Two identical parts cannot be manufactured due to variability in machines, materials & operators
Proper fit of various components ensures smooth functioning of product
Permissible tolerance in the dimensions depends on the functional requirements
Fig. 1: Effect of tolerance on production cost
“ If from a batch conforming to the same dimensions, surface finish and material
properties, any one can be selected at random to be used in place of another, with equal
probability that the selected part will assemble
and function satisfactory, then the parts in the
batch are said to be interchangeable”.
Interchangeability is controlled by workshop practices
During production checking will confirm to the required standard within the workshop
A set of tools and gauges are made so that
repeated adjustments or refinements can be
made for each components
Factors affecting the interchangeability of a component are specified by a drawing
Manufactured parts are independent of skill, tooling or knowledge within a particular
work-shop
Standard guidelines for choosing exact product dimensions within a given set of constraints
Advantage of using preferred numbers:
• Increases the probability of compatibility
• Minimize the number of different sizes to be manufactured or kept in stock.
Manufactured products are roughly equally spaced on a logarithmic scale. Some common series are:
• R5 series 101/5 1.00, 1.60, 2.50, 4.00...
• R10 series 101/10 1.0, 1.25, 1.6, 2.0...
• R20 series 101/20 1.0, 1.12, 1.25, 1.4...
• R40 series 101/40 1.0, 1.06, 1.12, 1.18...
R
10, R
20& R
40: Thickness of sheet metal or wire diameter
R
5, R
10& R
20: Speed layout in machine tools R
20& R
40: Machine tool feed
R
5series: Capacities of hydraulic press
Used when allowable difference is smaller than the normal permissible manufacturing conditions
Parts are manufactured to a wider tolerance
Components are classified into groups
Matched groups of mating parts are assembled
E.g. Assembly of ball and bearing units
Nominal Size:
• Size designation for general identification and is usually expressed in common fractions.
• Nominal size of mating shaft & hole is equal
Basic Size or Basic Dimension:
• Theoretical size from which limits of size are derived by the application of allowances and tolerances.
• Four decimal equivalent of nominal size
Actual Size:
• Measured size of the finished part.
Tolerance is:
• Total amount by which a specific dimension is permitted to vary
• Difference between the maximum and the minimum limits for the dimension.
• For Example a dimension given as 1.625 ± .002
means that the manufactured part may be 1.627” or
1.623”, or anywhere between these limit dimensions.
Fig 2: Basic size, deviations, limits & tolerances
Deviation:
Difference between the basic size and the hole or shaft size.
Upper Deviation:
Difference between the basic size and the permitted maximum size of the part .
Lower Deviation:
Difference between the basic size and the minimum
permitted size of the part.
The high limit is placed above the low limit.
In single-line note form, the low limit precedes the high limit
separated by a dash
Tolerance is required because of:
◦ Variation in the properties of the material.
◦ Inherent inaccuracies of production machines.
◦ Operator errors for e.g. inaccuracies
in settings up of machines
Wide tolerances result in reduced cost due to:
◦ Fewer defects
◦ Low tool cost: Fabrication of special tooling is easier
◦ Easy setup
◦ Less skilled and cheaper labor
◦ Lower machine maintenance
◦ Lower inspection cost
Tight tolerances result in
◦ Increased interchangeability: Also results in improved serviceability
◦ Better fits result in
Better aesthetics of assembled products
Improved functionality, durability and reliability
Unilateral Tolerance: Tolerance distribution is on only one side of the basic size
Fig 3: Unilateral tolerance designation
Bilateral Tolerance: Tolerance distribution lies on either side of the basic size
Fig 4: Bilateral tolerance designation
Unilateral system is preferred because:
◦ Easy and simple to determine tolerances
◦ GO gauge may be standardized
◦ Easier in manufacture
◦ Tolerances may be revised without altering the fit
Least Material Condition denotes:
◦ Lower limit of the Shaft
◦ Upper limit of the Hole
Maximum Material Condition denotes:
◦ Lower limit of the Hole
◦ Upper limit of the Shaft
Signifies the range of tightness or looseness that may result from the application of a specific combination of allowances and tolerances in mating parts .
Types of fits :
• Clearance fit
• Interference fit
• Transition fit
Shaft is always smaller than the hole.
Allowance is positive
Allows rotation or sliding between the mating parts
Fig. 5: Clearance fit
Loose Fit:
◦ Used when precision requirement is minimal.
◦ It provides minimum allowance
◦ E.g. loose pulleys, agricultural machineries etc.
Running Fit
◦ The dimension of shaft should be smaller enough to maintain a film of oil for lubrication.
◦ It is used in bearing pair etc.
Slide Fit or Medium Fit
◦ Used where great precision is required.
◦ Provides medium allowance
◦ E.g. tool slides, slide valve, automobile parts, etc.
Shaft is larger than the hole
Actual interference of material.
Allowance is always negative.
Fig. 6: Interference fit
Shrink Fit or Heavy Force Fit
◦ Maximum negative allowance.
◦ Used in fitting of rims etc.
Medium Force Fit
◦ Medium negative allowance.
◦ Considerable pressure is required for assembly
◦ E.g. Car wheels, armature of dynamos etc.
Tight Fit or Force Fit
◦ Assembly is done by hand hammer or light pressure.
◦ Slight negative allowance exists
◦ Gives a semipermanent fit
◦ E.g. keyed pulley and shaft, rocker arm, etc.
May result in either a clearance or interference condition .
Fig. 7: Transition fit
Push Fit or Snug Fit
◦ Zero allowance
◦ Light pressure is required in assembly
◦ Moving parts show least vibration.
Force Fit or Shrink Fit
◦ Used when the two mating parts are to be rigidly fixed so that one cannot move without the other.
◦ Assembly requires high pressure or expanding of hole by heating.
◦ It is used in railway wheels, etc.
Wringing Fit
◦ A slight negative allowance exists
◦ It is used in fixing keys, pins, etc.
Tolerance Allowance Upper limit – Lower limit
(of shaft/hole) MML of hole – MML of shaft Permissible variation of the
size/dimension Prescribed difference between two mating parts
Individual component is
involved Two mating parts are involved.
Difference between upper and
lower limits of dimensions Intentional difference between the sizes of the shaft and hole Influenced by the method of
manufacture and provided
because exact duplication is not possible
It is provided on the mating parts to get the desired
functional requirement
It is an absolute value May be positive or negative
Hole Basis System Shaft Basis System Basic hole with lower deviation
zero is chosen Basic shaft with upper deviation zero is chosen
Size of shaft is varied to get the
desired fit Size of hole is varied to get the desired fit
Limits on the hole are kept
constant Limits on the shaft are kept
constant Preferred for mass production
for economic reasons Not suitable for mass production
Easy to get the desired fit by
varying shaft size Difficult to get the desired fit by varying hole size
Gauging external features is
easy Gauging internal features is
difficult
Fig. 9: Types of fits in hole & shaft basis system
Fig. 8: Types of fits in shaft basis system
Refers to a set of tolerances that can be produced with an approximately equal production capability.
The actual total tolerance allowed within each grade depends on the nominal size of the
dimension.
Smaller tolerances can be achieved for
smaller dimensions, and vice versa.
Tolerance
Grade IT6 IT7 IT8 IT9 IT10 IT11 IT12 IT13 IT14 IT15 IT16 Tolerance 10i 16i 25i 40i 64i 100i 160i 250i 400i 640i 1000i
Std. Tolerance Unit (i)* = 0.453 (D)
1/3+ 0.001D
*(for sizes upto 500 mm)
Where
D = √ (D
max* D
min)
D
max= Upper diameter of diameter step
D
min= Lower diameter of diameter step
Above Upto Above Upto
- 3 80 120
3 6 120 180
6 10 180 250
10 18 250 315
18 30 315 400
30 50 400 500
50 80
Tolerance
Grade IT1 IT2 IT3 IT4 IT5 IT6 IT7 IT8 IT9 Tolerance 2I 2.7I 3.7I 5I 7I 10I 16I 25I 40I
Std. Tolerance Unit (I) = 0.004D + 2.1 Where
D = √ (D
max* D
min)
D
max= Upper diameter of diameter step D
min= Lower diameter of diameter step
Tolerance
Grade IT10 IT11 IT12 IT13 IT14 IT15 IT16 IT17 IT18 Tolerance 64I 100I 160I 250I 400I 640I 1000I 1600I 2500I
Above Upto Above Upto
500 630 630 800
800 1000 1000 1250
1250 1600 1600 2000
2000 2500 2500 3150
Fundamental Tolerance Application
IT01- IT4 For production of gauges & measuring instruments
IT5- IT7 Super finishing operations, precision engineering applications, grinding etc.
IT8 – IT11 Turning, Boring, Rolling, Extrusion etc.
IT12 – IT14 Sheet metal working & Press working IT15 – IT16 Casting, Stamping, Flame cutting etc.
A set of tolerances that varies according to
the basic size and provides a uniform level of
accuracy within the grade.
Fig.: Typical disposition of different types of fundamental deviations
Used to specify the tolerance and fits for mating parts.
E.g. 40 H7-g6
• First 2 digits indicate diameter in mm (40)
• Capital letter indicates fundamental deviation of hole
• Lowercase letter indicates fundamental deviation of shaft
• Numbers following letters indicate IT grade .
Calculate the limits of tolerance and allowance for the fit 25H
8d
9Determine
•
Tolerance on Hole
(Ans: 33 microns)•
Tolerance on Shaft
(Ans: 52 microns)•
Allowance
(Ans: 65 microns)•
Type of fit
(Ans: Clearance)A hole and shaft system has the following
dimensions 50H11c11. Multiplier for grade 11 is 100; Fundamental deviation for shaft c = – (95 + 0.8D) microns and diameter lies in the range 50 to 80 mm
Sketch the fit and show all dimensions on it.
Calculate the allowance and maximum &
minimum clearance/ interference
Shafts Grades Description of Fits Application
a, b, c 11 Very large clearance Generally not used d 8, 9, 10 Loose running Loose pulley
e 7, 8, 9 Loose clearance Electric Motor bearings, heavily loaded bearings
f 6, 7, 8 Normal running Lubricated bearings, pumps ad small motors, gearboxes g 5, 6 Precision running Lightly loaded shafts,
accurate bearings h 5 to 11 Extreme clearance (for
non rotating parts) Sockets & spigots of joints Preferred clearance fits: H11c11, H9d9, H7g6, C11h11, D9h9, F8h7, G7h6
Shafts Grades Description of Fits Application js 5, 6, 7 Slight clearance or slight
interference Very accurate location, couplings, gears
k 5, 6, 7 No clearance or little
interference Precision joints likely to be subjected to vibration m 5, 6, 7 Slight interference Forced assembly
n 5, 6, 7 Slight interference and
very little clearance Semi permanent or tight fit assemblies
Preferred Transition fits: H7k6, H77n6, K7h6, N7h6
Shafts Grades Description of Fits Application P 6, 7, 8 True interference (light) Fixing bushes R 5, 6, 7 Interference but can be
dismantled Keys in keyways
S 5, 6, 7 Semi permanent/
permanent fit Valve seating, Collars on shafts
T, u High degree of interference Permanent assemblies Preferred interference fits: H7p6, H7s6, P7h6, S7h6, U7h6