Hydraulic Power Unit Reservoir Sizing: Start With The Machine Cycle

A quotation may say “30 L tank” without saying what happens to the oil when the machine moves. At full extension, two cylinders may leave the pickup close to the low-level mark. On rapid retract, their cap ends can exhaust into the return line at a rate above pump delivery. Those states test different parts of the unit. If production cycles then become more frequent, a steady temperature rise is another reason to revisit the offer.

The starting documents are a circuit sketch, a motion time line and the proposed tank section. Mark where oil is stored at every relevant position, where the fastest return passes, and the lowest and highest running surfaces in the tank. The proposed pump must be supplied at the low point. The return route must handle the high-flow event without sending aerated oil straight to suction. The heat balance belongs to the sustained duty, even if the short motion tests look fine.

BLINCE offers a custom hydraulic power unit with a tank, motor, pump and valves for cylinder or motor applications. Its public page does not give a reservoir drawing, pump curve, usable level range or thermal rating for the particular configuration in a buyer's quotation. The calculations here are a way to prepare that configuration review. They are not a BLINCE tank rating.

What does the capacity number measure?

Four volumes are easily confused. The shell's geometric volume is space inside the tank. Normal fill is the amount of oil at a stated fill mark and actuator position. Initial system fill includes oil that will occupy hoses and actuators after installation. The usable running range is the oil volume between the highest permitted and lowest permitted operating levels. These numbers can be quite different for one tank. Ask which one appears in the offer.

Imagine a nominal 18 L shell as a constructed example. The exterior may be rectangular, but a sloped base, internal wall or formed cover changes how litres correspond to millimetres on the sight gauge. A large region below the pickup might count in the shell volume while doing little for usable range. A high fill line could reduce air space needed when oil returns. For an irregular shape, ask for the maker's fill chart; outside width × length × height measures the envelope, not oil available to the circuit.

The low and high endpoints need different evidence. At the low point, compare the pump's actual inlet condition with its selected data, using the specified oil at the relevant cold or hot viscosity and the worst permitted attitude. At the high point, returned oil needs room without overflow or loss of required breathing space. A mark made while every cylinder is retracted may mislead an installer who fills the unit with a cylinder extended. Put actuator and accumulator states beside the level marks.

Long hoses can make the initial system fill much larger than the oil in the reservoir alone. Once charged, most of that line volume stays in the circuit. The level chart still has to account for moving cylinders, accumulator charging, elastic line expansion and any drain-back after shutdown. Label initial fill and running range separately; filling with a cylinder extended can leave too much oil in the shell after it retracts.

Draw the cycle before choosing a tank

The pump nameplate is not a time line. Draw a normal cycle and the fastest one the controls allow. Leave the holds and overlaps visible. A fixed pump circulating across a restriction during a hold belongs in the heat estimate, even though nothing moves. Mark an accumulator discharge or decompression event on the return trace. Bosch Rexroth's power-unit design paper includes actuator volumes and such return peaks in its power-unit specification guidance.

Two cylinders extending together may create the lowest level. If one retracts before the other extends, returning oil can offset the second withdrawal. That smaller swing is valid only if the circuit and controls enforce the sequence. “The operator normally uses them one at a time” is not a control interlock. Draw each permitted overlap; the most oil-consuming state governs the low-level calculation.

Follow the return lines on the schematic. One branch may join after the filter; another may bypass the cooler. A short decompression pulse could govern the tank entry while the long production stroke governs heat. Write a separate maximum next to each component rather than carrying the pump nameplate flow through the entire drawing.

Calculate cylinder oil withdrawal

Take bore D, rod diameter d and stroke S from the actual cylinder drawing. The following are ideal swept volumes for a double-acting cylinder; leakage, compressibility, hose expansion and other actuators are outside this first calculation.

For a constructed pair of cylinders, D = 80 mm, d = 45 mm and S = 500 mm. Divide cubic millimetres by 1,000,000 to express the results in litres:

The tank sees the difference between the two chamber volumes. The numbers belong to this constructed geometry, not to a BLINCE model.

Now suppose a proposed drawing shows 1.2 L between normal fill and the lowest approved running level at the worst permitted attitude. The two-cylinder result is already 1.590 L. That proposed range fails this constructed low-level check before adding an accumulator or other motion. The next design choice is not an extra 0.39 L scribbled onto the order: confirm the actual fill chart, then revise tank geometry, an approved fill range or the permitted motion sequence.

The same formula is not a shortcut for every actuator. A single-acting cylinder may return its full chamber oil under spring, gravity or load force. A telescopic cylinder has several stages. Accumulator oil volume depends on the pressure range and precharge. A motor normally circulates flow rather than storing a cylinder stroke volume, but its lines and case drain still affect filling and routing. Put each actual component on the machine time line before summing tank inventory.

The fastest return is a separate calculation

The 1.590 L level swing is a volume, not a return-flow rating. During retract, the large cap chamber empties while the smaller annulus fills. For the 80/45 mm example, their area ratio is 1.463:

That is one cylinder-port result. Other returning branches may join it.

At the filter, the flow could be different. The cap line might bypass it, join another branch ahead of it or pass a cooler first. Mark the junctions and identify the fastest permitted retract. Then use the selected element's flow, pressure-drop and bypass data at the relevant oil viscosity. “18 L/min pump” does not establish those return conditions.

At the tank, a fast return moves the oil surface and exchanges air through a breather on a vented unit. The high-level mark and breather location have to fit that movement. Air carried in the oil itself can have come from an inlet leak, turbulent return or decompression. Its route through the tank matters more than a volume divided by flow figure.

For example, 24 L of active oil at a steady 30 L/min gives 0.8 minute of nominal turnover. A return jet crossing straight to the pickup could spend much less time in the tank; the calculation does not give every bubble 48 seconds. Donaldson's reservoir discussion explains the role of dwell and engineered deaeration. The section drawing and actual return event decide what that principle means for this unit.

Read the section drawing from return to suction

Top-view spacing between two fittings is a weak measure of separation. Under the cover, a return tube can end beside the pickup. A baffle can force oil through an opening close to the suction port. At the low end of the running range, an outlet that was submerged at normal fill may break the surface. The section drawing needs both oil-level extremes and the positions of the actuators that produce them.

Pump-inlet acceptance comes from the selected pump's data at the actual viscosity, speed and attitude. The suction line beyond the tank fitting therefore belongs on the same sketch. Parker's tank-optimisation brochure shows how tank internals and filtration are reviewed together, including physical validation. The BLINCE reservoir-baffle guide is useful if the divider geometry is the open question.

Take three drawings with the same nominal tank volume. The first places entry and pickup close together. The second divides them, but all flow crosses one narrow opening. The third uses a diffuser and a separate suction region. The useful question changes across the drawings: return-to-suction travel in the first, passage loss at cold peak return in the second, and diffuser submergence and performance in the third. A volume figure alone ranks none of them.

Noisy suction deserves a reading at the pump. Cold-only noise directs attention to viscosity, line loss and strainer condition. Noise on a permitted slope or with cylinders out makes the low-level surface and pickup position more relevant. Parker's Oildyne power-unit manual gives separate total and usable tank figures and describes low level, overflow after a fill at the wrong cylinder position, and dry running as different problems. Its model numbers are examples, not BLINCE limits.

Calculate heat from the duty, not from tank litres

A tank's oil mass stores heat; its exposed surface and any cooler reject heat. Here is an illustrative loss at one component, separate from any BLINCE build:

That 0.467 kW lasts as long as the assumed flow and drop last. Add other restrictions, leakage and drive losses for a machine-level estimate. More oil changes the warm-up time; continued loss still requires a heat-rejection route.

The duty sheet can carry both the oil-level states and these heat inputs:

For an existing unit, record a temperature trace at one named point alongside ambient, load and cycle rate. A faster cycle changes the time spent losing power, even if the hardware nameplates stay the same. For a new build, test the estimate on the assembled machine at the agreed duty. The BLINCE oil-cooler sizing guide covers cooler selection once the loss and cooling conditions are known.

The shell's surroundings matter. An exposed tank and one enclosed under a hot machine deck will not reject heat identically. At cold start, viscosity may instead govern suction and return restriction. Place a proposed heater relative to the lowest oil level, and list both cold-start and hottest-running conditions. If air is visible in the return oil, note that observation beside the temperature trace from the same machine state.

Fit the package around the machine

The preferred level range may point to a taller tank, while the only available space is beneath a low guard. Increasing width instead may place return and suction fittings close together. A larger exposed shell costs space and initial oil; an engineered compact unit needs more evidence about its diffuser, cooling and inlet. Put the physical envelope beside the oil-level and flow-path drawings. This turns “tank size” into a package decision.

Pump position changes the route to its inlet. On a retrofit, the existing hose height and bore remain relevant even when the tank shell is replaced. Compare the proposed line and pump location with the pump maker's inlet limits at cold and hot operating states.

The assembled drawing should show how the filter is removed, how the tank is filled and drained, and where cooling connections pass. An external return element also needs line support and a cold-oil pressure-drop check. Trace the fastest return through any filter and cooler in its actual order. Donaldson's filtration overview discusses flow variation and the difference between return-line and in-tank arrangements; the chosen component rating must come from its own data.

On a vented unit, level movement exchanges air through the breather. Mark any dust, spray or washdown exposure where that breather will sit. Record the permitted slope and vibration. A sealed or pressurized design needs its own system basis and protection, beyond a change to the filler cap. Check the machine guard against the removal path for the filter and access to the drain.

Investigate foam before ordering a bigger shell

Consider a constructed retrofit: the machine's cycle rate increases, but the pump and tank nameplates do not change. Foam appears. “Double the tank volume” is one proposed response, yet the observations needed to judge it are still missing. Does foam follow a fast retract, or develop during prolonged hot running? How low is the oil when both cylinders are out? Is the return outlet exposed at that level? Has the suction line or breather changed? Each answer leads to a different drawing or measurement.

If the running level drops below the selected pump's approved minimum during combined extension, compare the cylinder balance with the permitted sequence and the tank's level chart. The highest level after return also matters; simply moving the fill mark upward may cause overflow. If level remains in range yet the return jet reaches the pickup, inspect the entry and divider. Extra volume without a different internal route may not address the short circuit. If oil heats steadily with neither a low-level event nor visible aeration, revisit the losses created by the faster duty and the available heat rejection.

A report can stay narrow: “Foam after the second rapid retract at 45 °C oil temperature; level above the marked minimum; pump inlet pressure not measured.” This is an illustrative reporting line, not a BLINCE field result. It preserves the observation and the missing measurement without declaring the tank at fault. Where the machine lifts or holds a suspended load, use its approved isolation and safe-test procedure. Reservoir sizing cannot validate the load-holding circuit.

Put two reservoir offers on the same basis

“30 L” on two quotations does not necessarily describe the same thing. It could be shell volume, oil at the fill mark, or a nominal model designation. The comparison starts with a drawing, not the price column. On that drawing, mark the lowest oil surface while the actuators are in the state that removes the most oil from the tank. Then mark the highest surface after oil returns. The available distance between those marks is what the circuit uses. Neither supplier can settle it by repeating the nominal capacity.

Use one duty sheet for both offers. Include the machine's fastest permitted retract and any overlapping motion; a return component may see a larger flow than the pump's delivery. Ask for pump performance at operating pressure and speed, rather than a catalog maximum such as “up to 40 L/min.” The return-to-pickup route in a compact shell may decide its suitability. If that route is poor, extra shell volume alone may leave recently returned oil near the inlet. The BLINCE custom-unit listing identifies the type of equipment available; it does not publish a pump curve or a reservoir section for the build being compared.

What accounts for a 20 L gap between bids on the same machine? Check the motion assumptions first. If the controls allow both cylinders out at once, the tank sees both rod-displacement volumes before either returns. A quote based on alternate motion answers a different duty. One supplier may also have designed a return diffuser that the other has not. Put those differences on the quotation record and compare them with the actual controls and piping. The familiar “three times pump flow” figure can reserve space early; it contains no cylinder-volume, return-pulse or heat calculation.

What to send BLINCE about the tank

An initial packet can fit on a circuit sketch and a cycle chart. Give concurrent movements overlapping bars on the chart; mark an interlock at the control that enforces it. Add pump behavior during a hold and the unit's installation envelope. For a retrofit, include the old unit drawing or marked photographs. Missing measurements can be named in the supplier's first reply.

For an engineering review, use the following fields. A blank is more honest than a number copied from a similar machine.

A single tank photo cannot reveal an internal return tube. A pump nameplate does not give the flow into the tank at the fastest cylinder retract. If a fault motivated the inquiry, describe it by state: “foam appears on the second fast retract with oil at 45 °C” is a useful observation, while “tank too small” is already a diagnosis. Measurements should be labelled with their locations. A pressure measured ahead of a valve cannot answer a question about pressure after a return cooler.

Ask the supplier to return the chosen configuration and drawing, including gross and normal-fill volumes, permitted running levels, the pump inlet basis, ratings for the actual return path and the internal route from entry to pickup. Separate calculated values, catalog ratings and machine-test results. The BLINCE custom hydraulic power-unit page is the commercial destination for that configuration request. The request should travel with the machine cycle, not with an isolated desired tank size.

The two-cylinder power-unit guide addresses combined flow and motion order when two cylinders share a unit. Use its sequence result in the tank review. The AC versus DC pump guide helps with supply and drive duty, a separate part of the package choice.

Agree on the test before building the tank

The proposed drawing should name the states that will be tested. Otherwise a supplier may run an unloaded stroke at room temperature while the machine's hard condition is a hot, rapid retract followed by continuous cycling. The acceptance limits must come from the selected pump, fluid, filter, cooler and machine specification. A generic oil-level or temperature number cannot be pasted onto an unselected build.

Keep the measurement point in the record. “140 bar” at an upstream valve port is not a return-pressure reading. “60 °C oil” at the tank wall cannot be compared directly with a hot-line sensor. A no-load stroke gives no temperature trend or peak-return pressure under production duty. Use the machine's approved isolation and test procedure for gauges and pressurized lines; suspended loads require their separate controls.

Write the failed operating state beside the reading, then choose the next check from that state:

After a circuit or tank change, use the same machine state and instrument locations for the comparison run.

Questions that remain after the first calculation

Can a flow multiple set the reservoir capacity?

For an early layout drawing, a flow multiple gives a space allowance. For the purchase order, use the cycle. Cylinder oil withdrawal governs one level extreme, a return surge can govern the filter and entry, and ongoing power loss governs the thermal duty. The volume allowance survives only if the selected tank layout passes those separate checks.

Would a larger tank solve pump noise?

Only if the new geometry and level address the cause of poor inlet supply. Cold-start noise may involve viscosity and suction restriction; noise on a permitted slope points to pickup coverage as another possibility. Take the inlet reading while the noise occurs and compare it with the selected pump's limit. Retaining a high, narrow suction hose during a tank replacement leaves its restriction in the circuit.

Why might the return filter need more flow capacity than the pump delivers?

A retracting double-acting cylinder exhausts through its larger cap area while receiving oil through the smaller rod-side area. For the 80 mm bore and 45 mm rod example, the ideal area ratio is 1.463. At 18 L/min into the rod port, the calculated cap-port exhaust is about 26.3 L/min. The filter sees that flow only if the selected valve and return piping route it through the filter; identify the branch on the schematic before specifying the element.

How should a compact reservoir's diffuser be checked?

A diffuser changes how the return jet enters the oil. Its position and submergence therefore belong on a section drawing at both level extremes. Check air separation and the pump inlet under the intended duty. The diffuser cannot recover oil held in an extended cylinder or remove the circuit's heat.

Which volume goes on the purchase order?

Attach the approved configuration drawing and identify gross volume, normal fill and allowed operating levels as separate figures. State the actuator positions that produce the extremes. The initial fill quantity for the whole circuit is another entry. On a replacement, the same nominal litres can conceal a different pickup height or return entry.

A larger cylinder on the existing unit: what must be recalculated?

Recompute cap volume, rod volume and the sequence. The new cylinder may produce a lower level during extension and a larger return flow on retract. Check the existing tank, pump, drive, valve, return components and cooling duty against those states. A matching pressure rating alone does not approve the change; load-holding and failure behavior also need their own circuit review.

The useful document at the end of the inquiry is a marked machine cycle beside the proposed reservoir section drawing. The tank's level range, return path, pump inlet and heat duty can then be judged for the same build.

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Disclaimer

This article is a general engineering guide. Final component selection should be based on machine drawings, measured hydraulic data, working conditions, safety requirements, and confirmation from a qualified hydraulic engineer or supplier.

Blince Hydraulic Team

Blince Hydraulic is an industry-leading company dedicated to precision-engineered fluid power manufacturing and custom hydraulic solutions. Backed by decades of deep field expertise in industrial machinery and thousands of successful global deployments, our engineering team focuses entirely on high-performance hydraulic component manufacturing, including specialized orbital motors, high-pressure travel drives motor, and robust directional control valves. Our production infrastructure utilizes state-of-the-art multi-axis CNC machining systems and is fully ISO 9001 certified to guarantee repeatable volumetric accuracy across every single manufacturing run.

We deliver fast, highly dependable, and cost-efficient hydraulic solutions to heavy industry distributors, machinery OEMs, and maintenance crews across more than 150 countries. Whether your active project calls for a small-volume batch of customized shaft profiles or a large-scale production run of severe-duty cast iron gear pump, we configure our flexible production schedules to meet your target lead times with total pricing predictability. Partnering with Blince means securing maximum system efficiency, elite material quality, and uncompromised fluid power professionalism.

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