In my series of weekly hydraulics bulletins that go out to anyone who opts in to receive them, there are two messages that always provoke a response.
The first of these two bulletins sets out to explain how a double-acting cylinder becomes a displacement cylinder when the piston seal leaks and the load is acting to retract the piston rod. This message includes a link to a simulation video for the reader to watch and hopefully, think about.
Then a week later, a follow up message goes out, asking the recipient if they ‘got it’ – not the message itself, but the CONCEPT involved.
This follow up message provokes all sorts of replies. Most telling me I’m wrong, I don’t know what I’m talking about, that the simulation model doesn’t reflect the real situation, the piston rod must retract if the piston seal leaks and so on.
These responses from those who don’t ‘get it’ has prompted me to come up with what I think is the ultimate mini-test around the physics involved when the piston seal leaks in a double acting cylinder.
And I guarantee that if you give this little test to a group of technicians or mechanics, the majority would be tripped up by it.
So here’s the scenario:
You’ve been called in to troubleshoot a hydraulic problem on a bulldozer. The complaint from the operator is when the dozer blade is raised, it drifts down. When you get to site, the operator demonstrates the issue. Sure enough, with the blade raised off the ground, cylinder drift is quite noticeable.
Then the operator shows you something else. He lowers the blade to the ground and proceeds to lift the front of the machine off the ground with the blade. With the front of the machine lifted off the ground, there is no drift at all.
How would you explain this? And what would you do to fix it?
Some might say I’ve laid a perfect trap here for the uninformed to walk straight into. And they’d be right. But so there is absolutely no confusion about the correct answer, I’ve made this 5-minute simulation video that explains the situation.
Totally agree that the pressure will equalized from blind to rod end and lock it in position, seen it several times. Two simple ball valves if you can install them will totally prove it. Lift the blade the close both valves and watch what will take place.