Definition: The maximum and minimum permissible sizes within which the actual size of a component lies are called limits.
Limits are fixed with reference to the basic size of that dimension.
Upper limit (The high limit ) for that dimension is the largest size permitted and the low limit is the smallest size permitted for that dimension.
Need for limit system:-
The correct and prolonged functioning of manufactured products depends upon its correct size relationship between various components of the assembly.
This means that the parts must fit together in a certain way.
Example: Valve Assembly
Hence purpose of limit system is to establish the types of fits and recommend the dimensions of the mating parts
Tolerance can be defined as “the permissible variation in size or dimension “of a part.
Or Tolerance is the difference between the upper limit and lower limit of a part.
The difference between upper limit and the lower limit of a dimension represents the margin for variation in workmanship, and is called a “Tolerance zone”.
NOTE: The tolerance is always a positive quantitative number.
a shaft of 25 mm basic size may be written as 25 + 0.02.
Upper limit = 25 + 0.02 = 25.02 mm
Lower limit = 25 – 0.02 = 24.98 mm
Tolerance = upper limit – lower limit
= 25.02 – 24.98 = 0.04 mm
System of writing Tolerance (Toleranced dimensions):-
There are two systems of writing tolerance:-
1. Unilateral system
2. Bilateral system
1. Unilateral system:-
When the two limit dimensions are only above or only below the nominal size (basic size) then the tolerances are said to be Unilateral.
Example: +0.03 +0.01 -0.00 -0.01
25+0.02, 25 +0.00 , 25 -0.01 , 25 -0.02 etc…
2. Bilateral system:-
When the limit dimensions are given above and below the nominal size (basic size) then the tolerances are said to be bilateral.
Example: 25+0.02 , 25 +0.00 etc…
Note:- Unilateral tolerance is preferred over bilateral tolerances because the operator can machine to the upper limit of the shaft (or lower limit of the hole) still having the whole tolerance left for machining before the parts are rejected.-
It is easy and simpler to determine deviations
GO gauge end can be standardized as the holes of different tolerance grades have the same lower limit and all the shafts have same upper limit.
Relationship between Tolerance and Cost: –
If the tolerance are made closer and closer, the cost of production goes on increasing, because to manufacture the component with closer tolerance.
A compound tolerance one which is derived by considering the effect of tolerance on more than one dimension.
Limits of Size:-
In deciding the limits for a particular dimension it is necessary to consider following.-
1. Functional requirements: the intended function that a component should perform
2. Interchangeability: replacements of the component in case of failure/ damage without difficulty
3. Economy in production time and cost.
Thus degree of tolerance provided on the mating components calls for a compromise.
Number of standards on limit and fit systems has been published to help the designer in selecting the uniform limits and fits.
Maximum and minimum Metal Limits
(Or Maximum and Minimum Metal conditions): –
If the tolerance for the shaft is given as 25+0.05, the upper limit will be 25.05 mm and the lower limit will be 24.94 mm.
The shaft is said to be have Maximum Metal Limit (MML) of 25.05mm, since at this limit the shaft has maximum possible amount of metal.
The limit of 24.95 will then be the minimum or “Least metal Limit” (LML) because at this the shaft will have the least possible amount of metal.
(Terminologies used in Limits and Fits)
The term shaft refers not only to the diameter of a circular shaft but also to any external dimension of a component.
The term shaft not only refers to the diameter of the circular hole but also any internal dimension of a component.
When an assembly is made of two parts, one is known as male-surface and the other mating part as female (enveloping) surface.
The male surface is called as shaft and the female surface is called as hole.
Basic Size or Nominal Size:-
It is the standard size of a part in relation to which all limits of variation are determined. the basic size is same for hole and shaft.
actual size is the dimension as measured on manufacturing part.
it is straight line drawn horizontally to represent the basic size. In the graphical representation of limits and fits, all the deviations are shown with respect to the zero line (datum line).
The positive deviations are shown above zero line and negative deviation are shown below zero line.
deviation is the algebraic difference between the size (actual, maximum, etc) and the corresponding basic size.
it is the algebraic difference between the upper (maximum) limit of size and the corresponding basic size.
It is positive quantity when the upper limit of size is greater than the basic size and negative quantity when the upper limit of the size less than the basic size.
It is denoted by „ES‟ for hole and „es‟ for shaft.
it is it is the algebraic difference between the lower (minimum) limit of
size and the corresponding basic size.
It is positive quantity when the lower limit of size is greater than the basic size and negative quantity when the lower limit of the size less than the basic size.
It is denoted by „EI‟ for hole and „ei‟ for shaft.
either the upper or lower deviation, which is the nearest one to the zero line for either a hole or a shaft.
It fixes the position of Tolerance zone in relation to the zero line.
basic shaft is the shaft whose upper deviation is zero. Thus upper limit of the basic shaft is the same as the basic size. It is denoted by letter „h‟.
basic hole is the hole whose lower deviation is zero. Thus lower limit of the basic hole is the same as the basic size. It is denoted by letter „H‟.
Definition of fits, types of fits :-
fit may be defined as a degree of tightness or looseness between two mating parts to perform a definite function when they are assembled together.
Accordingly, a fit may result either in a moveable joint or a fixed joint.
For example: a shaft running in a bearing can move in relation to it and thus forms a moveable joint, whereas, a pulley mounted on the shaft forms a fixed joint.
Types of fits (Classification of fits):-
On the basis of positive, zero and negative values of clearance, there are three types of fits:-