shaft misalignment and its contribution to mechanical failures

shaft misalignment and its contribution to mechanical failures

Shaft misalignment is considered the second most prevalent source of vibration after imbalance, it occurs due to poor alignment between corresponding components, such as coupling halves, clutches, shafts, pulleys, etc. In more technical terms, the misalignment of axes and couplings can be defined as the condition in which the geometric centerline of two coupled axes does not coincide with the axis of rotation.

These deviations can take three different forms:

  • Parallel or radial misalignment;
  • Angular or axial misalignment;
  • Combined misalignment;

Parallel or radial misalignment occurs when the centerlines of the axes are unparallel. On the other hand, for angular or axial misalignments, this deviation is observed by the angle formed between the centerlines in a plane in space. Combined misalignment, which is the most common of the situations, occurs when there are parallel and angular deviations in the same set of centerlines.

In the case of shaft alignment, rigid and flexible couplings can generally be used. Although flexible couplings are preferable because they compensate for part of the misalignment, there are generally accepted standards for shaft misalignment with various types of coupling.

Typical symptoms of Misalignment:

  • Premature bearing, seal, shaft or coupling failure;
  • Excessive radial and axial vibration;
  • High housing temperature close to the bearings or high oil discharge temperature;
  • Excessive amount of oil leakage in the bearing seals;
  • Foundation screws loose (“false support”)

AXIS ALIGNMENT AND ITS IMPORTANCE

According to research carried out with participants (mostly maintenance and reliability professionals) of the International Maintenance Conference IMC-2012 on the most recurring machine failures, misalignment stands out in the first place.

Machine Failures:

Meanwhile, some studies indicate that machine downtime in Brazilian industries caused by problems related to improper shaft alignment reach more than 50%. In addition, it is believed that 90% of machines operate outside the recommended alignment tolerances, which can lead to a number of problems with machine performance, cost and degradation of other components.

PROBLEMS CAUSED BY Misalignment

  • Regarding performance:

With a misalignment condition, the increase in temperature, noise and vibration dissipate part of the energy that should be converted into work, which leads to a reduction in efficiency of the misaligned machine.

  • Regarding the cost:

There is a cost to produce such dissipated energy, which directly impacts the energy consumed by an electric motor, for example. During its start-up, the electric motor consumes more energy (due to its state of inertia) and a misalignment makes it difficult to enter the operating regime. This increases consumption, creating problems in the dimensioning of protective devices. In addition, the engine starts to consume more energy to do its job, generating a greater expense in the electric bill.

Correct alignment can reduce energy consumption by up to 15% (perhaps even more). In a quick estimate, the power consumption of a three-phase AC electric motor is given by:

equação de potência

Pot = 1.73volts amp efi PF / 1000 kW

Considering a 25 HP motor in the following conditions: voltage = 380 V, eff = 90% and PF = 0.9. With a current consumption before the alignment of 36 A and after the alignment of 32 A, while operating 350 days/year (representing 8400 h). This gives a power value of 2.13 kW, consumed due to misalignment.

equação do consumo de energia no alinhamento

Pot = 1.73380 (36-32) 0.902 1000 =2.13 kW

Assuming the price of kWh is equal to R$ 0.10, the annual savings generated by correcting this misalignment are R$ 1,790.00.

Econ =2.13 kW8400 h0.10 R$/kWhR$ 1.790,00/year

  • Regarding the degradation of other components:

Unfortunately, costs are not just restricted to energy consumption. The degradation of other components caused by misalignment can lead to the premature replacement of components:

– Bearings: the machine element that suffers most from the misalignment of an axis, receiving a much higher load than it was designed for. As well as the emergence of axial loads that damage, for example, ball bearings, which are normally not designed to receive axial loads.

 

– Seals: the sealing elements do not achieve optimal contact with the shaft, leading to leaks and contamination. This is caused by excessive wear to a certain part of the sealing element which causes it to cease to function. It is observed that a misaligned shaft can cause a reduction of up to 70% in the life of a retainer, for example.

– Couplings: misalignment can cause couplings to overheat, leading to the drying of the rubber parts (commonly used in these elements).

The figure below represents a qualitative distribution of the most recurring consequences in mechanical components due to the condition of machine misalignment.

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