Should it be a sliding bearing or a ball bearing?
—Comparison of linear ball bearings and linear sliding bearings
Like all bearings in linear technology, there are fundamental differences between sliding bearings and ball bearings. In the past, ball bearings were a primary choice in design selections. Thanks to the development of innovative high-performance engineering plastics, sliding bearings can now operate without lubrication, making them particularly suitable for applications with special requirements.
The term "sliding bearing" historically represents one of the simplest and oldest types of bearings. When transporting stones used to build the pyramids, the ancient Egyptians placed the stones on specially hardened slopes, lubricated with a mixture of mud and water, and then dragged the stones to complete the transport. The contact surface, the track, and the ropes served to guide linear movement. Today, this ancient principle still applies to the tool guidance systems in modern equipment and factory construction. While a larger contact surface ensures higher static loads, it also means that only with the traction of a large number of laborers and animals can the enormous resistance hindering movement be overcome.
With the advent of the Industrial Revolution, metal sliding bearings and ball bearings became increasingly important. The development of ball bearings was based on reducing friction and the required power. Because the point contact technology reduced the number of intersecting contact surfaces, low-friction guidance of shafts or axles was achieved. Reduced friction also reduced heat generation. At the same time, wear and lubricant requirements were lower compared to sliding bearings. A fundamental component of all ball bearings is the so-called rolling element. Rolling elements are typically composed of several steel balls. In linear ball bearings, the rolling elements move axially along the bearing races. In the linear direction, the load is always transmitted through the inner ball bearing races, but the load on the outer ball bearing races is reduced by counteracting the kinetic forces. Generally speaking, the more balls used, the greater the load capacity.
Ball bearings require lubrication due to the need for contact between the balls. This makes them more susceptible to maintenance, especially dirt and moisture, which is why they are typically fitted with dust covers or sealing plates. The internal balls and cage-like structure also make them relatively vulnerable to external shocks and vibrations. Therefore, they cannot achieve smooth operation or noiselessness. The inertia of the balls can also reduce operating speed. However, overall, until specialized materials enable sliding bearings to become higher-performing variants, and until the original disadvantages (i.e., lubrication and maintenance requirements) are transformed into advantages, ball bearings remain a significant technological innovation that maintains their status as the best alternative.
The development potential of plastics technology
The development of high-performance engineering plastics has also opened up new opportunities in the field of sliding bearings. Because the friction of the material components has been optimized, sliding bearings can now be produced using polymers, even with insufficient lubrication. Sliding elements and counter-rotating components made of engineering plastics possess excellent wear and friction coefficient characteristics. Compared to metal bearings, which are prone to corrosion and require constant lubrication, or especially in outdoor applications where additional grease is needed, plastic bearings are more suitable for general applications. Under similar conditions, plastic bearings have a certain degree of tolerance to humidity and heat. Systems such as balers equipped with engineering plastic sliding bearings benefit from the long service life and high wear resistance of plastic products. Costly maintenance downtime or equipment failures are no longer a problem.
Engineering plastic bearings in linear technology
The advantages of engineering plastic sliding bearings in linear motion and drive technology due to the technical advantages of ball bearings will be illustrated with diagrams and examples.
Because of the larger contact area, lower surface pressure, and the ability to use flexible shafts or shafts made of other materials such as aluminum alloys and carbon fiber, one of the many advantages of sliding bearings is further weight reduction.
Since there is no mechanical rolling of hard friction components and no collision between rolling elements, there will be less noise and vibration from sliding operation.
Using linear sliding bearings allows guide rails to be spliced together, thus easily creating longer strokes, because the movement of sliding elements is easier to pass through guide rail joints than rolling.
One significant material-related disadvantage of all ball bearings is the limitation on permissible speed and acceleration; the maximum values are limited, especially at lower loads.
In contrast, engineering plastic sliding bearings offer higher sliding speeds and accelerations, significantly extending their cycle life in numerous applications. However, their most crucial advantage lies in their lifespan; bearings made from high-performance engineering plastics are generally more durable than traditional ball bearings. Furthermore, their lifespan can be calculated using various programs. Significant progress has been made in precision and friction values in recent years. Many fields that previously used traditional ball bearings are now adopting engineering plastic sliding bearings.
Use in sensitive application areas
Maintenance-free, self-lubricating linear bearings, exemplified by the igus Drylin series, have long been used in various industrial sectors due to their wide range of applications. In production processes with specific requirements for robustness and insensitivity to environmental factors such as humidity, chemical media, or contaminants, systems using engineering plastic linear bearings are ideal solutions, from linear guides to complete linear drive mechanisms.
One of the first applications of self-lubricating and innovative sliding bearing technology was in the wood and furniture industry, where sawdust sticking to traditional lubricants was a major cause of downtime and high costs. Guide rails would become clogged, or sliding components would separate again due to lack of lubrication (a phenomenon known as "blocking"). Igus was one of the first plastics industry experts to take on this challenge, developing the industry's only low-friction, dry-running linear sliding bearing.
Meanwhile, the advantages of Igus's Drylin technology have paved the way for sliding bearings in more sensitive applications. The complete linear drive mechanism can be installed not only in hygiene-sensitive fields such as medical equipment and the food industry, but also in cleanroom technology and high-temperature systems, ensuring dirt resistance, reliable drive, and even enabling high-speed operation with low noise. The Drylin system also has design advantages over competing products. Igus is the only supplier to integrate sliding elements with a dirt channel, which allows dirt particles to be automatically cleared from the guide rails during operation. This prevents excessive dirt accumulation at the front of the bearing from obstructing operation. Wherever replacement is needed due to heavy dirt and wear, the sliding diaphragm can be easily replaced at minimal cost. Competitors' linear sliding bearings lack this dirt channel design, thus emphasizing a larger contact surface area but neglecting the additional load caused by dirt and dust accumulation. Unlike these competitors, Igus researchers, from a production cost perspective, utilize their in-house laboratory facilities to find the optimal ratio between surface pressure and reliable operation.
Linear bearing technology has made significant progress; for a long time, ball bearings represented the limit for linear drives. However, high-performance engineering plastic products with low friction have set a new standard for maintenance-free, self-lubricating applications. Sliding bearings are easier to use, more reliable in operation, and ultimately cheaper. For this reason, modern engineering plastic sliding bearings clearly prevail in discussions of "sliding bearings vs. ball bearings."
igus Cable Bearing Warehousing and Trading (Shanghai) Co., Ltd.
Factory Building No. 46, No. 11 Debao Road, Waigaoqiao Free Trade Zone, Shanghai
Postal code: 200131
Telephone: 021-5130 7642
Fax: 021-5130 3233
Free service hotline: 400 720 3100
Email: [email protected]
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