4.6 MW Ball Mill Replacement Gearbox: Engineering, Manufacturing and Inspection Case Study

Quick answer: this project involved the engineering, manufacture, inspection and delivery of a 4.6 MW heavy-duty replacement gearbox for an existing ball mill drive. The unit was developed around existing installation constraints and verified through design review, gear manufacturing records, assembly inspection, running tests and final release before shipment.

Replacing a large mill gearbox is fundamentally different from selecting a standard industrial reducer. For an existing ball mill installation, the new gearbox must transmit the required power and torque while also matching the existing shaft arrangement, mounting interfaces and operating conditions. That is why large replacement projects require both manufacturing capacity and disciplined engineering review.

This case study shows how KNGEAR handled a real industrial gearbox replacement project from drawing review through manufacturing, testing and shipment. Customer identity, project references, drawing numbers and commercial information have been removed.

Project Overview

The Main Challenge: Replacing an Existing Large Gearbox

For a new installation, surrounding equipment can often be designed around the gearbox. A replacement project works in the opposite direction: the gearbox has to work within constraints that already exist.

Shaft positions, centre heights, coupling interfaces, foundation dimensions, rotational direction and available installation space can all limit the design. In this project, the operating data and existing installation information were reviewed first, then a replacement gearbox layout was prepared for technical approval before manufacturing proceeded.

This review stage is critical for large industrial gearboxes. A dimensional difference that might be insignificant on a small standard reducer can create major installation problems on a multi-megawatt mill drive.

Two-Stage Helical Gear Arrangement

The gearbox uses a two-stage parallel-shaft helical gear arrangement. With an input power of 4,600 kW and input speed of 992 rpm, the required transmission ratio was approximately 7.06:1, giving an output speed close to 140 rpm and a nominal output torque above 300 kNm.

The engineering review considered gear tooth strength, flank loading, shaft deflection, bearing life, rotational direction and lubrication requirements. Gear calculations were reviewed using ISO 6336:2019 criteria. The project was therefore not treated as a simple dimensional copy of an existing unit.

Design Review Led to Engineering Refinement

One of the most important aspects of this project is that the gearbox did not move directly from an initial drawing to production. The design was reviewed and refined.

The review covered gear safety factors, bearing life, shaft arrangement, sealing and lubrication. Where appropriate, the internal configuration was adjusted to improve performance while retaining the dimensional envelope required for replacement.

This reflects KNGEAR’s approach to custom industrial gearboxes: existing drawings are valuable inputs, but they should not be copied blindly when a technical review can identify opportunities to improve reliability or manufacturability.

Case-Hardened and Ground Gears

The major gear components were manufactured from case-hardening alloy steel, followed by carburizing, quenching and precision gear grinding. Project correspondence recorded a finished tooth-surface hardness in the range of approximately 58–62 HRC.

The quality review did not stop at the statement “carburized and ground.” Carburizing records and gear grinding reports were reviewed together so that the effective hardened depth after finishing could be checked.

For one group of components, the recorded grinding removal was approximately 0.50 mm in total, or 0.25 mm per flank. For another group, the recorded total was approximately 0.80 mm, or 0.40 mm per flank. Carburizing test pieces were retained and shipped with the gearbox for traceability.

This level of process documentation is particularly valuable for high-power replacement gearboxes, where engineering teams need evidence that heat-treatment and finishing processes correspond with the design requirements.

Gear Contact and Backlash Inspection

After assembly, the mating gear pairs were checked for contact condition and backlash. The inspection criteria required more than 50% contact across tooth height and more than 70% across tooth length, and the recorded contact condition exceeded those acceptance requirements.

Recorded backlash values were approximately 0.28 mm for the input-stage gear pair and 0.32 mm for the output-stage gear pair.

These checks help verify that gear machining, shaft positioning, bearing installation and final assembly are working together as intended. For large custom gearboxes, assembly inspection provides evidence that individual component accuracy has translated into an acceptable complete gearbox.

Bearing Arrangement and Temperature Monitoring

The project used heavy-duty SKF rolling bearings. The design review considered both bearing life and the fixed/floating bearing arrangement needed to accommodate load and thermal expansion.

PT100 temperature monitoring points were incorporated around the bearing positions. For heavy-duty mill gearboxes, this provides useful condition information both during commissioning and subsequent service.

No-Load Running Test

After assembly, the gearbox underwent a documented no-load running test together with visual, dimensional and assembly inspections.

The recorded test measured an input speed of approximately 990.4 rpm and an output speed of approximately 140.0 rpm, corresponding to a measured ratio of about 7.074:1. The recorded noise level was approximately 74 dB(A), and vibration measurements were taken at multiple gearbox positions against the specified acceptance criteria.

The inspection programme also recorded a localized temperature concern around the input-shaft sealing area during the two-hour test. Rather than treating the test as a formality, the condition was documented for engineering review before final release.

That is an important part of meaningful gearbox quality control. A useful inspection process is intended to identify abnormal conditions before shipment, not merely generate a pass certificate.

Housing and Sealing Integrity

The final inspection documentation also included a housing pressure test at approximately 0.35 bar. After the specified stabilization period, the pressure differential acceptance requirement was less than 2 mbar.

For grinding-mill applications, sealing is important because the gearbox has to retain lubricant while limiting the ingress of external contamination. The project therefore treated sealing as part of the gearbox design rather than as a minor component choice.

Lubrication Was Reviewed as Part of the Engineering Process

Lubrication requirements were reviewed during the project. The engineering review considered controlled oil supply to the bearings and gear meshes together with cooling and filtration requirements, rather than assuming that the original arrangement should simply be reproduced.

The completed gearbox incorporated lubrication piping. Later project feedback on the gearbox was positive, while a minor detail associated with the connection of the upper lubrication piping was identified for local refinement. This kind of field feedback is valuable in custom gearbox engineering because it can be fed back into future replacement designs.

Final Release, Packing and Shipment

Before shipment, the engineering and quality documentation was reviewed together with the outstanding technical questions related to manufacturing and inspection. Once the review was completed, the gearbox was released for shipment.

The unit was then protected and packed for international transport. Post-delivery feedback indicated that the gearbox and the improvements incorporated into the unit were positively received.

What This Project Demonstrates

The most important part of this case study is not simply the 4.6 MW power rating. It demonstrates the process required to manufacture a serious industrial replacement gearbox.

A large replacement project requires the manufacturer to understand the original installation, convert existing information into a manufacturable design, review gear and bearing calculations, control critical manufacturing processes, document heat treatment and gear grinding, verify assembly quality and conduct meaningful final testing.

That is the type of work KNGEAR focuses on: large industrial gearboxes, custom units and dimensionally compatible replacement gearboxes for demanding applications.

If an existing gearbox is obsolete, unsupported by the OEM, uneconomical to repair or difficult to source, KNGEAR can evaluate the available drawings, nameplate information, installation dimensions and operating data to determine a practical replacement route.

Need a Replacement for an Existing Industrial Gearbox?

For an initial engineering review, send the available gearbox drawings, nameplate data, input power and speed, ratio or output speed, shaft dimensions, mounting dimensions and application information.

Learn more about KNGEAR gearbox solutions, gear manufacturing, or contact KNGEAR to discuss a custom or replacement industrial gearbox project.

Ball Mill Gearboxes — Planning a new or replacement mill drive? See our Ball Mill Gearboxes engineering page for design inputs, replacement interfaces and inspection scope.