Sliding Contact Bearings

In this tutorial, you will become acquainted with sliding contact bearings, their design, and their application in the industry. You will see the different types of these bearings, learn of some of their basic design criteria, reasons for their use, and the importance of their maintenance.

Contents:

1. What are Sliding Contact Bearings?
2. Types of Sliding Contact Bearings
3. The Petroff’s Equation
4. The McKee’s Investigation
5. The Raimondi and Boyd Method
6. Temperature Rise in Sliding Contact Bearings
7. Comparison between Sliding and Rolling Contact Bearings
8. Bearing Failures and Remedies

What are Sliding Contact Bearings?

Sliding contact bearings have sliding motion between the shaft and the bearing housing, with some fluid to reduce the friction between the sliding surfaces. Sliding contact bearings are also known as sleeve bearings, journal bearings, or just simple plain bearings.

• Babbitt is material from a family of lead and tin alloys and is probably most used for crankshaft and camshaft bearings in internal combustion engines.
• The following figure shows a journal bearing at various operating conditions.
  
• The black annulus represents the bush and the grey circle represents the shaft placed within an oil film shown by the shaded region. The shaft, called a journal, carries a load W on it. The journal being smaller in diameter than the bush, it will always rotate with an eccentricity.
• When the journal is at rest, it is seen from the figure that due to bearing load P, the journal is in contact with the bush at the lowermost position and there is no oil film between the bush and the journal.
• Only when the shaft starts rotating is there a build buildup fluid film between the sliding surfaces, with the load of the shaft being balanced by the frictional forces between the sliding layers.

Types of Sliding Contact Bearings

Some common categories of Sliding contact bearings are discussed below.

• A journal bearing is a sliding contact bearing working on hydrodynamic lubrication and supports the load in the radial direction. The portion of the shaft inside the bearing is called journal and hence the name journal bearing.
• There are two types of hydrodynamic journal bearings, namely, full journal bearing and partial bearing. The following figure shows the two types of bearings.
  
• In full journal bearing, the angle of contact of the bushing with the journal is 360°. Full journal bearing can take loads in any radial direction. Mostly they are utilized in industrial applications.
• In partial bearings, the angle of contact between the bush and the journal is always less than 180°. Most of the partial bearings have a 120° angle of contact. The partial bearing can take loads in only one radial direction.
• Partial bearings are simpler in construction. It is easy to supply lubricating oil to the partial bearing. The frictional loss in a partial bearing is less. Therefore, the temperature rise is low.
• A clearance bearing is a bearing in which the radius of the journal is less than the radius of the bearing. Therefore, there is a clearance space between the journal and the bearing. Most of the journal bearings are of this type.
• A fitted bearing is a bearing in which the radius of the journal and the bearing are equal. The fitted bearing must be partial bearing and the journal must run eccentric to the bearing to provide space for lubricating oil.
• The footstep bearing or simply step-bearing is a thrust bearing in which the end of the shaft is in contact with the bearing surface.
• The collar bearing is a thrust bearing in which a collar integral with the shaft is in contact with the bearing surface. In this case, the shaft continues through the bearing. The shaft can be with a single collar or can be with multiple collars.

The Petroff’s Equation

The Petroff’s equation is used to determine the coefficient of friction in journal ratings based on the assumption that the shaft is concentric with the bearing and the bearing is subjected to light load. In practice, however, such conditions do not exist.

f = \((2π^2)(\frac{r}{c})(\frac{μn_s}{p})\)
Where,
f=coefficient of friction
r=radius of the journal
c=radial clearance
μ=coefficient of viscosity
n_s=journal speed
p=Unit bearing pressure

The McKee’s Investigation

To visualize the transition from thin-film lubrication to thick film hydrodynamic lubrication, the McKee brothers developed an experimental curve titled the μN/p curve. The following figure shows the curve.

A bearing characteristic number is a dimensionless group of parameters given by the following equation.
Bearing Characteristic Numbe r = \(\frac{μN}{p}\)
The bearing characteristic number is plotted on the abscissa. The coefficient of friction f is plotted on the ordinate. The coefficient of friction f is the ratio of tangential frictional force to the radial load acting on the bearing.
To avoid seizure, the operating value of the bearing characteristic number should be at least 5 to 6 times that when the coefficient of friction is minimum. For fluctuating load, it must be at least 15 times that when the coefficient of friction is minimum.

The Raimondi and Boyd Method

In the Raimondi and Boyd method, the performance of the bearing is expressed in terms of dimensionless parameters. These dimensionless parameters are tabulated systematically and are available in all engineering design data handbooks for reference. The following figure shows a journal bearing which will help in understanding the parameters.

In the figure, O and O’ are the axes of bearing and journal respectively. The distance OO’ is called eccentricity and is denoted by the letter e. The radial clearance c is given by c = R–r.
The eccentricity ratio (ε) is defined as the ratio of eccentricity to the final distance.
The term h_o represents the minimum film thickness and the term \(\frac{h_o}{c}\) is called the minimum thickness variable.
The Sommerfield number is given by (r/c)² \(\frac{μn_s}{p}\) and contains all variables controlled by the designer.
The angle φ shown is called the angle of eccentricity or attitude angle. It locates the position of minimum film thickness for the direction of load.

Temperature Rise in Sliding Contact Bearings

Heat is generated in the bearing due to the viscosity of the lubricating oil. The frictional work is converted into heat, which increases the temperature of the lubricant. Assuming that the total heat generated in the bearing is carried away by the total oil flow in the bearing, the expression for temperature rise can be determined.
Heat Generated=(2πn_s)(fWr)10^-6 kW
Where W is the thrust load and r is the radius of the journal.
The heat carried away by the lubricating oil is given by the equation
Heat Removed=mC_p Δt

Comparison between Sliding and Rolling Contact Bearings

Bearings are classified depending on the type of contact they have, sliding or rolling. Each category has its characteristics and associated advantages over the other. These are discussed below.

• Rolling contact bearings are vulnerable to shock loads due to poor damping capacity. Hydrodynamic bearings are better suited for these conditions, which occur in connecting rod or crankshaft applications.
• Rolling contact bearings require a lower starting torque compared to hydrodynamic bearings. In hydrodynamic bearings, metal-to-metal contact occurs at the beginning, which results in higher starting friction.
• When the full hydrodynamic film has developed, the power losses due to friction are lower than that of rolling contact bearings.
• Rolling contact bearings require considerable radial space, while hydrodynamic bearings require more axial space.
• For the precise location of the journal axis, rolling contact bearings are preferred because the axes of the journal and the bearing are collinear. In hydrodynamic bearings, the journal moves eccentrically to the bearing and the eccentricity varies with load and speed.
• Rolling contact bearings, due to metal-to-metal contact, generate more noise compared with hydrodynamic bearings.
• The maintenance cost of hydrodynamic bearing is more. From cost considerations, rolling contact bearings are cheaper.

Bearing Failures and Remedies

Fatigue failures are not common in journal bearings, unlike ball bearings. The failures in journal bearings are mainly associated with insufficient lubricant, contamination of lubricant, and faulty assembly.

• Abrasive wear occurs when the lubricating oil is contaminated with dust, foreign particles, rust, or spatter. Proper enclosures for the bearing and the housing, cleanliness of lubricating oil, and use of high viscosity oil are some of the remedies against this type of wear.
• When the rotating journal touches the bearing, excessive rubbing occurs resulting in melting and smearing of the surface of the bearing. This type of failure is in the form of surface melting and flow of bearing material. The main causes for this type of wear are inadequate clearance, excessive transient load, and insufficient oil supply.
• The corrosion of the bearing surface is caused by the chemical attack of reactive agents that are present in the lubricating oil. The remedy is to use oxidation inhibitors as an additive in the lubricating oil.
• Misalignment and incorrect type of fit are the major sources of difficulties in journal bearings. When the fit is too tight, bore distortion occurs. When foreign particles are trapped between the bearing and the housing during the assembly, local bore distortion occurs. Correct selection of the fit and proper assembly procedure are the remedies against this type of wear.