Fig. 1.1 Graphical Illustration of Fits and Tolerances
ILLUSTRATION OF BASIC SIZE DEVIATIONS,
LIMITS AND TOLERANCES
Actual machine size - between the maximum
and minimum limits
Maximum permissible machine size corresponding to basic
Fig. 1.1, Ø 30 mm is = Ø 30 + 0.035 = Ø 30. 035 mm.
Basic size Maximum limit
Minimum allowable machine size corresponding to basic
In Fig. 1.1, Ø 30 mm is = Ø 30 - 0.215 = Ø 29. 785 mm.
Basic size Minimum limit
The difference between the maximum and minimum limits
of a size is tolerance.
In Fig. 1.1 the tolerance is = Ø 30.035 - Ø 29.785 = 0.25m.
Max. limit Min. limit Tolerance
(Actual, or maximum, or minimum) – (Basic size).
UPPER DEVIATION - (Maximum limit) – (Basic size)
In Fig.1.1, the upper deviation is = Ø 30.035 - Ø 30
= 0.035 mm.
LOWER DEVIATION - (Minimum limit) – (Basic size)
In Fig. 1.1, the lower deviation is = Ø 29.785 - Ø 30
= - 0.215 mm.
ACTUAL DEVIATION - (Actual measured size) – (Basic size)
In Fig. 1.1, the actual deviation = Ø 29.925 - Ø 30
= - 0.075 mm.
• A straight line drawn for the basic size is called zero line.
• Upper and lower deviation are referred to zero line.
• Zero line is drawn horizontally.
• Above zero line - positive deviations.
• Below zero line - negative deviations.
The zone bounded by the upper and lower limits of the
In Fig. 1.1, tolerance zone is shown by hatched line.
• The relationship between the mating surfaces of
the parts,because of the difference in dimensions
• Thus fit has a characteristic effect on the
performance of mating parts.
• Example: width of the slot & the thickness of the
(Hole dimension) – (Dimension of shaft that fit to it).
Maximum - hole size is maximum and the shaft size is
Minimum - shaft size is maximum and the hole size is
CLASSIFICATION OF FITS
Fig. 1.2 Clearance Fit
Min hole – Max Shaft = + ve – clearance fit.
Fit with positive clearance between the hole and the shaft.
Fit with negative clearance between the hole and the shaft.
Fig. 1.3 Interference Fit
Max hole – Min Shaft = - ve – interferance fit
Fit established when the dimensions of the hole are
such that there exists either a positive clearance or a
negative clearance when the shaft is fitted into the hole.
TRANSITION FITS AND INTERFERENCE FITS
Hole and Shaft
Quality of Fit Typical uses
H6 j5 fine
Very small clearance is obtained - used for fits where a
slight interference is permissible - coupling spigots and
recesses, gear rings clamped to steel hubs.
H7 j6 normal
H8 j7 coarse
H6 k5 fine
Fits averaging no clearance where slight interference can be
tolerated , with the object of eliminating vibration - ball
bearings races of light duty.
H7 k6 normal
H8 k7 coarse
H6 m5 fine
Fits averaging slight interference - used for ball bearing
races of medium duty.
H7 m6 normal
H8 m7 coarse
H6 p5 fine
Light press fit for nonferrous parts which can be dismantled
when required-bearing bushes-press fit for steel, cast iron
or brass to steel assemblies- bush in a gear.H7 p6 normal
H6 r5 fine
Drive fit Medium drive fit for ferrous parts and light drive fit for
non-ferrous parts that can be dismantled.H7 r6 normal
H6 s5 fine
Permanent or semi permanent assemblies of steel and cast
iron with considerable gripping force- collars pressed on to
shafts, valve seatings etc. For light alloys this gives a press
H7 s6 normal
H6 u5 fine
High interference fit a thorough investigation into the
degree of grip and the stresses in the part must be made.H7 u6 normal
SYSTEMS OF FITS
HOLE BASIS SYSTEM: the hole is constant in
diameter and various types of fits are obtained by
suitably varying the limits of the shaft.
SHAFT BASIS SYSTEM: the shaft is constant in
diameter and various types of fits are obtained by
suitably varying the limits of the hole.
Fig. 1.5 Hole Basis System
HOLE BASIS SYSTEM
Single hole, whose lower deviation is zero.
Minimum limit of the hole will be equal to its basic size.
MANUFACTURING PROCESS USED
H5 Fine Boring, Fine Grinding, Honing
H6 Fine Boring, Honing, Hand Reaming
H7 Int. Grinding, Broaching, Reaming
H8 Boring, Machine Reaming
H9 Boring & Reaming
Not for diameter fits; used for milled widths,
Not used in fits; punching, coarse drilling drawn
HOLE BASIS SYSTEM - Design Example
Hole basic size - 20 mm diameter.
Clearance of - 0.100 mm.
Hole tolerance - 0.025 mm.
Shaft tolerance - 0.050 mm.
Minimum limit of the hole is Ø 20 mm.
Maximum limit of the shaft = Lower limit of the hole –
= Ø 19.900 mm
Minimum limit of the shaft = Maximum limit of the shaft –
Tolerance on the shaft.
= Ø 19. 850 mm.
Max. limit = Ø 19.900 mm.
Min. limit = Ø 19. 850 mm.
Maximum limit of the hole = Maximum limit of the hole +
Tolerance on the hole.
= Ø 20. 025 mm.
Max. limit = Ø 20.025 m.
Min. limit = Ø 20. 000 mm.
Why the Hole Basis System is Preferred?
Holes are produced by drilling, boring, reaming,
Shafts are either turned or ground.
Shaft basis system - Holes of different sizes are required,
(requires tools of different types and
Hole basis system - Only one tool is required, to
produce the hole and the shaft can
be machined to any desired size.
SYSTEMS OF TOLERANCING
1. UNILATERAL SYSTEM
• Deviations are allowed in only one direction from the
• The deviation in the other direction is zero.
• Hence, either the maximum limit or the minimum
limit will be equal to the basic size.
• Deviations are allowed in both the directions from
the basic size.
• The deviations are allowed in both the directions.
• One of the limits - above the basic size.
• Other limits - below the basic size.
2. BILATERAL SYSTEM
1. Gopalakrishna K R, “Machine Drawing”, Tenth
Edition, Bangalore, 1996.
2. Faculty of Mechanical Engineering, PSG College of
Technology, “Design Data Book”, Coimbatore, 1993.
3. ASME Y 14.5M- 1994, “Dimensioning and
Tolerancing”, ASME, New York, 1995.