Design a Plain Surface Bearing Using Boundary Lubricated Approach

Journal Bearings

A simple journal bearing is composed of a rotating shaft (or journal) held by a stationary housing with a thin layer or oil separating the two.

From: Fluid Mechanics (Sixth Edition) , 2016

Journal bearings

Peter R.N. Childs , in Mechanical Design Engineering Handbook (Second Edition), 2019

Abstract

The purpose of a bearing is to support a load, typically applied to a shaft, whilst allowing relative motion between two elements of a machine. The two general classes of bearings are journal bearings, also known as sliding or plain surface bearings, and rolling element bearings, sometimes also called ball bearings. The aims of this chapter are to describe the range of bearing technology, to outline the identification of which type of bearing to use for a given application, and to introduce journal bearing design with specific attention to boundary lubricated bearings and full film hydrodynamic bearings. The selection and use of rolling element bearings is considered in Chapter 6.

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Journal Bearings

P.R.N. Childs , in Mechanical Design (Third Edition), 2021

2.1 Introduction

The purpose of a bearing is to support a load, typically applied to a shaft, whilst allowing relative motion between two elements of a machine. The two general classes of bearings are journal bearings, also known as sliding or plain surface bearings, and rolling element bearings, sometimes also called ball-bearings. The aims of this chapter are to describe the range of bearing technology, to outline the identification of which type of bearing to use for a given application, and to introduce journal bearing design with specific attention to boundary lubricated bearings and full-film hydrodynamic bearings. The selection and use of rolling element bearings is considered in Chapter 3.

The term "bearing" typically refers to contacting surfaces through which a load is transmitted. Bearings may roll or slide or do both simultaneously. The range of bearing types available is extensive, although they can be broadly split into two categories: sliding bearings, also known as journal or plain surface bearings, where the motion is facilitated by a thin layer or film of lubricant, and rolling element bearings, where the motion is aided by a combination of rolling motion and lubrication. Lubrication is often required in a bearing to reduce friction between surfaces and to remove heat. Fig. 2.1 illustrates two of the more commonly known bearings: a journal bearing and a deep groove ball bearing. A general classification scheme for the distinction of bearing types is given in Fig. 2.2, developing on a taxonomy originally produced by Hindhede et al. (1983).

Figure 2.1. A journal bearing and a deep groove ball bearing.

Figure 2.2. Bearing classification.

As can be seen from Fig. 2.2, the scope of choice for a bearing is extensive. For a given application, it may be possible to use different bearing types. In a small gas turbine engine, rotating at say 50,000   rpm, either rolling bearings or journal bearings could be used although the optimal choice will depend on a number of factors such as life, cost, and size. Fig. 2.3 can be used to give guidance for which kind of bearing has the maximum load capacity at a given speed and shaft size, and Table 2.1 gives an indication of the performance of the various bearing types for some criteria other than load capacity.

Figure 2.3. Bearing type selection by load capacity and speed.

Courtesy Neale, M.J. (ed.). (1995). The Tribology Handbook. Butterworth Heinemann.

Table 2.1. Comparison of bearing performance for continuous rotation.

Bearing type	Accurate radial location	Combined axial and radial load capability	Low starting torque capability	Silent running	Standard parts available	Lubrication simplicity
Rubbing plain bearings (nonmetallic)	Poor	Some in most cases	Poor	Fair	Some	Excellent
Porous metal plain bearings oil impregnated	Good	Some	Good	Excellent	Yes	Excellent
Fluid film hydrodynamic bearings	Fair	No. Separate thrust bearing needed	Good	Excellent	Some	Usually requires a recirculation system
Hydrostatic bearings	Excellent	No. Separate thrust bearing needed	Excellent	Excellent	No	Poor special system needed
Rolling bearings	Good	Yes, in most cases	Very good	Usually satisfactory	Yes	Good when grease lubricated

Courtesy Neale, M.J. (ed.). (1995). The Tribology Handbook. Butterworth Heinemann.

This and the following chapter provide an introduction to bearings. Lubricant film sliding bearings are introduced in Section 2.2, the design of boundary-lubricated bearings that are typically used for low-speed applications are considered in Section 2.3, the design of full-film hydrodynamically lubricated bearings is described in Section 2.4, and Chapter 3, Rolling Element Bearings considers rolling element bearings. For further reading, the texts by Khonsari and Booser (2017), Harris (2001), and Brandlein et al. (1999) provide an extensive overview of bearing technologies.

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Crankshaft bearings

In Tribology Handbook (Second Edition), 1995

SELECTION OF TYPE OF PLAIN BEARING

Journal bearings
Direct lined	Insert liners
Accuracy depends on facilities and skill available	Precision components
Consistency of quality doubtful	Consistent quality
First cost may be lower	First cost may be higher
Repair difficult and costly	Repair by replacement easy
Liable to be weak in fatigue	Will generally sustain higher peak loads
Material generally limited to white metal	Range of available materials extensive

Thrust bearings
Flanged journal bearings	Separate thrust washer
Costly to manufacture	Much lower first cost
Replacement involves whole journal/thrust component	Easily replaced without moving journal bearing
Material of thrust face limited in larger sizes	Range of materials extensive
Aids assembly on a production line	Aligns itself with the housing

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Tribology for Energy Conservation

Hiromu Hashimoto , in Tribology Series, 1998

1 INTRODUCTION

Hydrodynamic journal bearings have been widely used to support high speed rotating machinery such as turbines and compressors because of their superior durability and load carrying capacity. Therefore, the bearings are important machine elements for enhancing the quality of the rotating machinery. As the performance characteristics of high-speed, hydrodynamic journal bearings operated in both laminar and turbulent flow regimes are governed by a number of bearing parameters, the bearing designers usually try to select the design variables within constraints by a trial and error method using many design charts obtained from the bearing characteristic analysis. However, this approach only guarantees acceptable solutions, and it does not necessarily produce the optimum solutions. Moreover, even in the case that the bearing designers can get the optimum solutions successfully by such an approach, a considerable amount of working time and cost will be needed to complete the optimum design of high-speed journal bearings.

On the optimum design of hydrodynamic journal bearings, Rohde[1] determined the minimum film thickness which optimizes the load carrying capacity of an infinite length journal bearing operated in the laminar flow regime by the use of a variational technique. Asimov[2] applied the Newton-Raphson method to determine the length and diameter of journal bearings in the laminar flow regime which minimize the objective function defined as a weighted sum of friction loss and shaft twist, in which a short bearing approximation was used to simplify the analysis. Beightler et al.[3] treated the same problem by Geometric Programming, in which they extended the objective function to include a weighted temperature rise term. Seireg and Ezzat[4] applied the Gradient Search method to determine the optimized length, radial clearance and average viscosity of journal bearings in the laminar flow regime which minimize a weighted sum of the supply lubricant quantity and fluid film temperature rise. In these previous papers on the optimum design of journal bearings, however, the laminar flow condition was assumed in determining the optimum design variables, so the variables obtained are unrealistic for high-speed journal bearings operated in the turbulent flow regime.

In this paper, the optimum design procedure of high-speed, hydrodynamic short journal bearings operated under the laminar and turbulent flow conditions is developed based on three kinds of optimization methods such as Successive Quadratic Programming, Genetic Algorithm and Direct Search[5].

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Fundamentals of hydrodynamic journal bearings: an analytical approach

S. Balakrishnan , ... H. Rahnejat , in Tribology and Dynamics of Engine and Powertrain, 2010

18.1 Introduction

Journal bearings are one of the most common types of hydrodynamic bearings. Their primary purpose is to support a rotating shaft. They are used in various subsystems in engines and power trains, for example for support of both crankshaft and camshaft. They are also used in the rocker shaft of rocker-arm valve train systems. The piston pin-bore bearing also acts as a form of journal bearing, so do the big-end bearings, covered in Chapters 19 and 20. Journal bearings usually operate in a hydrodynamic regime of lubrication as the generated pressures are low compared with those experienced by ball and rolling element bearings, gears and cam–follower pairs. Unlike these counterforming pairs, where the area of contact is very small, the journal conforms reasonably well to the bearing bushing (but not completely), allowing a wedge shape to form a film of lubricant, drawn into the contact. Thus, a large area of contact results in generated lubricant pressures which are typically at least an order of magnitude less than those in concentrated counterforming contacts. These are usually from a few to tens of MPa, which are normally insufficient to cause localised deformation of surfaces in contact, unlike elastohydrodynamic conditions in the rolling element bearings and cam–follower pairs (see Chapters 6 and 16). The film thickness is also of the order of a few to several micrometres unlike a few tenths to a couple of micrometres in ball and rolling element bearings.

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Accessories

Royce N. Brown P. E. , in Compressors (Third Edition), 2005

Bearings and Seals

The journal bearings must have stiffness and damping properties sufficient to prevent bearing-contributed vibrations and to result in proper gear contact. The stiffness and damping properties of the journal bearings affect the rotor system dynamics. Normally, stabilizing bearings such as tilting pad and three-lobe are needed to prevent shaft oil whirl. The tilting pad bearing can be seen in Figure 5-38 in Chapter 5. Figure 8-21 shows a typical three-lobe journal bearing.

Figure 8-38. Radial shaft vibration probes.

Figure 8-21. Three-lobe journal bearing.

(Courtesy of Turbocare, a division of Siemens Westinghouse Power.)

If a thrust bearing is used, it should be of a tilting pad, self-leveling design. With a double-helical gear unit, thrust bearings, if used at all, should be on the low-speed shaft only, to accommodate loads in both axial directions. Journal and thrust bearings must be split for removal and installation without having to remove the coupling hubs.

Seals should be non-contacting, multiple-point labyrinths. The housing and seals should be drilled and the housing tapped to accommodate a dry gas purge connection. Appropriate shoulders or shaft fingers should be immediately inside the seal to aid in the prevention of oil leaks.

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Casting machine – basic design

Clifford Matthews BSc (Hons), CEng, MBA , in Case Studies in Engineering Design, 1998

Journal bearings

In a journal bearing, the shaft rotates inside a loose-fitting bearing shell of softer, often porous, bearing material. Lubricant, such as oil, grease or a low-friction compound like PTFE or graphite is used between the surfaces. The shell is sometimes split into two halves. The bearing shell is fitted tightly into its static housing to stop it revolving with the shaft. For simple rotation applications, journal bearings are designed to be parallel within close limits (about 0.1 mm), thereby avoiding excessive loads. A simple journal bearing is shown in Fig. 6.6.

Figure 6.6. Bearing types

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Introduction of advanced technologies for steam turbine bearings

P. Pennacchi , in Advances in Steam Turbines for Modern Power Plants, 2017

15.1.1 Journal bearings

Journal bearings are classified themselves on the type of the sliding surfaces:

•

if the journal bearing has fixed sliding surfaces, then it is defined as a "sleeve bearing" (Fig. 15.1);

Figure 15.1. Sleeve bearing.

Source: Courtesy of Eurobearings Srl.

•

otherwise, the bearing has several pivoted pads, which can tilt freely and it is defined as a "tilting pad journal bearing" (TPJB).

15.1.1.1 Sleeve journal bearings

The basic shape of sleeve bearings is the cylindrical bearing, in which the cross-section of the bearing surface is a circle. Actually, this kind of bearing is not of use in steam turbines, since it is prone to causing instability, in particular oil-whip [5], and is has been replaced for a long time by other designs, i.e., by multilobe bearings or by TPJBs.

Multilobe bearings have a cross-section composed of two (Fig. 15.2) to four (Fig. 15.3) circular arcs, forming the so-called "lobes." In the case of two lobes, the bearing is sometimes dubbed as "lemon-shaped." Two-lobe bearings may have a pocket machined in the upper half, called the pressure-dam, whose aim is to impose an additional downward load on the shaft, which contributes to stabilizing the rotor (and to increasing the bearing dynamic stiffness).

Figure 15.2. Two-lobe lemon-shaped sleeve bearing with pressure-dam.

Source: Courtesy of Eurobearings Srl.

Figure 15.3. Bottom half of a four-lobe sleeve bearing.

15.1.1.2 Tilting pad journal bearings

TPJBs may have several pads, around both the halves of the bearing shell (Fig. 15.4) or only in the lower one (Fig. 15.5). Moreover, the pads may be equal or different between them. Equal pads are the most common case, while, for instance, asymmetric three-pad TPJBs have been used in large steam turbines employed in nuclear power plants.

Figure 15.4. Tilting pad journal bearing.

Source: Courtesy of Eurobearings Srl.

Figure 15.5. Load between pads (LBP) tilting pad journal bearing.

Source: Courtesy of Eurobearings Srl.

Symmetric TPJBs, with the load applied on the bottom lobe (load on pad (LOP) configuration, Fig. 15.7) or between the two bottom lobes (with load between pads (LBP) configuration; Figs. 15.5 and 15.9) have the best performances in terms of stability of the rotor-bearing system.

LBP configuration is preferred when high load capacity is required. In this case, the damping is also higher than in the LOP configuration, due to the larger support area.

TPJBs have also some drawbacks, like hot oil carry over [4], risk of flutter of unloaded pads [6,7] (i.e., those in the upper half), higher costs, and more difficult determination of clearances than sleeve bearings. Pad fluttering is a somewhat difficult phenomenon to overcome and is defined as the unstable vibration of the pad floating back and forth between the pivot point and the journal continuously during shaft rotation. It is related to another phenomenon known as "spragging" of bearing pads [8].

However, the advantages of TPJBs outweigh their disadvantages and their use is growing in steam turbines.

The free tilting of the pad is about a pivot, which can be ideal, i.e., the pad rocks about a straight line on the back side (which has a geometrically ruled surface) of the pad that is in contact with the bearing shell. In this case, the TPJB is said to be of the rocker type (Figs. 15.6 and 15.7). If the pivot is machined on the pad back (Fig. 15.8) or it is realized by hardened metal inserts in the pad back and in the shell (Fig. 15.9), the TPJB is said to be of the pivoted type.

Figure 15.6. Front and back side of a rocker type pad of a tilting pad journal bearing.

Figure 15.7. Section of a rocker type tilting pad journal bearing, with load on pad (LOP).

Figure 15.8. Pivoted pad. Note the CuCr1Zr alloy coating base.

Source: Courtesy of Eurobearings Srl.

Figure 15.9. Pivoted type tilting pad journal bearing with load between pads (LBP).

Source: Courtesy of Eurobearings Srl.

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Recessed Hydrostatic Journal Bearings

W. Brian Rowe DSc, FIMechE , in Hydrostatic, Aerostatic and Hybrid Bearing Design, 2012

Application of Journal Bearings

Recessed journal bearings offer high rotational accuracy, high stiffness, and low temperature rise. Cylindrical bearings are the most common configuration. Alternatives include conical and spherical journal bearings, as described in Chapter 1. Recessed cylindrical bearings are a popular choice although plain bearings have advantages, as described in Chapter 10.

Two configurations are shown in Figure 9.1. The typical bearing is the simpler geometry without axial drainage grooves between the recesses. Axial drainage grooves reduce projected area and detract from hydrostatic and hydrodynamic load support. A possible advantage of axial grooves is greater through-flow for the same clearance.

Figure 9.1. Recessed Journal Bearings and Thin-Land Flow Factors.

Four, five, or six recesses are typical, recess depth typically being 20 times the gap at the bearing lands. Each recess is controlled by its own restrictor so that each recess with its surrounding land acts as a thrust pad; a whole bearing consists of n pads, of which n/2 pads support vertical loads and n/2 pads support horizontal loads.

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Compression Equipment (Including Fans)

A. Kayode Coker , in Ludwig's Applied Process Design for Chemical and Petrochemical Plants (Fourth Edition), 2015

Bearings

Shaft journal bearings for compressors operating in most process services and some air applications are located "outside" the case, rather than being an inside bearing. This is important for maintenance, as well as for reducing the problems in keeping oil out of the gas stream; although this problem still exists, but not to such a great extent. Thrust bearings of the Kingsbury type are one example of a good bearing for this equipment (Figures 18-49A and 18-49B). The double acting bearing can absorb thrust loads in either direction.

Figure 18-49a. Kingsbury-type thrust bearing.

(Used by permission: Elliot^® Company.)

Figure 18-49b. Kingsbury-type thrust bearing for centrifugal compressor.

(Used by permission: Kingsbury Machine Works, Inc.)

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Design a Plain Surface Bearing Using Boundary Lubricated Approach

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