Pilot systems used in mining equipment showcase the brute force of hydraulics, as well as the precise control.
By: Carl Dyke, Contributing Editor
I have been in the mining pits. It can be a rough and dusty environment. Two million pound, 5,000-hp shovels quickly and gracefully load 4,000-hp haul trucks that drive off weighing over one million pounds. As those trucks steer along the haul road, graders with hydraulically operated blades as wide as two highway lanes do their best to smooth the bumps. Back at the mining face, 900-hp dozers lift, drop and angle their blades while helping to feed the shovel. It takes those haul trucks only half of a minute to hoist and dump their loads, so they’ll be back at the shovel in no time.
When you watch the lumbering motions of the shovel or the simple hoisting action of the haul truck from a distance, it’s hard to appreciate that there are dozens of hydraulic pumps at work on each of those machines.
You have to see the massive directional valves for yourself to appreciate the control of 300 gpm for body hoisting and well over 1,500 gpm for some combined shovel motions. There are also some very small hydraulic components in sub-circuits where there is almost no flow occurring. Yet without these components functioning correctly, the heavy lifts and forceful motions of the grader, dozer, haul truck and shovel will simply not occur.
The operator’s light touch
If it’s hard to appreciate all of the massive hydraulic flows from a distance, it’s not much easier from inside the sound insulated operator’s cab. Shovel motions such as boom lift begin with a very light touch, pulling back just slightly on the right hand joystick. A digital instruction, or an analog signal of only a few milliamps is sent from the joystick to an electronic controller. Somehow, that signal results in smooth, feathered control of a pair of boom lift cylinders that are several feet in diameter each.
From the operator’s seat in the haul truck, only light and gentle movements are needed on the steering wheel to keep the massive wheels on the steering axle pointed in the right direction. Similar to the light efforts of the shovel operator, the haul truck operator hoists and dumps a 400-ton load with just a simple upward pull on a small lever.
Examples of pilot control
Valve piloting provides enormous amplification of the operator’s efforts. Piloting minimizes operator fatigue while allowing hydraulic systems to do all of the forceful controlling work and, of course, the heavy lifting. In some cases, an operator simply wouldn’t have the physical strength needed to manually activate such large valves.
To bring massive hydraulic flows online quickly when the shovel needs to swing towards the haul truck, or when the haul truck operator needs to steer quickly around an obstacle, the swash plate inside multiple piston pumps may have to swivel from neutral all the way to full stroke in a fraction of a second. Again, pilot pressure valves are needed to control another spring returned valve. In this case the valve is a controller for the pump displacement.
Pilot control is force control
Pilot control of hydraulic components is not often fully appreciated as a key subsystem. Pilot components are small, and the hydraulic flows passing through them are tiny. In fact, most pilot systems in mining machines are not meant to control flow, but rather they are force controlling. An example of force control is how hard to push on the end of a spring returned, proportional flow, directional valve spool. In older generations of hydraulic machines, and on some smaller and simpler machines in the current day, the valve spool is moved directly by hand, via a mechanical lever. If you have a keen sense of observation, you sometimes notice that a lever-operated spool valve requires more of your physical strength as you move the spool further away from neutral. Return springs inside the valve oppose the operator’s effort.
On large machines, the increasing and decreasing force required to operate a valve spool, against return springs, is handled by the pilot pressure system. On some shovel models, there are two stages of pilot control. A two-stage system is needed where the primary stage is a highly accurate, electrically operated, pressure reducing valve, but not adequately sized to handle the very short burst of flow that is needed to move a large directional valve spool to a new position. The secondary pilot stage is also a pressure-reducing valve with the same basic function as the primary stage—except that this larger sized valve with its heavier spring cannot be operated electrically.
Moving directional control valve spools into position is just one example of a force controlling, pilot scenario. The main steering amplifier valve on most haul trucks is essentially another (if slightly unique) piloted valve spool example. Displacement controls on large pumps use a similar pilot method as well. Naturally, force control is accomplished in pilot systems via pressure control.
A very central component in controlling pilot pressures is no larger than a soda pop can; often it is only half of that size. A screw-in or slip-in valve cartridge is a common style, with only the armature tube and solenoid visible above the manifold or cavity. For shared porting and hose connection efficiency, as many as eight of these valves are built up as one assembly with a common manifold.
Pilot pressure valves are actually pressure reducing valves
The valve in question is a pressure-reducing valve. When the solenoid is left unpowered, it provides no pressure on its outlet higher than what is found in the tank. As the proportional current level through the solenoid increases via operator joystick movement and through electronic controller circuitry, the pressure-reducing valve produces a progressively higher outlet pressure. These valves are often referred to simply as pilot pressure valves.
The range of pressures and the exact pressure value produced at any one moment on the outlet of a pilot pressure valve are a forceful translation of the operator’s lever and pedal motions. Even though we have translated a small electrical signal from levers to a forceful hydraulic pressure, we’re still working with very small components. Ultimately, the goal will be to control large valves that can handle large flows.
The pilot pressure pump
The source component in a pilot system is the pump. If a typical pilot system has too little or no flow as stated earlier, then why is a pump needed? If you’ve learned that a pump does not account for the pressure value present in a hydraulic system, then you’ve got one piece of the puzzle. Remember, resistance to flow from narrow passages (orifices, valves, undersized hoses) or from heavily loaded cylinders is what accounts for the pressure value on a gauge that precedes those components. If you noticed the word ‘flow’ in the previous sentence, then obviously a pump has a role to play. If there is no flow from a source component (the pump), then there is no flow to resist, and thus no pressure.
There are other ways to create and store potential energy (pressure) for use in valve piloting. These can include a raised boom cylinder with the weight of the raised boom and cylinder rod pushing down on a trapped column of fluid. The use of fluid from a charged accumulator is another method. Indeed, these methods of providing pilot pressure do exist for some mining machines.
In the case of a raised boom, the pilot pressure will dissipate once the boom rests on the ground. This is a handy trick as an alternate energy source for the piloting of directional valves to help lower a raised dozer blade to the ground, should the prime mover or pumps fail. Once the charge oil has been drawn from a pilot system accumulator, it will need to be recharged. At some point, a pump will come into play in the proper functioning of a pilot system.
On hydraulic shovels, the most common component for providing a source flow for a pilot system is a small gear pump. Earlier, we noted that pilot pressure valves typically consume little if any flow.
What is the path for the pilot pump’s flow, if not through the pilot pressure valves? The pilot pump flow is directed back to tank over the poppet of a spring-loaded, pressure relief valve. This relief valve is located on a tee fitting on or near the outlet of the pilot pump. As the pilot pump is very small, and the typical hydraulic tank on a shovel is very large, the continuous flow over the pilot pressure relief valve does not contribute much heat to the fluid volume, nor consume too much input energy.
How a pilot pressure valve works
The hose or tube that continues onward from the pilot pump and relief valve towards the manifold of pilot pressure valves supplies a maximum pilot pressure value. The exact maximum value that is set on the pilot pump relief valve is supplied to all of the pilot pressure valves. When activated individually by the partial lever/pedal movements of the operator, some but not all of that supplied pilot pressure is transferred over to the outlet port of the pilot pressure valve.
As the operator slightly increases lever or pedal movement away from the neutral position, increased current through the solenoid of the pilot pressure valve causes the armature to push the spool downward, which connects the pilot pump pressure to the valve outlet. This connection is only made for a brief moment. If the operator only moves the boom lift lever by a tiny amount (desiring a slow rate of cylinder extension), then the pilot pressure valve for boom lift only allows a portion of the maximum available pilot pressure to build on its outlet.
When the operator moves the lever back towards the neutral position (desiring a slower rate of boom lift), the decreased solenoid current allows the spool in the pilot pressure valve to momentarily move upward to connect the valve outlet to tank, decreasing the outlet pressure.
The pilot pressure valve only opens slightly, and for very brief periods of time for any new operator lever position. Why is this? With each change in lever position, the operator desires a different rate of cylinder travel or hydraulic motor speed. These speeds are determined by how much flow is allowed through a large, proportional, directional valve. The spool in such a large valve is pushed (piloted) from one end or the other, to the correct amount of flow opening using a tiny amount of fluid from the pilot pressure valves. What causes this large spool to stop travelling once the required level of flow is passing through? Springs and the pilot pressure valves take care of this action.
Complete pilot systems in action
- Let’s look at the action and response of the full system.
- The primary pilot pressure valve opens for a very brief moment, connecting pilot pump to the outlet of the pilot pressure valve.
- Fluid travels from the pilot pressure valve to the pilot end of a directional valve spool, or to a secondary pilot pressure valve.
- The spool now starts to move. As it moves, the spool compresses a spring on the opposite side of the valve spool.
- The compressed spring causes an increase in resistance to the tiny pilot flow, which causes the pressure value to rise.
- This increased backpressure is sensed nearly instantly inside the primary pilot pressure valve. The surface area on the larger flange-like, annular piston surface in the pilot pressure valve forces the spool to move upward against the solenoid force, closing off the path from pilot pump to pilot pressure valve outlet.
If the pilot pressure valve was to spend any significant amount of time (a few seconds would be very long) in the open position, as the operator selects a slightly increased signal, the maximum available pilot pressure would force the directional valve spool to move as far as possible. This would mean maximum flow to the cylinder on the shovel where the operator may have only wanted a very slow motion. This makes it clear that the outlet of any pilot pressure valve must only be open to the pilot pump supply pressure, or to the tank pressure, for a very brief moment only, whenever the operator makes a slight increase or decrease motion on a control lever.
The flow rate through a primary pilot pressure valve is limited by the tiny oil volume needed to move a directional valve spool or a pump displacement control spool, or the spool in a secondary pilot pressure valve. In addition, the primary pilot pressure valve must open and close quickly to establish and hold the correct outlet pressure. These two factors combined dictate a pilot pressure valve design with a small spool that travels only 1⁄16 in. (a few millimeters) in each direction, at the most.
What we have learned is that the large hydraulic flows that pass through massive directional valves to move boom lift cylinders and other large actuators on mining machines, are initiated by tiny spool movements inside of very small pilot valves.
The manufacturers of these tiny, precision components contribute a very large effect. As with all hydraulic systems, doing everything possible to keep valve-jamming contaminants out of the fluid makes for reliable pilot systems, where the operator can smoothly move hundreds of tons per machine motion with only the twitch of a finger.