Hydraulics and Carrier Matching

Understanding hydraulic power

For hydraulic forestry attachments:

Hydraulic Power (kW)
≈
Flow (L/min) × Pressure (bar)
÷
600

Worked example:

220 L/min × 300 bar ÷ 600
= 110 kW theoretical hydraulic power

Another:

350 L/min × 320 bar ÷ 600
= 186.7 kW theoretical hydraulic power

Actual usable attachment power is lower because of hydraulic losses — through hoses, couplers, valve blocks and return lines, and through heat.

Why a single flow figure is not a specification

Therefore:

"My excavator has 300 L/min."

is not enough information.

The buyer needs:

• continuous attachment flow
• pressure at that flow
• return pressure
• case drain requirement
• cooling capacity
• hydraulic hose size
• coupler restriction
• pump control
• permitted continuous duty

The distinction that matters most is peak versus continuous. Many carriers can briefly reach a headline flow figure. Far fewer can sustain it at working pressure, in Australian summer conditions, for a full shift, without the oil temperature climbing into the range where seal life and pump life begin to suffer.

A mulcher, in particular, is a continuous-duty attachment. It does not draw peak flow occasionally; it draws close to maximum flow continuously for hours. That is a fundamentally different demand from a bucket or a hammer.

Matching in practice

A practical matching sequence looks like this:

1. Establish the attachment's continuous flow and pressure requirement from the manufacturer.
2. Establish the carrier's continuous auxiliary flow at that pressure — not its peak.
3. Confirm cooling capacity at sustained full load at 35-40°C ambient.
4. Confirm hose and coupler sizing will not throttle the flow you just specified.
5. Confirm case drain provision where the attachment requires it.
6. Confirm the return line pressure is within the attachment's limit.
7. Confirm the carrier's permitted continuous duty for that circuit.

If any of the seven cannot be answered, the match is unproven — regardless of what the sales sheets say about tonnage class.

The lift chart is the other half

Hydraulic capability determines whether the attachment will work. The lift chart determines whether it can be used safely at the reach you need.

That remaining figure is what is available for timber. For grapple saw work in Australian hardwood, it is very often the binding constraint — not the saw's cutting diameter.

When a supplier says an attachment "fits 20 tonne excavators", the questions are:

• which exact models?
• at what boom configuration?
• with which coupler?
• what counterweight?
• has the lift chart actually been checked with the attachment fitted?

Why the check gets skipped

Carrier and attachment are specified in different languages. A carrier is described by operating weight, engine power and auxiliary circuit flow and pressure; an attachment is described by required hydraulic power, cutting or processing capacity and mass. Translating between them takes a calculation, and the calculation is easy to defer until after the purchase.

The consequence of deferring it is distinctive, and it is worth stating plainly because it is so often misdiagnosed. An attachment on an inadequate circuit does not simply work a bit more slowly. It works slowly and runs hot and consumes wear components faster, and the operator experiences all three as an attachment problem. Contractors replace teeth, bearings and sometimes the attachment itself before establishing that the carrier was never able to run it.

Four checks, not one

Hydraulic power is the first of four separate questions, and passing it does not answer the other three.

1. Power. Continuous flow multiplied by working pressure, less an allowance for losses, compared against the attachment's stated requirement. Use continuous flow and working pressure at that flow — not peak flow and not relief pressure, which the machine never delivers together.

2. Cooling. A circuit able to supply the flow may still overheat supplying it all day. Mulching, grinding and continuous saw work are sustained rather than intermittent duties, and they are the cases where cooling rather than power becomes the limit — worked through under cooling is a separate calculation below.

3. Plumbing. Return-line sizing, case-drain provision, coupler capacity and hose runs all affect what actually reaches the attachment. Undersized couplers and long hose runs take a larger share than most buyers expect, and a case drain that is absent or incorrectly connected can destroy a motor quickly.

4. The lift chart. Hydraulic adequacy says the attachment will run. It says nothing about whether the carrier can hold what the attachment cuts, at the reach and slew angle the work actually uses. This is a separate calculation, and on clearing and vegetation work it is usually the binding one.

Specify headroom deliberately

A combination that passes on paper with no margin will disappoint in the field, because the calculation is built on an efficiency assumption rather than a measurement.

Where a manufacturer publishes a flow range rather than a single figure, the bottom of the range is where the attachment runs and the upper half is where it performs. Where a requirement is quoted as a range, ask which duty each end refers to — intermittent figures are commonly lower than continuous ones, and sustained work spends most of its time at the top.

The cost of headroom is paid once at purchase; the cost of its absence is paid in wear and lost production for the life of the combination.

The questions to put to both suppliers

Ask the carrier supplier:

• Continuous auxiliary flow and the working pressure at that flow, for this exact configuration
• Cooling capacity for sustained auxiliary duty, and whether an additional package is available
• Return-line and case-drain provisions as fitted
• The lift chart, and confirmation of how it changes with a coupler and rotator fitted

Ask the attachment supplier:

• Required flow and pressure, and whether the figures are for intermittent or continuous duty
• Cooling requirement at the ambient temperatures you work in
• Coupler, hose and case-drain requirements
• Attachment mass including rotator and any adaptor

Get both sets in writing before the order. The answers are cheap beforehand and expensive to discover afterwards, and a supplier reluctant to provide them in writing has told you something useful.

Run the numbers in the hydraulic power calculator, and see attachments for the capability side of the same decision.

A worked match

The calculation is short. It is skipped because it requires figures suppliers do not always volunteer, not because it is difficult.

An attachment requires 45 kW at the motor for continuous duty. A candidate carrier publishes a continuous auxiliary flow of 160 L/min at a working pressure of 250 bar.

Hydraulic power before losses:

Power (kW) = flow (L/min) × pressure (bar) ÷ 600

           = 160 × 250 ÷ 600
           = 66.7 kW

That is the ceiling, and no installation delivers it. Applying a conservative allowance for losses in hoses, couplers, the rotator and the attachment motor:

At 80% system efficiency:  66.7 × 0.80 = 53.3 kW
At 70% system efficiency:  66.7 × 0.70 = 46.7 kW

Against a 45 kW requirement, the match passes at 80% efficiency with reasonable margin and barely passes at 70%. Since the efficiency figure is an assumption rather than a measurement — and long hose runs, undersized couplers and a filter approaching service all push it the wrong way — this combination has less headroom than the first calculation suggests.

Requirements differ by attachment class

Attachment classes sit in broadly different parts of the flow and pressure range, and knowing roughly where helps you rule combinations in and out before requesting detailed figures.

The pattern worth extracting: intermittent attachments are usually limited by the carrier's structure, and continuous attachments by its hydraulics and cooling. Which kind you are fitting tells you which set of questions to ask first.

Cooling is a separate calculation

Oil temperature is where an adequate-on-paper match most often fails in the field. Oil temperature rises with sustained work, viscosity falls, internal leakage increases, and the delivered power falls with it — so a machine that passes a power check in the morning can be materially down on capability by mid-afternoon in summer.

Three questions settle it:

• What cooling does the attachment manufacturer specify for continuous duty at your ambient temperatures?
• Does the carrier's standard configuration meet that, or is an additional cooling package available and required?
• What does the machine's oil temperature actually do over a full working shift in hot conditions? This is worth observing during a demonstration rather than assuming.

Plumbing: where a sound match quietly fails

Power and cooling can both pass while the installation still underperforms or damages components.

Hose sizing and length. Every metre and every restriction takes a share. Long runs to a boom-mounted attachment lose more than buyers expect.

Coupler flow capacity. Couplers are rated, and an undersized one throttles the circuit regardless of what the pump can deliver. This is a common and easily avoided fault.

Return-line sizing. Return restriction raises back-pressure, which the motor sees as lost power and heat.

Case drain. Many attachment motors require a dedicated low-pressure return. Absent or wrongly connected, it can destroy a motor quickly — and it is not always obvious from the coupler set that one is missing.

Control provision. The cab needs the right proportional controls for the attachment's functions. Fitting an attachment the operator cannot modulate precisely defeats the point of buying a controllable one.

Ask the attachment supplier for all five requirements in writing, and ask the carrier supplier to confirm the machine as configured meets each. Where both have confirmed in writing, a later problem has an owner — which is the practical reason for insisting on it.

Get it in writing from both ends

Where carrier and attachment come from different suppliers — which is the normal case in attachment-based fleets — a performance problem spanning both has two suppliers who can each reasonably point at the other.

The control is simple and it costs nothing: obtain written confirmation from both, before the order, that the combination meets each of the four checks. Where both have signed off, a later problem has an owner. Where neither has, you own it by default.