This article takes an in depth look at Linear Bearings.
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Linear bearings are crafted to sustain the load of a carriage as it moves along a singular axis, offering a low-friction interface for seamless sliding on guide rails. Within a linear guide system, the carriage travels in either a straight or curved path along the guide rail, which is embedded in the linear bearing.
These bearings come in various forms, including rolling elements and fluid-based devices, to minimize friction. They ensure high precision, stable mounting, and smooth motion. Linear bearings find applications in 3D printers, sliding doors, and other automated systems that require precise rail movement.
A linear bearing is a pivotal part of a linear guide assembly. Its usage spans cutting machinery, XY positioning tables, machine slides, industrial robots, and instrumentation systems. Movement can be powered by a motor-driven ball screw, lead screw, pneumatic cylinders, hydraulic cylinders, or manual force, with motion limited to a single axis in the X-Y plane. Hydraulic and pneumatic cylinders are also fundamental components of the XY bed in computer numeric controlled (CNC) milling machines.
Linear bearings are primarily categorized into two types: rolling linear bearings and plain linear bearings. The following sections will explore the components, operational principles, and design considerations of each type in detail.
Rolling linear bearings are widely used for linear motion due to their low friction characteristics. These bearings incorporate balls or rollers that are situated between the grooves of the bearing and the guide rails, facilitating smooth movement.
The diameter of the balls or rollers affects the speed of the linear guide: larger diameters generally allow for higher speeds. Additionally, the contact angle—measured horizontally—impacts the bearing’s load capacity in various directions. A contact angle of 45° offers balanced support for loads in radial, reverse radial, and lateral directions, with the angle influencing radial load capacity positively and lateral load capacity negatively.
Rolling linear bearings come in various designs, each offering different classifications based on their structural characteristics.
Linear ball bearings, also known as ball bearings, feature spherical rolling components such as steel balls. They are favored for their low friction, extended lifespan, and high precision. As one of the most common types of rolling linear bearings, their spherical design makes them versatile for various linear bearing applications.
Cylindrical rolling elements are another type used in linear bearings. These bearings offer greater load capacity, rigidity, and resistance to shocks and impacts compared to ball bearings. However, they tend to have higher friction due to the increased contact area and are more susceptible to misalignment issues.
Needle linear bearings utilize cylindrical rollers, known as needles, with a length-to-diameter ratio ranging from 3:1 to 10:1. These bearings offer superior rigidity and load capacity compared to ball or cylindrical bearings because the load is spread across a larger number of smaller rollers. The reduced size of the rollers enhances the contact area and minimizes deformation.
The geometry of the track in a linear bearing influences the number of contact points between the balls or rollers and the raceway, affecting both the load capacity and friction generated by the bearing.
In a gothic arc profile, the ball makes four contact points with the raceway—two points each on the bearing and guide rail grooves. This profile is more compact and can handle higher moments and heavier loads compared to the circular arc profile with the same raceway size. However, it also experiences greater differential slip, which increases friction. Differential slip occurs due to the varying circumferential lengths between the inner and outer contact diameters, leading to different rolling speeds and resulting in slippage. Consequently, driving the ball bearing requires a greater force to counteract this effect.
In a circular arc profile, the ball contacts the raceway at two points—one on the bearing groove and one on the guide rail groove. This design results in reduced differential slip and lower friction compared to the gothic arc profile. However, the circular arc profile typically offers lower load capacity than the gothic arc profile.
The linear guide profile defines the cross-sectional shape of the guide rails, which in turn influences the design of the bearing employed in the linear guide system.
Round rail profile linear guides use cylindrical shafts as guide rails and require linear bushings. A linear ball bushing, commonly referred to as a linear bushing, features a cylindrical structure that encases recirculating balls moving along the cylindrical shaft guide rail. The bearing includes a cage to manage the balls' movement. Its straightforward design, compact size, and ease of installation make it a practical choice.
Ball spline is a variant of the linear ball bushing. It features a shaft with axial grooves that align with corresponding grooves in the spline nut. These grooves prevent shaft rotation and enable torque transmission. Ball splines are designed to handle greater moment loads and are capable of supporting overhung loads effectively.
Square rail profile linear guides feature rolling elements positioned on two opposing sides along the length of the guide rail. In comparison to round rail profiles, square rails offer greater load capacity, enhanced stiffness, and improved resistance to shocks and vibrations. They also provide greater moment load capacity.
Guide roller-based linear systems use individual ball-bearing rollers with a groove in their outer race, which run along a corresponding steel track. The track typically features a V-shaped edge, and the rollers are equipped with a matching V-groove. In some configurations, the roller groove is designed to accommodate rollers on a cylindrical shaft.
These roller and track components can be integrated into machine frames with limited space. The adaptable design allows for various configurations to optimize support and performance based on the load's position.
The rollers are designed to resist contamination, as they are sealed to protect the ball path and remain isolated from the track. Contaminants between the track and roller are displaced as the roller moves, with the inner part of the roller moving slower than the outer part, helping to push debris away from the guide rail.
Vee linear guide systems are ideal for straightforward linear movements. They are easy to integrate, assemble, and maintain, making them suitable for demanding environments where contamination is unavoidable.
DualVee guides are designed for harsh and challenging conditions where other systems may fail. Their long length, low noise, and smooth operation provide superior performance and stability in stressful environments.
Drawer slide guide systems feature C-shaped slides and carriages made from sheet metal. Each carriage glides along its slide with the aid of two sets of intermediate ball bearings—one on each side of the carriage and slide. Unlike profile rail linear guide systems, the rolling elements in drawer slides do not recirculate but are held in place within the carriage by a ball cage.
In a recirculating linear bearing, balls circulate continuously within a looped raceway, enabling the carriage to move smoothly along the guide rail's length. This design allows for movement across any length of the guide rail. Recirculating linear bearings typically feature multiple raceways to support this function.
The raceway is the straight channel through which the balls or rollers move. The configuration of the raceway in a linear bearing plays a crucial role in how the bearing responds to applied torsional forces and impacts the overall performance of the linear guide.
In a face-to-face configuration, the balls or rollers make inward-facing contact with the guide rails, forming an X-pattern. This setup provides uniform load capacities in all directions but has a lower resistance to applied moment loads.
In a back-to-back arrangement, the balls or rollers contact the guide rails facing outward, forming an O-pattern. This configuration provides increased leverage on the guide rail, offering enhanced resistance to applied moment loads, as well as greater stiffness and rigidity.
In a non-recirculating linear bearing, individual rollers are fixed in place within a frame known as the cage, which is housed inside the bearing. These rollers are spaced at equal intervals by a retainer or separator, preventing direct metal-to-metal contact between them. Cages can be constructed from either metal or plastic materials.
Linear motion in these bearings is confined to the bearing's length. The rolling elements contribute to linear movement by rotating solely around their own axes. This constrained motion allows non-recirculating linear bearings to offer high load capacity and stiffness, along with smooth operation and precise travel accuracy.
Non-recirculating linear bearings come in the following types:
Non-recirculating linear ball bearings feature metal balls that are secured within a cage. The corresponding grooves on the bearing and guide rail may have either a circular or gothic arc track geometry.
Flat-type roller bearings use cylindrical or needle rollers arranged horizontally within the cage. The rollers' axes are oriented perpendicular to the direction of linear motion.
V-type roller bearings feature a V-shaped track with a 90° angle for the carriage. Each side of the V-profile accommodates a row of cylindrical or needle rollers.
Crossed roller bearings feature cylindrical rollers arranged so that each roller's axis intersects at a 90° angle with the adjacent ones, creating a crisscross pattern. This design provides increased load capacity, enhanced resistance to vibration and shock, and a longer service life. However, their assembly can be challenging.
Plain linear bearings operate through direct sliding contact between two surfaces, without the use of rolling elements. Compared to roller linear bearings, they feature a simpler construction and operating mechanism, making them more cost-effective. The larger contact area results in lower surface pressure, allowing them to support higher loads, weigh less, and better absorb shocks and dampen vibrations.
However, they have higher friction. Friction limits the speed of the linear guide and increases its wear. Hence, lubrication needs to be maintained. Different sliding materials or materials with a self-lubricating coating are frequently used to reduce the coefficient of friction. They also have lower travel accuracy, which makes them unfit for high precision systems.
Plain linear bearings include the following types:
Box-way slides are a type of linear bearing featuring a T-shaped profile formed by a stationary base and a moving saddle. The base acts as the guide rail, while the saddle serves as the carriage. Adjustable gib plates are placed between the base and saddle to apply preload and eliminate clearance. Box-way slides offer increased load capacity due to the larger contact area between the saddle and base.
A dovetail slide is a linear bearing featuring a base with a V-shaped tongue that fits into a matching saddle, providing full contact between the two components. Dovetail slides offer a high load capacity but do not allow for preloading. Instead, gib plates can be installed along the saddle's length to adjust for any clearances.
Linear sleeve bearings, or plain linear bushings, are hollow cylinders that support the journal (shaft guide rail) sliding along their inner surface. This surface is often coated with self-lubricating materials, such as PTFE. These bearings can accommodate both axial and radial loads but generally offer lower load capacity and stiffness compared to box-way and dovetail slides. They are commonly used in light to medium-duty applications.
Non-Contact Linear Bearings operate without direct contact between the carriage and guide rails, resulting in reduced friction. This design leads to longer service life and the capability for higher speeds. There are two main types of these bearings:
Fluid linear bearings utilize a thin layer of rapidly moving pressurized fluids, such as oil, air, or water. There are two primary types: hydrostatic fluid bearings, which use pumps to pressurize the fluid and lift the carriage off the guide rail, and hydrodynamic fluid bearings, which rely on the high-speed motion of the carriage to generate the necessary fluid pressure.
These bearings offer high load capacities and operate with minimal noise, making them ideal for high-speed and high-precision applications. However, they are generally more expensive and require more maintenance compared to other linear bearing types. Their performance can be affected by fluid leakage or exposure to extreme temperatures.
Magnetic linear bearings use magnetic force to levitate the carriage above the guide rail, allowing for smooth, frictionless motion. They offer high load capacities due to the strength of the magnetic forces. However, the electromagnets used in these bearings can pose a risk to nearby electronic components by potentially causing interference or damage.
Linear bearing components are made from the following materials:
Steel, an alloy mainly consisting of carbon and iron, is the most commonly used material for linear bearings. Steel bearings are valued for their excellent mechanical properties, including high strength and rigidity, which enable them to support heavy loads and ensure smooth, precise motion. Carbon steel and stainless steel are typical types used in these bearings. Higher carbon content in steel enhances its hardness, which can influence the performance of the linear bearing.
Aluminum is a lightweight, high-strength metal known for its corrosion and chemical resistance. It is softer and more cost-effective than steel. While aluminum linear bearings have a lower load capacity compared to steel bearings, they still offer smooth and precise motion.
Plastic linear bearings are softer, more affordable, and exhibit a lower coefficient of friction compared to metal bearings. Common plastics used in these bearings include nylon, polyethylene, and PVDF, often coated with self-lubricating materials like PTFE. They may also be reinforced with fibers and fillers to improve their load-bearing capabilities. While plastic bearings can work with softer shaft materials, they typically have lower load capacities and are limited to use within room temperature ranges.
Bronze, an alloy primarily made of copper and zinc, with additional elements like manganese and phosphorus, is a softer metal. Bronze linear bearings offer a higher load capacity compared to plastic bearings. However, due to metal-to-metal contact, they generate more friction, which requires regular maintenance to ensure adequate lubrication.
Ceramic linear bearings are commonly made from materials such as silicon nitride, aluminum oxide, zirconium oxide, and silicon carbide. These bearings offer high rigidity, ensuring precise travel and accuracy even at high speeds. Their hardness enhances service life and abrasion resistance while minimizing particle generation from component friction. Ceramic bearings are also suitable for use in vacuum environments and with electrostatic discharge (ESD)-sensitive equipment.
In recirculating linear bearings, ceramic rolling elements are used to support higher speeds.
Composite bearings feature a metal backing combined with a plastic sleeve or a PTFE liner. The polymeric component eliminates metal-to-metal contact, reducing friction while retaining the bearing's high load capacity. The metal backing helps with heat dissipation.
It is common to use different materials for the bearing and the guide rail. The guide rail material is typically more resistant to friction reduction. Wear is primarily focused on the contact surface of the linear bearing, which is the softer component. In contrast, guide rails, shafts, and bases (for plain linear bearings) are often made from harder materials like hardened steel, ground steel, or anodized aluminum.
We have covered various types of linear bearings, their construction materials, and how these factors influence load capacity, speed, and service life. Here are additional considerations for selecting, operating, and maintaining linear bearings:
PV rating refers to a specification that indicates the maximum allowable combination of surface pressure and sliding velocity a linear bearing can handle while operating effectively. This rating accounts for heat and wear generated by friction. For example, higher speeds can decrease the maximum permissible load capacity of a linear bearing. The PV value, which is the product of the operating surface pressure and speed, must always be lower than the PV rating.
A cleanroom is a controlled environment designed to minimize airborne pollutants, contamination, and particulates. It is used for the manufacturing of products such as food, beverages, pharmaceuticals, semiconductors, electronics, and medical devices.
In cleanroom settings, recirculating linear bearings can produce fine dust from metal fragments due to high-speed metal-to-metal contact among rolling elements. Therefore, non-recirculating linear bearings are often preferred for cleanroom applications as they feature cages that separate rolling elements, minimizing dust generation. Plain linear bearings are also suitable.
Lubrication presents another challenge in cleanroom environments. External lubricants like oil and grease must be kept to a minimum to avoid contaminating the cleanroom products. Thus, bearings made from plastic or composite materials are favored. Additionally, any lubrication used must be compatible with cleanroom standards and the washdown or clean-in-place systems used within the cleanroom.
Outgassing refers to the release of trapped gases and vapors from solid materials through vaporization or sublimation at low pressures. This phenomenon can increase the pressure in a material's surrounding environment and disrupt the ability to create or maintain low pressures in a vacuum. Common materials that outgas include plastics, ceramics, porous metals, elastomers, and certain lubricants.
To mitigate outgassing, it is beneficial to use linear bearing materials that have undergone a bake-out process. Bake-out involves heating the materials to around 200°C for several hours to drive off volatile substances. However, not all materials can tolerate this temperature. Additionally, lubricants may outgas as well, so employing self-lubricating coatings and solid lubricants that are compatible with vacuum environments is essential.
Air linear bearings differ from conventional mechanical linear bearings, which rely on rolling or sliding elements. Instead, they use a pressurized air or oil film to support loads, eliminating mechanical contact that can produce friction or heat. These bearings are particularly suited for applications demanding high precision and rigidity.
Air linear bearings are classified into two types: hydrodynamic and hydrostatic, based on their method of generating the supporting film. Both types use a gaseous medium, typically air, to support loads. In environments where air quality is a concern—such as in clean rooms—alternative gases like nitrogen may be used to avoid issues like moisture-induced corrosion.
Hydrodynamic linear bearings use a thin film of fluid or air to support rotating components, often referred to as fluid film bearings. This design minimizes friction and wear by maintaining a separation between the stationary and rotating surfaces, which extends the lifespan of the bearings.
In hydrodynamic linear bearings, the gap between surfaces is established by the motion of the bearings themselves. During startup, they require external pressure to prevent friction. These bearings are designed to handle both radial and thrust loads.
Common types of hydrodynamic linear bearings include circumferential groove bearings, pressure bearings, and multiple groove bearings. They are utilized in various applications such as steam turbines, electric motors, cooling pumps, rock crushers, as well as clutches, blowers, and other auxiliary machinery.
Hydrostatic linear air bearings utilize a positive air pressure supply to create a gap between the rotating and stationary surfaces. Like hydrodynamic bearings, hydrostatic linear air bearings are classified as fluid film bearings.
Hydrostatic bearings are known for their high stiffness and long service life, making them suitable for precision machinery. Since they do not depend on lubrication for maintaining relative motion, they can support heavier loads at lower speeds and offer direct control over stiffness and damping coefficients.
The main advantage of air linear bearings is the elimination of friction, wear, and heat generation due to the lack of mechanical contact between rotating and stationary surfaces. This absence of contact means that lubrication is unnecessary, reducing particle generation and producing less noise compared to rolling or sliding bearings.
Air bearings can achieve higher speeds and accelerations because they do not have recirculating elements. They offer exceptionally precise motion with minimal scale errors. The fluid film fully supports the load, providing high stiffness and accuracy.
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