AC vs. modern hydraulics

I am confused on the difference between AC (such as what would be on a CA or WD45) and the 2-way hydraulics of today. Other than AC being just one way, is there any difference between the two? I always thought AC was high pressure and low volume, while the modern hydraulics are low pressure and high volume. Am I correct here? Any help would be greatly appreciated.

JarrodACFan

Low pressure is a relative term. 50s vintage JD two cylinders ran 1200 PSI. New generation JD ran 2250 PSI and split the earlier cylinders. My MF-135 runs about 3000 psi and AC runs 3300, I think. 3300 is plenty to split commodity 2500 psi hoses and cylinders most common in the farm stores. Then porta power tools run up to 10,000 psi so its important to know the actual maximum pressure when choosing hydraulic couplings, fittings, hoses, valves and cylinders.

Gerald J.

Just be careful running the old "hi-pressure" hydraulics on newer hoses and cylinders.  When I put the pump back in my D15, it was putting out 3700 PSI at idle.  That will not be good on a hose or cylinder rated for 2500 PSI.

WD's and WD45's were rated at 3400 PSI....that doesn't mean someone hasn't shimmed it to be more than that. D15's and D17's were rated at 3700 PSI......same deal....I had one shimmed to 4200 PSI one time to be able to pickup big round bales of hay.....with the engine idiling; it would kill the engine when the lift arms hit the top of the stroke.

One more important differance. The early high pressure hydraulic systems used up until the D-17IV are the closed center type. A-C did not use this type again until the 7000 series tractors. In closed center hydraulics, the pumps stops pumping when the control go to neutral (closed). All larger tractors today are closed center. On the A-C tractors from the D-17IV- 220 have open center hydraulics. When the control is in neutral, the center is open and the oil re-circulates. The pump is pumping full volume all the time. If you are going to use a two way valve for a remote cylinder, be sure to get the correct valve. Closed center hydraulics are actually the modern type because the pump disengages to save energy and heat build up when the hydraulics are not being used. A-C was actually ahead of the curve on the early tractors but the volume was very low.

You can hear the pump disengage when the arms reach the top.

I'll hafta look into that, too... I was under the impression that my Series 1's pump was always active when the clutch was released.  If they set the pump to 'disengage', how are they doing it?  Do they just lift and hold the pump's poppets?

Larger volume/lower pressure fluid-power systems are generally open-center, using gear, gearotor, or swash-plate style pumps.  The latter can be 'open-centered' by neutralizing the swashplate angle, but these pumps aren't cheap, nor do they have the durability of the gear/gearotor design... and the gear-types can only be disengaged by an additional clutch mechanism.  The most common closed-center system is a vane-type pump, which is common practice on power-assist steering on cars and trucks.

The disadvantage to a closed-center system, is that during cold weather, the pump doesn't circulate fluid, so fluid doesn't come up to temperature very well.  In large-capacity vane-type systems... like the original system in my Clark IT-60, the presence of closed-center main pump means the engine starts well in cold weather, followed by the vane pump cavitating and self-destructing because the oil won't draw through properly.

I'll hafta take a closer look at my D-17's setup before being satified that the pump is set up to operate as an open-center setup.  It wouldn't surprise me if A-C built it to disengage under that circumstance, but I noticed any unloading mechanism like that in my diagrams.

Thanks for all the help guys, I am learning something every day on here!

  The plunger pumps used on the WD through D19 tractors are a hybrid of an open and closed center system.  They have an unloader valve that shuts off the main three pumping plungers once the unloading pressure has been reached, and a smaller Traction Booster plunger that continues to pump tiny amounts through a relief valve.  The Traction Booster section of the pump is the reason that the operators manuals recommend moving the lift/lower lever to the neutral or hold position when the hydraulic system is not in use.  

  Like everything else that I love about these tractors, the hydraulic system is simple, rugged, and ingenious compared to the competition of the day. Score one for the Allis-Chalmers engineering team!

It was my understanding that open center vs closed center was more in the control valves than the pumps.  I think thats why were getting so confusing here.

Closed center is just that.  When the valve is centered it is closed and no oil may flow.  Because of that a pump that can unload in some way is required or it would dead head and blow something or constantly bypass at the relief valve.

Open center means that when centered, the valve is open to the return path and the pump never stops pumping oil, it just stops producing high pressure.

In good closed center systems the pump is destroked to hold pressure high for instant motion response, like in the 4020 power steering system. Then when pressure drops the piston strokes increase to hold up the pressure. All that pressure control is at the pump. There are no pressure relief valves as parts of the cylinder control valves and those valves only take oil when feeding it to a cylinder (and if double acting return oil to the sump or reservoir from the other side of the cylinder).

In an open center system the pump is simple, can be a gear pump, that moves a constant volume of oil (at a constant shaft speed) and it depends on pressure relief valves in the cylinder control valves to prevent an excessive pressure rise from a loaded cylinder (or one that has hit the end). Oil not sent to the cylinder is passed through the valve back to the reservoir or sump along with oil returned from the other side of the cylinder.

It probably takes less shaft horsepower to circulate oil at very low pressure in the open center system which is simpler to build than it takes to hold pressure up with the complex pump control of the closed center system, though cylinder operating valves are less complex. Often we have to buy open center valves and add block off plugs for the open center port and the pressure relief port so we don't save on the "simpler" closed center valve. When there are high pressure leaks (split pipes, worn pump, or worn valves bypassing pressure to the return) it can take considerable horsepower to hold up pressure in the closed center system which is very effective at heating the hydraulic oil because they leak all the time. In the open center system such leaks only show up when the cylinder load is high and the cylinder control valve is not in the neutral position.

Many more recent tractors use a hybrid, sometimes called a pressure compensated system that holds a moderate pressure, like 250 psi without circulating the oil, and when a valve operates a cylinder the pump pressure is raised to the standard pressure for that tractor to be sure to push the cylinder as hard as it can. That takes more complex pumps and valves with (I think) a pressure sampling line from valve to pump control.

Gerald J.

You got it.  Use a closed center valve and leave your CA's hydraulic control in the raise position all the time.