Chapter 7 - Bearings Flashcards
Main function of a bearing
Provide a smooth, low-friction motion between two surfaces.
Used to support shafts and allow their free rotation.
Types of motion
Rotary
Linear
Rotary motion
Shaft turning in a housing
Linear motion
One machine element moving across another.
Types of Loads
Radial
Thrust or Axial
Radial Load
Right angles to the shaft or bearing’s axis of rotation.
Thrust or Axial Load
Loads are applied parallel to the axis of rotation.
Two basic types of bearings
Plain
Rolling Element
Bearings that support loads perpendicular to their axis are called
Radial Bearings
Support loads parallel to their axis of rotation are termed
Thrust Bearings
Types of bearing from design point of view:
Plain bearing
Ball bearing
Roller bearing
Plain bearings
Most basic type of bearing.
Limited to lighter loads and slower speeds.
Functions as a tight-fitting cylinder, usually stationary, that encloses and supports a moving member, generally shaft or journal.
Support radial or thrust loads.
Journal/sleeve = radial type
Plain Bearing Applications
Industrial machinery
Automobiles
Home applications
Construction equipment
Plain-Bearing Lubrication
Generally are lubricated with grease. Boundary Mixed-film Full-film or hydrodynamic Hydrostatic
Boundary Lubrication
Bearing and shaft rub together with only a thin film of lubricant on the surfaces. Grease lubricated bearings generally operate this way.
Mixed-film Lubrication
Part of load (where shaft and bearing are closest) is supported on a boundary film and the remaining part is supported by hydrodynamic pressure.
Full-film or hydrodynamic Lubrication
A thick, continuous film of self-pressurized lubricant separates the shaft from the bearing with no metal-to-metal contact.
Hydrostatic Lubrication
External pumps supply pressure to the lubricant, providing a full film to prevent metal-to-metal contact.
Self-Lubricating Bearings
In applications where relative motion is insufficient to circulate oil or grease through a bearing.
Used where temperatures exceed the scope of conventional lubricants.
Rolling-Element Bearings
Rely on either balls or rollers.
Their rolling motion produce less friction than plain bearings.
Low starting friction.
Available for radial loads, thrust loads, or a combination.
Types of Ball Bearings
Angular contact bearings
Thrust ball bearings
Unground ball bearings
Thin section bearings
Angular contact bearings
Accepts high thrust load in combination with moderate radial load. Single row.
Thrust ball bearings
Carry primarily thrust loads.
Unground ball bearings
Provides high-speed and high-load-carrying capabilities.
Have fewer parts, cost less, and can reduce installation costs.
Available in radial, thrust, and combination loads.
Cheap
Thin section bearings
Where space and weight must be conserved.
Ball and roller types.
Designed for light to medium-duty drives operating at medium and slow speeds.
Roller Bearings
Generally support heavier loads than comparably sized ball bearings.
Capable of handling high-impact or shock loads.
More sensitive to misalignment.
Roller Bearing Types:
Cylindrical
Needle
Spherical
Tapered
Cylindrical roller bearings
Has the highest radial load capacity and speed capability.
Non-locating. Enables axial movement.
For general use, a roller length equal to the roller diameter provides the best balance of load and speed capabilities.
Needle roller bearings
Have a much smaller diameter-to-length ratio.
Carry heavy radial loads in a minimal amount of radial space.
Load capacity is higher than most single-row ball or roller bearings.
Types of Needle Roller Bearings
Drawn-cup
Caged
Cam (or track) follower
Drawn-cup Needle roller bearing
Roller ends are shaped so that lips on the outer race keep them from falling out.
Higher load-carrying capacity than the basic type because the grease-retained rollers have spherical ends.
Caged needle roller bearing
Designed for heavy-duty, high-speed
applications.
The heaviest-duly version.
Cam (or track) follower
A special, heavy-duty needle roller bearing with a heavy outer race.
There are two basic mounting types: one with an integral stud for cantilever mounting, the other, an inner race for yoke mounting.
Spherical roller bearings
Usually containing one or two rows of rollers within a spherical outer raceway, spherical roller bearings.
Support high radial or combined radial and thrust loads.
Suitable for heavy-duty applications.
Tapered roller bearings
Carry heavy radial loads, thrust loads, or both.
Can tolerate some minor misalignment.
For more capacity, two row bearings are used.
Tapered roller bearings uses
Appliances Machine tools Mobile equipment Turbines Industrial machinery
Mounted Bearings
Eliminates the need to select and assemble components installed in a sturdy housing.
Provides shaft support for radial, thrust, or combination loads.
Types of Mounted Bearings
Pillow blocks
Flanged cartridges
Cylindrical cartridges
Take-up units
Pillow Blocks
Most common type of mounted unit.
Have the plane of the mounting base parallel to the shaft.
Used on vertical or horizontal surfaces or on an inclined plane.
Flanged cartridge blocks
Usually mounted on vertical surfaces where a shaft pauses through a machine frame at a right angle.
Mounted Plain Bearings
They are commonly used in linkages and low-speed applications where low friction is required.
Mounted Rolling-Element Bearings
Typically have spherical outer, surfaces that compensate for angular shaft misalignment.
The simplest types mount with a slip fit collar and setscrew.
Magnetic Bearings
Can obtain frictionless motion, unlimited speed and undetectable vibrations.
Parts of Magnetic Bearings
Bearing Actuators
Sensors
Controller and control algorithms
Types of Magnetic Bearings
Radial magnetic bearing
Magnetic thrust bearing
Active magnetic bearings
Radial magnetic bearing
Generates an attractive force to levitate the shaft.
Consists of laminated actuators, each having two poles wound by a coil.
Magnetic thrust bearing
Solid steel disc is attached to the shaft with an annular stator on either side.
Active magnetic bearings
Shaft position sensors are used to provide feedback to the control system.
Typically, inductive sensors in five axes (four radial, one thrust) are used.
Magnetic Bearing Characteristics
No Lubricant.
Suited to machines operating in a vacuum, at high or low (cryogenic) temperatures or in corrosive process fluids.
Much higher surface speeds.
Bearing Standards Organization
American Bearing Manufacturers Association
Two standardized load ratings
basic load rating
static load rating
Basic load rating
Defines the load that a group of bearings can endure for one million revolutions.
Static load rating
Load that corresponds to a total permanent deformation of rolling clement and raceway of 0.0001 of the rolling-element diameter.
Rating life
L10
90% of a group of bearings will complete at a given load and speed before fatigue failure occurs.
Bearing life
Usually inversely proportional to hearing load and speed.
Virtually impossible to predict the exact life of a bearing.
Shaft and Housing Fits
The inner ring (ID) must properly fit the shaft and the outer ring (OD) must properly fit the housing bore.
Bearing selection major factors
Load Speed Life Noise and vibration Misalignment Temperature Environmental conditions
Inspection of Bearings
Check of bearings in operation
Inspection of bearings after operation
Inspection when machine is running
Bearing Temperature Operating sound of Bearing Vibration of Bearing Lubricant Selection Relubrication
Non-locating means
Enables axial movement of the inner and outer ring to accommodate both thermal and axial expansion of the shaft and tolerances in the assembly
