Category Archives: Gearbox

Case Study #4 – Vibrating Screen Gearbox Bearing Defect

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Hello Everyone,

Here is the fourth of the five case studies brought to you with the Reliability Training Institute.

In this case study we show that by measuring the correct vibration parameters you can resolve defects in harsh applications, even an inner raceway defect on a vibrating screen. Hopefully, you are finding them of use and helpful.

These case studies are to support my book ‘Enhanced System Reliability Through Vibration Technology’ and my new role as an RMS Trainer with the RMS Reliability Training Institute.

Many thanks to Dr Jezdimir Knezevic from MIRCE Science for his enlightening discussion (and MIRCE Science) and to Dean and Stuart at RMS for all their support.

This case study and more can also be viewed on the RMS Blog.

What’s the highest Acceleration levels on Gears you have recorded?

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What’s the highest Acceleration levels on Gears you have recorded?

Apologies for the lack of posts over the month, it’s been a very busy time with the family.

This post is a short one just inquiring what levels of Acceleration impacting you have seen in the field from a gear set. This is one I came across a couple of months ago.

 

Summary:

  • Vibration data was collected with FMax of 10 KHz and Sample rate of 32,768.
  • The shaft rotates at 2990 RPM.
  • The gears are small and both have 65 teeth, their function is to transfer drive to an oil pump in a gearbox.
  • We recorded a Peak to Peak of 51.67 G’s compared to another identical asset running at 9.176 G’s.
  • Two weeks later the gears had a functional failure and stopped doing what they’re were designed to do.

 

 

Vibration Data:

This is the 1.3 Second Acceleration Time waveform comparing the suspect unit and an identical unit.

 

 

Images of Gears after Functional Failure:

Images of the gears on inspection.

 

 

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Gearbox Failure due to Metal Spray Fatigue

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Hi All, this is the last post for 2017 – Enjoy

Background:
We were called to inspect a gearbox as the client had reported an abnormal sound. This was a very large old extruder high torque gearbox with a single input and dual output shafts.

Executive Summary:
Through onsite vibration analysis we were able to pinpoint the shaft that was generating the abnormal noises, this enabled the bearings for the shaft to be pre-ordered so they arrived at the repair shop the same time as the gearbox. This ensured a quick turnaround was completed with minimal production loss.

On Site Initial Assessment:
The gearbox vibrational levels as measured under full load conditions were >20mm/s RMS. This is considered “Vibration Causing Damage” as per ISO 10816-3. The Acceleration Peak to Peak impactions at Gearmesh #1 was excessive at 162G’s. There was also indications of misalignment on the 1st intermediate shaft and considerable looseness present. The 1st intermediate shaft ‘binds’ for 1/4 to 1/3 of a revolution when turned by hand.

Vibration Data:
The Input shaft high frequency Acceleration spectra clearly shows a high 2x gearmesh frequency for the gearmesh 1. This indicates there is misalignment within the gearing setup. The sidebanding at 19.20Hz indicates that it is relative to the 1st intermediate shaft.

The plot above is the Acceleration Spectrum from the Gearbox NDE Horizontal.

The Peak to Peak measurements on the Acceleration Time Waveform below indicates the Acceleration forces are within the 1st Intermediate shafting. The total reading of 162G’s is highly destructive and is impacting at 19.2Hz, the 1st intermediate shaft speed.

The Velocity spectrum taken from the NDE of the 1st intermediate shaft shows a considerable amount of run speed harmonics attributed to the shaft speed. This is an indication of looseness.

Cause of Failure:

On inspection the tab washer on the first intermediate shaft outer bearing had failed. In addition the suspected gear on the 1st intermediate shaft was extremely loose. It was found that this shaft had been previously repaired with metal spray and this had failed. On closer inspection the stress raiser appears to be around the keyway, as there was no strengthening welds around the keyway to support the metal spray.

Strip Down Images:

This is an image of the gearbox internal layout.

Images of the failed tab washer found in the bearing cap from the 1st Intermediate shaft.

Image of the key that supported the 1st intermediate gear that was loose.

Metals spray coating that was under the 1st Intermediate shaft gear. This failed initially at the metal spray coating at the keyway.

 

 

Worm and Wheel Gearbox Premature Failure

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Hi all,

Here is an interesting one. History is a worm wheel conveyor drive (Radicon Type). This operates in a rough environment and it ran for 8 months after maintenance and started to make a lot of audible noise.

It has a MJT 2 ¾ and a LJT 3 ½ on the input shaft. Coupling was the rubber pin type.

On inspection, there looks to be poor worm/wheel contact with some hair line surface cracks in the wheel gear. The gearbox input worm shaft NDE bearing has very bad damage.

We are thinking three possibilities for the root cause: Impact during mounting resulting in spalling at ball pitch and/or Spalls (Hertzian Fatigue) due to excessive thrust loading due to assembly or alignment errors. Also the possibility of transportation/storage damage. 

Anyone got any input?

Vibration Trends:

 

Acceleration trend showing increase after maintenance and the sharp increase.

 

 

PeakVue Trend showing a similar increase as the Acceleration trend.

 

 

Velocity Spectrum with high BPFI matches.

 

On Inspection Gear:

Incorrect gear meshing indications.

Visual surface hair line cracks.

 

Visual Inspection Outer Raceway:

Top left of image looks to be a hole in the raceway, there were two at 180 degrees opposite.

 

Microscope image of the outer raceway defect.

 

Visual Inspection of the Inner Raceway:

Images from around the inner raceway.

 

Microscope image of the inner raceway defects.

 

Visual Inspection of the Rolling Elements:

Two of the rolling elements, they all have various level of similar damage.

Microscope images of the rolling elements defects.

Vibrating Screen Exciter Gearbox – Bearing defects

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Hi all,

How do you monitor vibrating screen exciter gearboxes for deterioration and reliability risks? Do any of you monitor vibrating screens exciters for bearing defects using routine vibration analysis, how do you cope with the harsh environment? Or do you just use oil analysis?

The screens in question are the in line type with two gearboxes with weights at the ends of the gearbox shafts, direct driven by a motor via carden shaft. These screens vibrate around 298mm/s RMS in the highest direction of motion.

I found a beauty of a defect when I was conducting a vibrating screen structure survey, I decided to collect the usual routine vibration data from the exciter gearboxes via a flat magnet mount and found an inner race defect! Site actually pulled it 2 days later due to increase in noise and temperature. This gearbox was only installed two weeks prior.

The unit was removed from service with the bearing in the early stages of failure, prior to catastrophic failure and secondary damage. I would be interested in others thinking for the root cause of this infant failure?

 

VA Data:

No historical data as this was a one off survey. The top plot is the PeakVue spectrum and this displays one order and harmonics together with a match for the bearing inner raceway defect frequency (BPFI).

The bottom plot is the PeakVue acceleration time waveform, and this displays dominant one order activity. In PeakVue this means that something is modulating at 1 Order i.e. Inner Race defect.

 

Bearing Images:

The long and short shaft fixed bearing had an inner race localised spalled area at the inner ring centre shoulder, on one side of the raceway. Failure due to flaking of the inner raceway. I think the most likely cause is ISO 15243:2004 – 5.1.2 subsurface initiated fatigue due to overloading (Axial shock load).

The above is the short shaft fixed bearing.

The above is the long shaft fixed bearing.

I believe these images show rolling fatigue flaking that may be caused early by over-load, excessive load due to improper handling, poor shaft or housing accuracy, installation error, ingress of foreign objects, rusting, etc.

Root Cause:

As for the cause of subsurface initiated fatigue is, among other things, caused by surface distress. Under the influence of loads in rolling contacts, described by the Hertzian Theory, structural changes will occur and micro-cracks will be initiated at a certain depth under the surface i.e. subsurface.

There are another two major causes of bearing flaking; (1) Fatigue Life and (2) Improper Handling. (1) Fatigue Life: This is discounted as the cause as the bearing has only been running for two weeks before being removed from service. (2) Improper Handling: There are no signs of any ‘True Brinelling’ with marks on the inner raceway equal to the distances between the rolling elements.

So what is your opinion of the root cause?