Flange mounted bearings are used when the shaft axis is perpendicular to the bearing mounting surface.  They incorprate a sealed bearing that is preassembled 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 beairngs 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.

Housing Style

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, loacated radially around the bearing axis.  Four-bolt flanged bearings typically have round or square housings.                               

2-bolt housing, 3-bolt housing, 3-bolt offset housing, 4-bolt square and round housings

Bearing type

Bearings used in flange mount housings are referred to as insert bearings.  Many different bearing types can be incorporated in a flange mount housing.  The proper selection type depends on the application and operating specifications it requires.

Ball bearings have a ball as the rolling element. They are used to provide smooth, low friction motion in rotary applications. Construction consists of an inner and outer ring, balls, and usually a cage or ball separator.

Roller bearings have a roller as the rolling element.  They are used to provide smooth, low friction motion in rotary applications and have higher radial load capacities than ball bearings.  Construction consists of an inner and outer ring, rollers, and usually a cage or roller separator.  There are three principal types of roller bearings.

• Cylindrical roller bearings have high radial-load capacity and moderate thrust loads.
• Spherical roller bearings are the most common roller bearing used in flange mounted bearings.  They contain two rows of friction-reducing, barrel-shaped rollers located between an outer ring with a spherical raceway and an inner ring composed of two inclined raceways.
• Tapered roller bearings contain an inner ring, outer ring, and a number of tapered rollers. Looked at individually, the inner and outer bearing rings are also tapered, resembling a cone segment. Due to the large surface area contact, tapered roller bearings are able to withstand heavy axial and radial loads. 

Needle roller bearings have a needle roller as the rolling element. They are similar to cylindrical roller bearings but have a smaller diameter-to-length ratio. By controlling the circumferential clearance between rollers, or needles, rolling elements are kept parallel to the shaft axis. Needle roller bearings are designed for radial load applications where a low profile is desired.

Plain bearings and sleeve bearings (also referred to as bushings or journal bearings), are used to constrain, guide, or reduce friction in rotary or linear applications. They work by means of sliding action as opposed to the rolling action used by ball, roller, and needle bearings. Lubrication is crucial to the operation of plain bearings to avoid overheating due to friction.

Important dimensions

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.

Overall height above mounting surface - Dimension "H"

Housing Material

The bearing housing provdes 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 includeAcetal, Nylon / Polyamide and PTFE / Teflon.

• Nylon, comprising several grades of polyamides, is a general-purpose material in wide use; it is tough and resistant and has good pressure ratings.
• PTFE (polytetrafluoroethylene) 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. Brand names include Celcon® (Hoechst Celanese), Delrin® (Dupont), Thermocomp® (LNP), Ultraform® (BASF), and Acetron® (DSM Polymers).

 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 maintentance or bearing replacement for worn or damaged bearings or shafts. 

Shaft Securing Method

• Setscrew -- Bearings secured with a setscrew  is secured to the shaft by a setscrew, located in an inner ring, which is e bearings have setscrews for securing the inner ring to a shaft.
• Locking collar bearings have a locknut for securingthe inner ring toa shaft.
• No securing method bearings have no provisions forsecuring to a shaft.

Performance Specifications

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 (C_O), 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.

The relationship between rating life, load rating and load is:

L₁₀ = (C/P)³

where:

L₁₀ = rating life (10⁶ rev)

C = basic load rating (lb)

P = equivalent radial load (lb)

To obtain the rating life in hours:

L_{10 hrs.} = 16667/N * (C/P)³

where:

N = speed (rpm)

The Equivalent Radial Load is:

P = XF_r + YF_a

where: 

F_r = radial load (lb)

F_a = axial load (lb)

X and Y = For radial contact ball bearings, calculate P with X = 1.0 and Y=1.0. Then recalculate P with X = 0.56 and Y from the chart below. Use the larger value of P to determine L₁₀ life.

where:

n = number of balls
d = diameter of balls

Engineering load data describes the effect of reaction loads on the bearings. Reaction loads are caused by forces acting on the shaft supported by the bearings. These forces are a result of the weight of the shafting and components mounted on the shafting, bending loads due to belt or chain pull, gear reaction loads, and off center or unbalanced loads. All the loads must be combined into a single resultant load of known magnitude and direction for each bearing. The magnitude and direction of the load can be calculated by taking the moment about the other bearing.

Operating Temperature is determined by the bearing cage, seal and grease material. When operating at high temperatures (excess of 300°F), grease may purge between the seal lip and the bearing inner ring.

Lubrication

As in all bearings, proper lubrication is required to reduce wear and friction.  Commercial lubricants are grouped into three generic types: greases, oils and solid films. Lubrication choices include manual lubrication and self-lubricating.

• Manual Lubrication requires a pump or a grease fitting mounted onthe housing to deliver lubrication to the friction points of the bearing.
• Self- lubricating bearings are constructed of a material such as oil-impregnated bronze, or graphite / metal alloy that does not require external lubrication.

Resources

Industrial Mounted Bearings (from Hub City; pdf)

Engineering Data (from RBC Bearings)

What is a Roller Bearing? (from wiseGEEK)

Image credits:

Grainger | Applied Industrial Technologies | Timken | Hayley