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Rethinking the Hydraulic Reservoir

Any engine or equipment manufacturer will agree: space is a valuable and limited resource.

This is why when designing hydraulic systems, design engineers often consider the fluid reservoir an afterthought, and somewhat rightfully so: it’s seen as a passive vessel that holds the excess hydraulic oil when not needed by the system. As such, the hydraulic reservoir is frequently placed wherever it will fit, in the final stages of vehicle or system design.

This thinking is understandable, at least to some degree. The hydraulic oil reservoir should not get in the way of an efficient system. It must be large enough to accommodate changing fluid levels and flow rate, and if that means that it’s placed somewhere less than convenient for the design, that’s often viewed as an acceptable compromise.  But one could make the argument that there is value in making every component — even those often seen as afterthoughts — as efficient as possible.

So we asked the question, What if there was a better way?

A custom-fit reservoir solution can be smaller and more form-flexible than conventional reservoir designs.

What if the hydraulic reservoir could be a net bonus for the entire hydraulic system and make the lives of the engineering team easier? The old “If it ain’t broke, don’t fix it,” adage is typically applied here; design engineers have other places under the hood that deserve attention. But a ready-made hydraulics reservoir solution that is more efficient and adds performance and reliability is something that every design team would jump at. By creating a reservoir that is custom fit for the system and built from components made to work together, it’s possible to make the whole system more efficient while making the most of the given space. This idea reduces the headache that comes with fitting the vessel and its varied components into that space.

Large for a reason

Many reservoirs are on the larger side because reservoir capacity is typically predicated by the dwell time of the returning fluid. This means that if the system flows 100 gallons per minute, a reservoir with a two-to-one flow ratio would need to hold 50 gallons. Every gallon that goes back into the reservoir will sit for at least one minute before it returns to the system, so that entrained air has time to rise out of the fluid.

Integrated reservoir solutions can help enhance total hydraulic system performance.

These numbers are mere examples, as opposed to specific cases. The point is that dwell time dictates reservoir size because system designers need to allow time for the air to dissipate. In many ways, entrained air — free air that leaked into the system typically at the pump inlet or via worn cylinder seals, that mixes with the oil as it becomes pressurized — can be as harmful to the system as contaminants like dirt and water. The last thing you want is air returning to the system where it can cause vibration, excessive noise, unnecessary wear and tear, increased fluid oxidation, and diminished precision when using controls.

Being better at what it does

If we look at how the reservoir does its job, we can make it more efficient at that job. Traditionally, contaminant filtration is a separate process from deaeration. Contaminants are filtered out at the reservoir inlet and the air removes itself while the fluid rests in the vessel. But that natural deaeration process often forces the engineers’ hands when deciding how large to make the reservoir.

Say you want the reservoir to be half the size, using a six-to-one flow ratio. This means you could downsize to a 20-gallon reservoir for a 120-gallon-per-minute flow system. Traditionally, this is a challenge because if the vessel is smaller, you don't have as much time for air bubbles in the fluid to rise up to the top naturally. But if you use a multifunction hydraulic filter that strips out entrained air at the same time as it filters the contaminants, you don’t have to let the fluid rest as long, meaning that you can design smaller reservoirs.

Hydraulic Oil Deaeration %
Lower aeration percentage results in better hydraulic performance. Donaldson deaeration filters maintain aeration at 1.5% or lower. At higher levels of aeration, loss of efficiency and noise become an issue.*

When compared to traditional reservoirs, this smaller, more efficient reservoir offers multiple benefits to both the original equipment (OE) manufacturer and its end-users. For the OE, a smaller and more flexible-in-size reservoir can fit in more places within the system, maximizing design efficiency. It also uses less material, which lowers costs. And if the reservoir holds half as much fluid, that’s a lot less fluid needed in the entire system, further lowering production (or first fit) costs.

For equipment owners, less fluid held by a system means they will need to purchase less hydraulic oil over the life of the system. Equipment owners and operators often talk about how efficient deaeration of hydraulic fluid delivers a quieter machine, more precise control and less wear and tear, meaning longer equipment life and lower costs of ownership. These are all benefits that are directly traced back to the OE’s efficient and smart design.

Simpler. Smaller. Smarter.

Our hydraulic reservoirs are designed to optimize space and performance – up to 50% reduction in reservoir size. By engineering to the available space on the equipment and integrating deaeration filtration into the system, our reservoirs are optimally sized for your platform.

The technology is here

The good news for equipment manufacturers is that this hypothetical hydraulic reservoir system is no longer hypothetical. There’s no longer a need to create extra space for a bulky, inflexible cube-shaped reservoir, nor is there a need to source components simply to fit a reservoir into a space it’s not designed for.

So, is there a better way to build hydraulic fluid reservoirs? Yes. It’s possible to create not just smaller reservoirs but “right-sized” reservoirs, simply by rethinking the whole process. It’s now possible to take an equipment design and build a turnkey reservoir system specifically for that design. This gives engineers more control and removes the pressure to make room for a bulky, cumbersome reservoir, ensuring that that reservoir is an integral part of the system.

Imagine a hydraulic reservoir that comes fully realized and headache-free; ready to install into your space; all components included. And it adds layers of protection from both entrained air and other contaminants.

That’s rethinking the hydraulic reservoir.


*Test conducted in a lab setting, simulating a continuous air leak into the system for 30 consecutive minutes.
This article was original published in July 2020 at


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