An excavator’s hydraulic system was designed around digging, and digging is a forgiving duty cycle. The machine loads the pump, then slews, then dumps, then repositions, and the oil gets a rest several times a minute. Hang a mulcher or a stump grinder on the same machine and that rhythm disappears. The attachment asks for most of the available power continuously, for as long as the operator keeps the head in the material, on a forty degree afternoon, with a radiator pack full of mulched bark. Heat is the result, and in Australian conditions it is the single most common reason a correctly specified attachment underperforms in summer.
Operating rule
Every kilowatt of hydraulic power that does not reach the rotor, screw or cylinder ends up as heat in the oil. On a continuous-duty attachment, size and verify the cooling system with the same seriousness as the flow and pressure figures, and treat a rising oil temperature as a production limit, not a warning light to be ignored.Continuous duty and cyclic duty are different machines
The families on this site split cleanly into two heat profiles, and the split matters more than the tonne class. Forestry mulchers, Dipperfox stump grinders, auger drives and compactors are continuous-duty. They run a motor under load for minutes or hours at a time and there is no natural pause in the cycle. Tree shears, forestry grapples and OMEF pruners are cyclic. They demand a burst of flow and pressure to close a cylinder, then almost nothing while the operator slews, places and repositions.
The practical consequence is that a carrier at the bottom of an attachment’s permitted flow band is usually acceptable for a cyclic attachment and rarely acceptable for a continuous one. A shear that closes slowly costs seconds. A mulcher running near the relief valve all day cooks the oil. Grapple saws sit awkwardly between the two: the grapple and rotator are cyclic, but the saw motor runs under load for the duration of every cut and, on large timber, that is not brief.
Where the heat actually comes from
Hydraulic heat is not a mysterious by-product. It is the energy the system moves that does no useful work at the tool, and it has a small number of identifiable sources.
Pressure drop across restrictions is the largest. Undersized hoses, long hose runs, couplers rated below the working flow and a restricted return line all convert pressure into heat directly. Oil crossing a relief valve is the most expensive version of this, because the pump is doing full work and the tool is receiving none of it. A carrier that is regularly on relief because the operator is feeding a mulcher too aggressively is effectively running an oil heater at full power.
Motor and pump inefficiency contribute continuously. Every hydraulic motor leaks a little oil internally by design, and that leakage, along with normal mechanical and volumetric losses, appears as heat. Bearings, belts and gearboxes on the attachment itself add mechanical heat that conducts back into the oil and the housing. The arithmetic is the same formula that appears in hydraulic matching, applied to the pressure that is lost rather than the pressure that is used.
Illustrative. A 30 bar loss at 200 L/min is about 10 kW of heat going into the tank, which is a meaningful load on a mid-size excavator's cooling pack.
Ambient conditions then set the ceiling. A cooler rejects heat in proportion to the difference between the oil and the air passing through it. On a still, dusty, forty degree day, the same pack that coped in June has far less capacity, and the attachment has not changed at all. This is the mechanism behind the familiar complaint that a machine mulches well all winter and starts shutting down after Christmas. Use the hydraulic power calculator to see how quickly the heat load rises as flow and pressure climb.
Oil temperature limits and the damage that follows
Ask the carrier dealer and the attachment supplier, in writing, for the maximum permitted continuous oil temperature and the temperature at which the machine will derate or shut down. These are not the same number, and an attachment supplier may also specify a lower figure than the carrier for the sake of the motor’s seals.
The damage from running hot is progressive rather than sudden, which is what makes it easy to ignore. Oil thins as it warms, so internal leakage in pumps and motors rises and volumetric efficiency falls, which produces more heat and less work in a self-reinforcing loop. Seals and hose liners harden and shrink, and shaft seals on saw and rotor motors are the first to weep. Oxidation accelerates, and a widely used fluid-power rule of thumb holds that oxidation rate roughly doubles for every ten degrees Celsius above about sixty, which turns into varnish on spool valves and sticking, unpredictable control. Filters load faster, additive packages deplete, and the eventual bill is a pump or a motor rather than a drum of oil.
Cooler sizing, fan performance and the cooling pack
Three physical things decide whether the carrier can reject the heat. The first is cooler capacity, meaning the size and configuration of the oil cooler and whether the machine was built with a forestry or high-flow package. Many excavators can be ordered or retrofitted with a larger oil cooler, and on a machine that will spend hundreds of hours mulching this is far cheaper than the production it protects. The second is airflow. Fan drive type matters: a fixed fan turns with the engine and loses airflow as soon as the machine drops revs, while a hydraulic or variable-speed fan can maintain cooling independently, and a reversing fan can clear the pack without a person and an air hose.
The third is cleanliness, and it is the one entirely within the operator’s control. Mulching, grinding and clearing generate exactly the fine dry material that packs into a radiator core: bark fines, leaf litter, dust and, in the wrong season, seed. A pack that is half blocked has lost half its capacity, and it will have blocked gradually enough that nobody noticed. Daily cleaning of the cooling pack is a production task, not a housekeeping nicety, and it is also a direct fire risk control, because packed combustible fines sitting against hot surfaces is how machine fires start.
Carrier derating, flushing lines and the attachment’s own protection
Modern excavators protect themselves by derating: reducing pump output or engine power as oil temperature rises. That is desirable behaviour, but it is important to understand what it looks like from the seat, because it presents as the attachment losing performance for no obvious reason. An operator who has not been told this is happening will usually respond by pushing harder, which makes it worse. Ask what the derate thresholds are and whether the machine logs them, because a machine that has spent the afternoon in derate is telling you the system is undersized for the duty.
On the attachment side, the case drain is part of the cooling strategy, not just a drain. Shearex lists oil cooling through a flushing line on the VM range, which continuously exchanges a small volume of oil through the motor housing to carry heat away. That line needs to run back to tank as a dedicated low-pressure path with a full-flow coupler, for the reasons set out in hydraulic matching: any restriction becomes back pressure on the motor’s shaft seal, and a hot seal under back pressure fails quickly.
Reading the symptoms
Heat announces itself in a consistent order. Oil temperature climbs steadily through the first hour and then does not stabilise. The attachment loses performance late in the day rather than early, and recovers after a break. Couplers and hoses near the attachment become too hot to touch. Cycle times lengthen on the same material. Then the machine starts derating, and finally something weeps: a motor shaft seal, a rotator seal or a hose end.
A head that bogs from the first minute is not a cooling problem; that is a flow or pressure mismatch. A head that works well for forty minutes and then progressively slows is almost always thermal, and the fix is in the cooler, the pack, the return line or the duty cycle rather than in the attachment.
Summer work practices and monitoring
The cheapest cooling improvement is usually a change in how the day is organised. Heavy continuous work moved into the cooler part of the morning, with handling, transport, servicing and lighter cyclic work in the afternoon, can recover hours that would otherwise be lost to derate. A short, deliberate pause with the engine at moderate revs and no load lets the cooler catch up far faster than idling, and is cheaper than a shutdown.
Beyond that, make oil temperature a measured number rather than an impression. Fit a gauge or use the machine’s monitor, and have the operator record the temperature hourly alongside the ambient temperature during the first weeks of a new attachment. Clean the cooling pack daily, or more often in dusty or bark-heavy material. Check oil level and condition on a schedule, use the oil specification the carrier manufacturer requires rather than whatever the yard has, and treat a sudden change in the temperature pattern as a fault to investigate. The total cost of ownership of a continuous-duty attachment is far more sensitive to a pump replacement than to a season of extra filters.
Confirm before ordering
- Maximum permitted continuous oil temperature for both the carrier and the attachment, in writing
- The carrier's oil cooler specification, and whether a larger forestry or high-flow cooling package is available
- Fan drive type, and whether a reversing or variable-speed fan is fitted or available
- Whether the attachment requires a flushing or case-drain line, its size, and the maximum permitted back pressure
- Return line size and coupler rating at the attachment's working flow
- The carrier's derate thresholds and whether derate events are logged
- Hose lengths, hose internal diameter and coupler flow ratings included in the installed price
- A demonstration run long enough in ambient summer conditions for oil temperature to stabilise
- Daily cooling-pack cleaning access, and whether it can be done without removing panels or guards
Frequently asked questions
My mulcher works well in the morning and slows every afternoon. What is wrong?
That pattern is thermal almost every time. The system is generating more heat than the cooler can reject once ambient temperature rises, so oil thins, efficiency falls and the machine may begin derating. Check the cooling pack for packed fines first, then the return line and coupler sizing, then whether the carrier has a larger cooler option.
Does a bigger excavator solve an overheating problem?
Not necessarily. A larger carrier usually brings more pump capacity and a larger cooler, but if the attachment is being run near the relief valve or fed through undersized hoses and couplers, the extra flow simply makes more heat. Fix the restriction and the duty cycle before buying tonnes.
Is it worth fitting an auxiliary oil cooler?
It can be, particularly where the carrier has no larger factory cooling package and the attachment will work long continuous hours in summer. Have it specified by the carrier dealer rather than improvised, because an added cooler is another restriction in the circuit and its position in the return path matters.
How often should the cooling pack be cleaned in mulching work?
Daily as a baseline, and more often in dusty, bark-heavy or seed-laden material. Many operators clean at every refuel. A reversing fan reduces the effort but does not remove the need to inspect the core properly.
Do tree shears and grapples need the same cooling attention?
Less, because their demand is intermittent and the oil recovers between cycles. They can still overheat a carrier if the circuit is badly restricted or the relief setting is wrong, but a shear will rarely be the reason a machine runs hot. Continuous-duty attachments are where cooling decides production.
What oil temperature should I treat as a working limit?
Use the figure the carrier manufacturer publishes for that machine, and the attachment supplier's figure if it is lower, rather than a general number. What matters commercially is that the limit is known, visible to the operator and recorded, so that a change in the normal pattern is noticed early.
Get a recommendation, not just a price
Send us your excavator model, hydraulic figures, tree species and typical diameters. We will come back with a carrier-matched configuration and a written specification you can compare against any other quote.