Questions and answers (Machine Design)
Q1.
What is meant by tolerance? How many types of tolerance is there?
A1. Tolerance is
the difference between maximum and minimum dimensions of a component, i.e.,
between upper limit and lower limit. Depending on the type Of application, the
permissible variation of dimension is set as per available Standard grades.
Tolerance is of two types, bilateral and unilateral. When Tolerance is present
on both sides of nominal size, it is termed as bilateral; Unilateral has
tolerance only on one side.
Q2.
What are the types fit? Describe the differences.
A2. The nature of
assembly of two mating parts is defined by three types of fit System, Clearance
Fit, Transition Fit and Interference Fit. Clearance Fit: In this type of fit,
the shaft of largest possible diameter can be fitted easily in the hole of
smallest possible diameter.
Interference
Fit :
In this type of fit, irrespective of tolerance grade there is always a
overlapping of the matting parts.
Transition
Fit:
In this case, a clearance is present between the minimum dimension of the shaft
and the minimum dimension of the hole. However, the fit is tight, if the shaft
dimension is maximum and the hole dimension is minimum. Hence, transition fit
have both the characteristics of clearance fit and interference fit.
Q3.
What are preferred numbers?
A3. Preferred numbers
are the numbers belonging to four categories of geometric progression series,
called basic series, having common ratio of, 5 10 ≈1.58, 1010 ≈1.26, 2010 ≈1.12
and 4010 ≈1.06 Preferred numbers of derived series are formed by multiplying or
dividing the basic series by 10, 100 etc. These numbers are used to build-up or
manufacture a product range. The range of operational speeds of a machine or
the range of powers of a typical machine may be also as per a series of
preferred numbers.
Questions and answers(Spring)
Q1.
What are the objectives of a spring?
A1. The objectives of a
spring are to cushion, absorb, or controlling of energy arising due to shock
and vibration. It is also used for control of motion, storing of energy and for
the purpose of measuring forces.
Q2.
What is the curvature effect in a helical spring? How does it vary with spring index?
A2. For springs where
the wire diameter is comparable with the coil diameter, in a Given segment of
the spring, the inside length of the spring segment is relatively shorter than
the outside length. Hence, for a given magnitude of torsion, shearing strain is
more in the inner segment than the outer segment. This unequal shearing strain
is called the curvature effect. Curvature effect decreases with the increase in
spring index.
Q3.
What are the major stresses in a helical spring?
A3. The major
stresses in a helical spring are of two types, shear stress due to torsion and
direct shear due to applied load.
Q4.
Do the helical spring experience reverse loading? What is the loading type called
when varying load acts on a helical spring?
A4. The helical spring
experiences only repeated load. It cannot experience reverse loading, because
the spring will lose contact with the end supports.
Q5.
What modification in Soderberg diagram is required when it is used for design of
helical springs?
A5. In the earlier
Soderberg diagram, we have used in the design for varying loads on the machine
member, had only stress amplitude in the endurance limit representation, since,
endurance limit value was for complete reversed loading. Here, in spring
design, we use endurance limit value for repeated loads only. Hence, we have
both stress amplitude and mean stress value of equal magnitude,
te/2 . Therefore, the
endurance limit representation in Soderberg diagram changes to
te/2.te/2
Q6.
What should be the safe frequency of a helical spring?
Ans6- Safe frequency for
design should be at least 20 times less than the fundamental frequency of the
spring to take care of more number of harmonics.
Q7.
What are the forms of leaf Spring ?
A7. Leaf springs are of two forms: cantilever and
simply supported type.
Q8.
What does the term “uniform strength” in the context of leaf spring mean?
A8. If the leaf spring has a shape of uniformly
varying width (say Lozenge shape) then
the bending stress at all section remains uniform. The situation is also identical as before in case of varying
thickness, the thickness should vary non-uniformly
with length to make a beam of uniform strength (L/h2 = constant). These leaves require lesser material, have more
resilience compared to a constant
width leaf. These types of springs are called leaf springs of uniform strength.
Q9.
What is “nipping” in a laminated spring?
A9. In general the differential curvature between
the master leaf and the next leaves is provided in a laminated spring, where,
radius of curvature being more for the master leaf. This construction reduces
the stress in the master leaf as compared to the other leaves of the spring in
a laminated spring. This type of constructional feature is termed as nipping.
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