VIBRATION MEASUREMENT AND VIBRATION ANALYSIS
Periodic vibration measurements and vibration analyzes made under the name of predictive preventive maintenance services is an important determinant for extending the life of the machines used in a factory and ensuring correct maintenance.
Periodic vibration data measured from certain points on the machines and the beds of these machines are monitored against time. From the data obtained at the point where a critical rise in the vibration level is observed, it is easily revealed what the source of the problem is and how the intervention should be interpreted. Vibration may have different causes. Values taken at different frequencies indicate different problems. By examining these frequencies with the vibration analysis, the reasons for the increase in the vibration level are revealed, and the necessary intervention is done as a point shot. In this way, the life of the machines can be efficiently removed to the longest possible levels.
Various rotors, fans, fan motors, heavy industrial parts, motor rotors, mill fans, ventilation systems, stone crushers and so on. In many sectors with “field balancing” many advantages such as quality working conditions and long-life use is obtained.
Advantages of On-Site Balancing
Benefits of on-site balancing:
- There is no need to remove the part, it is applied in original working conditions.
- There is no shipping cost.
- Fast, no time loss such as disassembly / reassembly, transportation.
- Since actual operating conditions are measured, all environmental factors, possible effects of other rotating components are observed and a precise solution is obtained.
- Periodically, the longest operating conditions are achieved.
WHAT IS BALANCE?
Unbalance is directly related to the terms “balance” and “weight”. We can define the industrial balance as the mass distribution of the rotor in the axis of rotation. The mass distribution around the rotation axis is called this rotor unbalance. The rotational speed of the rotor is directly proportional to the effect created by this imbalance. As the rotor rpm increases, the effect of the unbalance on the center of gravity will increase, this will create various disturbing vibration noises.
The vibration generated on the rotor because of unbalance will wear the rotor bearing system in a short time, and will cause various abrasions and noises. This situation will shorten rotors life with all its rotating parts. After a time there will be bearing failures in the bearing system due to unbalance vibrations, deterioration of the structure of the parts, and a non-effective working environment. This will lead to a decline in the quality of work, especially in the area of work safety. For example, if this is a machining bench, the precision of the resulting product will be adversely affected.
Vibrations due to unbalance can be eliminated by balancing. It is strongly advised to perform periodically balancing in terms of quality of work and long-lasting use in continuous working environments. Balancing is done to balance the mass distribution of the rotor and minimize the center of gravity force caused by unbalance. Different types of balancing techniques are available depending on the rotor structure. The main types of unbalance are;
It is usually observed in disc shaped rotors (thin form). There is unbalance in a single plane, and the rotor oscillates parallel to the rotor axis due to this imbalance. It is solved by balancing in a single plane (center of gravity plane). According to the rotor condition, method of weight addition or weight reduction is used.
If unbalance is measured on two different planes on a rotor with different weight values and angles, this is called dynamic unbalance. In dynamic balancing in addition to static balancing, there is also vibration in vertical axis to the rotor axis. Unbalance is measured in two planes, balance correction is made in two planes with weight addition or weight reduction method
WHAT ARE THE REASONS OF VIBRATION?
Often it is thought that vibration can be eliminated by balancing, but vibration is not the only cause of unbalance. Only one of the causes of vibration is unbalance.
Balancing operation does not correct anything except unbalance
There are many things that can cause vibration:
Misalignment, bent shaft, shaft had friction on stator, damaged bearings, turbulence, cavitation, oil return, insufficient lubrication, loose connections, worn teeth, stator windings, broken rotor bars and unbalance.
If the engine that drives any part is connected with a coupling, there is a risk for parallel and angular misalignment. This causes vibration on radial and axial direction at 1X and 2X rotational speeds.
Deformation in the motor or part causes the bearings to move out of the axis (alignment). Bearings that are not perpendicular to the shaft are also problematic.
Particularly if it is necessary to mount a roller with a narrower borehole using a hammer to mount the bearing, it can damage the bearings. Cracks in hollow ball bearings or cracks in ball and bracelets may occur during assembly. If the ball bearing is replaced by a hitting or a hammer instead of a mounting bracket, the resulting bearing may deteriorate and the ridge and the mileage may not sit upright. In this case there may be no visible damage, but as the material changes shape, wear will begin to occur in a much shorter period of time.
What can we expect from a wrongly fitted motor?Vibration, sound, friction, short life…
There may be a flat bending on a shaft working at full load at room temperature, especially if there is an irregular warming effect. Also excessive belt tension may cause one shaft to bend, and shortly it may cause bearing failure. If it is not turning at a slow speed, it will sag in long distances. Other shaft problems may be the result of machining errors.
Ideally, a fan should have equally shaped and equally spaced propellers. In practice, each prop is different from each other. As a result, one part of the fan produces more thrust than the other. Rotating pushing force causes unbalance. Non-uniformly mounted components, such as a fan or pump that have no equal distance between blades, produce vibration due to thrust or pressure. A fan of this type can be balanced but it will be vibrating again at full load.
Hydraulic or Aerodynamic Unbalance
For example, an eccentrically mounted pump causes vibration at 1x rpm and this vibration appears to be caused by unbalance. It works smoothly in dry operation but vibrations will occur when working in water. These effects are often referred to as hydraulic or aerodynamic unbalance.
Another problem with pumps is the limited intake, which creates a low pressure area and causes cavitation in the pump. Cavitation causes material wear and poor performance. There are other reasons for cavitation, but it is mainly due to the fact that cavitation operates from the optimum flow of the pump and operating away from the main conditions.
Cavitation produces high frequency noise that is not associated with rpm. This pressure imbalance will cause erosion and therefore shorten the life span.
Bearings may become unstable if there is too much radial clearance or very small radial load on the oil layer bearings. As a result, the rotation speed around the shaft bearing will fall below 50% of the shaft speed. The main problem is that there is not enough damping to dissipate the vibration energy.
Excessive vibration will occur in machines that are not properly fixed on the ground. It controls the energy of the machine’s energy and the inadequate connection causes vibration at very high amplitudes. Sometimes the vibration is 2X or 3X times the spindle rotation speed, but sometimes it can occur at different frequencies with respect to the resonance of the part in the system. This problem can be caused by the non-uniformity of the mounting surface or by the use of fine adjustment washers instead of a single piece of thickness below the engine feet. The machines must always be rigidly fixed to a smooth floor without any gap.
Excessive lubrication causes the bearing roller and the rollers to slip out of rotation and this can cause over heating – excessive lubrication is a common problem. Lack of lubricant causes noise and rapid wear. Proper use of oil (consistency, sex) is a critical issue. The cause of many problems is due to incorrect lubrication or the use of contaminated oils.
Worn and Damaged Gear
Gears should be manufactured with very tight tolerances for quiet and vibration-free operation. Minor errors resulting from manufacturing and assembly can lead to uneven loading and rapid wear. Abnormal wear on the component surfaces cause vibration. If the gear ratios are not properly selected (prime number gears), the gears will cause undulations on the surface of the component – causing the drive to hit the gear teeth of the drive gear, causing frequent wear, noise and vibration. Example: In gear ratio 2/1, gear # 1 on the drive and gear # 1 on the other drive hit in both cycles.
Unbalance causes radial loads on the bearings. Unbalance is a property of rotor, and it is not changed according to the motor speed. The vibration changes in direct proportion to the rotation speed. The centrifugal force varies according to the square of rpm speed.
How Does The Rotor Become Unbalanced?
After some time, material accumulation and abrasions occur on the working rotor. Foreign matter damage and thermal effects can also cause bent shaft and rotor stator lotions. Maintenance often means “change in working conditions”. The demounting process, changing the bearings, cleaning and reassembling will cause different situations. This will inadvertently lead to undesirable consequences.
If there is no reason to change the vibration or unbalance, overloads (torque or radial) may have caused a crack along the shaft. If this crack is not fixed, the result will be a broken shaft. Unbalance is a property of mass distribution (with bearings) of shaft assembly. If the unbalance has changed, some mechanical changes have definitely caused this change. After maintenance, the rotor must be absolutely balanced, the mass distribution must be made suitable and the assembly must be done in that way. After the final assembly, unbalance should be checked again, so that the balance will be checked on the other turning components after installation, and if not tolerated, the final balance should be done again. These operations may vary according to the type of parts and their specifications.