In order to serve all these functions, bearings make use of a relatively simple structure: a ball with internal and external smooth metal surfaces, to aid in rolling. The ball itself carries the weight of the load the force of the loads weight is what drives the bearing’s rotation. However, not all loads put force on a bearing in the same manner. There are two different kinds of loading: radial and thrust.
A radial load, as in a pulley, simply puts weight on the bearing in a manner that causes the bearing to roll or rotate as a result of tension. A thrust load is significantly different, and puts stress on the bearing in an entirely different way. If a bearing (think of a tire) is flipped on its side (think now of a tire swing) and subject to complete force at that angle (think of three children sitting on the tire swing), this is called thrust load. A bearing that is used to support a bar stool is an example of a bearing that is subject only to thrust load.
Many bearings are prone to experiencing both radial and thrust loads. Car tires, for example, carry a radial load when driving in a straight line: the tires roll forward in a rotational manner as a result of tension and the weight they are supporting. However, when a car goes around a corner, it is subject to thrust load because the tires are no longer moving solely in a radial fashion and cornering force weighs on the side of the bearing.
Types of Bearings
- Ball Bearings
- Deep-Groove Ball Bearings.
- Angular Contact Ball Bearings
- Self-Aligning Ball Bearings.
- Thrust Ball Bearings.
- Spherical Roller Bearings.
- Cylindrical Roller Bearings.
- Tapered Roller Bearings.
- Needle Roller Bearings.
Ball Bearings utilize balls as the rolling elements. They are characterised by point contact between the balls and the raceways. As a rule, ball bearings rotate very quickly but cannot support substantial loads.
Deep-Groove Ball Bearings
The most commonly used bearings are Deep-Groove Ball Bearings. Thanks to their simple design, they are easy to maintain and not as sensitive to operating conditions thus are used in a wide range of different applications.
In addition to radial forces, they absorb axial forces in both directions. Their low torque also makes them suitable for high speeds.
Angular Contact Ball Bearings
Angular Contact Ball Bearings are characterised by a contact angle. This means that forces are transferred from one raceway to the other at a particular angle.
Angular-contact ball bearings are therefore suitable for combined loads, where high axial forces have to be transferred in addition to radial forces.
Self-Aligning Ball Bearings
Self-Aligning Ball Bearings include a double row of balls guided by a cage and double row inner ring raceway but have the special feature of a continuous spherical outer ring raceway allowing the inner ring / ball complement to swivel within the outer ring. This is what enables a degree of self-alignment in the application.
This type of bearing is recommended when alignment of the shaft and the housing (misalignment) are a problem and the shaft could deflect. Self-aligning ball bearings are most suitable for absorbing radial forces.
Thrust Ball Bearings
Thrust Ball Bearings consist of two bearing discs with raceways for the balls.
Thrust ball bearings were developed solely for absorbing axial forces in one direction, meaning they can locate the shaft axially in one direction.
Roller Bearings are characterized by line contact. Line contact offers higher load rating than ball bearings of the same size; however the speed ability is lower than a ball bearing due to the increased friction of a contact line.
Spherical Roller Bearings
Spherical Roller Bearings are very robust and work on the same principle as Self-aligning bearings with the exception that they use spherical rollers instead of ball rollers allowing higher loads to be supported. This can compensate for misalignments between the shaft and the housing.
Spherical roller bearings are suitable for absorbing high radial loads and moderate axial loads.
Cylindrical Roller Bearings
Cylindrical Roller Bearings use line contact between the rolling elements and the raceways, which optimizes the distribution of stress factors at the point of contact. This arrangement means that cylindrical roller bearings have a very high radial load rating.
Depending on the design, they may also be able to transmit limited amounts of axial loads.
Tapered Roller Bearings
Tapered Roller Bearings have tapered raceways in the inner and outer rings with conical rollers arranged between them.
Due to the contact angle, tapered roller bearings can absorb high radial and axial forces in one direction.
Tapered roller bearings are often combined in pairs to support axial forces in both directions.
Needle Roller Bearings
Needle Roller Bearings are a special type of cylindrical roller bearing which contain long, thin rolling elements, known as needle rollers. The ratio of diameter to length is between 1:3 and 1:10.
Needle roller bearings have a high load rating and are only suitable for radial forces.
HOW BEARINGS ARE MADE
THIS IS WHAT YOU NEED TO KNOW ABOUT HOW BEARINGS ARE MADE:
WHY THERE’S NO SUCH THING AS A 1/4″ BALL:
It seems that very few people actually know how ball bearings are made. For those of you looking for something better, this is important. In the manufacturing of bearings, there are three basic machining operations. Inner race, outer race and the balls themselves. It is possible to manufacture balls with repeatable dimensional consistency, but it is not possible to do the same with the races. This is due to a number of factors, including variations in the machines themselves, but include others such as wear on the grinding wheels and even variations in temperature. As a result, bearing manufacturers don’t really try to maintain the same dimensions from run to run, and instead rely on different sized balls to assemble the bearing to the required tolerances. Bigger gap between the races? No problem, just use a slightly larger 1/4″ball. Looking for a “tighter” fit? Use a larger ball. The variation in ball sizes often are separated by millionths of an inch, well beyond the ability to measure by most of us. So what does this mean to you? Remember the following:
Three different bearings of the same size can have three different sized balls.
Balls are not interchangeable from bearing to bearing.
Accidentally putting just one ball that is larger than the rest can spell disaster!
It is extremely difficult to measure the differences with ordinary micrometers.
You cannot simply buy “a ball”, either steel or ceramic and expect it to be the right size.
So if you are in the market for assembled ceramics, it’s all about ball quality and the “fit”. To do it right takes a healthy assortment of ceramic balls, which adds up to a sizeable investment quickly. Everyone starts with a pre-made bearing, and the steel balls are simply swapped out with ceramics. Now there are an infinite number of possibilities of inner and outer race/ball combinations and each will require one particular ball size to meet the required tolerances. So having only three of any size category simply won’t cut it. It takes more than that. Ceramic balls are expensive (good ones anyway), and this is an easy way to cut costs. So the next time you hear about a failed ceramic horror story, consider the above. Don’t ask the bearing maker what type of balls they use, instead ask how many 1/4″ balls they have on hand.
HERE’S SOME TIPS ON BEARING REMOVAL & INSTALLATION FROM CASES:
Bearing Removal Tips
WHAT’S THE BEST WAY TO REMOVE BEARINGS FROM CASES?
We are asked quite often which is the best way to remove bearings in cases and we are quick to point out that whenever possible, we use heat (and cold) and try at all costs not to use presses and/or pullers. The reason for this is it is far too easy to damage the bore the bearing is in especially if it’s aluminum. But the “how much” and “how to” bring heat in is critical. We exclusively use an oven. Preferably a convection oven, where a fan circulates the heat and keeps things much more uniform.
We typically place the items in an oven set at 350 deg./F. We place them so the bearings are at the top and quite often when they reach temperature, you’ll hear them fall out. For most applications, we limit the oven time to 30 minutes An infrared temperature gun is very valuable here. If there are any bearings left in the cases, we (carefully) drop the cases on a hard floor from about 1″. This usually dislodges the stubborn ones. In the event there’s one being difficult, we prefer a slide puller, the one we use we purchased from McMaster Carr.
Inexpensive Table Top Convection Oven
A WORD ABOUT BALL BEARING “PRESS-FITS”.
Few people realize how critical the clearances either on the outer bearing bore or inner bore on bearing life and performance. Because if you have a shaft that’s .001″ larger than the bore, that inner race grows by that amount. Now take a bearing bore that’s .001″ smaller than the outer race and that outer race will now become smaller. Bear in mind that there needs to be enough clearance between the balls to allow space for the lubricant to work. If that clearance is not there, you are now forcing hard contact between the balls and races. The test we use to determine if all is well is to direct compresses air across the balls to initiate rolling. If the bearing turns freely, all is well. If it resists turning, it’s time to check the clearances.
Temp Guns Save Time And Avoids Damage
IF YOU MUST USE FORCE
Most bearing bore damage occurs when the bearing is “cocked” slightly and is forcefully pulled from the case. Be aware of that and correct if necessary. Whenever possible, we prefer to use a slide puller along with a heated case. If you’re having trouble, use your temp gun and verify you’re at 325-350 deg./F. Surprisingly, 10 to 20 deg. makes a big difference.
Internal Bearing Slide Puller
TO INSTALL BEARINGS, HEAT IS THE KEY.
When pressing bearings in place, there’s a lot of things that can go wrong. First off, you should only apply even force on the outer races. And how often does one have the correct press ring on hand? So the easy way is to grab a socket and press on the inner race. Bad plan. Second it’s easy to slightly “cock” the bearing thereby damaging the bore in the process. Third it’s difficult to know when the bearing has “bottomed out” with a press. So we apply more force and go by “feel”. A better approach is to heat the cases, and if you want, put the bearings (or races) in the freezer. That way they drop right in place!
THINGS TO NEVER DO
1) Never place aluminum parts on hot plates of any kind. It is next to impossible to heat evenly and even harder to control the temperature. Remember, when you approach 400 deg./F, (from metallurgical standpoint) you’re in the danger zone.
2) Never use a torch of any kind, again for the above reasons, plus the fact that aluminum dissipates heat rapidly and you wind up overheating areas that also damages the metallurgy of the aluminum.
3) Never heat the bearing itself. It makes it grow and become harder to remove.
4) Never try to locally cool the bearing to shrink it. The cases will cool as well and everything gets tight.
The coating is not adversely affected by common hydrocarbon-based solvents such as brake clean, carb cleaner or parts solvents. Do stay away from water-based cleaners as many of them are very caustic and can harm the coating. We recommend blowing out the bearings, followed by a WD40 flush, compressed air, an application of brake clean, finishing with compressed air.
All bearings need lubrication and should never be run “dry”. As to the type of lubrication, that depends on the application, the load and the environment. Remember, you only have viscosity separating the bearing surfaces, so this is not the time to get too carried away with “thin”. Another consideration is location. If you have a bearing in an axle housing that rarely gets looked at, grease is a better choice. In general, oils are fine for ball bearings (only) in go karts, quarter midgets, bandoleers and so forth. We DO NOT recommend any oils with tapered bearings, unless the hub is designed for it.
For tapered wheel bearings, the choice of grease is not as important as the amount you use. We typically recommend taking a bearing cone in one hand and then apply some grease around the rollers. Then place the bearing into the race and rotate it. When there’s a uniform film covering the rollers and races, you’re done. “Packing” in general is not necessary.
Whenever possible, we advise using heat to remove/install bearings. For cases, housings and covers, simply place in an oven heated to 300-350 deg./F. This will not damage the bearings, cages or seals. DO NOT use a hot plate or torch to apply heat. You will locally overheat things and easily do metallurgical damage. A heat gun can be used, but do not overheat things. Use an IR temp gun to keep track of things.
SET-UP & ADJUSTMENT
For tapered wheel applications, recommended end play is .001” to .002”. DO NOT allow any pre-load. When spacers are used with ball or angular contact bearings, it is CRITICAL to get the spacing correct. Meaning the balls must contact the ball races EXACTLY in the center. Any offset will quickly result in premature failure. If you’re unsure about things, it’s best to leave them out.
For tapered rear-gear applications, be sure to use much less pinion preload. Our bearings will not “loosen up” over time. We recommend two things: One, that total rotating torque to be around 12 in/lbs. with 6 of that on the pinion bearings. Two, in most cases, a pre-load of about .001” will get you very close. If it runs hot, it will be due to excessive pinion preload.
How to Measure Bearings
Most bearings have their part numbers engraved on the bearings themselves, but these often rub off over time and so it may be necessary to measure the bearing yourself to determine the correct part number.
You can measure the dimensions of a bearing by using a vernier caliper like the one below or measuring accurately with a ruler.
All bearings will have an inner diameter (ID), an outside diameter (OD) and width (W). Bearings are sometimes referenced by these dimensions, in that order: ID x OD x W.
Measure the ID
Insert the outer anvils of the caliper (the ones on top of the picture above) into the bore as shown and open the caliper until it is a good fit, but not tight, now read the value on the caliper. In the absence of a caliper a good quality steel ruler will suffice.
Measure the OD
To measure the outside diameter of a bearing place the jaws of the caliper around the bearing and close it until its a good fit but not tight, now read the value from the caliper.
Measure the Width
In the same manner as you measured the outside diameter measure the width of the bearing.
Plummer blocks are bearing blocks for self-aligning roller or ball bearings. This range is designed to handle higher load rating and moderate speed and allow dismantling of shaft without having to move the housing mounting as well.
What is a mounted bearing?
Pillow Block Bearings
Pillow Blocks & Flanges Pillow block bearings, flange bearing units, bearing blocks, and take-up bearings units all consist of a housing with a bearing mounted in it. They are available in a variety of materials, mounting configurations and various bearing features. Each mounted unit, including a mounted bearing, acts as a system to position the bearing securely for reliable operation. Read more about each type of pillow block bearing using the links below.
Flange Mounted Bearings
Flange mounted bearings are used when the shaft axis is perpendicular to the bearing mounting surface. They incorporate a sealed bearing that is pre assembled into a flanged housing. The housing contains a precision ground surface perpendicular to the bearing axis and two, three or four mounting holes, depending on the style. The bearing can be unbolted and removed, which makes bearing replacement easier and faster than traditional rotary bearings that must be press fit into a housing. Flange mounted bearings can also support heavy loads, which protects the shaft from deflection, which could cause vibration or other damage.
Two-bolt flange mounted bearings are diamond shaped and have two holes for mounting bolts, one on each side of the bearing. A line drawn through the axis of the mounting holes forms a straight line that runs through the axis of the bearing.
Three-bolt flange mounted bearings have three mounting holes, either arranged radially around the bearing axis at 120 degrees from each other, or on a triangular flange that is offset from the bearing axis.
Four-bolt flange mounted bearings have four mounting holes, located radially around the bearing axis. Four-bolt flange bearings typically have round or square housings.
The size of the bearing is important to consider when making a selection.
Shaft size specifies the maximum diameter of the shaft or the bore diameter of the bearing.
Height above mounting surface is important to take into consideration for low clearance applications.
The bearing housing provides a method for securing the bearing while in use, maintains bearing alignment and also protects the bearing during operation. Housings can be made from a variety of materials with different properties.
Cast iron refers to a family of materials whose major constituent is iron, with important trace amounts of carbon and silicon. Cast irons are natural composite materials whose properties are determined by their microstructures – the stable and metastable phases formed during solidification or subsequent heat treatment. The major microstructural constituents of cast irons are: the chemical and morphological forms taken by carbon, and the continuous metal matrix in which the carbon and/or carbide are dispersed.
Pressed steel is a low carbon steel, which has been pressed rather than machined.
Plastic refers to numerous organic, synthetic, or processed materials that are mostly thermoplastic or thermosetting polymers of high molecular weight and that can be made into objects, films, or filaments. Common plastic materials include Acetal, Nylon / Poly amide and PTFE / Teflon.
Nylon, comprising several grades of poly amides, is a general-purpose material in wide use; it is tough and resistant and has good pressure ratings.
PTFE (poly tetra fluoro ethylene) is an insoluble compound that exhibits a high degree of chemical resistance and a low coefficient of friction. It is sometimes marketed in proprietary classes of materials such as Teflon, a registered trademark of DuPont Dow Elastomers.
Acetal polymers are semi-crystalline. They offer excellent inherent lubricity, fatigue resistance, and chemical resistance. Acetals suffer from outgassing problems at elevated temperatures, and are brittle at low temperatures. Glass filled, and added lubrication grades are available, flame-retardant grades are not.
Stainless steel is chemical and corrosion resistant and can have relatively high-pressure ratings.
Bearing and Housing Features
Offset Hole Pattern from the bearing centerline, as opposed to radially symmetrical around the axis of the bearing. Offset holes are normally only found in 3-bolt flanges.
Spherical/ Self-aligning bearing come in two types
Internal and external.
Internal bearings have a grooved outer-ring that is ground as a spherical surface. In external bearings, the spherical surface is on the outside of the outer ring. This matches a concave spherical housing. These bearings allow for minor shaft / bearing misalignments.
Split Bearing housing and/or bearing is split into two pieces and bolted together, facilitating easier maintenance or bearing replacement for worn or damaged bearings or shafts.
Shaft Securing Method
Set Screw Bearings secured with a set screw is secured to the shaft by a set screw, located in an inner ring, which is e bearings have set screws for securing the inner ring to a shaft.
Locking collar bearings have a lock nut for securing the inner ring toa shaft.
No securing method bearings have no provisions for securing to a shaft.
There are many specifications to consider when selecting a flange mounted bearing.
Maximum speed is the high speed the bearing can safely function at before failure. It is influenced by load characteristics, bearing lubrication, and temperature.
Bearing life, also known as the rating life L10, is a statistical measure of the life which 90% of a group of apparently identical ball bearings will achieve or exceed. For a single bearing L10 also refers to the life associated with 90% reliability. Median Life, L50, is the life which 50% of the group of ball bearings will achieve or exceed. Median life is usually not greater than five times the rating life.
Bearing loads are a combination of radial loads and thrust forces. If the bearing is required to absorbed thrust forces in additional to radial loads, the following considerations much be made concerning the magnitude of the thrust force. When the thrust loads are half of the radial load, the selection should be made based upon the applied radial load. When thrust loads are equal to or greater than half of the radial load, the selection should be made based upon using the total load (radial and thrust loads together) as the equivalent applied radial load.
The basic dynamic load rating (C), or “dynamic capacity,” for a ball bearing is a calculated, constant radial load. The load is applied to identical bearings with a stationary outer ring for one million revolutions of the inner ring. The basic static load rating (CO), or “static capacity,” is that uniformly distributed load. The load produces a maximum theoretical contact stress on the most heavily loaded ball of 609,000 psi. At this contact stress a permanent deformation of ball and raceway occurs. This deformation is approximately 0.01% of the ball diameter in inches.
Plain bearing on a 1906 S-Motor locomotive showing the axle, bearing, oil supply and oiling pad
A plain bearing, or more commonly sliding bearing and slide bearing (in railroading sometimes called a solid bearing or friction bearing), is the simplest type of bearing, comprising just a bearing surface and no rolling elements. Therefore, the journal (i.e., the part of the shaft in contact with the bearing) slides over the bearing surface. The simplest example of a plain bearing is a shaft rotating in a hole. A simple linear bearing can be a pair of flat surfaces designed to allow motion; e.g., a drawer and the slides it rests on or the ways on the bed of a lathe.
Plain bearings, in general, are the least expensive type of bearing. They are also compact and lightweight, and they have a high load-carrying capacity.
The design of a plain bearing depends on the type of motion the bearing must provide. The three types of motions possible are:
Journal (friction, radial or rotary) bearing.
This is the most common type of plain bearing; it is simply a shaft rotating in a bearing. In locomotive and railroad car applications a journal bearing specifically referred to the plain bearing once used at the ends of the axles of railroad wheel sets, enclosed by journal boxes (axleboxes). Axlebox bearings today are no longer plain bearings but rather are rolling-element bearings.
This bearing provides linear motion; it may take the form of a circular bearing and shaft or any other two mating surfaces (e.g., a slide plate).
A thrust bearing provides a bearing surface for forces acting axial to the shaft.One example is a propeller shaft.
Integral plain bearings are built into the object of use as a hole prepared in the bearing surface. Industrial integral bearings are usually made from cast iron or babbitt and a hardened steel shaft is used in the bearing.
Integral bearings are not as common because bushings are easier to accommodate and can be replaced if necessary. Depending on the material, an integral bearing may be less expensive but it cannot be replaced. If an integral bearing wears out, the item may be replaced or reworked to accept a bushing. Integral bearings were very common in 19th-century machinery, but became progressively less common as interchangeable manufacture became popular.
For example, a common integral plain bearing is the hinge, which is both a thrust bearing and a journal bearing.
A bushing, also known as a bush, is an independent plain bearing that is inserted into a housing to provide a bearing surface for rotary applications; this is the most common form of a plain bearing. Common designs include solid (sleeve and flanged), split, and clenched bushings. A sleeve, split, or clenched bushing is only a “sleeve” of material with an inner diameter (ID), outer diameter (OD), and length. The difference between the three types is that a solid sleeve bushing is solid all the way around, a split bushing has a cut along its length, and a clenched bearing is similar to a split bushing but with a clench (or clinch) across the cut connecting the parts. A flanged bushing is a sleeve bushing with a flange at one end extending radially outward from the OD. The flange is used to positively locate the bushing when it is installed or to provide a thrust bearing surface.
Sleeve bearings of inch dimensions are almost exclusively dimensioned using the SAE numbering system. The numbering system uses the format -XXYY-ZZ, where XX is the ID in sixteenths of an inch, YY is the OD in sixteenths of an inch, and ZZ is the length in eighths of an inch. Metric sizes also exist.
A linear bushing is not usually pressed into a housing, but rather secured with a radial feature. Two such examples include two retaining rings, or a ring that is molded onto the OD of the bushing that matches with a groove in the housing. This is usually a more durable way to retain the bushing, because the forces acting on the bushing could press it out.
The thrust form of a bushing is conventionally called a thrust washer.
Two-piece plain bearings, known as full bearings in industrial machinery, are commonly used for larger diameters, such as crankshaft bearings. The two halves are called shells. There are various systems used to keep the shells located. The most common method is a tab on the parting line edge that correlates with a notch in the housing to prevent axial movement after installation. For large, thick shells a button stop or dowel pin is used. The button stop is screwed to the housing, while the dowel pin keys the two shells together. Another less common method uses a dowel pin that keys the shell to the housing through a hole or slot in the shell.
The distance from one parting edge to the other is slightly larger than the corresponding distance in the housing so that a light amount of pressure is required to install the bearing. This keeps the bearing in place as the two halves of the housing are installed. Finally, the shell’s circumference is also slightly larger than the housing circumference so that when the two halves are bolted together the bearing crushes slightly. This creates a large amount of radial force around the entire bearing, which keeps it from spinning. It also forms a good interface for heat to travel out of the bearings into the housing
Plain bearings must be made from a material that is durable, low friction, low wear to the bearing and shaft, resistant to elevated temperatures, and corrosion resistant. Often the bearing is made up of at least two constituents, where one is soft and the other is hard. In general, the harder the surfaces in contact the lower the coefficient of friction and the greater the pressure required for the two to gall or to seize when lubrication fails.
Main article: Babbitt (metal)
Babbitt is usually used in integral bearings. It is coated over the bore, usually to a thickness of 1 to 100 thou (0.025 to 2.540 mm), depending on the diameter. Babbitt bearings are designed to not damage the journal during direct contact and to collect any contaminants in the lubrication.
Split bi-material bushings: a metal exterior with an inner plastic coating
Bi-material bearings consist of two materials, a metal shell and a plastic bearing surface. Common combinations include a steel-backed PTFE-coated bronze and aluminum-backed Frelon. Steel-backed PTFE-coated bronze bearings are rated for more load than most other bi-metal bearings and are used for rotary and oscillating motions. Aluminum-backed frelon are commonly used in corrosive environments because the Frelon is chemically inert.
|Temperature range||P (max.)[psi (MPa)]||V (max.)[sfm (m/s)]||PV (max.)[psi sfm (MPa m/s)]|
|Steel-backed PTFE-coated bronze||−328–536 °F or −200–280 °C||36,000 psi or 248 MPa||390 (2.0 m/s)||51,000 (1.79 MPa m/s)|
|Aluminum-backed frelon||−400–400 °F or −240–204 °C||3,000 psi or 21 MPa||300 (1.52 m/s)||20,000 (0.70 MPa m/s)|
A common plain bearing design utilizes a hardened and polished steel shaft and a softer bronze bushing. The bushing is replaced whenever it has worn too much.
Common bronze alloys used for bearings include: SAE 841, SAE 660 (CDA 932), SAE 863, and CDA 954
|Temperature range||P (max.)[psi (MPa)]||V (max.)[sfm (m/s)]||PV (max.) [psi sfm (MPa m/s)]|
|SAE 841||10–220 °F (−12–104 °C)||2,000 psi (14 MPa)||1,200 (6.1 m/s)||50,000 (1.75 MPa m/s)|
|SAE 660||10–450 °F (−12–232 °C)||4,000 psi (28 MPa)||750 (3.8 m/s)||75,000 (2.63 MPa m/s)|
|SAE 863||10–220 °F (−12–104 °C)||4,000 psi (28 MPa)||225 (1.14 m/s)||35,000 (1.23 MPa m/s)|
|CDA 954||Less than 500 °F (260 °C)||4,500 psi (31 MPa)||225 (1.14 m/s)||125,000 (4.38 MPa m/s)|
A cast iron bearing can be used with a hardened steel shaft because the coefficient of friction is relatively low. The cast iron glazes over therefore wear becomes negligible.
In harsh environments, such as ovens and dryers, a copper and graphite alloy, commonly known by the trademarked name graphalloy, is used. The graphite is a dry lubricant, therefore it is low friction and low maintenance. The copper adds strength, durability, and provides heat dissipation characteristics.
|Temperature range||P (max.)[psi (MPa)]||V (max.)[sfm (m/s)]||PV (max.)[psi sfm (MPa m/s)]|
|Graphalloy||−450–750 °F or
|750 psi or 5 MPa||75 (0.38 m/s)||12,000 (0.42 MPa m/s)|
Unalloyed graphite bearings are used in special applications, such as locations that are submerged in water.
Main article: Jewel bearing
Known as jewel bearings, these bearings use jewels, such as sapphire, ruby, and garnet.
Solid plastic plain bearings are now increasingly popular due to dry-running lubrication-free behavior. Solid polymer plain bearings are low weight, corrosion resistant, and maintenance free. After studies spanning decades, an accurate calculation of the service life of polymer plain bearings is possible today. Designing with solid polymer plain bearings is complicated by the wide range, and non-linearity, of coefficient of thermal expansion. These materials can heat rapidly when used in applications outside the recommended pV limits.
Solid polymer type bearings are limited by the injection molding process. Not all shapes are possible with this process, and shapes that are possible are limited to what is considered good design practice for injection molding. Plastic bearings are subject to the same design cautions as all other plastic parts: creep, high thermal expansion, softening (increased wear/reduced life) at elevated temperature, brittle fractures at cold temperatures, and swelling due to moisture absorption. While most bearing-grade plastics/polymers are designed to reduce these design cautions, they still exist and should be carefully considered before specifying a solid polymer (plastic) type.
Plastic bearings are now quite common, including usage in photocopy machines, tills, farm equipment, textile machinery, medical devices, food and packaging machines, car seating, and marine equipment.
Common plastics include nylon, polyacetal, polytetrafluoroethylene (PTFE), ultra-high-molecular-weight polyethylene (UHMWPE), rulon, PEEK, urethane, and vespel (a high-performance polyimide).
|Temperature range||P (max.) [psi (MPa)]||V (max.) [sfm (m/s)]||PV (max.) [psi sfm (MPa m/s)]|
|Frelon||−400 to 500 °F (−240 to 260 °C)||1,500 psi (10 MPa)||140 (0.71 m/s)||10,000 (0.35 MPa m/s)|
|Nylon||−20 to 250 °F (−29 to 121 °C)||400 psi (3 MPa)||360 (1.83 m/s)||3,000 (0.11 MPa m/s)|
|MDS-filled nylon blend 1||−40 to 176 °F (−40 to 80 °C)||2,000 psi (14 MPa)||393 (2.0 m/s)||3,400 (0.12 MPa m/s)|
|MDS-filled nylon blend 2||−40 to 230 °F (−40 to 110 °C)||300 psi (2 MPa)||60 (0.30 m/s)||3,000 (0.11 MPa m/s)|
|PEEK blend 1||−148 to 480 °F (−100 to 249 °C)||8,500 psi (59 MPa)||400 (2.0 m/s)||3,500 (0.12 MPa m/s)|
|PEEK blend 2||−148 to 480 °F (−100 to 249 °C)||21,750 psi (150 MPa)||295 (1.50 m/s)||37,700 (1.32 MPa m/s)|
|Polyacetal||−20 to 180 °F (−29 to 82 °C)||1,000 psi (7 MPa)||1,000 (5.1 m/s)||2,700 (0.09 MPa m/s)|
|PTFE||−350 to 500 °F (−212 to 260 °C)||500 psi (3 MPa)||100 (0.51 m/s)||1,000 (0.04 MPa m/s)|
|Glass-filled PTFE||−350 to 500 °F (−212 to 260 °C)||1,000 psi (7 MPa)||400 (2.0 m/s)||11,000 (0.39 MPa m/s)|
|Rulon 641||−400 to 550 °F (−240 to 288 °C)||1,000 psi (7 MPa)||400 (2.0 m/s)||10,000 (0.35 MPa m/s)|
|Rulon J||−400 to 550 °F (−240 to 288 °C)||750 psi (5 MPa)||400 (2.0 m/s)||7,500 (0.26 MPa m/s)|
|Rulon LR||−400 to 550 °F (−240 to 288 °C)||1,000 psi (7 MPa)||400 (2.0 m/s)||10,000 (0.35 MPa m/s)|
|UHMWPE||−200 to 180 °F (−129 to 82 °C)||1,000 psi (7 MPa)||100 (0.51 m/s)||2,000 (0.07 MPa m/s)|
|MDS-filled urethane||−40 to 180 °F (−40 to 82 °C)||700 psi (5 MPa)||200 (1.02 m/s)||11,000 (0.39 MPa m/s)|
|Vespel||−400 to 550 °F (−240 to 288 °C)||4,900 psi (34 MPa)||3,000 (15.2 m/s)||300,000 (10.5 MPa m/s)|
- igus, iglidur Specially developed polymer bearing materials with life prediction
- Ceramic bearings are very hard, so sand and other grit that enters the bearing are simply ground to a fine powder that does not inhibit the operation of the bearing.
- Lignum vitae is a self lubricating wood and in clocks it gives extremely long life. Also used with bronze wheels in ship rigging.
- In a piano, various (usually) wooden parts of the keyboard and action are linked together by center pins typically made of German silver. These linkages usually have felt, or more rarely, leather bushings.
See also: oiler (occupation)
A graphite-filled groove bushing
The types of lubrication system can be categorized into three groups:
- Class I — bearings that require the application of a lubricant from an external source (e.g., oil, grease, etc.).
- Class II — Bearings that contain a lubricant within the walls of the bearing (e.g., bronze, graphite, etc.,). Typically these bearings require an outside lubricant to achieve maximum performance.
- Class III — bearings made of materials that are the lubricant. These bearings are typically considered “self-lubricating” and can run without an external lubricant.
Examples of the second type of bearing are Oilites and plastic bearings made from polyacetal; examples of the third type are metalized graphite bearings and PTFE bearings.
Most plain bearings have a plain inner surface; however, some are grooved, such as spiral groove bearing. The grooves help lubrication enter the bearing and cover the whole journal.
Self-lubricating plain bearings have a lubricant contained within the bearing walls. There are many forms of self-lubricating bearings. The first, and most common, are sintered metal bearings, which have porous walls. The porous walls draw oil in via capillary action and release the oil when pressure or heat is applied. An example of a sintered metal bearing in action can be seen in self-lubricating chains, which require no additional lubrication during operation. Another form is a solid one-piece metal bushing with a figure eight groove channel on the inner diameter that is filled with graphite. A similar bearing replaces the figure eight groove with holes plugged with graphite. This lubricates the bearing inside and out. The last form is a plastic bearing, which has the lubricant molded into the bearing. The lubricant is released as the bearing is run in.
There are three main types of lubrication: full-film condition, boundary condition, and dry condition. Full-film conditions are when the bearing’s load is carried solely by a film of fluid lubricant and there is no contact between the two bearing surfaces. In mix or boundary conditions, load is carried partly by direct surface contact and partly by a film forming between the two. In a dry condition, the full load is carried by surface-to-surface contact.
Bearings that are made from bearing grade materials always run in the dry condition. The other two classes of plain bearings can run in all three conditions; the condition in which a bearing runs is dependent on the operating conditions, load, relative surface speed, clearance within the bearing, quality and quantity of lubricant, and temperature (affecting lubricant viscosity). If the plain bearing is not designed to run in the dry or boundary condition, it has a high coefficient of friction and wears out. Dry and boundary conditions may be experienced even in a fluid bearing when operating outside of its normal operating conditions; e.g., at startup and shutdown.
A schematic of a journal bearing under a hydrodynamic lubrication state showing how the journal centerline shifts from the bearing centerline.
Fluid lubrication results in a full-film or a boundary condition lubrication mode. A properly designed bearing system reduces friction by eliminating surface-to-surface contact between the journal and bearing through fluid dynamic effects.
Fluid bearings can be hydrostatically or hydrodynamically lubricated. Hydrostatically lubricated bearings are lubricated by an external pump that maintains a static amount of pressure. In a hydrodynamic bearing the pressure in the oil film is maintained by the rotation of the journal. Hydrostatic bearings enter a hydrodynamic state when the journal is rotating.Hydrostatic bearings usually use oil, while hydrodynamic bearings can use oil or grease, however bearings can be designed to use whatever fluid is available, and several pump designs use the pumped fluid as a lubricant.
Hydrodynamic bearings require greater care in design and operation than hydrostatic bearings. They are also more prone to initial wear because lubrication does not occur until there is rotation of the shaft. At low rotational speeds the lubrication may not attain complete separation between shaft and bushing. As a result, hydrodynamic bearings may be aided by secondary bearings that support the shaft during start and stop periods, protecting the fine tolerance machined surfaces of the journal bearing. On the other hand, hydrodynamic bearings are simpler to install and are less expensive.
In the hydrodynamic state a lubrication “wedge” forms, which lifts the journal. The journal also slightly shifts horizontally in the direction of rotation. The location of the journal is measured by the attitude angle, which is the angle formed between the vertical and a line that crosses through the center of the journal and the center of the bearing, and the eccentricity ratio, which is the ratio of the distance of the centre of the journal from the centre of the bearing, to the overall radial clearance. The attitude angle and eccentricity ratio are dependent on the direction and speed of rotation and the load. In hydrostatic bearings the oil pressure also affects the eccentricity ratio. In electromagnetic equipment like motors, electromagnetic forces can counteract gravity loads, causing the journal to take up unusual positions.
One disadvantage specific to fluid-lubricated, hydrodynamic journal bearings in high-speed machinery is oil whirl—a self-excited vibration of the journal. Oil whirl occurs when the lubrication wedge becomes unstable: small disturbances of the journal result in reaction forces from the oil film, which cause further movement, causing both the oil film and the journal to “whirl” around the bearing shell. Typically the whirl frequency is around 42% of the journal turning speed. In extreme cases oil whirl leads to direct contact between the journal and the bearing, which quickly wears out the bearing. In some cases the frequency of the whirl coincides with and “locks on to” the critical speed of the machine shaft; this condition is known as “oil whip”. Oil whip can be very destructive.
A lemon bore
Oil whirl can be prevented by a stabilising force applied to the journal. A number of bearing designs seek to use bearing geometry to either provide an obstacle to the whirling fluid or to provide a stabilising load to minimize whirl. One such is called the lemon bore or elliptical bore. In this design, shim sare installed between the two halves of the bearing housing and then the bore is machined to size. After the shims are removed, the bore resembles a lemon shape, which decreases the clearance in one direction of the bore and increases the preload in that direction. The disadvantage of this design is its lower load carrying capacity, as compared to typical journal bearings. It is also still susceptible to oil whirl at high speeds, however its cost is relatively low.
A pressure dam
Another design is the pressure damn or damned groove, which has a shallow relief cut in the center of the bearing over the top half of the bearing. The groove abruptly stops in order to create a downward force to stabilize the journal. This design has a high load capacity and corrects most oil whirl situations. The disadvantage is that it only works in one direction. Offsetting the bearing halves does the same thing as the pressure dam. The only difference is the load capacity increases as the offset increases.
A more radical design is the tilting-pad design, which uses multiple pads that are designed to move with changing loads. It is usually used in very large applications but also finds extensive application in modern turbomachinery because it almost completely eliminates oil whirl.