B

 

Balance (Impeller)

Mechanical Balance

    Even distribution of impeller mass (not including the fluid contained within the impeller), surrounding the exact center of rotation.

Mechanical Unbalance

    Uneven distribution of impeller mass surrounding the center of rotation, including incorrect location of true center.  Mechanical unbalance arising from uneven mass distribution of the material out of which the impeller is made, can be caused by: voids in the casting or molded impellers, variations in the materials used in fabricated impellers, material inconsistencies, errors in the mold itself such as thickness variations in vanes or impeller shields, or an incorrect center location of the center of rotation.

Hydraulic Balance

    Even distribution of fluid volume (and therefore mass), contained within the impeller, surrounding the exact center of rotation.  Hydraulic balance is essential to correct pump and driver performance.

Hydraulic Unbalance

    Uneven distribution of fluid volume (and therefore mass), contained within the impeller, surrounding the center of rotation.  Because this unbalance rotates with the impeller, the resulting vibration changes frequency with rotation speed, and becomes more intense as speed increases due to inertia.  If a pump impeller is hydraulically unbalanced, mechanical balance is meaningless, the pump will experience abnormal and unbalanced radial thrust loads causing vibration and possibly damage to the pump and pump driver (prime mover).

Bearings

Purpose

Bearings serve three main purposes in pumps and motors:

  1. Bearings maintain correct alignment and position of the rotating parts in relation to the stationary parts in a machine.

  2. Bearings reduce friction, thereby allowing shaft rotation and load bearing with reduced wear, heat, and vibration.  By reducing friction, bearings reduce the amount of energy required to turn the shaft, increase machine life, and decrease maintenance costs.

  3. Bearings accept static and dynamic loads in either or both principle directions, axial and radial.  Static loads are gravity induced mass loads bearing upon the pump or driver components while at rest.  Dynamic loads originate from mechanical or hydraulic unbalance within the pump while operating, and from hydraulic pressures originating in the system into which the pump has been installed.

Basic Bearing Types and Functions

    Bearings commonly used in pumps and motors, can be conveniently divided into two major types: rolling element bearings and fluid film bearings.  The two types of bearings work on two different principles.

 

  1. Rolling Element - Bearings that reduce friction by a rolling action.      Rolling Element bearings are also known as "Precision" bearings because the rolling elements and the element guides must be manufactured to high degrees of accuracy for dimension and smoothness.  The name "Precision" intonates the weakness of these bearings which is the fact that the bearings are vulnerable to dirt and contamination.

  2. Fluid Film - Bearings that reduce friction by means of a fluid film between two surfaces.  Some Fluid Film bearings are called "Hydrodynamic" bearings, pointing to the fact that these bearings contain design characteristics that force a pressurized fluid film between the sliding elements.

 

    Fluid Film and Rolling Element bearings each have known distinctive abilities allowing engineers and designers to decide which bearing is best for specific application requirements.

 

    Bearings accept loads from two principle directions, or any combination of the two principle directions.  Axial loads apply pressure to the components parallel (in line) with the shaft or axle.  Radial loads apply pressure to components perpendicular (or sideways) to the shaft or axle.

 

    Bearings can be designed and implemented to accept loads that are primarily radial, primarily axial, or any combination of the two principle directions of thrust.  Axial load dedicated bearings are commonly called thrust bearings.

 

Rolling Element Bearings

    Bearings containing elements that roll or turn with the load.  Friction is reduced by means of a rolling action of spherical components or elements.

    Rolling elements roll along raceways that guide and contain the rolling elements.  The rolling elements may be contained in a cage to maintain clearances between the rolling elements, and also to evenly distribute the rolling elements within the raceway to maintain even load distribution within the bearing.

    Rolling element bearings may be provided with three different configurations: open, shielded, and sealed.  Open bearings have no covers over the rolling elements.  Shielded bearings may have a cover on one or both sides of the bearing (single and doubled shielded), the cover being made usually of steel.  Both open and shielded bearings can and should be re-lubricated periodically.  Sealed bearings incorporate a permanent non-removable elastomer cover on both sides of the bearing, sealing lubrication in while sealing out dirt and contamination.

    A few of the common descriptive names for rolling element bearings are: Ball Bearings, Needle Bearings, Cone Bearings, Cup and Cone Bearings, Spherical Roller Bearings, Spherical Tapered Roller Bearings, or just Roller and Tapered Roller Bearings.

 

Advantages of Rolling Element Bearings

  1. Rolling element bearings are precision devices, capable of maintaining tighter tolerances and smaller clearances between moving and stationary components, than fluid film bearings.

  2. Rolling element bearings maintain consistent clearances and tolerances for longer periods of time than fluid film bearings.

  3. Rolling element bearings, operate with less friction than fluid film bearings.  Depending on the application, heat rise is less, vibration lower, and efficiency higher, than with fluid film bearings.

  4. Rolling element bearings are more resistant to loss of lubrication than fluid film bearings.

  5. Rolling element bearings may tolerate frequent starting and stopping of rotation better than fluid film bearings, depending on the application, and only as long as starting and stopping does not involve large momentary or unusual direction thrust loads that can occur at start or stop.

 

Disadvantages of Rolling Element Bearings

 

  1. Rolling element bearings do not tolerate dirt or contamination as well as sleeve bearings.

  2. Rolling element bearings may cost more to purchase, implement, and maintain, than fluid film bearings.

  3. Rolling element bearings may require more frequent maintenance and higher maintenance skill levels, than fluid film bearings.  Therefore rolling element bearings may be less tolerant of abuse and poor maintenance, than fluid film bearings.

  4. Rolling elements can corrode quickly if lubrication is lost, or if bearing seals are compromised or leaking, sometimes due to poor lubrication practices by maintenance technicians.  Corrosion on the rolling elements quickly destroys the bearings.

  5. Rolling element bearings do not tolerate cyclic loads as well as fluid film bearings.  Pumps and systems into which pumps are installed, often produce cyclic loads.

 

Fluid Film Bearings

Including the following names for, and types of fluid film bearings:

Sleeve Bearing

Oil Film Bearing

Hydrodynamic Bearing

Journal Bearing

Tilting Pad Bearing also known as Kingsbury Bearing

 

    A bearing relying on the principle of a fluid film separating the sliding action between two surfaces.  These types of bearings are based on lubricating film theory.

    Commonly used bearing materials are copper alloys (bronzes, brass, beryllium copper, etc.), but steel, stainless steel, and rubber are also used.  The alloy may be in the form of a powder and then pressed into a shape by heat and/or pressure, called a sintered material, or simply cast or machined to the desired shape and size.

 

 

 

Advantages of Rolling Element Bearings

  1. Rolling element bearings are precision devices, capable of maintaining tighter tolerances and smaller clearances between moving and stationary components, than fluid film bearings.

  2. Rolling element bearings maintain consistent clearances and tolerances for longer periods of time than fluid film bearings.

  3. Rolling element bearings, operate with less friction than fluid film bearings.  Depending on the application, heat rise is less, vibration lower, and efficiency higher, than with fluid film bearings.

  4. Rolling element bearings are more resistant to loss of lubrication than fluid film bearings.

  5. Rolling element bearings may tolerate frequent starting and stopping of rotation better than fluid film bearings, depending on the application, and only as long as starting and stopping does not involve large momentary or unusual direction thrust loads that can occur at start or stop.

 

Disadvantages of Rolling Element Bearings

 

  1. Rolling element bearings do not tolerate dirt or contamination as well as sleeve bearings.

  2. Rolling element bearings may cost more to purchase, implement, and maintain, than fluid film bearings.

  3. Rolling element bearings may require more frequent maintenance and higher maintenance skill levels, than fluid film bearings.  Therefore rolling element bearings may be less tolerant of abuse and poor maintenance, than fluid film bearings.

  4. Rolling elements can corrode quickly if lubrication is lost, or if bearing seals are compromised or leaking, sometimes due to poor lubrication practices by maintenance technicians.  Corrosion on the rolling elements quickly destroys the bearings.

  5. Rolling element bearings do not tolerate cyclic loads as well as fluid film bearings.  Pumps and systems into which pumps are installed, often produce cyclic loads.

 

 

    Another material used for sleeve bearings is babbitt, an alloy consisting of tin, antimony, and lead.  The material is soft, has inherent lubricating qualities, and has a low melting point allowing the material to be cast into the finished machine part to form a perfect fit.  Babbitt requires a much larger surface than copper or other alloys to bear the same thrust load, and therefore babbitt is not commonly used as it once was, but babbitt is still in use today.

    Sleeve bearings can be made as self lubricating, manual periodic lubrication, product lubricated, or a pressurized lubrication system may be used.

    Sleeve bearings used in turbine pumps are either product lubricated or the pump may have a pressurized lubrication system.  When the pumpage itself is an excellent lubricant, such as water or oil, product lubricated sleeve bearings are often the best solution for turbine pumps for numerous reasons:

  1. Product Lubricated bearings never require lubrication, reducing maintenance costs, reducing the likelihood of improper lubrication practices, and eliminating the possibility of neglect.

  2. Sleeve bearings tolerate dirt better than rolling element bearings.

  3. Sleeve bearings tolerate large amounts of axial movement (movement caused by growth and normal thrust bearing wear).  In contrast, rolling element bearings tolerate very little axial movement of the shaft.  As thrust bearings wear they allow more axial movement, which can then quickly damage rolling element radial thrust bearings in contrast to sleeve bearings which are unaffected by wear allowing more axial movement.

  4. Sleeve bearings initially cost less than rolling element bearings, and sleeve bearings cost less to maintain.

  5. In turbine pump applications, and because of the inherent feature advantages mentioned above, sleeve bearings exhibit a longer life expectancy (MTBF) than would rolling element bearings.

  6. Turbine pumps produce very little radial thrust, and the thrust that is produced has a cyclic nature, which is a strong point for sleeve bearings (as opposed to rolling element bearings).

 

Tilting Pad Thrust Bearing - Also Known as Kingsbury Bearing

    Dr. Albert Kingsbury developed the bearing that bears his name, the Kingsbury Bearing.  Dr. Kingsbury obtained the US patent for the bearing in 1910, after years of development and trial, often at his own expense, while teaching at various universities and also while working for Westinghouse.  Westinghouse obtained a license from Dr. Kingsbury to manufacture the bearing for their own equipment, and to sell the bearing.  Demand for the bearing was greater than Westinghouse wished to supply, so Dr. Kingsbury established his own company to produce and sell the bearing, Kingsbury, Inc., which is still operating today.

    The bearing is also called the Tilting Pad Thrust Bearing, which accurately describes the basis of the design.  As the bearing collar rotates with the load, segmented pads facing the collar tilt on a pin, resulting in the leading edge of each pad to move away from the collar, lubricant collects in the space between the collar and pad, and the lubricant is then forced between the pad and the collar.  The load is born by the lubricant forced between the titling pad and collar.

    Kingsbury thrust bearings are often used on product lubricated submersible motors because the bearings work well with water as the lubricant.  Kingsbury thrust bearings are also used on ship propulsion systems, gas and steam turbines, compressors, and gear boxes.

    Kingsbury thrust bearings are particularly useful in pumps and prime movers encountering frequent high cyclic and transient axial thrust loads.  High cyclic and transient thrust loads are caused by any type of cavitation and by systems producing frequent pressure surges from valve closures.  The oil film between the tilting pads and the collar absorbs high intensity and high speed loads better than rolling element bearings.  Kingsbury bearings are also excellent supporting high static thrust loads, loads rotating at high speeds, and loads that must operate continuously.

    Kingsbury thrust bearings may be contraindicated on pumps and prime movers having a high amount of static thrust load mounted vertically, such that the mass bears against the bearing, and then that vertically mounted load must be started and stopped very frequently.  This may involve a pump with many stages, mounted vertically, and then started and stopped constantly.  The reason for the contraindication is because the collar must be rotating to produce a pressurized lubricating film separating the collar from the tilting pads.  At rest and when the load first begins to rotate, there is no pressurized lubricant film between the pads and the collar.

Sealed Bearings

    Rolling Element Bearings manufactured with a complete oil tight seal.  The seal retains the lubricating oil or grease inside the bearing.  Never attempt to lubricate a sealed bearing.  Forced lubrication can destroy the seal by pushing the seal into the internal elements of the bearing.  The destruction of the seal integrity may cause heating from friction, loss of lubricating oil, and contamination from the outside (water, dust, etc.).

    Typically sealed bearings are found on smaller motors.  Sealed Bearings work well on small motors and can be more desirable than other types of bearings.  One of the limiting factors for sealed bearings is the shaft size on which the bearing rides, the larger the shaft the less likely that a sealed bearing will work well.

 

Shielded Bearings

    Rolling Element Bearings manufactured with either one or two "shields".  Some shielded bearings can be lubricated, others cannot.  Never attempt to force lubricate a shielded bearing held tightly in a bearing chamber not intended for lubrication or with the sealing plug in place.  Forced lubrication with a grease gun without removing the drain plug can destroy the bearing by forcing the shield into the rolling elements.

 

Best Efficiency Point  - BEP - See Efficiency

 

Boiling Point

 

    The temperature at which the Saturated Vapor Pressure of a liquid equals the surrounding ambient gas pressure.  The relationship of boiling point and pressure is direct, as pressure increases or decreases, the boiling point also increases or decreases.

Example 1 - The boiling point of pure water In an open container exposed to sea level atmospheric pressure = 2120 F.  Sea Level air pressure is approximately 14.7 psi, or 33.9 Feet of Head.

 

Example 2 - The boiling point of that same open container of water at an elevation of 5,000 feet = 202.90 F.  Atmospheric pressure at 5,000 feet elevation is about 12.2 psi, or 28.2 Feet of Head.

 

Example 3 - The boiling point of pure water in a car radiator, under 16 psig of pressure = 2500 F.  In terms of absolute pressure, 16 psig + 14.7 atmospheric pressure = 30.7 psia, or 70.9 Feet of Head.

 

If you do not understand the difference between psig and psia go HERE.

 

 

 

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