Logging and timber harvesting machinery rely on load-sensing electrohydraulics for efficient, productive operation.
Contributed by Carl Dyke, LunchBox Sessions
In agriculture, like any other large-scale commodity, productivity and efficiency are keys to profitability and competitiveness. In corn harvesting, for example, it is not uncommon now to find harvesters with 2,000 hp and harvesting headers wider than 40 ft. Timber harvesting is similar to agriculture in that a living plant has to be cut from a growing area quickly and efficiently. It’s a very dynamic operating environment. Of course, the stalk of the plant is substantial compared to a row crop like corn. Instead of multi-row harvesting width, trees are still largely cut one at a time. So how is harvesting speed and efficiency achieved in the forest?
The only reason to start the engine on any track-type timber harvesting machine is to turn the shaft of a number of hydraulic pumps. Hydraulic pumps create the flow required by cylinders to produce linear motions and by motors to create rotary motions.
Complex hydraulics make it look easy
Moving around on an uneven forest floor or on a slope certainly requires a powerful drive. The fussy hydraulic actions really become evident to the uninitiated as you watch a boom reach out with a harvester head, align itself to the tree to be cut, clamp on, fell the tree and then tip it down to the ground. Without missing a beat, the head will remove all the branches in a process called delimbing, then buck the log to various lengths. It’s amazing to watch because it all appears to happen so quickly and effortlessly. In reality, there are a number of production demands made on the operator and there is a complex hydraulic system at work that integrates quite a few sensors and controls. The power requirements for the hydraulic system can change substantially from moment to moment with all of the variables in a forest.
When reading through the brochures and specifications for feller bunchers and tree harvesters one quickly understands that logging machinery requires high flow rates. Speedy harvesting on some machines means 220 gpm (840 lpm) total hydraulic flow provided by up to four or even five pumps. On the majority of machine models the popular and rugged axial piston pump gets the job done.
The idea is not only to displace the flow rate of oil required at the operator’s chosen speeds, but to do so as efficiently as possible. Simple flow production is a challenge that is easily conquered by the basic variable-displacement, pressure-compensated piston pump. You might know that type of pump controller as a pressure cut-off adjustment. The idea is that as the system pressure reaches the maximum desired value, the pump reduces its displacement. The pump’s internal controls provide a bias towards maximum displacement. The resistance (pressure) of the hydraulic system at any given moment is sensed just inside the pump’s outlet port. When that system pressure is near or at the maximum as permanently set on the pump’s compensator adjustment, the pump enters an active state where the swash plate angle is swiveled away from maximum by the large diameter internal control piston. This swiveling to a reduced angle shortens the stroke length of the main displacement pistons and thus the pump’s flow rate drops. If the sensed system pressure falls far enough as the swash plate begins to swivel away from maximum, then the compensator’s adjustable spring pushes the control spool back towards the normal position. This allows the bias spring or a smaller diameter control piston/bias spring combination to push the swashplate back towards the maximum angle and thus increase the pump’s flow rate once again. This pump control scheme means that the output flow rate of the pump will match the demand as set by proportional valve spools as chosen by the operator’s lever actions for direction and speed of an actuator. It also means that a correctly sized pump will often be operating at or near the maximum system pressure. But at least some energy has been saved by not always pumping at the maximum possible flow rate.
Load-sensing systems are key
Load sensing systems multiply the efficiency by adding a second adjustable control spool that is responsive to load changes. Instead of a permanently adjusted pressure cut-off, imagine a technician who keeps a wrench on the adjustment screw and increases the setting on the pump whenever the work is getting more difficult. (In our example, this might mean the trees are becoming larger, or the slope being climbed gets steeper.) Our imaginary technician is there to instantly increase the maximum allowable system pressure to help the machine tackle the changing job conditions. As the trees to be cut get smaller or as the machine travels downhill that same technician is still back there under the engine cover receiving the operator’s instructions to turn down that maximum pressure setting on the pump controller. These adjustments save you from pumping at pressure levels that are wasteful for the easy conditions. Pumping at an excessive flow rate or at an excessive pressure means wasted input energy, which is typically diesel fuel.
In this case, with a pump controller adjustment happening on the fly, the pump flow rate and the maximum system pressure are being adjusted.
With a load sensing pump controller you don’t need that technician to ride along making adjustments on the fly. The load sensing control on the pump receives a pressure signal from the section of the valve bank that is working with the highest load pressure at any given moment. Small shuttle valves in a set of passages and tubes called the signal network make way for this one highest pressure application to communicate to the pump. This is the pump’s controller ‘sensing the load.’ As the load pressure increases on that one section, that pressure assists a spring inside the pump’s controller to dynamically set the pump to a higher pressure in the same way as the ride-along technician in the earlier example. The outlet pressure in this case is not set to the maximum as in the pressure cut-off feature, but rather only a few hundred psi higher than the load pressure.
This load sensing arrangement can often be found to save at least 10% of the fuel bill as opposed to only using pumps with a single pressure adjustment. For a machine that can be found operating for several shifts a day during the timber harvesting season, this fuel saving can add up quickly. For those operators harvesting timber in the summertime, the fuel savings in a load sense system is also directly tied to lower heat levels in the hydraulic system.
Ensure careful valve designs
A timber harvesting OEM will take great pains to select a bank of proportional directional valves that will work well with a load sensing pump. The valve bank will need to have the correct inlet section with a port for the load sense signal pressure. The spools will have to be chosen to provide the correct center position for the actuator being used. The valve sections may need to have work port relief valves and anti-cavitation check valves.
Of key importance will be the choice of the pressure compensating valves in the spool sections. A typical load sense valve bank such as the Parker L90 will offer a few options. A basic pre-spool pressure compensator will make sure that the flow rate to a particular actuator function is prioritized and maintained. On the other hand, the choice might be for pressure compensating valves that help to share flow to all actuators when the piston pump is near a maximum flow state. This flow sharing may mean that all active valve sections run their attached cylinder or motors a little slower, but at least none go into a stalled condition as might happen if all flow is prioritized to a single flow path.
Know system flow limits
One of the key features that timber harvesting machine operators mention is the machine’s ability to swing at full speed while operating a harvesting or processing head. Again, this type of feature speaks to being able to supply full flow to several functions at the same time. In some difficult design challenges, the best solution is a separate and dedicated hydraulic system for a key function such as swing.
Flow limits can still be tested to the max when the system pressures are near the peak and the engine speeds start to droop. It is not uncommon on many feller bunchers and timber harvesters to encounter a scenario where the 220 gpm (840 lpm) total flow mentioned earlier would require a prime mover with 700 hp (520 kW) if all pumps were also operating at 4,500 psi (310 bar) system pressure. If the engine of some of the largest machines only offers 350 hp (260 kW) there is a potential for engine stall. In this case the largest pump, often tasked with supplying travel, swing and boom flows, will feature one additional type of control where the maximum flow rate of the pump is mechanically limited when the highest system pressure occurs. This horsepower limiting control feature allows the machine designer the luxury of allowing high possible flow rates and high pressure for individual pumps during tough jobs or for very high production rates. The trade-off is to sacrifice machine speed during the less common moments when all applications are used at the same time and at high pressure.
Harvesting timber is a competitive business requiring logging operators to work efficiently and select the best machines for the job. A consistent theme running through the industry is the use of well-designed load sensing hydraulic systems to save fuel while supporting the demand for flow in multiple applications at the same time. Retrofitting machines for a load sense system would require intense revision — unfortunately, the conversion is not as straightforward as simply purchasing a pump with load sense capability. The valve bank and any logic systems must also accommodate the load sense system. The expense of improved efficiency pays off quickly in high-production environments, as a well-tuned load sense system may shave as much as 10% from the machine’s fuel bill, while continuing to provide the power needed for even the toughest loads.