What is the difference between an air and a hydraulic cylinder?

[Barry Wolk]

What is the difference between an air cylinder and a hydraulic cylinder? None of the air cylinders seem to exceed 12" while hydraulic cylinders seem to be readily available in much longer lengths.

I would like to use air cylinders to extend a bumper rearward a minimum of 20" which would require a 28" hydraulic cylinder. Can I use a hydraulic cylinder as an air cylinder?

[Bhigdog]

Yes, you can. If you are going to actuate it a zillion times you should lube it with "air oil" now and then. BTW McMaster-Carr lists air cylinders with a 20" stroke in bore sizes from 1.25" to 4.25"

[Guest Silverghost]

Hydraulic Cylinders usually are buuit to handle much higher pressures 500psi +++ They also have packings and seals that can stand up to & are lubed by Hydraulic fluid!!!

Most Air cylinders can only handle low air pressures and can often run dry...without any oil!!! (Teflon seals & Packings). Others use a mist of Air tool Oil.

What in the world are you using these for???

Are you building a "Low Rider"???

[Barry Wolk]

My "Toybox" trailer has an unprotected rounded rear end. The design calls for the lower half of the clam-shell door to drop to the ground to clear the spare tire and ramps.

The cylinders would be mounted to the frame rails and attached directly to a rear bumper that would retract out of the way while loading/unloading. When fully extended they would act as shock absorbers.

[Bhigdog]

To be effective as absorbers do your math as to the kinitic energy (e=mv2) you wish to absorb vs the piston size x pressure. You may (or may not) be surprised.....Bob

[Barry Wolk]

Please explain further.

[Bill Stoneberg]

Barry, my physics in college has escaped me, but I believe what he is trying so say, is that unless you run a fairly large cylinder at a fairly high pressure, you wont be able to absorb the hit put on to your bumper by a car.

[Mark Shaw]

Barry,

Pressure times area equals force; The pressure limit on most pneumatic systems is 150 PSIG, therefore,it would take lots more piston area to push a heavy bumper than a hydraulic ram operating at 500 PSIG.

Regarding seals; For our larger pneumatic cylinders, we hard chrome plate the ID of steel cylinders and use molydisulfide grease to lube the piston seals and noryl (nylon dirivitive) piston wear rings. (Low temperature grease is used in Alaska and Canada.) To save weight, we also offer fiberglass reinforced plastic cylinders (this if for AWWA standards typically used in municipal applications).

These large cylinders are part of a scotch yoke type (heavy duty) actuator used for high torque valve automation requirements. Many of our competitors do not chrome plate the ID of the cylinders and experience pitting corrosion due to condensation within the cylinder bore. This is caused by the pressure cycling of humid air.

This may be too much info, but you asked.

Mark Shaw

[Bhigdog]

To determine if a given size cylinder will absorb the energy of a given impact you must calculate the energy imparted by the impacting object. The formula is energy = mass times the velocity of the object squared. If I do the math correctly, (disclaimer, I've just had a nice big glass of port) that means a 4000 pound object (car) moving at 5 mph (7.22 fps) will impact with about 209,000 ft. pounds of energy. Now all you have to do is calculate what amount of energy say a 4" cylinder charged to 200# of air will absorb keeping in mind that as the cylinder is compressed the trapped pressure will rise, thusly resisting further compression, and the impacting object will be slowing thus losing energy exponentially. Ideally the cylinder will completly absorb the imparted energy just before bottoming out or rupturing from the pressure rise. How you calculate the capacity of the cylinder is way beyond me but it is done routinely in many engineering problems ranging from a simple shock absorber to the space shuttle slamming onto the runway at 500 fpm sink rate and 200,000 lbs weight. My guess is there is a formula somewhere where the numbers are simply plugged in and the capacity of given cylinder/pressure is spit out. Any rocket engineers listening?...........Bob.

[Barry Wolk]

I'm not so much protecting the vehicle from vehicular impact as much as I am trying to protect the back end of this beast while backing up.

Upon further research I found a 24" x 1.5" air cylinders in the McMasters catalogue.

Thanks.

[Guest bkazmer]

you're going to be a bit off on the energy -it's 1/2 m v2. For the cylinder to work, it has to the absorb the energy (accelerate the piston) at an appropriate rate too. Energy/time = power.

[Guest Silverghost]

Barry: It would be better to instal a CCD back-up camera & LCD monitor if his is the REAL issue!

[Barry Wolk]

The wiring for the camera is already installed. Why would you doubt my statement?

[Friartuck]

To answer this primary question, I think air cylinders are limited in length because the amount of volume increases when fully actuated. I don't think you're able to get the same force of an air cylinder when fully extended as when fully retracted. Hydraulic's do provide even pressures. Remember, air compresses and fluids/oils don't.

As far as the camera idea, I discovered a 5.5 inch LCD screen version that has reverse video feature which means it's now like a rear view mirror: Objects on the left side of the screen are in fact on the left side of the trailer.

Chris

[Barry Wolk]

Terrific idea! What's the brand.

I got the ramp system working today. If you send me your email address I can send a QT movie of it in operation.

[Barry Wolk]

Here's the link.

Ramp Test 3-3-06

[Friartuck]

The TFT Color LCD Monitor is from ACCELEVision. Model is LCDP56 which means it's 5.6 inch across. Seems Accelevision makes the monitors for minivans, etc. It happens this model has the reverse video feature. I think other larger models are also available. This method could also be used for a street rod with limited rear view mirror visibility (i.e. an electronic rear view mirror).

Chris