Fit Tolerances and Tolerance Pairs

This article, relevant to technical drawing and manufacturing processes courses, covers tolerance pairs and fit tolerances. The maximum and minimum sizes that two mating machine parts need to have in order to work together as intended are expressed with tolerance pairs.

Fit Tolerances

The tolerance on the dimensions that let two machine parts work together determines under what conditions the two parts will operate together. These conditions range from very loose to very tight.

The zero-clearance position “H” is denoted with an upper- or lowercase letter.
Upper- or lowercase letters from “A” to “G” are used for clearance fits,
Upper- or lowercase letters from “J” to “Z” are used for interference (tight) fits.

(Note: the upper- or lowercase letters I, L, O, Q are not used in tolerance notation)

Tolerance Pairs

In the ISO tolerance system, fits obtained by mating various shafts to the same standard hole, or various holes to the same standard shaft, form the fit groups. Accordingly, in the hole-basis system there are the groups H6, H7, H8, H9, H10, H11, H12, and in the shaft-basis system there are h5, h6, h8, h9, h10, h11, and h12.

In machine building, H6, H7, H8, H11 and h5, h6, h8, h9, and h11 from these fit groups generally cover all needs.

Clearance Fit Tolerance Pairs

H7-h6: Used for machine parts assembled with lubricated surfaces. Example: pulley hubs, retaining collars, clutches, and moving machine-tool parts that need to be replaced frequently.

H7-g6: Sliding fit tolerance. Example: gears, clutches, connecting-rod bearings, pistons that need to move axially

H8-h9: Easy running fit. Example: transmission shaft retaining collars, belt pulleys, gears, clutches, etc.

H7-f7: Running fit tolerance. For example: machine tool main bearings, crankshafts

H7-e8: Tolerance for clearance fits. Example: fits that need to keep moving freely under heat

H7-d9: Tolerance pair used for very loose (large clearance) fits

Interference (tight) fit tolerances

H7-j6: A “tap-in” fit tolerance: easy tight fits assembled by tapping with a wooden mallet. Example: belt pulleys, gears, hand wheels, bearing bushings, etc.

H7-k6: Belongs to the easy hammer-fit type of fit tolerance: for example, pulleys, bearing inner races, hand wheels, etc.

H8-n6: A hard hammer-fit type of fit tolerance: for example, pulleys, clutches, couplings, etc.

H7-n6: Tolerance pair used for fits assembled by pressing: example — pulleys that will never be removed again, rotors pressed onto a motor shaft, gears that will be pressed onto a journal, bushings pressed into hubs, and similar fitting operations.

Press-fit tolerances

These are fits made by pressing, or by heating and expanding the socket, and that will never be disassembled again.

H7-r6: Fits that won’t shift under a moderate, constant force. Example: pressing a steel band onto a cast-iron hub, bushings pressed into connecting-rod bearings, etc.

H8-u8: Fits that won’t shift under a large, constant force.