This article takes an in depth look at Linear Bearings.
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Linear bearings are a type of bearing that "bear" or support the load of the carriage during its single-axis linear movement and provide a low friction sliding surface for the guide rails. In a linear guide, the carriage is the component that travels in a straight line curved line, back and forth, along the length of the guide rail, which is fitted and inserted into the linear bearing.
The various types of linear bearings are a rolling element or fluid device that reduces friction. They offer excellent precision, better mounting, and much smoother even motion. Linear bearings are used in 3D printers, sliding doors, and several automated applications where guiding rail motion is required.
A linear bearing is a critical component of a linear guide assembly. Its applications are found on cutting machinery, XY positioning tables, machine slides, industrial robots, and instrumentation systems. Either a motor driven ball screw, lead screw, pneumatic cylinders, hydraulic cylinders, or manual force can be used to drive the motion with single axis motion limited in the X-Y plane. Hydraulic and pneumatic cylinders are widely used as the basis of the XY bed of computer numeric controlled (CNC) milling machines.
The types of linear bearings are divided into two main classifications, which are rolling linear bearings and plain linear bearings. The components, working principles, and design aspects of each type will be discussed in the succeeding chapters.
Rolling linear bearings are the most common type of linear bearings and provide the least frictional surface for linear motion. They utilize balls or rollers as the rolling elements that are housed between the mating grooves present in the bearing and guide rails.
The ball or roller diameter is proportional to the linear speed of the linear guide. As the ball diameter increases, the linear speed of the guide also increases. The contact angle, measured on the horizontal, affects the loading capacity of the linear bearing in a specific direction. The contact angle is directly proportional to the radial loading capacity and inversely proportional to the lateral loading capacity. A 45° contact angle can support loads at radial, reverse radial, and lateral directions equally.
There are numerous types of rolling linear bearings and several ways to classify them based on their design:
Linear ball bearings, or ball bearings, have spherical rolling elements (e.g., steel balls). They have low friction, longer service life, and high accuracy. They are the most popular type of rolling linear bearings. The spherical geometry allows them to be utilized in many linear bearing designs.
Linear bearings can have cylindrically shaped rolling elements. They have higher load capacity, rigidity, and shock and impact resistance than ball bearings. However, they have more friction since the contact area is larger, and they also have high sensitivity to misalignments.
Needle linear bearings also have cylindrical rollers called needles that have a length-to-diameter ratio between 3:1 and 10:1. They have higher rigidity and load capacity than balls or cylindrical bearings because the load is distributed to a greater number of smaller rollers. The smaller roller size increases the contact area and reduces deformation.
The track geometry of a linear bearing determines the number of contact points the balls or rollers make to their raceway, as well as the load capacity and generated friction of the linear bearing.
In a gothic arc profile, the ball makes four contact points on the raceway, two contact points each for the bearing and guide rail groove. The gothic arc profile is more compact, has a higher moment capacity, and can support heavier loads than the circular arc profile for a similar raceway size. However, the differential slip is also larger for this profile; this creates higher friction. Differential slip refers to the circumferential length between the inner and outer contact diameters. This causes a ball to have different rolling speeds in its contact diameters, making it slip as it rolls. To overcome this, a larger force is required to drive the ball bearing.
In a circular arc profile, the ball makes two contact points on the raceway; one contact point each for the bearing and guide rail grooves. It has lower differential slip, therefore, it creates less friction. However, they have a lower loading capacity than the gothic arc profile.
The linear guide profile refers to the cross-sectional shape of the guide rails. This dictates the design of the bearing used in the linear guide assembly.
Round rail profile linear guides have cylindrical shafts as the guide rail and entail the use of linear bushings. Linear ball bushing, generally known as linear bushing, is a type of linear bearing that has a cylindrical body that houses the loops of recirculating balls that run axially around the cylindrical shaft guide rail. A cage is present in the bearing housing to organize the movement of the balls. It has a simple construction and compact size and is easy to install.
Ball spline is a subtype of linear ball bushing. The shaft has grooves that run axially around its length; they match the grooves present in the spline nut. The grooves prevent rotation of the shaft and facilitate the transmission of torque. Ball splines have increased the moment load and can withstand overhung loads.
Square rail profile linear guides have the rolling elements located on two opposite sides that cover the length of the guide rail. Compared to a round rail profile, they have higher load capacity, higher stiffness, and can handle shock and vibrations better; they also have a moment load capacity.
Guide roller based linear systems consist of individual, ball bearing based rollers with a groove in the outer race that run on a mating steel track. The track usually has a v-edge, and the roller has a compatible v-groove. In some designs the roller groove is large enough for the rollers to ride on a cylindrical shaft.
These roller and track components can be integrated into the frame of a machine where space is limited. The versatility of the design allows for different configurations to give the best support and performance considering the position of the load to be moved.
The design of the rollers make them resistant to contamination since they are sealed to keep contamination from the ball path and isolated from the track. Any contamination that lands between the track and roller is swept away as the roller rides along. The inner part of the roller moves slower than the outer portion, a motion that forces debris off the edge of the guide rail.
The vee linear guide system is excellent for simple linear movement. They are easy to integrate, assemble, and maintain and are the best choice for demanding applications where contamination is present and cannot be avoided.
DualVee designs are used for harsh and stressful environments where other technologies are incapable of being effective. The long length, low noise, and smooth motion of dualvee guides offers excellent performance and stability.
Drawer slide guide systems consist of C-shaped slides and carriages formed from sheet metal. Each carriage moves along its slide via two intermediate sets of ball bearings, with one between each side of the carriage and slide. Unlike profile rail linear guide systems, the rolling elements do not recirculate and are simply constrained to the carriage by a ball cage.
In a recirculating linear bearing, the balls travel continuously in a looped raceway within the bearing, allowing the carriage to move across the length of the guide rail. It can travel any distance within the guide rail, regardless of its length. Recirculating linear bearings usually have multiple raceways.
The raceway is the straight pathway in which the balls or rollers travel. The raceway arrangement of a linear bearing significantly affects its reaction to applied torsional moments, as well as the performance of the linear guide.
In a face-to-face arrangement, the contact of the balls or rollers on the guide rails faces inward, creating an X-pattern. It offers equal load capacities in all directions but has reduced capacity in resisting applied moment loads.
In a back-to-back arrangement, the contact of the balls or rollers on the guide rails faces outward, creating an O-pattern. This has greater leverage against the guide rail. It offers greater resistance to applied moment loads and higher stiffness and rigidity.
In a non-recirculating linear bearing, the alignment and position of individual rollers are fixed in a frame called the cage (inside the bearing housing). The rollers are separated at an equal, fixed distance by a retainer or separator. Cages prevent the metal-to-metal contact between the rollers. They can be made of metal or plastic.
The linear motion is limited to the length of the bearing. The rolling elements facilitate linear motion by only rotating about their axes. The restricted motion enables non-recirculating linear bearings to have high load capacity and stiffness while providing smooth motion and high travel accuracy.
The types of non-recirculating linear bearings are the following:
Non-recirculating linear ball bearings have metal balls fixed on the cage. The mating grooves of the bearing and the guide rail can have a circular or gothic arc track geometry.
Flat-type roller bearings have cylindrical or needle rollers lying on their horizontal axis in the cage. The axes of the rollers are perpendicular to the direction of linear motion.
V-type roller bearings have a V-shaped pathway, forming a 90° angle, for the carriage. A row of cylindrical or needle rollers lies on each side of the V-profile.
Crossed roller bearings have cylindrical rollers in which each cylinder‘s axis forms a 90° angle with its adjacent cylinder. This forms a crisscross pattern among the rollers. They have higher load capacity, higher resistance to vibration and shock, and longer service life. However, they are difficult to assemble.
Plain linear bearings rely on the sliding contact of two surfaces without the help of rolling elements. Compared to roller linear bearings, they have simpler construction, simpler operating mechanisms, and are much cheaper. The contact area is larger; this results in lower surface pressure. They have higher load capacity, lighter weight, and can absorb shocks and damp vibrations more effectively.
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.
The types of plain linear bearings are the following:
Box-way slides are a type of linear bearing that consist of a mating base and saddle; these give a T-shaped profile when fitted together. The base is the stationary part that serves as the guide rail, while the saddle is the moving part that acts as the carriage. Adjustable gib plates are installed between the base and the saddle to create preload and remove the clearance between them. Box-way slides have higher load capacity because of their larger contact area between the saddle and the base.
A dovetail slide is a type of linear bearing that consists of a base with a protruding V-shaped tongue and a mating saddle that have full contact with each other. Dovetail slides have a higher load capacity. Preloading is not possible with dovetail slides but gib plates can be installed along the length of the saddle to compensate for the clearances.
Linear sleeve bearings, also known as plain linear bushings, are hollow cylinders in which the journal (shaft guide rail) slides in their inner surface. The inner surface is usually coated with materials with a self-lubricating surface (e.g., PTFE). Linear sleeve bearings can handle both axial and radial loads. However, they have lower load capacity and stiffness than box-way and dovetail slides and are frequently used in light to medium-duty applications.
Non-Contact Linear Bearings are frictionless bearings that eliminate the contact between the carriage and guide rails. Thus, these bearings have a much longer service life and higher speeds. These bearings have two types:
Fluid linear bearings use a thin layer of fast-moving pressurized fluids such as oil, air, and water. There are two types of fluids. Hydrostatic fluid bearings require the use of pumps to pressurize the fluid in order to lift the carriage from its guide rail. On the other hand, hydrodynamic fluid bearings rely on the high-speed motion of the carriage to pressurize the fluid.
Fluid linear bearings have high load capacity and generate little noise during operation. They are suitable for high-speed and high-precision applications. However, they are much more expensive and require higher maintenance than other linear bearings. Their functionality is disrupted when leakage occurs or when they are exposed to extreme temperatures.
Magnetic linear bearings rely on magnetic force to levitate the carriage from its guide rail. They also have higher loading capacities due to strong magnetic force. However, the electromagnets present in these bearings can damage the mounted electronic components.
The materials for constructing linear bearing components are the following:
Steel, an alloy primarily composed of carbon and iron, is the most popular material selection for linear bearings. Steel linear bearings are known to have excellent mechanical properties (i.e., high strength and rigidity); this can support heavy loads and provide smooth and precise motion. Carbon steel and stainless steel are common types of steel used in constructing steel linear bearings. Increasing the carbon content increases the hardness of steel; this affects the performance of the linear bearing.
Aluminum is a lightweight but high-strength metal. It is corrosion and chemical resistant. It is softer and less expensive than steel. However, aluminum linear bearings have lower load capacity than steel linear bearings. Aluminum linear bearings are also capable of providing smooth and precise motion.
Plastic linear bearings are softer, cheaper, and have a lower coefficient of friction than metallic bearings. Typical plastics used in linear bearings are nylon, polyethylene, and PVDF, and they are usually lined with a self-lubricating coating (e.g., PTFE). They are sometimes reinforced with fibers and fillers to enhance their weight-bearing capabilities. Plastic linear bearings can be used with softer shaft materials. However, they generally have lower load capacities and are limited to room temperatures.
Bronze is an alloy mainly composed of copper and zinc, with other additives such as manganese and phosphorus. It is a soft metal. Bronze linear bearings have a higher load capacity than plastic linear bearings. Because of the presence of metal-to-metal contact, however, they generate greater friction than plastic bearings; this necessitates maintenance of sufficient lubrication.
Ceramic linear bearings are typically fabricated from silicon nitride, aluminum oxide, zirconium oxide, and silicon carbide. They have high rigidity, which maintains travel accuracy and precision at high speeds. They have a high hardness, which increases their service life and abrasion resistance and reduces the generated particles from sliding of bearing components. They are also compatible with vacuum and ESD-sensitive devices and equipment.
In recirculating linear bearings, ceramic rolling elements are utilized for their higher speeds.
Composite bearings consist of a metal backing and a plastic sleeve or a PTFE liner. The polymeric component eliminates metal-to-metal contact; this lowers the coefficient of friction while maintaining the high load capacity of the bearing. The metal backing allows the bearing to dissipate its heat.
It is common practice to use different materials for the bearing and the guide rail; with the guide rail, it is usually harder to reduce friction. Material wear is concentrated in the contact surface of the linear bearing, which is the softer component. The guide rails, shafts, and bases (for plain linear bearings) are commonly constructed from hardened steel, ground steel, and anodized aluminum, which are all harder types.
So far, we have discussed the different types of linear bearings and their materials of construction as well as their impact on load capacity, speed, and service life. The following are other considerations in linear bearing selection, operation, and maintenance:
PV rating is a specification of a linear bearing that designates the maximum combination of surface pressure and sliding velocity that a linear bearing can withstand while operating properly. It is based on heat and wear caused by friction. For instance, higher speeds reduce the maximum permissible load capacity of a linear bearing. The PV value is the product of the operating surface pressure and speed which must be always lower than the PV rating.
A cleanroom is a controlled environment wherein airborne pollutants, contamination, and particulates are monitored and filtered out. It is a facility wherein food, beverage, pharmaceutical, semiconductor, electronic, and medical products are manufactured.
Recirculating linear bearings generate fine dust from metal fragments created by high-speed metal-to-metal contact of the rolling elements. Hence, the use of non-recirculating linear bearings is a good option for cleanroom applications since they are equipped with cages that separate rolling elements. A plain linear bearing is also an option.
Lubrication is another concern for cleanroom linear bearings. External lubrication such as oil and grease must be minimal because it could contaminate the products inside the cleanroom. Hence, bearings made from plastic and composite materials are preferred. Lastly, lubrication must be compatible with cleanroom facilities and with the washdown or clean-in-place system of equipment inside the cleanroom.
Outgassing is the release of entrapped gases and vapors in solid materials through vaporization or sublimation at low pressures. It causes the pressure of a material‘s surrounding environment to rise and hinders a vacuum environment from creating or maintaining low pressures. Plastics, ceramics, porous metals, elastomers, and lubricants outgas at low pressures.
Outgassing can be prevented by using linear bearing materials that have undergone bake-out. Bake-out is a heat treatment process employed to gasify or vaporize volatile substances in the material‘s matrix at about 200°C; this lasts for several hours. However, not all materials can withstand this temperature. Lubricants can also outgas, thus, the use of self-lubricating vacuum-compatible coatings and solid lubricants is necessary.
Air linear bearings deviate from traditional linear bearings, which are mechanical rolling or sliding. They use a film of pressurized air or oil to support loads without the use of mechanical elements that can generate friction or heat. Air linear bearings are ideal for applications that require exceptional precision and stiffness.
The two classifications of air linear bearings are hydrodynamic and hydrostatic, which differ according to their source of film. Both types use a gaseous medium, typically air, as a means for supporting their load. When there is concern regarding the quality of the air, such as moisture that can cause corrosion, other gasses, such as nitrogen, are used, a solution that is common in clean rooms.
Hydrodynamic linear bearings float rotating components on a thin film of fluid or air and are referred to as fluid film bearings. The separation between the stationary and rotating surfaces results in less friction and wear, which leads to a longer useful life for hydrodynamic bearings.
With hydrodynamic linear bearings, the gap between the surfaces is created by the motion of the bearings. During startup, they require external pressure to avoid friction and are designed for radial and thrust loads.
The basic types of hydrodynamic linear bearings are circumferential groove bearings, pressure bearings, and multiple groove bearings. Hydrodynamic linear bearings can be found in steam turbines, electric motors, cooling pumps, and rock crushers as well as clutches, blowers, and auxiliary machinery.
Hydrostatic linear air bearings use a positive air pressure supply to provide the clearance between the rotating and stationary surfaces. As with hydrodynamic linear air bearings, hydrostatic linear air bearings are referred to as fluid film bearings.
The various types of hydrostatic bearings have high stiffness and long bearing life and are used for fine finishing machinery. Due to the fact that hydrostatic bearings do not rely on lubrication to maintain relative motion, they can carry heavier loads at lower speeds and provide direct stiffness and damping coefficient.
The primary benefit of air linear bearings is the removal of the potential of friction and wear as well as heat generation because of the absence of mechanical contact between the rotating and stationary surfaces. Without mechanical contact, there is no need for lubrication, which reduces particle generation and produces less noise than rolling or sliding bearings.
With the elimination of recirculating elements, air bearings can reach higher speeds and accelerations. Since air bearings produce very small scale errors, they provide extremely accurate motion. The fluid film fully supports the load enabling them to have higher stiffness.
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