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

Review

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 − + 0.1 50 −0 +0.2 or 50 −0.2 +0

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

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

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

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

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

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

Possible fits (clearance)

Possible fits (transition)

Possible fits (interference)

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 H7 25 g6 Fundamental tolerance 0.021 0.013 Fundamental deviation - 0.007 Max diameter 25.021 24.993 Min diameter 25 24.98 25 −0 +0.021 mm 24.993 −0.013 +0 mm

General tolerance