Vibration Technology

Different kinds of technology can be used for the handling, loading, unloading and storage of powders and granules, as well as their sifting, mixing and homogenisation. Vibration technology, based on the principle of use of vibration as an energy vector, can be used to carry out these single operations.

Vibration phenomena can be created using electric motors equipped with eccentric rotating masses, or through the release of pressure waves from pneumatic hammers.

An electric motor vibrator is composed of three main parts:

• a motor body (1)
• eccentric masses placed at the ends of the drive shaft (2)
• end covers for the rotating masses (3)

The eccentric masses used in motor vibrators are usually circular discs characterized by a misaligned hole, or half circles.

The stress of a motor vibrator is generated by the centrifugal force produced by the rotating eccentric masses.

Indicating with "m" the rotating mass, with "e" the misalignment of the rotating mass respect to the shaft rotational axis and with a the angular speed of the mass, centrifugal force Fc is expressed as:

The use of motor vibrators allows two kinds of vibration to be obtained:

• rotational, using only one device;
• directional, using two devices with the same characteristics, rotating in opposite directions, connected rigidly to the same support.

Using only one vibrator, the centrifugal force picks up, moment by moment, a different direction and the structure to which it is connected, and therefore the material contained in it, takes on an elliptical trajectory.

Using two motor vibrators, the centrifugal forces of the masses, which rotate in opposite directions, have the components on the rotational plane - equal but characterized by opposite direction - cancelling each other out. The components, which are perpendicular to the plane - equal and characterized by the same direction - are added together and as a result give a centrifugal constant directional force. This situation imposes a uni-directional motion on the structure connected to it.

Generally speaking, the dynamic behaviour of a system comprising a vibrator and a mass that moves on it is very complicated. This complexity is mainly due to the fact that interaction between the motor vibrator and the mass is three-dimensional, that the mass tends to absorb the energy supplied by the vibrator in a more or less reversible manner, and that between the mass and the motor vibrator there is a structure that cannot normally be considered infinitely rigid.

Handling capacity of these kind of devices depends on several paramenters, mainly on:

• the frequency and intensity of vibration, the choice based on the material's properties;
• the wall friction angle;
• the inclination of the hopper.

Vibration systems are particularly suitable for a variety of applications, such as: flow aids, sifting systems, conveyor systems.

Authors

Dr. S. Ghelfi
Dr. A. Zucchelli

University of Bologna
Faculty of Engineering
Mechanical, Nuclear and Aerospace Department (DIEM)

Date June 1999