This article will take an in-depth look at linear rails.
The article will bring more detail on topics such as:
This chapter will cover the concept of linear rails, including their design and functionality.
Linear rails are highly effective for moving items through production processes with exceptional precision and minimal friction, making them ideal for tasks such as creation, packing, and distribution. These devices are commonly used across various industries.
A linear rail system is engineered to support the movement and load of equipment in either vertical or horizontal directions. It is a straightforward mechanical component that performs its task effectively, facilitating smooth and secure movement of goods during production or packaging processes.
Linear rails are referred to by a variety of names, including linear guide rails, linear guides, linear guideways, linear slides, and linear guiding systems. A linear rail efficiently transfers weights along a predetermined horizontal or vertical course with the least amount of friction or resistance.
Linear guide rails are normally made of corrosion-resistant high-strength, toughened, and galvanized steel. Before installing a roller runner, the metal is formed and contoured using a cold drawing method. Profiled rail guides are typically the best choice for large loads since they are designed to produce a very precise linear motion. Rail guides are available in a variety of sizes, starting with minuscule linear rails for moving small components in tight spaces.
Linear rails can be designed in varying lengths, from short sections to lengths exceeding 2 meters, depending on the application's requirements. In the industrial sector, two main types of linear guides are commonly used: ball bearing runners and roller runners. The linear guidance system consists of two key components: the rail and the runner. The runner moves back and forth within the rail, and the assembly's moving parts typically include recirculating balls or roller bearings. These components are equipped with threads to secure the items being transferred.
Linear slides utilize various types of bearings, including rolling element bearings, ball bearings, and roller bearings. Additionally, there are plain surface bearings, which encompass metal-to-metal, dry lubrication, hydrostatic, and aerostatic lubrication types. Magnetic bearings also play a role in linear motion systems.
Rolling element bearings are the most commonly used type among the primary classes. Compared to greased and magnetic bearings, rolling element bearings offer greater durability and versatility, performing effectively in both dynamic and static conditions. Their performance and service life are well-predicted by industry standards and best practices. In contrast, magnetic and hydrostatic bearings are more specialized, typically used in laboratory settings and specialized instruments.
In linear slides, rolling element bearings are categorized based on the recirculation method of the rolling elements. In systems without recirculation, the travel distance of the linear slide is limited by the length of the rolling element row. Here, the rolling elements rotate with the carriage but do not complete the entire travel path.
Non-recirculating rolling elements move at half the speed of the carriage and cover half the distance. In setups with recirculation, the carriage features a return path that allows the rolling elements to circulate along a looped course. This design enables both the rollers and the carriage to move simultaneously along the guide rail. For recirculating linear slides, ball bearings are commonly used as the rolling elements.
The carriage is the moving component that is guided by the bearings and supports the tool, instrument, or sub-assembly in linear motion. Its movement is confined primarily to the X-Y plane.
Power screws or screw drives are employed in carriages that move along the Z-axis. Typically, the carriage is linked to a drive unit that provides the necessary force or torque to advance it.
The gliding surfaces of plain bearings or rolling elements slide against these fixed surfaces. Plain surface bearing guide rails are essentially flat surfaces with or without lubrication. They can also have a cylindrical shape, which is known as a shaft or journal. The races in rolling element bearings are designed to balance the contact covered area with the magnitude of contact stress. Because this is more noticeable in ball bearings, they have a racing profile that is divided into two types: circular arch and gothic arch.
Ball bearings are housed in two types of arches: circular and gothic. In circular arches, the ball contacts the race at two points, while in gothic arches, the contact occurs at four points. Gothic arches theoretically offer better load-bearing capacity.
However, gothic arches are more susceptible to differential slip, which arises from bent races with varying rolling diameters. This discrepancy results in different rolling speeds and increased sliding friction. Differential slip is more pronounced in gothic arches due to the distinct differences in rolling diameters. Consequently, circular arches are generally preferred. Gothic arches are often used in smaller systems where higher load capacities are required compared to circular arches of the same size.
In systems with recirculating rolling elements, these elements are mounted on both the front and rear sides of the carriage. The end caps direct the rolling elements from the load-bearing area to the return path, ensuring smooth and continuous motion.
End caps are designed to incorporate lubrication mechanisms for the recirculating bearings within the carriage races. This lubrication is essential for maintaining smooth operation and extending the life of the bearings.
Seals integrated into the end caps serve to block external contaminants such as dirt and metal debris from entering the bearing races. Since dirt is abrasive, it can cause significant damage to the surfaces of the guide rails and bearings.
Protective covers, such as bellows, safeguard the guide rail surfaces from damage. These are especially important for machinery that handles metal chips, abrasive substances, and coolants, such as lathes and milling machines, where debris is prevalent.
Impact dampers are installed at the ends of the carriage to act as a safety mechanism in cases of excessive travel, helping to prevent damage to the system.
For linear slides equipped with drive units, an integrated control system manages the carriage's movement. This system regulates the drive unit or actuator by delivering power based on operator controls or feedback signals from sensors and switches.
The component that provides or transmits the forces that move the carriage is known as the drive unit or actuator. Ball screw, toothed belt, rack and pinion, linear motor, and pneumatic systems are some of the driving units offered.
Position sensors provide feedback to the controller and driving unit. Their primary functions include preventing the carriage from exceeding its intended range of motion and determining its exact position.
This section will cover the specifications pertinent to linear rails.
A single-axis system has one axis and moves exclusively along the X-axis. Conversely, vertical lift devices operate along the Z-axis. Multi-axis positioning systems consist of stacked or connected units that move along two axes within the X-Y plane, typically orthogonally.
Examples include a carriage moving along the X-axis paired with another carriage moving along the Z-axis. Three-axis motion is achieved with three orthogonal axes in three-axis systems. Key travel characteristics for linear slides and linear stages include the X-axis linear travel, Y-axis linear travel, and Z-axis linear travel.
The accuracy of a slide is influenced by the type of bearing or guiding system employed. Linear bearings are capable of delivering highly precise and repeatable motion.
This refers to the total distance that the slide can traverse from one end to the other.
Load capacity indicates the maximum weight that a slide can handle without sustaining permanent damage.
This denotes the maximum speed at which the carriage can move along its axis.
The load capacity, moment rating, and stiffness of the slide depend on its design and the bearing or guiding system used.
For applications requiring high stiffness and repeatability, a ball screw drive with a motor may be necessary. Manual positioning can be achieved with a ball or lead screw and handwheel. Pneumatic and hydraulic drives are suitable for quick actuation when precision and repeatability are less critical.
Maintaining proper lubrication is crucial for any application involving linear guides or roller and ball bearings. In cleanrooms, the lubricant might need to be classified as "permanent" and approved by the FDA, depending on the cleanroom's class. Additionally, the same seals that keep the bearing clean will also help maintain the cleanliness of the working environment. If contaminants cannot enter from the outside, lubricants inside the bearing will not be able to escape either.
This is particularly important in more demanding conditions such as high-speed metalworking, where metalworking fluids used for cooling might infiltrate poorly sealed linear motion components, potentially contaminating or washing out the lubricant. To address this, reinforcing seals, applying hard chrome plating, using corrosion-resistant steel types, and implementing regular lubrication changes may be necessary.
Linear rails support and guide moving parts as they move in a reciprocating linear motion along a specific direction. They are categorized based on their friction characteristics into sliding friction guides, rolling friction guides, elastic friction guides, fluid friction guides, and more.
Automation machinery, like machine tools supplied from Germany, bending machines, laser welding machines, and so on, employ linear bearings. Linear bearings and linear shafts are, of course, used in tandem. There is no need for transitional media between moving parts and permanent elements of linear guides, as there is with linear rails, which are mostly utilized on mechanical systems with high precision requirements. Instead, rolling steel balls are used.
Rolling steel balls are effective for moving parts like tool holders and carriages due to their ability to handle high-speed movement, low friction, and high sensitivity. However, excessive force on the steel ball can lead to prolonged preloading times, which increases the kinematic resistance of the bracket.
Design engineers often inquire about the lifespan of linear rails. Understanding how to calculate theoretical life expectancy and factors that may impact it is essential.
The theoretical or nominal life expectancy of a linear bearing represents its maximum possible lifespan, usually calculated in terms of how far something is likely to travel. It’s calculated based on the load size and the bearing’s ability to handle that load over time.
This method relies on Hertz's theory of impact, which enables the calculation of the maximum surface pressure between two curved bodies. From this, the dynamic load capacities, which are influenced by surface factors, are determined. The Nominal Life calculation for both guides and screws follows the rolling bearings approach from DIN ISO 281.
Nevertheless, this calculation is insufficient for predicting the actual lifespan of a bearing in real-world conditions. Therefore, DIN ISO 281 also defines how to compute what is known as "Modified Nominal Life Expectancy." This calculation estimates the probability that a sufficiently large sample of identical bearings operating under consistent conditions will achieve or exceed the theoretical life expectancy before experiencing material fatigue, by applying a life expectancy coefficient to the previous formula.
In this calculation, a 90% survival rate (the industry standard) is assigned a coefficient of 1, meaning that a higher survival rate will result in a reduced life expectancy. Consequently, the formula is adjusted to reflect this.
In other terms, achieving a 99 percent probability of bearing survival means that the Modified Nominal Life Expectancy is reduced to one-fifth of the life expectancy typically associated with a 90 percent survival rate. These figures serve as a preliminary reference for assessing the actual longevity of a linear rail in practical applications. The anticipated lifespan of linear motion components is influenced by three key factors: environmental conditions, operational demands, and installation practices.
Numerous potential combinations are available to address specific needs, including various bearing types, recirculating or non-recirculating designs, bearing contacts, race profiles, drive mechanisms, and precision control systems. Nonetheless, certain combinations are favored due to their simplicity, load-bearing capacity, rigidity, and versatility. These configurations are continually refined to suit their intended applications. Below are some of the most frequently utilized linear slides.
These linear rails feature basic surface bearings that depend on lubrication and have a low friction coefficient. They are named for their dovetail-shaped projection, which interlocks with a corresponding negative geometry.
The projection is typically fixed to the stationary rail or base, while the carriage features the corresponding negative geometry. This configuration is often referred to as a dovetail table. Dovetail rails are robust, capable of handling both radial and lateral forces. They are frequently used in large machine tools like lathes, shapers, and milling machines.
Boxway rails, similar to dovetail rails, utilize basic surface bearings. Unlike the dovetail design, boxway rails are characterized by a square gib with T-shaped flanges on the top.
Because of the increased contact area between the carriage and the rail, boxway rails are capable of supporting heavier loads compared to dovetail rails.
This design utilizes cylindrical surfaces rather than interlocking shapes. The cylindrical surfaces are referred to as bushings and journals. A bushing is a hollow cylindrical component integrated into the carriage, while the journal is an extended shaft that acts as the guide rail mounted on the base.
Sleeve bearing slides offer ease of operation and can manage loads applied from any direction. However, they are less robust compared to dovetail and boxway slides, making them more appropriate for light to moderate load applications.
Similar to sleeve bearing slides, this type incorporates ball bearings instead of basic bushings. Inside the bushings, recirculating ball bearings are used. These bearings can recirculate tangentially or radially. Tangential recirculation has a return path oriented sideways or tangentially relative to the shaft, facilitating a more compact design. In contrast, radial recirculation features a return path perpendicular to the axis, allowing for more rows to handle greater loads.
Bushings are categorized based on their design into closed and open types. Closed bushings provide support only at the ends of the shaft, while open bushings offer support from underneath. The additional support underneath helps reduce shaft deflection when carrying heavy loads.
This is a widely used type of rolling element slide. Linear ball rails are similar to linear ball bushings but use a runner block instead of bushings. The runner block can also incorporate a return path for recirculating the balls.
Linear ball rails offer greater load capacity and flexibility compared to linear bushings. The races are mounted directly on the base, which can lead to guide rail misalignment. Furthermore, race profiles can be customized in various designs to enhance either load capacity or minimalistic requirements.
This type employs rollers positioned at angles of 45° and 135° to the horizontal, as suggested by its name. The rollers may be organized in a single row with alternating 90° orientations or in multiple rows, where each row is perpendicular to the others.
This type provides greater load capacity compared to similarly sized ball rails, thanks to the larger contact area of the roller bearings.
This distinctive linear slide combines ball bearings with power screws. The common Acme-threaded power screw drive meshes with the nut integrated into the carriage through sliding contact.
Using balls as rolling element bearings in a ball screw further minimizes friction. The nut is equipped with a recirculation return pathway.
This section will explore the uses and advantages of linear rails.
Linear guides offer exceptional travel precision due to the precise machining of one or both rail edges, which act as reference surfaces. The use of two, four, or six rows of rolling elements—whether spherical balls or cylindrical rollers—ensures high stiffness and minimal bearing block deflection. These attributes combine to make linear guides ideal for applications demanding high precision, rigidity, and durability.
Linear rails are capable of supporting overhung loads even with just one rail, thanks to the load-supporting balls (or rollers) positioned on each side of the rail. (For overhung loads, round shaft linear guides should be used in pairs.) This capability allows many systems to utilize a single linear rail, thereby saving space and reducing alignment issues among other components. Below are some examples of applications that benefit from a single linear rail.
Because of their ability to sustain moment loads, linear rails are frequently used as the guide mechanism for actuators driven by belts, screws, or pneumatic cylinders.
They can also handle travel speeds of up to 5 m/sec, which is critical in belt and pneumatic systems.
Linear rails are ideal for guidance when loads are positioned directly below the rail and bearing block, which is often the scenario with overhead transport systems.
Thanks to their substantial load-bearing capacity, heavy loads can be moved effectively, and the rigidity of the linear rail enhances the overall stability of the system.
A gantry system features two X axes and sometimes two Y and two Z axes. Each axis typically operates via a screw or belt and pulley mechanism and is composed of a single linear rail.
Although each axis uses only a single linear rail, excellent moment capacities can be achieved when two axes operate together (for example, X and X').
To handle significant moment loads, linear rails can be used in pairs to distribute the moment load into forces applied to the bearing blocks. In this configuration, the drive system can be placed between the linear rails, resulting in a very compact overall system. Examples of dual linear rail applications include:
Stages typically require high precision, so ensuring accurate travel and minimal deflection is essential.
To maximize stiffness and extend bearing life, dual linear rails are often employed, even when the load is primarily centered on the stage with minimal moment loading.
For high-quality production, machine tools, similar to stages, demand exceptional travel accuracy and rigidity. Deflection is reduced by employing two parallel rails, frequently equipped with two bearing blocks on each rail.
Given the immense loads machine tools endure, distributing the weight across four bearing blocks helps to prolong bearing life.
Cartesian robots typically feature one linear system per axis, requiring each axis to manage substantial moment loads. Therefore, most cartesian robot axes are constructed using linear actuators with two parallel linear guides to accommodate these demands.
Six-axis robots are well-suited for tasks that require extensive reach and rotational movement in multiple directions. Additionally, dual-rail systems can function as a "seventh axis," enabling the robot to relocate to a different station or work area by transferring the entire unit.
A key benefit of linear rails in many applications is their ability to be linked together for extended travel lengths, often exceeding 15 meters.
Linear rails offer several advantages compared to other guide mechanisms, with their load-bearing capacity, movement accuracy, and structural rigidity being the most significant.
Nevertheless, the advantages of linear rails generally surpass their disadvantages.
Linear slides, also known as linear guides or linear-motion bearings, are bearings that allow for smooth, friction-free motion in a single axis. Linear slides use rolling element bearings, plain surface bearings, and magnetic bearings as its working principles. A linear slide's main components are the bearings, carriage, and guide rails. The linear motion guide system is made up of other parts such as drive units, sensors, controllers, lubrication systems, and others. Dovetail, boxway, sleeve bearing, linear bushing, linear slide, crossing roller, and ball screw slides are the most popular types of linear slides.
From a purely performance perspective, linear rails are the best. They offer greater precision, better mounting, and smoother motion and reliability. However other linear guides, including round shaft systems, plain bearing guides, and even crossed roller slides, may be suitable and less expensive for applications that do not require the load capacity, rigidity, or travel accuracy of a linear rail.
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