2023 August the First Week KYOCM Technical Knowledge: Rolling bearings for centrifugal pumps and their new technologies

Abstract: This article introduces the basic structure and working conditions of the most widely used centrifugal pump, with a focus on the functions of bearings in centrifugal pumps and the continuous improvement of new bearing technologies to meet the performance of centrifugal pumps.

Keywords: Rolling bearings; Centrifugal pump; New technologies; lubrication

1. Introduction

A pump is a mechanical device widely used for lifting, compressing, and transporting liquids or gases. In Japan, the output value of pumps is 330 billion yen, while the annual output value of global pumps is expected to be 300 billion yen. According to different working principles, pumps can be roughly divided into three categories: centrifugal pumps (power pumps), positive displacement pumps, and other specialized pumps (see Figure 1). A centrifugal pump utilizes the blades on the rotating impeller to generate high head and high delivery pressure. Positive displacement pumps are usually divided into rotary pumps and reciprocating (piston) pumps, which transmit fluid in one direction. There are also specialized pumps such as jet pumps and air pumps. Nevertheless, centrifugal pumps are still the most common, as they can provide users with various ranges of flow delivery capacity and pressure. This article will focus on discussing centrifugal pumps and their functions, as well as bearing technologies that meet the requirements of centrifugal pumps.

Figure 1 Classification of pumps based on their working mechanism

2. Basic structure of centrifugal pumps

When the fluid flows through the blades and enters the turbine box, the centrifugal pump increases the velocity of the fluid by rotating the blades and centrifugal force on the impeller. When the fluid is discharged, the turbine converts kinetic energy into static pressure (pump head). Figure 2 depicts the basic structure of a centrifugal pump. A centrifugal pump consists of a turbine box and an impeller installed on a rotating shaft, which is supported by bearings. All loads applied to the shaft are also transmitted to the pump side bearings (front end bearings) and the motor side bearings (rear end bearings). A mechanical seal is used between the pump body of the drive shaft and the front bearing to prevent leakage. The basic characteristics of pump bearings are:

(1) the front end bearings bear radial loads, while the rear end bearings bear both radial and axial loads.

Figure 2 Basic Structure of a Centrifugal Pump

(2) In order to facilitate the installation of the mechanical sealing device between the impeller and the front end bearing, there must be sufficient clearance between the impeller on the shaft and the front end bearing. This structure causes radial load to be generated near the impeller, which in turn generates torque load on the front bearing.

(3) Under normal working conditions, the shaft will expand due to heating. In order to avoid excessive additional load on the bearing due to expansion, clearance fit is usually used for the installation of the front end bearing. To ensure the reliable installation of the rear end bearing on the shaft, interference fit is usually used for the installation of the rear end bearing.

(4) Pump bearings are usually lubricated with grease, but under high temperature working conditions, oil bath lubrication can also be used. Based on the above working characteristics, the front end bearings usually use deep groove ball bearings or cylindrical roller bearings with high radial bearing capacity, while the rear end bearings usually use paired angular contact ball bearings that can withstand both radial loads and large bidirectional axial loads.

Basic working conditions of centrifugal pumps

3.1 Speed

The shaft of most centrifugal pumps is driven by an AC motor through a coupling. The speed of the shaft can be expressed as

N=f × 6 × 2/p

In the formula: p is the number of magnetic poles; F is the rated frequency; N is the rotational speed, r/min. The standard value for rated frequency is 50 Hz or 60 Hz. Due to the number of magnetic poles being 2 or 4, the average speed is usually 1500 to 3600 r/min.

3.2 Load

There are two basic forms of load borne by bearings used in centrifugal pumps: one form of load comes from the imbalance of self weight and rotating components; Another form of load comes from the static pressure (pump head) generated before the pumped fluid leaves the discharge port. The design of the impeller (including single stage and multi-stage) and the structure of the turbine greatly affect the head/pressure of the pump. When the static pressure difference between the front and rear ends of the impeller (blade) is significant, the axial load acting on the bearing will be significant. Therefore, pumps are designed in various forms to reduce this axial load. The load example of a heavy-duty petroleum/chemical centrifugal pump is as follows: when the aperture is 50 mm and the inlet pressure is 2.5 MPa, the radial load of the front end bearing is 550 N; The radial load of the rear bearing is 350 N; The axial load of the rear bearing is 4500 N.

4. Requirements and related technologies for bearings

4.1 High load-bearing capacity

Pump manufacturers have always faced the requirement of reducing costs and reducing pump size. When the structure of the pump is designed to be more compact, it is also necessary to maintain the original output capacity of the pump. Therefore, bearings with higher load-bearing capacity and more compact structure are needed. In order to meet this requirement, NSK's high load bearing series products (see Figures 3 and 4) have emerged, with a basic rated dynamic load increased by about 10%. Measures such as increasing the diameter of the steel ball and optimizing the internal structure of the cage have also increased the bearing life by 30%.

Tables 1 and 2 list the external dimensions and basic dynamic load ratings of these bearings. For example, the basic dynamic load rating of the standard 6306 bearing is 26700 N, while the high load bearing reaches 29800 N, which means that the basic dynamic load rating has increased by approximately 12%. Due to a 12% increase in load-bearing capacity during the same working time, the lifespan of the bearing has been increased by 40% under the same load conditions.

Figure 3 Sectional view of standard deep groove ball bearings and high load deep groove ball bearings

Figure 4 Sectional View of Standard Angular Contact Ball Bearings and High Load Angular Contact Ball Bearings

4.2 Long lifespan

In the case of lubricant contamination, surface origin fatigue spalling may occur. Surface induced fatigue spalling is caused by the intrusion of foreign objects, resulting in indentation and rapid stress concentration around the indentation. Figure 5 illustrates the process of surface initiated fatigue spalling. UR treatment can effectively reduce stress concentration. This heat treatment process can appropriately increase the content of residual austenite in the bearing material, thereby alleviating the occurrence of surface origin peeling. Therefore, even when operating in polluted environments, the durability of bearings has been significantly improved.

Figure 5 The occurrence process of surface initiated fatigue spalling

As shown in Figure 6, in the case of lubricant contamination, the service life of bearings treated with UR has increased by two times compared to standard bearings. UR treatment can also reduce the Shear stress that mainly causes bearing rolling peeling. In addition, the compressive stress on the surface can effectively prevent the diffusion of cracks. The service life of bearings treated with UR has increased by more than 1.5 times compared to standard bearings (see Figure 7).

Figure 6 Rolling Fatigue Life of 6206 Bearing under Lubricant Pollution Conditions

Figure 7 Fatigue Life of 6205 Bearing under Clean Lubrication Conditions

4.3 Anti slip

The fluctuation of liquid flow rate, as well as the rotation and imbalance of the impeller, can generate loads on the drive shaft of the pump, and it is difficult to completely eliminate them. When the load becomes too high, low-speed sliding occurs between the outer ring of the bearing and the bearing seat, resulting in wear. The development of slip can cause excessive vibration of the shaft, leading to abnormal vibration and damage to the impeller. The usual anti slip measures include using a tight fit with sufficient interference. However, for pumps, it is difficult to adopt a tight fit because it makes pump assembly difficult and cannot overcome the additional load generated by shaft heating and expansion. The anti slip bearings provided by NSK solve this problem. There are two grooves on the outer ring of the anti slip bearing, in which an O-ring is installed. The friction generated by the O-ring can prevent the occurrence of slip (see Figure 8).

4.4 Low fever

The American Petroleum Institute's 610 standard provides detailed requirements for centrifugal pumps. The limit temperature of lubricants is one of the important parameters in the requirements of centrifugal pumps and other pumps covered by this standard, as it affects the temperature rise of bearings. The following are the main factors that affect temperature rise:

(1) Reasonably select lubricating grease suitable for working conditions and seal in the correct amount of grease. Excessive or insufficient grease can significantly affect temperature rise.

(2) Negative clearance can cause excessive temperature rise. In order to ensure sufficient clearance for the bearing, it is necessary to fit the shaft and bearing properly (usually using ordinary clearance).

(3) Incorrect installation can cause a sharp increase in bearing temperature, and special attention should be paid to avoiding relative tilting of the inner and outer rings.

5 Conclusion

This article discusses the design and working conditions of centrifugal pumps, as well as the functions and related new technologies of rolling bearings in centrifugal pumps. Centrifugal pumps are widely used in the petroleum, chemical, power, construction, and water supply industries. In the future, people will develop centrifugal pumps into composite systems by improving their comprehensive analysis and design capabilities, replacing single pumps, and will have a more compact structure, better performance, and higher reliability. NSK will continuously improve its existing technology to provide pump manufacturers with bearings that meet the latest development needs of pumps.

More about KYOCM Spherical Roller Bearing：

A spherical roller bearing is a rolling-element bearing that permits rotation with low friction, and permits angular misalignment. Typically these bearings support a rotating shaft in the bore of the inner ring that may be misaligned in respect to the outer ring. The misalignment is possible due to the spherical internal shape of the outer ring and spherical rollers.[1] Despite what their name may imply, spherical roller bearings are not truly spherical in shape. The rolling elements of spherical roller bearings are mainly cylindrical in shape, but have a (barrel like) profile that makes them appear like cylinders that have been slightly over-inflated.

http://www.kyocm.com/products/Spherical-Roller-Bearing/739.html