Alloy bushes survive the wear and tear of mining

Euclid Hitachi is moving towards respecifying the kingpin bushes for all of its dump trucks, based on the novel alloy ToughMet.

Some of the world's largest mobile machinery is used in open cast mining.

Dump trucks, for example, can be bigger than a house, weigh around 180t and carry a payload of 250t or more.

Needless to say, the engineered components have to be extremely rugged to withstand the high loads encountered, and the harsh environment of a mine poses its own problems.

Furthermore, maintenance is not easy on such large equipment, and downtime is extremely costly.

As a result, reliability and long service intervals are paramount to productivity and the customer's bottom line.

Euclid Hitachi manufactures a range of dump trucks, the latest of which will be the EH5000.

An area of dump truck design that has traditionally been problematic is the upper and lower kingpin spindle bushings on the front suspension.

These can wear, leading to slackness in the steering and, ultimately, damage to other components.

Replacing a bushing that has a bore of 9.5in, a wall thickness of 1in and a length of 8in is not a simple matter, especially if the work has to be carried out in the field, throughout the world including areas such as the bauxite mines of Brazil.

Initially Euclid Hitachi had used a lubricated steel bushing on a steel kingpin, but if the lubrication became marginal, the resulting steel-on-steel arrangement soon started galling.

A change to aluminium bronze overcame the galling problems, but these bearings suffered from high wear rates.

When a sales engineer from Brush Wellman suggested using ToughMet, an alloy of copper, nickel and tin, the Euclid Hitachi engineers were keen to investigate the feasibility of another material change.

Extensive tests were run on a test rig built by Euclid Hitachi, which essentially replicated one side of the front suspension of a large dump truck.

Using this rig, it was easy to reproduce the conditions in the field and to predict the life of ToughMet bushings.

Remarkably, the tests showed that ToughMet exhibited only one-third the wear rate of aluminium bronze, which would translate directly to a three-fold increase in the service interval.

The grade of ToughMet specified was 3 CX 105, a cast spinodal copper-based alloy with 15% nickel and 8% tin, giving a yield strength of 724MPa.

This unusual alloy has the strength of steel and the low-friction and lubricity that would normally be associated with leaded bronzes.

As a result, compact bearing arrangements can be designed, and the bearings will continue to function even if starved of lubricant.

Unlike bronzes, ToughMet is also not susceptible to damage by contaminants should these find their way into the bearing.

For the conversion to Toughmet from aluminium bronze, no changes were required for the design of the bushings apart from detailed modifications at the request of the machine shop.

Interestingly, Euclid Hitachi has found that there has been no overall cost premium for moving to the higher-specification bearing alloy.

Indeed, the company is realising specified competitive advantages through its association with Brush Wellman.

This is the result of improved supply chain management factors.

Lead times have been cut in the supply of material and the value of the Euclid Hitachi inventory has been reduced as they previously had to carry large inventories of aluminium bronze tubing.

In addition, Brush Wellman uses its patented continuous casting process known as Equacast which enables the successful manufacture of the spinodal systems in large cross sections of the alloy.

This is performed to the inside and outside diameter specified by Euclid Hitachi, so there is only minimal machining time and very little valuable material is wasted as swarf.

Brush Wellman can accommodate customisation of the dimensions of a tube in a mill run so that it either matches the ideal size required or provides a machining envelope around two or more similar sized parts, such as the upper and lower kingpin bushings.

Following the success experienced with the newest and largest dump trucks, Euclid Hitachi is now moving towards respecifying the kingpin bushes for all its dump trucks, right down to its "smallest", a 32t unit.

On the large EH5000 machine ToughMet has also been specified for the front suspension torque tube bushes, but so far there are no plans to do the same on the smaller machines.

Mac Knight, the Senior Design Engineer at Euclid Hitachi working on the EH5000 and EH4500, comments: "The combination of the excellent bearing properties of ToughMet and the way in which Brush Wellman is prepared to deliver as and when we want, means that we are far better off with ToughMet than we were with aluminium bronze.

Our customers also appreciate the extended service intervals because downtime on dump trucks is hugely expensive for mine operators".

Knight also praises the technical support he has received from Brush Wellman, saying: "The Brush Wellman engineers were really supportive throughout the development program; they have an immense depth of knowledge about ToughMet and were able to help us make the most of this incredible material".

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