Centrifugal pump

This example shows the bearing selection process applied to an application case in which modification is required to a centrifugal pump.

The pump manufacturer wants to improve the efficiency of an existing centrifugal process pump by modifying the impeller. As a result, the bearing loads will be greater, and so the current bearing selection needs to be checked to verify that it can cope with the change.

Each step of the example is described in an expandable/drop-down section below. The steps in the example follow the sequence in the bearing selection process. Refer to the bearing selection process for a full description of each process step.

Performance and operating conditions
Performance and operating conditionsBearing type and arrangementBearing sizeLubricationOperating temperature and speedBearing interfacesBearing executionSealing, mounting and dismounting

The application drawing is shown in fig. 1.

The operating conditions are:
  • rotational speed: n = 3 000 r/min
  • lubrication:  
    • method: oil bath
    • oil viscosity grade: ISO VG 68
  • for the non-locating support – a cylindrical roller bearing, NU 311 ECP:
    • max. radial load: Fr = 3,29 kN
    • estimated operating temperature: T = 70 °C (160 °F)
  • for the locating support – a pair of universally matchable angular contact ball bearings, 7312 BECBP, arranged back-to-back:
    • max. radial load: Fr = 1,45kN
    • max. axial load: Fa = 11,5 kN
    • estimated operating temperature: T = 85 °C (185 °F)
Following pump industry standards, the basic rating life L10h should be at least 16 000 h at maximum load conditions.

Bearing type and arrangement
Performance and operating conditionsBearing type and arrangementBearing sizeLubricationOperating temperature and speedBearing interfacesBearing executionSealing, mounting and dismounting

A cylindrical roller bearing is used as the non-locating support and a pair of universally matchable single row angular contact ball bearings are used as the locating support.

The cylindrical roller bearing, of type NU, is used for the following reasons:
  • It can accommodate, within itself, thermal expansion of the shaft.
  • The inner ring is separable from the outer ring, with rollers and cage – this simplifies assembly of the pump and the use of interference fits on both inner and outer ring.
For the pair of universally matchable single row angular contact ball bearings:
  • Ball bearings with a 40° angle are well suited to accommodate high axial loads and medium to high speeds.
  • The bearings are arranged back-to-back, inner rings clamped and mounted with an interference fit to the shaft. Because the clearance of the pair is controlled by clamping the inner rings, the outer rings can be positioned in the housing between a shoulder and a cover, without the need for precise clamping.
Both bearing housing seats are machined in one clamping position, which guarantees good alignment. Misalignment is less than 2 minutes of arc, which is within the acceptable misalignment limits for the angular contact ball bearing pair and cylindrical roller bearing.

Conclusion

The current selection of bearing type and arrangement is adequate for this application.

Bearing size, non-locating support
Performance and operating conditionsBearing type and arrangementBearing sizeLubricationOperating temperature and speedBearing interfacesBearing executionSealing, mounting and dismounting

The given operating conditions, and the effects of rolling contact fatigue, indicate that bearing size should be determined using the basic rating life and SKF rating life.

Product data for NU 311 ECP → product details.

Basic rating life




From Loads, P = Fr. Therefore, the load ratio C/P = 156/3,29 = 47.



The bearing is oversized. 

SKF rating life


 

1Lubrication condition – the viscosity ratio, κ

κ = ν/ν1

Given:
  • oil viscosity grade = ISO VG 68
  • operating temperature = 70 °C (160 °F)
then, using diagram 1, ν = 20 mm2/s

Given:
  • n = 3 000 r/min
  • dm = 0,5 (55 + 120) = 87,5 mm

then, using diagram 2, ν1 = 7 mm2/s

Therefore κ = 20/7 = 2,8


2Contamination factor, ηc

Given:

  • contamination conditions are typical (i.e. open bearings, no filtering, wear particles and ingress from surrounding)
  • dm = 87,5 mm
then, using table 1, ηc = 0,2

Given:
  • Pu = 18,6
  • P = Fr = 3,29 (→ Loads)
then ηc Pu/P = 0,2 x 18,6 / 3,29 = 1,13

3SKF life modification factor, aSKF

Given:
  • κ = 2,8
  • ηc Pu/P = 1,13
  • NU 311 ECP is an SKF Explorer bearing
then, using diagram 3, aSKF = 50

Given:
  • L10h > 1 000 000 h
then L10mh > 50 x 1 000 000 h

then L10mh >> 1 000 000 h indicating that the bearing is oversized for the operating conditions.

Minimum load

The fact that the basic rating life and SKF rating life are both very high and above the required bearing life indicates that the bearing may be too lightly loaded.

Using the minimum load equation from Loads, the minimum radial load (Frm) required to avoid skidding and roller slip for cylindrical roller bearings is given by:

Given:
  • dm = 87,5 mm
  • kr = 0,15 
  • n = 3 000
  • nr = 6 000 
then:

Frm = 0,94 kN < Fr = 3,29 kN

Conclusion

The bearing is oversized / lightly loaded. Options are:

  • Continue to use the current bearing. There is no risk that the bearing will be damaged due to being too lightly loaded.
  • Downsize the bearing, and in so doing reduce cost. Consider one of the following:
    • Keep the shaft diameter the same, but use the smaller NU 2 series bearing NU 211 ECP ( → product details).
    • Reduce the shaft diameter one step, provided the shaft design permits (strength and stiffness), and use the smaller NU 2 series bearing NU 210 ECP ( → product details).
    However, both of these downsizing actions require design modifications to the adjacent components.
Bearing size, locating support
Performance and operating conditionsBearing type and arrangementBearing sizeLubricationOperating temperature and speedBearing interfacesBearing executionSealing, mounting and dismounting

The given operating conditions, and the effects of rolling contact fatigue, indicate that bearing size should be determined using the basic rating life and SKF rating life.

Product data for 7312 BECBP → product details.

Basic rating life




From Loads → Load carrying capacity of bearing pairs:

C = 1,62 Csingle bearing = 1,62 x 104 = 168,5 

From Loads, for bearing pairs arranged back-to-back:  

Fa/Fr = 11,5/1,45 > 1,14

So use:

P = 0,57 Fr + 0,93 Fa = (0,57 x 1,45) + (0,93 x 11,5) = 11,52 kN

Therefore, the load ratio C/P = 168,5/11,52 = 14,6. 



SKF rating life




1Lubrication condition – the viscosity ratio, κ

κ = ν/ν1

Given:
  • oil viscosity grade = ISO VG 68
  • operating temperature = 85 °C (185 °F)
then, using diagram 1, ν = 13 mm2/s

Given:
  • n = 3 000 r/min
  • dm = 0,5 (60 + 130) = 95 mm

then, using diagram 2, ν1 = 7 mm2/s

Therefore κ = 13/7 = 1,8

The next higher viscosity grade, ISO VG 100, would give κ = 2,5. But this would result in the NU 311 ECP bearing having κ > 4, which, during cold starts in particular, would give unwanted high κ values.


2.  Contamination factor, ηc

Given:

  • contamination conditions are typical (i.e. open bearings, no filtering, wear particles and ingress from surrounding)
  • dm = 95 mm
then, using table 1, ηc = 0,2

Given:
  • Pu = 2 x 3,2 = 6,4 
  • P = 11,52 (→ Basic rating life, above) 
then ηc Pu/P = 0,2 x 6,4 / 11,52 = 0,11 


3SKF life modification factor, aSKF

Given:
  • κ = 1,8
  • ηc Pu/P = 0,11 
  • 7312 BECBP are SKF Explorer bearings 
then, using diagram 4, aSKF = 5 

Given:
  • L10h = 17 400  h
then L10mh = 5 x 17 400 = 87 000 h


Conclusion:

The pair of 7312 BECBP SKF Explorer bearings are of a suitable size.

Lubrication
Performance and operating conditionsBearing type and arrangementBearing sizeLubricationOperating temperature and speedBearing interfacesBearing executionSealing, mounting and dismounting

The pump has an oil bath. This is typical of process pumps, because of their requirement for long service intervals. In this pump, for simplicity, the locating and the non-locating support bearings are lubricated by the same oil bath.

As determined in previous steps, κ is 1,8 for the pair of angular contact ball bearings and 2,8 for the cylindrical roller bearing, and so the viscosity grade of the selected oil is adequate.

Operating temperature and speed
Performance and operating conditionsBearing type and arrangementBearing sizeLubricationOperating temperature and speedBearing interfacesBearing executionSealing, mounting and dismounting

Determine whether a detailed thermal analysis is required (→ Thermal equilibrium) by checking that:
  • the rotational speed is less than 50% of the bearing limiting speed:
    • This is true for the non-locating support.
    • For the locating support, it is 56 %, which is just slightly above the limit. That is, for a pair of single row angular contact ball bearings, the limiting speed is reduced by 20% (→ Permissible speed), and so 3 000/(0,8 x 6 700) = 0,56.
  • the load ratio C/P > 10:
    • This is true for the locating and non-locating supports.
  • there is no pronounced external heat input:
    • The pump is located in an environment where the ambient temperature is 20 to 30 °C (70 to 85 °F).
    • The pump medium is at ambient temperature, so no additional heat flows to the bearings.
Therefore no further thermal analysis is needed.

Bearing interfaces
Performance and operating conditionsBearing type and arrangementBearing sizeLubricationOperating temperature and speedBearing interfacesBearing executionSealing, mounting and dismounting

Because the loads on the bearings will be greater, as a result of the modification to the pump, you should check the bearing seat tolerances to make sure the bearings are mounted with adequate fits.

Given the standard steel shaft and cast iron housing, the bearing loads, speeds and temperatures, which are all within standard conditions, you can apply Fits for standard conditions.

Shaft tolerances 
You can find shaft tolerances for seats for radial ball bearings in table 3 and for radial roller bearings in table 4 (from Fits for standard conditions).

Given:
NU 311 ECP7312 BECBP
Condition of rotationrotating inner ring loadrotating inner ring load
P/C ratio0,020,07
Bore diameter 55 mm60 mm


Results:
 Bearing seat
 BearingDimensional
tolerance
Total radial
run-out tolerance
Total axial
run-out tolerance
Ra
NU 311 ECPk6ⒺIT5/2IT50,8 µm
7312 BECBPk5ⒺIT4/2IT40,8 µm


Housing tolerances
Any wear developing during service may lead to imbalance of the impeller, leading to an indeterminate direction of load on the outer rings of both bearings.

You can find tolerances for seats for cast iron and steel housings, for radial ball bearings, in table 5 (from Fits for standard conditions).

Given:
NU 311 ECP7312 BECBP
Condition of rotationindeterminate direction of loadindeterminate direction of load
P/C ratio0,020,07
Outer diameter 120 mm130 mm


Results:
 Bearing seat
 BearingDimensional
tolerance
Total radial
run-out tolerance
Total axial
run-out tolerance
Ra
NU 311 ECP
K7ⒺIT6/2IT63,2 µm
7312 BECBPK7ⒺIT6/2IT63,2 µm


Axial location 
The current design has adequate axial location. Make sure that the lock nut that locates the inner rings of the angular contact ball bearings is sufficiently tightened. Apply the clamp force uniformly around the circumference. To avoid distortion of the inner rings, limit the clamping force to 19 kN (C0/4) and respect the abutment dimensions (→ product details). This is necessary to achieve the desired axial clearance in the bearing pair.

Bearing execution
Performance and operating conditionsBearing type and arrangementBearing sizeLubricationOperating temperature and speedBearing interfacesBearing executionSealing, mounting and dismounting

Checking the initial internal clearance

The current design uses bearings with Normal initial clearance. The fits for the inner and outer rings, and a temperature difference between the inner and outer rings of 10 °C (20 °F), reduce the internal clearance. Other influences on the internal clearance are negligible.

1. Initial internal clearance

NU 311 ECPPair of 7312 BECBP
min./avg./max.40 / 55 / 70 μm22 / 32 / 27 μm
 → Bearing data. Values obtained from table 6.→ Bearing data. Axial values obtained from table 7, converted to radial (axial x tan 40°).


2. Clearance reduction caused by interference fits

Use: 

Δrfit = Δ1 f1 + Δ2 f2Clearance reduction caused by interference fits

Obtain values for:
  • factors, f1 and f2 → diagram 6
  • probable fits for shafts, Δ1 → table 8 
  • probable fits for housings, Δ2 → table 9

Results:



NU 311 ECP  Pair of 7312 BECBP
d/D
0,46 0,46
f1

0,78 0,78
f2

0,86 0,86
Δ1min./avg./max.-32 / -19 / -6 μm -26 / -16 / -6 μm
Δ2min./avg./max.-20 / 0 / 20 μm -21 / 1 / 23 μm
Δrfitmin./avg./max.-42 / -15 / -5 μm -38 / -12 / -5 μm


3. Clearance reduction caused by temperature difference

Use:  

Δrtemp = ΔT dm x 12 x 10-6 → Clearance reduction caused by temperature difference between shaft, bearing rings and housing

Results:

NU 311 ECP  Pair of 7312 BECBP
dm87,5 mm 95 mm
Δrtemp -11 μm -11 μm


4. Operating clearance 

NU 311 ECP  Pair of 7312 BECBP
min./avg./max.
-13 / 30 / 55 μm
 -27 / 17 / 4 μm


For a cylindrical roller bearing, negative clearance (i.e. preload) is generally not recommended.

Pairs of angular contact ball bearings should have an average operating clearance close to zero (ranging between small clearance and light preload), particularly when the pair is loaded predominantly axially. A small range is required to:
  • limit preload – to limit friction, reducing viscosity ratio, and so reduce bearing life
  • limit clearance – to avoid ball skidding
This manual calculation does not consider smoothing of the mating surfaces, nor elastic deflection under load, nor the probability of extreme values occurring at the same time.

Analysis using more advanced SKF software gives operating clearance results:


NU 311 ECP  Pair of 7312 BECBP
min./avg./max. 
3 / 34 / 59 μm 
 -10 / 11 / 24 μm 


These results indicate that Normal internal clearance is suitable.

Cage selection

Given the estimated operating temperature of 85 °C (i.e. the higher temperature of the two bearing supports), speed well below the limiting speed, and considering availability and price, the standard rolling element guided polyamide cages are confirmed as adequate.

For historical reasons, in some geographical areas, brass cages are preferred for angular contact ball bearings. These are available as standard from SKF. This also applies to the cylindrical roller bearings.

Conclusion


Non-locating support:

The NU 311 ECP bearing, currently used in the centrifugal pump, is adequate. As an alternative, the NU 311 ECM bearing could be used. Downsizing of the bearing is possible. 

Bearing execution is described by suffixes in the bearing designation (→ Designation system).

Designation suffixes:

SuffixDescription
Internal designECoptimized internal design incorporating more and/or larger rollers and with a modified roller end / flange contact designed to minimize friction
Cage designPglass fibre reinforced PA66 cage, roller centred

Mmachined brass cage, riveted, roller centred
Clearance classNormal


Locating support: 

The pair of universally matchable 7312 BECBP bearings, currently used in the centrifugal pump, is adequate. As an alternative, the 7312 BECBM could be used.

Bearing execution is described by suffixes in the bearing designation (→ Designation system).

Designation suffixes:

SuffixDescription
Internal designB40° contact angle

Eoptimized internal design – reinforced rolling element set
External design / clearance classCBbearing for universal matching; two bearings arranged back-to-back or face-to-face; have Normal axial internal clearance
Cage designPglass fibre reinforced PA66 cage, ball centred

Mmachined brass cage, ball centred


Sealing, mounting and dismounting
Performance and operating conditionsBearing type and arrangementBearing sizeLubricationOperating temperature and speedBearing interfacesBearing executionSealing, mounting and dismounting

Sealing

The current pump design uses radial shaft seals to keep the oil bath lubricant in the pump and to protect the bearings from contamination (fig. 1).

You can use seals SKF HMS5 (fig. 2) and HMSA10 (fig. 3) (→ HMS5 and HMSA10 seals). These are suitable for both oil and grease lubricated applications. The temperature range and speed capability of the nitrile rubber compound used for these seals suits the operating conditions of the pump.

For more information about shaft and housing bore specification, refer to Shaft requirements and Housing bore requirements.

If the seal counterface becomes worn, you can repair the shaft with a wear sleeve, such as SKF Speedi-Sleeve.

Hot mounting of the bearings

The bearings are mounted with an interference fit on the shaft and a transition fit in their housings. You can mount the bearings easily by heating their inner rings to 100 °C (210 °F) and the housing seats to 50 °C (160 °F). For heating the inner rings, use an SKF induction heater or electric hot plate (→ Bearing heaters). 

Shaft alignment

To maximize pump life, the pump and its electric motor need to be well-aligned (→ Alignment tools).

Overall conclusions
The existing bearings can be used in combination with the new impeller design.

Downsizing of the cylindrical roller bearing is recommended.

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