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February 17, 2010

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Case study: the instability of mini-excavators

John Mcilhagga describes how his company worked together with clients, suppliers and equipment manufacturers to address a perennial problem in the construction and utilities industries — instability of mini-excavators.

At Balfour Beatty Utility Solutions (BBUS), operations centre on the maintenance and provision of essential utility assets, and an integral aspect of this is the need to expose water mains, gas mains, or underground cabling. This involves excavation and the use of mini-excavators. The company’s current fleet of such machines stands at 250 and is made up of both hired and owned machines.

It is well known within the utility industry that, in certain operating conditions, there is the potential for mini-excavators to overturn. This is not an everyday occurrence and, in the past, was simply attributed to ‘operator error’. As a responsible company, however, BBUS felt duty-bound to examine the circumstances in which such an event could occur and devote itself to finding a solution to the industry-wide problem of mini-digger instability.

Pull up a chair
Within the framework of the Balfour Beatty Group’s ZERO HARM initiative (see panel) BBUS decided it needed to combat this problem. It quickly realised that, to do so, it needed to involve not just its own employees and immediate partners but also those deemed to be stakeholders within the BBUS ‘village’ (when considering how to achieve ZERO HARM by 2012, the analogy often used by the company is: ‘how do you eat an elephant?’ the answer being to ‘invite the whole village’!)

In this instance, the ‘village’ is populated by client organisations, plant-hire companies and manufacturers, including Scottish and Southern Energy, A-Plant, Gap, Kubota, Takeuchi and JCB. Early discussions were held with the ‘village’ and it soon became apparent that insufficient information was available regarding instability testing of mini-excavators. As a result, BBUS and fellow stakeholders took the decision to bring in expert advice and contacted OPERC, the Off-highway Plant and Equipment Research Centre.

Determining the issues
To start, Dr David Edwards, of Loughborough University, and Barry Robinson MBE, of OPERC, provided an academic perspective and assurance that the BBUS findings could be fully interpreted. The company’s health and safety team held numerous meetings with Dr Edwards to develop a remit for a research project to look into what circumstances must exist to facilitate mini-excavator instability. Together with fellow stakeholders, it then outlined to OPERC what the collective requirements were.

This stage involved discussion of all the factors that had historically been suggested as reasons for machine rollover. As no conclusive testing had ever been done before, there was only speculation and general intuition regarding what caused instability on which to base a remit. By discussing these factors, however, the partners were able to determine research methods that would produce scientific and comprehensive results.

A whole range of instability factors was debated — for example, one line of discussion considered whether cab-type mini-excavators are more susceptible to instability incidents than canopy-type vehicles. The general hypothesis was that having an enclosed cab on top of the slew drive would raise the centre of gravity and make a machine more vulnerable than if it possessed merely an open canopy.

Whether or not to have extendable tracks was also confronted. It was assumed that having them (and using them) would give a larger ‘footprint’, which would lend better stability to a machine, but it was decided that conclusive testing was needed to scientifically prove this theory.

Also subject to debate were the hydraulic systems used by machines. In this case, stakeholders unanimously decided that it was necessary to determine whether a displacement pump, or gear system was more likely to cause instability.

A thorough investigation into the importance of incline to rollover susceptibility was also deemed necessary, to calculate whether other factors, such as inertia, contribute to machine instability and, if so, how influential they are. This was a particularly contentious issue that had never been conclusively proven one way or the other, so it was central to the research.

Research methods
Broadly speaking, there are two types of problem with mini-excavators — longitudinal instability and lateral instability — and both have been behind various incidents in the industry in recent years. The research therefore considered both types, to ensure that results were as conclusive as possible. As speculation over instability factors was largely based on previous incidents, as much information as possible was collected on these incidents. Scrutinising historical data was felt to be a good starting point, with any indications complementing the new research.

The research undertaken can be divided into three individual, but interlinked, sections. Firstly, accident investigations of previous incidents were collected from BBUS and other OPERC members. This qualitative data was submitted to Dr Edwards in advance of the next stage, so that any initial findings could be factored into the scope of later research.

The second stage of the research involved two days of tests, which took place at Apple Industrial Training in Mansfield and were organised and carried out by Barry Robinson. In all, 11 different mini-excavators were tested.1

First of all, a slewing test (where a machine is rotated on its axis to mimic everyday activity) was conducted, which involved the following steps:

1    On level ground with dozer blade down and grading bucket empty; slewing across and against the tracks one metre off the ground.

2    Operation is repeated but at two metres off the ground.

3    On level ground with dozer blade raised and grading bucket fully heaped; slewing across and against the tracks one metre off the ground.

4    Steps 1 to 3 repeated, but with a 100-150mm block on the front of the tracks during the operations.

Both longitudinal and lateral instability were addressed, and the exact same process was carried out with all 11 machines. All possible configurations of each machine (e.g. tracks extended if they had them) were deployed to ensure full validity of the testing.

The performance of each machine under each set of conditions was recorded on a Likert scale, which allowed each configuration to be ranked and quantitative data to be collected. Any comments or observations were also noted down and taken as qualitative data.

Next, operatives tested each machine (again, in all their configurations) by carrying out real work in research-designed conditions. They were then invited to score each machine (and each configuration) against a range of stability criteria, including general operations, responsiveness to controls, and bucket-to-blade clean-up. The Likert scale used by operatives to score the machines meant overall instability factors could be calculated.

All testing was digitally filmed so it could be viewed and thoroughly analysed at a later date, thus providing another source of quantitative data.

The third stage of research has yet to be completed, but involves the use of a tilt table to determine the exact gradient at which rollover becomes a factor. The findings of this stage are expected to back up those of the first two stages.

Once all qualitative and quantitative data had been collected by OPERC, a comprehensive and scientific report was compiled. The data accumulated from the test days clarified a number of previously debated, or not fully understood, stability factors, including:

Cab versus canopy
Results did not reveal any significant difference between these two machine types. In fact, they suggested that a cab positioned over the slew ring could potentially ‘hold’ the machine to the ground, providing greater stability than a canopy. In addition, a full restraining system, one which holds the operative within the cab/canopy, would offer greater protection. Consequently, BBUS will be proposing a four-point restraint system to its manufacturers.

Extendable tracks
It was discovered on the test days that extendable tracks outperformed regular tracks significantly. All operators commented that they were more comfortable operating each machine when tracks were extended. Additionally, when the digital recording was analysed it showed there were fewer instability incidents when tracks were fully extended.

Machine geometry

Tests showed that some of the machines did not have a dozer blade that was designed well enough to complete a full bucket-to-blade clean-up — an important function of the mini-excavator. If the blade and bucket are not sufficient to ‘clean up’ the excess spoil this could frustrate the operative, which may lead to over-exertion of the machine’s capabilities and thus potential instability incidents.

Hydraulic systems

Results from the slewing test suggest that the displacement pump is the more adept system, less inclined to contribute to instability. On the test days, operatives scored this system highly compared with the gear pump, claiming it to be smoother, easier to control, and less jerky when slewing.

Zero tail-swing mini-excavators

In Mansfield, one zero tail-swing machine was tested and its superiority in avoiding human-machine collisions and machine-machine collisions was clear. It was also very adept at operating on level ground and is certainly an option worth considering, in certain conditions. However, zero tail-swing machines are, by nature, undoubtedly less stable than machines with rear ballast.

Incline versus inertia

The testing method involved placing blocks underneath one track to gauge the incline at which a mini-excavator becomes unstable. It was found that, on a level, suitably load-bearing surface, a 75mm difference in height between tracks could reasonably be expected to turn a machine over.

All machines tested were under two tonnes and given that 75mm is a very minimal incline, the suggestion is that incline is not a significant contributing factor to machine rollover. In other words, if a machine can tip over at just 75mm then other factors are more influential — such as machine configuration, weight, width of the tracks, geometry, power, and the load being manipulated.

When purchasing machines, durability and price are undeniably influencing factors but the BBUS research has shown that stability is also very important and must be high on the agenda. It has also demonstrated that the bespoke standards to which manufacturers are currently testing are not sufficient; they do not cater for operator misuse (e.g. rapid movements), which ultimately causes instability. The current tilt-table method alone will not suffice, as inertia is a more influential instability factor and so must be incorporated into manufacturers’ testing processes before machines go to market.

Of course, more questions need to be asked to ensure future instability incidents do not occur. For example, is a mini-excavator needed in the first place, or can risk be factored out simply by eradicating the machine? Also: is it necessary to use a two-tonne machine? Would a three or four-tonne machine, statistically proven to be more stable, be appropriate for the work? Is the specification correct for this particular work?

BBUS believes analysis of case studies and the testing days has produced some conclusive results, which will facilitate further work with manufacturers towards creating a ZERO HARM mini-excavator, and ultimately help achieve the goal of ZERO HARM by 2012. By working together with contractors, clients, suppliers and manufacturers risk can be factored out and best practice shared for the benefit of all.

1    These were provided by Kubota, Takeuchi and JCB

John Mcilhagga is general manager, health and safety at Balfour Beatty Utility Solutions.

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