Rotating unbalance
One speaks of an imbalance in the rotary bodies, the mass is not rotationally symmetrically distributed. In other words, this means that the mass inertia axis deviates from the axis of rotation. Imbalances lead to vibrations and increased wear, which is why they are balanced by counterweights. A typical example are automotive wheels, the essential imbalance is caused by the tires.
On the other hand, imbalances are often used deliberately for technical purposes.
A distinction is made between static and dynamic unbalance. Mostly, both forms occur simultaneously.
Static imbalance
A static unbalance occurs when the axis of rotation does not pass through the center of gravity of the rotary body. It is a special case of dynamic unbalance. Is characteristic of a static unbalance in that the plane in which the imbalance is situated, coincides with the radial plane of the center of gravity and therefore circular generating mechanical vibrations at right angles to the rotation axis during rotation.
Dynamic unbalance
Dynamic imbalances occur when the axis of rotation does not coincide with one of the stable main axes of inertia of the part. The axis of rotation is tilted in focus.
Dynamic unbalance occur only in operation. You express yourself in a bending moment, the so-called unbalanced moment on the rotational axis. They evoke at the ends of the axis by 180 degrees shifted, circular vibrations. The center of gravity of the rotating body remains at rest, while the axis wobbles because of the opposite circular movements.
It is this form of imbalance eg car wheels are often called " flutter" true. Therefore, they are dynamically balanced, which can lead to both sides (inside and outside ) of the rim wear counterweights.
Terms of imbalance effects
Imbalances are not only a negative effect, they must be used consciously in the art in many areas. So be cleaned eg with vibrators container and Silowandungen, materials compacted and driven with electric vibrators vibrating conveyor, promote or sort the bulk materials. In mills plansifter be treated with imbalances in vibration. The forces of inertia of reciprocating engines can be reduced or eliminated with balance shafts. Also massagers and vibrating alert of mobile phones are working with one of a small motor in rotation offset imbalance. The unbalance hole serves to lead drill through rocky to clayey sediments and to investigate as to deeper layers. With a vibratory hammer the pile ( sheet piles, steel beam ) is driven into the ground. Also in the vibrating plate ( construction machine ) is based on the compaction effect unbalance forces.
Critical speed
As a critical rotational speed that speed is designated, in which the forces of the rotating unbalanced offset a machine part or the whole machine in the resonance oscillations. The straight trend in resonance system is a spring -mass system; It consists for example of the rotor and shaft of a motor / a turbine or from the entire engine and its base and its suspension.
The excitation by oscillations with the same frequency as the critical speed represents a danger to rapidly rotating machines (turbines, centrifuges, etc.) represents the effects of such stimulation are good balancing, damping suspension or by a particularly rapid passage through the critical speed at start-up reduced. The critical speed itself can also be varied by altering the stiffness. Reducing the rigidity takes place e.g. through resilient suspension, which also lowers the critical speed.
In vibratory feeders and shakers you put the other hand, the resonant frequency of the oscillatory system intentionally in the field of critical speed or vice versa.