Fit system

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The fit system is an aid in mechanical engineering to avoid a large number of tolerances . Fit systems help optimize production.

purpose

The basic idea of ​​such a system is to manufacture one of the two workpieces belonging to a fit as uniformly as possible and to move the associated tolerance into the other workpiece.

A distinction is made between two methods, the standard bore and the standard shaft .

Unit bore

The dimensions for ISO fits in the standard bore system are precisely defined in DIN 7154 .

In the case of the standard bore (H), the diameter of a bore , for example that of a shaft-hub connection , is always tolerated with an H field according to the ISO tolerance system; the diameter of the associated shaft is assigned to any tolerance field position as required. This can be easily achieved in practice by turning .

The process is characterized by the cost-effective use of the tools used and is predominant in mechanical engineering.

The reason is that conventional drilling tools have two or more tool cutting edges which, in addition, cannot be geometrically changed or adjusted without great effort. In addition, the measurement and thus the quality control of the inside diameter of a bore is more complex than the measurement of an associated outside diameter.

Unity wave

The dimensions for ISO fits in the standard shaft system are precisely defined in DIN 7155 .

The standard shaft (h) provides for a tolerance with an h field according to the ISO tolerance system for the individual diameters of the shaft shoulders. The inside diameters of the components on it are then assigned to individual tolerance fields. The process is rarely encountered; a typical application is, for example, the long waves of transmissions and on textile machines or in agricultural equipment.

standardization

According to ISO , the tolerances of the standard bore for the shaft are specified with lower case letters , whereby the standard bore must always be tolerated with an H. In the case of the uniform wave , the situation is exactly the opposite; the tolerance of the shaft is indicated with h , the tolerances of the associated bores are given in capital letters .

A complete selection of fits for the standard bore system is defined in DIN 7157 . The same standard defines only clearance fits for the unit shaft system.

Selected fits

According to the "unit bore" fit system

Clearance fits
Transition fits
  • H7 / j6: parts can be joined with a rubber mallet: easily removable gears, bearing bushes
  • H7 / m6: parts can be joined with little pressure: wheel flanges on wheel centers
  • H7 / n6: Parts can be added with a machinist's hammer: gears and couplings on pins
Interference fits
  • H7 / p6: parts can be joined with pressure: feather key connections
  • H7 / r6: Parts can be joined with greater pressure: Shaft-hub connections, lever connections
  • H7 / s6: Parts with greater pressure and additional heating can be joined: shaft-hub connections

See also