How to Size Hydraulic Pumps for High-Flow Excavators

Start with the Attachment, Not the Machine

The bucket determines the pump flow requirement. A 1.0 m³ bucket with a sharp cutting edge demands more flow than a 1.0 m³ bucket with a rounded edge and heavy soil. The difference is not just in the label. It is in the time the blade spends in the cut versus the time it spends in the air.

Before touching any pump datasheet, define three numbers:

• Bucket working volume (m³)
• Target digging cycle time at the operating depth (seconds)
• Maximum system pressure the main relief valve is set at (bar)

These three figures drive every calculation that follows. If you skip them, you will end up with a pump that is either starved or flooded, which is worse than a mismatched pump.

Define the Flow Demand for Each Circuit

Excavator hydraulic systems are not a single loop. They are a bundle of parallel circuits feeding different functions. The main boom arm and stick circuits usually share the highest flow demand because they work together during the cut. Travel, swing, and auxiliary circuits draw less flow but have different pressure profiles.

A practical breakdown looks like this:

• Boom and stick circuits: 60 to 75% of total flow
• Travel circuit: 10 to 15%
• Swing circuit: 5 to 10%
• Auxiliary and idle: 5 to 10%

This split varies by machine design. Some machines use a separate travel pump. Others use a single variable pump with a pressure-compensated travel valve. Your sizing calculation must match the architecture you are specifying, not a generic textbook split.

Apply the Flow Multiplier Method

The multiplier method is the standard industry shortcut. It takes the theoretical flow from the bucket geometry and multiplies it by a factor that accounts for cutting resistance, soil type, and operator technique.

The formula is:

Required flow = Bucket volume x Cutting speed factor x Soil resistance factor

Typical values, expressed as multipliers:

• Light soil or loose material: 1.2 to 1.5
• Medium soil or normal digging: 1.5 to 2.0
• Heavy soil or rock breaking: 2.0 to 2.5
• Auxiliary work or grading: 1.0 to 1.3

For example, a 0.8 m³ bucket digging medium soil at a target cycle of 12 seconds needs a flow multiplier of roughly 1.8. The calculation gives you a baseline flow for the boom and stick circuits. You then add the other circuits and apply a safety margin.

Account for Pressure and Flow Together

Flow and pressure are not independent. A pump that delivers 100 L/min at low pressure will deliver less at high pressure. The pump curve is a trade-off curve. If you size for flow at the wrong pressure point, the pump will either be undersized or will run at an inefficient part of its curve.

Check the manufacturer’s pressure-flow chart for the candidate pumps. Find the flow at your relief valve setting, not at the rated low-pressure point. Most excavators run their main circuits at a relief pressure between 280 and 350 bar. If your pump curve drops more than 15% from the 100 bar point to the 300 bar point, you need to re-evaluate.

A common mistake is to select a pump based on its maximum flow at zero back pressure. That flow will never appear in the system.

Consider Duty Cycle and Heat Buildup

High-flow excavators run long shifts. The pump, valves, and hoses all generate heat. If the pump is sized for peak demand only, it will run at full open for most of the shift. That generates heat faster than the cooler can dissipate.

Duty cycle affects pump selection in two ways:

• Continuous flow at high pressure increases internal wear and heat
• Frequent throttling at partial flow increases valve spool wear and pressure ripple

A variable-displacement pump with load-sensing compensation is the standard choice for high-flow machines. It reduces flow when the valve is not demanding full capacity. A fixed-flow pump with a bypass valve is used on some machines, but it is less efficient for long-duration work.

Check the cooler capacity for the selected pump flow. If the cooler is sized for 80 L/min and you select a pump that delivers 120 L/min, the oil temperature will climb. Hot oil degrades seals and shortens pump life.

Select the Pump Configuration

The most common configurations for high-flow excavators are:

• Single variable-displacement pump with load-sensing valve
• Two fixed-flow pumps with a pressure-compensated travel valve
• Two variable-displacement pumps with independent control for boom and travel

Each has trade-offs. The single pump design is simpler and cheaper to maintain. The two-pump design gives better travel speed and isolates travel heat from digging heat. The dual-variable design offers the best efficiency but requires more complex valve control.

For a machine that spends most of its time digging, the single variable pump is usually sufficient. For a machine that moves large loads over distance between digging cycles, the two-pump design is the better choice.

Check Valve and Hose Capacity

The pump is only one part of the flow path. The main valve, the hoses, and the cylinder bores all have flow limits. A 120 L/min pump feeding a valve rated for 100 L/min will create pressure drop and heat at the valve. The hose size must match the peak flow to keep velocity under the recommended limit.

Typical hose size guidance:

Flow (L/min)	Minimum hose inner diameter (mm)
0 to 60	12
60 to 100	16
100 to 140	20
140 to 180	25
180 to 250	32

These are general minimums. Check the specific hose manufacturer’s data for burst pressure and flow rating at the expected length. Long hose runs increase friction loss. A 15 m hose run of 20 mm internal diameter at 140 L/min will have more pressure drop than a 3 m run of the same size.

Common Sizing Mistakes

These are the errors that show up in the field:

• Sizing for the largest bucket without checking the machine’s maximum cylinder stroke. A big bucket on a short-stroke boom cannot use all that flow.
• Ignoring the auxiliary circuit. A machine with a large hydraulic hammer or a long reach may need more flow than the digging calculation suggests.
• Selecting the pump based on the lowest pressure circuit. The main boom circuit runs at the highest pressure. Size for that.
• Forgetting the swing circuit. A heavy attachment on a slow swing circuit will make the operator dig slower, which changes the flow demand.
• Not checking the tank capacity. A 200 L tank with a 140 L/min pump will recirculate heat quickly. The tank size affects how well the system can cool between cycles.

Verify Before You Commission

After selecting the pump, valve, and cooler, run a pressure-flow test. This is not a guess. It is a measurement.

The test procedure:

1. Install the pump and valve assembly on the machine with the correct oil level.
2. Set the main relief valve to the design pressure.
3. Run the engine at 1800 rpm and hold it there.
4. Open the boom and stick valve to full open.
5. Record the flow at the pump outlet and at the valve inlet.
6. Record the oil temperature after 30 minutes of continuous operation.
7. Repeat the test with the travel valve open and the boom and stick idle.

The flow difference between the pump outlet and the valve inlet should be under 5% of the pump flow. If it is higher, the hose or valve is restricting flow. The oil temperature after 30 minutes should be below the manufacturer’s maximum operating limit. If it is higher, increase the cooler capacity or reduce the pump flow.

If the test passes, the pump is correctly sized. If it fails, go back to step two and adjust the relief valve or the pump selection.

The goal is a pump that delivers the required flow at the required pressure without overheating the system. That is the whole job.