- External loads are environmental influences to which the
pressure transmitters are subject during operation of the working
equipment (harvesters on fields, excavators on sites) or during
cleaning (automatic washers, steam jet cleaning).
- Shock: During operations in fields or on construction sites roughnesses /
reliefs in the landscape cause impulses that have an effect on
the pressure transmitter. In addition there are many other
factors that might cause shocks, e.g. impacts, dropping of the
object or in general collisions of two objects. The MH-2 resists
shock loads of up to 500 g without its functionality being
affected. (For comparison: A car driving into a tree at 100 km/h
corresponds to a shock load of 20 ... 100 g)
- Vibration: The engines of vehicles generate vibrations, i.e.
vibratory stresses over a prolonged period, which the pressure
transmitter is exposed to. Depending on the working load the
frequency and consequently also the vibratory stress changes. 10
g corresponds to the vibration that is generated at the engines
of an airplane during takeoff. Within a wide range of
frequencies, the MH-2 is suitable for applications with
vibratory stresses of up to 20 g, even under resonance
conditions.
- Dirt: During operations in fields or on construction sites
the outside of the pressure transmitter gets dirty.
- Moisture: If the vehicle is subjected to varying weather
conditions (snow, rain) or environmental conditions (when
driving through water holes), the pressure transmitter has to be
specially protected against the penetration of moisture. When
the vehicle is cleaned in washing facilities or by steam jets,
the instrument is also exposed to moisture.
- Internal stresses and loads: Pressure peaks, changing loads,
cavitation and micro diesel effects which occur in the hydraulic
circuit of the machine are understood under internal stresses and
loads. Concerning mobile hydraulics these stresses and loads are
extreme. Every MH-2 is equipped with the capillary damping system
CDS (taper of the pressure channel), which helps to avoid the
following stresses.
- Pressure peaks: They occur as a result of the control system of the
working hydraulics. The working hydraulics control system
comprises hydraulic pump, hydraulic accumulator and switching
and control valves. To control an excavator jib (consumer in the
hydraulics circuit), for example, the necessary operating
pressure is delivered through the control valve and is taken
from the hydraulic accumulator. If the control valve switches
quickly (< 1 s) with a larger change in the cross section,
pressure peaks will occur which can be essentially higher than
the operating pressure and cannot be measured due to the
extremely quick pressure rising time. These pressure peaks are
caused by the pressure which builds up at the speed of sound (in
the pipe through the hydraulic oil). Since a hydraulic system is
elastic the lines are expanded and more oil has to be pressed
through the line. This accelerated volume of oil generates a
pressure peak which impinges on the end of the pipe (this means
to say pressure oscillations are generated).
- Changing load: Load change is the cycle from pressure
build-up to zero pressure (when raising the excavator jib
pressure build-up, lowering the excavator jib, pressure drop = 1
cycle). In mobile hydraulics applications changing load counts
from 107 to 108 per life circle are
demanded from the pressure transmitter sensors!
- Cavitation/micro diesel effect: To be understood under
cavitation is the generation of a cavity in liquids. To be
understood under the generation of a cavity is the release of
the air trapped in the hydraulic oil (5-12 %) which can produce
an air bubble and a mixture of oil in the hydraulic system.
Cavitation is caused by dropping pressure. This, in turn, is
caused by redirecting the hydraulic oil from the working
cylinder into the hydraulic accumulator (reducing the pressure).
As a result of the elasticity given in the hydraulic system a
negative pressure is produced which causes its release
(cavitation). When the pressure drops this mixture is compressed
and thus combustion (burning) occurs at a high temperature.
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