Method to calculate the

press fit force

Sometime you shall project a new productive l ine but

you haven't any idea about which size of servo

presses is necessary instal l to press fit your parts. I f

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 dramatical ly 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 wil l 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 l imit.

F= A*

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 mil l imetres (mm^2)

: is the material 's yield point which is made the part

expressed in Newton per square mil l imetres

(N/mm^2)

Calculate this force value is very easy only if the

material fol lows the Hooke's Law, but in case the

material pass from the elastic behaviour immediately

to the rupture consider wil l 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 l imit force

before appear some creek or permanent de-

formations.

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

1

between the two parts and is possible calculate with

this formula :

P=

I

d

Eo

do

2

+ d

2

do

2

- d

2

+Vo

*

+

d

Ei

di

2

+ d

2

di

2

- d

2

-Vi

*

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**d*pi*L

where :

Fp: means the press fit Force expressed in N

P: means the pressure of contact expressed in

N/mm^2

: means the friction between hub and shaft

d : means the nominal diameter of joint (hub, shaft)

expressed in mm

pi: 3,1 4

L: length of the joint between hub and shaft

expressed in mm.

di

d

do

2

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 liabi l ity for any damages for

gods or person about the use of the formula

expressed above.

3Auloma Holding S.r. l .

Via Mussolina 1 074

4001 8 - S. Pietro in Casale

ITALY

info@uloma.com

www.auloma.com

Auloma provide

servo presses

until 500 kN