Building a better PCP compressor

[mobilehomer]

You need a reversible cylinder, and some way to drive it....

Bob

Something along the lines of a log splitter. Micro switch at each end to set stroke, spring return. Auto pressure shutoff.

[antithesis]

Trav did say DRIVEN by the screw mechanism, I dunno exactly what sort of arrangement he had in mind, heck maybe he don't know yet, he's trying to milk the brain trust over here lol...

The power of the screw to overcome large mechanical forces is indeed impressive, as anybody who over torqued a 22xx end cap could tell you, maybe the compressed air turns the screws which push the balls through the sealed nut, making the output of air a potentially completely different source? Maybe using the spent lpa used to drive the screw?

Jeez Travis you really know how to provoke a thought experiment

[rsterne]

Ken,  I was trying to point out that you need a power source to supply the hydraulic pressure.... In a log splitter that is generally an electric motor, or more commonly a gas engine.... With any booster I have seen, you need to release the driving pressure at the end of the compression stroke.... In an air driven booster, that air is vented to the atmosphere and lost.... In an hydraulic one, you would either return it to the oil reservoir (requiring repressurizing with a motor), or have some other way to vent the pressure to reverse the motion.... A log splitter requires continuous power input, you can't just run the motor at the beginning and then split wood all day with no additional power input, right?....

You could certainly build an hydraulic booster.... it is the power source that may be to bulky/heavy to be practical.... I'm guessing that Travis is trying to figure out the lightest and most compact way to provide the high force levels required for a booster, while remaining portable.... but that is just a guess at this point.... A 1" piston on a 4500 psi booster requires over 3500 lbf. to drive it....

Bob

[Rob M]

could build the unit where the end user buys the motor.. Basically like a PTO shaft where the end user can use a gas, 2 phase or single phase motor in whatever shaft size is predetermined.. this in essence splits the shipping weight in half, and also increases packing efficiency in theory..

from Bobs comments , and a few others, Piston retraction is the main issue here.. So its either electrical switches, a complex reverse  valve, a bleed off , or something i havent thought of ..
eletrical retract is  not the simplest but well proven ( think rapid retract on a cnc machine.. It does lower cycle time )

[Rob M]

another idea, is to copy the mechanism in a commercial tapping head.. They auto reverse at a certain torque load, so once they piston hits lets say 5000 lbs of force, the mechanism automatically reverses ( but the motor is still driving in the same direction

[rsterne]

The boost pressure varies, and so does the load.... I suppose you could build it to "bottom out" on the threads on every stroke, but the loads would be very high, and may cause premature wear (or even binding)….

Bob

[Rob M]

So if we're assuming the boost pressure varies each stroke . Some strokes would stop shy of full compression then back to electronic limit switches that reverse the feed instantly.. matter of fact a proximity switch would be better. It's not reliant on contact so there would be some margin for error during development. 

[Rob M]

I thought this machine was a compressor and not a booster as a compressor has a larger audience anyway.  Requiring the buyer to already own tanks doesn't seem like a massive market

[antithesis]

I thought this machine was a compressor and not a booster as a compressor has a larger audience anyway.  Requiring the buyer to already own tanks doesn't seem like a massive market

those were the lines I was initially thinking along, or as stated previously, maybe two optional parts of a whole system....if in some way the rod for a piston were one of the circulating balls, and were able to follow the ball return channel, it would be a slow stroke to be feasible but I could see how maybe two synced screws guiding a shaft crossing through the piston rod could be practical, and would in fact return itself

[oldpro]

 My thoughts on this are 1. produce a totally self reliant PCP pump that doesnt need water or external air source. 2. be portable and easy to repair. 3. be less expensive than a traditional compressor.
 So with a screw mechanism you can easily produce huge driving forces needed to move a piston. I propose a two or three stage screw mechanism. One screw to drive ambient air to 500 psi a second screw to drive it to 2000psi and a third that may actually work off the second to drive it up to 4500 psi approx. This whole mechanism could easily be driven by a 110v Ac or 12v Dc motor.

[Rob M]

well then ideally youd want the second stage on the back stroke , to make use of the retraction time.. one stage pushing forward to a check valve and small bleeder line that feeds the 2nd stage , then , the ballscrew reverses and compresses that high pressure air on the backstroke. Not sure where the 3rd stage fits in yet. unless of course the first stage was precharged with shop air , making the system a 2 stage .

[antithesis]

Thank you for clearing some of that up, I was really kinda grasping at straws here.  While I have a good grasp of the mechanical principles involved, and very much in my comfort zone outside the box, the actual execution will take some some technical engineering knowledge and number crunching .. mechanical drawings, blueprints.....but the mechanical interface and linkage is probably going to be a high maintenance point....I still kind of like the idea of the balls themselves driving air through the stages and all encased in a sealed cylindrical housing.. maybe something that combines these in discrete stages..the highest pressure stage likely a more conventional piston type unit driven via the same shaft as the screw stages through a relatively simple gearbox, and to prevent overheating  incorporate a few pressure and temperature limit switches..

Have you ever looked at the possibility of a scroll compressor? These are notoriously robust, although I'm not completely sure how efficient they are regarding high pressure differentials, I think at least for the initial stage or two it would be worth examining

[rash]

@antithesis

CompAir, Kellog-American, and Campbell-Hausfeld (I believe) had a plant in Pittsburgh at one time.

CompAir made rotary screw compressors that delivered hundreds of cfm @hundreds of psi.  Don’t know if similar technology could be applied to high pressure air.  It was somewhat like a Wankel engine used as a pump.

[antithesis]

@antithesis

CompAir, Kellog-American, and Campbell-Hausfeld (I believe) had a plant in Pittsburgh at one time.

CompAir made rotary screw compressors that delivered hundreds of cfm @hundreds of psi.  Don’t know if similar technology could be applied to high pressure air.  It was somewhat like a Wankel engine used as a pump.

Worth looking into...only perk to living in a city, at least one built on a foundation of heavy industry and technology, sourcing alot of components and raw materials is relatively easy compared to the few other places I've been in the past

[int3man]

Well you could get a Roots Blower.  But they are Volume not Pressure related. 
I would stick with the Area Differential, Area Pressure device design.
Like the one we did all those years ago.  It has a new life, it is now compressing Nitrogen on a daily basis.

https://www.divegearexpress.com/haskel-mini-sport-booster?gclid=CjwKCAiAzJLzBRAZEiwAmZb0aqLP6hdaacEShNxqYzWZuQJsWIet4NELhMuvbfmpT9s_-Jp0Zl_fQRoCYsIQAvD_BwE

That one is a little rich but the one we found was $695 on eBay?  Mr. Hollowpoint had a video on it.  He was using it manually to charge his Big Bores.
Michael :- )