Secrets of imbalance 2 - How to find heavy spot position. (displacement, velocity, acceleration)


In this video I would like to explain the relation between heavy spot position and vibrations in acceleration, velocity and displacement. Understanding of that will help you in balancing jobs.

We use the Adash VA5 analyzer. We measure the acceleration signal and the analyzer recalculates it to velocity and displacement signals. We also use the APS measurement. It gives us the amplitude and phase on speed frequency.

In negative maximum the moving must stop, because the moving direction must be changed from downward to upward. From this point the upward moving begins. It is the positive moving. It means the velocity must be positive too.

When the movement passes through the middle point, it has the highest velocity. It increased velocity up to this point, it means it accelerated. From this point it must decrease the velocity, it means to brake. Why? It clear it must stop in this time. The movement must change the direction in this time. It means the velocity must be zero again. In this interval a downward movement takes place. The velocity is negative.

And the important question. How to find the heavy spot position from velocity measurements. We know that the displacement phase value exactly tells us, where the heavy spot is. In this example the phase is zero degrees. It means the heavy spot is in the same position as the reflective tape. The velocity phase is 90 degrees. It is clear, that we must deduct the 90 degrees from velocity phase and this phase is the angle of heavy spot position. When we deduct 90 degrees we are in the same situation like with displacement phase value. Before I begin talk about acceleration, I should tell you some words about force and spring. The motor with flexible foundation is on the picture. It is mounted on springs. The motor does not run now. It is in resting position or zero position. I apply force upwards, for example by the crane. The springs stretch and makes the opposing force. The force magnitude is proportional to change of spring length. It means bigger stretch makes bigger opposing force. When the plus force made by crane disappears, the spring force minus F returns motor back to the resting position. The same process happens when the spring is compressed. Again bigger compression makes bigger string opposing force. Now we apply the rigid foundation. Springs are removed and bolts are used. The question is: has anything changed? Answer is no. The bolt is spring also. But much much stiffer.

This is the similar example. We have the rotor, which is not rotate. It is in resting position. It can move just in vertical direction. As in last example I can stretch the spring or compress the spring. When the force disappears, the strings returns back to resting position.

Now the rotor is running. The heavy spot makes the centrifugal force. At some points the force is big and stretches or compress the spring. At other points the spring opposing force is bigger and returns rotor back to the resting position. Our rotor can move just in vertical direction. It means just vertical part of centrifugal force is used.

Now we a little understand what it is the force. It is pulling or pushing. The basic physical formula tells us, that the force is equal to the mass multiplied by the acceleration. The mass is the constant. It means that the acceleration is directly proportional to the force. It means when we speak about force, we speak about the acceleration.

The direction of vibration sensor is the same as direction of tacho sensor. It simplified the examples. If both directions would not be the same, then we must add or deduct their angel value from the final phase value. I think, that we to keep this rule in practice is easy. Please understand well, what this rule means. The sensor can be mount on many places on machine. We do not care where the place is. The direction of sensor is important. In one place I can mount sensor in several directions. Each direction needs different position of tacho sensor.

And the final revelation. You can tell me: yes now I can estimate the heavy spot position after the first run in balancing. But what to do with such information. It is very useful. It allows you to mount the trial mass on the opposite side. It means the vibration in trial run should decrease. And it is good. When you mount the trial mass randomly, then the vibration can increase. And it could be danger and machine could be seriously damaged. Or the control system stops the machine because of high vibration. I am looking forward to your comments. It helps us decide what video to make next time. So, see you next time and thanks for watching.