Better Farming February 2017

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76 The Business of Ontario Agriculture Better Farming February 2017
HOWIT WORKS
Many years ago, it was
common practice to
examine hydraulic fluids for
unusual colour changes. We rubbed
these fluids between our fingers to
check for that lubricity feel and then
checked the magnet on the drain plug
to see if metal particles had collected
there. Those were the days when we
used gear pumps that had high-
clearance values. These pumps only
provided fluid power for lift rams or
three-point hitches.
Those days are long gone. We now
have piston-type pumps with
variable output that must provide
clean hydraulic fluid continuously to
hold loads or provide variable flow
rates to drive hydraulic motors.
The clearances between the
pistons and cylinder bodies are
measured in microns (µm) not the
thousands of an inch that some of us
used many years ago. Let us put
those figures in perspective.
Here is an example. Have you
checked the thickness of a page of
the Better Farming magazine? Not
likely, so I did it for you. My mi-
crometer shows that the page
thickness is 0.0025 inches. A mil is
one thousandth of an inch.
When 0.0025 inches is multiplied
by 1,000 that gives me the thickness
in mils which becomes 2.5 mils for
the page. Each mil is 25.4 µm. So, the
page is 63.5 µm thick. You might
argue that you can see the edge, but
trust me if that edge was in particles
you cannot see particles that are
smaller than 100 microns in diame-
ter.
Many newer piston-type hydraulic
pumps are now designed with the
tolerances most of us associated with
diesel-injection pumps. The pistons
must be selectively hand-fitted in a
constant temperature fluid bath. To
avoid contamination damage, or
jamming, the fluid must be kept free
of particles as small as 10 µm, with
30 µm particles being the upper
limit. Please keep in mind these
small particles are all well into the
non-visible range.
Piston pumps
A typical variable displacement
piston pump is shown in the accom-
panying photo. Note there are no
rings or seals on these pistons. A
small amount of clean hydraulic fluid
will bypass these pistons to lubricate
and cool them. A central orifice in
each piston allows fluid to pass down
and lubricate the foot pads that run
against a variable angle foot plate that
is tilted or angled to vary the pump-
ing rate.
Variable rate pumps are required
Specialized hydraulic filters can protect
precise mechanical components
To ensure the optimal performance of piston pumps, the hydraulic fluid needs to be kept free from
contamination. Consider the who, when and how of maintaining these systems.
by RALPHWINFIELD
These are the inner workings of a typical, continuously variable
output hydraulic pump. Leakage/lubrication is dependent
entirely on piston clearance.