HYDRAULIC FLUIDS
Pushing the performance limits in hydraulics through tribological optimisation of lubricants and surfaces
Boris Zhmud and Mattias Sund, Tribonex AB
Hydraulic systems are ubiquitous in various industrial applications where their ability to produce high forces with precise movements is valued. One common application is construction equipment (excavators, bulldozers, cranes, loaders). Other common applications include industrial machinery (hydraulic presses and machine tools), automotive (brakes, power steering), aviation (landing gear mechanism, flight controls, cargo doors), marine (hydraulic controls and deck machinery), etc.
One of the key advantages of hydraulics is that, in a compact design, they can pack sufficiently high power and allow precise torque control at low speeds. The amount of torque generated by a hydraulic motor is proportional the fluid pressure and piston area, and the speed is controlled by the fluid flow rate. A small hydraulic motor with 100 cc/rev displacement can generate a torque of 1000 Nm. Large motors such as Hägglunds CBm can produce torques up to 2 million Nm and over 2 MW power.
Figure 1: Illustration of Pascal’s Law.
Hydraulic systems operate based on Pascal’s Law (Figure 1), which states that pressure applied to a confined fluid is transmitted equally in all directions. If we connect two cylinders with pistons and apply force F to the left piston with the cross-section area A, exerting pressure P = F/A to the fluid inside the cylinders, the same pressure will act on the right piston. Hence, using interconnected cylinders of different diameter allows force amplification:
In practice, however, the output force will always be somewhat lower than predicted by the Pascal equation because of losses. Main losses in a hydraulic system include friction, viscous dissipation, and fluid leakage.
26 LUBE MAGAZINE NO.189 OCTOBER 2025
The key components of a hydraulic system include a pump, cylinders and motors, hydraulic fluid, valves, hoses, and pipes. The pump is used to pressurise the fluid. The pressurised fluid moves through hoses and valves to actuate cylinders or motors.
Hydraulic fluids There are several classes of hydraulic fluids (Bock 2017). Most hydraulic fluids are mineral-oil-based. For some special applications, fire-resistant hydraulic fluids are required, recognisable by the HF prefix in the label, e.g. HFC 46. Such fluids are needed, for example, when hydraulic equipment operates near
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