Aluminum, cast iron or high-pressure pump? Which hydraulics to choose for a boom mower
In machines such as boom mowers and flail mowers for excavators you encounter three basic hydraulic power concepts. The first is the aluminum system, most often found in budget solutions. The second is the cast-iron system based on a gear pump and a gear motor. The third is the high-pressure system, usually with a piston pump. Technical literature has long shown that gear pumps are valued for their simple and compact design, while multi-piston pumps are used where higher operating pressures are required.
On paper, the high-pressure system looks very good. It delivers high pressure, good efficiency and high power density. Research papers on axial piston pumps confirm their advantages: high efficiency, resistance to high pressures and a wide range of applications in working machines. At the same time, they are more complex systems and more sensitive to operating conditions than simple gear pumps.
That is why, when purchasing, it is not worth looking only at the bar figure. In practice the whole balance counts: hydraulic power, cost of the system, durability, oil cleanliness, filtration, service availability and overhaul cost. From this perspective the cast-iron system with a gear pump and gear motor very often comes out on top.
The three basic hydraulic systems used in boom mowers
Aluminum system
This is the cheapest group of solutions. It typically operates in the range of about 180 to 190 bar and 90 to 100 l/min. Such a system attracts buyers with its low entry price and can be sufficient for lighter work. However, it must be honestly said that in terms of durability and headroom for heavy operation it is the weakest of the three options. In long work on road shoulders, ditches, brushwood and in difficult terrain its limitations show up faster.
Cast-iron system with gear pump and gear motor
This is the mid-price solution, but often the best one in use. A typical operating point that makes sense in heavier boom mowers is around 230 bar and 120 l/min. Gear pumps are widely used precisely because they have a simple structure, are compact and fit well into systems where predictable operation matters.
High-pressure system
Here we usually talk about piston pumps and pressures of around 320 bar at a flow rate of approximately 80 l/min. These solutions are technically justified where high efficiency and high resistance to pressure load are required. Educational materials and scientific publications clearly indicate that multi-piston pumps work in higher-pressure systems because they ensure better sealing and good operating parameters.
The bar figure alone does not settle the matter
In hydraulics, power is not determined by pressure alone. The relationship is simple: hydraulic power is the product of flow and pressure. In educational materials for fluid power it is written as P = Q · p, and when converted for bar and l/min we get the practical formula P [kW] = p [bar] × Q [l/min] / 600.
For the three compared systems this gives:
Aluminum system
180 bar × 100 l/min = 30 kW
190 bar × 90 l/min = 28.5 kW
Cast-iron gear system
230 bar × 120 l/min = 46 kW
High-pressure system
320 bar × 80 l/min = 42.7 kW
The conclusion is simple. A typical cast-iron 230 bar / 120 l/min system provides more hydraulic power than a typical high-pressure 320 bar / 80 l/min system. This is important from the customer’s standpoint because it shows that higher pressure does not necessarily mean a more powerful machine. You have to assess the whole system, not just one specification from a brochure.
What technical literature and research say about piston pumps
Looking at scientific publications on axial piston pumps, the picture is fairly consistent. They are designs offering high efficiency, resistance to high pressures and high power density. Studies published in technical journals also show that the overall and volumetric efficiency of axial piston pumps depends strongly on pressure, rotational speed, oil temperature and viscosity. Rising inlet temperature and dropping viscosity worsen their operating parameters.
From a practical standpoint this means that the piston pump is a good solution, but a more demanding one. In a machine working hard and long, beyond efficiency itself the cost of maintaining that efficiency starts to count.
Why the cast-iron system is often the most economically justified
Here begins the most important part for the end user. In real operation you do not buy just the pump — you buy the whole system and the whole machine life cycle.
Gear pumps are structurally simpler and widely available. This is not a sales slogan but a feature of the technology itself, confirmed in educational materials and engineering work. Simpler construction usually means easier service, greater workshop availability and less dependence on highly precise overhauls.
Axial piston pumps, in turn, require more accurate control of operating conditions. Bosch Rexroth emphasizes that modern hydraulic components — especially proportional valves, servo valves and axial piston pumps — are very sensitive to solid-particle contamination. Even one-step degradation of the cleanliness class means doubling the particle concentration in the oil, which can lead to increased wear, deteriorated operation and failures.
The same is confirmed by manufacturer technical manuals. For axial piston units Bosch Rexroth recommends keeping the oil cleanliness class at least 20/18/15 according to ISO 4406, and even better at higher temperatures.
The practical conclusion is simple. The more precise and highly stressed the system, the more its proper operation costs. That is why in service practice high-pressure systems are usually more expensive not only at purchase but also in hoses, fittings, filtration, overhauls and downtime. The exact price difference depends on the manufacturer, component class and machine configuration, but the economic direction is clear: high efficiency costs more.
High pressure makes sense, but not always for the mower itself
A higher-power system makes sense when the machine is not used solely for light grass cutting. In ditch desilting, road shoulder cleaning, work in dense brushwood or mulching thicker branches, more power at the rotor gives a real advantage. The machine keeps revs more easily, stalls less and copes better with heavier material.
This means that a high-power system is justified not only for classic boom mowers but also when the attachment has to perform heavier ground and terrain work. In practice this also applies to machines such as flail mowers for excavators, where the attachment runs on the carrier’s existing hydraulics.
When the high-pressure system really makes sense
The answer here is quite simple. A high-pressure system makes the most sense when the carrier already has such a supply. Excavators and other working machines, where high-pressure hydraulics are already available, are a good example. In such a situation using a high-pressure motor for the attachment is logical, because the ready pump and ready system can be reused.
It is precisely in such a scenario that flail mowers for excavators can run on high-pressure hydraulics with a real justification. A separate power supply is not built from scratch; the carrier’s existing one is used.
If, however, the customer is only choosing the entire system for a boom mower, the situation is different. Going for a high-pressure system is then often going into higher costs without a proportional usage benefit. With a deliberate ground-up selection, a more reasonable choice is often a cast-iron gear pump and cast-iron gear motor.
What the customer really gains with a cast-iron 230 bar / 120 l/min system
First, they get 46 kW of hydraulic power, fully sufficient for heavy work in many municipal, road and drainage applications. This is more power than in a typical 320 bar / 80 l/min set.
Second, they get a system that does not operate at extremely high pressure, so the entire hydraulic equipment is less stressed. In practice this translates into a lower cost of the whole system.
Third, they get a solution that is simpler to service. With gear pumps it is easier to access parts, repairs and workshops. With piston pumps not only the part itself matters but also the very high quality of overhaul and the system’s service culture.
Practical conclusions
If the lowest entry price is the priority, the aluminum system can be a sufficient solution, but its lower durability must be accepted.
If the carrier already has high-pressure hydraulics and the attachment is to work heavily and across many tasks, the high-pressure system may make sense.
If, however, the choice concerns building the system from scratch, with a goal of reasonable purchase cost, good power, durability and predictable operation, in many cases the most justified remains the cast-iron system with gear pump and gear motor, working around 230 bar and 120 l/min.
It is not the most spectacular choice on paper. It is the most rational choice in practice.
FAQ
Does higher pressure always mean a better boom mower?
No. Hydraulic power is decided by pressure and flow together. That is why a 230 bar / 120 l/min system can deliver more power than a 320 bar / 80 l/min system.
When is it worth choosing a piston pump?
Mainly when the carrier already has high-pressure hydraulics, or when the attachment has to operate in very demanding conditions and high efficiency at high pressure is required.
Why are piston pumps more demanding?
Because they are precise, work at high pressures and are more sensitive to oil cleanliness and filtration quality. Manufacturers specify concrete oil cleanliness requirements for them according to ISO 4406.
Should flail mowers for excavators have a separate hydraulic system?
Not always. If the excavator already has suitable high-pressure hydraulics, it usually makes more sense to use the existing supply than to build a separate system.
Sources and literature
- Educational materials on fluid power, covering gear and multi-piston pumps.
- Bosch Rexroth, requirements for oil cleanliness and sensitivity of axial piston pumps to contamination.
- Scientific publications on axial piston pumps and their efficiency.
- Technical dissertation on gear pumps and their design.
Main keyword: boom mowers
Secondary keywords: flail mowers for excavators, hydraulics in boom mower, gear pump for boom mower, piston pump for mower, high-pressure system in mower