Sliding contact bearing pdf

In thrust bearings, the load acts along the axis of rotation as shown in Fig c. Sliding contact bearings, and. Rolling contact bearings. In sliding contact bearings, as shown in Fig a , the sliding takes place along the surfaces of contact between the moving element and the fixed element.

The sliding contact bearings are also known as plain bearings. In rolling contact bearings, as shown in Fig b , the steel balls or rollers, are interposed between the moving and fixed elements. The balls offer rolling friction at two points for each ball or roller. The sliding contact bearings in which the sliding action is guided in a straight line and carrying radial loads, as shown in Fig a , may be called slipper or guide bearings.

Such type of bearings are usually found in cross-head of steam engines. The sliding contact bearings in which the sliding action is along the circumference of a circle or an arc of a circle and carrying radial loads are known as journal or sleeve bearings. This type of bearing is commonly used in industrial machinery to accommodate bearing loads in any radial direction.

This type of bearing has less friction than full journal bearing, but it can be used only where the load is always in one direction. The most common application of the partial journal bearings is found in rail road car axles.

The full and partial journal bearings may be called as clearance bearings because the diameter of the journal is less than that of bearing. When a partial journal bearing has no clearance i. The sliding contact bearings, according to the thickness of layer of the lubricant between the bearing and the journal, may also be classified as follows:.

Thick film bearings. The thick film bearings are those in which the working surfaces are completely separated from each other by the lubricant. Hydrostatic lubrication: A system of lubrication in which load-supporting fluid film is created by an external source like, pump, supplying sufficient fluid under pressure.

In the diagram, it is an example of a Hydrodynamic lubrication system. When the shaft is at rest, there is metal-to-metal contact.

When the shaft moves at a lower speed there is still metal-to-metal contact. At higher speeds, there is complete separation between the shaft and bearing and there is no metal-to-metal contact. Therefore, as the shaft rotates in an anticlockwise direction at high speed in a bearing, it will move towards the right of the bearing making no metal-to-metal contact.

Turned Ground Nickel plated Lapped. The surfaces where the bearing fits into the machine must be very round, and the sides must be flat. The surface that the balls roll on is ground first, and then lapped. This means that a very fine abrasive slurry is used to polish the races for several hours to get almost a mirror finish. At this point, the races are finished, and ready to be put together with the balls. Explanation: The following re the different conditions for different types of bearing Sr.

A stable lubrication and there is no metal to metal contact A stable lubrication because there is some amount of metal to metal contact An unstable lubrication because there is some amount of metal to metal contact An unstable lubrication because there is no metal to metal contact.

Answer Detailed Solution Below Option 1 : A stable lubrication and there is no metal to metal contact. Explanation: Thick film lubrication: It is the condition of lubrication where two surfaces of the bearings are completely separated by the film of liquid, in this type of lubrication, the load is taken completely by the oil film.

Thick film lubrication is of two types Hydrodynamic Lubrication: In this type of lubrication the load supporting fluid film is created by the shape and relative motion of the sliding surfaces. Hydrostatic Lubrication: In this type of lubrication the load supporting fluid film is created by a external source, like a pump, supplying sufficient fluid under pressure. Answer Detailed Solution Below Option 3 : viscosity, speed and bearing pressure. Explanation: Several dimensionless numbers are used to understand the performance of bearing.

Sommerfield number : It is used to co-relate the working condition of a different machine which are operating under the same bearing.

Flat pivot bearing Flat collar bearing Conical pivot bearing Trapezoidal pivot bearing. Answer Detailed Solution Below Option 4 : to decrease intensity of pressure.

Explanation: 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 the 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 Foot step 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 foot step bearings. Collar bearings are thrust bearing having a suitably formed face or faces that resist the axial pressure or decreases the intensity of pressure of one or more collars on a rotating shaft. Sleeve bearings Hydrodynamic bearings Thin lubricated bearings Ball and roller bearings.

Concept: The antifriction bearing consists of rolling elements, races, and cage. Rolling elements are available in different shapes such as balls, parallel rollers, taper rollers, barrels, and needles. They are made of chromium or chrome-nickel steel with ground or polished surface. The load of the rotating member is carried by the rolling elements. Screw jack Aeroplane engines Crane Steering mechanism. The ball screw is also called a ball bearing screw or recirculating ballscrew.

It is a mechanical linear actuator that translates rotational motion to linear motion with little friction. It consists of a screw spindle, a nut, balls, and an integrated ball return mechanism. Ball screws are used in aircraft and missiles to move control surfaces, and in automobile power steering to translate rotary motion from an electric motor to axial motion of the steering rack.

They are also used in machine tools, robots, and precision assembly equipment. Radial load only Axial load only Both radial and axial load and the ratio of these being less than unity Both radial and axial load and the ratio of these being greater than unity.

Answer Detailed Solution Below Option 4 : Both radial and axial load and the ratio of these being greater than unity. 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. Monel metal Phosphor bronze White metal Silicon bronze.

Explanation: White metal or Babbit metal: The white metal is used for making bearings that are subjected to heavy load. The white metal is an alloy of Lead and Lithium metal. Zero film bearings. The zero film bearings are those which operate without any lubricant present. Hydrostatic or externally pressurized lubricated bearings. The hydrostatic bearings are those which can support steady loads without any relative motion between the journal and the bearing. This is achieved by forcing externally pressurized lubricant between the members.

Hydrodynamic Lubricated Bearings We have already discussed that in hydrodynamic lubricated bearings, there is a thick film of lubricant between the journal and the bearing. The load can be supported by this fluid pressure without any actual contact between the journal and bearing. The load carrying ability of a hydrodynamic bearing arises simply because a viscous fluid resists being pushed around.

Under the proper conditions, this resistance to motion will develop a pressure distribution in the lubricant film that can support a useful load. The load supporting pressure in hydrodynamic bearings arises from either: 1. The flow of a viscous fluid in a converging channel known as wedge film lubrication. The resistance of a viscous fluid to being squeezed out from between approaching surfaces known as squeeze film lubrication. Assumptions in Hydrodynamic Lubricated Bearings The following are the basic assumptions used in the theory of hydrodynamic lubricated bearings: 1.

The lubricant obeys Newton's law of viscous flow. The pressure is assumed to be constant throughout the film thickness. The lubricant is assumed to be incompressible.

The viscosity is assumed to be constant throughout the film. The flow is one dimensional, i. Important Factors for the Formation of Thick Oil Film in Hydrodynamic Lubricated Bearings According to Reynolds, the following factors are essential for the formation of a thick film of oil in hydrodynamic lubricated bearings: 1. A continuous supply of oil. A relative motion between the two surfaces in a direction approximately tangential to the surfaces.

The ability of one of the surfaces to take up a small inclination to the other surface in the direction of the relative motion. The line of action of resultant oil pressure must coincide with the line of action of the external load between the surfaces.

The most common case is that of a steady load, a fixed nonrotating bearing and a rotating journal. When the journal rotates slowly in the anticlockwise direction, as shown in Fig. In the absence of a lubricant, there will be dry metal to metal friction. If a lubricant is present in the clearance space of the bearing and journal, then a thin absorbed film of the lubricant may partly separate the surface, but a continuous fluid film completely separating the surfaces will not exist because of slow speed.

The center of the journal has moved so that the minimum film thickness is at C. It may be noted that from D to C in the direction of motion, the film is continually narrowing and hence is a converging film. The curved converging film may be considered as a wedge shaped film of a slipper bearing wrapped around the journal.

A little consideration will show that from C to D in the direction of rotation, as shown in c , the film is diverging and cannot give rise to a positive pressure or a supporting action. The following Figure 5 shows the two views of the bearing shown in c above, with the variation of pressure in the converging film. Figure 5 Lubricants The lubricants are used in bearings to reduce friction between the rubbing surfaces and to carry away the heat generated by friction.

It also protects the bearing against corrosion. All lubricants are classified into the following three groups: 1. Liquid, 2. Semi-liquid, and 3. The liquid lubricants usually used in bearings are mineral oils and synthetic oils. The mineral oils are most commonly used because of their cheapness and stability.

The liquid lubricants are usually preferred where they may be retained. A grease is a semi-liquid lubricant having higher viscosity than oils. The greases are employed where slow speed and heavy pressure exist and where oil drip from the bearing is undesirable. The solid lubricants are useful in reducing friction where oil films cannot be maintained because of pressures or temperatures.

They should be softer than materials being lubricated. A graphite is the most common of the solid lubricants either alone or mixed with oil or grease. It is the measure of degree of fluidity of a liquid. It is a physical property by virtue of which an oil is able to form, retain and offer resistance to shearing a buffer film- under heat and pressure.

The greater the heat and pressure, the greater viscosity is required of a lubricant to prevent thinning and squeezing out of the film. The fundamental meaning of viscosity may be understood by considering a flat plate moving under a force P parallel to a stationary plate, the two plates being separated by a thin film of a fluid lubricant of thickness h, as shown in Fig.