HOW TO GET THE MOST OUT OF A GEARBOX INSPECTION

INTRODUCTION

(English version of the article published in Spanish in this blog on March 11, 2019)
Maintenance and Production personnel are usually well aware of the financial consequences of unscheduled shutdowns due to gearbox failures in critical equipment. Given today´s technology, the best way to prevent unscheduled shutdowns is through condition monitoring systems with sensors fixed to the gearbox. These systems continuously measure, record and analyze vibrations, temperatures, oil pressures and other variables to detect impending damage on gears, bearings and other gearbox components. In a perfect world, condition monitoring would give us the necessary information to take timely corrective action, thereby avoiding unscheduled shutdowns. However, not all gearboxes are equipped with these systems, and even when they are, an inspection by a qualified technician may be necessary to determine the underlying cause of failure. Depending on a plant’s systems, inspections may be scheduled based on the number of hours the equipment has been in operation; when unusual vibration, noise, temperature, lubricant contamination or other anomalous conditions indicate there is a risk of failure in the near future; or to determine the cause of an incidental or recurring failure.

Proper planning and preparation are essential to perform a quick and thorough inspection of all gearbox components for an accurate assessment of their condition, cause of initial damage, expected remaining useful life, immediate actions that may reduce the probability of failure before the next scheduled shutdown and remedial measures that may offer longer time between gearbox related shutdowns in the future. This article discusses the roll that non-gear-specialist maintenance and production personnel play in planning, preparing and collaborating with gear specialists to make inspections of critical gearboxes as quick, thorough and accurate as possible.

CAUSES OF FAILURE

The identification and analysis of gear failures is beyond the scope of this article, there is plenty of literature on the subject [references]. Here, we will only offer a list with some of the most common causes of gear failure:

Cause	Damage
Contact fatigue Sub-Case Fatigue	Initial macro pitting Advanced pitting Spalling Macropitting Tooth breakage
Bending stress fatigue	Root fillet cracks Partial Tooth factures Tooth Fractures
Lubrication film brake down	Fine grain spalling Micropitting Latent micropitting Scuffing Plastic flow Tooth Burning Fretting corrosion and false brinell
Lubricant contamination	Tooth corrosion Abrasive scoring and ware
Electric arcing	Electric pitting
Manufacturing defects	Premature pitting and spalling Premature tooth braking Hub destruction Grinding fractures
Overload (insufficient design service factor, unexpected peak or shock  loads, increase in output demand)	Cracks Fractures Macro pitting, pitting and spalling Plastic flow

PLANNING AND PREPARATION

BEFORE THE SPECIALIST HAS ACCESS TO THE GEARBOX,

DO NOT:

1.       Remove the gearbox from the machine if at all possible. A better diagnose can be rendered if, prior to inspecting the internal components, the specialist is able to observe and measure vibrations, temperatures and oil pressures under regular operating conditions and a tooth contact check to assure proper load distribution over the tooth flanks.

2.       Open the gearbox. It is important for the specialist to inspect the gear casing for leaks and cracks while the gearbox is still closed with its seals and bolts as they have been during operation.

3.       Clean or drain the lubricant without taking a complete set of samples. A proper oil analysis requires samples from different spots, including lubricant input and output ports for the oil cooling system, top, middle and bottom of the oil sump, gear lubrication nozzles, filters, magnetic particle traps and lubricant storage drums. The condition of the oil in the different locations may provide information regarding the probable sources of lubricant contamination and on where and how the failure originated.

PLAN:

1.       Gather as much information as available on the gearbox and its application: manufacturer, model, size, serial number, power rating, speed ratio, date purchased, date of commissioning, drawings, parts lists, service history and normal operating temperatures, oil pressures, speeds and power actually delivered by the motor.

2.       Describe in detail the reason for the inspection (suggested by the service manual, unusual noise, temperature or vibration, etc.).

3.       Request from the gear specialist a description of the tasks to be performed, a time table and a list of spare parts, tools, equipment and personnel required for the inspection.

4.       Reach a clear understanding with the gear specialist regarding the inspection procedures, the laboratory tests to be performed and the reports that will be submitted upon completion.

5.       Schedule the necessary test and shut down periods.

6.       Make sure the spare parts, tools, equipment and personnel will available during the inspection

DURING THE INPECTION:

1.       Be available to take care of any contingencies.

2.       Coordinate the timely availability of spare parts, tools, equipment and personnel.

3.       Overview and record the sample gathering, lab testing, inspection and report presentation procedures.

AFTER THE INSPECTION:

1.       Review all reports

2.       Overview the startup procedure

3.       Keep all recorded data for future reference

4.       Follow the operation and maintenance procedures recommended as a result of the inspection

CONCLUSION

Rather than something spectacularly new, this article follows sound practices that have proven their effectiveness through many years and across many industries. Planning and preparation are an essential part of any productive activity and the inspection of gears is no exception.

References

1.- AGMA GEARTECH HANDBOOK

2.- The Gear Reducer Book. Peerless-Winsmith, Inc 1980

3.- Bob Errichello, Jane Wuller. How to Analyze Gear Failures, Machinery Lubrication

4.- Failure Analysis Gears-Shaafts-Bearings-Seals. Rexnord 1978