MECH2110 Methods of Mech & Space Engineering

Mating Parts : These are parts that are supposed to fit together in an assembly. The degree of fit is important, e.g. clearance fit, transition fit, or interference fit (ie the shaft larger than the hole)

Basic Hole System : The smallest hole diameter is the basic diameter from which the tolerance limits are applied. This is widely used in the industry because standard drills, reamers, etc are designed to give standard hole sizes.

Basic Shaft System : The largest diameter of the shaft is used as the basic dimension from which the tolerance limits are applied. This system is used only in those applications where shafts come in standard sizes.

Upper Deviation : Difference between the maximum permissible size and the basic size

Lower Deviation : Difference between the minimum permissible size and the basic size

Fundamental Deviation : The distance from the basic size to the closest end of the tolerance zone

The fundamental deviation is designated by a letter: upper case for holes and lower case for shafts.
H (or h for shafts) corresponds to a fundamental deviation of ZERO.

Tolerance is the difference between the maximum and minimum permissible sizes of a part. AS1654 specifies 18 grades of tolerance (01, 0, 1, 2, … , 16). The lower the number the tighter is the tolerance. Table A-1 in the SAA HB7 gives some common grades.

Example

Give tolerances for a shaft-hole pair with a 38-mm basic size. Use the hole basis system. Specify tolerances for a close running fit

Use H8f7
Table A-2 for d=40 mm (note that Table A-2 gives the tolerance range in mms)
Hole tolerance range (H8) = [0 39] (given in mms)
Shaft tolerance range (f7) = [25 50]

Applying to a basic size of 38 mm
dmin = 38 + 0
dmax = 38 + 0.039
Hole diameter range = [38.000 38.039]

dmax = 38 - 0.025
dmin = 38 - 0.050
Shaft diameter range = [37.950 37.975]

Example

Give tolerances for a shaft-hole pair with a 96-mm basic size. Use the hole basis system. Specify tolerances for a medium drive fit

Use H7s6
Table A-2
H7 : [0 35] (given in mms)
s6 : [ 79 101]
Applying to a basic size of 96 mm

HOLE
dmin = 96 + 0
dmax = 96 + 0.035
Hole diameter range = [96.000 96.035]

SHAFT
dmax = 96 + 0.079
dmin = 96 + 0.101
Shaft diameter range = [96.079 96.101]

Minimum Interference = 96.079 - 96.035 = 0.044 mm
Maximum Interference = 96.101 - 96.000 = 0.101 mm

Example

Suppose the “hole” in the previous example is a ring of 10 mm thickness. We will heat the ring to fit onto the shaft. To what temperature should we heat it to give a radial clearance of 0.5 mm in the worst-case combination of hole and shaft dimensions?

The thermal expansion coefficient for steel is 1.17 x 10^-5 ^oC^-1.

What would be stress in the ring and the pressure on the shaft after the fit?

The stress :

The pressure on the shaft can be found from :

p : Pressure

t : Plate thickness

6	Grinding
7	High quality turning
8	Centre lathe turning
9	Horizontal or vertical boring
10	Milling; rolling; extrusion
11	Drilling; rough turning
12	Tube drawing
16	Sand casting

	ISO Symbol
	Hole Basis	Shaft Basis	Remarks
Clearance Fits	H11/c11	C11/h11	Loose fit. Wide tolerance.
	H9/d9	D9/h9	Free running. Not when accuracy is important.
	H8/f7	F8/h7	Close running fit.
	H7/g6	G7/h6	Sliding fit. Not meant for the two parts running against each other but suitable for sliding adjustments.
Transition Fits	H7/h6	H7/h6	Snug fit but easy assembly
	H7/k6	K7/h6	Accurate location with some intereference
	H7/n6	N7/h6	Use when larger interference is permissible
Interference Fits	H7/p6	P7/h6	For rigidity and correct alignment but not for power transmission
	H7/s6	S7/h6	Medium drive fit for ordinary steel parts. The tightest fit for cast iron parts
	H7/u6	U7/h6	High intereference. Shrink fit recommended. Force fit for heavy parts only

Standard Fits