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Technician checking hydraulic hoses and test equipment on an excavator attachment circuit

T01 · Hydraulic setup · measured data

How to measure excavator auxiliary hydraulic flow for an attachment

The short answer

Use a calibrated flow meter and load valve at normal operating temperature, test the intended auxiliary circuit in both required directions, and record flow at the attachment manufacturer’s working pressure. The machine’s advertised pump output is not the number the attachment receives.

Procedure

Complete the work in this order

  1. 01

    Collect the machine data

    Record model, serial range, operating configuration, coupler, auxiliary circuit type, published pump output and relief setting. Photograph the monitor settings and couplers. These details let the supplier distinguish a machine limitation from a setting, plumbing or test problem.

  2. 02

    Warm the hydraulic system

    Bring the excavator to its normal working temperature using a safe operating cycle. Cold oil can make a marginal circuit appear stronger than it is. A forestry mulcher or grinder imposes sustained load, so a cold five-minute reading is not representative.

  3. 03

    Connect a rated flow meter

    Use hoses, fittings and a meter rated above the machine’s maximum pressure and flow. Connect in the same path the attachment will use, including the intended quick couplers. Do not improvise a restriction or open a pressurised line to estimate flow.

  4. 04

    Load the circuit progressively

    Run the engine at the operating speed intended for the attachment, then increase the load valve gradually. Record litres per minute at several pressures, particularly the attachment’s nominated working pressure. Stop if oil temperature, hose movement or noise becomes abnormal.

  5. 05

    Check return and repeatability

    Measure return pressure where the attachment specifies a limit and repeat the test after sustained operation. Test reverse direction for double-acting functions. Save the result with date, oil temperature, engine speed, monitor setting and the person who performed the test.

Why published pump flow is not usable attachment flow

An excavator specification may state total pump output, combined pump flow or a maximum auxiliary setting. Steering, travel, boom functions, control valves, hose diameter and relief behaviour all sit between that number and the attachment motor. The useful figure is the stable flow available at the pressure the tool needs while the machine is hot.

A flow-only reading is incomplete. A circuit delivering a high number at almost no resistance may collapse as the load valve approaches working pressure. Record a flow-and-pressure curve or at least several paired readings. That evidence lets an attachment supplier select a motor or model without treating the carrier class as a substitute for hydraulic data.

Interpret the result conservatively

Do not order a tool whose minimum requirement equals the best momentary result. Allow for hot oil, filter condition, normal component wear and other functions the operator uses during a cycle. Continuous tools also need cooling capacity; stable flow for ten minutes does not prove the carrier can reject heat across a full summer shift.

If the result is low, check monitor limits, circuit mode, coupler restriction, hose sizing, filters and relief settings with a competent technician. Increasing pressure or flow without confirming component ratings can damage the excavator. The goal is a verified operating envelope, not the largest number the system can produce briefly.

Common mistakes

Using total pump output

That figure may combine pumps and does not describe the attachment port under load.

Testing cold oil

The result can overstate sustained performance and hide a cooling problem.

Ignoring return pressure

A restricted return creates heat, poor motor performance and seal failures.

Changing relief settings casually

More pressure is not free capacity and may exceed the rating of hoses, valves or the attachment.

Continue the job

Hydraulic flow calculator

Compare measured results with every attachment category.

Carrier compatibility

Combine hydraulics with weight and lift capacity.

Identify the circuit

Confirm what the plumbing can actually control.

More field procedures

Keep building the machine record

Technician checking hydraulic hoses and test equipment on an excavator attachment circuit
T01

Measure auxiliary flow

Use a calibrated flow meter and load valve at normal operating temperature, test the intended auxiliary circuit in both required directions, and record flow at the attachment manufacturer’s working pressure. The machine’s advertised pump output is not the number the attachment receives.

Open tutorial →
Excavator auxiliary hoses, couplers and hydraulic service points being inspected
T02

Identify the hydraulic circuit

Trace the hoses and controls rather than counting couplers. Confirm whether flow is one-way or reversible, whether a low-pressure case drain returns independently to tank, and whether two attachment functions can operate at the same time. Then verify the result against the machine manual and a functional test.

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Forestry excavator holding a cut tree section with an attachment at working radius
T03

Size an attachment to a carrier

Treat published carrier tonnes as an initial filter. The final match requires attachment mass with bracket and rotator, excavator lift capacity at the actual radius and sector, usable hydraulic flow and pressure, transport configuration, guarding, and the maximum material the head will retain.

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Forestry attachments and excavators prepared in a machinery yard for installation and commissioning
T04

Install and commission an attachment

Commission the ordered attachment as one installed system. Verify identity and mass, mechanical fit, hose routing, hydraulic settings, control logic, guarding and transport before loading it on representative material. Record baseline pressures, temperatures, cycle times and operator limits for future troubleshooting.

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Excavator-mounted stump grinder with hydraulic supply, return and case-drain hoses visible
T05

Set up a case drain

Route the motor’s case-drain port through a dedicated, correctly sized, low-restriction line to the approved tank connection. Confirm the attachment manufacturer’s maximum drain pressure and measure it at working temperature. Never cap the port or tee it casually into a pressurised return.

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Forestry attachment wear parts, tools and components arranged on a workshop bench for inspection
T06

Inspect a used attachment

Identify the exact model and serial first, then inspect structure, pins, bearings, wear system, hydraulics, controls and supplied fitment as separate cost lines. Test the head hot on representative material and price every missing bracket, hose, controller and overdue wear item before comparing it with a complete new quotation.

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Forestry machinery fleet in an Australian equipment yard representing ownership and tender costs
T07

Calculate a tender rate

Choose the unit the client accepts, estimate completed units per productive hour from comparable work, calculate the full hourly cost of carrier, attachment and crew, then add mobilisation, wear variability, downtime, overhead, risk and margin explicitly. Do not turn a demonstration cycle into an all-in hectare or stump rate.

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Forestry attachment wear parts, hand tools and service items organised on a workshop bench
T08

Build a wear-parts kit

Stock parts by failure consequence, consumption rate and replenishment lead time—not by what is cheapest. Carry enough teeth, blades, chains, hoses, couplers, fasteners and service tools to protect the next work period, with every item matched to the exact attachment serial range and a reorder point recorded.

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