Just in time for Halloween, Cross Company celebrates the successful application of a hydraulic hybrid vehicle drive system – the “Creep Drive” by Poclain Hydraulics.
The Task
A truck chassis can be the basis for countless pieces of productive equipment in our society. Anyone who builds or uses the following equipment should pay attention:
- Gravel spreaders
- Bridge & Tunnel Inspection trucks
- Street Stripers & Paint removal trucks
- Remote control concrete mixer trucks
- Any chassis needing remote control capability
- Rail service trucks
- Tree Trimmer trucks
- Water sprinkling trucks
- Chip spreaders
- Aircraft de-icers
- Livestock mixer/feeder trucks
- Asphalt sealcoating trucks
These trucks often need to move much more slowly than they normally would travelling down the interstate. So how can the same truck that goes 70 MPH on the highway operate smoothly between 0 and 5 MPH? Or change from normal operation to remote control?
The Problem
Just putting the truck in “drive” and feathering the brakes may work for some applications, but when there’s a delicate task being performed out on a limb, you need something with more precise variability. Like… a hydraulic drive system.
In purely off road applications, hydraulic drive systems are the go-to solution because of their high torque output and infinite variability. In purely on-road applications, however, a mechanical drive system is always chosen because of its lighter weight, lack of a hydraulic reservoir, and larger overall range of ratios. So what if there were a way to marry the two? One’s flaws could be made up for by the other’s strengths, you know the drill…
There are other hydraulic hybrid drives out there besides the Poclain Creep Drive. The catch is, none of them were designed with the intention of being used for slow and smooth control. Other systems such as the Parker RunWise or the Kinetic Infinidrive were designed for constant stop and start applications like garbage trucks or city buses. They reclaim wasted energy by charging an accumulator with the vehicle’s inertia while braking. Then, they use the stored pressurized fluid to drive the vehicle while at low speed. There’s no on-demand shifting between hydraulic and mechanical drive and there’s no remote control capability.
The Poclain Creep Drive system is different. It’s either on, or it’s off. It can be remote controlled. It’s less about efficiency and more about precise positioning. These other systems share the “hydraulic hybrid” status, but they are entirely different in application.
If your task can be handled purely with a mechanical transmission, then there’s probably no reason to continue reading. It’s easier and cheaper to just do that. But if you need remote control and/or precise positioning in a truck that’s used to get to the work site and do the work once it’s there, there’s really nowhere else to look than Poclain.
The Solution
In order for two drive systems to be used on the same vehicle, some creative engineering had to be performed. For the mechanical transmission to function, the drive shaft must still transmit motion to the drive axle in an unencumbered way. A normal in-line hydraulic motor cannot be used because it would have to spin at high RPM with no oil flow while the mechanical transmission is functioning.
For the hydraulic drive system to function, the hydraulic motor must not be allowed to transmit motion back into the mechanical transmission, but must transmit motion to the same set of wheels. This essentially requires that each system be mutually exclusive. Each system needs a fool proof, on demand “towing” mode.
Poclain Hydraulics has engineered an inline motor that incorporates a “clutch” for on-demand engagement. The truck drive shaft is cut and coupled with the input and output of this “CDM” motor. Its internal design is such that when the motor is disengaged, it is little more than a shaft riding in a bearing carrier. When the motor is engaged, the rotating group is connected to the shaft that was previously free spinning. To provide flow to the circuit, a variable displacement, closed loop piston pump is run from the PTO of the mechanical transmission.
The brain for logic functions is Poclain’s SmartDrive PLC. It comes pre-programmed for the application and will prevent any overlap in the use of the mechanical and hydraulic drive systems.
The previously mentioned components are sized for each application and supplied as a package. This reduces the risk of compatibility issues since they have been tested and functioning in the field for years. There’s also no question of where product support needs to come from since the entire system is from one manufacturer.
With the core components in place, the only details left are user input related. Poclain offers a packaged control box with a basic array of controls known as the CreepDrive Box:
This box is best suited for in cab or on-chassis operation. If the application requires remote control, say from the operator standing on the ground next to the work being done, then a remote control solution is easily integrated.
One key provider for Cross Company in this area is Kar-Tech. They provide pre-programmed receivers and transmitters in countless configurations. Map the inputs and outputs to match what the SmartDrive controller wants to see, and the creep drive doesn’t miss a beat. Just carry one of these and have a 30,000 lb truck at your fingertips from up to 2,500 feet away! (Not actually recommended, just possible).
In Summary
The hydraulic hybrid “Creep Drive” shines when the following features are required:
- Full mechanical drive performance on the road
- On-demand hydraulic drive capability
- Remote control capability
There are other hydraulic hybrid systems out there, but none that we are aware of that function quite like Poclain’s.
