CP & E  
Contractors Plant & Equipment  
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   PAGE 18   

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[The Latest Evolution in Mining Trucks from Caterpillar . . .cont]

 

Mechanical Drive v Electric

Both models have improved safety features, more servicing from ground level, higher speed on grade, increased power, new cab and new styling. However, both retain mechanical drive systems although Caterpillar is prototype testing an electric drive version of the 795F but will not be available until 2011 at the earliest, depending on performance criteria resulting from field evaluation.

Caterpillar justifies its decision to maintain mechanical drive because of the performance and productivity benefits of high efficiency over a wide range of applications and faster cycle-time.

Mechanical drive also has fewer performance restrictions being able to cope with gradients to +/- 26%, a stall torque of 29%, powerful braking and retarding on grades of -17%. Operators also tend to prefer the 'automotive' type feel.

Mechanical drive systems, with current technology, are also more fuel efficient and have lower operating costs. There is also a considerable weight saving - as much as 10 tons.

However, Caterpillar accepted that for some customers, electric drive was the preferred option. Hence, the development of the 795F-AC as an addition to the range.

The potential to develop the concept in areas such as energy recovery, hybrid technology, a trolley adaptation and for opportunities in automotive and military applications are also significant.

Even so, Caterpillar's adventure into electric drive was conditional on protecting 'the brand'.

As a result, the alternator is derived from the company's expertise in power generation; the controls and hardware were developed in-house, the power inverter was developed through a collaboration agreement with Mitsubishi Electric, the motors, from a supplier but built to Caterpillar design and specifications. The result is a drive line that the company claims to be 'unique'.

With 100% electric or blended all wheel retarding, the operator has greater control and there is less wear on the braking system to reduce costs. The wheel motors are mounted on the rear axle and the high voltage alternator is chassis mounted for easier servicing and even weight distribution.

The Cat 797F-AC drive system also features completely new high speed final drives designed for durability in tough environments.

3rd Generation Cat 797

Since its introduction in 1988, the Cat 797 has continuously evolved to remain the leader in 360 – 400 ton size class, selling close to 500 units. During extensive field testing and evaluation in working mines, the F series version increases cycle time performance by 13% over the B series.

A contributing factor to this is the latest Tier II compliant engine - the C175 –20 - which delivers 4000 gross horsepower, an increase of more than 12% over the previous Cat 3524 engine. The overall weight, at 623,690 kg remains the same as does the standard top speed.

However, the new version has a 14% higher speed on 12% grades, contributing to faster cycle times.

The power train has been redesigned to put more power on the ground. The Cat seven speed, planetary power shift transmission is matched with the C175-20 engine to deliver constant power over a wide range of operating speeds.

The Electronic Clutch Pressure Control (ECPC) [1] provides better performance, smoother shifting, longer clutch life and a more comfortable ride. The lock-up torque converter [2], which engages at 8 km/h, combines maximum rimpull and cushioned shifting of torque converter drive to deliver more power to the wheels.

The final drive [3] with its planetary power shift transmission to deliver maximum power to the ground.

Electronics control systems work together with the mechanical drive to ensure operations remain within fixed operating parameters. For instance, a body up-shift inhibitor prevents the transmission from shifting above the pre-programmed gear without the body fully lowered.

The machine can move forward slowly but the speed is limited to avoid excessive racking when pulling away from the dump area.

There is also an overspeed protection device that senses engine conditions and automatically up-shifts one gear to prevent overspeeding. There is a downshift inhibitor, controlled throttle shifting and reverse speed inhibitor.

Truck Body Options

There are four mine-specific body options and a configurable liner system to enable managers to configure the trucks for optimum performance depending on the material. On all the bodies, the shell is designed to withstand loading impacts and increase wear resistance.

The optional lining systems are designed to keep additional weight down while effectively giving the level of wear protection required.

Comparing load volume specifications, the 797F offers a heaped capacity on the MSD II body from 240 - 267 m³.

 

[cont]