Low Speed and High Speed Rotors
Basic Theory and Definitions |
|||
| Balancing is the process of attempting to improve the mass distribution of a rotor, so that it rotates in its bearings without uncompensated Couple (Moment) Unbalance may be found in a rotor whose diameter is less than 7 to 10 times its width. In the case of a cylinder, it is possible to have two equal masses placed symmetrically about the centre of gravity, but positioned at 180° from each other. The rotor is in static balance, i.e. there is no eccentricity of the centre of gravity, but when the rotor turns, the two masses cause a shift in the inertia axis, so that it is no longer aligned with the rotation axis, leading to strong vibrations in the bearings. The unbalance can only be corrected by taking vibration measurements with the rotor turning and adding correction masses in two planes. | |||
| The difference between static balance and couple balance is illustrated. It can be seen that when the rotor is stationary, the end masses balance each other. However, when it rotates, a strong unbalance is experienced. Dynamic Unbalance, illustrated is a combination of static and couple unbalance and is the most common type of unbalance found in rotors. To correct dynamic unbalance, it is necessary to make vibration measurements while the machine is running and to add balancing masses in two planes. Rotors are classified as being either rigid or flexible. This Application Note is concerned with rigid rotors only. A rigid rotor is one whose service speed is less than 50% of its first critical speed. Above this speed, the rotor is said to be flexible. A rigid rotor can be balanced by making corrections in any two arbitrarily selected planes. I he balancing procedure for flexible rotors is more complicated, because of the elastic deflections of the rotor. |
|||
Gallery |
|||
|
|||
