Common Problems with Traditional Hydraulic Systems
Modern hydraulic systems are simply amazing for their power density, robust operation, and all-around influence in heavy duty equipment and vehicles. But for all the advantages of modern hydraulic systems, there are some serious problems that persist. We will go over common problems and how they may be addressed.
Problem #1 – Inefficiency
The largest and arguably most costly problem is the terrible inefficiencies of fluid power systems. A typical hydraulic system incurs energy losses at multiple points throughout the work cycle. From the prime mover (traditionally an IC diesel engine), pump, valves, hoses, and actuators, the losses are comprised of several different types.
We will start with the prime mover. In mobile applications, the prime mover is typically a diesel engine due to its reliability and torque output. The best way to optimize efficiency of an internal combustion engine is to decouple it from the pump, and allow it to operate within its optimum efficiency, torque, and rpm range. This scenario lends itself to hybrid or zero emission all-electric architectures for vehicles. By changing the engine load from one that must follow the torque duty cycle (coupled) to one that can operate independent of the torque requirement (decoupled), we can easily increase the prime mover efficiency by approximately 25% . When the prime mover is an electric motor, there is a similar scenario regarding efficiency. According to the U.S. Department of Energy, the maximum efficiency of an electric motor is around 75% of full rated load . Again, unfortunately most fluid power systems do not allow the motor to operate at that condition very often. If the electric motor is only loaded to 20% of its rating, then the efficiency losses can be as high as 80%. For a great in depth read about electric motor efficiency, check out “Premium Efficiency Motor Selection and Application Guide” put out by the U.S. Department of Energy’s Advanced Manufacturing Office .
Current methods to size an electric motor for fluid power systems is all about compromise. They are sized to handle worst case load situations but in reality, it will be operating throughout its percentage of full load range. One way to increase efficiency of the motor losses is to operate in a power-on-demand configuration. The motor would only be supplying the speed and torque required by the work being done. While synchronous induction motors are by far the most widely used motor type in the world, they do not lend themselves to operating in a power-on-demand function. This is due to needing a minimum speed to attain a magnetizing current and thereby be able to produce torque. If you must operate at lower speeds, an induction motor will not suffice. Moving to a permanent magnet motor is a great step in achieving power-on-demand functionality, but it comes at the price of requiring a motor controller (sometimes called a “drive” or an “inverter”). This creates a cost prohibitive barrier for most applications. There are ways to drive down the cost of motor controllers that will be discussed in a future article.
Following the prime mover down the chain is the hydraulic pump. Like the prime mover, the pump’s performance is largely dependent on the duty cycle of the system. When running full speed and fully loaded, many piston pumps can approach 90% efficiency, unfortunately most systems rarely (if ever) operate in this condition. Usually, they see a duty cycle that fluctuates from zero work being done to moderate or higher levels of work. This means that the overall pump efficiency may be closer to 60% or 70%. One easy way to improve the energy lost at the pump (although you would not improve the actual pump’s mechanical or volumetric efficiency) is to again, operate it in a power-on-demand situation. This means that the pump is only producing flow or pressure when work is being done and only the flow and pressure that is required at that instantaneous point in time. This is similar to decoupling the engine as described above and can remove all non-optimum duty cycle losses.
Next in the chain are the control valves. These take the form of directional control valves, pressure control valves, flow control valves, etc. A typical hydraulic circuit has a complex manifold of valves, and they all rely on Bernoulli’s principals of pressure and velocity in a fluid. Specifically, it states that an increase in speed of a fluid results in a decrease in pressure (pressure drop). This pressure drop is a decrease in the potential energy of the fluid and results in heat being generated, i.e. energy lost. In almost all hydraulic valves, we must create an increase in velocity in order to control the direction, flow, or pressure, thus the energy losses in these components are inevitable. In a future article, we will discuss a simple way of dealing with the energy losses in valves, by simply removing them from the system!
• The best way to optimize efficiency of an internal combustion engine is to decouple it from the pump, and allow it to operate within its optimum efficiency, torque, and rpm range.
• Increase the efficiency of an electric motor and hydraulic pump by operating in a power-on-demand configuration.
• Pressure drop is a decrease in the potential energy of the fluid and results in heat being generated, i.e. energy lost.
Problem #2 – Complexity
Over the years, fluid power systems have consistently become more and more complex. With the explosion of integrated electronic valves, sensors, and controllers in the last decade or two, the complexity of a system has been compounded. A hydraulic system is very dynamic by nature, and when one component has a problem, it typically can cause a chain reaction of effects throughout the system. This creates problems not only for troubleshooting (we discuss that below) but also tuning, sensor feedback filtering, maintenance, and cost among other things. A good example of this problem is when a pump load sense orifice is constricted by a partial blockage. This partial blockage can present itself as various symptoms throughout the system depending on the dynamic load or flow rate being called for. Due to Bernoulli’s pesky principle (states that an increase in fluid speed results in a decrease in pressure or vice versa), a constantly changing flow rate will constantly change the pressure drop across a LS orifice. Normally this is compensated for within the system design, but when an unforeseen obstruction or blockage occurs (especially one that is transient in nature) the dynamic system effects can be troublesome.
• fluid power systems have consistently become more and more complex.
• A hydraulic system is very dynamic by nature, and when one component has a problem, it typically can cause a chain reaction of effects throughout the system
Problem #3 – Difficult to Troubleshoot
Troubleshooting….. there are more troubleshooting articles, opinions, tips, tricks, how-to’s, etc. out there than any other item related to fluid power. Every expert or technician has an opinion on how best to go about troubleshooting a given system. The main problem with troubleshooting is every hydraulic system is different: different pumps, different valves, different controls, different pressures, different duty cycles, different flow rates, different.. well… everything! No two systems are alike, even if they are designed the same with exact components. This fact, along with the very dynamic nature of fluid power systems, creates a nightmare for the troubleshooter. Troubleshooting also involves the blame game, i.e. which component supplier is most likely responsible for whatever the problem is at hand. The pump manufacturer will blame the valves, the valve people will blame the controls guy, etc. This will cloud the waters and divert from the true culprit of the system issue leading to incorrect diagnosis and/or solution implementations. Instead of going into all the reasons that a system could fail or not perform correctly, let’s talk about components and their likelihood to fail or cause problems starting from the least likely to the most likely.
1. Reservoir: its steel so not much to say.
2. Hoses and tubes: these either work or not. They either hold pressure or they leak.
3. Fittings: again, see above.
4. Coolers and heat exchangers: slightly more complex but typically either the fan is running or not. Pretty easy to troubleshoot.
5. Hydraulic Cylinders (actuators): not much can go wrong here either. According to Fluid Power World, they can fail by damaged or deteriorated seals or physical damage . Blowby can create some more difficult issues to troubleshoot, but a seasoned technician should be able to identify that.
6. Hydraulic motors: these can be a bit more difficult. They can be robust and simple like the fixed displacement variety, or difficult to troubleshoot like adjustable axial piston motors. The most common issues involve the swashplate control valves or control electronics.
7. Hydraulic pumps: now we are getting to many-a-problem in a hydraulic system.
Problem #4 – Prone to contamination
The typical clearance between some internal components in a modern piston pump or servo valve is less than 2 microns. To put that in perspective, a human hair is around 50 microns, a red blood cell is about 8 microns, and 40 microns is about the smallest particle visible to the naked eye! That means that a particle around the size of a red blood cell has the potential to bring an entire hydraulic power unit to a standstill. A typical hydraulic system filter is sized to remove contaminants down to about 10 microns with many high-end systems requiring filtration down to 5 or even 2 microns.
We have engineered the Hydrapulse to fulfill as many applications as possible as a non-custom solution. However, we understand that every project is unique and may call for something a little different. E-mail or call us to discuss your project and how we may be able to design a custom solution for you.
Hydrapulse, Inc. works with OEM’s to create semi-custom solutions tailored to meet your needs. Whether it is a custom displacement pump, communication interface, or firmware configuration, the Hydrapulse team can offer a semi-custom solution to fit your specs. Contact us to discuss your OEM opportunity.