2. Design of Machine Elements - Limits and Fits.pptx

Design of Machine Elements
Dr. G.Praveen Kumar
Assistant Professor
Mechanical Engineering Department
IIITDM Kurnool
25/07/22 Dr. G.Praveen Kumar, IIITDM Kurnool 1

Design
of
Machine
Elements

• Whenever we are going to design an element we need decide various
aspects like
• Shape
• Dimension
• Material, etc.
• Ultimately it should be able to successfully manufactured to give appropriate
shape to the product as per the design
• Will discuss on ideas of manufacturing
Design and Manufacturing

•Concept of limits and fits
•Preferred numbers
•Various manufacturing processes

• First thing we need to think while designing a machining element is its
size
• E.g. if a shaft is designed to have a diameter of 20 mm. Means
manufacturing department should be able to make a shaft of 20 mm
• However it is very difficult to make a shaft with diameter exactly 20
mm
• Hence, while designing a tolerance band is defined
• This in general refers to as limits and fits.
•

Fig. 1. Interrelationship between tolerances and limits
• Figure 1 explains the terminologies
used in defining tolerance and
limit
• For the convenience, shaft and
hole are chosen to be two mating
components
• The zero line is the basic size or
the nominal size.

Tolerance:
• Tolerance is the difference
between maximum and minimum
dimensions of a component
• i.e. between upper limit and lower
limit
• Tolerance is of two types,
• Bilateral
• Unilateral
• When tolerance is present on
• both sides of nominal size, it is
termed as bilateral;
• Unilateral has tolerance only on
one side. Fig. 2 Types of tolerance

Allowance :
• It is the difference of dimension between two
mating parts.
Upper deviation:
• It is the difference of dimension between the
maximum possible size of the component and its
nominal size.
Lower deviation
• Similarly, it is the difference of dimension between
the minimum possible size of the component and
its nominal size.
Fundamental deviation
• It defines the location of the tolerance zone with
respect to the nominal size.
• It is the nearest deviation from the basic size.

• A machine part when manufactured has a specified tolerance
• When two mating parts fit with each other, the nature of fit is dependent
on the limits of tolerances and fundamental deviations of the mating parts
• The degree of tightness or looseness between the two mating parts is
known as a fit of the parts.
• The nature of assembly of two mating parts is defined by three types of fit
system,
• Clearance Fit
• Transition Fit
• Interference Fit
Design and Manufacturing – Fit System

Fig. 3. Schematic view of Fit system

CLEARANCE FIT : In this type of fit, the largest permitted shaft diameter is less than the
smallest hole diameter so that the shaft can rotate or slide
according to the purpose of the assembly.

INTERFERENCE FIT: It is defined as the fit established when a negative
clearance exists between the sizes of holes and the shaft.
In this type of fit, the minimum permitted diameter of
the shaft is larger than the maximum allowable diameter
of the hole.
In case of this type of fit, the members are intended to be
permanently attached.
Ex: Bearing bushes, Keys & key ways

TRANSITION FIT : In this type of fit, the diameter of the largest
allowable hole is greater than the smallest
shaft, but the smallest hole is smaller than the
largest shaft, such that a small positive or
negative clearance exists between the shaft &
hole.
Ex: Coupling rings, Spigot in mating holes, etc.

• Two ways of representing a fit system
• Hole basis system
• Shaft basis system
• Hole basis system
• The dimension of the hole is considered to be the datum
• Shaft basis system
• Dimension of the shaft is considered to be the datum
• The holes are normally made by drilling, followed by reaming. Therefore, the dimension
of a hole is fixed due to the nature of the tool used.
• However, the dimension of a shaft is easily controllable by standard manufacturing
processes.
• Hence, the hole basis system is much more popular than the shaft basis system

If the hole basis system is used, there will be reduction in
production costs as only one tool is required to produce the Hole
and the shaft can be easily machined to any desired size. Hence
hole basis system is preferred over shaft basis system
Hole basis system

Shaft basis system
In this system, the basic diameter of the shaft is constant while the hole size is varied
according to the type of fit. It may, however, be necessary to use shaft basis system where
different fits are required along a long shaft.
For example, in the case of driving shafts where a single shaft may have to accommodate to
a variety of accessories such as couplings, bearings, collars, etc.,
it is preferable to maintain a constant diameter for the permanent member, which is the
shaft, and vary the bore of the accessories

• Basic shat and Basic hole: The shafts and holes that have zero fundamental
deviations. The basic hole has zero lower deviation and the basic shaft has zero
upper deviation
• Hole designation: By upper case letters from A, B, ……, Z, Za, Zb, Zc (excluding I, L,
O, Q, W and adding Js, Za, Zb, Zc) – 25 numbers
• Shaft designation: By lower case letters from a, b, ….. z, za, zb, zc (excluding i, l, o,
q, w and adding js, za, zb, zc) – 25 numbers

• Figure shows the schematic view of a standard limit and fit system
• Tolerance is denoted as IT and it has 18
grades( IT01,IT0 and IT1 to IT16)
• Greater the number, more is the
tolerance limit
• The fundamental deviations for the hole
are denoted by capital letters from A and
ZC, having altogether 25 divisions.
• Similarly, the fundamental deviations for
the shaft is denoted by small letters from
a to zc
• Here H or h is a typical case, where the
fundamental deviation is zero having an
unilateral tolerance of a specified IT
grade.

• The values of standard tolerances and fundamental deviations can be obtained
from design hand book
• The choice of tolerance grade is related to the type of manufacturing process;
• For example, attainable tolerance grade for lapping process is lower compared to
plain milling
• Choice of fundamental deviation largely depends on the nature of fit, running fit
or tight fit etc.
• Manufacturing processes involving lower tolerance grade are generally costly

Fig. Typical zones of fit

Prob1: The main bearing of an engine is shown in Fig. 1. Calculate (i) the
maximum and minimum diameters of the bush and crank pin; and (ii) the
maximum and minimum clearances between the crank pin and bush. Suggest
suitable machining methods for both.

Prob2: The valve seat fitted inside the housing of a pump is shown in Fig. 2.
Calculate (i) the maximum and minimum diameters of the housing and valve
seat; and (ii) the magnitude of the maximum and minimum interferences
between the seat and housing.

Prob3: The valve seat fitted inside the housing of a pump is shown in Fig. 3.
Calculate (i) the maximum and minimum diameters of the housing and valve
seat; and (ii) the magnitude of the maximum and minimum interferences
between the seat and housing.

From Tables 3.2 and 3.3b, the tolerances for the small end of connecting rod and bush are as follows:
Connecting rod (inner diameter) (15H6) = 15.000 15.011 mm Bush (outer diameter) (15r5) = 15.023 15.031 mm
Maximum interference = 15.031 – 15 = 0.031 mm Minimum interference = 15.023 - 15.011 = 0.012 mm

Prob4: The exhaust valve of an IC engine is shown in Fig. 4 There is a clearance
fit between the valve stem and its guide and an interference fit between the
valve seat and its housing. Determine (i) diameters of the valve stem (ii) inner
diameters of guide for valve stem (iii) the clearances between the stem and
guide (iv) diameters of the valve seat (v) inner diameters of housing of the valve
seat (vi) the interferences between the valve seat and its housing.

From Tables 3.2 and 3.3a,
Limiting dimensions of valve stem (5d8) = 4.952 4.970 mm
Limiting dimensions of guide for valve stem (5H7) = 5.000 5.012 mm
Maximum clearance = 5.012 - 4.952 = 0.06 mm Minimum clearance = 5 - 4.97 = 0.03 mm From
Tables 3.2 and 3.3b, Limiting dimensions of valve seat (20s5) = 20.035 20.044 mm
Limiting dimensions of housing (20H6) = 20.000 20.013 mm
Maximum interference = 20.044 – 20 = 0.044mm Minimum interference = 20.035 – 20.013 = 0.022 mm

Preferred numbers (Renard series)
• A designed product needs standardization
• It means that some of its important specified parameter should be common in
nature
• E.g., the sizes of the ingots available in the market have standard sizes. A
manufacturer does not produce ingots of sizes of his wish, he follows a definite
pattern and for that matter designer can choose the dimensions from those
standard available sizes.
• Motor speed, engine power of a tractor, machine tool speed and feed, all follow a
definite pattern or series
• This also helps in interchangeability of products

Preferred numbers (Renard series)
• It has been observed that if the sizes are put in the form of geometric
progression, then wide ranges are covered with a definite sequence
• These numbers are called preferred numbers having common ratios

Typical values of the common ratio for four basic G.P. series are given below

Examples for preferred number

The types of common manufacturing processes are
Common manufacturing processes

The types of shaping processes are
The types of machining processes are

The types of joining processes are
The types of surface finishing
processes are

The non-conventional machining processes are

2. Design of Machine Elements - Limits and Fits.pptx

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