One of the many considerations to be made by track builders and maintainers deciding on their next road rail vehicle (RRV) for hire, lease, or purchase, is which drive system is most preferable for the application. Rail Express speaks with Manco Engineering Group’s Craig Munro about the options.
Three types of drive systems are defined in the Australian Standard for RRVs: Type 1 – self powered rail wheels, Type 2 – friction drive, and type 3 – direct drive.
Craig Munro is Manco Engineering Group’s Chief Engineer for Rail. He tells Rail Express each drive system has a set of pros and cons which must be considered before leasing or purchasing an RRV.
Self-powered rail wheels
Self-powered rail wheels provide both traction and braking through the rail wheels. During rail operations road wheels or crawler tracks are lifted above the rails; there is no contact between the road and rail wheels.
“Although a higher capital investment is required for a Type 1 RRV, the benefit is its adaptability,” Munro explains. “Rail modules can be manufactured to handle one, two, or all three common rail gauges (narrow, standard and broad) as there is no dependence on road wheels to provide traction.”
The extra stability provided by a full rail base means Type 1 RRVs don’t require stabiliser legs to support an elevated work platform (EWP).
“Tractive effort is generally provided via hydrostatic closed loop pumps and motors, either utilising direct wheel mount hydraulic motors, or a chain drive system,” Munro continues.
“With today’s advanced hydraulic systems, this drive system can provide electrically controlled, infinitely variable speed with high torque and excellent traction. A Type 1 RRV equipped with an EWP can be easily driven on rail from the workman’s basket via interfaced electronics, something not readily achieved with Type 2 or Type 3 drive systems.”
Braking can be controlled either manually by the operators, or it can be safely interfaced with the drive system, automatically applying and releasing based on the drive circuit pressure readings, Munro adds.
Type 2 friction drive systems indirectly derive braking and traction from the road wheels to the rail wheels; this is achieved via contact from the road wheel directly onto the rail wheel, through contact from the road wheel onto a rail wheel extension hub, or via a layshaft transferring the tractive effort to the rail wheels through the use of a gearbox or chain drive.
“Issues with braking, and predominately concerns with roll away when transitioning to or from rail mode have seen this style of drive method fall out of favour in recent years,” Munro says. “While all-wheel braking can be employed to mitigate some of these issues, friction drive rail gear is now seen as old technology.”
Additionally, to be driven ‘cab first’ on the railway, a Type 2 RRV’s engine must be driven in reverse gears, which can over time lead to drive issues with the carrier vehicle, as standard gearing is not designed for extended use in reverse.
“Additionally, small contact patches on vehicle tyres can lead to premature wear and even failure of the contact tyres,” Munro says. “Additional service and maintenance procedures must be in place to monitor this, and if not, could lead to tyre blow outs.”
Type 3 direct drive RRVs rely on at least one pair of road wheels to remain in direct contact with the rail tracks to provide tractive effort and braking.
The steer axle on the vehicle is generally lifted clear of the rail track on larger vehicles, although all road wheels may remain in contact with the rail on smaller RRVs (e.g. an RRV based on a Toyota Landcruiser).
“Type 3 systems can have economic benefits over Type 1 systems through lower hydraulic and structural requirements,” Munro says. “However, direct drive systems have inherently lower traction due to the rubber-steel contact, especially in wet conditions where grip can drop to below a quarter of what can be achieved in steel-steel contact under the same conditions.
“Where significant grades are encountered, a self-powered rail wheel drive system may be the better choice.”
Operators considering a Type 3 RRV also have to understand how the load is shared between the carrier vehicle tyred axles and rail axles, Munro says. “If too little load is transferred to the rail axle, derailment can occur, especially with vehicles that have tandem rear drive bogies. Manufacturers are taking steps to address load share with active suspension systems coming to market. Manco Rail commissioned several RRV in early 2017 with airsuspension and continues to evolve these systems.”
Theoretically, direct drive systems would be capable of attaining top speeds similar to those attainable on the road, but safety at higher speeds must be carefully considered.
Similar to Type 2 friction drive systems, another setback of direct drive is tyre wear.
“Only a small contact patch is made with the rail track, and with a higher loading than seen with friction drive systems,” Munro explains. “This can ultimately lead to localised wear and/or tyre failure.”
Furthermore, as Type 3 RRVs do not lift road wheels inside the relevant structure gauge, additional measures must be undertaken to avoid causing damage to trackside componentry during operation. When using an EWP, chocking of the vehicle suspension systems and/or stabiliser legs may be required, and control of the RRV’s movement from the workman’s basket is not readily achieved, and an in-cab operator will need to drive the RRV along the rail tracks while the EWP is in use, which can lead to communication errors, lower productivity, and the increased chance of hazards for operators.
“Generally, direct drive systems are confined to use only on standard and broad-gauge rail tracks due to the need for direct road wheel to rail contact,” Munro summarises. “Smaller vehicles, such as the Toyota Landcruiser, can operate on narrow-gauge tracks with direct drive, however these smaller vehicles are generally not able to meet the needs of operators requiring EWP access to overhead wires or to transport large loads to worksites.
“Direct drive systems are a serious limitation for operators wanting a versatile machine capable of operating on all three track gauges using multi-gauge hi-rail gear.”
Contact: craig (at) manco.co.nz