Variable displacement A/C compressor converted to hydraulics? 1

[aj7897]

Hey guys,

I'm currently developing a prototype that will include some hydraulics and I'm considering utilizing a variable displacement axial piston pump (henceforth referred to as a VDAPP). I'm not a hydraulic engineer, so my knowledge of them is somewhat limited. I want one to play around with, and maybe incorporate into my prototype. My crazy idea is to go to the local pick-n-pull junkyard and pull a Delphi compressor out of a late model GM - I've heard they have electronically controlled VDAPPs. Then I will fill it with mineral oil, control it with a potentiometer, and make it drive a hydraulic ram.

Is this absurd? Or could it be done with some modifications? I have access to all types of CNC machines if I need custom parts. If it's a stupid idea, than is there somewhere I can get a VDAPP (like these) without buying in bulk?

Thanks for the help!

Andrew

 

[MikeHalloran]

A/C compressors typically work at well under 500 psi.
Common hydraulic systems typically work at pressures above that.
You will just break the compressor when you load it.

You've probably noticed that VDAPPs are expensive. That's because of the number of precision parts required to make them. Not CNC precision, either; hand lap, selective fit, bands of light, all that stuff.

Better to use a cheap fixed displacement pump and drive it with an electric motor and put a VFD on the motor.



Mike Halloran
Pembroke Pines, FL, USA

 

[Oldhydroman]

I wouldn't risk running a "compressor" on an incompressible fluid (mineral oil) - I think you'll end up with a box full of bits. And, as Mike says, the rated working pressure of an AC compressor is very much lower than that found in a typical hydraulic system.

The problem with a fixed displacement pump/electric motor/VFD combo is that most hydraulic pumps have a minimum operating speed below which the output is too lumpy and the controls don't work properly. This means you can't get the equivalent of "zero displacement" with an ordinary pump. Another problem is that a variable displacement pump can change its output from minimum to maximum in a matter of milliseconds. If you try to do this with a VFD then the acceleration and deceleration is so severe you need a stronger drive shaft than most pumps possess. I know Yuken have done a lot of work on this idea but their product is geared toward energy efficiency not low cost.

The link is to Bosch Rexroth variable displacement pumps - if you like that sort of thing but don't like their prices then try Metaris, Huade, Casappa or Daikin (there are lots of others). Or you could contact your local hydraulic pump repair center and see if they have a suitable "rebuilt" unit. They might even have a peculiar model left on the shelf that no-one else wants and you could pick this up for a song. Then use your CNC machining facility to manufacture the bespoke shaft couplings and mounting brackets that you will need to connect the unusual beast to your prime mover.

PS, it might sound crazy but have you tried ebay?

DOL

 

[lukin1977]

I´ve looked at the rexroth link. Some pumps are "open circuit" and some are "closed circuit".
Can anyone explain me what this means?

Thanks

lukin1977

 

[Oldhydroman]

An open circuit pump will suck oil from the tank and push it round the circuit, the oil returning from the cylinders and motors etc. goes back to the tank. Generally speaking, a variable displacement "open circuit" pump will vary between zero displacement (no flow) and maximum displacement (full flow).

A closed circuit pump pushes oil round the circuit and the returning oil goes straight back to the pump inlet (not back to tank). A small [open circuit] pump called the boost pump (or charge pump or replenishing pump) sucks oil from the tank and pushes this into the low pressure side of the main circuit in order to make up for leakage losses and to maintain a minimum pressure at the inlet of the variable pump. Generally speaking, a variable displacement "closed circuit" pump will vary start in neutral / zero displacement (no flow) and be capable of adjusting either side of neutral to get maximum positive displacement (full flow in one direction) and maximum negative displacement (full flow in the other direction). Closed circuit pumps are usually used with hydraulic motors and give the best efficiency because there are no flow control valves getting in the way - motor speed is controlled by varying the pump displacement directly.

To aid confusion some manufacturers refer to these as "open loop" or "closed loop" pumps - the confusion comes about because these terms usually refer to techniques of control. And indeed the pump displacement control could be "open loop" or "closed loop" regardless of the pump's hydraulic configuration.

DOL

 

[aj7897]

Thanks for the responses (especially the brand suggestions). I'm actually planning on using the VDAPP as a CVT (like the Honda TRX500 Rubicon ATV). The TRX500 uses a fixed displacement pump to drive a VDAPP, thus varying the "gear ratio". I will only need the VDAPP, since I am going to convert the fluid flow into linear motion. The VDAPP will drive a hydraulic ram, which will compress a coil spring.

Basically, I am trying to control the force of a linear motion. Picture this: a 10 ft cable is wrapped around the VDAPP's driveshaft. When the cable is pulled by an external force, then varying the swashplate angle (with a solenoid) will determine how much fluid flows into the hydraulic ram, and therefore how much the spring is displaced per revolution. When the external force is decreased, the spring will expand, driving the VDAPP in reverse and retracting the cable.

So when 10 ft of cable is pulled out, it could require a lot of work (enough to compress the entire spring), or just a little work (only compress the spring partially). The energy is returned by the return force of the spring (minus efficiency losses). Maximum power put into the system will be about 1000 lbf-ft in one second (1300W, 1.7HP). And I can gear it however I want. So I would imagine I can use a pretty small pump, and maybe the AC compressor idea will work.

The TRX500 transmission costs about $1800 and the Rexroth units I've found start around $1200. Is there a small, relatively inexpensive VDAPP on the market that will work for 1.7HP? Thoughts on my idea now that I've clarified?

Thanks.