Vibrating screen

This example shows the bearing selection process applied to an application case in which a vibrating screen manufacturer is selecting the bearings for a new machine.

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 new machine is a free circular motion vibrating screen. The vibrator unit is composed of a shaft with two bearings and counterweights. This means the main radial load rotates with the shaft while the outer ring is stationary. The application drawing is shown in fig. 1.

The relevant performance requirements, operating conditions and input parameters for the bearing selection are:
  • mass of screen box without charge: G = 6 100 kg
  • shaft diameter: 140 mm
  • rotational speed: n = 756 r/min
  • angular velocity (n x 2π/60): ω= 79,2 rad/s
  • radius of vibration: r = 8,1 mm
  • distance between the centres of gravity of the counterweights and shaft axis: R = 80 mm
  • distance between the bearings: 3 m
  • lubrication method: grease
  • operating temperature of the bearings: T = 75 °C (165 °F)
  • environment: the screen may be located outdoors, in harsh, dusty and humid conditions
  • required SKF rating life: 20 000 hours
Bearing type and arrangement
Performance and operating conditionsBearing type and arrangementBearing sizeLubricationOperating temperature and speedBearing interfacesBearing executionSealing, mounting and dismounting

A locating/non-locating bearing arrangement is used. The bearing on the drive side is the locating bearing. This limits axial displacement of the transmission pulley, which saves energy and increases belt life. The opposite bearing is non-locating to accommodate axial displacement due to thermal expansion of the shaft.

The distance between the bearings is 3 metres and the vibrating screen structure is made of welded and bolted steel parts. Shaft deflection and misalignment of supports under load require bearings that can compensate for misalignment.

Spherical roller bearings are selected for this new vibrating screen (fig. 2), which is the typical solution. They can carry high loads and accommodate misalignment between the inner and outer ring without any reduction of their service life.
Bearing size
Performance and operating conditionsBearing type and arrangementBearing sizeLubricationOperating temperature and speedBearing interfacesBearing executionSealing, mounting and dismounting

A shaft diameter of 140 mm is needed to transmit the required shaft drive torque and limit shaft deflection.

SKF supplies bearings in the 223 series for vibratory applications. Based on the required shaft diameter, the 22328 CCJA/W33VA405 is selected. We will check its size using the SKF rating life.

Product data for 22328 CCJA/W33VA405 → product details.

For vibrating screens, the equivalent dynamic bearing load, P, can be estimated using:



= 186 kN

The load ratio C/P = 1357 / 186 = 7,3.

SKF rating life


 

1Lubrication condition – the viscosity ratio, κ

κ = ν/ν1

The rated viscosity v1 = 10 mm2/s (diagram 1).

A viscosity ratio, κ, of about 4 is targeted to operate in full-film lubrication conditions, therefore v should be about 40 mm2/s.

You need to verify the viscosity ratio after you have selected your lubricant.

2. Contamination factor, ηc

Given:
  • contamination conditions are typical (i.e. open bearings, no filtering, wear particles and ingress from surrounding and harsh environment)
  • dm = 220 mm
then, using table 1, ηc = 0,2

3SKF life modification factor, aSKF

Given:
  • κ = 4
  • ηc Pu/P = 0,2 x 132/186 = 0,14
  • 22328 CCJA/W33VA405 is an SKF Explorer bearing
then, using diagram 2 for radial roller bearings, aSKF = 1,3


= 1,3 x (106 / (60 x 756))(7,3)10/3 = 21 500 h > 20 000 h

Conclusion

SKF bearing 22328 CCJA/W33VA405 is a suitable size to meet the rating life requirements.

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

Selecting grease or oil

Table 2 under Selecting grease or oil provides limits for the ndm factor, up to which grease lubrication is normally a suitable solution in terms of relubrication intervals at normal temperatures.

Input values:
  • spherical roller bearing in the 223 series
  • C/P = 7,3
  • ndm = 756 x (140 + 300)/2 = 166 320
From table 2, the recommended ndm limit for C/P ≈ 8 is 150 000, which is somewhat below the actual ndm factor. The operating conditions are at the limits where grease lubrication is suitable, and you can expect short relubrication intervals. But this is not an issue for vibrating screens, and you can select grease lubrication.

Grease selection

You can find a suitable SKF grease using the SKF bearing grease selection chart [PDF]. Grease selection criteria are:
  • temperature: 75 °C (165 °F) → M
  • speed: ndm ≈ 166 000 → M to H
  • load: C/P ≈ 8 → M
  • severe vibrations
  • humid outdoor conditions: good rust inhibiting properties
SKF LGEP2 is a suitable choice provided a viscosity ratio, κ, of 4 is confirmed.

LGEP2 has the following properties:
  • ν = 200 mm2/s at 40 °C (105 °C)
  • ν = 16 mm2/s at 100 °C (210 °C)
  • operating viscosity at 75 °C (165 °C) is around 40 mm2/s, based on diagram 3.
  • κ = ν/ν1 = 40/10 = 4 is confirmed

Relubrication interval and quantity

Experience suggests relubricating the bearings in the vibrating screen every 75 h with 30 g of grease. The short intervals are needed to push out contamination, while the reduced quantity limits heat generation caused by high grease volumes.

Using the standard relubrication interval from diagram 4 and input values gives:
  • ndm bf = 166 320 x 2 ≈ 330 000
  • C/P ≈ 8
The relubrication interval is 1700 h. This needs to be reduced, with contamination and vibration considered. (→ table 3), confirming approximately the experienced values used for vibrating screen bearings.

Relubrication quantity is:
Gp = 0,002 D B = 0,002 x 300 x 102 = 61 g

Standard relubricating the bearings every 75 h with 30 g will maintain adequate lubrication condition.

Initial grease fill

The free volume in the bearing, which should be filled with grease is approximately:



V = 3,14/4 x 102 x (3002 - 1402) x 10-3 - 36,5/0,0078 = 957 cm3

For a filling degree of 50% you need about 430 g grease per bearing.
Operating temperature and speed
Performance and operating conditionsBearing type and arrangementBearing sizeLubricationOperating temperature and speedBearing interfacesBearing executionSealing, mounting and dismounting

Experience from similar applications is broad and a bearing operating temperature between 70-80 °C (160-175 °F) can be assumed.

The screen charge is at ambient temperature and there are no other external sources generating heat. The speed is < 50% of the limiting speed. Although the load ratio C/P < 10, no detailed thermal analysis is required.

The actual operating temperature should be checked on the real machine.

The bearing frictional losses are 1 900 W per bearing, calculated with the SKF Bearing Calculator.
Bearing interfaces
Performance and operating conditionsBearing type and arrangementBearing sizeLubricationOperating temperature and speedBearing interfacesBearing executionSealing, mounting and dismounting

The radial load turns in phase with the rotating inner ring, while the outer ring stands still. Therefore, the inner ring has a stationary load condition and the outer ring a rotating load condition. An interference fit is needed between the outer ring and the housing. A loose fit can be used between the inner ring and the shaft.

The standard fit recommendation is:

Dimensional
tolerance
Total radial
run-out tolerance
Total axial
run-out tolerance
Ra
Shaftg6ⒺIT5/2IT51,6 µm
HousingP7ⒺIT6/2IT63,2 µm

There are reasons for choosing dimensional tolerances other than the standard fits:
  • Choose f6Ⓔ for easy axial displacement of the inner ring. To reduce the risk of fretting corrosion, consider hardening the shaft seat.
  • Select P6Ⓔ (tighter tolerances) to improve outer ring support and bearing service life.


Additional recommendations

The following additional factors are recommended:
  • The bearing centre should be aligned with the frame centre of the vibrating screen (fig. 3).
  • The housing wall thickness should be greater than 40% of bearing width.
  • Design the housing to be as symmetrical as possible, so it has the same thickness on both sides of the vibrating screen frame, in order to avoid housing deformation (fig. 4).
  • Machine threads in the housing to make it easier when dismounting the housing from the screen body and the bearing from housing by the use of bolts (fig. 5 and fig. 6).
Bearing execution
Performance and operating conditionsBearing type and arrangementBearing sizeLubricationOperating temperature and speedBearing interfacesBearing executionSealing, mounting and dismounting

The bearing selected for this application is a spherical roller bearing for vibratory applications. (→ Designs and variants)

These bearings are identified by the designation suffixes VA405 and VA406. They have a C4 internal clearance, which is required because of the interference fit of the outer ring in combination with the temperature difference between inner and outer rings, particularly during start-up situations. Their hardened window-type cages reduce friction and wear in the bearing when operating under rotating outer ring load and high acceleration conditions, resulting in a lower operating temperature and longer lubricant life.

The VA406 execution is intended for the non-locating support and has a PTFE coated bore. This helps to prevent fretting corrosion, which can occur because of the loose fit and vibration.
Sealing, mounting and dismounting
Performance and operating conditionsBearing type and arrangementBearing sizeLubricationOperating temperature and speedBearing interfacesBearing executionSealing, mounting and dismounting

  • Vibrating screen designs generally use labyrinth seals to protect the rolling bearings. With this type of seal, it is important to maintain a sufficient quantity of grease in the labyrinth gaps, so that dirt and moisture are kept away from the bearings. Quantities and relubrication intervals should be adjusted according to the operator's observations.
  • Check the total radial run-out of the housing seat when the housing is mounted to the screen frame. Inadmissible deformation might occur and can require corrective action.
Overall conclusions
  • The 22328CCJA/W33VA405 bearing meets the rating life requirement.
  • SKF grease LGEP2 is appropriate for the given operating conditions.
  • Maintenance and condition monitoring aspects have not been included in this example. For additional information about SKF offers for vibrating screens refer to Vibrating screens under Industry solutions.
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