InfraStructures - August 2008 - Hydrostatic Drive Helps Conventional On-Off Highway Trucks Get Traction

Hydrostatic Drive Helps Conventional
On-Off Highway Trucks Get Traction

Scott Betzen, Poclain Hydraulics
Special Collaboration

Poclain Hydraulics is best known for its hydrostatic transmission systems used in construction, agricultural, and other off-highway vehicles. However, last year at bauma 2007, held in Munich, Poclain Hydraulics introduced a new drive system that gives all-wheel-drive capability to conventional on-highway trucks.

Dubbed the AddiDrive Assist, the drive system uses a Poclain hydrostatic pump powered through a power takeoff from the vehicle's transmission. Hydraulic fluid from the pump is routed through control valves to a hydrostatic motor mounted within the envelope of the truck's left and right front axle assemblies.

System operation
In normal operation, the Assist is disabled, so the front wheels spin freely, and the system consumes no power. However, when the vehicle encounters difficult terrain, the driver need only stop the vehicle and engage the AddiDrive Assist system. In this mode, a vehicle can more easily negotiate steep inclines, deep mud, snow, and similar conditions.

Some trucks already have fulltime all-wheel drive capability through a mechanical power train. However, because they are active full time, they rob fuel economy whenever they are used when not needed. Compared to permanent all-wheel drive trucks, AddiDrive Assist is 8400 kg lighter, provides a lower center of gravity (which improves vehicle stability), and allows a tighter turning radius.

Other part-time systems require the driver to exit the truck, raise the truck's cab, and physically engage the front-wheel drive system. This technique is not only cumbersome and unproductive, but requires extra overhead clearance so the cab can be raised.

The AddiDrive Assist system is currently offered in TGA series trucks with a HydroDrive option manufactured by MAN Nutzfahrzeuge Group, Munich.

New breed of hybrid drive
Poclain also used bauma to introduce a new hybrid hydraulic drive that bucks the trend of other hydraulic hybrids. Typically, hydraulic hybrid drives use accumulators to regenerate energy used in braking of vehicles that undergo frequent starts and stops. However, Poclain's AddiDrive CreepDrive is intended to benefit vehicles that often operate in a low-speed, or creep, mode. Essentially, then, CreepDrive provides the benefits of a hydrostatic transmission's lowspeed control with the fuel economy and high-speed capabilities of a geared transmission.

When the CreepDrive is disengaged, the vehicle's powertrain operates normally, and no energy is used by the CreepDrive. To engage the CreepDrive, a clutch in a transfer box disengages the vehicle's two-section drive shaft at its midpoint. A hydraulic motor then powers the rear section of the drive shaft to turn the wheels at low speed. The motor is powered by a hydraulic pump, which is driven from a PTO on the vehicle's transmission.

Once the CreepDrive is disengaged, the clutch re-engages to positively transmit power from the front section of the drive shaft to the rear. In this mode, the CreepDrive is completely removed from the powertrain, so it poses no drain on fuel economy. Switching from normal drive mode to creep drive simply requires throwing a switch.

Applications for the CreepDrive include road marking vehicles, road sweepers, asphalt layers, railway and bridge maintenance trucks, and aircraft de-icing vehicles. Potential benefits include improved fuel economy, lower noise and emissions, longer life of vehicle brake and clutch linings, and higher safety and productivity because drivers can focus more attention on operations and less on speed control.