Agricultural hydraulic design systems must be energy efficient, easy to operate and maintain, and of course reliable.
Contributed by Carl Dyke, CD Industrial Group, Inc.
Designers of hydraulic systems for agricultural machines face some difficult challenges. This is especially true for high horsepower tractors with not only steering and brake functions, but where a great variety of hydraulic implements will be connected and towed. With the operating parameters of a farmer’s collection of cross-branded, specialty implements unknown to a tractor designer, the hydraulic system may be either inefficient due to over-sizing and over-design—or it might fail to perform adequately if under-designed.
Self-propelled row crop harvesters, as well as fruit and vegetable harvesters now approach the size of many pulling tractors with engines larger than 500 hp. Some forage corn harvesters have engines with more than 1,000 hp. For some of these machines, the only reason to start the engine is to power hydraulic pumps. Others split the mechanical functions between a direct drive gearbox and a hydraulic system. System design must balance the challenge of evenly distributing hydraulic flow to all of the subcircuits, while retaining the capability of bringing all of the hydraulic power to bear on motor functions for quick travel between growing areas.
The common challenge across the entire spectrum of agricultural machinery is to create a hydraulic system that is energy efficient, easy to operate and maintain, and of course reliable. Partial automation is an absolute minimum necessity for large-scale machines where the operator’s attention can be drawn in so many directions.
First up: energy efficiency
Assuming that hydraulic motor torque requirements and displacements have been calculated and that cylinders have been sized to produce the required lifting forces (within a normal range of hydraulic pressures), the choice of pump size and style will be the next major decision. For a small scale farm, a utility type of machine may incorporate a fixed displacement gear pump with an open center valve bank for the sake of simplicity and low cost. Even though the pump may be pressure-unloaded with flow directed back to tank when no hydraulic function is in use, higher-than-needed fuel consumption still occurs due to the full flow rate.
When only part of the total flow available from the pump is needed for a cylinder—deliberately extended at slow speed by judicious use of the valve lever—the excess flow is forced over a spring-loaded pressure relief valve. The partially shifted valve spool closes the pump unload path to tank, and places a partially open valve in the flow path as a major restriction and pressure load. The pump is forced to work at full flow and at maximum pressure, even if the cylinder is only lifting a light load. In other words, the flow requirement for cylinder speed may only be a fraction of the total flow available, yet the pump can only pump at the full rate. In addition, the cylinder may only need to lift a light load (requiring minimal pressure), yet the system overall will be at maximum pressure due to the restriction of the partially opened directional valve. Fuel consumption is highest when pumping at full flow and full pressure, but this may be acceptable for a simple tractor with manual steering, and where hydraulic cylinder motion is occasional while working with the machine.
For large, production-scale machinery with continuously running hydraulic functions such as planting or harvesting circuits, and with the need for forceful steering actions at any time, the variable displacement piston pump remains the standard choice. A pressure-compensated and load-sensing piston pump avoids the inefficiencies of a simple gear pump. It provides a flow rate that automatically matches cylinder and hydraulic motor flow demand, across multiple parallel circuit paths, and also sets the maximum system pressure only slightly higher than what is currently needed for the highest operating load.
Pressure feedback is delivered to the load sense compensator on the pump from the highest pressure circuit within the system. This feedback adjusts the pump on the fly. With the maximum pump displacement sized just slightly larger than the anticipated maximum flow, fuel efficiency is delivered by a pump that can continuously adjust pressure and flow as needed.
The load sensing feature also offers greater flow accuracy within the system overall. Minor changes in engine speed do not show up as flow variances because the load sense compensator can adjust the pump automatically. If the hydraulic system is operating motors on timed production functions on a harvester, then this flow accuracy has a direct impact on production and quality.
The pressure compensator is a control mechanism on the pump that reacts when a preset maximum pressure level is reached. Instead of using a system relief valve as the primary means of pressure limiting, the compensator reduces the pump displacement and keeps it near zero until the excessive pressure condition (e.g. an overload at cylinder or motor) dies down. When pressure drops down, the compensator allows the pump to increase displacement once again. The pressure compensator is similar to a standard relief valve in that it keeps the system pressure from rising above a safe level. But the pressure compensator does this by reducing the pump output, rather than inefficiently diverting extra flow back to tank. The pressure compensator provides a similar safety feature as a relief valve, but with the added bonus of energy savings.
The flow and directional control valve banks on most modern tractors and harvesters feature electronic control. With variable current solenoids, the valves are capable of proportional flow to set cylinders and motors in motion at the desired speed.
The valves can hold a cylinder in a locked position (e.g. crop cutting height or plow depth), or allow a cylinder rod to move freely in and out so that an implement can float up and down with ground contours. For smaller, directly attached implements, this raise-and-lower cylinder is found on the tractor’s three-point hitch. For larger towed implements with their own cylinders and hydraulic motors, pairs of quick couplers from tractor valves commonly referred to as “remotes” are found at the rear of the tractor to receive hydraulic hose connections.
A pressure compensator is typically found within each section of the valve bank. Most often this extra valve component is found on the inlet side of the main spool. The compensator senses the load pressure of the cylinder or motor in motion, and adjusts the pressure at the inlet of the main valve spool to ensure a steady pressure differential across the spool, thus maintaining a steady flow rate even when system pressure or cylinder load pressure changes. This means that the operator does not have to quickly move a valve lever in reaction to a hydraulic function that is changing speed during motion. The pressure compensator eliminates the problem.
The valve bank contains a network of shuttle valves with tiny steel balls or poppets inside. These shuttle valves (aka ball resolvers) allow for the load pressure from only the highest loaded valve section to be transmitted to the load sense compensator on the pump, via a special signal hose. The use of the load sense compensator on the pump and the valve bank with shuttle valves as described completes a closed loop control scheme with the use of a fluidic pressure line to deliver feedback from the load to the pump.
Load-sensing systems are considered to be among the most fuel efficient for continuously running hydraulic systems. They can also be challenging systems to correctly adjust for tractors pulling implements of infinite design variation. There are lots of components involved, making system troubleshooting a complex process.
Recent innovations such as Parker Hannifin’s Split Pump Intelligent Flow Architecture use one piston pump for each major function on the machine, allowing for even greater energy savings. These types of systems, already proven for loaders and some excavators, will slowly make their way onto agricultural machines. The pump flow rate and the proportional valve spools are electronically controlled. This offers options around combining pumps for use with one implement cylinder during fast motion requirements, or for separate functions when operating several different cylinder motions at the same time. The internal parts of the valve bank are simpler and the use of a load-sense signal hose between pump compensator and the valve bank is eliminated in favor of faster electronic pump control.
Energy saving designs for hydraulic systems on large agricultural machines also translate to lower oil temperatures, which lower or even remove the need for large coolers and fans. A size reduction for fluid reservoirs is also possible for these systems.
Ease of operation
Hydraulic functions on an agricultural machine are considered easy to operate when the operator does not have to make continuous corrections to cylinder positions (e.g. crop cutting height, steering) or adjust flow rates to hydraulic motors. Quality and production for plowing, planting or harvesting are the top priorities on large-scale machines. The operator’s brainpower is best used to monitor the big farming picture. With the high travel speeds and the wide crop rows possible on high horsepower machines, the operator still has lots to do. Operator-programmed controllers are extremely common. These controllers can be programmed to remember how much time it takes for hydraulic cylinders to unfold an implement. This “timed detent” feature can then be used later with the push of a button to accomplish the same operation.
The minor points of machine control and position correction should be automatic with an allowance for operator override. Automation and operational ease is the result of typical electronic controller and touch screen use with electrohydraulic valves. The possibilities for a machine that in the past was hard physical work to operate, continue to expand with steering guidance systems now moving well beyond the servo-operated steering wheel. Unlike the world of construction machinery, where steering is often controlled by joystick on many models, the familiar steering wheel connected to an orbitrol valve (aka hand metering unit) is slow to disappear from agricultural machines. There are however, entirely automated steering controls and valves connected in parallel to the steering wheel on some machines.
Designers should now give standard consideration to the installation of linear position sensors inside the cylinders on the steering axle or in the cylinders used for articulated steering. Non-steering cylinders are also worth considering for the installation of linear position sensors, to enable more position memorized automation. Some newer sensors that are suitable for double rod steering cylinders can mount on the outside of the cylinder tube with only the magnet inside on the piston. The electronic control equipment on the machine may not yet be configured for fully automated, robotic steering, but many systems already offer a level of GPS-based guidance for the long straight runs across the field, allowing the operator to avoid fatigue.
Ease of maintenance and repair
Ease of maintenance and repair is a valued feature that many farm operators look for. While easy-to-access fill ports and easy-view sight glasses are welcomed, many speak with notable praise for the machine designer who makes adjustments and wrench work easy. With 45° elbow fittings, or wide radius tube elbows at the cylinder ports, the connections are easy to work on when damaged hoses have to be replaced. Rows of closely spaced hoses and fittings clamped along the machine frame can work out sufficiently if the fittings are offset from each other to allow room for wrenches. Mounting filters for easy removal and change-out is not always the most visually pleasing layout, but mechanics appreciate it. It also helps to make sure that maintenance tasks are not skipped.
Even something as basic as filter placement can have a big impact on maintenance; if filters are accessible and easy to remove/change out, this simple maintenance task is less likely to be overlooked.
Agricultural machinery continues to be a key product for the application of hydraulic systems. As innovative components and system designs that make for more efficient operation are being released from time to time, the use of automation is increasing very rapidly. This is especially true for large-scale machines designed to help the farm operator with production yield, time usage and human input energy.
It is exciting to see innovations from other industries, such as mining and construction, crossing over into the agricultural world and making a difference in the average farmer’s day. As innovations in hydraulic system design continue to filter into agriculture, farmers will enjoy the rewards of even greater efficiency, ease-of-operation, and hopefully, reductions in maintenance complexity.