Maximum Piston speed

[subsearobot]

I realize this is a question with numerous constraints, but here goes.

Is there a maximum piston speed above which I need to be concerned? If there is, will looking at the PV characteristics of the piston seal and guide bushings be an adequate design check?

At present, I am designing around a Bimba 500 psi oil cylinder. I called Bimba tech support, and the gentleman stated that 8(!) in/sec would be a maximum *fluid* speed in my lines to avoid cavitation; he would say no more. When pressed, he stated that they recommend a maximum *piston* speed of ~20 in/sec for their pneumatics. I would guess that being lubricated with oil, hydraulics could sustain much higher speeds. true?

I am hoping to move the piston at up to 30in/sec. Fluid temperatures are anticipated to be low. loads will be relatively low, with the exception of ~.1 sec accelerations, where the ram will see full system pressure along with translation of the piston.

The piston will be in sea water, so the seals simply need to last longer than the external packaging, which has unknown aluminum alloy end-caps coupled to SS304 body. galvanic corrosion is active.

 

[hydtools]

The speed is limited by the seal capability. The paramters include pressure and velocity as well as temperature and seal material.
See here

http://www.marcorubber.com/ucups.htm

for examples.
I have run pistons 35 ft/sec through Disogrin polyurethane seals.

Ted

 

[hydromech]

The person at Bimba tech support is talking rubbish.

8"/sec fluid velocity will not cause cavitation. 8000"/sec might, but then that will depend on the fluid vapour pressure and the line pressure. Most modern hydraulic systems are run with fluid velocity at around 50 ft/sec, and the risk is turbulent flow which pushes up the pressure losses.

It is linear velocity of the rod and piston seals that will ultimately dictate the speed of the cylinder. Making the ports big enough to fill the cylinder will push the cylinder size up. Pushing the cylinder size up will increase the contact area of the seals and thus reduce the allowable speed.

If the cylinder is not load holding, ask about low friction seals.

Adrian

 

[PNachtwey]

8"/sec fluid velocity will not cause cavitation. 8000"/sec might, but then that will depend on the fluid vapour pressure and the line pressure.

It isn't the speed that causes cavitation it is the the sudden decelerations when a over haul load is stopped by quickly reducing flow.

This shouldn't be a problem in this example since the hammer is stopped when it impacts the anvil, not by cutting flow flow to the cap or driving end of the cylinder.

I am hoping to move the piston at up to 30in/sec. Fluid temperatures are anticipated to be low. loads will be relatively low, with the exception of ~.1 sec accelerations, where the ram will see full system pressure along with translation of the piston.

Accelerating to 30 inches per in 0.1 second is an acceleration rate of 300 inches per second^2 which is about 0.77g which is less than the acceleration due to gravity.
The acceleration and speed should be easy to achieve with gravity alone.



Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

 

[PNachtwey]

Oops, got threads mixed up, but moving at 30 inches per second is done all the time. Accelerating at 300 inches per second^2 will depend on the net force and mass being accelerated.

Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

 

[EdDanzer]

The cylinder problem you have is not in the acceleration or the maximum speed it is the stopping at the end of the cylinder movement. The inertia of the piston and seal will cause part failure, the piston will come loose on the rod or the piston seal will shear into the piston to barrel gap.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[subsearobot]

Yes, as soon as the Bimba tech said (and repeated) the 8"/sec fluid speed statement, I thought, "I need a more knowledgeable opinion. Eng-Tips, here I come- again!"

Ed,
I am designing the travel limits to be the cylinder endcaps. (I am using the full stroke of the piston for each cycle). Will this help with the piston-rod separation issue? should I consider cushioned endstops?

thanks once again, you guys are a great resource. I realy appreciate the time you put into considering my questions.

 

[hydtools]

Consider cushion stops.

Ted

 

[EdDanzer]

Do not use any hydraulic cylinder as a stop if there is much inertia in the system unless you want to spend a lot on development and repair. We are currently working on a 5 ½” bore x 3 ½” rod cylinder that has seen over 250,000 lbs of extension force and enough side loads to compress the 2” wide wear ring length over .005” causing metal to metal contact.

It is ok to cushion the non anvil end of the cylinder as the hammer contacts an end stop. The anvil end of stroke should have extra travel. One of the anchor ends should be shock isolated.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com