The article presents
a review of modern methods of producing hydraulic components that can work with
water as a working medium. The presented technology of coating and production
of plastic elements is an alternative to the currently used stainless steel
construction. Large-scale testing of cylinders and valves made in coating
technology has been carried out. Innovative methods for designing pumps made of
plastic have been developed, and were confirmed by laboratory tests. Introduction
Research conducted on the use of water as a working medium not only in power
hydraulics, but also in other branches of industry (fire extinguishing,
refrigeration, etc.) confirm the existence of problems with its use [1, 2, 3].
However, the advantages far outweigh its disadvantages. That is why it is so
important to use water hydraulics in natural environment friendly applications
[4]. Especially if their price will not be a big obstacle. Currently, the cost
of components made of stainless steel 3 ÷ 4 times exceeds the price of standard
oil hydraulic elements [5, 6]. The first solution to its reduction is the
technology of chemical-physical coatings integrating the Diamond Like Carbon
coating technology (DLC) Coating with innovative technology with
self-lubricating properties of the Columnar Nanostructured Coating (CNC). The
research of these coatings was one of the topics of the EU project called
“Novel high-pressure water hydraulic equipment for application in the mining
and mining sector” with the working name “Hydrocoat”. Tests were carried out in
the fluid power laboratory which currently is a part of the Laboratory of
Techno-Climatic Research and Heavy-Duty Machines at the Cracow University of
Technology. The second method is the use of plastics. The research of hydraulic
pumps and other elements made of plastic bodies is carried out by the Fluid
Power Research Group at the Wrocław University of Science and Technology.
Materials The parts produced for oil hydraulics were specially treated. The metal-contacting
parts were covered with special patented protective layers. This technology is
treated as a solution enabling an adaptation of the already existing valves and
cylinders to work in a water hydraulic system. These layers are the following:
the Diamond Like Carbon (DLC) and Columnar Nanostructured Coating (CNC) with
self-lubricating properties. Their use contributes to the lowering of the
friction force and also provides a layer protecting steel from corrosion [2,
7]. In addition, using the experience gained Terotechnology 2017 Materials
Research Forum LLC Materials Research Proceedings 5 (2018) 200-205 doi:
http://dx.doi.org/10.21741/9781945291814-35 201 in the research of water
hydraulic systems, the seals were replaced in the tested cylinders. The new
seals were made of materials that could cooperate with water, such as: ULTRALEN
90 and KEFLOY 22 [1]. In turn, the use of plastics in the construction of pumps
brings design, technological, operational and economic benefits such as
reduction of mass, simplification of construction, self-sealing ability,
improvement of tribological properties between cooperating elements, increase
of resistance to contaminants in the working fluid, increase of the ability to
damp vibrations and reduce noise [8]. And above all, pumps made of plastics can
work with various working fluids, such as hydraulic oils, water, emulsions,
nanofluids and chemical fluids [2, 9]. As a plastics easily available on the
market, cheap and easy to process polyoxymethylene POM was chosen [10].
Cylinder tests The aim of the research was to confirm the effectiveness of the
developed cover technologies. The Polish Standard PN 72 / M 73202 concerning
tests of oil hydraulic cylinders was used during the tests of a water hydraulic
cylinder. No standards have yet been developed for testing elements using water
as a working fluid. According to it, external leak tests and internal leak
tests were conducted, the friction force, volumetric efficiency, hydraulic
mechanical efficiency and total efficiency were determined [11]. Tests with a
static pressure load did not show measurable external or internal leaks.
Therefore, the determination of hydraulic mechanical efficiency was reduced to
determine the equivalent, total efficiency of the cylinder as a function of
piston velocity and working pressure. A special stand was designed for the
tests, where in the welded frame of C profiles, cylinders connected with a
trolley were mounted. The wheels of a trolley were run along the inner surface
of the frame. One of the cylinders was used for the drive, while the other was
used to generate the load. The displacement of the piston L, the pressure on
the piston side pa1 and the pressure on the rod side pa2 of the cylinder were
measured. During efficiency tests, the temperature of the water in the
hydraulic system was T = + 40 °C. They were repeated, for comparison, for other
values of temperature. To determine the impact of pressure on the total
efficiency, tests were carried out at the piston velocity of v = 0,2 m/s and
the working pressure pZ changing from the minimum to the nominal value. To
determine the influence of velocity on the total efficiency of the cylinder,
tests were carried out at the nominal load pressure and the speed changing from
0,05 to 0,2 m/s. Figure 1 shows three dimensional characteristics of the
determined total cylinder η efficiency for the extension and retraction
depending on the velocity of the v piston and the theoretical F force. The
force was calculated as a product of pressure and surface area on the piston
side of the cylinder during extension and on the rod side of the cylinder
during retraction. Fig. 1. Characteristics of the total cylinder efficiency
during extension and retracting [7] Terotechnology 2017 Materials Research Forum
LLC Materials Research Proceedings 5 (2018) 200-205 doi:
http://dx.doi.org/10.21741/9781945291814-35 202 Valves tests The problem of a
proper design of the gap between the movable elements of the spool directional
control valve is particularly difficult when water is used as a working medium
[12]. The low value of its viscosity coefficient causes the increase of
leakages and a friction force. For these reasons, a directional control valve
was constructed of four two-way ON/OFF valves. They were chosen because they
are durable and relatively uncomplicated. Four separate ON/OFF valves were
covered with a CNC coating and mounted in a specially designed aluminium block
(Fig. 2). To control the coils of these valves, a programmable electronic
module was designed and made. It enabled an independent activation and
deactivation of each ON/OFF valve. That is why the distributor could work in
any configuration of ways. Tests of the directional control valve consisted of
functional tests in a hydraulic system with a cylinder and determination of
flow characteristics for individual way of valve (Fig. 3) [13]. Fig. 2. View of
a four way, three position manifold body with four ON/OFF valves [13]. Fig. 3.
Flow characteristics for roads P A (red graph, negative values Q), P B (red
graph, positive values Q), A T (blue graph, positive values Q), B T (blue
graph, negative values Q) Pump tests The tests of pumps and rotary motors with
the use of covers did not bring the expected results. The exact analysis of the
problem revealed that the use of coatings presents difficulties in keeping the
strength and clearances between the elements. Therefore, it was considered
reasonable to make such an element from scratch, ideally, from an easily
accessible and easily machinable material such as plastics [14, 15]. It was
assumed that the general shape of a pump body should be formed as a prism with
a square base. By removing the material from this general shape and applying
the principles of global and local modification, the final shape of the pump
body was obtained (Fig. 4). In addition, using the generally known methods of
designing hydraulic machines and systems and the Finite Element Method (FEM),
the author’s methodology for shaping the bodies of hydraulic machines from
plastics was developed [14]. Terotechnology 2017 Materials Research Forum LLC
Materials Research Proceedings 5 (2018) 200-205 doi:
http://dx.doi.org/10.21741/9781945291814-35 203 Fig. 4. View of a gerotor pump
with a body made of plastics The assembled gerotor pump with a plastic body has
been subjected to experimental research on a test stand in order to determine
its basic hydraulic characteristics. The test stand enabled the measurement of
hydraulic parameters, such as flow and pressure at the inlet and outlet of the
pump, and measurements of mechanical parameters, such as torque and rotational
speed. Signals from measuring instruments were sent to the computer using a
laboratory signal amplifier. The CatmanEasy computer program was used to
archive and analyze the measurements on the computer. The tested pump was
driven by a DC electric motor with the power of 30 kW and the maximum speed of
n = 3000 rpm. A throttle valve was used to load the pump. To protect the pump
and electric motor from overload, an overflow valve was used. Fig. 5. The
characteristics of volumetric and total efficiency, depending on the output
pressure ηv, η = f (pout), for different rotational speeds n = 500 ÷ 1000 rpm
Figure 5 shows that the pump was working in the range of working pressure of p
= 0 20 bar and rotational speed of n = 500 1000 rpm. The ηv volumetric
efficiency varied in the range of ηv = 89 70%, and the total η efficiency
varied in the range of η = 73 50%. The relatively low efficiency prevented
further loading of the pump, which could lead to its seizure. Figure 4 shows
that the volumetric ηv and total η efficiency decreases with the increase of
the p output pressure and the n rotational speed.