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MDA Technologies 4/2014

MDA Technologies 4/2014

Machine Elements Glide

Machine Elements Glide or roll? Linear ball bearings and linear plain bearings compared Stefan Niermann As with all bearings in linear technology, the same basic distinction is made between two bearing types: gliding and roller bearings. The latter type is often named ball bearings after the main individual type within the group. Thanks to innovative high-performance plastics, today’s plain bearings can manage without lubrication, hence are particularly suitable for demanding applications. Author: Stefan Niermann, Head of the drylin Linear and Drive Technology Division, Igus GmbH, Cologne, Germany In historical terms, plain bearings are the simplest and oldest type of bearing. The ancient Egyptians transported the stones for their pyramids on slides which were drawn over special hardened transport ramps and lubricated with a mixture of sludge and water. Contact surface as well as slide and ropes acted as a guide for straight, translative movement. This ancient principle is still basically applicable for tool guidance in modern machine and plant construction. The large contact surface guaranteed a high static loadbearing capacity; at the same time it meant enormous resistance to movement which could only be overcome by the mass use of human and animal tractive force. When the industrial revolution came along, plain bearings and roller bearings made of metal became increasingly important. The development of roller bearings was based on reducing friction and thus the motive force required. The surfaces were reduced to point contact, and low-friction guiding of shafts or axles was achieved. Heat development was also able to be reduced thanks to the reduction in friction. At the same time, wear and lubricant requirements were lower compared to plain bearings. An elementary component of all roller bearings is the so-called roll body. Roll bodies are usually balls made of steel. In the case of linear ball bearings the roll bodies are moved in races in an axial orbit. In the linear direction, the load is always passed on through the inner ball race whereas the load on the outer race is reduced against the movement. As a general rule, it can be said that the more balls are used, the greater the load-bearing capacity Mutual ball contact requires ball bearings to be permanently lubricated. This makes them susceptible to maintenance and particularly sensitive to soiling and humidity, 34 MDA Technologies 4/2014

which is why they are often fitted with cover and sealing plates. The inner ball and cage structure also makes them relatively prone to external impact and vibration. Consequently, they are neither smooth-running nor quiet. The inertia of the balls also reduces the possible speed of operation. All in all, however, ball bearings were a significant technical innovation which remained the best alternative until special materials made plain bearings the better performing bearing variant, and the original disadvantages – namely lubrication and maintenance requirements – were turned into an advantage of plain bearings. The potential of plastic technology The development of high-performance plastics opened up new opportunities in the field of plain bearings as well. Tribologically optimised material components now permit the production of polymer plain bearings which do without lubrication completely. Glide elements and counterrotation partners made of tribo-plastics possess optimum wear and friction coefficient characteristics. In contrast to metal bearings, which are prone to corrosion and thus need constant oiling or greasing in outdoor applications in particular, plastic bearings are suitable for universal use. They are resistant to humidity and heat in equal measure. Systems such as packaging machines which are equipped with polymer plain bearings benefit from the long service life and high wear resistance of the plastics. Expensive maintenance stoppages or machine failure are no longer a problem. Polymer bearings in linear technology As an example, the technical advantage of polymer plain bearings (also known as slide MDA Technologies 4/2014 35

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