Hydraulic Matching for Forestry Attachments

More forestry attachment purchases go wrong on hydraulics than on anything else. The attachment is the right size, the carrier is in the right tonne class, and yet the shear closes slowly, the mulcher rotor bogs in anything heavier than regrowth, or the saw motor shaft seal fails in the first month. Almost every one of those outcomes was predictable from the two data sheets before the order was placed. This guide explains what the numbers mean, which ones matter for each family of attachment and what to send the supplier so the recommendation is based on your excavator rather than its tonne class.

Flow, pressure and power are three different numbers

Flow, measured in litres per minute, sets speed. On a mulcher or a Dipperfox stump grinder it sets rotor or screw speed; on a shear or grapple it sets how fast the cylinder strokes. Pressure, measured in bar, sets force and torque: how hard the blade is driven through the fibre, how much torque the motor produces before it stalls. Power is the product of the two, and it is the number the carrier’s engine ultimately has to supply.

The formula is most useful for exposing pairings that cannot work. Take the Dipperfox SC600, rated at 65 to 150 L/min and 150 to 250 bar on 6 to 12 tonne excavators with 35 to 74 kW of engine power. At the top of both ranges, 150 × 250 ÷ 600 is 62.5 kW. A 35 kW carrier can run the attachment, and Dipperfox lists it as acceptable, but it cannot deliver anything like the top of the flow band at working pressure, so it will drill more slowly than a carrier near the upper end of the class. The same logic applied to the Dipperfox SC850 Pro (110 to 230 L/min, 180 to 350 bar, carriers of 74 to 130 kW) gives 230 × 350 ÷ 600, or about 134 kW, which exceeds the largest permitted carrier. That is not an error in the data sheet; it simply shows that maximum flow and maximum pressure never occur together, and that the attachment’s output tracks what the carrier can actually deliver.

Mulchers push the arithmetic hardest. A Shearex VM-50SR wants 159 to 238 L/min of continuous flow. Assume, for illustration, that the carrier holds 350 bar at the top of that band: 238 × 350 ÷ 600 is about 139 kW of theoretical hydraulic power, on an attachment whose stated minimum carrier power is 59 kW. The VM-65SV at 344 L/min and the same assumed pressure comes to roughly 200 kW. These figures tell you why a 15 tonne excavator at the bottom of the class will mulch, but at the bottom of the head’s production band, and why oil temperature becomes the limiting factor on a hot afternoon (see hydraulic cooling and heat). The hydraulic power calculator does this arithmetic for any flow and pressure pairing.

Continuous flow is not pump flow

An excavator brochure quotes total pump output at rated engine speed with no load on the circuit. The auxiliary circuit gets a share of that, and the share falls as pressure rises because the pump’s power limiter, the engine’s speed droop and the valve spool all take something back. Dipperfox is explicit that carrier flow must be available under load, and Shearex publishes both a minimum and a maximum continuous flow for every VM and HM model for the same reason. The number you need from the carrier dealer is the auxiliary flow at the attachment’s working pressure, with the engine at the speed you will actually run, in the work mode you will actually use. Attachments that cycle, such as a tree shear or an OMEF pruner, tolerate a carrier at the low end of the band because the demand is intermittent. Continuous-duty attachments, meaning mulchers, stump grinders, augers and compactors, do not.

Pressure: enough, and not too much

Every attachment publishes a minimum and maximum pressure, and both matter. Below the minimum the cylinder lacks force and the motor lacks torque, so the shear stalls at the last third of the cut and the grinder screw hesitates in root wood. Above the maximum, seals, motors and gearboxes are at risk. The ranges vary widely across the families on this site: the OMEF CS pruners run at 180 to 250 bar, the Shearex VM range is listed up to approximately 410 bar, and the OMEF CPT180 compactor wants only 100 to 120 bar. Fitting a low-pressure compactor to a circuit whose relief is set for a hammer is a common way to destroy a vibration unit, and it is the reason the relief setting of each auxiliary circuit should be confirmed and, where needed, adjusted at commissioning. The Trevi Benne WE10 illustrates a different trap: earlier literature listed around 310 bar and 100 L/min, and the current build must be confirmed rather than copied from an old brochure.

Count the circuits before you order

A single-acting hammer circuit is not the same as a double-acting auxiliary, and a second double-acting circuit is not the same as a case drain. The OMEF CS pruners need two double-acting lines, one for the cutting cylinder and one for 360-degree rotation. The OMEF BTD disc trimmers are described by the factory as needing a single-acting line with no drain. Shearex’s guidance for the VM range is the clearest example of how options change the requirement: the standard hydraulic door uses pressure and return plus a case drain with electrical control for the diverter, tilt is better served by a second double auxiliary line, and tilt plus grapple is best run from two double auxiliary circuits plus case drain. A grapple saw adds rotator, grapple, saw motor and chain lubrication, normally shared through diverter valves and electrical switching. If the carrier has one auxiliary circuit and the attachment you have specified needs two plus a drain, the shortfall has to be priced as plumbing, valves and cab wiring, not discovered on delivery day.

Return pressure and the case drain

Hydraulic motors on mulcher rotors, saw units, auger drives and stump-grinder transmissions leak a small amount of oil past their internal parts by design, and that oil has to get back to tank through a case drain at very low pressure. Tee the case drain into the main return, or run it through an undersized coupler, and every restriction in the return line (filter, cooler, coupler, long hose) shows up as pressure on the motor’s shaft seal. The result is a seal that weeps, then fails, then dumps oil down the side of the rotor housing. Ask the attachment supplier for the maximum permitted back pressure on the return and the drain, ask the carrier dealer what the return line actually produces at working flow, and run the drain as a dedicated line to tank with a full-flow coupler.

Electrical control is part of the hydraulic package

Diverter valves, proportional rotation, saw triggers and door control all need switches, wiring and often a control module in the cab. On a carrier that has never run a multi-function attachment, this is frequently the largest unbudgeted line in the installation. Establish who supplies the cab controls, whether the joystick buttons are usable, whether a second machine can be wired the same way if the attachment moves between carriers, and how the electrical package is protected when the attachment is uncoupled and left on site.

Hoses and couplers set the ceiling

A high-flow attachment fed through hoses and couplers sized for a hammer loses pressure to friction, and lost pressure becomes heat. The symptoms are a rotor that never reaches speed, oil that runs hot within the hour and couplers too hot to touch. Hose internal diameter, hose length to the attachment, coupler type and coupler flow rating all need to match the flow, and the quick coupler on the stick needs to be able to pass it. This is also where standardising across a fleet pays: one coupler type and one hose specification across every attachment on the carrier reduces changeover time and connection errors (see quick couplers and attachment changeover).

What to send the supplier

A supplier who receives the following can make a carrier-specific recommendation and put the hydraulic requirement in writing. A supplier who receives only “20 tonne excavator” can only guess.

• Carrier make, model, year and serial, with boom and stick lengths and counterweight fitted
• Quick hitch make and model, and its weight
• Auxiliary circuits fitted: number, single or double acting, flow and pressure of each at working rpm and under load
• Case drain fitted, and its line size and coupler
• Return line size and any known back-pressure figure
• Existing electrical controls in the cab and any spare joystick functions
• Attachment options wanted: rotation, accumulator, door, tilt, grapple, heel, saw motor size
• Species, typical and maximum diameters, annual hours and ambient conditions

Symptoms of a mismatch

A shear that closes slowly, or hesitates at the end of the stroke, usually has too little flow or too little pressure; in hardwood it may have both. A mulcher rotor that bogs and stalls in material within the head’s optimal target diameter is short of flow under load or is being starved by hose losses. Oil that reaches its temperature limit within an hour of steady mulching points to a carrier working at its relief valve, an undersized cooler or restricted return. A grapple saw that cuts slowly and dulls chain quickly may have the wrong saw motor displacement for the carrier’s flow. Weeping motor shaft seals point at case drain or return pressure. Each of these has a cost in production, and each is cheaper to fix on paper before the order.

Confirm in writing before ordering

• Minimum, recommended and maximum continuous flow for the attachment, and the flow the carrier delivers at that pressure under load
• Minimum and maximum operating pressure, and the relief setting of each auxiliary circuit
• Number and type of circuits required for the ordered configuration, including case drain
• Maximum permitted return and case-drain back pressure
• Hose sizes, hose lengths and coupler type included in the price, and who fits them
• Electrical control package included, and whether the cab needs a module or extra switching
• Saw motor or drive displacement selected for the carrier's flow, where options exist
• Commissioning inclusions: relief setting check, flow test, temperature check under load

Frequently asked questions