Fluid power has become a vital component in our ability to perform work: it harvests our crops, takes our waste to the landfill, moves the landing gear, entertains and protects us, and all with a power density and flexibility that is unmatched in any other power transfer system. However, when it comes to efficiency, fluid power systems are not very high on the list.
A recent study has shown an average efficiency in fluid power systems to be less than 30% with a cost of billions of dollars in wasted energy. A great deal of work has been done and continues to be done in developing more efficient components: cylinders with lower breakaway, pumps and motors with greater volumetric and mechanical efficiency, compressors driven with variable speeds, and digital variable-displacement pumps. These are all beneficial, but they often come at a substantial cost, which must be weighed against the potential energy savings.
A pump that has a high volumetric and mechanical efficiency in a circuit where it is dumping across a relief valve is operating at 100% inefficiency. Even a load-sensing or digital pump is 100% inefficient at minimum displacement. An inefficient circuit with efficient components is still an inefficient system.
This article series will ask us to think differently about our fluid power systems. We will look at energy in a different way and discover ways to transform fluid power energy to make our systems more efficient and then be able to make better use of the new and improved components available to us.
We are about to enter a new era in the use of fluid power. To be successful in understanding the changes that are coming, we will need to re-think some of our most cherished notions about power transfer by fluids. For some of us, this will be very uncomfortable. We know what we know, and our formulas have been working just fine. What could possibly be new in fluid power? The laws of physics have not been changed, and it is unlikely some new laws have been discovered. So what will this new era bring to light that we don’t already know?
It’s not that we need to learn new things. We simply have to think a little more deeply about the things we already know and have sometimes taken for granted.
One of the most common misunderstandings about fluid power is the relationship between flow and pressure. It is hard to convince someone that increasing the pressure in a system will not increase the speed when the technician just finished increasing the pressure in the system and things are going faster. Most folks are unfamiliar with Daniel Bernoulli and his discovery that there is a relationship between pressure and flow when an orifice is involved. If the system in question has resistive flow controls, or even if it only has plumbing that functions as a restrictive orifice, increasing the system pressure, upstream from the restrictions, will increase the flow to the actuator. It was the increase in flow resulting from the increase in pressure that made things go faster.
However, there is an inextricable relationship between pressure and flow that must be understood when we are talking about power transfer. Any fluid power designer is familiar with the rotary power formula P = p x d x N / K where P is power, p is pressure, d is displacement, N is RPM, and K is a constant. Using this formula, we see that there is an absolutely discrete relationship among the variables; for any given power requirement at a given RPM, there must be a discrete product of pressure and displacement. With a variable flow source, such as with a variable displacement pump or an accumulator, an increase in pressure with a constant motor displacement, would cause an increase in flow and RPM.
A power-limiting pump is a variable-displacement pump set at a fixed pressure that varies its flow to match a power output. If this type of pump is used to drive a fixed-displacement motor under constant load, you could establish the motor RPM by the pressure setting of the compensator, without the use of a flow control, according to the power formula. A change in speed would require a change in pressure.
Let’s try it out: Get out your calculator or open up a spreadsheet and plug in the power formula. For power, put in 10 kW; for pressure, put in 20 MPa. Displacement will be 10 cc, and the K constant is 60,000. When we solve this for RPM, we get 3,000. Now, increase the pressure to 21 MPa. What do you get for RPM? That’s right. It drops to 2,857. It is true that there was a decrease in flow to reduce the speed, but it was responding to an increase in pressure.
Wait a second! Are you telling me that an increase in pressure produced a decrease in flow? That’s crazy! Everybody knows that an increase in pressure will cause an increase in flow and therefore an increase in RPM, right? Well, apparently not. We set up the formula so that there is a constant power input. The formula requires a discrete relationship among the variables, and the only thing we allowed to change with the pressure was the RPM. An increase in the torque demanded an increase in pressure. With constant power, the only option was to slow the motor.
A variable-displacement, pressure-compensated pump that is not power limiting will typically have a greater flow capacity than is required by a fixed-displacement motor. The compensator will be set at a pressure that is higher than the maximum load requirement. If the load on the motor is constant, the speed of the motor can be controlled by a simple needle valve or a pressure-reducing valve. The needle valve confirms Bernoulli’s observation: a specific flow will be provided because of the constant pressure drop across the needle valve. A pressure-reducing valve confirms the power formula: constant power, RPM, and displacement require a constant pressure in order for the motor to have a constant speed. But, in this case, if the pressure setting of the compensator or the pressure-reducing valve is raised, there will be more flow and the speed of the motor will increase. If the load on the motor is reduced, there will also be an increase in motor speed.
Very few applications have motors that have constant loads. If a constant speed is required from the motor under varying loads and/or with a varying flow and pressure source, a pressure-compensated flow control or a load-sensing system can be used. These are designed to provide a constant pressure drop, regardless of load or supply pressure, resulting in a constant flow and RPM at the motor.
Exactly the same situation exists for linear motion. A power formula could be written: P = K x V x d² x p where P is power, K is a constant, V is velocity in in/sec, d is diameter, and p is pressure (the constant includes solving for the area). Again, we see that there is an absolutely discrete relationship among the variables; for any given power requirement at a given velocity, there must be a discrete product of pressure and area.
We’ll give this a try: P will be 10 HP, K is 0.000119, d is 2 inches, and p is 2000 psi. Who wants to solve for V? Ok, what did you find? That’s right: 10.5 in/sec. Now, increase the pressure to 2200 psi and what happens? The velocity drops to 9.5 in/sec. So, again we see that, given a constant power input, an increase in pressure will result in a reduction in speed. The point is, we need to think a little more deeply about our power formulas. In future articles, we will look at making velocity, angular or linear, the constant.
With both hydraulics and pneumatics, the variable power source is always at a higher pressure than is required by the load. If the pressure were only that induced by the load, it would be a static situation. Directing the high pressure of the flow source directly to the actuator would cause the actuator to accelerate until the discrete relationship within the variables is satisfied, or, in the case of pneumatics, until the critical backpressure is reached. To prevent this, flow controls are installed. These flow controls add an additional restriction, producing a pressure drop so what enters the actuator satisfies the power formula.
In a pneumatic system, both the flow control and the pressure-reducing valve (regulator) restrict the rate of flow to or from the actuator. Of the two, only the regulator reduces the energy consumed. The regulator limits the total number of air molecules that enter the actuator while the flow control only limits the rate of flow until the actuator stops. Once the actuator stops, the flow control allows more molecules in until the actuator reaches line pressure.
In hydraulic systems, neither the flow control nor the pressure-reducing valve limits the energy consumption. Potential energy has been given to each unit of relatively incompressible hydraulic fluid. When that energy is greater than what is needed by the actuator, the energy must be removed to prevent acceleration. Both the flow control and reducing valve remove the excess energy by extracting the energy as heat.
So, back to “Flow Means Go.” There is no “Go” with flow unless it is accompanied with sufficient pressure. There is too much “Go” when flow has more than the required pressure. If you were to ask most any hydraulic professional how to make a motor or a cylinder go faster, you would likely receive the answer, “Add more flow.” This is normally my answer, too, especially when doing training in basic fluid power. This is correctly based on our understanding of the relative incompressibility of hydraulic fluid. A fixed-displacement actuator, be it a motor or a cylinder, will require a certain volume of fluid to move a given distance. The rate of speed is determined by the flow rate to the actuator. We are so accustomed to this concept that we take for granted the need to toss away the excess energy supplied by the pump or accumulator. The energy loss is simply the cost of control.
We need to think differently. Instead of separating flow and pressure as two different entities, we need to combine them into a single energy unit. Much like in electricity, where the volt/amp is the energy unit, we need to think in terms of pressure/volume as our energy unit. When we think this way, we are closer to finding a way to use flow without the expense of pressure. We can determine energy requirements in terms of units of pressure/volume. We can understand the potential energy stored in accumulators and, hopefully, not be too willing to toss the excess away.
Dan Helgerson, CFPAI/AJPP, CFPS, CFPECS, CFPSD, CFPMT, CFPCC, is Fluid Power Journal’s technical editor. He can be reached at firstname.lastname@example.org. Stay tuned for more articles in this series, and keep the conversation going by visiting Dan’s blog, Watts It All About.