Innovations in backhoe loaders
The backhoe loader continues to be one of the most versatile machines at construction sites around the globe. This versatility is due, in large part, to hydraulics.
Take a tractor for a base, attach an excavator at one end, a loader at the other, and you have a backhoe loader. That sounds simple enough, but building all this functionality into a single machine requires clever system design and strategic use of hydraulic components.
As backhoe loaders have advanced, so too have the demands placed on them. Today, attention must be paid more than ever to efficiency and controllability - not to mention ergonomics and lower exhaust and noise emissions. Hydraulics encompasses all of these.
Power management
Rarely is the power available from a backhoe loader's gasoline or diesel engine sufficient to drive all hydraulic functions at full displacement and maximum pressure. If a hydraulic system were operated at full power all the time, most of the power would be wasted. Therefore, a common practice is to specify a variable-displacement pump in conjunction with appropriate controls to allow using only as much power as is needed. In many cases, the widely applied variable-displacement axial-piston pump with medium pressure capability can be selected, provided the appropriate control options are specified.
Power control (torque limiting) - When sizing a hydraulic pump without power control, the pump's displacement must be based on the corner horsepower - maximum flow at maximum pressure. This corner horsepower should not exceed power available from the engine, otherwise the engine will stall. Not using all the hydraulic power available wastes energy.
To circumvent this limitation, pump controls can be implemented to limit the torque consumed by the pump regardless of its corner horsepower. This is accomplished by limiting the pump's discharge pressure based on its displacement. Therefore, higher flows can be achieved at lower pressures and lower flows at higher pressures, thus preventing the engine from stalling or becoming overloaded.
Another benefit is enhanced machine operation. Higher corner horsepower allows faster working speeds from higher flow rates or higher forces from higher pressures. Also, engine power is applied more effectively, leading to higher fuel economy, lower noise and exhaust emissions, and no engine overloading or stalling.
Dual horsepower control (dual torque limiting) - When a backhoe loader is used as an excavator, nearly all engine power is used for the backhoe's hydraulic implements. But when used as a loader, only a portion of the engine power is used for hydraulic implements. This is because engine power must be shared between the loader's hydraulic implements and the drive transmission.
Dual power control allows shifting between high- and low-pressure operating modes. This is done by utilizing the power control valve's dependency on the control flow rate to modify the power output setting. Increasing control flow to the power control valve raises pressure in the control line and increases the power output setting. Conversely, delivering less flow reduces pressure and, therefore, the power output setting.
Whereas setting a standard power regulator demands a compromise between excavator and loader operation, power control utilizes the full potential of both operating modes. Moreover, power output setting can be adjusted to the particular engine speed (engine power output). This is a big advantage in residential areas where machines may only be operated at lower speeds for in conforming with noise restrictions.
Speed sensing power control - One of the latest developments of cost-effective controls is the electroproportional displacement control. With this control, the pump's displacement is governed by an electric current applied to a control solenoid. It is compact and directly connected via a pilot spool to the pump's displacement control cylinder. It acts as an adjustable maximum displacement stop. to p[provide failsafe operation, if all electric power is lost, the pump automatically returns to maximum displacement, ensuring oil flow to steering unit and braking devises.
Speed-controlled diesel engines are commonly used in backhoe loaders; engine speed drops in accordance with load applied to it. Detecting the position of the accelerator pedal or the stroke of the injection pump via an angle sensor provides an indication of the power available. Sensing the actual diesel engine speed provides a measure of load on the engine. Comparing both signals in a control unit can indicate how much engine power is actually being utilized.
A diesel engine is overloaded if it undergoes an excessive drop in speed. Should this occur, the control unit reduces the solenoid current of the pump's controller, adjusting the pump to a lower displacement. This, in turn, reduces the load on the diesel engine, allowing it to recover. However, if the engine speed doesn't drop enough, engine power is not fully utilized. In this case, the control unit will increase the solenoid current, thereby bringing increasing pump displacement. Thus, all the power the engine can deliver will be fully harnessed.
The power consumed by hydraulic implements is automatically adjusted to the power available, regardless what power is used by the gearbox. However, with dual power control, performance always is a compromise. Tests have shown that a smaller pump with electroproportional displacement control can achieve performance of a larger pump equipped with dual power control - especially when in loader operation.
Summary
The power management concept of speed sensing based on an electroproportional displacement control guarantees full-time optimum engine power utilization without overloading or stalling.
Optimum machine utilization keeps operating costs down, also making this system an attractive option compared with the costs of the technically inferior fixed displacement pump system. Ultimately, decisions regarding the hydraulic system will be based on cost of products compared to the operating cost and performance.
Limiting pump torque
The horsepower control valve (torque limiter) is a pressure relief valve connected to the swashplate's swivel cradle via two springs. Its opening pressure, therefore, is governed by the position of the swivel cradle. When flow from the pump is low, opening pressure is high, when pump delivery is high, opening pressure is low. The valve is piloting the flow regulator and limits pressure in the LS-line.
Because the control fluid is delivered through a nozzle, the power regulating valve is able to generate a pressure drop in the control line as soon as it reaches opening pressure, thus affecting the flow regulator so that the pump swivels into position. Once the power control is active, the flow regulator becomes a remote pressure controller. Flow is reduced as the pump swivels into position, increasing the opening pressure (control pressure) and thus keeping pump drive torque constant.