Fits and Tolerances
Dr. manal amin
ENG 111
Tolerancing – Control of Variability
 Goals
 Understand the description and control of variability through
tolerancing.
 Use standard tables for tolerancing and control of fit
Gateway Engineering Education Coalition
Definition of Tolerance
 Tolerance is the total amount a dimension may vary. It is the
difference between the maximum and minimum limits.
 There is no such thing as an "exact size".
 Tolerance is key to interchangeable parts.
Gateway Engineering Education Coalition
Direct Limits and Tolerance Values
Can be:
Limits: Upper limit – 3.53
Lower limit – 3.49
Unilateral – vary in only one direction
3.49
+.04
0
0
-.0X
+.0X - 0
Bilateral – vary larger or smaller (may
or may not be same amount)
3.50
+.03
-.01
+.05 -.01, +.10 -.20
Gateway Engineering Education Coalition
+/- 0.05
Important Terms – Single Part
 Nominal Size – a general size, usually expressed as a common




fraction (1/2”)
Basic Size – theoretical size used as starting point (.500”)
Actual Size – measured size (.501”)
Limits – maximum and minimum sizes shown by tolerances
Tolerance – total allowable variance in dimensions (upper
limit – lower limit)
Gateway Engineering Education Coalition
Important Terms – Single Part
Gateway Engineering Education Coalition
Important Terms – Multiple Parts
 Allowance – the minimum clearance or maximum
interference between parts
 Fit – degree of tightness between two parts
 Clearance Fit – tolerance of mating parts always leave a space
 Interference Fit – tolerance of mating parts always interfere
 Transition Fit – sometimes interfere, sometimes clear
 Tolerance – total allowable variance in dimensions (upper
limit – lower limit)
Gateway Engineering Education Coalition
Fitting Two Parts
Tolerance of B
Part B
Tolerance: Clearance or
Interference
Tolerance of A
Part A
Gateway Engineering Education Coalition
Shaft and Hole Fits
Clearance
Interference
Gateway Engineering Education Coalition
Shaft and Hole Fits
Transition
CLEARANCE FIT
+ .003
Gateway Engineering Education Coalition
Standard Precision Fits: English Units
 Running and sliding fits (RC)
 Clearance locational fits (LC)
 Transition locational fits (LT)
 Interference locational fits (LN)
 Force and shrink fits (FN)
Gateway Engineering Education Coalition
Gateway Engineering Education Coalition
Gateway Engineering Education Coalition
Gateway Engineering Education Coalition
Gateway Engineering Education Coalition
Basic Hole System or Hole Basis
 Definition of the "Basic Hole System":
 The "minimum size" of the hole is equal to the "basic size" of
the fit
Gateway Engineering Education Coalition
Fit Calculations
 Clearance = Hole – Shaft
 Cmax = Hmax – Smin
 Cmin = Hmin – Smax
Both Cmax and Cmin >0 – Clearance fit
Both Cmax and Cmin <0 – Interference fit
Cmax > 0, Cmin < 0 – Transition fit
 Allowance = Hmin - Smax (i.e., Cmin)
Gateway Engineering Education Coalition
Fit Calculations
 System Tolerance = Cmax - Cmin (Sometimes called
Clearance Tolerance)
Gateway Engineering Education Coalition
Metric Limits and Fits
 Based on Standard Basic Sizes – ISO Standard, see the
Appendix material (Appendices 8 - 12)
 Note that in the Metric system:
Nominal Size = Basic Size
 Example: If the nominal size is 8, then the basic size is 8
Gateway Engineering Education Coalition
Metric Preferred Hole Basis System of
Fits
Gateway Engineering Education Coalition
EXAMPLE
Consider that part no 4 assembled with part 2 with fit, what is
the type of fit and then calculate:
a- The max. and min. dimensions for both items
b- The tolerances in both items,
c- The max. and min .clearance or interference
Gateway Engineering Education Coalition
Gateway Engineering Education Coalition
Gateway Engineering Education Coalition
Gateway Engineering Education Coalition