Componentes de Transmisión Mecánica: GEARBOX INSPECTION FOR NON-EXPERTS

By: Eduardo Niño de Rivera

Edition by: John Amendola Sr.

Artec-Machine Systems.

INTRODUCTION

We tend to think of gearbox inspections as a thorough examination of all the components of a gearbox performed by a specialist. Although the final diagnosis and the recommended course of action should be performed by specialists, unusual noise, a change in vibration patterns or a sudden increase in temperature may require an immediate inspection of gearbox components. If time, cost and/or not having a specialist available make it necessary for plant personnel with limited experience to carry out the inspection, obtaining useful data to convey to the specialists requires planning, preparation and following proper procedures. This article offers advice to non-specialists on how to carry out an onsite gearbox inspection to obtain the maximum benefit out of it.

IMPACT ON PRODUCTION

The importance of avoiding an unexpected failure from a particular gearbox depends on the production lost during the associated shut down. It would be impractical to try to keep detailed records of all incidents and interventions on each gearbox in a plant, however, these records must be kept for those whose unexpected failure may represent large losses to the company. We suggest the following classification for maintenance priorities.

A: A failure of these gearboxes may shut down a production line or an entire plant, with production losses worth thousands of dollars per hour. These gearboxes must be under continuous observation, preferably through condition monitoring systems.

B: Gearboxes in non-critical processes, that may be repaired or replaced without disrupting production.

C: Gearboxes in auxiliary processes that may be stopped for extended periods of time without a major impact on production.

Priority	Description	Maintenance
A	A shut down brings heavy production losses	Continuous observation, programed in depth inspections, condition monitoring
B	Non-critical processes, may be repaired or replaced without disrupting production	Routine observation, programed general inspections
C	Auxiliary processes, may stop for long periods without major impact on production	Sporadic observation, inspection as required

The first step toward an effective control of group A gearboxes is to keep record of basic identification data (see chart below), the incidents and the interventions that have been performed on it.

COMPANY

PLANT

LINE

MACHINE

GEARBOX

MANUFACTURER

MODEL

SIZE

SERIAL NO.

RATING

GEAR RATIO

INPUT SPEED

RPM

OUTPUT SPEED

RPM

SERVICE FACTOR

MANUALS

DRAWINGS

BACKGROUND

The first level of inspection could come from maintenance and production personal in daily contact with the gearbox. At this level, defects that hinder the machines’ proper performance, increase maintenances costs, reduce the useful life of components, and cause unexpected shutdowns can be identified. Joel Levitt considers friction, dirt and looseness as the defects that contribute the most to machine decay. Once identified, these defects can easily be corrected increasing plant productivity [9, 10]

“the Battle For Reliability”, Joel Levitt [9], reproduced with the author’s permission

After an initial run-in period, a gearbox will settle into a “normal” operating behavior, displaying steady levels of noise, vibration and temperature. Ideally these levels are measured and monitored regularly, but this is often not the case, and it is up to the plant personnel to be familiar with what the normal operating condition “feels like” and to report when the machine does not seem to feel well.

ROUTINE INSPECTIONS:

Whether the condition of the gearbox is being monitored or not, whenever plant personnel are near a gearbox, they should look for and report any sign of distress:

“the Battle For Reliability”, Joel Levitt [9], reproduced with the author’s permission

· Keep the casing clean. A gearbox covered in dust will lose heat dissipating capacity and it will operate at a higher temperature, reducing useful life of gears, bearings, seals and lubricating oil.

· Listen to and touch the gearbox. Noise, vibration and temperature should always be within acceptable levels.

· Check the level of the lubrication oil, it must always remain within the level indicated by the manufacturer. If it needs regular replacement, check for leaks and high operating temperatures.

· Look for signs of overheating, like case temperatures measured with an infra-red thermometer, smoke coming out of shafts or seals, discoloration of metal components, burnt paint, surfaces too hot to touch or water evaporating too fast when sprinkled on bearing housings or shafts.

· Look for oil leaks in seals, gaskets, mating surfaces and throughout the circulating oil system.

· Check tightness of bolts on the base, housing split and bearing side plates.

· Check for sings of displacement between adjacent components (gaps, cracks in paint, markings on surfaces).

These simple habits can have a great impact on the time between repairs and provide waring for potential failures. An inspection can then be scheduled and prepared for, avoiding costly unexpected shutdowns

SCHEDULED INSPECTIONS

Beyond daily observation, we recommend scheduled in-detail inspections to record above data. noise, vibration and temperature measurements should be recorded and compared to previous measurements to identify any changes, with special attention to upward trends in any of these parameters. Such a trend could mean that a failure condition is developing and that a more thorough inspection may prevent a catastrophic failure and the corresponding collateral damage. During these inspections oil filters and magnetic traps must be checked and cleaned, and an oil analysis must be performed by a certified laboratory.

VISUAL INSPECTION OF GEARS:

Ideally, visual inspections should be conducted by a specialist, but under certain circumstances, such as a sudden change in temperature, noise or vibrations, a non-specialist must perform a visual inspection. The following are recommended practices for a fast and thorough inspection that may lead to well-informed decisions regarding the immediate course of action.

GATHER INFORMATION:

It is always a good practice to gather as much information as possible, specifically, name plate data, drawings and manuals that should have been provided by the manufacturer. The other important source of information are the observations, data and reports that made the inspection necessary. The combination of these two sources will provide an idea of the size and nature of the task ahead.

OUTSIDE DYNAMIC INSPECTION (DURING OPERATION):

If possible, the gearbox should be observed during operation to check for:

· Cleanliness.

· Noise.

· Vibrations (if the proper equipment is available).

· Signs of overheating (take temperatures if possible).

· Casing integrity (look for cracks or oil leaks)

· Bolt tightness.

· Signs of displacement relative to the base.

· Signs of displacement between mating parts.

· Oil levels.

· Oil samples and temperatures (if accessible).

· The condition of the circulating oil system (pipes, hoses, ports, valves, gages, pumps, etc.).

· The condition of sensors and electric connections.

· Spray a soap in water solution into the base, the solution will start bubbling if the base is not set properly on the baseplate or the foundation, a condition known as soft foot.

PLANNING:

Discuss with the specialists that will provide the diagnosis and recommendations, what data and other information will be required and how it must be presented. The working plan should include:

· The objectives of the Inspection (what will be delivered, data, information, reports).

· Complying with safety protocols.

· Special tools and equipment required.

· Personnel requirements (numbers and skills).

· Machine down time allocation.

· Clean space required.

· Tasks to be performed (personnel, tools, time and sequence).

OUTSIDE GEARBOX STATIC INSPECTION (WHEN STOPPED):

Once the gearbox has cooled down, further inspection may take place. Start by making sure that all safety protocols are in place, then thoroughly clean the casing to ensure that no dust or other contamination can fall into the gearbox when the inspection plate or the cover are removed.

Make sure the necessary brakes are applied to the motors and the machine to prevent them from running loose once they are disconnected from the gearbox. Mechanically disconnect the gearbox from the motor, the brake and the machine to allow the gearbox shafts to rotate freely.

MEASURING END PLAY:

Place a dial indictor on a fixed surface with the stylus touching the shaft’s end face, push the shaft into the casing with enough force to make sure it is placed as far as it can go and slowly rotate to make sure the bearing balls or rollers make proper contact with their races, set the dial indicator to zero, pull the shaft out as far as it can go and rotate to measure proper ball or roller contact, record and compare the reading with the end play specified by the gearbox manufacturer.

MEASURING BACKLASH:

It is not usually necessary for this type of inspection, but it is good practice to keep track of backlash evolution over time as an indicator of bearing and tooth wear. If the maintenance manual provided for the gearbox includes a backlash specification and a way to measure it, follow the procedure outlined in the manual and compare your measurement to the manual’s specification. If there is no backlash specification, it can be measured by clamping a straight edge to the shaft coupling and measuring how much the input shaft can freely rotate when the output shaft is held in a fixed position Total Indicator Reading (TIR). Ideally it is measured at the pinion’s pitch circle diameter. In order to be able to compare consistent date obtained at different times, the measuring procedure and measuring position must be the same on every inspection.

INTERNAL VISUAL INSPECTION:

Before removing the inspection cover or the casing top, make sure the casing and its surroundings are clean and no dust or other contamination may fall into the gearbox once it is opened; remove all contents from your shirt pocket to prevent them from accidentally falling into the gearbox; and go through the check list of required tools, equipment, available space and personnel.

Remove the inspection cover following the manufacturer’s instructions. If the instructions are not available, slowly loosen the bolts in a crossed pattern to evenly release the pressure from the cover. Always use the supplied separating or lifting threaded holes and remove the cover perpendicular to its support surface to avoid damaging the cover or the casing. Take care not to allow gaskets or sealing compound to fall into the gearbox and do not allow the cover to swing, putting people and equipment at risk.

Photograph courtesy of Artec-Machine Systems.

In many gearboxes, the contact surfaces of all gear teeth are in plain sight when the inspection cover or the top of the casing are removed, in others, a borescope may be required to see some of these surfaces. Although a non-specialist may not be able to recognize the type of damage or what caused it, it is usually not difficult to see wear patterns, pitting, cracked teeth, scratches, rust or other damage to the gear teeth. Their description and photographs can be sent to the specialists for analysis and recommendation.

In some cases, the contact patterns obtained during original assembly at the manufacturers plant are still visible, if not, a new no-load inspection may be done (details will be covered in a future article).

It is important to learn if the gear teeth are case hardened or thru hardened. This information should be noted on the gear data sheets or drawing. If information is not available a measured check can be done with a hardness checker on the end of the tooth face very near the surface or top land of the tooth. Otherwise, a simple hand file passed of a tooth tip is a fair way to compare. Hardened surfaces do not scratch easily. This simple test provides the inspector an indication of tooth wear. Thru hardened gears will polish over time and initial macropitting may be tolerated for continued use with periodic inspection of the trended nature of the surfaces. If the pits grow spalling will develop an indication that the gear is failing.

Non-progressive macropitting Progessive macropitting

Pictures by ANSI/AGMA (1010-F14 [6] publication)

Macropitting in case hardened gears are more critical and can quickly lead to rapid deterioration of the surface. Hardened surface gears can develop different surface distress such as micropitting or scuffing which may be an indication of inadequate load distribution due to bearing wear, structural support, misalignment, or changes is external load or environmental conditions. Corrective action may arrest the distress and if the damage were not severe, the gear may remain in operation.

KEEP A LOGBOOK:

It is important to keep records of the condition of the gearbox and all its components, as well as of all the activities performed on it. When possible, take photos of the rotor elements and bearings and archive their respective condition into the report for future comparison.

Should the anomalous operation continue after finding no evidence of damage, a specialist must be called in to do a second inspection as soon as possible.

REPLACING THE COVERS:

Ideally, a maintenance manual with instructions to close the gearbox would be available, otherwise, the procedure to remove the inspection cover or the top of the casing should be followed in reverse sequence. In the Process DO NOT:

· Bang any parts.

· Allow foreign objects or matter to fall into the gearbox.

· Damage dowel pins.

Make sure all bolts and lubrication connections are properly tightened; filters, gaskets, sealing compound and seals are in place; and all instruments are working in order.

START UP:

Again, follow the manual’s procedure when available (this topic will be covered in a future article)

SUMMARY:

A sudden increase in noise, vibration, temperature, or metal debris in the oil may warrant an immediate inspection of the gearbox. Time, cost or non-availability of an expert, may make it necessary for plant personnel with limited experience to perform a visual inspection of the gear teeth condition. Getting the most of such an inspection requires good communication with the expert that will do the final analysis and recommendation, and proper planning, preparation, execution, and record keeping. Finally, there is no substitute to following sound maintenance practices and common sense to perform these tasks in a safe and productive manner.

REFERENCES

1.- https://www.horsburgh-scott.com/resources/PDFs/hs-maint-manual.pdf Manual de mantenimiento de Horsburgh Scott

2.- https://www.powertransmission.com/articles/0314/Best_Practices_for_Gearbox_Assembly_and_Disassembly/

3.- https://www.machinerylubrication.com/Read/28765/how-to-inspect-a-gearbox-

4.- https://www.engineerlive.com/content/top-10-tips-industrial-gearbox-inspection-and-maintenance

5.- https://fieldservicesengineering.co.za/gearbox-maintenance/

6.- https://webstore.ansi.org/SDO/AGMA?gclid=Cj0KCQiA7oyNBhDiARIsADtGRZYWHCXT9-PAX-wSTWUucvcovdbsBX5FYce-NdGmpqgtYP6F96ecO4waAsavEALw_wcB ASI/AGMA 1010-F14 Appearance of Gear Teeth – Terminology of Ware and Failure. Febrero 2020

7.- https://www.geartechnology.com/issues/1192x/faure.pdf Classification of Type of Gear Tooth Wear – Prat I

8.- https://www.geartechnology.com/issues/0193x/faure.pdf Classification of Types of Gear Tooth Wear

– Part II

9.- The Battle for Reliability, Joel Levitt, 2021, Springfield Resources

10.- The Quest for Defect Elimination, Joel Levitt, 2020, Springfield Resources