6.1 Hydraulic systems
Hydraulic systems work in a similar way to pneumatic systems,
but they are different in three respects:
- The use of an non-compressible fluid as opposed to a compressible
gas means that much higher pressure can be produced within the
system. The increased pressure also means that the equipment is
more robust and difficult to damage;
- Instead of working against atmospheric pressure, hydraulic
fluid is conserved, and so hydraulic cylinders must be pumped
in both directions. This means that the control valves are more
- The use of a more viscose and more dense fluid as opposed
to a gas means that hydraulic systems move slower than pneumatics.
6.2 Hydraulic pumps
The source of power in a hydraulics system is the pump. This takes
hydraulic fluid from the sump/tank and raises it to a higher pressure.
The fluid then circulates through the system, much in the same
way as pneumatic systems.
Like any other pump the hydraulic pump has an inlet pipe - this
sucks hydraulic fluid in. The pistons or impellers in the pump
then raise the pressure of this fluid, and it emerges at the fluid
outlet (this is often the smaller of the pipes connected to the
pump, and it is easily identifiable as the hottest if the pump
is running). Caution must be exercised if you tamper with an operational
pump - as with pneumatic systems if tampered with the high-pressure
output from the pump could spray hot fluid over you, or burst/fail
explosively. It is often easier, if the source of fluid is not
accessible, to deal with the hydraulic control systems rather
than take on the pump itself.
As noted above, because of the higher pressures used in hydraulic
systems, the components of that system tend to be incredibly well
built. This is especially so for the pump. To disable a hydraulic
pump is quite difficult - you must either remove its energy source
(either an electric motor, electrical connection, or an engine),
or you must try and take it apart. The latter can be very difficult.
A third option would be to spike the fluid with abrasive, but
this might not have a very quick effect.
Typical hydraulic pumps (fig. 13/14)
Manual valve and manifold (figure 15) and Electrical valve
and manifold (figure 16)
6.3 Valves and switches
Different types of hydraulic system have different types of control
mechanism. At its simplest a system might comprise a sump, a pump,
a ram (piston), and a 'changeover' switch which pumps fluid alternately
to each end of the ram to move it back and forth. More complex
systems may have one or more pumps, serving a large number of
rams or hydraulic motors (similar to air motors).
How the control systems are actuated can also differ; some are
mechanical - you pull a lever to activate the valve; some are
electrical - you press a button to activate the valve; and some
are electromechanical - that is an electrical system might activate
a road or cable which then pulls a lever which activates the valve.
How you tackle these different control systems will depend upon
the target. Mechanical systems can be dismantled. Breaking them
is very difficult because they are normally made from cast metal
bolted together with high-tensile steel bolts. Electrical systems
are simpler - the electrical connections are vulnerable, and the
control panel itself may be easily modified. Normally the electrical
part of the valve is in a separate housing on the top or side
of the valve, and is often less sturdily built - in these cases
more direct action against the valve is possible.
Electromechanical systems are difficult to explain, because they
differ so widely. Tackling them is really a combination of the
two approaches outlined above - you tackle both the electrical
sections of the system and the mechanical parts.
Whatever the switching system, all valves will be connected to
some sort of 'manifold', where oil under pressure is distributed
to different parts of the hydraulic system. Often the manifold
is part of/located beneath the main switches. Damaging the manifold,
or removing the valves from it, is therefore a sure way of disabling
the whole system.
Construction and designs of hydraulic hoses and hydraulic
connectors (figures 17-20)
One of the needs in the hydraulic system is to move energy - the
oil under pressure - to where it is needed. This is done using
two types of pipe:
- Rigid Pipes: These are generally made of metal, with a diameter
and thickness representative of the quantities and pressures of
oil they carry;
- Flexible pipes (hoses): These are made of rubber composite
materials - and can be armoured inside to make then puncture of
There are two basic options for sabotage - you either cut or drill.
Cutting can be difficult, particularly with large heavy metal
pipes. Likewise drilling can be also difficult on reinforced/armoured
With hoses there is another option though - you can unscrew them.
Generally hoses have metal screw connectors on each end - most
of these can be easily unscrewed if you have a spanner big enough
for the job. Of course just removing a hose does not cause a lot
of damage, so often the most effective method is to remove the
hose (carefully plugging the holes to stop too much fluid leaking
out), fill the hose with an abrasive such as sand, iron filings
or small steel screws, and then replace the hose.
Cutting larger pipes and hoses can be time consuming - although
small hoses may cut easily with bolt cutters, or good side cutters.
In these instances you might consider drilling the pipe/hose with
the small (2mm) drill - the end result is that the owner of the
equipment will have to replace the part, which saves you the job
of removing it yourself. Be aware that metal pipes can be welded
up, so if you do drill metal pipes make a few holes, or better
still, drill the hole right on the side of a screw fitting or
joint of some kind (this makes welding more difficult as it affects
the use of the screw fitting).
Typical hydraulic ram (figure 21)
Hydraulic rams do the actual 'work' in hydraulic systems - thus
because they must withstand great stress they tend to be extremely
well built, much more so than air cylinders.
In general I would say that it's not worth taking on a hydraulic
ram if you are short or time - it's much quicker to just go for
the pump, sump or pipes. However, it you do have time you could
try drilling through the ram casing/tube, or trying to damage/remove
the end bush. Another option is to remove the flexible hoses and
swap them around, or remove the pipe/hose, inject abrasive materials
into the ram, and then replace the pipe.
Don't forget also Newton's Third Law of Motion - 'for every
action there is an equal and opposite reaction'. The hydraulic
ram must be fixed to something in order to anchor it. If you can
undo these fixings so that the ram is not anchored to anything,
it doesn't do its job.
6.6 Sump and fluid
Perhaps the greatest flaw of the hydraulic system is its need
for oil - but unfortunately oil is relatively cheap so depriving
the system of oil has no great long term effect. Also, removing
oil can have serious environmental consequences. So we have to
work in ways which eliminate/minimise the spill of oil, but still
render the system unusable. This essentially means finding ways
to contaminate the oil to make it useless.
There are three straightforward methods...
- Where time is short, just fill the sump with material - nuts,
bolts, sand, dirt - anything that will clog the system (this is
my preference in any case). Water doesn't have any particular
damaging effect on the system, but it will prevent the system
- If you can remove the oil filter (if there is one) to spike
the filter papers, you can put abrasive substances like polishing/grinding
powder, or sand, into the sump. Over time this will cause damage
throughout the system.
- If you don't mind getting messy, and you can get access to
the outlet from the sump, pack clay or some plastic substance
into the pipe - as much as you can. If you are lucky the pump
won't be able to suck oil, and if it does, it will clog the oil
filter or the pump.
The only thing to beware is that when fiddling with the sump,
try not to splash too much fluid on your skin, or more crucially,
in your eyes. Some people have a sensitivity to the substances
in the hydraulic fluid which brings them out in a rash, and getting
oil in your eyes can cause serious damage.
6.7 Basic sabotage of hydraulic systems
How you tackle a system depends on what it is, where it is, and
most importantly how much time you have...
- On all types of system go for the sump first, because spiking
that will disable the whole system. Also, in terms of damage,
the sump will have to be removed and cleaned, with perhaps a few
other components, which will cost a lot of time and money.
- Next go for the control systems/electrics - this will equally
stop the whole system, and will cost money to fix. The problem
comes with mobile equipment because increasingly the cabs of vehicles
are alarmed. Stationary equipment is much more easily damaged
in this respect, if you can get access to the controls.
- Hoses next, if they are the sort that you can cut or drill
easily. As noted earlier, don't just thing about cutting - unscrewing
hoses, particularly on things like JCBs where the hoses are well
made, is much quicker.
- Rigid pipes last because they can sometimes take time to drill,
and you will have the least effect.
Also you must think of 'quality'. Take a few minutes to study
the system and work at which hoses connect to what, or which control
systems actuate what ram. That way you can tackle the most crucial
parts of the system.
Another thing to consider is whether the system is active or not,
and whether, even if the system is turned on, the pipes/hoses
are under pressure due to gravity. An associate was mildly hurt
when cutting the hose on a JCB - the jib of the JCB was raised,
and the oil in the pipe held the jib up against gravity. When
the hose was cut, the jib came down - very quickly!
Likewise, cutting a pipe/hose in an active system can be very
dangerous, if not because of the effect on the system, then because
of the hot oil under pressure it sprays out in every direction.
In volume I there is an illustration of how to drill pipes under
pressure - this should only be attempted on small pipes you can
be reasonable sure carry little pressure.