The Importance of Balancing and Weighing a Bulk Materials Handling Machine

During the commissioning process for a balanced mobile Bulk Materials Handling Machine (BMHM) such as a bucketwheel reclaimer, stacker, bucketwheel stacker reclaimer or shiploader, an important step is to balance and weigh the machine. Although it may be tempting to forego this vital step due to schedule demands and the impending commencement of operations, it is imperative that proper balancing and weighing be carried out to ensure the safe and reliable operation of the machine for many years to come.

Validates the Design Intent

Balancing and weighing of a new machine on site allows for comparisons to be made between the measured and design values. Any discrepancies regarding the luffing structure balance and total weight of the machine may require further investigation and rectification prior to commencement of operation. This practical validation step is an important hold point before handover. The Australian and international standards for mobile BMHM do not allow the as-constructed weight to be more than a 5% variation to the design value, in which case the design checks and calculations must be repeated.

Avoiding False Confidence

If only balancing or weighing is undertaken, and not both, this does not provide the machine’s owner a complete picture of their new investment. As an example, weighing alone might indicate the machine’s mass matches the design value. But this might give the project team a false sense of confidence, because without also undertaking appropriate balance and centre of gravity checks on-site the machine’s stability cannot be verified and it might be improperly balanced, which may not be obvious from weighing alone.

Achieve the Fatigue Service Life

The stability, balance and centre of gravity is critical for a machine that undertakes luffing and/or slewing motions, such as a bucketwheel reclaimer, stacker or shiploader. If the machine is not properly balanced, the fatigue stresses caused by each luffing and/or slewing motion can be amplified and the slew bearing load distributions may also be outside its design load envelope. As such, an incorrectly balanced machine will have a shortened slew bearing service life and premature onset of fatigue cracking.

Confirm Wheel Loads on Rail

The loads imposed on the wheels and rails for a bulk materials handling machine depend not only on the machine’s overall weight but also its balance and overall centre of gravity. An overweight or incorrectly balanced machine will have excessive wheel loads which can lead to wear issues for the wheels and rails. It can also mean that the long travel drive power may not be adequate for the machine to operate efficiently. These increased loads can also have significant consequences for the wharf or rail foundations, which were designed and built for a specific design wheel load spectrum.

The Best Methodology

The best method for balancing and weighing will depend on the design and configuration of the particular machine, therefore using an incorrect methodology can give erroneous or inconclusive weight and balance results. There are many factors to consider which add together to create complexity. Taking measurements with the boom horizontal is sufficient for some machines but not for others. If the conclusion is that the machine is overweight or improperly balanced, what is the path forward? Adding or removing counterweight can have significant implications elsewhere on the machine that need to be fully understood.

In recognition of the importance of weighing and balancing, the requirement of Australian Standard AS4324.1 (2017) states that:

“After a machine has been constructed, the mass and centre of gravity of the as-built machine shall be accurately determined”

— Clause 5.9 of AS4324.1 (2017)

The team at LEC are experienced in preparing suitable procedures to validate the luffing structure balance and weighing of the entire machine in accordance with the requirements of AS4324.1 as well as other international standards such as ISO 5049-1 and FEM Section II (2 131 / 2 132).

Click here to contact LEC and discuss the best methodology for weighing and balancing your particular bulk materials handling machine.

Return to Service of a Derailed Grab Unloader

Due to an unforeseen event, a clamshell bucket grab unloader moved away from its parked position and derailed, causing visible damage to the pier leg and bridge structure.

The derailment resulted in an offset between the shear leg and the pier leg that caused the bridge structure to skew 10 degrees from its neutral position. This excessive skew angle resulted in contact between the pier leg and the bridge structure which caused visible structural deformation and local plate buckling. The structural damage due to this incident was investigated using three-dimensional finite element analysis (FEA) to simulate the contact condition and to compare the FEA model’s results with the detailed site survey measurements. This facilitated the calculation of the locked-in structural forces in the grab unloader at its derailed position. The buckling modes predicted by the FEA results correlated with the buckling modes visible on the derailed grab unloader.

Structural damage on the grab unloader, caused by the derailment incident.

These FEA results formed the basis for the re-railing methodology, in consultation with the heavy-lifting contractor, which involved pulling the shear leg structure to align with the pier leg structure and then lifting & rotating the pier leg structure to return the long travel wheels back onto the rail. Temporary supports were installed to ensure the safety and stability of the grab unloader during the re-railing process.

Structural plate buckling caused by the derailment incident.

Once the machine was returned to its normal operating position on the rails, several repairs were necessary to remediate the structural damage caused during the derailment. The design of these structural repairs was undertaken using detailed finite element modelling and analysis (FEA) to ensure that the proposed structural repairs met or exceeded the design requirements of the original structure.

The grab unloaded was successfully returned to operation after the completion of the structural repairs and recommissioning of the mechanical, electrical and control systems.

Why do I need an independent design review?

Every machine is unique

When procuring a new Mobile Bulk Materials Handling Machine (BMHM), such as a bucketwheel reclaimer, stacker reclaimer, stacker, bridge/portal-type reclaimer or ship loader, the machines configuration will depend on many factors:

• Required throughput;

• Stockpile layout and footprints;

• Yard conveyor arrangements;

• Bund or wharf rail gauge;

• Properties of the material to be handled.

It would therefore be very unlikely to find an “off-the-shelf design” that would suit all the required design parameters for a particular mine or port site.

Higher rates of structural failure

A new BMHM is a major capital expenditure asset with an expected design service life of 25+ years, however, they experience a higher rate of structural failure when compared to other heavy industrial structures.

Historical evidence shows that catastrophic structural failures can occur at any time during a machine’s service life, including during the commissioning stage.

Safety and cost benefits

Engaging an independent or third-party design reviewer during the early stages of procurement can provide many benefits:

• Implement “Safety in Design” early in the design phase;

• Satisfies legislative requirements (duty of care / due diligence);

• Anomalies in the machine configuration and potential constructability issues can be identified and notified to the supplier (OEM) early in the design phase;

• Design code compliance issues can be identified in the design phase and promptly rectified;

• Costly production delays due to design issues and associated on-site remedial work can be minimised;

• Identifying design issues early in the design phase minimises cost overrun and schedule delay;

• Structural modifications can be incorporated during fabrication without undue cost or schedule penalty;

• Minimise the risk of commercial and/or legal disputes which potentially lead to expensive litigation.

Prevention is better than Remediation

LEC personnel have undertaken independent detailed design reviews, structural condition assessments and failure investigations for more than 50 bulk materials handling machines since 1993. Often this involved detailed Finite Element Analysis (FEA) using 2D and 3D elements.

Based on our experience, the detailed design review often identifies structural design issues related to:

• Safety in Design;

• Omissions and ambiguities in the Technical Specification documents;

• Materials and constructability issues;

• Serviceability issues;

• Member strength issues;

• Member and local plate buckling issues;

• Fatigue service life compliance.

These design issues can generally be resolved with the Original Equipment Manufacturer (OEM) during the design phase, thus eliminating potential schedule drift and cost overruns.

An independent detailed design review can be carried out in accordance with the following standards, generally nominated in the client’s technical specification for the new Bulk Materials Handling Machine:

• Australian Standard AS 4324.1

• FEM Section II (2 131 / 2 132)

• ISO 5049-1

Click here to contact LEC and discuss your requirements for an evaluation of a machine’s technical specification and an independent design review.