Hello,
I would like to replace an existing air operated press with a hydraulic press, but I don't know what kind of speeds are practical with
hydraulics. I need cycle rate of 60 per minute with a 2" diam by 1.5" stroke cylinder at 500/600 psi, max pressure will only be at the last 1/8". I presently have a Vickers power pac that is rated at 6gpm and is set at 500/600psi. One of the things I'm concerned about is how to recycle the oil to keep the same oil from going back and forth without getting back to the tank reservoir for cooling. I have a bank of four valves available, could I use one valve to supply the pressure side and another as a return. I want as little heat build up as possible to avoid dimensional changes.
Any help would be appreciated,
Charlie

Your existing pump is so big and if it is a fix displacement pump, you will encounter a lot of power losses because you have to control the movement(speed). During pressing, some unused flow will be diverted to the relief valve(power loss), and most if not all open circuit application has to circulate thru reservoir.

Try to read Bud T e-book on basic hydraulics and you can compute your cycle time. An estimated 1 GPM pump is even more than enough to get your 60 cycles.

To consider air, does the force needed can be produced using your compressed air pressure?

quote:

An estimated 1 GPM pump is even more than enough to get your 60 cycles.

Extend AND Retract or just Extend?

EDIT....

Extend volume = 4.71 cubic inches
time to extend 1/2 a sec ...time to retract 1/2 sec?
So 9.42 cubic inches per sec?

One GPM = 231 Cubic inches per minute
231 divided by 60 = 3.85 cubic inches per sec

3 GPM = 11.55 cubic inches per sec

There certainly appears to be excess flow available to get the required 60 cycles per minute or one cycle every second ...assuming that my math and assumptions are correct..

EDIT EDIT ...NOTE:- I do not know enough to tell if a valve could react fast enough and supply the flow required by the cylinder to allow it to accelerate and deccelerate in the given time.

Regards Woody

This message has been edited. Last edited by: Woodygb,

my estimate is both, by imagining the size... , he does not have rod size too... oh, ok, my bad...

Thats too fast!!! 60/min... your simple hydraulic design is not possible, Peter can do that using servo... maybe

Yes Woody, that would probably enough using high tech, hehehe...

BTW, better not change the pneumatic system

Also, dont use Bud T e-book, sorry Bud T, hi, GLK!!!

This message has been edited. Last edited by: maglub,

Take a look at the Basic Ebook here:
http://www.hydraulicspneumatics.com/200/eBooks/
and look in the Formula section for how to size a hydraulic cylinder circuit. There are flow formula there that should allow you to determine the flow required to move your cylinder at the required rate.

If you mount your valves directly to the cylinder ports or at least with the shortest lines possible there will be no problem with cylinder oil exchange. Mounting the valves close to the cylinder ports with short lines also allows the least amount of line losses and heat generation.

I have used a Toggle arrangement driven by a small air cylinder that worked well for your application. The cylinder can be low power since the tonnage part is during the Mechanical Advantage portion of the Toggle arrangements stroke. You can also extend the cylinder and continue through center to make one punch action, then retract the cylinder to do the next punch operation.

Thank you all for your help!
I am presently using a 1.25" x 12" air cylinder at 80 psi to drive a toggle link press. My speed right now is 50 cycles per minute. I have a lot of things going on during one cycle, loading parts, ejecting parts from the die, removing parts and while I realize that a flywheel type punch press would be the way to go as far as speed, they aren't very forgiving. And after making millions of parts, I've definitely proved the old saying "if it can go wrong it will go wrong". I was hoping I could use a pressure switch to disable the press if things were to get out of time and it was to have a crash,
Bud T, my valves are manifold mounted on the power pak, so I can't move them ,but I will look at putting the power pac as close to the press as possible, and if I keep my hose sizes to the min. I should be able to get some fresh oil in the cylinder using that idea, thanks.
Maglub, is it the valve reaction time that makes this a bad idea? The oil flow speeds on a little chart I have look to be inline. If I do as BudT suggested and keep the lines as short as possible, cut the stroke a little, then a 2" bore X 1.25" stroke with a 1" dia double ended rod would require a total of 1.5 gal per minute with a feet per second in a 1/4" hose of 5fps. In fact, I would consider mounting a valve to the cylinder and build a new 1.5/2.0 gal power pac if I could make this work. The reason I want to ditch the pneumatic system is its a real air hog and I'd like to be able to run two machines at a time without buying a new compressor.
I really appreciate everyone's time and I hope I haven't been to long
winded.
Charlie

Charlie,
Could you provide a figure for the force that your cylinder must supply.

Regards Woody

Hi Woody,
I did a static pressure test (slowly loading a press until something happened) using a simple lever type press and came up with 1000/1200 pounds, dynamically I would think it would be less, but then I'm no engineer.
Thanks,
Charlie

OK ..Just checking that you have enough Ooomph in your hydraulic ram.
I was intially confused by your air cylinders size / force ...I now assume that it has a 10-1 ratio thus giving around 1000 pounds of force.

Bud T seems confident that you can get the required cycle time and I've done some number crunching and can see no reason why your proposed setup will not work.

Disclaimer
NOTE:- That I have limited knowledge of these things....so all the above is IMO.

Regards Woody

Woodygb, you scared me to post on this thread , anyhow forums are just opinions...

I promise myself not to make any more computation using "nothing" but my little brain doing some estimates...

IMO, assuming an X GPM can give the speed and the Y pump/motor pressure capability can do the work/cycle, a very simple hydraulic system will give more problem in terms of power loss.

No doubt this system can be run smoothly with a high tech servo system, but it will be very expensive... hi you there! .

But if a conventional system is to be used, and utilizing what Charlie has in his stock, the system will work with a little "banging". But IMO an accumulator has to be used to attain better production target with lesser power loss.

During operation, and with this kind of set-up(using pump only/no accumulator), an constant pressure system has to be used to attain the desired production rate, which could result to higher power losses(more heat thru the relief valve pressure drop). But with an accumulator system, there will be lesser losses since pump will be used only to charge the accumulator(sized properly) and can unload at low pressure(venting operation of relief) dictated by a pressure switch. Pump performance characteristics has to be considered also when

Still acceleration and deceleration will be harder to control especially with a short stroke. But if a carefully designed cylinder with a cushion on both sides can be used, the system might work just fine. The amout of intensified pressure due to the cushion on rod side can be sustained if the system pressure is not too high. Also cushioning stroke is very short and could happen only at the end of stroke.

IMO, a 4/2 way spring offset single solenoid valve with open center crossover is better than a 4/3 double solenoid.

You dont need to worry about fluid returning from the actuator to the tank, they dont create much heat, except when there is too much pressure drops cause by the valves.

Hydraulic system still will be very costly than a pneumatic system.

Thank you Woody and Maglub,
Sounds like it "could" work. Which is good as I already built the press earlier in the year. lol I will carefully study your recommendations Maglub, I do know I have double solenoid valves, pretty sure they are closed center, but I can change that, have the cushions also, but if I cut the stroke I'd lose the one at the top.
I agree on the initial cost difference between pneumatic and hydraulic, but times being as they are, I came by this 2hp Vickers Power Pak for $100.00 and it's like new. And if it's two horse will run a press, it will replace a five horsepower air compressor that runs 4 minutes on and three minutes off.
I really do appreciate everyone who took the time to respond. I'll get to work plumbing everything up, heck I was 90% there, but that damn last 10% is hell. Starting is a whole lot more fun than finishing.
Now if I only had BudT's stamp of approval, life would be good.
Charlie

Lol, a close center 4/3 has a close center cross over/transition( positive spool overlap)... Hope shock will be minimal... But at least you have a center position... Pump is variable? Or you have vented relief? Or the manifold has a bypass valve? Hope so...


Goodluck

Hi Maglub,
I'm trying to find out exactly what I do have and have been perusing BudT great ebook for some pointers as well.
Here's what on the name plate of the Vickers System Pak
TK14VP VB6S M P4 /s2c/s2c/s2c/s2c/6F6--F1-B1-069

I believe the pump is PVB6S, which about all I could find means piston pump and 6 gpm, seems like from a search last year the V means variable, but I couldn't find it this last search.
It has four valves mounted and they are all
DG4V-3-2c-m-wb-40 . I don't know what any of that means.
There is a relief valve on the pump that I used to turn the pressure up to 1,500 pounds, but the pump sounded like it was badly cavitating, so I immediately turned it back down to 500 pounds, where it runs nice and quite. BTW, the oil, which is a vegetable oil, was low in the tank and after the cavitation episode was full of bubbles, I'm thinking that might have caused the problem. I'd like to replace the vegetable oil with something less expensive, for this high speed application what weight oil should I use?


Thanks for the help,
Charlie

Pump is variable and 6 gpm @ around 1800 rpm. Your valve as far as I can remember with 2c is close centered double sol. The noise is due to low level oil only and hope you did not try running it for long.
Your pump has no relief valve but rather a pressure compensator or any type of control depending on the type which can be determined on part#. there are PU designs that rely on the pressure com, or pump control options, which IMo is not right. The valve block that you have is possibly equipped with a relief valve(stackable/sandwich) and could probably below the first sol valve.
You can use a 68 grade hyd fluid, don't forget to clean the reservoir. I suggest you test the valves and use the one having the least internal leakage, then blind the rest with a blanking plate(DO3 interface) which can be easily fabricated.
You can test the PU pump pressure/flow compensation capability and flow that could be used if the operation requires a constant pressure/flow, if there is a relief valve. In order for the press/flow comp to operate, the relief must be set higher, and can be double check using a clamp ammeter.
To find out how much the max pressure the motor can sustain @ rated pump flow(5-6 gpm), you can screw in( set) the pump comp adj screw higher(I suggest slightly above your target operating pressure) than the relief valve setting( watch out for ammeter reading and please set your OL relay to rated current). Since you can raise the pressure( though cavitating) in this initial/ original condition, you can lock the pump control to desired pressure, then reduce relief setting to zero. Make sure that all A/B outlet ports are plugged for extra precautions. Change fluid and clean/flush the system befor doing this.
Btw, you can open all A/B ports when testing for internal leak(safe place/point) of each valves. My point here is that close center valves can cause a differential cylinder to creep at center position, that if unnoticed might cause damage/accident. That's why I recommend a 4/2 valve. Although you can use a 4/3 valve, but continously energizing the retacting coil will reduce it's life.

I'm mobile, and pardon if I said a lot of wrongs, will edit later when I reach home.

Wow! Thanks for the all info Maglub!
You have given me plenty of info to study. You are indeed a master hydraulic clown! But more importantly a good man to take your valuable time to help your fellow man. We've got some good folks on this internet!
Thank you!
Charlie