Chapter 13
SYSTEM OF FITS AND TOLERANCES
The standard reference temperature is 20 C for industrial measurements and, consequently, for dimensions defined by the system.
Due to the inevitable inaccuracy of manufacturing methods, a part cannot be made precisely to a given dimension, the difference between maximum and minimum limits of size is the tolerance.
When two parts are to be assembled, the relation resulting from the difference between their sizes before assembly is called a fit (The degree of tightness or looseness between two mating parts )
Tolerance
• How to decide tolerance?
– Functional requirements of mating parts
– Cost of production
– Available manufacturing process
• Choose as coarse tolerance as possible without
compromising functional requirements
Max Hole size – Basic Size = Upper Deviation Min Hole size – Basic Size = Lower Deviation
HOLE
Max shaft size – Basic Size = Upper Deviation
Min shaft size – Basic Size = Lower Deviation
CLEARANCE FIT (LOOSENESS)
Maximum shaft dimension < Minimum hole dimension
INTERFERANCE FIT (TIGHTNESS)
TRANSITION FIT
Obtained by overlapping of tolerance zones of shaft and hole …… Does not guarantee neither clearance nor interference fit
HOLE BASED
SYSTEM-Size of hole is kept constant, shaft size is varied to get different fits. Hole based system is recommended for general use
To obtain different types of fits, it is general practice
to vary tolerance zone of one of the mating parts
SHAFT BASED
SYSTEM-Size of shaft is kept constant, hole size is varied to get
different fits. Shaft based system is sometimes
required, especially when two or more fits are required on the same shaft
FIT PRESENTATION
A fit is indicated by the basic size common to both components, followed by symbol corresponding to each component, the hole being quoted first.
FIT PRESENTATION
SYSTEM-H : lower deviation of hole is zero
h : upper deviation of shaft is zero
E.S. – upper deviation E.I. – lower deviation
Representation of
Tolerance
1) Letter Symbol
The selection of letter freezes one limit of hole / shaft
(how much away from Basic size)
45 E8/e7
Basic Size
One can have different possible combinations; eg. 45H6g7,
Representation of Tolerance
2) Number or Grade
IT01, IT0, IT1,….IT16
Tolerance Grade defines range of dimensions (dimensional variation) There are manufacturing
constraints on tolerance grade chosen
H : lower deviation of hole is zero
Representation of
Tolerance
The selection of
Tolerance grade number freezes the other limit of hole / shaft
Together (Letter & Grade) on both mating components
decide quality of fit
H7 : Tol Grade 7 mean 21μ variation (H means upper deviation zero)
Representation of Fit
0.021 0.022 0.013 Φ30.000 Φ30.021 Φ30.022 Φ30.035
p6 : Tol Grade 6 means 13μ variation (p means upper deviation is 22 μ)
Estimate kind of fit
Φ 30 H 7s 6 Φ 30 H 8e 6 Φ 30 F 8r 6 Φ 30 G 7r 6FITS
Clearance Fits
- Loose running fit for wide commercial tolerances or allowances on
external members. Hole Basis - H11/c11, Shaft Basis - C11/h11
- Free running fit not for use where accuracy is essential, but good for
large temperature variations, high running speeds, or heavy journal
pressures. Hole Basis - H9/d9, Shaft Basis - D9/h9
- Close Running fit for running on accurate machines and for accurate
moderate speeds and journal pressures. Hole Basis - H8/f7, Shaft Basis
- F8/h7
- Sliding fit not intended to run freely, but to move and turn freely and
locate accurately. Hole Basis - H7/g6, Shaft Basis - G7/h6
- Locational clearance fit provides snug fit for locating stationary parts;
but can be freely assembled and disassembled. H7/h6, Shaft Basis -
H7/h6
Selection of Preferred Fits for Your Applications
– Hole and Shaft Basis
Transition Fits
- Locational transition fit for accurate location, a compromise between
clearance and interference. Hole Basis - H7/k6, Shaft Basis - K7/h6
- Locational transition fit for more accurate location where greater
interferance is permissible. Hole Basis - H7/n6, Shaft Basis - N7/h6
Interference Fits
- Locational interference fit for parts requiring rigidity and alignment
with prime accuracy of location but without special bore pressure
requirements. Hole Basis - H7/p6, Shaft Basis - P7/h6
- Medium drive fit for ordinary steel parts or shrink fits on light sections,
the tightest fit usable with cast iron. Hole Basis - H7/s6, Shaft Basis -
S7/h6
- Force fit suitable for parts which can be highly stressed or for shrink
fits where the heavy pressing forces required are impractical. Hole
Basis - H7/u6, Shaft Basis - U7/h6
Selection of Preferred Fits for Your Applications
– Hole and Shaft Basis
PRACTICAL
FITS AND TOLERANCES
The components of the toleranced dimension shall be indicated in the following order:
a) the basic size, and b) the tolerance symbol.
If, in addition to the symbols it is necessary to express the values of the deviations or the limits of size, the additional information shall be shown in brackets.
Permissible deviation
•If a dimension needs to be limited in one
direction only, this should be indicated by adding “min” or “max” to the dimension.
•The upper deviation or the upper limit of size
shall be written in the upper position and the lower deviation or the lower limit of size in the lower
position, irrespective of whether a hole or a shaft is toleranced.
•The tolerance symbol for the hole shall be placed before that for the shaft or above it, the symbols being preceded by the basic size indicated once only.
