3.3.1. Selection and calculation of fits

Fits

Fits are fundamental for the function of parts. When selecting fits, you have to determine where you need tight fitting, to e.g. to transfer torque, or where you need loose fitting, so parts can still be moved relatively to eachother.

Instruction for selecting fits:

  • First, you have to determine for which touching surfaces within an assembly a fit is needed. (e.g. holes and shafts with the same basic size, refer 5. lecture).

  • Then select a adequate fit from this chart according by the function that should be fulfilled.

    This chart contains information and rules for proper application of different tollerance grades.
  • General inormation and basics for fits (like possible tolerance grades, etc.) are found in DIN EN ISO 286-1.
  • Charts of deviations for respective tolerance grades are found in DIN EN ISO 286-2.

Tolerance grade

Characteristics

Application examples

Hole

Shaft

Selection
Interference fit

H7/s6

H7/r6

R7/h6

S7/h6

 H7/r6

Interference fits can only be assembles using high pressure or shrinking. Additional safety against relative rotation is not needed. Clutches on ends of shafts, bushing in hubs, permanent assembly of bolts, anchor bodies on shafts
Transition fit

H7/n6

N7/h6

H7/n6

 Tight fitting components that can only be assembled using high pressure. Additional safety against relative rotation is needed. Gears, bearing hubs, armatures on shafts, driven bushes

H7/m6

M7/h6

 Can only be disconnected by using force, e.g. by using a hammer. Safety against relative rotation is needed. Parts on machines that have to be exchanged without damage, e.g. gears, clutches, bolts, pulleys, etc.

H7/k6

K7/h6

H7/k6

 Can be assembled with small amount of force. Additional saftey agains relative rotation and translation is needed. Pulleys , gears and clutches on shafts for medium workload

H7/j6

J7/h6

H7/j6

 Parts that can be easily assembled by hand if well lubricated. Safety against rotation and translation is needed. Parts needing frequent disassembly, like bushing, hubs, pistons on a piston rod, etc.
Clearance fit

H7/h6

H7/h6

H7/h6

 Parts that can be easily moved by hand if well lubricated. Milling cutters in milling machines, gasket/seal rings, etc.

H7/h9

H8/h9

H8/h9

 Parts that can be easily slid over eachother and across long sections on shafts Clutches, levers, wheels, adjusting rings, etc.

H7/g6

G7/h6

H7/g6

 Minimal clearnce for easy sliding, for high accuracy. sliding wheels in gearboxes, slidable clutches, guiding of shafts

H7/f6

F8/h6

H7/f7

 Parts that slide easily with substantial clearance, proper lubrication is made easier. Most commonly used tolearnce for bearings in engineering, when using two bearings for shafts

H8/f7

F8/h9

F8/h9

 Parts that have substantial to large clearance, so that they can easily be moved relatively to each other. For shafts using multiple bearings, pistons in cylinders, bearings for pumps

H7/f6

F8/h6

H7/f7

 Parts fith large clearance Shafts with multiple bearings, that don't need exact concentric adjusting.

Annotations:

  • Bold tolerance grades are preferred when using tolerancing during this CAD course.

  • Adding tolerances to models in CAD is described in geometric tolerancing in the tutorial.

Calculation (using the example of a shaft and bushing on a fan):

  • Identification of the guidance of the shaft as fit

  • Determining the function of the fit: substantial play for ease of lubrication.

  • Selected tolerance grade: 7 H7/f7

  • Determining the lower and upper deviation (ger. "unteres Abmaß - Au" and "oberes Abmaß - Ao" (for the respective tolerance grade using DIN EN ISO 286-2):

    Ao (H7, 7 mm) = 15 µm

    Au (H7, 7 mm) = 0 µm

  • Calculating maximum and minimum size of the hole:

    Maximum size = N + Ao = 7,000 + 0,015 = 7,015 mm

    Minimum size = N + Au = 7,000 + 0,000 = 7,000 mm

  • Determining the upper and lower deviation for the shaft (for the respective tolerance grade using DIN EN ISO 286-2):

    Ao (f7, 7 mm) = -12 µm

    Au (f7, 7 mm) = -28 µm

  • Calculating maximum and minimum size of the shaft:

    Maximum size = N + Ao = 7,000 - 0,013 = 6,987 mm

    Minimum size = N + Au = 7,000 - 0,028 = 6,972 mm

  • Calculating maximum and minimum clearance (or interference) of the fit:

    Maximum clearance = maximum size_hole - minimum size_shaft = 7,015 - 6,972 = 43 µm (play!)

    minimum clearance = minimum size_hole - maximum size_shaft = 7,000 - 6,987 = 13 µm (play!)