A few weeks ago we published this tip

"A popular misconception about hydraulic cylinders is that if the piston seal is leaking, the cylinder can creep down. Fact is, if the piston seal is completely removed from a double-acting cylinder, the cylinder is completely filled with oil and the ports are plugged, the cylinder will hold its load indefinitely”.

Alan Wallace of Iron Ore Canada sent us the following feedback:

"The truth (IMHO) is that the piston seal is the only thing that causes no flow between the two sides of the piston (when the ports are closed) and hence holds the load in place. Without the piston and 0 flow past the piston seals the cylinder will creep."

The author Brendan Casey replied:

I guess this is like believing the world is flat and then being told its round. You are not the only one that struggled with this concept. But let there be no 'error' here...

FACT: A double-acting cylinder will hold its load indefinitely without a piston seal.

When you calculate the difference in oil volumes either side of the piston the concept becomes clear. You can't fit a quart of liquid into a pint cup. There are however a couple of exceptions. The first is where a load is HANGING on a double-acting cylinder. With the cylinder fully retracted, there is enough volume on the piston side to accept all the oil from the rod side. So if the piston seal is by-passing and the load is heavy enough to overcome the vacuum that will develop, the cylinder will creep. The second exception is a double-ended cylinder, due to the equal volumes on both sides of the piston.

If you are still not convinced, I have a video available on CD-ROM that demonstrates this concept by removing a backhoe boom cylinder from a machine, removing the piston seal from the cylinder and then re-installing the cylinder on the machine. As we now know, the boom defies gravity and remains suspended!
http://www.hydraulicsupermarket.com/hydraulic-fundamentals.html (Editors note: Ouch -- a Tip reply that pitches a video - not Reliabilityweb.com's style")

Anyway - we want to get to the bottom of this and ask for your help in setting the record straight.

Thanks

Terry O

Alan Wallace is wrong about about the piston holding the load up on top of a hydraulic cylinder. It used to be common on manure loaders for real old farm tractors to use single acting cylinders with no piston on the rod. The only seals were the rod seals. On those cylinders the piston rods would have a stop attached to keep the end from traveling past the rod seals.

Alan should send his hat back. Even contemporary hydraulic elevator cylinders do not have piston seals, just a slotted brass or bronze bushing to keep the end of the rod from scoring the ID of the barrel.

quote:

A popular misconception about hydraulic cylinders is that if the piston seal is leaking, the cylinder can creep down

quote:

Even contemporary hydraulic elevator cylinders do not have piston seals

I find these statements dangerous. It are over simplified statements based on looking at components isolated, instead of the hydraulic system in its totally.

I would not stand under the derrick of a drilling rig with leaking mast raising cylinders, or working on the boom of an excavator, without it being properly denergized and immobilized.

I know that we are going to lean manufactury etc.., but leaving piston seals out in cylinders

This message has been edited. Last edited by: svanels,

Lean manufacturing has nothing to do with it – standard practices and knowledge based design does.

Hydraulic elevators use rams, not double acting cylinders.

Someone that does not understand the difference between a ram and double acting cylinder has no business either designing a system using one or heaven help us, working on a system with one, and, if you do not understand intensification and the extreme pressures that can be imposed on the rod seal when the piston seal of a double acting cylinder is damaged or compromised, please, just step away from the machine.

Consider a 3000 psi 5” bore double acting cylinder with a 2” rod. At 3000 psi it can extend with a force of 58,900 lbs. But, if both ports are blocked and the piston seals are bad and 58,900 lbs of force is applied to the 2” rod, there is a potential of 18,750 psi in the cylinder (58,900 lbs / 3.14 sq.in.). Naturally, it will never get that high, because something will blow well before then, usually the rod gland or rod seal.

But, if the cylinder were rated for 20,000 psi, it would resist the weight, not drift down, but move only as much at necessary to compress the fluid. Ta-Dahhhhhh

I fell over the phrase:
a popular misconception

Hydraulic design and maintenance is not for the mass, crowd out there, it is very specialized work.
But with these simplified statements, some idiot out there will put his life and others life in danger because he read something on the internet.

Are there more lunatic or idiot internet surfers than those who are intelligent users of internet info?

If I am looking at the hits on google for chupacabra the "goat sucking vampire" who recently moved to russia ...

I have serious doubts about the intelligence of the readers and writers who are surfing on the Information Higway

And I did not started about the hoaxes, which are circulated by millions of people in the genuine believe that they are helping the world..

I still remember the panic when this one was sent
Internet cleanup day

http://www.geocities.com/heartland/plains/6271/inter030.html

This message has been edited. Last edited by: svanels,