How the Construction Affects the Operation of the Gearbox, the Distribution of Loads, and Routine Maintenance

How the Construction Affects the Operation of the Gearbox, the Distribution of Loads, and Routine Maintenance

  • Katarina Knafelj Jakovac

    February 2, 2024

Power transmissions are mechanical assemblies that transfer power and/or motion from a driving machine to a driven one, and they are present in all types of manufacturing and processing industries.

The assembly consists of a driving and a driven mechanical element that rotate and achieve transmission either through direct engagement or indirectly via belts, pulleys, chains, and the like.


Image: Example of a two-stage gearbox by Wärtsilä

The most prevalent are mechanical power transmissions such as gearboxes, multipliers, or transmissions, while gear and belt transmissions are the most widespread types of transmissions.

Appropriate gearbox design, proper operation, timely detection and elimination of faults, and routine maintenance processes are the main elements that will achieve high reliability throughout the entire life cycle of the gearbox.

Successful operation and reliability of the gearbox are the equal responsibility of the designer, operator, and maintainer.

Basic Components of a Gearbox

A gearbox is a mechanical transmission device used to reduce the rotational speed of the input shaft compared to the rotational speed of the output shaft, for example, decreasing the speed of an electric motor when transferring revolutions to a fan.

Machines equipped with gearboxes are often considered auxiliary equipment. Auxiliary machines are frequently taken for granted when they are part of a larger system; however, they are complex and require maintenance.

The cross-section of a gearbox and its components are depicted in the image.

Image: Gearbox (Source)

A more complex type of gearbox consists of the following components:

1Donja polovica kućištaLower Housing HalfUnteres Gehäuse
2Ulazno vratiloInput ShaftEingangswelle
4Zupčanik 2. stupnjaGear 2nd StageZahnrad 2. Stufe
5Međustupanjska čahuraIntermediate SleeveZwischenhülse
6Zupčanik 1. stupnjaGear 1st StageZahnrad 1. Stufe
7Međustupanjska čahuraIntermediate SleeveZwischenhülse
8Konični ležajTapered BearingKegelrollenlager
9Poklopac koničnog ležajaTapered Bearing CoverAbdeckung Kegelrollenlager
10Zupčanik 3. i 4. stupnjaGear 3rd and 4th StageZahnrad 3. und 4. Stufe
11Poklopac kugličnog ležajaBall Bearing CoverAbdeckung Kugellager
14Kuglični ležajBall BearingKugellager
15Zupčanik 3. stupnjaGear 3rd StageZahnrad 3. Stufe
16Međustupanjska čahuraIntermediate SleeveZwischenhülse
17Zupčanik 4. stupnjaGear 4th StageZahnrad 4. Stufe
18Pogonjeni konusni zupčanikDriven Tapered GearGetriebenes Kegelrad
19Pokrov međustupanjskog vratilaIntermediate Shaft CoverAbdeckung der Zwischenwelle
20Međustupanjsko vratiloIntermediate ShaftZwischenwelle
21Zupčanik 1. i 2. stupnjaGear 1st and 2nd StageZahnrad 1. und 2. Stufe
22Pogonski konusni zupčanikDriving Tapered GearTreibendes Kegelrad
23Pokrov pogonskog vratilaDrive Shaft CoverAntriebswellenabdeckung
25Zupčasta spojkaGear CouplingZahnkupplung

Impact of Design on Gearbox Operation

The API 613 standard specifies design requirements for power transmissions intended for use in the oil, chemical, gas, and petrochemical industries.
Conditions affecting the gear tooth load and load distribution are influenced by the design and precision achieved in manufacturing, foundation deflection, even load distribution on gear teeth, alignment accuracy concerning the driving and driven machines, and flexible installation.

The transmission characteristics include the transmission ratio (the ratio of rotational speed between the input (driving) and output (driven) elements of the transmission) and the load distribution on the shafts.

An important factor affecting the alignment accuracy in two-stage gearboxes is the difference in the rotational speed between the front and rear bearings in the gearbox.

Bearing operation is influenced by components of forces generated by load and torque, as well as the component of the action of the gear and housing weight itself.

For certain types of gearboxes, when the static load acting on the front and rear bearings differs in intensity (unlike bearings subjected to equal-intensity loads), the resulting forces will not act in the same direction.

This will cause the front and rear bearings to operate in different positions within their clearance zones.

Gearbox bearings are located in the housing above the lubricating oil reservoir, causing their foundations to heat up during operation, leading to unintentional bearing movement.

On the other hand, a small amount of heat is generated during the operation of the driving and driven machines, which have a higher temperature than the ambient temperature.
In this case, it is inevitable that their operation also affects the operation of the gearbox bearings when, for example, a pump unit transitions from cold idle to operating temperature.

When operating conditions are achieved, the gearbox bearing housing will rise approximately 0.3 mm to 0.7 mm above the bearing housing of the driven machine.

Achieving compensation for thermal and elastic deflection of gear teeth is a crucial factor for reliable operation, allowing for an acceptable distribution of loads among the engaged gears.

Gear teeth in multi-stage gearboxes are designed to operate under the influence of forces and have side critical speeds far above operating speeds.

They will operate without vibrations if balanced according to the appropriate procedure. If steam or gas turbines are the main driving machines, balancing is particularly important for the first-stage reduction gear because it has the same rotational speed as the turbine.

Therefore, the first-stage reduction gear must have the same degree of dynamic balance as the turbine.

In general, gear teeth in gearboxes are made using the turning method. In this process, the cutting tool is a lathe knife, which has multiple rotating cutters on one or more guides, with teeth designed to form the selected basic gear.

During the manufacturing process, the action of rotating cutters cuts the teeth, creating a true spiral shape on the side of the tooth and gaps between the teeth. The knives also determine the dimensions of the teeth in relation to their shape.

Other parameters determining the tooth geometry, the number, and the angle of the helix are determined by selecting the variable gear ratio of the lathe gear without changing the type of cutting tool.

Group 16 (2).png
Image: Geometry and Markings of Straight Teeth Gears (Source: Eugen Oberšmit: Gearings and Gears)

By adjusting the helix angle, determined by the tooth profile in the plane of rotation, it is possible to use the selected tool to obtain the tooth profile in the plane of gear engagement and a specific number of teeth. Therefore, a standardized and limited number of turning tools are used for gear manufacturing.

Another machining process used for the production of large gears is shaping by cutting.

In this process, the shaping tool is either in the form of a toothed rack or similar to a small gear with cutters that remove material during rotation to shape the required tooth profile.

Group 17.png
Image: CoroMill® 172 Turning Tool by Sandvik Coromant for Gear Manufacturing

Post-cutting, various machining processes are applied to enhance the accuracy and uniformity of the gear tooth surfaces.

Grinding is a process where knives with carbide-tipped edges are used to remove irregularities on hardened components of the gearbox.

However, the demand for high precision in gearbox construction often hinders the application of grinding for the final processing of gears.

Rotational honing of gearbox gears is a method to improve the gear finishing process. Honing is a process similar to milling along the axial axis but uses an abrasive tool without teeth.

Specialized honing tools are required to maintain the accuracy of the gear profile. During the grinding process, the side surfaces of the gears are ground, simultaneously achieving the required tooth shape and helix angle.

The grinding process often allows modifications to the tooth profile, helix angle, and root clearances. Changes in the tooth profile enable achieving an optimal load distribution along the teeth and reduce noise when the gears are engaged.

Through the process of axial scraping, a gear with multiple rows of teeth is used to process the surface by scraping the teeth of the gear it engages.

The tool surface has rectangular recesses and a helix angle that differs by several degrees from the helix angle of the gear being processed. The resulting difference in angles allows precise cutting and milling along the edge.

Considering the high rotational speed of the gears during processing and the slower milling of the surface, precise cutting of the side surface of the teeth is enabled.

The result of this process is a finer tooth surface and a more precise shape of the helix compared to a gear made by a turning process.

Milling also allows correction of defects in the helix angle of the gear by selectively choosing the milling surface in contact with the engaged gear.

Image: Gear Manufacturing on a Milling Machine, a) horizontal tooth cutting b) vertical tooth cutting (Source)

Verification of gear meshing and uniform load distribution across the face width of the teeth is crucial for reliable operation and reducing the risk of accelerated tooth wear. Verification of gear meshing using a brake is used as an indicator of mutual contact between teeth.

The check can also be performed using applied layers of copper or by applying blue color.

The imprint left by the blue color or the impression on the copper layer serves as an indicator of meshing.

By using advanced vibration measurement and modulated frequency methods, the condition of the teeth can be monitored during all operating modes and different environmental conditions, recording the impact of all external and internal factors affecting tooth durability during gearbox operation.

In early gearbox designs, irregularities in gear teeth produced during manufacturing were sometimes the cause of vibrations.

However, the precision with which gears are manufactured today has eliminated this source of vibration.

In gearboxes connected to steam or gas turbines, three types of torsional vibrations are distinguished.

In the first type of torsional vibrations, angular vibrations are the greatest, while the largest torsional moment occurs on the gearbox. This mostly occurs during operation in systems with long couplings and can be very dangerous when it occurs in systems with shorter couplings.

The first type of torsional vibrations must be carefully assessed to ensure that the torsional moment added to the transmitted moment under stable operating conditions does not jeopardize the operation of the gearbox Inertia and elasticity factors of the turbine and gearbox have no significant effect on the first critical speed.

The second type of torsional vibrations occurs when two adjacent driving machines, such as turbines, vibrate in opposite directions, and this can happen during operation.

In this case, the torsional moment must be assessed in the same way as in the first critical speed.

Applying a solid drive system prevents the excitation of the second type of vibrations. In such a drive, two turbines are synchronized in operation by adjusting the dimensions of the shafts so that they have the same operating frequency as the gearbox.

As a result, all movements of the shafts of both turbines and the driven machine will not cause the excitation of the second type of vibrations due to the structural stiffness of the driving machine.

The third type of vibration is when the gearbox is prone to bending because it does not have a rigid structure. It usually occurs far beyond normal operation but can affect the operation.

Usually, excitations up to the 12th order are analyzed. Most cargo ship diesel propulsion systems require a flexible torsional coupling to mitigate the stiffness of the shaftline, which also has damping characteristics to reduce torsional vibrations in gearboxes.

Four-stroke engines have excitations of the order of ½, 1, 1 ½, 2, 2 1/2. Two-stroke engines have excitations of the order of 1, 2, 3, etc.

Usually, excitations up to the 12th order are analyzed. Most cargo ship diesel propulsion systems require a flexible torsional coupling to mitigate the stiffness of the shaftline, which also has damping characteristics to reduce torsional vibrations in gearboxes.

The gearbox housing's function is to provide adequate support for the bearings and an oil reservoir for forced lubrication of the gears.

The gears are immersed in oil up to a precisely determined depth. The depth must be maintained at a constant level because too shallow a depth leads to insufficient lubrication, while too deep a oil level causes splashing.

The immersion depth differs for high-speed transmissions, should not exceed one tooth height (H1 < H2), and for low-speed transmissions, it should be two to three tooth heights (H1 > H2), as shown in the image.

Screenshot 2024-02-01 at 17.58.01.png
Image: Forced Lubrication of Gears by Immersion, a) Lubricant Depth for Low-Speed Reducers b) Lubricant Depth for High-Speed Transmissions (Source: Scanned from the book by Eugen Oberšmit: Gearings and Gears)

All bearing loads are in planes perpendicular to the shaft axis. In many cases, especially in the connection with two-stage reducers, the bearing housing must support the bearings at different inclinations.

The next image shows a serviced gearbox with an open upper housing cover. The lower housing has been emptied of oil. New bearings have been mounted on the shafts.

Screenshot 2024-02-01 at 17.59.46.png
Image: Serviced gearbox with newly mounted bearings (Source)

For the proper operation of the gearbox and minimal wear of the gears, it is necessary for the shafts to operate continuously and in parallel to each other. The construction of the gearbox housing is one way to ensure the alignment of the drive and driven machine shafts with respect to the gearbox.

Due to the different directions of rotation of various shafts and the position of the loads on the gears of the gearbox, a force reaction can occur on the bearings at certain angles to the axis of rotation. Therefore, it is important to consider this event in the construction of the housing cover.

The housing is rigidly attached to the foundations to form a structure that will prevent the bending of the shafts of the gears.

Except for small auxiliary reducers, the housings have separate inspection covers to check the condition of the gears and to be able to inspect and replace bearings, damaged parts, and lubricating oil.

The construction and rigidity of the gearbox housing must be studied and compared with the structure and rigidity of the foundation on which the gearbox lies and the assembly where the gearbox housing is bolted to the foundation plate in the manufacturing plant.

It is important that the supporting assembly has the appropriate structural strength to prevent the occurrence of any measurable bending due to the action of different loads.

Couplings connecting the turbine shafts to the reducers are also important in determining the lateral critical speed in the turbine-reducer assembly and should be taken into account in the assessment.

The combination of an electric motor, couplings, reducer, and pump forms an assembly that will torsionally vibrate in reaction t impulses caused by the rotation of the pump rotor.

Problems in the operation of the gearbox are also caused by excessive coupling stiffness and improper centering, hence continuous improvements in construction and exchange of experiences between operators, maintainers, and designers are necessary to enable satisfactory installation.

During installation, it is necessary to follow the required procedures and recommendations of equipment manufacturers and apply best practices during alignment.

Load Distribution

Load distribution on different machines with a special emphasis on power transmissions is classified into 3 categories: J – uniform load (uniformly distributed), U – moderate load, and T – heavy load.

In the table, loads are classified by machine type and industry type.

Fans and blowersLoad distribution
Fans (radial and axial)J
Cooling tower fansU
Draft fansU
Turbo blowersJ
Chemical industry
Cooling drumU
Mixer / BlenderU
Reciprocating compressorsT
Turbo compressorsU
Overhead craneU
Crane with hookJ
Gearbox with internal and external gearsU
Centrifugal pumpsU
Piston pumpsT
Screw pumpsU
Dredger winchesT
Equipment on oil wellsT
Washing machinesU
Food industry
Bottle filling machinesJ
Can pressU
Metal shearsT
Raw material crusherU
Dough making machineU
Packaging machinesU
Sugar beet crushersJ
Construction machinery
Concrete mixersU
Cargo liftsU
Asphalt paving machineU
Generators and transformers
Frequency transformersT
Generators on welding devicesT
**Metal processing industry **
Sheet metal pullersT
Chain conveyorsU
Roller pressT
Pneumatic hammerT
Conveyor beltsU
Ingot machineT
Pipe welding machineU
Wire winding machineU
Forging pressesT
Drilling pressesT
Processing of rubber, plastic, paper, textiles, wood
Raw material pressesT
Raw material crushersT
Spinning machineU
Wool combing machineU
Band sawsT
Woodworking machinesJ
Painting and varnishing machinesU
Sample-making machinesU

Most Common Gearbox Failures

The most frequent causes of gearbox failures include overloading, pitting, shaft bending due to material fatigue, surface material wear on gears, abrasion, cavitation, and erosion.

Cavitation and erosion on the surfaces of gearbox materials lead to small cracks.

Pitting corrosion is a cause of gear failure given that they are exposed to significant surface stress in contact and numerous repetitive stress cycles.

Consequently, small cracks start on the gear surfaces or in the first layer of metal just below the surface. The crack grows until a piece of metal breaks off.

Cracks usually initiate at the surface where the oil film is thin and where metal surfaces come into contact. Cracks in the first layer below the surface are due to irregularities, i.e., inclusions in the metal.

Abrasive particles in the oil can also cause pitting as they adhere to the gear surface, increasing the stress intensity.

Wear occurs due to a lack of lubrication or when the oil film becomes too thin due to increased temperature, the use of oil with inappropriate viscosity, or the gearbox's operational overload.

During the gearbox's running-in phase, mild wear typically occurs as surface irregularities smooth out during the contact between surfaces.

During this phase, the gearbox should operate at reduced load to avoid excessive wear on the contacting surfaces. After completing the running-in phase, gear surfaces become smoother, reducing wear.

Abrasive wear of materials happens due to oil contamination, the presence of metal particles generated by gear wear, impurities entering from the outside during maintenance, leaks at sealing joints, or leaks through vents.

Impurities cause premature tooth wear. Vents on the gearbox housing may have filters, and all connections must be appropriately tightened to prevent impurity entry.

The first check for the gearbox involves examining the condition of the lubricating oil. Regular oil analysis is necessary, including checks on viscosity, water content, and the presence of metal particles.

In new gearboxes, metal particles left after assembly should be removed during oil circulation using a mesh or sieve. The initial oil change should occur after a trial run, and oil filters should be replaced or cleaned. Smaller gearboxes may have magnetic plugs that trap metal particles.

These plugs need to be inspected, and the amount of deposited particles should be checked and removed. Larger gearbox lubrication systems may use magnetic filters, requiring regular cleaning. Oil must always be kept clean.

Oil samples should be sent for spectrometric analysis to determine the metal wear quantity and to check for water presence and viscosity.

If there is a trend of increasing metal particle levels in the oil, a ferrographic analysis should be performed to determine the size and type of particles in the oil.

If large particles of impurities are discovered, it is necessary to open the gearbox and inspect the gears.

When disassembling the housing cover to inspect the gears, the entire oil quantity should be pumped out, and the housing should be thoroughly cleaned.

Before closing the cover, check that nothing has fallen into the housing. If the gearbox must remain open for a certain period, it should be covered with a protective nylon to prevent impurity entry.

During the examination of the teeth on the gears, slowly rotate the input shaft and thoroughly inspect all the gears in sequence.

Mark the initial gear with a marker to ensure that all gears are indeed inspected. Surfaces of gears in contact should have the same width throughout the tooth length.

If the width is smaller at one end and larger at the other end, there may be eccentricity. Detected damage on the teeth must be marked and photographed for future inspections. If the damage to the teeth is severe, it will be necessary to replace the entire gear.

Elements of Routine Maintenance Process for Gearboxes

To prevent and/or address failures in gearboxes within process and manufacturing plants, continuous maintenance activities are essential.

The routine maintenance process is a crucial part of daily operations in industrial facilities, encompassing proactive (predictive and preventive) and reactive maintenance (repairs). The process includes proactive (predictive and preventive) and reactive maintenance (repairs).

The individuals involved in this process include maintenance engineers, plant operators, and workshop personnel specialized in various fields such as electricians, mechanics, and instrument technicians. Additionally, specialists from different areas may be brought in as needed.

The size of the plant and the number of machines determine the workforce, with a coordinated effort among various professionals based on priorities.

Routine maintenance is challenging to sustain with a high level of quality and efficiency over an extended period due to several reasons. These challenges include dependence on decision-makers weighing current risks against long-term contributions, insufficient training of personnel, and involvement of a diverse workforce.

Other challenges include conflicting priorities, companies rewarding employees engaged in firefighting situations daily, repetitive tasks extending beyond scheduled timelines, urgent matters interrupting planned work and disrupting schedules, assigning simpler repetitive tasks to less technically skilled employees while assigning more complex tasks or handling emergencies to more skilled workers.

The fundamental organizational prerequisites for successful routine maintenance include the existence of a maintenance department operating during regular working hours, immediate reporting of urgent breakdowns at the beginning of the workday, resolving urgent breakdowns until the completion of the job and the machine's startup, and addressing high-priority planned tasks as needed, even on weekends.

Let's examine the key elements in the routine maintenance process and the resolution of repairs using the example of a machine such as a gearbox.

The diagram illustrates all the components of everyday routine maintenance.

Screenshot 2024-02-02 at 13.42.50.png

Image: Elements of Routine Maintenance Process (Source: Diagram created by the user)

Let's proceed step by step:

Notification of Gearbox Downtime and Fault Diagnosis

In most cases, a maintenance service employee notifies about the gearbox downtime and the occurred malfunction.

They are tasked with conducting a diagnosis or, if the operator on the production floor is uncertain about the type of malfunction, they should inform the maintenance service employee to come and diagnose the issue.

For complex machinery systems, it might be necessary to engage a specialized company that deals with specific diagnostic procedures.

A malfunction implies that the gearbox is no longer performing its intended function or that its operation deviates from the norm. For example, the gearbox may frequently trigger an alarm for elevated oil temperature, sensors may record increased vibrations, or the operator may hear unusual scraping noises during gearbox operation.

After diagnosing the malfunction, a fault report is generated, containing essential information such as the date and time of the malfunction, the technological designation of the gearbox, a specific and concise description of the malfunction, the consequences of the malfunction on the production process (environmental pollution, product loss, plant shutdown, etc.), and the operating conditions of the gearbox.

Setting Priorities and Scheduling Work

Determining priorities for repairs and scheduling work are two elements that are planned together to address the following questions:

a) Which malfunctions are urgent and need to be resolved as soon as possible?
b) If a malfunction is not urgent, within what timeframe must it be rectified?

If all malfunctions are urgent, it creates system and resource overload, leading to higher costs. Therefore, it is essential to assess the justification of each urgent repair and, if possible, address as many malfunctions over an extended period.

Apart from urgent malfunctions, there are others that have priority for resolution within a few days, two weeks, a month, or several months, depending on planned semi-annual/annual services.

Asset and Maintenance Management Software (CMMS) provides options for selecting the priority of malfunction resolution in its menu.

Planning and Executing Work

The next logical step is work planning for fixing the gearbox malfunctions. Unplanned repairs and mistakes in prioritization take up to 4 times more time and resources compared to planned works with predefined priorities.

The main goal should be to achieve the maximum number of scheduled works according to specified priorities.

Characteristics of planned works include having all necessary resources (tools, materials, spare parts, specialized workers, gearbox preparation for works, work permits, auxiliary machinery, transport) available at the scheduled time and location to completely eliminate the malfunction.

The work execution schedule must be organized to minimize downtime and unnecessary extensions.

Before planning works, a review of the gearbox location is necessary, considering preparatory works and required resources based on whether the repair will be carried out in a workshop or on-site.

An estimated time for actions before putting the machine back into operation (gearbox mounting on foundations, coupling connection, gearbox oil filling, air bleeding, etc.) should be evaluated.

During work planning, it is crucial to specify whether scaffolding, transport vehicles (crane, truck, forklift, etc.), special protective equipment, etc., are required for the execution of works.

Subsequently, a work order is drafted, outlining the sequence of works with accompanying resources, tools, material lists, transport vehicles, and all necessary workers by profession.

The work order is sent to all involved personnel and stored in CMMS for accessibility when needed.


Evaluation, Measurement, and Process Adjustment

After complex and lengthy repairs and starting the gearbox in operation, most maintainers are simply relieved that the job is finally done and they can move on to repairing other machines.

Instead of pausing to question why it was so challenging and arduous, we are just happy to leave it behind and start working on other machines from tomorrow onwards.

A more detailed analysis would highlight all flaws and irregularities, but due to work overload, numerous obligations, and other disruptions, such an approach has unfortunately become the rule rather than the exception for most maintenance workers in many companies.

Many business processes include evaluation forms as a standard part of operations after completing a project or activity, precisely determining which steps caused delays or losses. However, in the practicalities of daily work, we simply lack the time or resources to delve into such analyses further.

Even when an attempt is made to introduce process evaluation into the routine maintenance process after resolving a malfunction, it often turns into additional paperwork requiring the involvement of many people and additional burdens on an already packed schedule.

Evaluation should allow employees to report on all situations not in line with expected procedures and all negatives that led to delays or losses to work more efficiently and learn from past mistakes.

Evaluation should indicate the percentage of unplanned works that occurred, the percentage of additional works that appeared in the defect elimination process, the percentage of works that were in line with the planned ones, and a comparison of planned and achieved work, materials, and resources.

Evaluation should be conducted every Monday for the previous week, noting all irregularities to identify recurring incorrect steps and how to correct or eliminate them and improve planning in the future.

This continuous process helps in improving the routine maintenance of machines and equipment.

The final step is adjusting the routine maintenance process based on the conducted evaluation.

Every increase in efficiency requires constant adjustment because, in the practice of maintenance work, having two identical weeks in terms of required resources, materials, or works has never happened.

If the amount of required resources in the workshop or on-site is identical every day, it means you either have too few or too many. If you have too few, the risk increases, and if you have too many, it means resource management is not as efficient as it should be.

Katarina Knafelj Jakovac
Katarina Knafelj Jakovac social media icon
February 1, 2024

Katarina Knafelj Jakovac holds Master degree in Mechanical engineering with long term work experience in Oil industry. She is Certified Reliability Leader specialized for mechanical equipment and operational excellence. Author of blog Strojarska Radionica (Mechanical Workshop) where she shares professional knowledge and personal experience in maintaining various rotating machines, machine systems and process equipment. Adores mechanics, thermal engineering and internal combustion engines. She is dedicated to the continuous improvement of machine maintenance and quality management of physical assets.