M1 Bearings

Question 1

What is a bearing

Answer 1

A bearing is a mechanical device that supports the moving parts of a machine. It guides or confines the motion of the moving parts and prevents them from deflecting in the direction of the applied load.

Question 2

What characteristics do bearings need?

Answer 2

• Low friction, particularly at startup
• Support both radial and thrust loads and high speeds of rotation
• Consistent performance under varying loads and speeds
• High load carrying capabilities
• Minimal wear when running
• Easy to lubricate

Question 3

Plain Bearings

Answer 3

AKA Sleeve bearings
Simplest form of bearing. Cylindrical cross section, composed just of the bearing surface with the shaft journal sliding over the bearing surface.
- Lightweight, compact, high load carrying capacity

Question 4

Rolling Element Bearings

Answer 4

Carries a load by placing round elements between two rings. The elements roll or tumble as the rings rotate.

Question 5

Plain bearing material requirements

Answer 5

• Durable
• Resistant to corrosion and wear
• Low coefficient of friction
• Can withstand elevated temperatures

Question 6

Plain bearing materials

Answer 6

• Bronze: easy to machine and resistant to corrosion
• Cast Iron: low coefficient of friction but not good for corrosive environments
• Graphite: dry lubricant, low coefficient of friction, little maintenance. Often alloyed with copper for strength, durability, and better heat dissipation. Unalloyed for underwater applications
• Plastic: lightweight, resistant to corrosion, and little maintenance. Softens at high temp, brittle at low temp, high coefficient of thermal expansion.

Question 7

Babbitt

Answer 7

AKA white metal
Used as the surface layer in steel-backed sleeve bearings. Soft, highly resistant to galling.
Two common alloys: tin/antimony/copper (internal combustion engines, resistant to pounding); lead/antimony/tin (better for constantly rotating machinery)

Question 8

Types of Plain Bearings

Answer 8

1. Integral
2. Bushing
3. Two Piece

Question 9

1. Integral

Answer 9

Built into the machine. Generally a hole is bored into the casing or component of the machine that is prepared into the bearing surface.
Least expensive type of sleeve bearing, but entire component must be replaced if worn out

Question 10

2. Bushing

Answer 10

Most common form of sleeve bearing; an independent sleeve is inserted into the housing.
1. Solid
2. Flanged
3. Split
4. Clenched

Question 11

3. Two Piece Bearing

Answer 11

AKA full bearing
Composed of 2 halves (shells) located inside of a housing. The shells are located and held in place by a tab, dowel, similar arrangement. Length is slightly longer than that of the housing so need amount of pressure to install properly. Shell’s circumference is slightly larger than housing so that when the housings are bolted together, the bearing crushes slightly. This creates a large amount of radial force and prevents the bearing from spinning.

Question 12

Tri-metal bearing

Answer 12

Shell is composed of steel and inner face is plated with a coating of bronze, then a thin layer of Babbitt (bearing surface). Sometimes added thin layer of bronze that will wear away during initial machine startup.

Question 13

Considerations for Plain Bearings

Answer 13

• High start-up torque means plain bearings aren’t suitable for machines that start and stop frequently
• Requires 3x more energy to maintain full fluid film
• Difficult to lubricate with grease
• Vertical or thrust applications require expensive specialty bored
• Generally optimized for a single speed
• More suitable for the heavy loads when power density is extremely high

Question 14

Construction of Rolling Element Bearings

Answer 14

1. Inner ring
2. Outer ring
3. Rolling element
4. Cage

Question 15

1. Inner Ring

Answer 15

The smaller of the two bearings. It has a groove on the outside diameter to form a path for the balls. Smooth surface, precision finished, with high tolerances. The surface is called a raceway. Normally mounted on the shaft and is the rotating element.

Question 16

2. Outer ring

Answer 16

Has a groove on the inside diameter that forms a path for the balls. Called the outer race, same high precision finish. Normally placed inside a housing and is held stationary.

Question 17

3. Rolling elements

Answer 17

Used to separate the inner and outer rings, permit the bearing to rotate with minimal friction. The radius of the element is slightly smaller than the races to allow the elements to contact the rings at a single point. Dimensions are held to very tight tolerances, surface finish and roundness.
- May be balls, cylindrical rollers, spherical rollers, or tapered rollers

Question 18

4. Cage

Answer 18

Separates the rolling elements while maintaining an even spacing. The cage guides the rolling elements in the raceways and prevents them from falling out.
Constructed of steel, brass, bronze, or phenolic.

Question 19

Separable bearings

Answer 19

May be broken down into their components for greasing and assembly

Question 20

Non-separable bearings

Answer 20

Rigid in design and will not come apart

Question 21

Single Row Deep Groove Bearings

Answer 21

One row of balls rolling in a single deep groove in each race.
- Used for radial loads or radial-thrust combinations

Question 22

Double Row Deep Groove Bearings

Answer 22

Two rows of balls rolling in a double groove.
- Higher capacity than single row

Question 23

Sealed Pre-Lubricated Bearings

Answer 23

Fitted with seals or shields to keep out dirt and debris and retain lubricant

Question 24

Single Row Angular Contact Bearings

Answer 24

Do not have grooves. A single row of balls roll between two high shoulders, one on the outer ring and one on the opposite side of the inner ring.
- Heavy thrust load

Question 25

Internally Self-Aligning Bearings

Answer 25

Two rows of balls rolling in a double channel inner race and a concave outer race. Can operate with the inner race at a considerable angle to the outer race, so will not fail with misalignment or distortion of the shaft. - Radial loads and moderate thrust loads

Question 26

Roller Bearings

Answer 26

High radial load capacity but low tolerance for misalignment or thrust loads. Can operate at high speeds.

Question 27

Spherical Bearings

Answer 27

High load carrying capacity. The rounded shape of the rollers and the concave surface of both races makes them self-aligning - full capacity of the bearing is available even with some misalignment. May be in a single row (one direction thrust) or double row configuration (either direction thrust).

Question 28

Tapered Bearings

Answer 28

Roller center lines and the raceway surfaces form an angle from the shaft center line. May be mounted singularly or in pairs. - Suitable for heavy combined thrust and radial load. Separable - outer race is mounted separately from the inner race & rollers

Question 29

Needle Bearings

Answer 29

Constructed from a series of small diameter rollers. Usually no inner race; the outer race forms a shell. Sometimes no outer race, the bearing runs directly on the shaft and housing. Normally held within a cage but not always. If no cage, internal friction is very high. - Light loads, no thrust, mostly to align or guide shafts, or to separate shafts if one is inside another

Question 30

Identifying Rolling Element Bearings

Answer 30

First digit: type code/ prefix Second digit: Series code Third + Fourth: bore size /diameter Suffix

Question 31

Lifespan of Rolling Element Bearings

Answer 31

Normally expressed in the number of revolutions or number of operating hours at a continuous speed that the bearing is capable of enduring before the first signs of metal fatigue.

Question 32

L10 Life

Answer 32

90% of random sample of bearings can be expected to meet or exceed a stated number of revolutions at a given size and load.

Question 33

Service life

Answer 33

Actual life of a bearing in real operating conditions before it fails

Question 34

Specification Life

Answer 34

Life specified by a testing authority based on hypothetical loads and speeds

Question 35

Static Seals

Answer 35

Seals that are in contact with stationary surfaces and their effectiveness depends on their deformation when installed. E.g. gaskets and o-rings

Question 36

Dynamic Seals

Answer 36

Seals that are in contact with sliding surfaces and are used to seal passages between machine components that move relative to one another. Specially designed to: - Retain lubricants - Exclude contaminants - Separate different media - Withstand differential pressures

Question 37

Radial Lip Seal

Answer 37

Composed of a synthetic rubber ring fitted into a steel retainer. Held in tension around the shaft by means of a spring, but does not come into direct contact with the shaft. Seal is formed by specialized geometry of the lip, drawing in lubricant as the shaft rotates and creating a hydrodynamic seal. There will be rubber to metal contact during start up and shut down, so rubber composition needs to be resistant to wear. - Unidirectional : keep lubricant in OR keep debris out

Question 38

Non-Contact Radial Seal

Answer 38

Achieves a seal by using a narrow, relatively long gap arranged axially, radially or in combination. As these passageways are long and complex, both the escape of fluids and the ingress of contaminants are blocked.

Question 39

Sealing on the Bearing Itself

Answer 39

In cases where there is insufficient space to fit a proper seal, the arrangement may be transferred to the bearing itself. If low contamination: a metallic shield is fitted between the inner and out race, the shield is rigidly attached to the outer race and does not touch the inner race. If high contamination: synthetic rubber ring is attached to the outer bearing race and the inner edge forms a seal against the inner race

Question 40

Bearing Installation

Answer 40

1. Mechanical Mounting 2. Temperature Mounting 3. Hydraulic Mounting 4. Oil injection method

Question 41

1. Mechanical Mounting

Answer 41

Suitable for press fitting bearings 80mm bore or less. The bearing is mounted on the shaft and a sleeve with the same diameter as the inner ring is fitted and forces the bearing down. - If miss hitting, can cause Brinelling

Question 42

2. Temperature Mounting

Answer 42

Expand inner ring to easily position on shaft. Raise temperature to 150F above ambient, but need to heat evenly and not overheat. - Induction heater, hot plates, oil bathes, ovens

Question 43

3. Hydraulic Mounting

Answer 43

Uses a hydraulic nut (steel ring with a groove in one side and an annular piston that rests in the groove). Oil is pumped into the nut and pushes the piston out with a force sufficient to mount the bearing.

Question 44

4. Oil Injection Method

Answer 44

Used for installing large bore (200 mm + diameter), delivers pressurized oil to shallow grooves on the shaft surface. A thin layer of oil reduces the fitting pressure of the bearing.

Question 45

Normal wear

Answer 45

Occurs at start up, before fluid film lubrication has been established and at shut down

Question 46

Abnormal wear

Answer 46

- Bearings improperly seated - Misalignment between moving parts - Shifted bearing caps - Out of round bearing seat - Scored or out of round journal or pin - Distortion in the supporting structure - Incorrect clearance (too much or not enough) - Corrosion

Question 47

Number one cause of bearing failure

Answer 47

Fatigue from overload. Contaminated lubricating oil

Question 48

Sleeve Bearing Failures

Answer 48

- Fatigue (cyclic loading) - Wiping (overheating, smearing or running of bearing material) - Corrosion (alloys containing lead) - Electrolytic Pitting (electrical leaking across the bearing) - Cavitation erosion (air bubbles traveling from low to high pressure and imploding)

Question 49

Rolling Element Bearing Failure

Answer 49

- Wear and pitting caused by foreign matter (dirt, grit, lint, dust, etc) - Improper mounting (denting, loose shaft fit, loose housing fit, excessive tight fits, out of round housings, poor finish) - Misalignment (bent shafts, out of square shaft shoulders, spacers, clamping nuts) - Brinelling (caused by a heavy load resting on a stationary bearing for an extended length of time, resulting in a permanent dent) - Vibration (rapid movement of rolling elements in raceway while at idle, removing lubrication between elements and the races) - Electrical damage (electrical current passes through a bearing, arcing and burning at point of contact) - Improper lubrication (lack of lubrication) - Bearing fatigue (result of the stresses produced by the rotating elements flexing the raceways - Bearing corrosion (moisture, acid action, poor wrappings, condensation) - Defective sealing - High temperatures - Storage (dampness)

Question 50

What are the advantages of a rolling element bearing versus a plain bearing? Where would each be used?

Answer 50

1. Suitable for high start-up torque, frequent stopping and starting 2. Requires less energy to operate 3. Easier to lubricate 4. Can more easily accommodate thrust applications 5. Can tolerate a wider speed range, more suitable for variable speed drives Roller bearing: used for Plain bearing: used for extremely high power density

Question 51

What are the different types of plain bearings? What are their advantages and disadvantages?

Answer 51

1. Integral: built into the machine, generally the hole in the casing or component is prepared into a bearing surface. Least expensive but if the surface wears out the entire component needs to be replaced. 2. Bushing: most common form, independent sleeve inserted into a housing. May be solid, flanged, split or clenched. 3. 2-piece bearings: sleeve bearings composed of two halves located inside of a housing.

Question 52

What is bearing crush and why is it used?

Answer 52

Bearing crush is used in 2 piece bearings to create a larger amount of radial force when the installed, which prevents the bearing from spinning in the housing. The shell's circumference is slightly larger than that of the housing so that when the two halves of the housing are bolted together, the bearing crushes slightly.

Question 53

What are the purposes of the components of a rolling element bearing

Answer 53

1. Inner ring: smaller of the two bearing rings, the outside diameter forms a path for the balls or rollers. The surface (inner race) is precision finished to high tolerance and polished to a smooth mirror-line finish. The inner ring is normally mounted on the shaft. 2. Outer ring: larger of the two bearing rings, the inside diameter forms a path for the balls or rollers. The surface (outer race) is also finished to high tolerances and polished. The outer ring is normally placed inside a housing and is usually held stationary. 3. Rolling elements: separate the inner and outer rings and permit the bearing to rotate with a minimal amount of friction. The radius of the element is slightly lower than that of the races so that the rolling elements are only in contact with a single point. Dimensions of the rolling elements are held to very tight tolerances as well as surface finish and roundness. May be balls, cylindrical rollers, spherical rollers, or tapered rollers. 4. Cage: separates the rolling elements while maintaining an even spacing. The cage guides the rolling elements in the raceways and prevents them from falling out. Cages may be constructed of steel, brass, bronze, or phenolic.

Question 54

How is the lifespan of a rolling element bearing denoted?

Answer 54

1. L10 Life : 90% of a random sample is expected to meet or exceed a stated number of revolutions at a given size and load. 2. Service life: actual life of the bearing in real operating conditions before failure. 3. Specification life: life specified by a testing authority based on hypothetical loads and speeds.

Question 55

Briefly explain the nomenclature used to identify rolling element bearings

Answer 55

First digit: type code/prefix. If R = imperial. e.g. 6 single row deep groove ball bearing Second digit: series code. e.g. 2 light Third + fourth digits: bore size in mm. 04+ multiply by 5 for the bore. e.g. 04 = 20 mm Suffix: specialty e.g. 2RS both sides sealed

Question 56

What is the difference between a static, radial, and radial non-contact seal?

Answer 56

Static: seals in contact with stationary faces, effectiveness depends on deformation when installed. e.g. gasket or o-ring Radial: consist of an outer steel ring pressed into a housing with a rubber ring bonded to it. The lip on the rubber ring draws in lubricant, creating a unidirectional seal and preventing the entrance of contaminants OR the leaking of lubricant. A spring holds the lip in positive contact with the shaft when shut down, resulting in metal-to-rubber contact at startup and shutdown. Radial non-contact: no point of contact with rotating parts. Passage ways in the seal prevent the leakage of lubricant or ingress of contaminants. Useful for high temp, high speed applications.

Question 57

Briefly explain how a rolling element bearing is installed on a shaft or in a housing

Answer 57

1. Mechanical: suitable for small bores < 80 mm. Small, straight bore bearings are mounted on the shaft and then a sleeve the same diameter of the inner ring is fitted. A hydraulic or mechanical press forces the bearing evenly onto the shaft. If the bearing is being pressed into a housing, the sleeve is fitted to the outer ring. 2. Temperature: use heat to expand the bearing inner ring so it can be easily positioned on the shaft. Best suited for straight bore arrangements. Apply enough heat to raise the temperature to 105F above ambient. Use hot plates, oil bathes, ovens, or induction heater. 3. Hydraulic: uses a hydraulic nut consisting of a steel ring with a groove in one side and an annular piston that rests in the groove. Oil is pumped into the nut and pushes the piston out with a force sufficient to mount the bearing 4. Oil injection: large bearings bore > 200 mm. Delivers pressurized oil via connecting ducts in the shaft to shallow grooves on the shaft surface. A thin layer of pressurized oil reduces the fitting pressure and simplifies mounting and dismounting.

Question 58

What precautions should be taken when installing a rolling element bearing?

Answer 58

1. Brinell (denting) when installing by applying blows or pressure on the outer rate 2. Loose shaft fit: rotation of shaft within inner ring can produce heat and small loose particles of metal will eventually get into the bearing, causing wear 3. Loose housing fit: similar to loose shaft fit 4. Excessive tight fits: can cause rings to crack, cause high operating temperatures and premature failure 5. Out of round housing: usually found in split housings causes localized overloading with abnormal wear on surfaces and retainer pockets. 6. Poor surface finish: bearing seat will break down causing loose fit

Question 59

Describe in detail what causes most bearings to fail.

Answer 59

Contaminated lubricating oil and poor installation

Question 60

Key Concepts

Answer 60

Bearings are an important part of all machinery. There are two types of bearings: Sleeve and rolling element bearings and each type has its advantages and disadvantages. Seals are used to keep dirt and water out of the bearing and to keep lubricant in. Bearings must be properly installed or they will fail prematurely.