Linear Motion Bearings and Transmissions Ltd

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All About Bearings

What is a Bearing?

An example of various types of bearings.

A bearing is any one of various machine elements that constrain the relative motion between two or more parts to only the desired kind of movement. This is usually to allow and promote free rotation around a fixed axis or free linear movement; it may also be to arrest any motion, such as by controlling the vectors of normal forces. Bearings may be classified according to the motions they allow and according to their principle of operation, as well as by the directions of applied loads they are able to handle.

The term "bearing" comes ultimately from the verb "to bear", and a bearing is therefore a machine element that allows one part to bear another, usually allowing relative motion between the two. The simplest bearings are nothing more than bearing surfaces, which are surfaces cut or formed into a part, with a certain degree of control over the quality of the surfaces form, size, surface roughness, and location. Many other bearings are separate devices that are installed into the part or machine. The most sophisticated bearings are very expensive, highly precise devices, whose manufacture involves some of the highest technology known today.

History of the Bearing

The invention of the rolling bearing, in the form of an object being moved on wooden rollers, is of great antiquity and could well predate the invention of the wheel.

It is often claimed that the Egyptians used roller bearings in the form of tree trunks under sleds, however, this is modern speculation. They are depicted in their own drawings in the tomb of their Pharoah as moving massive stone blocks on sledges with the runners lubricated with a liquid which would constitute a plain bearing.

Tapered Bearings

The earliest known example of a rolling element bearing is a wooden ball bearing supporting a rotating table from the remains of the Roman ships in Lake Nemi, Italy. The shipwrecks were dated to 40 AD.

The famous polymath, Leonardo da Vinci, incorporated drawings of ball bearings in his design for a helicopter around the year 1500. This is the first recorded use of bearings in an aerospace design. However, Agostino Ramelli is the first to have published sketches of roller and thrust bearings. An issue with ball and roller bearings is that the balls or rollers rub against each other causing friction which can be prevented by encapsulating the balls or rollers in a cage. The caged ball bearing was originally described by Galileo in the 17th century. The mounting of bearings into a set was not accomplished for many years after that. The first patent for a ball race was by Philip Vaughan of Carmarthen, Wales in 1794.

Bearings saw use for holding wheels and axles. The bearings used there were plain bearings that were used to reduce friction over that of dragging an object by making the friction act over a shorter distance as the wheel was turned.

The first plain and rolling element bearings were made of wood and closely followed by bronze bearings. Over their history bearings have been made of many types of materials including ceramic, sapphire, glass, steel, bronze, other metals and plastic. The metal and plastic bearings are the ones which are all used today.

Watch makers produce "jeweled" watches using sapphire plain bearings to reduce friction, therefore allowing more precise time keeping.

Even very basic materials can have good durability. For example, wooden bearings can still be seen today in old clocks or in water mills where the water provides a cooling and lubrication function. The first practical caged roller bearing was invented in the mid 1740s by horologist John Harrison for his H3 marine timekeeper. This uses the bearing for a limited oscillating motion but the inventor also used a similar bearing in a truly rotary application in a contemporaneous regulator clock.

Early Timken Tapered Roller Bearing with Notched Rollers

A patent on ball bearings, supposedly the first, was awarded to Jules Suriray, a Parisian bicycle mechanic, on 3 August 1869. The bearings were then fitted to the winning bicycle ridden by James Moore in the worlds first bicycle road race, Paris to Rouen, in November 1869.

In 1883, Friedrich Fischer, founder of FAG, developed an approach for milling and grinding balls of equal size and exact roundness by means of a suitable production machine and formed the foundation for creation of an independent ball bearing industry.

The modern, self aligning design of a ball bearing is attributed to Sven Wingquist of the SKF ball bearing manufacturer in 1907, when he was awarded Swedish patent No. 25406 for its design.

Henry Timken, a 19th century innovator in carriage manufacturing, patented the tapered roller bearing in 1898. The following year he formed a company to produce his innovation. Over a century the company grew to make bearings of many types, including specialty steel and an array of related products and services.

Erich Franke invented and patented the wire race bearing in 1934. He focused on a bearing design with a cross section as small as possible, which could be integrated into the enclosing design. After World War II he founded together with Gerhard Heydrich the company Franke & Heydrich KG, to push the development and production of wire race bearings. This company is now known as Franke GmbH

Richard Stribeck's extensive research on ball bearing steels identified the metallurgy of the commonly used 100Cr6 (AISI 52100) showing coefficient of friction as a function of pressure.

Designed in 1968 and later patented in 1972, Bishop-Wisecarvers co-founder Bud Wisecarver created vee groove bearing guide wheels, a type of linear motion bearing consisting of an external and internal 90 degree vee angle.

In the early 1980s, Pacific Bearings founder, Robert Schroeder, invented the first bi-material plain bearing which was size interchangeable with linear ball bearings. This bearing had a metal shell made from aluminum, steel or stainless steel, and a layer of Teflon-based material connected by a very thin adhesive layer.

Today ball and roller bearings are used in many applications which include a rotating component. Examples of these include ultra high speed bearings in dental drills, aerospace bearings, gearbox and wheel bearings on motor vehicles, flexure bearings in optical alignment systems and bicycle wheel hubs.

Common Bearings

The most common bearing is the plain bearing, a bearing which uses surfaces in rubbing contact, often with a lubricant such as oil or graphite. It may be nothing more than the bearing surface of a hole with a shaft passing through it, or of a planar surface that bears another; or it may be a layer of bearing metal either fused to the substrate or in the form of a separable sleeve. With suitable lubrication, plain bearings often give entirely acceptable accuracy, life, and friction at minimal cost. Therefore, they are very widely used in many industries.

However, there are many applications where a more suitable bearing can improve efficiency, accuracy, service needs, reliability, speed, size, weight, and costs of buying and operating machinery.

Therefore, there are many types of bearings, with varying shape, material, lubrication and principle of operation.

Principles of Operation of Bearings

There are at least six common principles of operation of bearings:

  1. plain bearing, also known by the specific styles: bushings, journal bearings, sleeve bearings, rifle bearings.
  2. rolling-element bearings such as ball bearings and roller bearings.
  3. jewel bearings, here the load is carried by rolling the axle slightly off-center.
  4. fluid bearings, here the load is carried by a gas or liquid.
  5. magnetic bearings, here the load is carried by a magnetic field.
  6. flexure bearings, here the motion is supported by a load element which bends.

Motions

Common motions permitted by bearings are:

  • axial rotation
  • linear motion
  • spherical rotation
  • hinge motion

Friction on the Bearings

Reducing friction in bearings is very important for efficiency, to reduce wear and to facilitate extended use at high speeds and to avoid overheating and early failure of the bearing. Effectively, a bearing can reduce friction by virtue of its shape, the material it is made from, or by introducing and containing a fluid between surfaces or by separating the surfaces with an electromagnetic field.

  • By shape, gains advantage usually by using spheres or rollers, or by forming flexure bearings.
  • By material, exploits the nature of the bearing material used. (An example would be using plastics that have low surface friction.)
  • By fluid, exploits the low viscosity of a layer of fluid, such as a lubricant or as a pressurized medium to keep the two solid parts from touching, or by reducing the normal force between them.
  • By fields, exploits electromagnetic fields, such as magnetic fields, to keep solid parts from touching.

Combinations of these can even be employed within the same bearing. An example of this is where the cage is made of plastic, and it separates the rollers/balls, which reduce friction by their shape and finish.

Loads on the Bearings

Bearings can vary greatly over the size and directions of forces that they can support.

Forces can be predominately radial, axial or bending moments perpendicular to the main axis.

Speeds of the Bearing

Different types of bearings have different operating speed limits. Speed is usually specified as maximum relative surface speeds, often specified ft/s or m/s. Rotational bearings usually describe performance in terms of the product DN where D is the diameter of the bearing and N is the rotation rate in revolutions per minute.

Generally there is considerable speed range overlap between bearing types. Plain bearings handle only lower speeds, rolling element bearings are faster, followed by fluid bearings and finally magnetic bearings which are limited ultimately by centripetal force overcoming material strength.

Play on the Bearings

Some applications apply bearing loads from varying directions and accept only limited play as the applied load changes. One source of motion is play in the bearing. Therefore, a 10mm shaft in a 12mm hole will have 2mm of play.

Allowable play varies greatly depending on the use. For example, a trolley wheel supports radial and axial loads. Axial loads may be hundreds of newtons force left or right, and it is acceptable for the wheel to wobble by as much as 10mm under the varying load. To contrast this, a lathe may position a cutting tool to ±0.02mm using a ball lead screw held by rotating bearings. The bearings support axial loads of thousands of newtons in either direction, and must hold the ball lead screw to ±0.002mm across that range of loads.

Stiffness of the Bearing

A second source of motion is elasticity in the bearing itself. Another example would be where the balls in a ball bearing are like stiff rubber, and under load deform from round to a slightly flattened shape.The race is also elastic and develops a slight dent where the ball presses on it.

The stiffness of a bearing is how the distance between the parts which are separated by the bearing varies with applied load. With rolling element bearings this is due to the strain of the ball and race. With fluid bearings it is due to how the pressure of the fluid varies with the gap.

Service Life of the Bearing

Fluid and Magnetic Bearings

Fluid and magnetic bearings are quite amazing and can have virtually indefinite service lives. In fact, there are fluid bearings supporting high loads in hydroelectric plants that have been in nearly continuous service since about 1900 and which show no signs of wear.

Rolling Element Bearings

Rolling element bearing life is determined by its load, temperature, maintenance, lubrication, material defects, contamination, handling, installation as well as other factors. These factors can all have a significant effect on the bearings overall life. The service life of bearings in one application was extended dramatically by changing how the bearings were stored before installation and use, as vibrations during storage caused lubricant failure even when the only load on the bearing was its own weight; the resulting damage is often false brinelling. Bearing life is statistical: several samples of a given bearing will often exhibit a bell curve of service life, with a few samples showing significantly better or worse life. Bearing life varies because microscopic structure and contamination vary greatly even where macroscopically they may appear to be identical.

Plain Bearings

For plain bearings some materials can give much longer life than others. Some of the earlier mentioned Harrison clocks still operate after hundreds of years because of the lignum vitae wood that was used in their manufacture, whereas his metal clocks are seldom run due to the possibility of wear.

Flexure Bearings

Flexure bearings rely on elastic properties of material. Flexure bearings bend a piece of material time and time again. Some materials fail after repeated bending, even at low loads, but careful material selection and bearing design can make flexure bearing life pretty much indefinite.

Short-life Bearings

Although long bearing life is often very desirable, it is sometimes not that necessary. Harris describes a bearing for a rocket motor oxygen pump that gave several hours worth of life, this was far in excess of the several minutes life that it actually needed.

L10 Life of the Bearing

Bearings are often specified to give an "L10" life. This tends to be an Americanism as outside of the United States of America, it may be referred to as a "B10" life. This is the life at which ten percent of the bearings in that application can be expected to have failed due to classical fatigue failure, or, alternatively, the life at which ninety percent will still be operating. The L10/B10 life of the bearing is a theoretical life and may not represent the actual service life of the bearing. Bearings are also rated using static loading value. This is the basic load rating as a reference, and not an actual load value.

External factors of the Bearing

The service life of the bearing is affected by many parameters that are not controlled by the bearing manufactures. Such as bearing mounting, bearing temperature, bearing exposure to external environment, lubricant cleanliness and the passing of electrical currents through the bearings.

Maintenance and lubrication of the Bearings

Many bearings require periodic maintenance to prevent premature failure, although some bearings such as fluid or magnetic bearings may require very little maintenance.

Most bearings in high cycle operations need periodic lubrication and cleaning, and may require adjustment to minimise the effects that wear has on them.

Bearing life is very often better when the bearing is kept clean and well lubricated. However, many applications make good maintenance quite difficult. For example bearings in the conveyor used in a quarry setting will be exposed continually to hard abrasive particles from the rocks and sand. Cleaning is of little use because cleaning is expensive, yet the bearing is contaminated again as soon as the conveyor belt resumes operation. Therefore, a good maintenance program might lubricate the bearings frequently but not actually clean them.

Bearing Packing

Some bearings use a thick grease for their lubrication needs, which is pushed into the gaps between the bearing surfaces, this is also known as packing. The grease is held in place by a plastic, leather, or rubber gasket or gland, that covers the inside and outside edges of the bearing race to keep the grease from escaping.

Bearings may also be packed with other materials. Historically, the wheels on railroad cars used sleeve bearings packed with waste or loose scraps of cotton or wool fibre that had been soaked in oil.

Ring Oiler for Bearings

Bearings can be lubricated by a metal ring that rides loosely on the central rotating shaft of the bearing. The ring hangs down into a chamber that contains the lubricating oil. As the bearing rotates, viscous adhesion draws oil up the ring and onto the shaft, where the oil migrates into the bearing to lubricate it. Excess oil is thrown off and collects in the sump or pool again.

Splash Lubrication for Bearings

Some machines contain a pool of lubricant in the bottom, with gears partially immersed in the liquid, or crank rods that can be swung down into the pool or sump as the device operates. The spinning wheels throw oil into the air around them, while the crank rods slap at the surface of the oil, splashing it randomly on the interior surfaces of the engine. Some small internal combustion engines specifically contain special plastic flinger wheels which randomly scatter oil around the interior of the mechanism. Pressure lubrication

For high speed and high power machines, a loss of the lubricant can result in rapid bearing heating and damage due to the friction produced. Also in dirty environments the oil can become contaminated with dust or debris that will increase the friction. In these applications, a fresh supply of lubricant can be continuously supplied to the bearing and all other contact surfaces, and the excess can be collected for filtration, cooling, and possibly to be reused. Pressure oiling is often used in large and complex internal combustion engines in parts of the engine where directly splashed oil cannot reach, such as up into overhead valve assemblies. High speed turbochargers also typically require a pressurized oil system to cool the bearings and keep them from burning up due to the heat from the turbine.

Types of Bearings

Type

Description

Friction

Stiffness†

Speed

Life

Notes

Plain bearing

Rubbing surfaces, usually with lubricant; some bearings use pumped lubrication and behave similarly to fluid bearings.

Depends on materials and construction, PTFE has coefficient of friction ~0.05-0.35, depending upon fillers added

Good, provided wear is low, but some slack is normally present

Low to very high

Low to very high - depends upon application and lubrication

upon application and lubrication Widely used, relatively high friction, suffers from stiction in some applications. Depending upon the application, lifetime can be higher or lower than rolling element bearings.

Rolling element bearing

Ball or rollers are used to prevent or minimise rubbing

Rolling coefficient of friction with steel can be ~0.005 (adding resistance due to seals, packed grease, preload and misalignment can increase friction to as much as 0.125)

Good, but some slack is usually present

Moderate to high (often requires cooling)

Moderate to high (depends on lubrication, often requires maintenance)

Used for higher moment loads than plain bearings with lower friction

Jewel bearing

Off-center bearing rolls in seating

Low

Low due to flexing

Low

Adequate (requires maintenance)

Mainly used in low-load, high precision work such as clocks. Jewel bearings may be very small.

Fluid bearing

Fluid is forced between two faces and held in by edge seal

Zero friction at zero speed, low

Very high

Very high (usually limited to a few hundred feet per second at/by seal)

Virtually infinite in some applications, may wear at startup/shutdown in some cases. Often negligible maintenance.

Can fail quickly due to grit or dust or other contaminants. Maintenance free in continuous use. Can handle very large loads with low friction.

Magnetic bearings

Faces of bearing are kept separate by magnets (electromagnets or eddy currents)

Zero friction at zero speed, but constant power for levitation, eddy currents are often induced when movement occurs, but may be negligible if magnetic field is quasi-static

Low

No practical limit

Indefinite. Maintenance free. (with electromagnets)

Active magnetic bearings (AMB) need considerable power. Electrodynamic bearings (EDB) do not require external power.

Flexure bearing

Material flexes to give and constrain movement

Very low

Low

Very high

Very high or low depending on materials and strain in application. Usually maintenance free.

Limited range of movement, no backlash, extremely smooth motion

†Stiffness is the amount that the gap varies when the load on the bearing changes, it is distinct from the friction of the bearing.

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