What happens to a cylinder with one blocked connection ? 1

[Hamid Karevan]

Dear Mechanical Engineeres

I had a discussion with some colleagues about what happens to the hydraulic cylinder in the shown attached photo.

I believe that as long as there is no internal leakage, nothing happens and the piston remains fixed. Others believe that a vacuum will happen in the upper side of the cylinder and the piston starts moving downward.

May I know your opinion please?

 

[HPost]

The load will drop...

The force on the rod will pull on the cylinder and the cylinder will extend until equilibrium is established. The amount of movement is dependent on the force applied.

The cylinder will stop dropping when the negative pressure acting on the full bore is equal to the force pulling on the cylinder.

If the force is high enough, the cylinder will extend and the pressure in the head side of the cylinder will drop below the vapour pressure of the fluid. The fluid will boil. Eventually, the hugely negative pressure will pull air in through the seals and the system will achieve equilibrium.

It will slow down, but will keep dropping until the cylinder is fully extended.

 

[hydtools]

Negative pressure cannot 'pull'. The atmospheric pressure acting on the rod will push.

Ted

 

[HPost]

I know that Ted...

But as part of the equation of motion, the effect is the same. Don't call it "pull" on the piston, just call it push on the load. 1 BAR absolute over the area of the load verses zero BAR absolute at the piston head.

It won't "pull" but it will be a negative force in the equation.

 

[Hamid Karevan]

Thank you.

What if we assume that the cylinder is perfectly sealed and there is neither internal leakage nor probability of entrance of air. Besides, both sides of the cylinder are full of oil.

May I know please the formula by which we can calculate how much the displacement of rod is?

 

[HPost]

The cylinder will not be perfectly sealed. Air will get in at some point. Seals are designed to keep high pressure in, where the internal pressure is lower than the external pressure, the seals will allow air to get it.

If we consider this as a thought experiment and suppose that there was no way for air to get in, then the load will drop until the upward force is equal to the downward force.

Downward force is mass x gravity

Upward force is the atmospheric pressure acting on the annulus of the cylinder head and also what acts on the end on of the rod. When they are balanced the load will stop.

The distance of the drop is derived from the total mass and the volume of the cylinder. Use Boyle's law to calculate the pressure change in the head side of the cylinder.

Small mass and big cylinder will hardly move because of the large area of the cylinder.

Large mass and small cylinder will drop loads. However, the maximum relative pressure delta can only be max of 101.325kPa. The cylinder will always bottom out as there would be nothing to stop it.

 

[jbthiel]

There is also the assumption that there is no air in the oil, but that is rarely the case. How much air is very difficult to predict as it depends on the oil, temperature, amount of time it has to let air release, etc. ISZ