Hydraulic oil spends most of its time in the reservoir, and as such, various tank design criteria provide benefits for the hydraulic system as a whole. When designing the optimum hydraulic reservoir, most of the considerations are in keeping the oil clean and cool. Reservoirs in industrial applications are spoiled by the extra space and cost allowed to them, but unfortunately, mobile reservoirs are given few ideal operating conditions.
Reservoirs are designed to achieve five primary functions, as well as a few secondary ones not discussed here. It seems self-evident reservoirs should be a vessel to house the hydraulic fluid, but this point is too elementary to include on this list. More accurately, the reservoir is the intermediary between exhaust and supply fluid to which the primary reservoir functions can take place.
The functions of a reservoir are to aid in cooling, allow de-aeration to take place, allow particles to settle out, provide ideal pump inlet conditions and as a mounting location for other components. A hydraulic circuit could function without anyone of these functions—however, not for long. The reservoir in a hydraulic system is critical for reliable operation, and when done right, allows the machine to operate continuously within its design parameters.
Squared off against the mobile hydraulic reservoir are various impediments preventing it from realizing its full potential. These impediments are reality for most mobile machines, and include price, size and weight. Sacrifices are made in the performance and reliability of mobile machinery because of these limitations, and they’re a result of the competitive nature of the industry and the regulations involved to reduce the size and weight of machines to meet fuel economy and emissions targets. These three limitations play a role in the design of mobile hydraulics, and I’ll discuss how each one affects the five primary functions of the reservoir.
Hydraulic reservoirs help cool hydraulic fluid in two ways; by radiating heat to the atmosphere via the tank walls, and by providing more time for this radiation to occur. The cooling effect of a hydraulic reservoir counts for a portion of the cooling in a hydraulic system, but its capacity to do so is limited the surface area of the reservoir and the difference in temperature between the fluid and the air. A 50-gal tank will have around 30 sq-ft of cooling surface, although 1/6 to 1/5 of that surface might be “air-to-air” cooling, by virtue that reservoirs are only filled to 80% capacity.
It makes sense that the larger difference in temperature between the ambient air and the hydraulic fluid, the more quickly the oil will cool. The difference in this effect is vast, and with our 50 gal example, cooling can vary from 1 to 3 hp worth of heat from 30 to 100° F differential, respectively. Two of our three limiting factors come into play with a mobile reservoir, and it’s simply impossible to install massive tanks on dump trucks, plows and stone-slingers, for example. Any additional component weight on these machines reduces payload and increase both fuel consumption and emissions.
The ideally sized hydraulic reservoir is at least three times pump flow. For example, if our pump provides 25 gpm, the tank should be at least 75 gal. The reality, is you’re more likely to see a 30 or 40 gal tank on a mobile machine, especially a truck-mounted one. Machines are often packed with components and gear, pushing the limits of GVWR as well as the real-estate to mount these components. A properly sized reservoir is just not an option in most cases, and this is a detriment to cooling. Our third limited factor, money, is often trumped by cooling requirement, and any money saved on a large reservoir is offset by the necessity of a cooler. However, at least a cooler is smaller and lighter than the equivalent volume and mass added by a double- or triple-sized tank.
A smaller reservoir reduces the dwell time of the fluid, which not only reduces the time available for cooling, but the time available for particles to settle out of the hydraulic fluid. Although every machine has a filter (or at least should), economics dictate filters are mobile machinery aren’t always of the highest quality. This means the general volume of particle contamination can be higher in mobile machinery. One of the functions of a reservoir is to provide time for fluid to settle before it is again sucked out to do work. If little time is available, as a result of a poor tank-size to flow ratio, particles can simply pass back into the circuit again where they cause further wear and exacerbate the problem.
The problem of particles “rivering” right back into the pump can be partially avoided by installing the reservoir with a baffle plate, forcing fluid through longer and indirect paths to the suction port. This ensures any given mass of oil molecules spends more time within the reservoir, allowing particles to settle out. Baffles do add cost to the tank, of course, but should be considered mandatory, because dwell time improves cooling and aeration quality as well.
Aeration is the is the introduction of air bubbles within the hydraulic oil. If left unchecked, this can progress so far as to cause foaming of the fluid. In more mild conditions, the entrained air bubbles will behave just as cavitation bubbles, causing miniature thermal implosions on the pressure side of the pump, damaging it and reducing efficiency.
Aeration bubbles can be treated as a form of contamination, which must be removed before the tiny air pockets can reach the pump. A larger reservoir has more time for bubbles to rise to the surface, and once again, a baffle is your best defense against aerations (beside prevention, of course). Because air bubbles rise, and baffle cut-outs are typically on the bottom sides of the reservoir, aeration can be blocked from entering the suction side of the reservoir. Once again, cost is the limitation here, and although a baffle sounds like a no-brainer, I’ve seen plenty of tanks without them.
Pump inlet conditions should be a critical design consideration for any hydraulic system. Fluid entering the pump should be clean, cool and free from aeration. Each design consideration already discussed plays a part in optimum pump inlet conditions, and I should mention a baffle is a key strategy in reservoir construction. A baffle prevents the rivering effect of fluid returning to the tank and essentially flowing straight out the suction port to the pump before it can be cooled or given time for aeration and contamination to settle out.
Once again, the economy of mobile hydraulics play a part here, because a large reservoir with baffle would be ideal for pump inlet conditions. It seems common sense to include a baffle in a tank, but I’ve seen them without, and the result was the exact condition described above. In this example, there was an in-tank filter assembly that reached down 10-in. into a 24-in. tall reservoir, which had located directly below it, the suction port. In this example, aeration wasn’t a concern and the filter was high-quality, but the oil temperature raised to 190° F in about ten minutes of operation. Oil was flowing straight from the filter outlet right back into the pump inlet, and heat increased exponentially. The remedy was a baffle, filter relocation and a liquid-to-air cooler.
Hydraulic reservoirs make a great place to mount other components of the circuit, and if you’ve ever seen an industrial power unit, sometimes nearly every component with the exception of actuators are mounted on the reservoir. Often with mobile machinery, because of the premium on large areas of free space, components are mounted wherever they can be. The only items guaranteed to be on the tank are the level/temperature gauge and filler/breather cap.
The idea of mounting components in the most convenient spot is actually an advantage with mobile hydraulics. Valves, filters, pumps and accessories are in various locations on the machine, often tucked away in a safe place, or conversely mounted in such a way to provide easy access for controls or gauges. Although one could argue it requires more labour to mount components far and wide, at least you can get away with a smaller reservoir and the advantage of lower mass.
Economy of size, mass and money are all considerations when applying a mobile hydraulic reservoir. However, from a standpoint of design, it wouldn’t be prudent of me to recommend anything other than the largest and most well-designed reservoir for the job. The importance of fluid conditioning is too critical, and cooling, contamination/aeration removal, and ideal pump inlet fundamentals should be heeded. Although difficult, and aside from tank-mounted components, these factors will only help improve the efficiency, reliability and reputation of your mobile machine.