Design of Machine Elements - short answer questions from AMIE exams (Summer 2020)
Answer any four.
Clutches
A clutch is a machine member used to connect a driving shaft to a driven shaft so that the driven shaft may be started or stopped at will, without stopping the driving shaft. The use of a clutch is mostly found in automobiles.In order to change gears or to stop the vehicle, it is required that the driven shaft should stop, but the engine should continue to run. It is, therefore, necessary that the driven shaft should be disengaged from the driving shaft. The engagement and disengagement of the shafts are obtained by means of a clutch which is operated by a lever.
Following are the two main types of clutches commonly used in engineering practice:
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| Square jaw clutch |
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| Spiral jaw clutch |
Friction clutches
A friction clutch has its principal application in the transmission of the power of shafts and machines which must be started and stopped frequently. Its application is also found in cases in which power is to be delivered to machines partially or fully loaded. The force of friction is used to start the driven shaft from rest and gradually brings it up to the proper speed without excessive slipping of the friction surfaces. In automobiles, a friction clutch is used to connect the engine to the drive shaft.
Positive clutches
The positive clutches are used when a positive drive is required. The simplest type of positive clutch is a jaw or claw clutch. The jaw clutch permits one shaft to drive another through a direct contact of interlocking jaws. It consists of two halves, one of which is permanently fastened to the driving shaft by a sunk key. The other half of the clutch is movable and it is free to slide axially on the driven shaft, but it is prevented from turning relatively to its shaft by means of a feather key.
Types of welded joints
Welded joints are divided into two groups—butt joints and fillet joints.
A butt joint can be defined as a joint between two components lying approximately in the same plane. A butt joint connects the ends of the two plates.
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| Butt joint |
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| Fillet joints |
Shot peening
In a large number of applications, the external force acting on the spring fluctuates with respect to time resulting in fatigue failure. Due to the poor surface finish of the spring wire, the fatigue crack usually begins with some surface irregularity and propagates due to tensile stresses. It has been observed that the propagation of fatigue crack is always due to tensile stresses.
In order to reduce the chances of crack propagation, a layer of residual compressive stress is induced on the surface of the spring wire. One of the methods of creating such a layer is shot peening. In this process, small steel balls arc impinged on the wire surface with high velocities cither by an air blast or by centrifugal action. The balls strike against the wire surface and induce residual compressive stresses.
Power transmission elements
Types of Mechanical power transmission elements are
- Shafts & Couplings
- Power screws
- Gears & Gear trains
- Brakes & Clutches
- Belts, Ropes & Pulleys
- Chains & sprockets
- Difference between thick and thin cylinders.
See full details about these elements in our study material available at https://amiestudycircle.com
Mild steel curves for steel
The stress-strain curve for mild steel consists of strain along the x-axis and stress along the y-axis. The stress-strain curve for mild steel consists of various stages such as
- Proportional Limit: When we applied load on mild steel first point we observe Point A which is called proportionality limit because stress and strain are proportional to the applied load and it follows hook’s law and this line will be straight.
- Elastic Limit: Limiting value of stress up to which a material behaves like a perfectly elastic called elastic limit. In the figure, point B is the elastic limit point. After removal of the load material can regain its original shape if does not cross point B.
- Yield Strength: Yield strength is a phase in the curve in which extension takes place more even application of small load or negligible load. CD curve shows yield strength of mild steel.
- Ultimate Strength: Ultimate strength is the maximums strength of a material that can bear without fracture. Stress on this point is called ultimate stress. Point E is the point of ultimate strength. After point E material can break suddenly even application of small load.
- Fracture Point: Point F is the fracture point in the curve. The fracture point is a point where the strength of material breaks and the strength of this point is called rupture strength.
Difference between thick and thin cylinders
Thin Cylinder
- The cylinder whose thickness of the wall is less than 1/10 to 1/15 of its own diameter is called a thin cylinder.
- It may be noted that whenever a cylinder is subjected to an internal pressure its wall are subjected to circumferential stress or hoop stress and longitudinal stress.
- In the case of the thin cylinder, the stresses are assumed to be uniformly distributed throughout the wall thickness.
Thick Cylinder
- The cylinder thickness of wall is greater than 1/10 to 1/15 of its own diameter is known as a thick cylinder.
- The thick cylinder are generally used to withstand high pressure. Sometimes even compound thick cylinder are used to with stand very high pressure or to contain chemicals under high pressure.
- The problem of a thick cylinder is somewhat complex and is solved by using Lame's theory.
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