GEAR BOX ANALYSIS

Without any doubt, a gearbox, shown in figure 1, is a very important part in a vehicle or any application that requires transmission of power. Transmissions machines consist of two main things
Power source
A system for power transmission.


Figure 1: A gear box showing the gear train
The gearbox employs gears as well as gear trains to provide torque and speed conversions from a power source that is in rotation to another device.

In a vehicle for example, the gear box is responsible for adapting the internal combustion engine’s output to the steering wheels. The engine rotates at a high speed which is not ideal for starting, stopping or slow travel and therefore the transmission helps to reduce the high engine speed to a slower speed with high torque that the drive wheels can handle. Other areas of application include in motor cycles and fixed machines.


How the gear box works

In order to understand how the gear box works, we can take an example of a manually operated car. First introduced by Emile Levassor and Louis-Rene Panhard in 1894, the manual transmission system has grown in leaps and bounds and today, we have very sophisticated transmission systems. Most modern cars however, offer an option of selecting four or five speeds in the forward direction and one reverse speed. The gearbox is bolted to the engine rear and together with the clutch, forms the transmission system.

The most popular gearbox is the constant mesh. It contains three shafts which include the input shaft, lay shaft and the main shaft. The input shaft is driven by the engine and consequently drives the lay shaft. The lay shaft in turn rotates the gears, but only freely until they are locked by a synchromesh device splined to the shaft. It is this synchromesh device that is operated by the driver through the gear selector rod. A baulk ring ensures that a gear is not engaged until there is a synchronisation of the gear speed and therefore protects the gear box against damage.

Gear box fault diagnosis 

Diagnosis is basically the identification of the faults of a machine based on some symptoms.
Taking care of the gear box is very important to the life of the associated machine and learning how to detect and analyse the gear box can help to prevent damage, wrong diagnosis and the associated huge economic loss that comes with a broken gear box. The first step in analysing the gear box involves fault diagnosis as this helps us to know where the problem is.
With the recent developments in electronic gadgets like computers, sensors and transducers, machine supervision has become almost automated. There exists a lot of diagnosis equipment that when connected to the machine are able to highlight where the fault is.

Typical causes of failure

Failure in the gearbox is likely to emanate from the following sources:

  • Manufacturing errors: These errors occur due to a fault in the manufacture of the gearbox. Sometimes the gearbox manufacturer can recall some them if there is a widespread complaint about a specific gearbox or if they realise that they made a mistake.
  • Application errors: These are as a result of problems like wrong mounting or installation, vibrations, improper maintenance and cooling.
  • Design errors: These occur due to reasons such as usage of wrong materials, materials of low quality, improper gear geometry and wrong specifications.

Gearbox analysis methods

1. Physical analysis

In this method, you use techniques such as X-ray, magnetic rubber, dye penetrates and microscopy among others to physically detect faults in the gearbox.

Limitations of physical analysis;
  • They cannot be performed, on most occasions without the removal of some of the components which can sometimes result in damage. 
  • They are also not ideal for analysis of large transmission systems.
Vibration analysis 

A key characteristic of all machines with moving parts is that they produce some sound and have vibration signatures related to their construction. Once the state of the machine changes, a change in the vibration signature also occurs and therefore this can be the first sign of a problem in the gear box. This forms the basis of many gear box condition monitoring systems. Vibration analysis uses signals that are picked from the gearbox casing. The gear meshing frequency (GMF) is very important and together with its harmonics can help to identify a fault.

Typically, mechanical vibration is caused by misalignment, distortions, looseness, defect bearings, couplings and gearing inaccuracies, forms of resonance, reciprocating forces, hydrodynamics or aerodynamic forces, unbalance, critical speeds, bad drive belts, stator and rotor misalignments, friction whirl and bent or defective rotor bars among others.

Faults that can be detected by vibration analysis include;
  • Damage to the gears: tooth meshing faults, cracked teeth and misalignment.
  • Electrical machines: structural resonance, unbalanced magnetic fields, broken or damaged rotor bars and air gap geometry variations
  • Rotors and shafts: Cracked shaft, loose components, unbalance and bent shafts.

As is clear, vibrations in the gearbox can be complicated and gives quite a lot of information. It is therefore important that one understands the information carried by the vibration by relating it to factors that have an influence to the vibration and the vibration frequency.
For successful vibration analysis, it is paramount that the following factors are taken into consideration;
  • The speed of rotation of the machine.
  • Location point of the monitoring transducer.
  • Load sharing characteristics.
  • Background vibration/noise level.
  • Dynamic interaction of the machine and other machines or items which are in contact.


2. Vibration analysis techniques

  1. Time domain analysis
    • This techniques analyses the phase and amplitude information of the vibration time signal and uses it to detect the gearbox fault.
  2. Frequency domain analysis
    • This technique uses the signal’s power spectral density difference, caused due to the fault in the gearbox to identify the damage, maybe to a gear tooth and/or bearing.
  3. Order analysis
    • Used to analyse vibration signals in reciprocating or rotating machines.  It is dependent on how the frequency changes as the rotation speed of the machine changes and uses this to detect the fault.
  4. Time synchronous averaging
    • This helps to identify a fault by reducing the complex time-domain vibration signal to estimates of the vibration for individual shafts and associated gears.


Advantages of vibration analysis.

  • This method is non-intrusive and therefore does not result in damage of some components.
  • Can be used to analyse large transmission systems

References

1. Aherwar, A., & Khalid, S. (2012). Vibration analysis techniques for gearbox diagnostic: a review. International Journal of Advanced Engineering Technology, 3(2), 4-12.

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