Mr Hebisch explains. "We resorted to the wear results and related service life calculations from Igus, so that all in all, at any time we felt we were on the safe side."

There are strict hygiene regulations to be observed in the food industry. Bevelled surfaces, the elimination of blind holes, lubricant-free lifting and slicing systems ensure a hygienic design. Product contamination by lubricants should not occur at any point when cleaning with water and different cleaning agents.

In addition to chemical resistance and low moisture absorption, it also depends on physiologically harmless materials, as they are located mainly in the product zone. And since the slicing machines sometimes work three shifts up to six days a week, their service lives need to be coordinated. Unscheduled maintenance should be excluded.

LUBRICATION-FREE LINEAR BEARINGS FOR DEPOSITING SYSTEM

Various lubrication-free open and enclosed linear plain bearings as well as plain bearings are used in the area of the depositing system for inserting the slices and also in the actual slicing machine.

"The decision between linear bearings or plain bearings depends on the application requirements in terms of performance, speed and service life," explains Florian Berg, technical sales consultant at igus GmbH. "In this environment, the topic of speed plays the leading role."

For this, precision shafts supplement functional capability. The liners of the linear plain bearings are characterised by their ease of use and interchangeability in everyday operation. The clip-on in the mounting hole has a positive locking. The axial securing is made possible by a ring groove and the rotation lock by engaging the locking pin in the appropriate hole.

"Originally we had expected problems in the area of propelling," recalls Mr Hebisch. "But they showed up in a completely different place." The toothed belt, with the fork, are synchronously driven and deflected. Roller bearings were initially installed in the deflection, and they had to be sealed. The sealing generated friction which had a negative effect on the drives. Consequently, roller bearings had to be omitted. Today, in the linear feed of the slices holder, external soft stainless steel shafts which are resistant to chemicals are used in combination with open linear films.

Additional lubrication-free and corrosion-resistant polymer bearings are used in the area of the depositing fork for mounting the slice holder. They ensure the pivoting movement of the drive to the pneumatic cylinder. The material is resistant to weak acids, diluted alkalis and many disinfectants used in the food industry; the intensive cleaning of machines in the food industry thus presents no problems for the polymer bearings.

The use of the polymer technology goes even further: soft stainless steel shafts and linear sliding elements secure the linear stroke of the pre-gripper for the forward feed of the slices. Polymer bearings are used in the pivotal mounting of the feeding tower. They are abrasion-resistant, insensitive to dust and dirt and very economical. All materials that come into direct contact with food are FDA-compliant and thus safe in the long-term.

Kai Jakobsen-Urwald, head of strategic purchasing at Treif, commented: "The slicing speeds have increased greatly in recent years. The lubrication-free polymer plain bearings withstand this rise without any problems. Due to the many positive experiences, they are also used very often in our other business areas."