Chapter 7 Rolling Contact Bearing-3

8/10/2019 Chapter 7 Rolling Contact Bearing-3

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Besides specifying the type and number of bearings, we have to

consider how to mount the pair of bearings, including

Axial position; Fit between bearings and other components;

Clearance adjustment;

Sealing and lubrication.

1. types of shafting

(1) Fixed at both ends

For 60000 type, 70000 typeand 30000 type bearing;

Both ends are fixed;

Axial clearance 0.25-0.4mm;

Slight thermal expansion.

7.4 Mounting of Bearings


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(2) One end fixed in two ways, the other end free

For shafts with great length or high thermal expansion;

Retainer

for shaft


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(3) Both ends free

For the shaft at low speed:

both ends are fixed.

For the shaft at high speed:

both ends are free.

For the high speed

shaft with double helical

gear drives;

To offset the machining

error.


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2. Types of bearing axially positioning

(1) Between inner race and shaft

Shoulder;

Retainer for shaft;

Shaft end ring +Bolts;

Round nut + Locking washer;

Tapered adaptive sleeve + Round nut + Locking washer.


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(2) Between outer race and hole

Retainer for hole;

Cover;

Shoulder in hole;

Retaining collar.

Shoulderin hole

Cover

Cover

Retaining

collar

Retainer

for hole


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Bearing bushing


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Proper radial clearance can be achieved by adjusting the fit

between bearing and shaft, bearing and housing. Radial clearance effects the accuracy and life of rolling

contact bearing.

Rolling contact bearings are standard components. The fit

between the inner race and shaft adopts basic hole system.The fit between the outer race and hole adopts basic shaft

system.

The load pattern, direction and magnitude, bearing types,

rotational speed and working conditions decide the bearing fit.

3. Fit of bearings


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Commonly, for the rotational race, a light interference fit can

be chosen, for example k5 or k6. For the stationary race, a light

clearance fitting can be chosen, for example H7 or G7.

Generally, under big size, great loading, serious vibration,high rotational speed or high temperature, interference fit could

be appropriate. Under frequent disassembling or free bushing,

clearance fit could be appropriate.

Tolerance of shaft dia.

Tolerance of bore size of bearing

Tolerance of hole dia.

Tolerance of outer race

dia. of bearing


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4. Adjustment of bearings

(1) Adjustment of radial clearance

Adjusting the thickness of gasket.


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Adjustable cover

Adjustable cover for clearance adjustment


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(2) Pre-tightening of bearing

By some pre-tightening methods, there exists slight

deformation among rolling elements, inner race and outer race.

If pre-tightened, the bearings have higher accuracy, higherstiffness, and longer life.

Adding metal gasket, reducing the width of inner race or

outer race, adjusting the length of bushing.

Round nuts are widely used for the convenience to adjust.


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(3) Adjustment of bearing combination

To locate the position of components on the shaft, such as

gear, sheave.

For bevel gear drive, the vertexes need to be coincided.

For worm gear drive, the principle plane of worm gear must

pass through the worm axis.

Gasket for adjusting the bearing clearance

Gasket for adjusting the axial position of shaft


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5. Installation of bearings

Assembling a bearing

Commonly, the installation force should be applied directly

to the inner race of the bearing.

For large bearings, it may be necessary to heat it to expand its

diameter in order to keep the installation force within reason.


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The height of shoulder must be not

above height of inner race.

Pressure

Press

Shaft

Handle

Bolt

Nut

Claw

Disassembling a bearing

Bearing puller


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7.5 Practical Considerations in the Application of Bearings

To provide a low-friction film between the rolling elementsand the races of bearing and at points of contact with cages,

guiding surfaces, retainers and so on.

To protect the bearing components from corrosion.

To help dissipate heat from bearing unit.

To carry heat away from the bearing unit.

To help dispel contaminants and moisture from the bearing.

Rolling contact bearings are usually lubricated with either

grease or oil.

Grease: Under normal ambient temperature (approximately

21) and relative slow speeds (under 52.3 rad/s).

Oil: At higher speeds or higher ambient temperatures, applied

in a continuous flow, possibly with external cooling of the oil.

1. Lubrication


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Bearing

Type

Speed Factor dn/ (mm. r. min-1)

Oilgumming

Oildropping

Oilinjecting

Oilspray

grease

Deep groove

ball bearing

2.5105 4.0105 6.0105 >6.0105

(2-3)105

Angular

contact ballbearing

Cylindrical

roller

bearing

Taperedroller

bearing

1.6105 2.3105 3.0105 --

Thrust

bearing0.6105 1.2105 1.5105 --

Table 7-12 Lubrication recommendation for rolling contact bearings


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To operate in dirty or moist environments;

Seals or shields can be provided on either or both sides of the

rolling elements;

Shields are typically metal and are fixed to the stationary race,

but remain clear of the rotating race;

Seals are made of elastomeric materials and do contact the

rotating race;

Permanently lubricated: Bearings fitted with both seals and

shields, and pre-charged at the factory with grease.

2. Sealing


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The presence of seals also increases the friction in a bearing;

Sealing can be provided outside the bearing in the housing or

at the shaft/housing interface;

On high-speed shafts, a labyrinth seal is frequently used,

consisting of a non-contacting ring around the shaft with a

few thousandths of an inch radial clearance.

Contact type

Non-contact type

Combined type


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Contact type

Sealing ring seal:


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Non-contact style

Oil groove sealing Shield sealing


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Non-contact style

Labyrinth


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Combined style


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3. Bearing Stiffness

Stiffnessis the deflection that a given bearing undergoes

when carrying a given load.Commonly, the radial stiffness is most important.

Generally, the softer the bearing (low stiffness), the lower the

critical speed of the shaft assembly.

Stiffness is measured in the units used for springs, such asNewton per meter.

The stiffness values are quite high, with values of 87.56

103

to 175.12

103 N/m.

For a precise device, the stiffness of bearings need to be

considered.


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4. Shaft assembly design

Homework 20


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Homework-20

There are several design errors in the picture below. Try to mark

out the errors and finish the correct design in the upper half.

Cabinet for closed gear drive

Chapter 7 Rolling Contact Bearing-3

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Transcript of Chapter 7 Rolling Contact Bearing-3