I see numerous hydraulic applications, and I love observing the difference in execution from one machine to the next. The level of quality and implementation varies vastly, with some simple integrated “powerpacks” with tank, endhead, motor and pump all in one small unit, or gigantic power and control systems running hundreds of horsepower and thousands of gallons of fluid.
Regardless of complexity, it seems the quality of design and fabrication vary much more than I should hope. I’ll be honest; there are some persons and companies who should not be engineering and building hydraulic systems. I feel there are three types of mistakes to be made during the design and fabrication of a hydraulic system, which are as follows:
Application mistakes: These mistakes are usually related to production selection, such as incorrectly choosing one product where a more appropriate choice existed. A usually harmless example is in choosing a pilot-operated check valve over a counterbalance valve. PO Checks work most of the time, but can sometimes chatter. Counterbalance valves may cost marginally more, but are smoother in operation.
Some misapplications are more serious, such as choosing a closed centre valve to be partnered with a fixed displacement pump. The fixed pump has to flow somewhere when the valve is closed, and that somewhere will be over the relief valve, blatantly wasting energy. Don’t laugh, I have actually seen more than one system like that. Okay, you can laugh.
Implementation mistakes: These mistakes are normally made at the assembly and fabrication stage, where technicians have the opportunity to leave their legacy of cluelessness at every turn of the wrench or weld of a port. A less common error is the non-termination of return lines below oil level in the hydraulic reservoir. Although considered part of power-unit fabrication kindergarten, I have seen return lines plumbed directly to the top of a reservoir, allowing aeration to foam up a reservoir as fluid passes from the return port to the fluid level.
The most common set of mistakes related to fabrication is the lack of consideration for servicing. When a technician puts a D05 assembly within inches of the electric prime mover to inadvertently prevent a hex wrench from squeezing between the gap to adjust the relief valve … well, he’s clearly sociopathic and doesn’t like to have friends.
Design mistakes: As a designer, I understand the solid foundation one should have in math, physics and applications to create efficient and effective systems. But also, as a designer, the cringe that convulses my spine is stronger when I have to witness some of the design oversights I’ve seen. There is a lot of simple math in hydraulics — mostly related to geometry. If you are a designer or engineer who has not memorized the equation to calculate the area of a circle, perhaps you should consider a career in the humanities.
I am extra annoyed when I see undersized plumbing, because someone didn’t take the time to run the pressure drop calculations. I saw a recent application with 20 ft of pressure and tank lines run to a directional valve operating two cylinders. Pump flow was 20 gpm and both P and T lines were ¾ in. tube with 0.095 in. wall. After subtracting wall thickness, we now have just over ½ in. ID of tube to flow 20 gpm. This much flow will create 160 psi of pressure drop just flowing through this undersized tubing. If we factor in flow intensification from the retracting cylinders, flow could rise to 40 gpm, further increasing backpressure; all because someone didn’t feel like turning on their calculator.
The moral here is if you’re not qualified or experienced in fluid power, leave it to the professionals rather than trying to wing it. You’re not doing anyone any favours.