Rugged design goes underground

May 5, 2005
All industrial equipment must be safe and should be efficient and easy to use.

Jyurki Kulmala
Design Manager,
Normet Corp.
Iisalmi, Finland

Normet's RBO vehicle for use underground combines the rugged design and construction of heavy mobile equipment with the ride and comfort of an SUV.

Simplified schematic of the RBO's steering circuit show major fluid paths and major components.

But when it comes to vehicles for use in mines and other underground sites, rugged and reliable command are a high priority. Rugged construction means the vehicle must stand up to poundings from cargo, rough terrain, and be able to scale and stop on steep slopes. Reliability is paramount as breakdowns can bring production to a standstill and could be prolonged because these vehicles typically are used in remote areas hundreds of feet underground.

Normet Corp., Iisalmi, Finland, has developed a new vehicle designed exclusively for underground use. Dubbed the RBO, it is designed and built with Normet's experience of more than 40 years in underground mobile equipment, with roughly 90% of its products being exported. The result is a robust, safe, and powerful working machine that's as nimble and comfortable as an SUV. Until now, personnel and equipment had to be transported through mines and tunnels in vehicles not designed for underground use, such as cars and tractors. In most cases, these general-purpose vehicles don't last very long when exposed to extreme conditions underground.

The Normet RBO powertrain starts with a rear-mounted, electronically controlled 9- kW diesel engine, which enables the RBO to scale a 30° incline while carrying five workers and 1 ton of cargo. From there, power is transmitted to an electronically controlled automatic hydrostatic transmission (HST) with dual output shafts. The HST varies output speed and torque according to operator commands. The HST achieves a wide range of continuous speed and torque ratios, relieving the operator from having to shift gears. Each of the HST's output shafts drives a differential gearbox that is integral to the front or rear axle. The RBO's fully hydrostatic transmission is not unlike those used in heavy mobile equipment — a dramatic departure from lighterduty transmissions common to SUVs. The RBO's braking system also is more like that found in heavy equipment. Whereas SUVs typically use dry drum or disc brakes, the RBO uses multiple-disc wet brakes. Multiple discs have much greater surface area than standard disc brakes do, so multiple discs pack more braking capacity into a smaller space. Moreover, because they are bathed in oil to dissipate heat, wet discs tolerate much more aggressive braking and longer braking cycles. Pound for pound, then, disc-disc wet brakes outperform dry disc brakes, so the RBO can not only scale a 30° incline, but stop on a 30° decline. Rounding out the braking system are spring-applied (fail-safe) emergency and parking brakes.

The RBO isn't all about power, however. It has a low center of gravity, so it can be operated safely even on surfaces with a 40° lateral angle. Its chassis sits on parabolic leaf springs with hydraulic shock absorbers for a comfortable ride. Other niceties include sound insulation, fresh air blower with filtering, interior lighting, cabin heating, air conditioning, laminated safety windscreen, and halogen headlights — all designed for underground use to the last detail. So instead of being a commercial-duty vehicle modified for use underground, it's more of an underground vehicle modified for comfort and convenience.

The hydrostatic transmission itself consists of a variable-displacement axial-piston pump driven from the engine's flywheel. The pump puts out up to 75 cm3/rev at pressures of 420 bar. Its overcenter capability allows it to drive the hydrostatic motor full-speed forward, fullspeed reverse, with continuously variable speed in between. A small charge pump provides positive pressure at the hydrostatic pump inlet. The positive inlet pressure prevents cavitation when the pump draws in air. Cavitation can damage the pump and dramatically shorten its life. So the charge pump ensures longevity of the main pump no matter what the orientation of the RBO.

Fluid from the pump flows to an axial-piston motor, whose displacement is variable from 46 to 135 cm3/rev. The variable flow of the pump, coupled with the variable flow of the motor, provides a wide range of output torque and speed combinations transmitted to the output shaft. This can be accomplished with the engine running at speeds for power and efficiency. The RBO's electronic system controls HST input-to-output ratio (pump and motor displacements) based on operator commands and engine speed, which it also controls.

The motor has a doubleended through-shaft that transmits power to front and rear axles. Integral gearing in the axles accommodates differential speeds of individual wheels and transmits reduced speed and increased torque to the wheels. The wide speed and torque range of the HST eliminates the need for a transfer case, and the dual output of the HST motor provides full-time four-wheel drive.

In addition to propulsion and braking, hydraulics also provide hydrostatic steering.

Referring to the schematic, the system is powered by a gear pump that delivers 12 cm3/rev at pressure to 190 bar. Although not shown in the schematic, this pump also supplies the power braking system. The springloaded check valve just downstream of the pump ensures backpressure to further prevent cavitation.

A second spring-loaded check valve, in parallel with the return filter, allows fluid to bypass the filter should it reach its dirt-holding limit. When this occurs, the filter builds backpressure, allowing return fluid to overcome the spring force of the check valve and flow directly to the reservoir, which holds 100 liters.

The cross-port relief valves prevent piston seals from blowing out if external loads transmit excessive force on the cylinder's piston rod. These external loads can occur when the vehicle's wheels strike a large rock or rut in the road surface. The steering valve is a gerotortype hydraulic valve. As shown, fluid from the pump flows through the steering valve and right back to the reservoir.

Turning the steering wheel clockwise opens a fluid path from the pump to the right side of the cylinder. This causes the cylinder's piston rod to move to the left, which pivots the wheels to the right.

For additional information on Normet's RBO, visit

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