Friction Losses Reduce Efficiency

In all mechanical, hydromechanical, and pneumatic systems, friction losses are the main cause of poor efficiency. Friction losses cause part of the supplied energy to be converted into unusable heat, which can certainly lead to problems.

Friction Losses Are Unavoidable in Mechanical Systems

Wherever two moving elements come into contact, friction occurs. However, these do not necessarily have to be solid bodies. Fluids and gases also create friction along the walls of their lines or at the interfaces of modules and fittings. The result is always the same: the supplied energy is partially converted into heat, which is rarely usable. As a rule, it is dissipated through elaborate cooling systems or radiates unused into the environment.

For hydraulic systems, this means that friction reduces the pressure and, above all, the flow velocity in a hydraulic system. Increasing the pressure through higher pump output is only the second-best solution. Ideally, friction losses in hydraulic systems should always be minimized as comprehensively as possible.

Causes of Friction Losses

Friction itself is unavoidable. However, it can be promoted by certain factors. In straight pipelines, rough surfaces on the inside are a typical cause of friction. Pipes should therefore always be as smooth as possible to avoid or minimize this friction. Branches, T-pieces, or installed modules such as valves or fittings further increase friction losses. Unclean connections, seals protruding into the flow, offsets at couplings, poor welds, and protruding sealing screws are additional causes of unwanted friction. However, friction and friction losses also occur in pipes optimized for flow dynamics.

Mathematically Capturing Friction Losses

The friction losses occurring under optimal conditions can, however, be well captured mathematically. This makes it possible to incorporate them into the planned efficiency of a hydraulic system. Thus, the entire system can be optimized so that it can deliver its desired performance with minimal energy input. The basis for calculating friction losses is the so-called “pipe friction factor.” This, in turn, is calculated from the so-called “Reynolds number,” named after the physicist Osbourne Reynolds. It indicates the ratio of the inertia of a medium to its viscosity. The Reynolds number is formed from the factors:

• Density of the medium
• Flow velocity
• Characteristic length
• Dynamic viscosity

The pipe friction factor for laminar flows, i.e., flows that are free of turbulence, can be determined quite simply: it is 64/Reynolds number.

Most flows in hydraulic systems, however, are not laminar but turbulent. Their calculation is somewhat more complex, but equally straightforward. In this calculation, the pipe system is calculated section by section and then an overall value is determined. Part of this calculation is the roughness in the pipe system.

Practically Determining Friction Losses

The theoretical-mathematical determination of friction losses in a hydraulic system is quite complex. Nevertheless, it rarely actually occurs in the pre-calculated form, but always produces only an approximate value. Therefore, in practice, the actual efficiency and thus the occurring friction losses are determined through a simple target-actual comparison. The system is simply flowed through with a medium with a defined viscosity. From the velocity losses, the friction losses and thus the efficiency can be determined at the system output.