Bearing life

The size of a bearing to be used for a fan application is normally determined from its known load bearing capacity. This may need to be modified dependent on a minimum diameter neces­sary to satisfy shaft critical speed requirement.

In general the basic dynamic load ratings of the bearings will have been determined by the bearing manufacturer in accor­dance with the methods specified in ISO 281:1990. The life of a rolling bearing is defined as the number of revolutions which the bearing is capable of performing, before any signs of fatigue are evident on its rings or rolling elements. Such signs might be flaking or spalling of these elements.

At a constant rotational speed, it is then possible to convert the number of revolutions into an operating life for

Revs to fatigue

Revs /min x60

Life hours

Ll0h 60n IP

1000000 fcY

-10h ‘

-10

1000000 60n

Or

Bearing life

C = basic dynamic load rating N

P = equivalent dynamic bearing load N

P = exponent of the life equation

P = 3 for ball bearings

P = 1% for roller bearings

For fan bearings operating at constant speed it is usual to cal­culate with a basic rating life expressed in operating hours us­ing the equation

 

Bearing life

Equ 10.4

 

Bearing life

Equ 10.5

 

Where:

L10h = basic rating life (operating hours)

N = rotational speed (r/min)

At elevated bearing temperatures dynamic load carrying ca­pacity is reduced. This reduction is taken into account by multi­plying the basic dynamic load rating C by a temperature factor as shown in Table 10.2.

 

Bearing temperature °C

150

200

250

300

Temperature factor

1.00

0.90

0.75

0.60

Table 10.2 Temperature factors

 

Satisfactory operation of the bearings at elevated temperatures also depends on whether they have adequate dimensional sta­bility for the operating temperature, whether the chosen lubri­cant will retain its lubricating properties and whetherthe materi­als of the bearing seals, cages etc., are suitable.

It must be emphasised that this temperature is the temperature of the bearing race. Usually, unless the bearing is in the air stream, this is much below the air or gas temperature. Where the impeller is overhung on the shaft, there is often the possibil­ity of introducing an auxiliary cooling fan between the casing side and the inner bearing to reduce the heat transmitted along the shaft. A“spacer” coupling or slots in the shaft can perform a similar function.

The radial loads acting on the bearings are simply calculated using the theory of moments. It is assumed that the fan shaft acts as a beam resting in rigid, moment-free supports for fixed bearings, or simple supports if the bearings are contain in self-aligning housings. (See Chapter 8, Section 8.6.3.)

Whilst the “dead” weight of the impeller, shaft and where appli­cable, pulleys are known, there are other loads which are vari­able and have to be estimated. Thus the impeller weight will be augmented by a fluctuating load due to its residual out-of-bal­ance. This will have been allowed for at the design stage, but may increase due to erosion, corrosion, or dust build-up.

Many centrifugal and mixed flow fans are driven through vee belts, and these are also used to a lesser extent with axial flow fans. The effective belt pull is dependent on the transmitted torque and will be an important load in the determination of bearing radial loads. (See Chapter 11.)

One of the fan bearings will also be subject to an axial load due to the impeller end thrust. This is a function of the fan pressure, its distribution between the inlet and outlet ducting, the inlet area of the fan impeller and the momentum change due to the flowrate.

 

By experience we know that apparently identical bearings oper­ating under the same load and ambient conditions will have varying lives, even if they have been correctly installed and lu­bricated. Usually we use the so-called L10 (basic rating) life, which is the life at which a sufficiently large group of these bear­ings can be expected to have a 10% failure rate. The Li0 life for the application should be known and/or agreed between the parties to a contract.

In general small clean air fans will be designed with bearings rated to give an L10h life of 20,000 hours rising to 40,000 hours for a medium size light industrial fan. Heavy duty public utility fans are frequently designed for an L10h bearing life of 100,000 operating hours.

The average life of a sufficiently large sample of bearings under identical load and temperature conditions will be 5 times the L10 life.

It will be noted that an increase in rotational speed results in a reduction in operating life in hours.

The ISO Standard in fact specifies the basic rating life L10 in terms of millions of revolutions for a basic dynamic load rating and the formula which interconnects the various factors is:

 

Equ 10.3

 

Where:

L10

 

= basic rating life, millions of revolutions

 

If the resultant load is constant in magnitude and direction, the equivalent dynamic bearing load can be obtained from the gen­eral equation.

Bearing lifeP = XFr + YFa Equ 10.6

Where:

P

= equivalent dynamic bearing load (N)

Fr

= actual radial bearing load (N)

Fa

= actual axial bearing load (N)

X

= radial load factor for the bearing

Y

= axial load factor for the bearing

An additional axial load only influences the equivalent dynamic load P for a single row radial bearing if the ration Fa/Frexceeds a certain limiting value, but with double row radial bearings even light axial loads are significant.

Equation 10.6 is also applied for thrust bearings, which can take both axial and radial loads, e. g., spherical rollerthrust bearings. For thrust bearings, the equation can be simplified, provided the load acts centrally, viz.

P = Fa Equ 10.7

It will be appreciated that axial loads higher than design (due to excessive system resistance) will adversely affect bearing life. Double inlet, double width centrifugal fans have essentially bal­anced end thrusts and their bearings are therefore only subject to radial loads. Nevertheless a minimum axial load is necessary to ensure correct “centring” of the bearing, which often results from the blocking effect of a pulley in one inlet.

The L10h life is only achieved when the bearings are correctly in­stalled, correctly lubricated and correctly maintained. If the lu­bricant is unsuitable for the application and is replenished incor­rectly in both quantity and frequency then premature failure will occur. Over-greasing is often more harmful than under-greas­ing. Corrosion and external wear may also affect the bearings, and seals must be inspected to confirm that they are preventing the ingress of contaminants.

Posted in Fans Ventilation A Practical Guide


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