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Linear Bearings: What You Need to Know Full-time Job

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Linear Bearings: What You Need to Know

Linear bearings are rolling-element and fluid-film devices that reduce friction in motion systems where the motion acts along a straight — or sometimes curved — pathway. They are distinguished from radial bearings in which motion is rotary. Linear bearings are used in machine tool applications such as sliding doors, 3D printers, and automation settings where reducing friction and guiding linear motion is needed. They can be loosely grouped as ball and roller types that use the rolling motion of rolling elements and sliding types that rely on lubricant and low-friction surfaces. This article will discuss the distinct forms that linear bearings take and highlight some general applications.

SLIDE-TYPE BEARINGS

Sometimes called linear bushings, linear bearings are manufactured in sintered aluminum bronze, metal/polymer composites, carbon insert styles, polymer-lined sleeves, etc. and rely on a lubricating film to form between the bearing and the shaft while the two are in relative motion. Powdered-metal bronze bushings have been used for many years as die-post bushings. Their porous constructions, often with carbon plug inserts, are vacuum impregnated with oil that flows to and from the bearing under the frictional heat developed as the shaft moves and stops, providing a film of oil between the bearing and the shaft.

Polymer, self-lubricating linear bushings have captured some of the market that had been dominated by sintered metal bushings. The polymer itself — typically PTFE — provides slipperiness for the running shaft. These are popular in food packaging for their ability to run without lubricants and their ability to withstand washdown chemicals. They can run on unhardened shafts having high corrosion resistance. Because polymers are subject to cold flow at high loads and do not dissipate heat well, polymer is often bonded in thin sections to metal backers in the construction of these bearings.

Linear bushings can be crowned slightly to permit some angular adjustment of the bushing relative to the housing to adapt to shaft deflection. In more extreme cases, the bushing is supported in the housing by O-rings. Linear bushings are available as naked units or housed in pillow blocks and flanged units.

Bushings can be arranged as tandem installations in common housings to provide higher axial spread for the bearing surfaces. They are available as open and closed varieties, with the selection depending on how the shaft is supported. Shafts supported continuously over their lengths require open bushings that can clear the rod rails. Shafts supported on their ends can use closed bushings. Linear bushings are tolerant of dirt as a result of having no moving elements.

What Is a Rod End Bearing?

A rod end bearing is a common type of mechanical joint used on the ends of control rods. The steering columns in most cars, trucks and other vehicles, for example, feature tie rods with a rod end bearing. Of course, tie rods are designed to connect a vehicle’s steering rack to its steering knuckle. As a result, tie rods must be able to rotate according to the direction in which the wheel is turned. Rod end bearings allow tie rods to perform this rotation in a precise and controlled manner. To learn more about rod end bearings and how they work, keep reading. If you’re looking to purchase Rod Ends, Monroe has you covered.

Rod End Bearings Explained

Also known as a heim joint in the United States or a rose joint in the United Kingdom, a rod end bearing is a mechanical joint that features a rounded ball-like swiveling tip. They were invented in Germany during the 1930s to 40s for use in aircraft control systems. This promoted a company called H.G. Heim Company to patent and produce its own rod end bearings in North America, which is why the mechanical joint now has the moniker “heim joint.”

H.G. Heim Company has since closed its doors for business, but rod end bearings are still produced and used throughout the world. Automotive tie rods are just way in which rod end bearings are used. They are used in countless other applications in which an articulating joint is needed, including aircraft control systems, steering links, track rollers and more.

Rod end bearings are typically made using the following materials:

Steel

Stainless steel

High-carbon steel

Aluminum

Male vs Female Rod End Bearing: What’s the Difference?

A rod end bearing can be classified as either male or female depending on the way in which the threading is designed. Male rod end bearings are designed with external threading. In comparison, female rod end bearings are designed with internal threading. With interior threading, female rod end bearings can handle unique applications that aren’t possible with male rod ends bearings. Helicopters, for instance, often use female rod end bearings to adjust the direction of the blade. They allow pilots and aviation technicians to fine tune their blade adjustments.

You can see an example of both male and female rod end bearings in the photo above. The red rod end bearing is male because the threading is located on the exterior, whereas the black rod end bearing is female because the threading is concealed inside it.

Lead Screw

lead screw is a kind of mechanical linear actuator that converts rotational motion into linear motion. Its operation relies on the sliding of the screw shaft and the nut threads with no ball bearings between them. The screw shaft and the nut are directly moving against each other on a large contact area, so higher energy losses due to friction are produced. However, the designs of lead screw threads have evolved to minimize friction.

The lead screws are a cost-effective alternative to ball screws in low power and light to medium-duty applications. Since they have poor efficiency, their use is not advisable for continuous power transmission. Unlike ball screws, they operate silently with no vibration and have a more compact size. They are typically used as a kinematic pair (linkage) and actuation and positioning in equipment such as lathe machines, scanners, recorders, wire bonders, and disk drive testers. They are used to transmit forces in testing machines, presses, and screw jacks.

The components of a lead screw are the following:

Screw Shaft

The screw shaft is a cylindrical rod that has a single or series of grooves running helically around its length; this is referred to as the external thread.

Thread

The thread is the structure responsible for converting rotational motion into linear motion as the screw shaft and the nut slide with each other.

Nut

The lead screw nut is a cylindrical section that has an internal thread that matches the external thread of the screw shaft.Lead screws may be operated in two possible ways. One mode of operation is either the screw shaft or the nut rotates and moves linearly while the other component is fixed. This mode is commonly seen in printers and helical pairs. The other mode of operation is either the screw shaft or the nut rotates but does not move linearly. This mode is commonly seen in presses and lathes.

The design aspects of lead screws are the following:

Major Diameter

The major diameter is the largest diameter of the thread. The major diameter of the screw shaft is the distance between two opposite crests, while the major diameter of the nut is the distance between two opposite roots.

Minor Diameter

The minor diameter is the smallest diameter of the thread. The minor diameter of the screw shaft is the distance between two opposite roots, while the minor diameter of the nut is the distance between two opposite crests.

Crest

A crest is the raised helical structure in an external thread (screw shaft) and the recessed helical structure in an internal thread (nut).

Root

A root is the recessed helical structure in an external thread (screw shaft) and the raised helical structure in an internal thread.

Thread Depth

The thread depth is the distance from the root to the crest, measured radially.

Flank

The flank is the surface that connects the root to the crest.

Pitch Diameter

The pitch diameter, or the effective diameter, lies concentrically and approximately halfway between the major and minor diameters. It is the diameter of the imaginary cylinder whose circumference intersects half of the thread pitch.

Pitch

The pitch is the axial distance between two adjacent threads measured parallel to the axis. It is equivalent to 1/number of threads per inch.

Lead

The lead is the linear distance traveled by the screw shaft or nut along its axis in one complete revolution (3600 rotation). As the lead increases, the linear speed also increases, but the load capacity of the lead screw decreases, etc.

Ball Bearings - A Complete Buying Guide

Ball bearings are rolling-element bearings which use balls to maintain the separation and distance between the bearing races. They are designed to reduce rotational friction while supporting both radial and axial loads.

The types of ball bearings that we think of today were initially manufactured at the end of the 19th century. The Welsh inventor Phillip Vaughan was given the first patent for ball bearings and his was the earliest design to feature a ball running along the groove in the axle assembly. Another major development came in 1869 when the Paris-based bicycle mechanic Jules Suirray created the first ball bearing of the radial variety. This radial bearing was included in the manufacture of the bicycle that French racer James Moore rode to victory in the first Paris-Rouen race of 1869.

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Linear Bearings: What You Need to Know