With numerous designs to choose from, specifying the right linear guidance system can be a daunting task. Mike Hughes provides an overview of the types of system available, as well as their main design features and typical areas of application.
When it comes to selecting the right linear guidance system for an application, many engineers are not completely confident. Not only are there numerous designs to choose from, each with their own particular advantages and limitations, but also many engineers may avoid linear guidance systems altogether and instead revert to their traditional comfort zone of specifying a rotary alternative.
However, on most occasions, if engineers contact a reputable supplier of linear guidance systems to discuss their application, potential issues can normally be resolved and a suitable solution found. Companies can save themselves a lot of wasted design effort or prevent the system from being too costly (ie over-engineered).
The supplier can also inform the customer exactly what is and what isn’t possible and can provide valuable advice and guidance on integrated systems such as lubrication, measurement devices, mounting arrangements, corrosion-protected coatings and materials, as well as sealing arrangements.
What is a linear guidance system?
Linear rolling element guidance systems, more commonly referred to as linear guidance systems, are translational guidance systems based on the principle of rolling elements (eg, balls, rollers, needle rollers) between moving guidance elements. Such guidance systems ensure that the translation occurs with very low friction between one or more moveable sub-assemblies, while maintaining a direction of motion along a linear track (eg, profiled rail, guideway, cylindrical shaft).
Linear guidance systems are responsible for the guidance and transmission of force between machine parts moving in a translational direction and so have a significant effect on the overall performance capability and accuracy of a machine.
Linear guidance systems are available in many different designs, including flat cage guidance systems, linear rolling bearings, monorail guidance systems (linear recirculating ball bearing unit, linear recirculating roller or ball guidance system), shaft guidance systems, track roller guidance systems, miniature guidance systems, and driven linear units (eg, linear actuators and linear X-Y tables).
Guidance systems comprise of either balls or rollers that run between the moving component and the stationary component. Linear guidance systems can therefore be differentiated in terms of:
* The type of rolling element motion (ie, with or without recirculation of the rolling elements).
* The type of rolling contact on the raceways (ie, point contact or line contact).
This article will outline the most common types of linear guidance system, ie shaft, track roller, flat cage, miniature and monorail guidance systems. Other designs such as driven linear units (eg, linear actuators and linear tables) and ball/planetary roller screws are covered in separate articles.
Shaft guidance systems
Shaft and round guidance systems with linear ball bearings are one of the oldest types of guidance system based on rolling elements. These guidance systems comprise a hardened and ground shaft and one or more low friction linear bearings.
Linear ball bearings are used for the support and transmission of forces. Linear ball bearings run back and forth on the shaft as linear motion elements. The unlimited stroke and return of the balls from the loaded zone to the unloaded zone are facilitated by a cage. These bearings support high radial loads while having a relatively low mass, enabling the design of linear guidance systems with unlimited travel.
The shaft (solid or hollow) is typically mounted on a solid support rail. Bearings and units are typically available in light duty, heavy duty, compact, machined or plain bearings (plain bushes) versions, in both open and closed types, with or without seals.
Due to their low to moderate accuracy, these relatively simple guidance mechanisms are specified for applications such as power tools (e.g. mitre saws) and fitness/exercise equipment.
Track roller guidance systems
Track roller guidance systems can support forces from all directions (and moments about all axes), except in the direction of motion. The system elements – carriages, composite guideways and track rollers – can be combined in many different ways to achieve designs that are precisely matched to the application.
Most carriages run on track rollers with preset clearance values. These are normally supplied ready-to-fit and are tamper-proof in operation. The bogie carriage, used with curved guideways, is typically set clearance-free by means of eccentric bolts. Connectors can be provided for secure joining of multi-piece guideway sections.
The running accuracy of a track roller guidance system essentially depends on the straightness, accuracy and rigidity of the mounting surfaces. The higher the requirements for accuracy and smooth running of the system, the more attention must be paid to the geometrical and positional accuracy of the adjacent construction. The adjacent surfaces should be flat and have parallel faces. The displacement force of a track roller guidance system depends on the preload, lubrication and the particular application.
Due to their lightweight construction, track roller guidance systems are highly suitable for applications in handling systems, where low noise running, high speeds and long travel distances are required, in combination with low, uniform displacement resistance. Typical applications include multi-axis and gantry arrangements, machine guards, packaging machinery, automatic doors and tool changers.
Monorail guidance systems
For general machine design applications, monorail guidance systems are one of the most important types of linear rolling element guidance system.
Monorail guidance systems comprise one or more carriages running on a profiled guideway. The technical characteristics and areas of application are determined by the number, arrangement and contact geometry of the rolling element rows.
The carriage comprises the functional components: saddle plate, end pieces, rolling elements, end seals and sealing strips. The rolling elements in the carriage are guided by a rolling element recirculation system that comprises a forward section and a reverse section. The saddle plate and guideways are often made from hardened rolling bearing steel and have ground raceways. The rolling elements are in point or line contact with the guideway and carriage (depending on the rolling element type).
The end pieces of the carriage contain return devices that direct the rolling elements from the forward section into the reverse section. These also support the end seals. The rolling element system is protected from contaminants by sealing elements. Lubrication is provided via lubricant connections in the end pieces and/or lubrication pockets within the carriages.
Rigidity is an important characteristic of a monorail guidance system and is defined as the ratio of load to deflection. Rigidity is dependent on the type and size of the guidance system. The size of a monorail guidance system is determined by the demands made on its load carrying capacity (dynamic and static), operating life and operational security/safety (load safety factor).
Depending on the type of rolling elements, monorail guidance systems are described as either linear recirculating ball bearing and guideway assemblies, or linear recirculating roller bearing and guideway assemblies.
Linear recirculating ball bearing & guideway assemblies: Available in two-, four- and six-row designs, these units comprise rows of ball bearings and a full complement rolling element system. The rolling elements are either in two-point contact with the raceways (eg, six-row designs) or four-point contact (two-row designs).
Linear recirculating roller bearing & guideway assemblies: These units have a full complement roller set (ie the maximum possible number of rolling elements). This means the units provide extremely high load carrying capacity and high rigidity. These units are therefore specified when linear guidance systems must support extremely heavy loads, where high rigidity is required and where very precise travel is necessary. In preloaded form, these units are suitable for machine tools.
The technical characteristics and areas of application of monorail guidance systems are determined largely by the number, arrangement and contact geometry of the rolling element rows. Typical applications therefore include machine tool axes, robots, packaging machinery, large rollers in paper processing machinery, medical equipment, measuring equipment, palletisers and other mechanical handling systems.
Flat cage guidance systems
Flat cage guidance systems are linear bearings without rolling element recirculation. The rolling elements move at half the velocity of the table and so cover only half the distance.
Flat cage guidance systems comprise a pair of guideways between which angled needle roller flat cages, angled cylindrical roller flat cages or needle roller flat cages are arranged. Guideways are normally available in various profiles, for example, with an adjusting rib for setting of preload, or with an integral toothed rack for positive control of the angled flat cage.
End pieces at the ends of the guideways hold the cage in its nominal position at the ends of the stroke length in order to prevent the cage from creeping out of the load zone. Cage materials are typically light metal, steel, brass or plastic.
Due to their design, flat cage guidance systems are ideal for oscillating motion and where linear locating or non-locating bearings with extremely high load carrying capacity with restricted stroke length and very smooth running are required. Bearing designs with these guidance elements have high rigidity, high accuracy, low friction and a smaller design envelope compared to other linear guidance systems. Typical applications include gauging equipment and linear motion systems that require very low section heights.
Miniature guidance systems
A variety of miniature guidance systems are available, characterised by their extremely compact designs, high accuracy, high rigidity and high load carrying capacity. The more compact versions of linear recirculating ball bearing and guideway assemblies (see ‘monorail guidance systems’ section) are also classified as miniature guidance systems.
Miniature ball guidance systems
Miniature monorail ball guidance systems are suitable for applications with unlimited stroke length and are normally available with two or four rows of recirculating balls. Special wide versions are also common.
Miniature carriage units
These units are suitable for limited stroke lengths and comprise a carriage running on a guideway by means of roller flat cages. The carriage, guideway and cage are typically supplied in corrosion-resistant steel.
Miniature plain guidance systems
Miniature plain guidance systems have a carriage and guideway and are available in various sizes. Typically, the carriage has a lining of low friction sliding material.
Miniature linear guidance sets
Miniature linear guidance sets are often available for applications with limited stroke length. These typically comprise a pair of guideways separated by a cylindrical roller flat cage.
Due to their compact size, miniature guidance systems are suitable for a variety of applications such as machines that manufacture electronic components, as well as compact medical equipment.
In some cases, linear guidance systems need to operate in special conditions or harsh environments such as high temperatures, high speeds, high accuracy, low noise or unusual load conditions.
Many linear guidance systems can therefore be supplied with special, anti-corrosion or anti-wear protection materials and coatings, as well as special lubricants or integral measuring systems. Most suppliers also offer a range of linear accessories such as wipers, plugs, clamping elements, guideway covering strips, emergency braking devices and vibration damping elements.
Assuming that sufficient application information is made available, a supplier of linear guidance systems is often able to advise and guide customers in selecting the most suitable accessories, materials, coatings and lubricants for their application.
Mike Hughes is an Applications Engineer at Schaeffler (UK) Ltd, Minworth,n Sutton Coldfield, West Midlands, UK.