1. Mechanical Engineering Drawing MENG 204
Limits & fits
Summer Semester 2015
By:
Dr. Ghassan Mousa

2. Review

3. Tolerance
The maximum allowable variation in a dimension or in the size of the part. or
The difference between the mac and min dimensions
IT: international tolerance grade
Bilateral: the variation in both directions of from the basic size, the exact theoretical size.
used usually for parts that fits beside each others
Unilateral: the basic size is taken as one of the limits and the variation is only in one direction
used usually for parts that fits inside each others
100 −
50 −0 +0.2 or
50 −0.2 +0

4. Fits Hole N: nominal size GuH: hole max dim. GlH: hole min dim.
Th: hole tolerance
Shaft
N: nominal dimension
GuS: shaft max dim.
GlS: shaff min dim.
Ts: shaft tolerance
Th = GuH - GIH
Ts = GuS - GIS

5. Fits Fit: planned joining condition between hole and shaft GuH - GIS
GIH - GuS
GIH - GuS
The tolerance mating parts have a space when assembled
The tolerance mating parts sometimes have interference fit and sometimes clearance when assembled
The tolerance mating parts interfere when assembled

6. Fit system: basic hole system
The hole size is kept constant while the shaft size varies (more common)

7. Fit system: basic shaft system
The hole size is kept constant while the shaft size varies

8. Fundamental tolerance
Fundamental tolerance: a tolerance assigned to a fundamental tolerance grade, e.g. IT7 and a nominal dimension range, e.g. 30 to 50

9. Fundamental deviation
Fundamental deviation: determine the position of tolerance zone wrt zero line

10. Possible fits (clearance)

11. Possible fits (transition)

12. Possible fits (interference)

13. Fits example (hole system)
Find the shaft and hole dimensions for a sliding fit (hole basis system) with 25 mm basic size then specify the dimension of both members using unilateral tolerance.
Solution: hole shaft
25 H g6
Fundamental tolerance
Fundamental deviation
Max diameter
Min diameter
25 − mm
− mm

14. General tolerance