In its simples form this type of pendulum has a heavy bob, suspended at the end of a light inextensible, flexible string and the other end of the string is rigidly fixed to wall. 'm' is the mass of the bob and 'l' is the length of string. Following laws of simple pendulum are important.
Hence the time period t = 2 pÖ l / g
Belt
Drives:
Belts are used to transmit power from one shaft to another by
means of a pulley. When the driver rotates, it carries the belt due to
grip between its surface and the belt. The belt in turn carries, the
driven pulley which starts rotating. The grip between the pulley and the
belt is obtained by friction. This friction grip if required is increased
by tightening the belt. The amount of power transmitted depends upon
|
S. No |
Terms |
Definition |
| 1 |
Pitch Circle |
A imaginary circle which by pure rolling action
would give the same motion as the actual gear. |
| 2 |
Pitch Circle Diameter |
The diameter of Pitch circle. Gears are specified
by this PCD. |
| 3 |
Addendum |
The radial distance between the PCD and top of
tooth. Addendum circle is drawn through the top of teeth and concentric to
PC. |
| 4 |
Deddendum |
The radial distance been the PC and bottom of
teeth. |
| 5 |
Circular Pitch |
The distance measured from the circumference of
the pitch circle from a point in one tooth, to the corresponding point in
next tooth. |
| 6 |
Module |
it is the ratio of PCD to the number of
teeth. |
| 7 |
Diametrical pitch |
Inverse of Module. |
| 8 |
Total depth |
Addendum + Deddendum |
| 9 |
Tooth Thickness |
Width of the tooth measured along pitch
circle. |
| 10 |
Face of Tooth |
Surface of gear above the pitch
surface. |
| 11 |
Flank of Tooth |
Surface of gear above the pitch
surface. |
The main principle of working of the centrifugal governor is based upon the balancing of centrifugal force on the rotating balls by an equal and opposite radial force, known as the controlling force.
- Pendulum Type - Watt governor
- Load type
- Dead weight governors - Proel and portel governors.
- Spring controlled governors - Hartnell, hartung governors, Wilson-hartnell and Pickering governors.
Inertial governors :
They operate on different principle .Governor balls are arranged that the inertia forces caused by an angular acceleration & retardation of the governor shaft tend to alter their position. The obvious advantage of this type of governor is quick response to load variation .This advantage is offset however by the practical difficulty of arranging for complete balance of the revolving parts of the governor.
Equilibrium speed: Is the speed at which the governor balls are at
complete equilibrium and the sleeve does not tend to move up or
down.
Sensitiveness: If there is more displacement in sleeve for
the same speed then the governor is said to be sensitive. It is equal
to
2 (
N2 - N1 ) / ( N2 + N 1
)
Hunting: In this the governor switches between the maximum and
minimum position.
Lubrication :
The different types of lubrication between two surfaces having relative motion can be classified as
Fluid film lubrication
Boundary lubrication
Extreme boundary lubrication
Surface contact of the sliding members
Fluid film lubrication:
In this the moving or sliding surfaces are separated from each other by a thick film of fluid which is at least 1000 angstrom thick so that direct surface-to-surface contact and welding of junctions rarely occurs. This is also called as hydrodynamic lubrication.
Fluid friction is considerably less than metallic friction and under such circumstances, the viscosity of the fluid plays an important role in the design parameters of the bearing. It is very clear that the lubricant chosen should have the minimum viscosity under the working conditions. At the same time it should remain in place and separate the surfaces. The co-efficient of friction in such cases should be as low as 0.001 to 0.03. Fluid film lubrication prevails when there is high relative velocity between sliding surfaces. In a journal bearing the lubricating oil covers the irregularities of shaft as well as the bearing surfaces and the metal surfaces do not come into direct contact with each other.
Boundary lubrication:
This is also called as thin film lubrication. when t he relative velocity between the two sliding surfaces is very low, the fluid film will not be able to support the total load and under such circumstances boundary lubrication is done.
The clearance space between the moving surface is lubricated with a lubricating oil., a thin layer of which is adsorbed, on both the metallic surfaces. These layers avoids direct metal-to-metal contact. The value of co-efficient of friction is usually, 0.05 to 0.15.
The friction phenomena in this case is complicated and no exact theory is available for boundary lubricated bearings. However it has been found that certain metals and lubricants with less friction compared to others have the same viscosity improve the performance of such bearings.
Extreme boundary lubrication:
When the moving surfaces are under very high pressure and speed, a high local temperature is attained and under such condition the fluid film is completely broken because of decomposition or vapoursation and there is direct metal to metal contact at that high spots of the sliding materials. High load and speed in turn generates heat with the following mentioned effects.
Welded junction and metal tearing.
Deformation and seizure of surfaces.
Change in physical and chemical properties of metals and lubricants that renders lubricants ineffective.
The mechanism of this type of lubrication is given below. Special additives that are capable of withstanding very high load and temperatures are added to the lubricants. These additives react with metallic surfaces at prevailing high temperatures to form metallic chlorides, sulphides or phosphides. These metallic compound posses high melting points and serve as good lubricant under high temperature and pressure conditions. If by chances, the low shear strength films are broken by the rubbing action of moving parts, they are immediately replenished. Since a chemical reaction takes place in this lubrication, the metal surface under goes certain wear.
The function of lubricant is to reduce the loss of energy, to reduce surface deformation, war and tear, to increase the efficiency of engine, to reduce the frictional heat and thus prevents the expansion of metals, to reduce the maintenance cost of a machine etc.
Application of Liquid Lubricants:
Mechanical devices to supply lubricants are called lubricators. A simple form of lubricator is a container mounted over a bearing or other part and provided with a hole or an adjustable valve through which the lubricant is gravity-fed at the desired rate of flow.
Wick-feed oilers are placed under moving parts, and by pressing against them they feed oil by capillary action.
Horizontal bearings are frequently oiled by a rotating ring or chain that carries oil from a reservoir in the bearing housing and distributes it along the bearing through grooves or channels.
Bath oiling is useful where an oil-tight reservoir can be provided in which the bearing journal may be submerged; the pool of oil helps to carry away heat from contact surfaces.
Splash-oiling devices are used where gears, bearings, or other parts contained in housings have moving parts that dip into the lubricant and splash it on the bearings or into distribution channels.
Centralized oiling systems usually consist of a reservoir, pump, and tubes through which oil is circulated, while heaters or coolers may be introduced to change the viscosity of the lubricant for various parts of the system. Many oiling operations are automatically synchronized to start and stop with the machinery.
Application of Semisolid and Solid Lubricants:
Grease lubricants are semisolid and have several important advantages. They resist being squeezed out, they are useful under heavy load conditions and in inaccessible parts where the supply of lubricant cannot easily be renewed, and they tend to form a crust that prevents the entry of dirt or grit between contact surfaces. Grease is a mixture of a lubricant and a thickener; often it is made from a mineral oil and a soap. It may be applied in various ways: by packing enclosed parts with it, by pressing it onto moving parts from an adjacent well, by forcing it through grease cups by a spring device, and by pumping it through pressure guns. Solid lubricants are especially useful at high and low temperatures, in high vacuums, and in other applications where oil is not suitable; common solid lubricants are graphite and molybdenum disulfide.
CAM:
Is a rotating machine element which transmits oscillating or
reciprocating motion to the follower. They have line contact and constitutes
higher pair. According to the type of follower they are classified as
Last updated on Wednesday, December 17, 2003 , 08:30 PM