Pneumatic vs. Hydraulic Cylinders

Thanks that was well explained. It means I need hydraulics here because I need to make a working model.

GA to Lyn!

The other problem you have to consider is what happens if you get a leak or rupture. The (almost) incompressible hydraulic fluid squirts all over you and ruins your clothes. The very compressible air expands explosively, possibly injuring you, certainly damaging the equipment.

Air pressure is normally much lower than hydraulic, and thus would require larger diameter cylinders. As Lyn noted, positional control is poor with compressible fluids.

When you get in to high pressure hydraulics are very much safer than air.

With hydraulics what about Hydraulic Injection if something fails?????

Air pressure is generally not going to be that high. Most people don't have access to a dive type comperssor capable of hugh air pressure. Hydraulic pressure would have to be more. To be able to get it to do much i am sure you would need upwards of a few hundred psi and getting into the thousands. Then there is sheer cost air is cheap.. Minture hydraulics i have seen online for a model excavator type machine is in the US$3000-5000 mark per machine.

I am sure the original poster is not wanting as complex as that.

as the description of lyn ,thats very right and secondly with hydraulic you can control the piston at

any step but in the pneumatic you have to provide the mechanical stop to do this due

to higher density of hydraulic oil .the under load stop will be more mature then air because

of the high density counter(bearer side) secondly you have to use very bigger dia of cylinder ,comparing to hydraulics .

If all your doing is producing a working model, I would go to an air oil system. Oil gives you the control needed and air gives you the reduced pressure your might be seeking. Flow control of air/oil systems work very smoothly and can if sized right move a proportional amount of load. One old use of this technology is the lifts in garages from the 1960's. You should be able to google air/oil systems and pickup more info on these types on units.

Jay:

I don't know what your budget is, but I would go with hydraulic all the way. Could be pricier. It doesn't sound like you want to "engineer" a complete system, so I think that you can build a very versatile model or simple machine with hydraulics. I also think that hydraulic systems are easier to design than pneumatics (my opinion via experiences). You need to know the desired plane(s) of movement, functional load requirements, and your overall mounting strategy. After you have the basics down on paper, you can obtain the parts you need (cylinders, pump, electric/diesel motor to turn the pump, rated hoses, mounting hardware, and valve system). For ideas on a basic schematic on how a hydraulic system can work, use a "newer bulldozer" as an example. The blade raises up and down; the blade tilts left to right (and visa versa); the blade pans front to back (and visa versa). These 3 planes of movement can be used one at a time or in combination while moving dirt. The system provides a very effective functionality of the blade all from one hydraulic pump and multiple function valve system. For ideas, check out the link below. They have anything you would need for your project. You can also check out the pneumatic section.

http://www.northerntool.com/shop/tools/category_hydraulics

Budget is not an issue. I plan to go with hydraulics after what I have read here. I want to make a working model of a bucket loader, like the one in construction sites, but only the arm - it needs to lift load too.

Basically, consider this JCB

I need to make a bucket loader similar to that (or atleast try to). What I am thinking now is that I will use 4 hydraulic cylinders: one at the base for the left-right movement of the entire arm, the rest three for the movements of the arm in a give plane. I am using double-sided hydraulic pistons. I wish to use solenoid valves to control these cylinders movement. For four cylinders, will two solenoid valves be sufficient to control or will I need four? What specifications should they be? Then I would require a manifold to control the entire thing. Have I got it right? Looking for more comments.

Jay:

I would use a manifold and valve system, not solenoids if you can help it. I am not sure if you can get solenoids that can operate appropriately to give you smooth operation with multiple cylinders at the same time. With solenoids, you will probably have one big choppy-jerky opening and closing of the hydraulic flow gates. You will need to operate levers manually; some opening, some closing, (forward and reverse) to make the cylinders put the bucket in the right position, and you will have to vary the degree of "openess" for the valves to make the machine behave as you want. In other words, you won't want to run the hydraulic flow "wide open" with every movement.... hydraulics can be finessed easily and smoothly. In order to lift a load and move it, you need the cylinders to respond. That means controllable power from the pump and valve system. I like solenoids for pneumatic systems because in most cases, I am not looking for control....just open and closed states.

"You will need to operate levers manually; some opening, some closing, (forward and reverse) to make the cylinders put the bucket in the right position, and you will have to vary the degree of "openess" for the valves to make the machine behave as you want."

Thanks Krom. So basically solenoid valves can't offer a degree of openess, but can only be full-open or full-closed. Is that right? Given that the JCB operates electronically from inside, how is the "degrees of openess" achieved in an actual bucket loader?

I think your in over your head. I would suggest doing some research into the operations of these types of machines. It sounds as though your a novice at best with mechanical units. Designing any type of operational bucket loader or backhoe is quite involved. The mathematics to get the geometry correct on the mast alone is quite daunting. I am not saying not to try, but your a long way from being able to do this without some serious help.

Its true I have no working knowledge of hydraulic cylinders, and only a basic understanding of solenoid valve working. I have some others helping me, I posed the question on cr4.globespec hoping to get some helpful replies. Advice well taken though :)

Here are two free online e-books at www.hydraulicspneumatics.com

Fluid Power Ebook, Edition 1: Fluid Power Basics

Fluid Power Ebook, Edition 2: Fluid Power Circuits Explained

Their website has lots of helpful information, and a forum.