Sliding Contact Bearing MCQ Quiz - Objective Question with Answer for Sliding Contact Bearing - Download Free PDF

Concept:

The life of a ball bearing is related to the load by the following equation:

\( L_2 = L_1 \times \left( \frac{P_1}{P_2} \right)^3 \)

Where:

• \( L_1 \) = Initial life of the bearing
• \( L_2 \) = New life of the bearing
• \( P_1 \) = Initial load
• \( P_2 \) = New load

Calculation:

Given data:

• Initial load, \( P_1 = 45 \) kN
• New load, \( P_2 = 60 \) kN
• Initial life, \( L_1 = 6400 \) hours

Substituting the values into the equation:

\( L_2 = 6400 \times \left( \frac{45}{60} \right)^3 \)

\( L_2 = 6400 \times \left( 0.75 \right)^3 \)

\( L_2 = 6400 \times 0.4219 \)

\( L_2 = 2700 \) hours

Journal Bearings in Industrial Machines:

• Journal bearings support rotating shafts and transmit radial loads.
• The $l/d$ ratio is a key parameter that influences:
  • Load-carrying capacity.
  • Friction and heat generation.
  • Effective lubrication.

1. Optimal Range for $l/d$:

   • For general industrial applications, the $l/d$ ratio is often selected between 0.8 and 1.5 because:
     • Low $l/d$ (e.g. < 0.8): Causes higher side leakage of lubricant and lower load capacity.
     • High $l/d$ (e.g. > 1.5): Increases friction, heat generation, and difficulty in lubrication.

2. Design Considerations:

   • Short Bearings ($l/d < 1$):
     • Suitable for high-speed applications with moderate loads.
     • Easier lubrication but reduced load-carrying capacity.
   • Long Bearings ($l/d > 1.5$):
     • Suitable for heavy loads but at the cost of higher friction and heat generation.

3. Applications:

   • General industrial machines: l/d=0.8 to 1.5.
   • Heavy-duty applications: 
     l/d > 1.5 (e.g., turbines, compressors).
   • High-speed applications: l/d < 1 (e.g., small motors).

Explanation:

Bearing designation:

• In bearing designation systems, particularly those following the ISO 15 standard, the number designation typically consists of a series of digits that give information about the bearing type, size, and other characteristics.
• The last two digits of the designation number usually provide the bore diameter when multiplied by a factor (commonly 5).
• For the given bearing number 208, the last two digits are '08'.

To find the bore diameter, multiply these two digits by 5.

Bore Diameter = 08 × 5 = 40 mm

Explanation:

Bearings:

• Bearings are used to reduce friction between the rotating shaft and its support structure.
• When a shaft rotates anticlockwise at a slow speed in a bearing, the forces acting on the shaft can cause it to move towards the left side of the bearing.
• This misalignment leads to metal-to-metal contact, which can increase wear and potentially damage both the shaft and the bearing.

Why the Shaft Moves Left:

• When the shaft rotates anticlockwise at low speed, the bearing experiences a resistive torque due to friction between the shaft and the bearing surfaces. This frictional force can cause the shaft to move sideways, particularly if the bearing's design doesn't fully constrain axial movement (movement along the shaft’s length).
• As the shaft rotates, the non-uniform pressure distribution along the bearing surfaces can result in the shaft shifting to one side. This happens because the contact area between the shaft and the bearing isn't always perfectly symmetric, leading to a higher force on one side of the bearing, pushing the shaft in that direction.
• In some cases, especially if the bearing is not well lubricated or if the lubricant is insufficient for slow speeds, metal-to-metal contact may occur. This happens when the oil or grease film breaks down and the metal surfaces rub directly against each other, leading to wear and increased friction.

Explanation:

Collar bearings:

• Collar bearings are a type of thrust bearing specifically designed to support axial loads.
• They consist of a series of collars or rings that are mounted on a shaft.
• These collars bear against a stationary surface, such as a housing or another component, to support the axial load.
• The design of collar bearings allows them to handle significant axial forces while facilitating smooth rotation or linear movement of the shaft.
• Collar bearings are commonly used in applications where axial loads are predominant, such as in automotive transmissions, propeller shafts, and rotary tables in machine tools.
• Their ability to support high axial loads with minimal friction makes them an ideal choice for such applications.

Thrust Bearings

• A thrust bearing is a particular type of rotary bearing designed to support axial loads while allowing rotational or linear movement between two parts.
• These bearings are specifically designed to handle forces parallel to the axis of rotation, which distinguishes them from other types of bearings that primarily support radial loads.
• Thrust bearings operate by distributing the axial load over a larger surface area, reducing the friction and wear between the moving parts.
• The bearing components, including the rolling elements (such as balls or rollers) and races (inner and outer rings), work together to facilitate smooth movement while supporting the axial load.
• The design ensures that the load is evenly distributed, preventing excessive stress on any single point.

Advantages:

• Capable of supporting high axial loads with minimal friction.
• Enhances the longevity and reliability of the machinery by reducing wear and tear.
• Available in various designs to suit different applications and load requirements.

Disadvantages:

• Not suitable for supporting significant radial loads.
• Requires precise alignment and installation to function effectively.
• May necessitate regular maintenance and lubrication to ensure optimal performance.

Applications:

• Thrust bearings are widely used in various applications, including automotive transmissions, aerospace components, heavy machinery, and industrial equipment.
• They are essential in any system where axial loads need to be supported while allowing for rotation or linear movement.

Explanation:

The sliding contact bearing, according to the thickness layer of lubricant between bearing and journal are mentioned as follows.

Concept:

Hydrodynamic Lubrication:

• It is thick film lubrication in which fluid pressure is created by the relative motion between the moving parts.
• As fluid pressure is itself being created, there is no need for a pressurized supply of lubricant.
• In this, surfaces do not contact during the rotation as there is a thick film of the lubricant.
• It can be used with thrust as well as sliding contact bearing.

Hydrostatic lubrication:

• In hydrostatic lubrication, external pressure is supplied to separate two surfaces, while relative velocity between two surfaces is used to generate liquid pressure between two surfaces.
• Hydrostatic lubrication is generally costlier compared to hydrodynamic lubrication.
• In hydrostatic lubrication, the pump is used.
• Thick film lubrication is further divided into two groups
  1. Hydrodynamic lubrication
  2. Hydrostatic lubrication

Explanation:

• The most common chrome steels contain from 0.5 to 2% chromium and 0.1 to 1.5% carbon.
• The chrome steel is used for balls, rollers and races for bearings.
• A Nickel-Chrome steel containing 3.25% nickel, 1.5% chromium and 0.25% carbon is much used for armour plates.
• Chrome nickel steel is extensively used for motor car crankshafts, axles and gears requiring great strength and hardness.

Explanation:

For a journal bearing, the Petroff equation is used to determine the coefficient of friction.

\(f = 2{{\rm{\pi }}^2}\left( {\frac{r}{c}} \right)\left( {\frac{{{\rm{\mu }}{{\rm{N}}_{\rm{s}}}}}{P}} \right)\)

McKee’s equation:

A bearing characteristic number is given by McKee which helps is visualizing the transition from thin film lubrication to thick film hydrodynamic lubrication.

Bearing characteristic number = \(\left( {\frac{{\mu N}}{P}} \right)\)

The Sommerfield number (S) also known as bearing characteristic number is a dimensionless quantity used in the design of hydrodynamic journal bearings. It is very important in lubrication analysis because it contains all design variables. 

The Sommerfield number (S) is given by, 

\( S = {\left( {\frac{r}{c}} \right)^2}\frac{{μ {N}}}{P}\)

where μ = viscosity of the lubricant in N-s/mm2 or MPa-s, N = journal speed in rev/s, P = unit bearing pressure i.e. load per unit of the projected area in N/mm2, r = radius of the journal in mm, and c = radial clearance in mm.

Tapered roller bearings are capable of carrying both radial and axial loads but largely used for applications where radial load component predominates. They are often used in pairs to take the thrust load in both directions.

The taper roller bearing consists of rolling elements in the form of a frustum of cone. They are arranged in such a way that the axes of individual rolling elements intersect in a common apex point on the axis of the bearing.

The taper roller bearing consists of rolling part and inner and outer raceways in the form of conical surfaces. The outer raceway or outer ring is called ‘cup’ and inner raceway is called ‘cone’.

Following Points are to be noted about the load-carrying capacity of different types of rolling contact bearings: 

• Thrust Ball Bearing: Thrust ball bearing cannot take the radial load.
• Taper Roller Bearing: can take heavy radial and thrust loads.
• Angular Contact Bearing: can take both radial and thrust loads.
• Cylindrical Roller Bearing: can take radial load only
• Deep Groove Ball Bearing: takes load in radial as well as an axial direction

Explanation:

The following re the different conditions for different types of bearing

Concept:

Bearing

• A bearing is the machine element, which supports another moving machine element known as a journal.
• It permits relative motion between the contact surfaces of the members while carrying the load.

Classification of bearings:

Depending upon the direction of the load to be supported

• Radial bearing – The load acts perpendicular to the direction of motion of the moving element.
• Thrust bearing – The load acts along the axis of rotation. The bearing may move in either direction.

Thrust bearing

• A thrust bearing is used to guide or support the shaft which is subjected to a load along the axis of the shaft. Such types of bearings are mainly used in turbines and propeller shafts

There are two types of thrust bearings

• Footstep or pivot bearing – The loading shaft is vertical and the end of the shaft rests within the bearing.
• Collar bearing – The shaft continues through the bearing. The shaft may be vertical or horizontal with a single collar or many collars.

Flat collar bearings

• The shaft may be vertical or horizontal with single or multiple collars.
• In designing collar bearings, it is assumed that the pressure is uniformly distributed over the bearing surface.
• The bearing pressure for a single collar and water-cooled multi-collared bearings may be taken the same as footstep bearings.
• Collar bearings are thrust bearing having a suitably formed face or faces that resist the axial pressure or decrease the intensity of pressure of one or more collars on a rotating shaft.

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Following Parameters are important in the selection of Bearing

Length to diameter ratio (L/D) ratio: In the design of bearings the diameter of the shaft is designed by the strength and rigidity criterion not on the bearing capacity.

The length to diameter ratio (L/D) affects the performance of the bearing. As the ratio increases, the resulting film pressure increases. A long bearing has, therefore, more bearing capacity compared with a short bearing. A short bearing has greater side flow which improves the heat dissipation.

The long bearings are more susceptible to metal to metal contact at the two edges, when the shaft is deflected under load. The longer the bearing, the more difficulty is there to get sufficient oil flow through the passage between the journal and the bearing. Therefore the design trend is to use (L/D) ratio as 1 or less than one.

Bearing Pressure: It depends upon number of factors such as bearing material, operating temperature, the nature and frequency of load and service conditions.

Start-up Load: The unit bearing pressure for the starting condition should nit exceed 2N/mm²_. It mainly consists of the weight of the shaft and its attachments.

Radial Clearance: The radial clearance should be small to provide necessary velocity gradient.

c = (0.001) r

Minimum Oil Film Thickness: This is the lower limit beyond which metal to metal contact occurs.

h_o = (0.002) r

Explanation: