Proactive maintenance emphasizes the routine
detection and correction of root cause conditions that would otherwise lead
to equipment failure. In the case of hydraulic systems, there are three
easily detectable symptoms that give early warning of root cause conditions.
These symptoms are abnormal noise, high fluid temperature and slow operation.
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Abnormal Noise
Abnormal noise in hydraulic systems is often caused by aeration or cavitation.
Aeration occurs when air contaminates the hydraulic fluid. Air in the hydraulic
fluid makes an alarming banging or knocking noise when it compresses and
decompresses, as it circulates through the system. Other symptoms include
foaming of the fluid and erratic actuator movement. Aeration accelerates
degradation of the fluid and causes damage to system components through loss of
lubrication, overheating and burning of seals.
Air
usually enters the hydraulic system through the pump’s inlet. For this reason,
it is important to make sure pump intake lines are in good condition and all
clamps and fittings are tight. Flexible intake lines can become porous with
age; therefore, replace old or suspect intake lines. If the fluid level in the
reservoir is low, a vortex can develop, allowing air to enter the pump intake.
Check the fluid level in the reservoir, and if low, fill to the correct level.
In some systems, air can enter the pump through its shaft seal. Check the
condition of the pump shaft seal and if it is leaking, replace it.
Cavitation
occurs when the volume of fluid demanded by any part of a hydraulic circuit
exceeds the volume of fluid being supplied. This causes the absolute pressure
in that part of the circuit to fall below the vapor pressure of the hydraulic
fluid. This results in the formation of vapor cavities within the fluid, which
implode when compressed, causing a characteristic knocking noise.
The
consequences of cavitation in a hydraulic system can be serious. Cavitation
causes metal erosion, which damages hydraulic components and contaminates the
fluid. In extreme cases, cavitation can cause mechanical failure of system
components.
While
cavitation can occur just about anywhere within a hydraulic circuit, it
commonly occurs at the pump. A clogged inlet strainer or restricted intake line
will cause the fluid in the intake line to vaporize. If the pump has an inlet
strainer or filter, it is important for it not to become clogged. If a
gate-type isolation valve is fitted to the intake line, it must be fully open.
This type of isolation device is prone to vibrating closed. The intake line
between the reservoir and pump should not be restricted. Flexible intake lines
are prone to collapsing with age; therefore, replace old or suspect intake
lines.
High Fluid Temperature
Fluid temperatures above 180°F (82°C) can damage seals and accelerate
degradation of the fluid. This means that the operation of any hydraulic system
at temperatures above 180°F is detrimental and should be avoided. Fluid
temperature is too high when viscosity falls below the optimum value for the
system’s components. The temperature at which this occurs is dependent on the
viscosity grade of the fluid in the system and can be well below 180°F.
High
fluid temperature can be caused by anything that either reduces the system’s
capacity to dissipate heat or increases its heat load. Hydraulic systems
dissipate heat through the reservoir. Therefore, the reservoir fluid level
should be monitored and maintained at the correct level. Check that there are
no obstructions to airflow around the reservoir, such as a build up of dirt or
debris.
It is
important to inspect the heat exchanger and ensure that the core is not
blocked. The ability of the heat exchanger to dissipate heat is dependent on
the flow rate of both the hydraulic fluid and the cooling air or water circulating
through the exchanger. Therefore, check the performance of all cooling circuit
components and replace as necessary.
When
fluid moves from an area of high pressure to an area of low pressure without
performing useful work (pressure drop), heat is generated. This means that any
component that has abnormal internal leakage will increase the heat load on the
system. This could be anything from a cylinder that is leaking high-pressure
fluid past its piston seal, to an incorrectly adjusted relief valve. Identify
and change-out any heat-generating components.
Air
generates heat when compressed. This means that aeration increases the heat
load on the hydraulic system. As already explained, cavitation is the formation
of vapor cavities within the fluid. These cavities generate heat when
compressed. Like aeration, cavitation increases heat load. Therefore, inspect
the system for possible causes of aeration and cavitation.
In
addition to damaging seals and reducing the service life of the hydraulic
fluid, high fluid temperature can cause damage to system components through
inadequate lubrication as a result of excessive thinning of the oil film (low
viscosity). To prevent damage caused by high fluid temperature, a fluid
temperature alarm should be installed in the system and all high temperature
indications investigated and rectified immediately.
Slow Operation
A reduction in machine performance is often the first indication that there is
something wrong with a hydraulic system. This usually manifests itself in longer
cycle times or slow operation. It is important to remember that in a hydraulic
system, flow determines actuator speed and response. Therefore, a loss of speed
indicates a loss of flow.
Flow
can escape from a hydraulic circuit through external or internal leakage.
External leakage such as a burst hose is usually obvious and therefore easy to
find. Internal leakage can occur in the pump, valves or actuators, and unless
you are gifted with X-ray vision, is more difficult to isolate.
As
previously noted, where there is internal leakage there is a pressure drop, and
where there is a pressure drop heat is generated. This makes an infrared
thermometer a useful tool for identifying components with abnormal internal
leakage. However, temperature measurement is not always conclusive in isolating
internal leakage and in these cases the use of a hydraulic flow-tester will be
required.
The
influence of internal leakage on heat load means that slow operation and high
fluid temperature often appear together. This can be a vicious circle. When
fluid temperature increases, viscosity decreases. When viscosity decreases,
internal leakage increases. When internal leakage increases, heat load
increases, resulting in a further increase in fluid temperature and so the
cycle continues.
Proactively
monitoring noise, fluid temperature and cycle times is an effective way to
detect conditions that can lead to costly component failures and unscheduled
downtime of hydraulic equipment. In most cases, informed observation is all
that is required.
Credit
- http://www.machinerylubrication.com/Read/531/hydraulic-root-causes