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JNTU B.Tech IV DESIGN OF AGRICULTURAL MACHINERY Previous Question Paper (September - 2020 - R15)
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Discuss about different theories of failure under static load.
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Theories of failure are essential for predicting the safe limits of materials under static loading conditions. They provide criteria to determine whether a component will undergo permanent deformation or fracture based on the state of stress at a point.
The primary theories of failure are:
- Maximum Principal Stress Theory (Rankine's Theory): Failure occurs when the maximum principal stress reaches the yield strength of the material in simple tension. This is suitable for brittle materials where failure is caused by fracture. The condition is σ1 = σyt.
- Maximum Principal Strain Theory (Saint-Venant's Theory): Failure occurs when the maximum principal strain reaches the strain at the yield point in simple tension. This theory is rarely used in modern practice but is sometimes applied to brittle materials. The condition is ε1 = σyt / E.
- Maximum Shear Stress Theory (Guest's or Tresca's Theory): Failure occurs when the maximum shear stress in the component reaches the maximum shear stress at the yield point in a tension test. This is widely used for ductile materials. The condition is τmax = (σ1 - σ2) / 2 = σyt / 2.
- Maximum Strain Energy Theory (Haigh's Theory): Failure occurs when the total strain energy per unit volume reaches the strain energy per unit volume at the yield point in simple tension. This theory is applicable to ductile materials.
- Maximum Distortion Energy Theory (Von Mises-Hencky Theory): Failure occurs when the distortion energy per unit volume exceeds the distortion energy at the yield point in simple tension. This is considered the most accurate theory for ductile materials and is widely used in engineering design. The condition is (σ1 - σ2)2 + (σ2 - σ3)2 + (σ3 - σ1)2 = 2σyt2.
In agricultural machinery design, ductile materials like mild steel are commonly used for structural frames and linkages, making the Maximum Shear Stress and Maximum Distortion Energy theories the most relevant for ensuring safety against permanent deformation.
Briefly discuss about different terms used in limit system with a neat sketch.
Answer
In the limit system, the following terms are essential for defining fits between mating parts:
- Basic Size: The size to which limits or deviations are assigned.
- Tolerance: The difference between the maximum and minimum permissible limits of a size.
- Upper Deviation: The algebraic difference between the maximum limit of size and the basic size.
- Lower Deviation: The algebraic difference between the minimum limit of size and the basic size.
- Zero Line: The reference line representing the basic size from which deviations are measured.
A sketch would show the Zero Line, with the tolerance zone for a hole or shaft represented as a rectangle extending between the upper and lower limits relative to the basic size.
Design a cotter joint to support a load varying from 30 kN in compression to 30 kN in tension. The material used is carbon steel for which the following allowable stresses may be used. The load is applied statically. Tensile stress = compressive stress = 50 MPa; shear stress = 35 MPa and crushing stress = 90 MPa.
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Write the design procedure of cranked level with a neat sketch.
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A helical spring is made from a wire of 6 mm diameter and has outside diameter of 75 mm. If the permissible shear stress is 350 MPa and modulus of rigidity 84 kN/mm2, find the axial load which the spring can carry and the deflection per active turn.
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Write about different types of keys with neat sketches.
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A mild steel shaft transmits 50 kW at 500 rpm. It carries a central load of 1000 N and is simply supported between the bearings 3m apart. Determine the size of the shaft; if the allowable shear tress is 50 MPa and the maximum tensile or compressive stress is not to exceed 60 MPa. What size of the shaft will be required, if it is subjected to gradually applied loads. Take Km, Kt values are 1.5 and 1.0.
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Derive the equations for equivalent twisting and bending moments, if shafts are subjected to combined twisting and bending moments.
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Explain about fluctuation of energy of four stroke IC engine with a neat diagram.
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Write the step wise procedure for calibration of seed drill.
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Total draft of four bottom, 35 cm MB plough when ploughing 18 cm deep at 5 kmph speed is 1600 kg. (i) Calculate the unit draft in kg/cm2 (ii) What is actual power requirement? (iii) If the field efficiency is 75% what is the rate of doing work in ha/hr.
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Write about of types of furrow openers used in seed drills
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A tractor mounted row crop sprayer with boom having 30 x 40 cm nozzles is moving at a speed of 4.2 km/hr. The nozzle of the sprayer are set at a pressure of 7.4 kg/cm2 for an application rate of 1000 l/ha. The nozzles fitted on the sprayer are hallow cone type with spray angle of 750. Calculate the height of the spray boom from the deposition level so that each nozzle spray pattern at deposit level is 30% greater than the nozzle spacing.
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