Know-How-Center

Calculation of the Centrifugal Force

Rüttler: BRECONmade in germany

Calculation of the Centrifugal Force

For correct assessment and adjustment of the vibration equipment, first the calculation of the centrifugal force is necessary. It activates the motion of the every separate particle of the mass which has to be moved.

Is the centrifugal force too small, the particle will not move and will remain at it’s objectionable state.

Is the centrifugal force too high, e.g. for compacting undesirable motions are caused besides the necessary compaction.

To determine the centrifugal force, the mass mu of the eccentric disc, the distance e from the centre of gravity the centre of motion, and the frequency of mechanical vibration fm are important factors.

Fig. 8: Formation of working moment Mu = mu ∙ e

Fc = mu ∙  e  ∙ ω2 / 1000   

w = 2 Π ∙ fm

Fc  – Centrifugal force in kN
mu – Mass of the eccentric disc in kg
e   – Distance from the centre of gravity to the centre of motion in m
fm  –  Frequency of mechanical vibration of the free vibrating vibrator in l/s
ω  -   Winkelgeschwindigkeit (angular rate/speed/velocity) of the rotating eccentric disc in l/s

In catalogues only the minimal and maximal adjustable centrifugal force at synchronous speed as reference values are given because calculating is easier with this values.

The synchronous speed of a three-phase asynchronous motor is calculated out of the electrical operating frequency and the number of pole pairs. 

 

Motor Rotation Speed

ns = 60 ∙ fel/p

ns – Synchronous speed in l/min
p – Number of pole pairs
fel – Electrical operating frequency

When loaded, the speed of the asynchronous motor is reduced  by slip rate and results in operating speed.

N =ns ∙ (1-σ)


n – Operating speed (mechanical frequency) in l/min
σ – Slipping

At the slightly smaller operating speed Fc is reduced by the factor (1- σ)2.

The fact, that the three-phase asynchronous motor at load in comparison to idle speed decreases only slightly (by slip), is a decisive advantage.

In practice the mass of the eccentric weights is unknown, known are mass m, which has to be set to vibration, and the acceleration a. For the different areas of application there are multiple experience values available (see experience values).

Therefore Fc (kN) is determined according the equation

 

Calculation of the Centrifugal Force in Practice

Fc = m ∙ a / 1000

m – The sum of the masses, which has to be set into vibration, in kg:

  • The mass of the vibrator mR
  • The mass of the vibration equipment mT
  • 10-15% of the mass which has to be compacted ms

m = mR + mT + ms
a – Acceleration in m/s2 

In addition a characteristic value for rigidity and the resonance behaviour of the vibration equipment have to be considered.