Sandeep,

Most manufacturers of linear motion exciters will publish a dynamic moment (kg.cm) for each model size.

If the dynamic moment is divided by the total vibrating mass of the screen or feeder (kg), the answer is the resultant peak to peak vibrating stroke (cm).

For example if a screen weighs 2000 kg (including unbalanced exciters), and is fitted with two exciters each having a dynamic moment of 750 kg.cm, then:

(750 x 2) /2000 = 0.75 cm or 7.5mm peak to peak stroke.

(Instead of a dynamic moment, some manufacturers will list the static moment (kg.cm) which is equal to half the dynamic moment.)

Altering the speed (rpm) will not change the vibrating stroke – but the stroke and speed relationship (acceleration) must be held within definite limits to maintain the structural and mechanical reliability of the screen/feeder.

The acceleration limits will depend upon the design of the machine, and should be referred to manufacturer. However as a general rule the acceleration of linear motion units should be held below say 5.5 g’s.

To calculate:

‘g’ = (screen throw peak to peak (mm) x rpm x rpm) / 1,800,000

For example if a screen operates with a 10mm stroke at 950 rpm, then:

g = (10 x 950 x 950) / 1,800,000 = 5 ■

Greetings Sandeep,

Yes you are generally correct, but the throw does not necessarily have to be decided before the screen size (and weight) is fixed. Both these things need to be done together (taking into account the material type, size grading, mesh aperture sizes, feed moisture content etc. etc. etc).

The only thing I would question is your statement that the throw is “permanent and fixed for all horizontal screens”. Reputable manufacturers would normally provide a certain amount of throw adjustment, which is usually achieved by means of changing the amount (or position) of the unbalanced weights on the exciters. However it would always be wise to check with the manufacturer before increasing the factory settings.

Hope this helps ■