What's Better: Hydro or Air?

Hydro. It doesn't compress.

>>>'Hydro. It doesn't compress.'<<<

Words are important. What you have stated is not accurate.

Compared to gasses, liquids exhibit very low compressability, but they do indeed compress.

Benbenben

You are splitting hairs, sir!

Because a gas (air) is a compressible fluid, it is not preferred for quick response time applications. If quick response time is your goal, go with hydraulic.

Look no futher! Hydraulic is by far the best for your needs,The best liquid is water,which cannot be compessed,and an additive (suds) to prevent corrosion.

plus Hyraulic pumps are easier to build than air compressors.

Hydraulic rams & roof support systems are used extensively in the mining industry,

because of their fast reaction and their ability to withstand pressure.

Good luck.

>>>'...The best liquid is water,which cannot be compessed...'<<<

Water can definately be compressed. The compressability is small compared to a gas, but it can be compressed.

Benbenben

Definitely hydraulic. Run of the mill hydraulic pressures are 3000 psi, whereas run of the mill air pressures are 125 psi. So for a large amount of force in a small space, and for precise control, hydraulic is hard to beat.

Well... $10,000 for a hydraulic pump can be way under priced... I'm currently designing a system with three pumps, two of which cost $48,000, the third is a chep vane pump only costing $5,000.

It think this is a great answer, and voted it so.

However, pneumatics can often be more expensive than hydraulics. It is for this reason that log splitters are virtually always hydraulic. A cheap gear pump provides plenty of pressure with energy losses lower than typical pneumatic energy losses. No large pressure vessel is required for storing the pressurized working fluid. But the single largest cost driver is the cylinder. For equal force, an air cylinder needs to have an area about (3000psi/125psi) 24 times as large.

In practice we see pneumatics used mainly where forces are low, such as in robotics (and also in places where leakage of fluid can be a serious problem -- such as in the tire building industry, where pieces need to stick together reliably). But where forces are high, and where compact cylinders are desired, we see mainly hydraulics: construction equipment, large production equipment, presses, lifting jacks, brake systems, etc.

That last item also points up a couple issues. The first is that air brakes in trucks show the difficulty in fitting air cylinders into small spaces: the actuators are always outside the brakes drums, and they are very large. But also it shows an advantage of air brakes, that being that the trailer can be disconnected without hydraulic leaks and the need for bleeding air from the system. (On the other hand, swapping out modules, such as post hole diggers, in mobile equipment with hydraulic quick disconnects works pretty well. too.)

Pneumatics alone are poor for precise intermediate positioning purposes. The cylinder needs to be fully extended or retracted against a mechanical stop to establish a position. On the other hand, this can be an advantage of pneumatics as well. If one wants to provide a relatively constant force against something that can move around a little, pneumatics provides springiness, so the the force does not change a lot with small movements. With hydraulics the force can change dramatically with a small movement.

Got to get back to the shop to so some sanding with my hydraulic sander, after which I need to change a tire, using my hydraulic impact wrench.

Agreed. Which is why I asked to have the forces, and speed qualified. No decisions can be made either way without knowing this information.

However, pneumatics can often be more expensive than hydraulics. It is for this reason that log splitters are virtually always hydraulic.

That one I'm no so sure about. I think the reason has less to do with cost, and everything to do with power density, and the high forces needed. A Pneumatic cylinder will be much cheaper than the same sized hydraulic cylinder. air lines are cheaper than 3000# hydraulic hose, plastic push on air fittings are much cheaper than 3000# brass/steel/stainless fittings, valving is much cheaper as well.

Unfortunately, very precise proportional movement is not easily obtained with pneumatics (although not impossible), and not to mention the differences in available force from each system.

Pneumatics really shine when quick positioning, and low force movement is needed, like you said, in robotics, and many semi-automated processes in virtually every manufacturing plant in the world. The cost comment I made was really due to the idea that most shops already have a compressed air supply, so all you would need is the valving, and cylinder, VS a reservoir, electric motor, filters, oil, pump, cylinder, drip pan, heat exchanger, etc etc etc which would be needed for a hydraulic system.

Don't get me wrong, I'm a Hydraulics guy, 95% of the work I do is designing Hydraulic systems for Industry, 3% is pneumatic systems for automation, and the other 2% is well... thats when I just stare at the wall for a while.

In the end, NO ANSWER CAN BE GIVEN WITHOUT MORE INFORMATION, only broad generalizations about one system VS the other.

Gases (air) compress. Liquids (hydraulic oil) don't.

The answer to your question depends on the nature of the load, which you haven't described. In general, pneumatic systems are high speed and hydraulic systems are high force. For specialized applications there are hybrid systems that employ both air and hydro. Such a system might be used when you need high force from a compact package, but rapid acceleration for a short distance as in projection welding.