Linear Bearings

View A Video on Linear Bearings - A Quick Introduction

Linear bearings provide a sliding or rolling contact between the object being moved and the bearing, creating relative motion among parts of the machine. There are many common uses for linear motion bearings. They can be found in drawer glides and work station height adjustment mounts. Occasionally these are referred to as linear guides, but mostly they are used as part of an industrial motion control system, along with devices such as linear actuators. Other more general uses for various types of linear bearings include optic alignment, disk certification, precision dispensing, vision inspection, wafer testing and handling, precision gauging and automated production equipment. They are often made of materials such as bronze, plastic and stainless steel. In the United States, they are measured via the American measurement system, but in Europe, metric linear bearings are used. The three main types of linear bearings are linear ball bearings, linear roller bearings, including needle bearings, and slide bearings. Other types include precision linear bearings, miniature linear bearings, linear rotary bearings and linear air bearings. There are different options for linear bearing housings, including pillow block linear bearings. Linear bearings have become much more popular in recent years, largely due to industry advancements in robotic automation.

Linear ball bearings use a rolling element such as a hard round ball to separate 2 moving parts along a straight line that provides smooth, low friction linear motion in rotary applications. There are two types-radial ball bearings, which carry both radial and axial loads, while thrust ball bearings are solely designed for pure thrust loads. They are used for light to moderate loads. Linear roller bearings carry loads by placing round elements, usually cylinders, between two objects. They have more load capacity because the contact between the rollers and line spread the load out. They are self cleaning, inexpensive and simple to use. Needle bearings, a type of roller bearing, have cylinder rollers that are much smaller in diameter and use a thinly shaped needle roller. Slide bearings use low friction metals like Teflon and graphite inside of metal backing plates for linear movement and weight bearing. They are used on heated pipelines on oil production modules, in steel fabrication, and in oil or water transportation.

Air bearings are crossed roller slide assemblies that are controlled by a pair of air cylinders. They are low friction because the materials being moved ride on a cushion of air. They are round or rectangular, come with or without a vacuum and magnetic prload, and are used often in the aerospace industry and machine tools. Linear air cylinders are often made of aluminum and are used for tensile testing and precision positioning. Linear rotary bearings are used when accurate linear movement of large objects is required. They use a cylinder roller that is able to divide and handle heavy loads and are self-lubricating. They are used in car axels, machine shafts, clock parts and grinding machines.  Precision bearings are used in machine-tool spindles and in other high-precision applications. They are considered high in quality and have a tight bearing tolerance. Miniature bearings operate at low torque and provide high performance in applications where low noise, minimal run-out and long life are required. Pillow block bearings are housings that are mounted to provide load support for a rotating shaft. They are often made of iron and may be plain, which constrain, guide or minimize friction in rotary or linear applications, or ball, which produce smooth, low friction movement in rotary applications. 

Linear bearings use bushings, balls, slides, rails and grease and vary in size and strength. Manufacturers produce many different types of linear bearings used in conjunction with larger automated systems, such as positioning tables and robotic assisted assembly lines. One of these types, linear ball slides, is compact and has a small form factor and a short stroke. These slides have the ability to stroke up to one-third of the slide length. Crossed roller bearings, on the other hand, have a much larger load capacity than ball slides because of the contact line between each roller and the raceway. Linear bearings are easy to install and are typically low cost. Because there are so many linear bearing manufacturers, standardization is not common. Special consideration should then be made to determine what a bearing's width, length, inside diameter and load capacity needs to be for the specific application. Some of the linear bearings styles are self-cleaning and lubricating, while others need to be cleaned and lubricated with regularity to ensure successful utility.

Images Provided by Del-Tron Precision, Inc.

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Linear Bearings Types

• Ball screw bearings have a helical inner race that converts torque to thrust as the screw and nut turn and the component moves in a linear direction.
• Crossed roller bearings are one of three primary types of linear bearings. They have more load capacity than ball slides because of the contact line between each roller and the raceway.
• Flat ball cages are a linear bearing design in which the balls are trapped in a frame that spaces them at equal distances.
  
• Flat roller cages are similar to a crossed roller bearing, but the axes to all the other rollers are parallel instead of crossed.
  
• Gonio way bearings are curved to mimic the arc of a pendulum.
• Linear air bearings are frictionless bearings that use a thin film of pressurized air to support a load and allow free motion in one direction. Also called a fluid film bearing, air bearings have a layer of air that forms between the machine element and the bearing so there is no solid to solid contact. Because there is no contact, there is no friction, static or running, as well as no wear, no lubricants and no heat generation.
• Linear ball bearings are a type of rolling-element bearing that uses spherical elements to separate two moving parts along a straight line. Ball bearings consist of an assembly with spherical balls. They are considered linear or axial when the moving parts they separate travel along a straight line, as in a drawer opening.
• Linear ball slides, often referred to as "linear slides," are compact forms of linear bearings with small form factor and a similarly short stroke. Maximum stroke can be about one-third of the slide length, but the form factor justifies the stroke.
  
• Linear motion bearings are used to bear loads and direct linear motion.
• Linear roller bearings are a type of rolling-element bearing, meaning they carry a load by placing round elements between two objects.
• Linear rotary bearings are designed to allow an incoming shaft to rotate smoothly and with low friction as it moves simultaneously in a straight line. By combining multiple linear bearing technologies, these bearings are able to implement 360 degrees of rotation in addition to moving in a straight plane.
• Metric linear bearings are designed for settings in which an object must be moved in a straight line in one direction with high accuracy and are identified by metric units of measurement.
• Miniature linear bearings are bearings less than 10 millimeters in inner diameter that are designed to provide low friction motion in one straight direction. Including both rolling element bearings and plane bearings, this subset of linear bearings accomplishes the same tasks as its larger counterparts, only on a smaller scale.
• Needle bearings use long thin cylinders as rollers to reduce friction between rotating components. The cylindrical rollers have a high length-to-diameter ratio which means they have a relatively large surface area in contact with the outer surfaces as compared to ball bearings. However, needle roller bearings are more compact and have the highest load capacity for a given radial space of all rolling element bearings.
• Pillow block linear bearings are mounted bearings that provide accurate, low friction guidance and support for a shaft passing through the center of the bearing in a direction parallel to the mounting surface. Also called a plumber's block, these bearings allow the shaft to smoothly glide in and out of the housing.
• Plastic linear bearings allow objects to move smoothly along in a straight direction with a minimum of friction. Made solely from plastic, these bearings are able to withstand the same rigors that ceramic or steel bearings can while possessing certain advantages over the more traditional materials, like being corrosion resistant to chemicals and acids that would harm conventional bearings.
• Positioning stages or tables are comprised of two basic components: a precision linear slide (ball or cross roller), which serves as a linear bearing and guide, and a drive mechanism, which accurately moves and positions the slide top along the linear axis. 
• Precision linear bearings allow objects to be moved in one straight direction at high speeds with superior accuracy, low noise and vibration, and maximum life. Linear bearings in general are not capable of consistently accomplishing those tasks, and precision bearings are used in demanding situations for high-performance work with the least amount of machine downtime.
• Slide bearings are a type of bearing that use low-friction materials such as Teflon and graphite for linear movement and weight bearing. Most slide bearings use a "sandwich" design consisting of two plates of low-friction material in between metal backing plates.

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Linear Bearings Terms

Axial Clearance - The amount of play or movement displayed when a bearing's inner ring is moved axially in relation to the outer ring.
 
Basic Dynamic Load - The load under which a bearing can operate while rotating.
 
Bearing Analyzer - A device used to measure a bearing's performance, and for fault analysis after use.
 
Bearing Cage - A device used to separate the rolling elements of a bearing (also known as a retainer).
 
Brinelled - The result of rolling elements being forced onto inner or outer raceways, leaving indentations in the raceway which lead to premature bearing failure.

Carrier - Responsible for supporting and moving the load. Carriers are commonly attached to the linear slide with a bearing system.
 
Gib - A wedge-shaped piece of metal or wood designed to hold structural parts in place and/or provide a bearing surface.
 
Graphitic Corrosion - The deteriorization of bearing housings in grey cast iron products.
 
Grease Pocket - A special design feature of a cage/retainer that allows grease to accumulate and then slowly be fed to the rolling elements.
 
Load Rating - The maximum amount of weight that a bearing can withstand.
 
Lubricant - A substance is used to lubricate the moving surface of a bearing, protect against corrosion, conduct and transfer heat and keep contaminants from entering.
 
Pear Skin - A term that refers to the "fuzzy" appearance of the load-bearing surface of a bearing, which is caused by contamination.
 
Raceway - The portion of the bearing in which the ball or other part slides. Raceways provide the track of confined movement for the bearing mechanism itself.
 
Radial Clearance - The space between the rolling elements and the races in an unmounted bearing, designed so that the bearing has the necessary net clearance to operate.
 
Seal - A device to stop the ingress of contamination and the escape of lubrication.
 
Stay Rod - The long fasteners used to hold the retainers together when riveted retainers are used in a maximum bearing capacity.
 
Tensile Strength - The ratio of a material's strength to its cross-section.
 
Tribology - The study of friction, lubrication and wear on two moving surfaces in contact.
 
Weibull Distribution - A statistical analysis tool that is used to study bearing life and calculate the probability of failure.

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