The pump is the heart of a pressure wash system. Basically the
pump pushes water using pistons or plungers, inside a
cylinder. When the water is pushed out of the cylinder a small
valve closes, not allowing the water to come back into the
cylinder. Instead, new water is brought into the cylinder,
past a similar valve which opens when the piston pushes the
pressurized water out. This process is repeated many times per
minute, creating flow.
Since water can not be compressed and pumps are positive
displacement the water needs a place to go. The pump creates
the flow. The size of the orifice (nozzle) the flow is being pushed through
creates the pressure.
How much flow is a result of displacement and pump speed.
How much pressure depends on orifice size. This depends on
enough horsepower to
push the flow through the orifice, without slowing the pump
There are two types of pumps used in the pressure wash industry:
The rotary pump works by turning an angled plate which
has pistons held against it by springs. As the plate turns it
moves the pistons, which in turn push water through a valve. The
pistons are held tight to the plate by springs. Think of it as a
cam shaft in an engine, which open valves as it turns.
PISTON / PLUNGER
Piston and plunger pumps work in the same way to
produce flow. The sealing system is the major difference. In
a piston pump the sealing system (rings, packing etc) is attached to
the piston and moves with it during its stroke. The sealing system for
the plunger pump is stationary and the plunger moves through it during
The main difference to a rotary pump is
how the pistons are moved. While the rotary pump turns an angled
plate to move the pistons, the piston/plunger pump pistons are turned
by a rotating shaft with connecting rods attached to pistons.
There are no springs to pull the pistons back since the shaft does
both jobs. Think of it as the crankshaft in an engine, which
moves pistons up and down as it turns.
Regardless of the pump type, all pumps have two
separate sections. The water side and the oil
The oil side lubricates the metal parts as the pump
turns. There is not much to maintain here but from time to
you will want to check the oil level in the pump. Occasionally
the oil will need to be changed.
The water side is flooded with water to be pumped,
which also helps the cool the pump. The valves and seals in the
water side require periodic replacement to maintain pump performance.
Pump speed is an important factor when deciding on
your pressure washer. Because rotary pumps are made for the
lower-end big box seller, using gas engines, virtually all rotary
pumps run at 3400 RPM.
Plunger and piston pumps are, for the most part, made
for the commercial and industrial markets. Pump speeds can vary from a
few hundred RPM to 3400 RPM. Slower turning pumps will last
longer. However, they are larger, and more expensive.
Slower turning pumps can be driven by any speed motor through a belt
system or gear box
A positive displacement pump needs a place to push the
flow it is producing. If pressurized water flow from a pump was
never turned off the pump would not need a method of controlling this
flow But, this is very rare circumstance.
All pressure washers need a way to deal with the water
flow, considering the flow is turned on and off many times by the
operator opening and closing the trigger
gun. The unloader
valve solves the problem by diverting water through a bypass when
the flow is stopped.