So you’ve decided that your system needs to include a hydraulic circuit because electrical or mechanical power transmission just wouldn’t work for reasons of cost, force requirements, or otherwise. You understand the work that needs to be done by the actuators, but now need to specify components. This can be a series of painful and overwhelming decisions if you are not equipped to recognize the appropriate details.
A key component decision is the pump itself. Every hydraulic pump takes fluid from one side and moves it to the opposite side, but that’s about the only descriptor that applies universally. It’s up to you to decide countless other options that require an intimate understanding of the circuit. This blog is intended to outline the key questions that should be asked for every pump selection. Asking these questions should result in a productive and accurate approach to a new system for your company’s newest project.
Open Loop or Closed Loop?
In an open loop, the pump will draw fluid directly from a reservoir and provide flow to any number of actuators. The flow may or may not pass through valving to throttle it, direct it, or lower the pressure of it before being sent to the actuator(s). After flow passes through the actuator, it will ultimately return back to the tank.
In a closed loop, the pump draws from the flow that is coming back from the actuator that would be going back to the tank if it were an open loop. There is a small “charge” pump in addition to the main pump that draws oil directly from a reservoir. It then pumps oil into the main loop to keep it full and to force some oil back out through flushing valve(s) and pump/motor cases to be cooled, filtered, and recirculated. Closed loop pumps are generally more complicated internally because they need to tolerate pressure on both sides, A and B. There is no dedicated inlet or outlet.
Closed loops are generally used for propel or mixer type applications where the direction of motion is directly controlled by the pump rather than a valve. The advantages include smooth actuation and the added benefit of hydrostatic braking (the downstream pressure buildup that inherently slows down the actuator(s) when the pump de-strokes). The closed loop also requires fewer components, which can result in lower cost and machine weight.
A Schematic Representation of a Closed Circuit Pump (Poclain Hydraulics)
Fixed Pump or Variable Pump
A fixed pump has a set amount of fluid that will be transferred during each revolution. There is no coming on or off stroke. Gear, vane, and piston pumps can all be fixed displacement. These pumps are simpler in design and of lower cost than their variable cousins. Unless your application requires it, don’t overcomplicate the design by using a variable pump where a fixed pump will do.
Examples of Fixed Displacement Pumps (Wikipedia)
A variable pump has an adjustable amount of flow that can be transferred during each revolution. It can fluctuate between zero or maximum flow, sometimes in both directions. The type of pump displacement control will decide how this fluctuation is managed. Piston pumps are the main candidates to be variable because the swashplate angle can be changed smoothly and infinitely. Some vane pumps are also variable.
EATON Open Circuit Piston Pump (EATON 420)
What Type of Pump Control Do You Need?
If your application requires a variable pump, then you must decide what will control that varying displacement. Open loop variable pumps can be pressure compensated, load sensing, or mechanically controlled by a linkage. Closed loop pumps can be controlled by a hydraulic pilot signal, a proportional electrical signal, or a mechanical linkage. Either open or closed loop pumps can have feedback devices installed to communicate actual displacement to a PLC or similar device.
Bolt-on Control Modules for Variable Piston Pumps (EATON Duraforce HPV)
There are other features such as cold start valves, bypass valves, and FNR (forward, neutral, reverse) control that are also available based on your application.
How Much Pressure Does the Pump Need to Tolerate?
Pumps are generally separated into medium and high-pressure categories. You will need to know the pressure that will build as a result of the work you intend to do before selecting a pump to supply the flow. You will also need a relief valve in place to prevent accidental over pressurization. Knowing the pressure will also enable you to select the factory settings for pressure compensators, built-in relief valves, pressure overrides, power limiters, etc.
What Mounting Details Do You Need?
The pump itself with have a mounting flange, pilot feature, and shaft that will need to match the prime mover connection. The pump may also need to have an auxiliary mounting provision to accept a second pump that is driven through it. The mounting orientation will also affect decisions about port size and location on the pump.
Example Shaft Detail Illustrations (Poclain Hydraulics)
How Much Flow Do You Need?
Perhaps the most basic of pump questions is the flow requirement. Actuator size, speed, and number will play into this decision. If you undersize a pump and do not have carefully planned priority flow provisions for critical functions, then major safety issues can arise. Imagine trying to steer a tractor away from an obstacle only to find that your steering function did not have adequate flow due to a saturated pump.
Take Your Time and Do Your Homework
All of these considerations (and in some cases more) need to be made in order to properly specify a hydraulic pump. Don’t rush this process. It could result in paying too much for a component or making a mistake and losing credibility.
Those assembling the system won’t appreciate being asked to plumb a charge pump, load sense lines, pilot lines, or that remote filter when all they really needed was a gear pump with an inlet and an outlet. The purchasing department will also be happy with the minimized cost. Spare yourself the difficulty and specify the correct pump the first time by asking these questions. Our experienced team of professionals in the Mobile Hydraulics and Controls group is happy to help with any step of the design process for your next project.