Customer References

Customer Comments

Reducing Truck Body Deterioration Costs in the Mining of Abrasive Material

The Argyle Diamond Mine is an open pit operation located in the North East Kimberley region of Western Australia. It is operated on a 2-weeks-on and 2- weeks-off, fly-in/fly-out basis, and currently has an overall mining rate of 48M metric tons per year (53M US tons).

This remote area is characterized by consistently high temperatures and a short but intense wet season that yields about 1 meter (39”) of rainfall annually.

A large percentage of the material mined is a hard, blocky, and highly abrasive quartzite rock with a UCS of 250 MPA.

The mine commenced in December, 1985, mining at a scheduled rate of 12M metric tons per year, using 11 x CAT 777B haul trucks, loaded by CAT 992C front-end loaders. The 70-ton haul trucks were fitted with Heavy Duty steel bodies, lined with 12mm (1/2”) Bisalloy 360 wear plate kits (Rockwell 39-43R) and Dua plate hard-faced composite in the “dovetail” section.

The blocky material caused some deformation to occur on the body liners, which exacerbated the high wear rates induced by the high-silica rock. The first of these bodies failed at 1,000 hours of use and the remainder within 1,250 hours.

These bodies were refurbished on-site using a contractor, and were fitted with 25mm (1”) Bisalloy 360 wear kits and again Dua plate in the “dovetail” section. The bodies fitted with the 25mm wear kits were able to achieve a life of 2,500 to 3,000 hours between rebuilds. On the haul distances at that time, this represented 14,000-17,000 loads between rebuilds.

it was necessary to hold two spare bodies on-site and a contract work force of six (6) was permanently allocated to body repair. The average cost of rebuild at the time was AUS$35,000, and this was increasing with successive rebuilds.

In 1988, a SKEGA Suspended Dump Body (SDB) was imported and trialed on one of the CAT 777B haul trucks for a period of approximately 6,000 hours. The haul truck fitted with the SDB was 7 metric tons lighter than the average Tare weight of the remaining fleet equipped with steel bodies. Two weight studies indicated that the SDB truck had an average payload of 6.78 metric tons heavier than the fleet average-an almost 10% payload increase. The truck was noticeably faster when empty than the other trucks, and also ran below the fleet average in fuel consumption. it is acknowledged that fuel consumption at the time was variable and there was no study undertaken to attempt to scientifically evaluate the difference.

Carry-back, which was occurring in the front corners of the steel tray (approx. 3- 4 tons per load), was never observed to occur with the SDB unit.

two problems were encountered with the SKEGA SDB unit. The first was that during tipping, the front of the truck became light, particularly when dumping on a down slope. This was attributed to the extended tail fitted to this unit and the observation that the load did not slide out of the body the same way as it did from the steel-lined bodies.

The second problem was that substantial cracking started to occur on the main steel frame. All of the cracking was associated with the welded joints. Full scale testing by consultant metallurgists found major defects in many welds, with low penetration and voids occurring.

Repairs were undertaken and the frame was fully instrumented when it was recommisioned to establish the actual levels of stress being experienced during use. This testing indicated that the stress being experienced was within the design specification and that if properly welded, major problems should not occur.

Extrapolation of of the wear rates occurring on the suspended rubber body liner projected a life of around 8,000 hours, or 46,000 loads.

The trial of the SDB was considered promising enough that when a decision was made to upgrade the haul fleet in 1990 to CAT 785B trucks, they were all supplied with SDBs. In these trucks, the steel frames were manufactured in Australia by Forward Engineering who had extensive experience in the fabrication of dump bodies for the mining industry. No spare dump bodies were ordered for the CAT 785B trucks and only spare components were held.

During the life of these trucks a number of modifications were made to theoriginal concept aimed at maximizing the life of the rubber components.

A separate under-body support was developed for the tail area that positively located the suspension ropes and removed the need for grooving the tail area of the main mat, thereby increasing the available wear material and strength in this area.

Lower wear rates were achieved later by inverting the main mat so that the outside of the mat, as it was manufactured on the cylindrical mandrel, became the inside of the dump body. This change achieved some degree of compression on the inner mat face.

The CAT 785B fleet was operated for some 18,000 hours and the SDB achieved the following:

• Average mat life 10,000 hours (56,000 loads)
• NO major frame cracking
• Normal tipping performance
• Total elimination of carry-back

The CAT 785B trucks were sold in mid-1993 and replaced with CAT 789 units at the Argyle site which are fitted with Duratray SDBs.

The majority of the CAT 785B units have continued to work in Australia and have exceeded 40,000 hours, some reportedly still fitted with the rubber mats that were in place when they left Argyle. Although the operating conditions are not as harsh as those at Argyle, it stands testimony to the frame design and construction of the SDB.

The first CAT 789 trucks, again fitted with SDBs, arrived in 1993 and at the time of writing there are 13 trucks in the fleet. The loading of these trucks is predominantly undertaken by O&K RH2000 hydraulic excavators in face shovel configuration with bucket sizes ranging between 21m3 and 23m3. Back-up loading capacity is provided by a CAT 994 front-end loader.

The CAT 789 haul trucks are fitted with the Cat TPMS (payload monitoring system) linked to a modular dispatch system, allowing direct monitoring of truck loading. Some trucks have now reached 18,000 of operation and have achieved the following results:

• Average payload 177.2 metric tons (195 US tons)
• Average rubber mat life 14,000 hours (68,000 loads)
• Normal stability during tipping
• No carry-back
• No frame cracking

The only problem encountered in early use was that occasionally the body could not be hoisted. This occurred mainly when the truck was in fact under-loaded, but the payload placed forward in the body. The situation was rectified when the body was extended about 1m at the rear allowing the load to be located a little further rearward. AN electronically controlled two-stage pressure relief valve was also fitted to the truck hydraulic system. The valving allowed the use of higher pressure for initial hoist and normal pressure to be used for the full hoist cycle.

The manufacturer has recently modified the under-body support for the floor mat, which is expected to further increase mat life.

Cost of Ownership Comparison

Cost comparisons have been undertaken between the HD steel bodies and the Suspended Dump Bodies at various stages in their use at the Argyle site. These commenced with analysis and comparison directly of the bodies fitted to the 70T CAT 777B trucks, and subsequently by extrapolation of that data and supplier costing at each fleet change.

The Duratray SDBs have enjoyed a substantial cost advantage at each analysis.

For the cost comparisons, it has been assumed that the wear rates experienced in the first three (3) years of the operation of the steel bodies would have remained constant on a number of loads carried basis. the following procedure has been used to estimate the costs of HD steel bodies and SDBs fitted to the current fleet of 13 x CAT 789 haul trucks in use at the Argyle mine.

The results are shown in Table 1 below. Costs are shown in Australian dollars.

...On this basis, the Duratray SDBs achieved a 47% lower cost of ownership over
the 4-year period.

Life of Truck Comparison

The original steel trays fitted to the CAT 777Bs were experiencing severe cracking and deterioration of the main bodies after only 6 to 7 rebuilds (18,000- 20,000 hours) and rebuild costs were increasing. It was anticipated that the HD steel bodies would require replacement at a life of 24,000 hours. The current indicators are that the SDB frames will achieve a life of at least 40,000 hours at Argyle. On this basis:

...An estimated savings of 64% in total dump body cost.

There are a number of less direct benefits that have been observed during the last six years of operating a haul truck fleet fitted with Suspended Dump Bodies at the Argyle Mine.

Operating conditions with the mining and earthmoving industry vary enormously. This paper summarizes the experience at the Argyle Mine and the particular conditions found at that site. Other operations will have to assess the particular applicability of the SDB concept to the conditions encountered at each location.