Linear guide systems: when to use plastic linear bearings or recirculating ball systems

Linear guide systems can be divided into open and closed guides (according to the shape of the guide and the direction of the transferable forces) and into plain bearing or rolling bearing (according to the type of friction). In principle, a linear guide consists of two components: a leading and a guided part. The leading element is a profile rail or a round shaft. Guide carriages and round bushings are guided. The difference is in the type of bearing, rolling or sliding translation. Both linear guide types are available individually, as a unit as well as in combination with linear drives as a complete assembly.

Rolling bearing guides

Most rolling bearings operate according to the principle of a recirculating ball bearing system and are therefore also called recirculating ball bearing guides or recirculating ball bearings. Small steel balls are used as rolling elements, which move axially in a ball channel. There are two types: circular guides with ball bushings and rail-rail units with intermediate flat ball cages.

In the case of the profile rail guide, the rolling movement takes place by means of a rail and a ball guide carriage running on it. The carriage guides the recirculating steel balls, which are stressed in the direction of movement via the inner ball guide row, deflected and guided back within the carriage into the raceway in a load-free manner against the axial movement. This recirculation principle ensures that all balls are evenly loaded. The small contact surface of the rolling elements with the elements of the guidance leads to very low friction. Due to the point of contact and the associated high pressures on bearings and shafts, only hardened rails and shafts made of steel or stainless steel can be used. The ball rail guides can be provided with two-, four- or six-row orbits (for very high loads). The balls usually consist of steel, or ceramics when high speeds are also required. The runner blocks can be made of steel or aluminium.

In the case of round ball bearing guides, the rolling movement takes place by means of recirculating rows of balls in a bushing on a round shaft. This form is the most widely used linear guide.

Figure 1: Plain linear polymer bearing compared with a recirculating ball bearing system (round shaft).

Plain bearing guides

The concept of linear plain bearings differs from that of recirculating ball bearing systems in the nature of movement: plain bearings do not roll, they slide. This gives a larger contact surface resulting in lower surface pressure. Due to the large surface load distribution in comparison with ball bearing systems, low-cost, soft shaft materials such as aluminium and non-metallic components can also be used as a running partner (shaft) in addition to hardened stainless steel or hard-chromium-plated steel. In order to reduce friction, plastics optimised for friction and wear are applied on the sliding surfaces. These high-performance plastics consist of a thermoplastic base material which ensures good wear resistance. In order to increase the mechanical load-bearing capacity, fibres and fillers are added to the matrix.

By selecting the suitable material pairing of sliding surface and shaft or rail, the coefficients of friction of the plain bearings can be controlled and their service life extended. The polymer component can be fitted to the guide, the guide carriage or as a liner between the components. There are also sliding bushes made of solid material, and the entire bearing wall thickness acts as a wear zone.

Comparing linear guides

As a result of the different technical principles and materials, both rolling guides and linear plain bearings have their strengths and weaknesses.

Lubrication versus dry operation

A significant difference between linear plain bearings and rolling guides is the use of lubricants. In order to reduce friction as well as corrosion, the contact of the steel rolling elements in rolling bearings requires a permanent lubrication with grease or oil. The recirculating ball bearing system ensures that the amount of lubricant is evenly distributed; however, a linear bearing that has to be lubricated requires maintenance: approximately every three to six months the guide carriage must be relubricated via an adapter or a grease nipple. 38.5% of all rolling bearing damage is caused by a lack of, or incorrect, lubrication.

Lubricated rolling bearings are also sensitive to dust and dirt. Higher quality rolling bearings are therefore equipped with scrapers or guards for environments with gross dirt and chips. Longitudinal or end seals as well as cover strips can also prevent the penetration of contaminants into the interior of the ball carriage; however, the sealing elements increase friction and are very sensitive. If the lubrication is not sufficient, the sealing elements become brittle and crack, allowing dust to penetrate the housing.

The self-lubrication effect

Linear plain bearings made of plastic operate dry because micro-fine solid lubricants are embedded in minute chambers in the matrix of the polymer. As soon as the linear guide moves, homogeneously distributed solid lubricant particles are released by micro-abrasion, which settle in the microscopic troughs of the shaft surface and lubricate the guidance itself. Due to the self-lubrication effect, plastic plain bearings do not have to be serviced, and linear plain bearings can also be used openly in adverse environmental conditions. Dirt or dust particles cannot stick to lubricants or are pushed forward from the bearing during the next movement. Furthermore, liners with specific geometries act as dirt channels which push any foreign bodies from the guide track without hindering the operation of the bearing.

Dry operating linear bearing systems are extremely clean and hygienic, so that numerous polymer plain bearings are certified for cleanroom compatibility. Lubrication-free linear plain bearings are also suitable for use in the food industry, since they cannot lead to contamination of foodstuffs and the plastics are resistant to harsh disinfectants.

Precision

The special strength of the rolling guide is its high precision. The bearing clearance, which is achieved by precise manufacture and the applied pre-load, is about 0.001 mm to 0.01 mm for linear rolling guides.

Since linear plain bearings must always have a minimum clearance in order to counteract a braking effect, polymer plain bearings achieve a maximum bearing clearance of 0.02 to 0.15 mm. For this reason, their use in machine tool manufacturing, CNC machining or electronics production is not recommended.

Weight

Low weight is beneficial in designs where every gram counts. It is also useful when it comes to dynamic applications and the increase in the number of cycles, as in handling or automation tasks, since it increases acceleration. Heavy metals such as non-alloyed steel also have a very high weight due to their high density. In this regard, linear guide systems made of plastic perform better, since they are roughly five times lighter than steel and still display good toughness. The less mass you have to accelerate, the less energy you use.

Plain bearing guide systems are more flexible when compared with rolling bearings in the choice of materials for their guide partners. They can run on soft and non-metallic shaft or axis materials. Aluminium is lighter than steel, and a linear guide that slides on non-metallic shafts — for example, on carbon fibre-reinforced or glass fibre-reinforced plastic — is 40% lighter again than aluminium guides and 60% lighter than a steel rail. The flexible use of plastic solutions is also useful wherever physical loads are reduced or transport costs have to be reduced.

Figure 2: Relative densities for different bearing guide materials.

Corrosion protection

The corrosive process in steels can seriously affect the operation of a component. For this reason, many rolling guides use stainless steels, which have significantly better corrosion resistance than low-alloy and unalloyed steels. To some extent they also withstand aggressive media and do not require additional surface protection.

Polymer plain bearings, on the other hand, are corrosion-free. Due to their organic nature, they are resistant to inorganic media as well as mineral acids, alkalis and salt solutions. A guide with a shaft made of high-alloyed steel in combination with a plain bearing made of high-performance polymer is suitable for cleaning-intensive industries such as filling technology, chemical and electroplating industry or in situations involving seawater.

Speed

Linear bearings must be able to withstand high speeds and accelerations. Due to the inertia of the rolling elements the maximum speed of rolling bearings is 5–10 m/s. Plain bearings, on the other hand, reach up to 30 m/s and also perform better in terms of acceleration provided only small masses up to a maximum of 10 kg have to be moved, otherwise the friction heat caused by the higher contact pressure during the sliding movement will interfere. When using hard-coated aluminium as a guide, the operating temperature in the bearing point can be reduced by the high thermal conductivity of aluminium, allowing significantly higher loads and speeds.

Loads and impacts

Plain bearings made of thermoplastic polymers can handle loads with surface pressures of up to 150 N/mm². The fibre matrix of thermoplastic compounds does not yield even with radial pressures, and the dampening properties of the plastic also make them relatively insensitive to impacts, vibrations and shocks. In comparison, the soft and thin gliding layer of the metal bearings can be easily pushed away under high loads, edge pressures or vibrations.

Figure 3: Flexural strength of bearings constructed from materials.

Stroke lengths

In order to achieve greater stroke lengths, guide rails are lined up in a row. In the case of linear plain bearings, the rails are slightly chamfered at their ends and placed one behind the other. The grooves produced by the joints can be easily passed over by the sliding elements. Rolling guides, on the other hand, are mortised, measured and aligned in order to make the crossovers precise and to avoid damage to the shield and the balls.

Ball guides require a minimum stroke length so that all balls can be inserted and the lubricant can be recirculated. For very small strokes, the same balls are always in contact with the shaft in rolling bearings. As a result, they are stressed on one side, which impairs service life. Over short distances linear plain bearings are therefore more suitable because they operate independently of the stroke length, even as short as 2 mm.

Coefficient of friction

Rolling friction is 10 times lower than sliding friction. Since the coefficient of friction of the linear plain bearings is higher than that of the rolling guides, the required drive force is consequently also higher. This fact must be taken into account, especially in manually operated applications.

Figure 4: The 2:1 design rule for the distance of the driving force for linear plain bearings.

In order to avoid the possibility of an uneven movement sequence or blockage of the system, the position of the drive must be precisely planned for linear plain bearings. The greater the distance between the drive and fixed bearing, the higher the degree of wear and required drive force. Therefore, the 2:1 design rule should be applied: if the distance of the driving force to the fixed bearing is more than twice the bearing spacing (2:1 rule), the guidance will theoretically get stuck at an adhesion friction coefficient of 0.25. Thereby this principle applies regardless of the value of the load or drive force. On the other hand, rolling bearing guides are more flexible in application because of their lower coefficient of friction.

Costs

The production of polymer sliding elements using an injection moulding process as well as completely injection-moulded plastic bearings is economical, since plastic production is more energy efficient than metal production. In addition, there are restrictions with regard to the casting moulds, whereas more complicated mouldings can also be produced from thermoplastics with relatively little effort. Even extruded, coated aluminium rails are more cost effective to manufacture than the multipressed, ground and finally hardened profile rails made of steel or stainless steel used in rolling bearings.

By dispensing with external lubrication in plain bearings, the costs for lubricants can be saved. Production costs can also be reduced, since the maintenance-free, dry-operating linear plain bearings significantly reduce machine downtime. Overall, plastic plain bearings can reduce the costs of metal solutions by up to 40%.

Conclusion

Certain industrial requirements need different linear guides. If machine elements have to work with millimetre accuracy, or high precision and low friction are indispensable, then rolling bearings should be used. In the interior of machine tools, machining centres or even in the assembly of printed circuit boards in electronics manufacturing, rolling guides cannot be replaced because of their very high precision and stiffness. However, if accuracy plays a minor role and factors such as dirt resistance, corrosion resistance or jerk-free movements are required, a linear plain bearing made of high-performance plastic is the first choice.

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