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How calculate the press fit force

Determinate the press fit force to choose the right servo press

Sometime you shall project a new productive line but you haven't any idea about which size of servo presses is necessary install to press fit your parts. If the shape of parts are cylindrical or almost, is possible calculate the press fit force necessary to assembly together your parts.

Determinate the minimum force required

To determinate the minimum force required exist many formulas that can provide you an approximate value. This value could be considered good for determinate your purchase for don't buy an under sized servo press. The press fit force is produced by many magnitudes as: Interference, friction and the elasticity between the two parts. A small variance of one of these magnitudes can change dramatically the press fit force. For this reason became important a control of supply chain to avoid big differences in the features of your products. The first step to have a good press fit process will be the determination of the maximum compressive force applicable on your parts. Cheek in the data sheet of the material used to made the parts the yield point to know the elastic limit.

F= A*Ϭ

Where :

  • F: is the Force admissible before the plastic deformation expressed in Newton (N)
  • A: is the resistant area of the part expressed in square millimetres (mm^2)
  • Ϭ: is the material's yield point which is made the part expressed in Newton per square millimetres (N/mm^2)

Calculate this force value is very easy only if the material follows the Hooke's Law, but in case the material pass from the elastic behaviour immediately to the rupture consider will be necessary use the Von Misees criteria to calculate the maximum force allowable. However, to be sure to know what is the maximum compressive force applicable, our suggestion is cheek in laboratory the limit force before appear some creek or permanent deformations.

Press fit force calculation

The maximum force admissible F shall be more bigger than the Fp force value for fit the parts. To know prior an approximate value of force to press fit your parts is necessary know the pressure of contact between the two parts and is possible calculate with this formula :

Where:

  • P: means pressure of contacts expressed in N/mm^2
  • I: means interference between the hub and shaft expressed in mm
  • d: Nominal rim diameter expressed in mm
  • Eo: Hub Young's modulus expressed in N/mm^2
  • do: Hub outer diameter expressed in mm
  • vo: Hub Poisson's Modulus
  • Ei: shaft Young's modulus expressed in N/mm^2
  • di: shaft internal diameter expressed in mm
  • vi: shaft Poisson's Modulus

As is demonstrated in the formula above the pressure of contact depends mainly by the interference between the two parts joined in press fit process.
To calculate the minimum force necessary to press fit the parts now is necessary use this formula :

Fp = P*η*d*π*L

Where:

  • Fp : means the press fit Force expressed in N
  • P : means the pressure of contact expressed in N/mm^2
  • η: means the friction between hub and shaft
  • d: means the nominal diameter of joint ( hub, shaft ) expressed in mm
  • π: means 3.14
  • L: means the length of the joint between hub and shaft expressed in mm

In case the press fit force value Fp result more bigger than the maximum force admissible F before the deformation is necessary working on the interference during the introduction by a cooling down of the shaft or by a warming up of the hub. Another good practices easy to implement and very affordable is the use of lubricant to reduce the friction between the hub and shaft or with a reduction of the surfaces roughness. The last instance is the reduction of the joint length. The values obtained by the use of the formulas above are purely indicative. Auloma suggest test in laboratory to determinate the right press fit force value. Auloma reject any liability for any damages for gods or person about the use of the formula expressed above.

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Press Fit Force

Method to calculate the press fit force