Bosch Rexroth’s HET (High Efficiency Traction) hydrostatic drive system is said to allow for optimal traction in difficult conditions while retaining optimal energy efficiency, which are often conflicting goals during the development of single-wheel driven municipal vehicles and work machines.
Credit for the energy efficiency of HET is given to the all-wheel drive system only being activated when needed, automatically, with no driver intervention.
If in difficult terrain a wheel loses its grip with the ground, slip rapidly increases.
Rexroth’s MCR radial piston motor driving the wheel responds by increasing the oil flow. The rising oil flow causes a brief transmission of the torque to the series-connected MCR motor and wheel located on the other axle.
By using this “slip activation” of the AWD, the HET provides reliable response to the terrain and saves significant energy compared with full-time AWD, claims the company.
In less difficult terrain, such as on the road, the wheels in the driving axle generally have sufficient traction. The radial piston motors on the other axle then simply have “free-run.” Only the two permanently driven MCR motors transfer the power from the diesel engine. However, slip occurs not only when a wheel spins. Due to deformation of the rubber tires, slip is actually always present to a greater or lesser degree. Even such a comparatively slight slip will also result in an increasing oil flow by the driven radial piston motors. The increased flow is hydraulically clamped between the driving and free-running radial piston motor, resulting in a significant loss of efficiency for the entire travel drive. HET is said to completely eliminate this efficiency loss.
A specially developed valve block ensures that excess hydraulic oil caused by natural slip is directed to the low-pressure side of the circuit.
In addition, the HET valve block is not located in the main oil flow, thereby minimizing further losses. Integration of the purge function and other design features make the unit highly compact, while the plug-and-play solution reduces time and money associated with installation and piping.
The HET uses no sensors or control electronics. Besides the newly developed valve block, HET consists of proven standard components from Rexroth such as the A4VG or A10VG pump as well as MCR motors with constant or selectable displacement volume. This makes the HET viable for use in a wide variety of vehicle types such as municipal vehicles, minidumpers, or small front loaders. It is designed to cover the greatest possible number of requirements, including vehicle speed or installation space.
AWD in general is advantageous for commercial vehicles that deliver materials to construction sites, have to travel on softened ground, or need to traverse hills and passes in winter. The hydrostatic ancillary drive Hydraulic Traction Assistant (HTA) offers an energy efficient alternative to traditional AWD. HTA reduces the system weight by around 400 kg (880 lb) by comparison, says Rexroth. The lower weight translates into greater useful load capacity and reduces fuel consumption.
Two hydraulically driven wheel-hub motors increase traction when needed on any axis in the vehicle. For on-highway applications, the motors run in neutral mode to reduce drag losses to a fuel-saving minimum. Off-highway, the operator uses a toggle switch to activate traction mode, even while driving under load. In traction mode the HTA system provides additional power of around 50 kW (67 hp) and additional torque of up to 6000 N·m (4425 lb·ft) per wheel.
In creep speed mode, HTA improves maneuverability and provides additional working gears that can be used at low, continuous vehicle speeds. The two wheel-hub motors drive the vehicle forward or in reverse; the mechanical gearbox is in a neutral position for this purely hydraulic action.
Rexroth says that HTA can be integrated into existing vehicle concepts without any major modifications; does not affect the chassis design, maneuverability, or the turning radius; and provides advantages compared with mechanical AWD.
The HTA system solution consists of radial piston hydraulic motors integrated into the axle, an axial piston variable pump, a valve block, an electronic controller as well as oil cooler and hydraulic tank. All the components are from Rexroth.
HTA can be implemented in two- or four-axle vehicles and makes it possible to add the AWD function to vehicle types for which the purchase of a mechanical system would otherwise be prohibitively expensive. This includes vehicles that spend the majority of time on-highway and seldom require AWD—yet need to safely and reliably handle difficult terrain when called upon, such as tractors and towing vehicles, dumpers, fire trucks, concrete mixers and pumps, skip and bucket loaders, tank and silo trucks, and wood transporters.