On-Demand Hydraulic Assist for Vocational Trucks

By Scott Betzen of Poclain Hydraulics and Daniel Skelton of Terra Drive Systems

On-road vocational trucks — those that spend 95 percent of their time on the road and five percent in off-road work — often have very high efficiency on the road but almost no efficiency in off-road situations. When these vehicles encounter rough terrain or environmental conditions such as sand, mud or extreme temperatures, they struggle to operate and can get stuck because they can’t get enough traction.

These trucks typically use one of two drive technologies — rear-wheel or all-wheel mechanical drive systems. For example, we can compare a 6x4-type truck rear-wheel drive versus the mechanical all-wheel drive system. With the mechanical all-wheel drive loaded or unloaded, this truck is in a very good situation as far as driving. However, if you look at the off-road conditions, there can be issues with maneuverability, reduced turning radius, traction and changing ride height. These systems require a reduced payload because the axle, drop box, and other extra weight that goes on the truck reduces the allowable payload weight.

That extra weight — along with constantly meshing gears — is going to reduce the fuel efficiency as well. The one advantage the standard rear-wheel truck has over the mechanical all-wheel drive is in terms of driver comfort because it offers a lower cab height.

Another system, the hydraulic all-wheel drive, serves as an on-demand assist to give vocational truck drivers productivity in terms of minimized tire wear, fuel consumption, minimized downtime, and also payload. It also increases driver comfort and allows drivers to be confident that they can focus on the job and know that they're going to be safe and avoid dangerous situations.

A New Design Beats the Old

If we look at the hydraulic all-wheel drive, we get the best of both worlds. The system offers high performance off-road with or without a load, good maneuverability because the turning radius and the ride height is not changed, and the payload is a better situation because the weight of the hydraulic all-wheel drive is much less than that of the mechanical all-wheel drive. Add in fuel consumption and operator comfort and you have a truck that gives productivity, driver confidence and safety.

Hydraulic all-wheel drives include the best of electrohydraulic design. The system includes a pump to create flow. Electronics control the pump and change the pump command. Valves connect the flow from the pump to the motors and help ensure the engagement and disengagement of the motors.

Poclain hydraulic motors provide the system traction, and key components such as a tank cooler, filter and hose all need to be sized according to your duty cycle and the pump and motors that you have on your system.

In the wheel end, the motors have a special knuckle that has porting for the hydraulics to drive the pump. The system uses a cooler tank combo to minimize the space that is required because vehicles don't have a ton of real estate.

The motor is integrated into the axle and the system also features a modified hub and spindle with standard drum or disc brakes. Poclain uses a radial piston cam lobe hydraulic motor. The knuckles with special drillings take hydraulic fluid out to the cam lobe motor, and that powers in an on state, giving high pressure to have the pistons follow the cam and propel the wheel.
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Real-Life Conditions 

Without hydraulic all-wheel drive, if a truck goes into an off-road situation, the rear wheels can start to slip, lose traction, and potentially get the truck gets stuck.

If the driver engages the hydraulic all-wheel drive to begin assist mode, it starts to adjust the traction and the pump will provide the necessary flow to match speed. However, if you’re on slick, stable ground, or going onto the highway, the system will then disengage itself.

With the hydraulic assist option, you are able to use this truck as a hydrostatic drive and drive it with a joystick. So with the truck in neutral, it is driven hydrostatic only. The pump displacement is proportional to the command and the engine speed, so you'd be driving this with a joystick and you would be regulated by the software in ramps. So you would have acceleration ramps and deceleration ramps all provided in the software with your joystick.

Another advantage of hydraulic all-wheel drive is boost mode. In a traditional mechanical all-wheel drive, you'd have to spool the engine up into a higher torque range. With the hydraulic assist, we can change the displacement of the pump to provide more flow, create more pressure, and in turn, provide a more tractive effort and more torque in that front axle. But now in this case, given throttle command, you don't have to necessarily be moving to get this high pressure.
For more information about on-demand hydraulic assist, listen to Scott Betzen and Daniel Skelton’s interview on NFPA's Fluid Power Forum​ podcast.

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