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Bearings
Housings
Dimensions
Bearings
friction
dimensions
Bearings
Friction
Housings
The housing (sometimes called a pillow block) of a bearing is its outer casing.
The bearing has a nominal size such as “2 7/16" bearing.” This is the size of the bore of the bearing. Note this is not called a 2.4375" bearing other dimensions are also fractional, not decimal.
Cap
Cap bolt
Friction
Parts
Base bolt hole/slot
Anti-friction
Liner
Base
The basic parts of anti-friction bearings are two hardened steel rings, the
hardened balls or rollers, and the separator. A number of variations are in
use. Some types, such as needle roller bearings, may be used with or
without an inner ring, outer ring, or separators. If there is no inner ring,
the rollers fit directly onto the hardened shaft. For specific applications,
bearings may have other parts such as a snap ring used to set axial location.
• fabricated steel for bearing housings for special jobs.
• cast iron for general use with light to medium loads
• cast steel for general use. It is stronger than cast iron and can be used for
light to heavy loads.
The dimension from the base to the shaft centre (called the eye of the
bearing) is important for installation and alignment. Bearings of the same
size, design, and service weight from the same supplier have the same shaft
centre dimension. Bearings from different suppliers may have different
values for this and other dimensions.
Shields and seals
Roller elements
Styles
Joint design
Roller
Open
Shielded
Sealed
Needle
Ball
The choice of a bearing depends on the load, the rotational (or rubbing) speed, the direction of pull, and the support design.
Bearings
Friction
Styles
Bearings
friction
joint design
Ball bushing
You must consider the design of the bearing joint when choosing a bearing
for a specific job Bearing joints are designed in three general styles:
Housings are made of one of the following materials:
The choice of a bearing housing depends on the load, the rotational (or
rubbing) speed, the direction of pull, and the support design.
Regular or flat
Bearings friction
styles
flat/regular
Gib Joint
bearings
friction
styles
flat
solid
Bearing
friction
styles
flat
split
Split
Solid
The two kinds of flat bearings differ in their housings. They are solid
bearings and split bearings.
Flat joints
Angled joint
Solid bearings are used
when speed and load are
low. They are designed
to be used in locations
where the load is applied
to the top part of the
bearing. Solid bearings
are frequently mounted
in an inverted position.
They must be slid on or
off the shaft, which
means that the bearings
must be put on as an
assembled unit.
Split bearing designs vary from one manufacturer to another.
• A two-bolt bearing is used for light to medium loads.
• A four-bolt bearing is used for medium to heavy loads.
Bearings
Friction
styles
intergrated
Integrated
Angled
Bearings
Friction
styles
angled
Angle bearings are used for drives in either of the following situations:
• when the load is applied parallel to, or at a slight angle to, the horizontal
• when the bearing is mounted on a vertical support
Some heavy-duty machines in industrial plants have the bearing base cast as
an integral part of the machine. These bearings are either babbitted or fitted
with shells or liners like other journal bearings.
Babbitt
Liner materials
Working conditions
Bearings
Friction
liner materials
Bearings
friction
babbitt
Babbitt is a common anti-friction lining material for bearings. It melts at a
temperature of about 288°C (550°F) or less. In a liquid state, it fills all
cracks, voids, and irregularities in a casting, giving a smooth surface to
match the shaft surface. Babbitt can be used in badly worn bearing housings,
thus eliminating the cost of a new housing.
Journal bearings and pillow block bearings with anti-friction material liners
are commonly used for low to medium speed and for light to heavy loads.
They can be used for ultra-high speeds with air-jet lubrication only under
exact conditions—for example the main bearings on a turbine.
When selecting liner material, it should be:
• softer than the shaft material, so it deforms slightly under heavy loads
• of low coefficient of friction
• wear-resistant
• a good heat conductor and remain relatively stable with heat changes
• readily available.
Type of Load
Speed
Combination
Thrust
Radial
Ball bearings are generally suited to high-speed applications (up to
40 000 rpm) and roller bearings to low-speed applications. The speed of a
bearing is expressed as either rpm, or as “rubbing speed.” Some makes of
roller bearings are good for speeds up to 20 000 rpm. The normal range of
speeds is 2000 to 20 000 rpm.
Babbitt includes several alloys that contain various proportions of tin, copper, antimony and lead:
• Tin-based babbitts (copper or antimony, with up to 90% tin) are the
hardest and the toughest.
• Tin-based babbitt with a very high tin content is sometimes called nickel
babbitt. It is used in conditions of heavy service and extreme pressures.
• The introduction of a small percentage of lead to a tin-based babbitt
gives a slightly softer material.
• Lead-based babbitts are those in which the tin has been largely replaced
by lead (up to 10 percent tin and 75 percent lead). They are cheaper and
can be used for light loads at low speeds.
Metallic
Non-Metallic
Bearings
friction
liners
non metallic
Bearings
friction
liners
metallic
Metallic bearing materials must have a low coefficient of expansion. They
are also dimensionally stable in the presence of water. They may react
chemically with water, mild acids, alkalis, salts, or other materials.
Non-metallic liner materials are:
• nylon
• polyurethanes
• phenol laminates—such as Celoron™ and Micarta™
IMPORTANT TEMPERATURES
for babbitt
Bearings
friction
liners
non metallic
plastics
Bearings
friction
liners
non metalic
phenol
Babbitt for general use
Service
What is Creep?
Brass (copper/zinc) for higher speeds and heavier loading than babbitt
Phenol laminates have layers of cotton or other natural fillers bonded with phenolic resin. They are strong, shock resistant, and compatible with most
fluids.
343 to 371 C
275 to 285 C
25 to 28 C below melting point
The advantage of plastic bearings is that they are generally inert to most
mild acids and alkalis. The lubricant for nylon or plastic bearings can be oil or grease, but water is also used. With low speeds temperatures, some grades of nylon will run with no lubrication.
They have varying coefficients of heat expansion, all higher than those of
metallic bearing materials. In addition, some synthetic materials expand
when saturated with water. Before doing any critical fitting, check the
specifications of the material. Basic nylon has a thermal expansion rate
roughly ten times that of steel, and a fully saturated water expansion rate of
0.0256" per inch of material. Heat expansion is fairly constant through the
grades. This is important when dry fitting a nylon bearing which will run
with water lubrication.
Same bore diameter
complete melting point
Pour point
complete solidification point
Bronze (copper/tin) for higher speeds and heavier loading than babbitt
Sintered bronze (oilite) for a built in lubrication supply. oil to bronze is 1:2 by volume
Aluminum (common in hydrualic pumps)
Different service wieght
Clearance
Fitting a babbitt bearing for contact
Anti-friction bearings have a rolling contact between the shaft and
bearing using balls, needles, or rollers. In this type, the area of contact is
very small and the pressure on the rings and balls or rollers is quite high.
Hand tools for fitting bearings
The Fitting Process
chamfer
Contact area
cross hatch/
boundary lubrication
When a bearing is assembled, there must be a small amount of clearance
between the shaft and the bearing cap. This prevents a binding or clamping
action on the shaft
Hand tools, such as scrapers, used for
fitting bearings may be commercially
produced or made in the plant to suit
the millwright’s preference.
Friction bearings have a sliding contact between a shaft and the bearing.
A special, low-friction material lines a rigid housing. The lining directly
contacts and supports the shaft. In use, the shaft slides over the liner
material, separated by a thin film of lubricant. The area of contact is
relatively large and pressure on the bearing material is usually low.
Friction bearings are also referred to as plain bearings, sleeve bearings,
and journal bearings. The journal is the part of a machine’s shaft that is
inside the housing of a bearing.
reference MWM 9-11 for fitting procedure
Amount of Clearance
Shim
The amount of clearance set into a friction bearing depends on machine
design and company policy. Some machines with a constant load toward the
base of the bearing do not have a fitted cap. (The cap merely keeps out
foreign material and supplies the lubricant.)
As a general rule, for any shaft/bearing assembly:
• with constant one-direction load and rotation, bearing clearance can be from medium to loose.
• with reversing rotation and fluctuating load, bearing clearance can be from medium to tight (see Table 1).
• with reciprocating action, the clearance must be tight (see Table 1).
Size and Design
Self-Aligning
Separable vs. non separable
Internal
The bore of a bearing is indicated by the last two digits of the bearing
number.
• Up to 20 mm, the bore of a bearing is designated as:
00 = 10 mm
01 = 12 mm
02 = 15 mm
03 = 17 mm
Example: A 6200 ball bearing has a bore of 10 mm.
External
From 20 mm to 480 mm, to find the bore, multiply the last two figures of
the bearing number by five.
Example: A 6204 ball bearing has a bore of 04 x 5 = 20 mm.
Above 480 mm (number 6296), the bearing size is directly included in
the bearing number.
Example: A 62/500 (or 62500) ball bearing has a bore of 500 mm.
How about needle bearing?
Thrust control
Bearing Codes
Clearance
In a four-figure code:
• First figure indicates the type of rolling element.
• Second figure indicates the service weight, outside diameter and width.
This figure ranges from 0 to 4, indicating 00 series, ultralight to 400
series, heavy duty.
• Third and fourth figures indicate bore size.
• Initial clearance, or uninstalled clearance is the clearance in the
bearing as it comes from the box.
• Installed clearance is the clearance left in the bearing after installing it
on a shaft and in a housing; or the clearance left in a taper-bore bearing
that has been forced up the taper.
• Running clearance is the clearance in the bearing when the machine is
up to operating temperature and lubrication has been supplied.
Clearance is the total internal clearance between the balls or rollers in a
bearing and their raceways. This clearance has several functions:
• It compensates for expansion of the inner ring or for contraction of the
outer ring when interference fits are used.
• It compensates for differential expansion of the two rings when the inner
ring of the bearing operates at a higher temperature than the outer ring.
• It affects the end play of ball journal bearings and their capacity for
carrying axial loads. The greater the clearance, the greater the capacity
for supporting axial load.
Preformed bearing liners
In a five-figure code:
• First figure indicates the type of rolling element.
• Second figure indicates OD
• Third figure indicates the service weight, outside diameter and width.
• Fourth and fifth figures indicate bore size.
SKF–6208
SKF indicates the manufacturer
6 = Conrad
= single-row, deep groove ball bearing
2 = service weight (light)
08 = 08 x 5 = 40 mm bore
NTN–22208
NTN indicates the manufacturer
22 = double spherical roller, self-aligning
2 = service weight (light)
08 = 08 x 5 = 40 mm bore
Compared to the 6208 bearing, the 22208 has:
• different rolling elements indicated by 22
• the same OD indicated by the third 2
• the same bore indicated by 08
• different widths—single vs. double
SKF–6308
The 6308 has a different OD and a different width
than the 6208.
6 = Conrad
3 = heavier service weight than Example 1
(medium)
08 =8 x 5 = 40mm bore
NTN–23208
2 = rolling element
3 = series diameter (∅)
2 = service weight (light)
08 = 08 x 5 = 40 mm bore
Carb bearing
-K designates the common 1-in-12 taper. K30 designates the 1-in-
30 taper used for larger bearings. The position of the K and K30 is
important—it must be at the end of the code.
Friction (plain) bearings are usually considered radial load bearings with
limited thrust capacity. Thrust (axial force) is mainly controlled by the use of
other components.
Bushings
shells
• A shield is indicated by the letter Z at the end of the code.
• A rubbing seal is indicated by RS at the end of the code. (2RS indicates
rubbing seals on both sides)
• P numbers indicate tolerances: P2 indicates close (small) tolerance; P6
indicates large tolerance.
Bushings are one-piece liners and can be made from any of the common
bearing materials. Fit and clearance is usually set by the dimensions
machined into the housing and bushing
Shells are two-piece liners installed in a bearing housing. They are usually
held in position by pins or dowels, screws, special bearing designs,
compression, or combinations of thes
Pivoted Shoe bearing
And more....
Magnetic bearing
Before and after
Thrust washer
Radial Air bearing
Air bearing spindle
Installing and removing anti-friction bearings
Bearings are installed (fitted) square on the shaft. They are installed in
several ways. To determine appropriate fitting methods the housing and shaft
must be checked. Bearings may have a push fit or an interference fit.