Machinery Lubrication May June 2011 - Welcome to ECN Digital ...

HYDRAULICS AT WORK
BRENDAN CASEY
The increasing complexity of today’s hydraulic systems means
there is a lot to know about proactive and predictive maintenance
in order for optimum reliability outcomes to be achieved. It’s
my observation though, that this increasingly specialized knowledge
is grossly undervalued in the marketplace. And despite my
loud and regular protests to the contrary, it seems the Holy Grail of
all hydraulics-related skills is and always has been: fault-finding.
Yes, whether you and I like it or not, the crack trouble-shooter,
the man who can cast his eye over the most complex of hydraulic
systems and quickly zero-in on the fault without breaking sweat, is
‘top-gun’ in the hydraulics world. He’s a soldier of fortune.
Admired by some; envied by the rest of us.
But there’s no need to be awestruck. Hydraulic troubleshooting
can be challenging at times, but in the majority of cases it’s usually
straightforward. Consider the fundamental structure of any
hydraulic system for a moment: A positive displacement pump (or
pumps) producing flow which is directed to an actuator (cylinder
or motor) to move a load.
When pump flow meets actuator load, the resistance of the load and
the positive displacement nature of the pump means pressure is developed.
Once pressure is developed there are two possible
outcomes—both of which usually occur simultaneously to some degree:
1. The load moves (work is done); and
2. Flow escapes from the pressurized circuit to either the tank or
the atmosphere (power is lost).
But if the amount of power lost becomes noticeable, i.e. the
movement of the load becomes unacceptably slow—when
compared to its ‘normal’ speed, or pressure is noticeably lower
(due to the volume of flow being lost from the circuit) then by definition,
we have a problem.
So when you think about it, in the majority of troubleshooting
situations, the task is to find the missing flow, or more correctly,
the component which is allowing an abnormal amount of flow to
escape from the circuit—without doing any work. In other words:
it’s a flow-chasing exercise.
And here’s a list of the tools—and some of the cunning techniques,
available to a savvy flow-chaser:
Stop Watch
The speed of an actuator (or actuators) is determined by flow.
And so accurately timing how long it takes for an actuator to
complete its cycle—combined with a logical process of elimination,
can be used to isolate flow loss.
As an example, consider a hydraulic excavator with four mainpumps.
The operator complains that the machine is slow. While it’s
not certain to be one of the pumps, it’s usually not a bad place to
start. The pumps can be ‘tested’ individually (and relatively quickly)
by recording the cycle time for each, and comparing the results.
To do it, the attachment (stick and bucket) is fully extended and
the bucket is placed on the ground. The displacement controls of
three of the four pumps are isolated (electrically or hydraulically) -
so only the pump being clocked produces flow.
With the engine at high idle and starting with the bucket on the
ground, the operator fully summons the boom up on the joy stick
and the time it takes to fully extend the boom cylinders is recorded.
This process is repeated for the other three pumps and their times
recorded. Let’s say the results look like this:
Pump 1: 29 seconds
Pump 2: 36 seconds
Pump 3: 29 seconds
Pump 4: 30 seconds
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The Holy Grail of Hydraulics
Hydraulic Trouble Shooting Usually Straightforward
2 May - June 2011 www.machinerylubrication.com Machinery Lubrication