reservoirs for mobile equipment often use a dipstick to check fluid level because sight gages, though preferred, might be inaccessible or subject to damage.Mobile hydraulic reservoirs are expected to perform the same functions as their industrial counterparts — but usually under more adverse and less predictable operating conditions. Machine motion (which makes complex baffling systems necessary to prevent fluid sloshing) and extreme ambient temperatures are just two examples of the special problems designers of hydraulic systems for mobile equipment face.

Size and weight limitations may require mobile equipment to operate with reservoirs as small as the volume a pump discharges in a minute. This is roughly a third the size of a reservoir traditionally used in an industrial application. The space and shape limitations mobile equipment places on reservoirs requires that they often be custom designed. Cost, size, and weight must be minimized, while still maintaining adequate performance and efficiency.

Internal or external filters?

Return filters are often placed inside the tank to save space and to provide integral diffusion. One advantage of in-tank return filtration is that filling the tank through the filter helps ensure system cleanliness. However, be sure contaminants cannot fall into the reservoir when a return filter element is changed. Placing filters within the tank provides a neat design but may promote contaminating an area that is difficult to keep clean. While more difficult to plumb, external return filters keep contamination outside the tank, and they are more easily accessible for servicing.

Magnets should be placed in the reservoir to trap ferrous particles. Dams and suction strainers also can be added to increase the effectiveness of the reservoir as a contaminant controller. Particle dams, placed between the return and suction areas of the tank, help contain heavier particles that may have bypassed the return filters. Dams commonly consist of an angle plate that extends across the floor of the tank. The dam should be high enough to contain particles until the reservoir is routinely cleaned but low enough to prevent fluid from having to cascade over it. Dams also provide ideal mounting surfaces for magnets.

Locating a pump at or above fluid level and far away from the tank (more the rule than the exception with mobile equipment) usually prohibits the use of pump inlet filters. Suction strainers or filters should be considered as a form of last-chance pump protection when positive pump inlet conditions can be provided - as with a charge pump or pressurized reservoir. Pay attention to fluid temperature (especially during startup) when sizing suction filters if equipment will operate in cold climates and pumps cannot be disengaged during startup.

Vented or pressurized reservoir?

An important design consideration is whether to specify a vented or pressurized reservoir. The major deciding factors are the location and inlet requirements of the pumps. The fluid level of the reservoir in many mobile applications is below the pump inlet. At best, if there is vacuum at the pump inlet, the pump may have to be derated. If inlet line losses are great enough, cavitation will occur. In these cases, pressurizing the reservoir will help maintain pump performance.

Any of three methods can be used to pressurize a reservoir on most mobile equipment:

1. Use regulated compressed air from a machine's pneumatic system — the most effective method — if available.

2. Trap the air within the reservoir clearance volume (above the fluid) and depend on thermal expansion of the fluid to compress this air, and thus pressurize the reservoir. A reservoir pressure cap holds pressure within the tank and relieves excess pressure.

3. Tap pressurized air from the scavenge pump of a two-cycle diesel engine.

With pressurized reservoirs, consideration must be given to calculate stresses on reservoir walls, because even low pressures can exert substantial loads. For example, an internal pressure of only 3 psi applies a force of 1,800 lb on a 20- by 30-in. wall. This force, combined with weight of hydraulic fluid, plus G forces involved in mobile equipment, can produce stresses high enough to actually work harden a metal reservoir. Work hardening makes the metal more brittle, which eventually will cause leakage when the metal is exposed to continued stress.

Wall stresses should also be calculated for vented reservoirs. High stresses develop quickly in large areas of flat plate. And again, weight of the fluid can cause large deflections. Furthermore, mounting peripheral equipment, such as ladders, to a reservoir increases the need to specify stiffening members and thicker plate.

Cleaning and maintenance

Reservoir servicing must also be taken into account. There must be provisions to drain both return and suction areas of the tank, especially if a dam is installed to separate them. Pipe couplings often are used, but SAE O-ring ports provide better sealing. Valving should also be provided to close off inlet lines when replacing pumps or other components that are mounted below fluid level.

This is often wishful thinking, but access should be provided for cleaning and maintaining the interior of the tank. Ideally, hatches should be large enough to provide enough room for service personnel to maneuver cleaning tools. There should also be means for lighting each portion of the tank for inspection.

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* The industry standard for hydraulic reservoirs is contained in ANSI/(NFPA) T3.16.2 R1-1997 (R2005) Hydraulic fluid power – Design for Nonintegral Industrial Reservoirs, which is available from the National Fluid Power Assn. Click here for more information, or contact NFPA, Milwaukee, at (414)778-3344, or e-mail