hydraulics headscratcher

I was trying to use my hydraulic brake press today,(shop built by me, but that has nothing to do with the problem!) and had nothing but problems. I was bending 1/4" steel, but was unable to get a full 90 degree bend because pressure would only go to 5-600psi and no more. This is a 2 stage pump that I have tested and it will deliver 3000 psi, so that isn't the problem. What will happen though, and this is the head scratcher, is if I deadhead the punch into the die it will build full pressure, but with a piece of steel in place to be bent, will only develop the pre-mentioned 5-600 psi. This was with a 2"wide opening die, so there is about 1" of vertical travel of the punch. So this made me think there is something amiss with a cylinder, scored or something. So I got the idea to raise the die up by placing a 2" thick spacer under the die, so I would be using a different area of the cylinders, but the same thing happens, if I deadhead it, it will go to 3000psi, but with the 1/4" steel in the die, will only go to 500 psi. I have a Parker elec/hyd control on it, hard to see how that could be contributing to the problem. I will add that I rebuilt the cylinders last winter with all new seals, and could find no scoring in any place on them. Under normal conditions, the cylinders are extended about 6" when using typical punch/die combinations, and they were extended about 4" when I experimented with raising the die.

Anyone have any suggestions?

I suppose it is a dubble acting cilinder So if you loosen the connection a bit on the side where there is no pressure and oil is comming out a bit under pressure the seal is leaking

Peter

None return valve ball seat leaking perhaps ?

I had similar symptoms with a manual 2 stage pump on a frame press where if it was pumped down with vigour it would make the higher pressures, a more liesurely action resulted in only about half pressure which would slowly drift away once pumping ceased.

Replacing the none return ball, the spring & re-machining the seat cured the problem.

regards

Brian

Leak

I think Brian is on to the problem. If the pressure comes up suddenly, the ball seats solidly, but in a gradual buildup it seats gently and does not seal completely. I have a crane on my truck that has some of the same symptoms and have been waiting for cool weather to work on it. Of course now I have to find a new excuse or fix it. Seals will sometimes do the same thing.

Bill

Relative to the pump being "2 stage"....if it is an axial piston pump, it is possible that the swashplate is not shifting into the "high pressure" rotational position. Usually there's a hydraulic solenoid of sorts that is energized to make this happen...more or less it shifts the swashplate to a fixed stop (the stop is an adjustable threaded rod in simple concept).

Sooo. That opens up the door to either the solenoid being bad, or else the signaling device that determines when to shift to high pressure mode is not signaling.

yeah, but...

When the pressure builds like it should, it will do that regardless of how I attempt to do so. Meaning I can start the pump, lower it a few inches to deadhead the punch, and it will build pressure to max in a few seconds. I can also repeat this without changing the position of the punch/die, or I can bring the punch down to the die, pause, and it will still build pressure. It is only when I am trying to bend something that the pressure won't build. And I have tried every combination of lifting the punch, and quickly lowering it, then pausing, hitting the foot control repeatedly, you name it, it won't build pressure until by some miracle, the 5-600 psi will slowly bend the steel enough so that the punch gets close to the bottom of the die, and then the pressure will build to what it should be to bend, in this case, 1/4" plate.

But, anyway, thanks for the ideas, keep them coming!

Dale

The unanswered question, though, is "what is the signal that causes the pressure to build"

Distill that essence and you will find zen

Matt

I see what you are talking about, seems like I recall that 5-600 psi is the point where it shifts to high pressure low volume mode, but I can't see how the pump knows where the cylinders are in their stroke extension to make it decide not to shift modes. When I have time, I will try bending shorter pieces of steel and see if there is some difference in how it handles smaller pressure demands. But short of trying another pump, probably won't know for sure.

Dale

Following along with what Brain said...

I think what the difference is between bending a piece and going to "deadhead" as you say... is the transition and ramp up of the pressure that acts on the ball. (to visualize it - graph the pressure over time for deadheading and for bending)

In bending, you only build as much pressure as the yield strength of the material (give or take a bit). I would assume the required pressure start the bend would be initially more (a small spike in pressure) as the material yields and starts to bend the pressure would decrease a small amount and be somewhat constant to a certain point where it would then require additional force to continue/finish the bend (change in geometry of the apparatus, material yielding force required, etc). In that process - up until you have the lack of pressure problem, you have changes in pressures, none of which are likely near the max PSI you're looking for or need for the finishing of the bend. If that ball is able to leak at 600psi or there abouts - but not able to if it is quickly and firmly seated under higher pressure, you could have your problem. If you deadhead it... you go from extremely low psi to a hard stop and an immediate spike in pressures that is much more than the spike needed to start the bend, and there is no point at which the pressure decreases when deadheading unless something gives that shouldn't.

To add to that, I would think, in the case that the ball is leaking, that pressure that is able to leak past might create some pressure on the other side of the ball - giving force from the opposite side and in turn reducing the net pressure on the ball, allowing it to leak worse.

Im no expert, thats just what hit my feeble little mind when I read Brians post and your response.

I hope you figure it out!
Best of luck!
Wade

You might have the seal plate/piston loose inside the cylinder. When it goes to bottom it seals, in between it is loose. ???? That's my guess and I'm stickin' to it!

matt_isserstedt said:

The unanswered question, though, is "what is the signal that causes the pressure to build"

Distill that essence and you will find zen

Click to expand...

Precisely & succinctly put Matt

Wade illuminated my suggestion comprehensively. Not being a metal basher I am unfamiliar with press brakes but the hydraulic mechanisms are common enough. Two stages means something switches fluid from the lower pressure (larger bore) cylinder to the higher pressure (smaller bore) cylinder at a certain point, usually a simple spring loaded shuttle valve which may have associated none return valves. If the shuttle occasionaly sticks in its bore this would stop the transition to the high pressure cylinder but would load up the driving motor which should be audible, if however a none return valve in either the shuttle valving or the pump cylinder feeds fails to seat the pump will continue to deliver & recirculate fluid without noticeable increased load.

Assuming the pump is a proprietory unit a link to a manufacturers diagram may assist in diagnostic suggestions.

regards

Brian

Well, I'll be jiggered

I have gone from saying that the pump can't be the problem to now being 99% sure that it is. It may be some time before this gets to the top of the list of things that need to get done, but when I do, I will report back on the fix.

Thanks again
Dale Nelson

Test

Try repeatedly lifting the punch a bit and banging back down on the piece you are bending. Eventually you should get to a point that gives a pressure rise that is abrupt enough to seat the valve or shift, or whatever. Not the way to get production out or extend the life of the system, but another bit of data tending to confirm the pressure ramp theory ( or not ).

Bill

9100

Lifting and lowering the punch repeatedly was one of the many things I tried while using the brake. I had to bend 24 pieces, and maybe twice out of 500 attempts to make pressure, if the punch got close enough to the bottom of the stroke, it would build to approx 1000psi, which is about what is required to bend 14" of 1/4" A36 plate in a 2" die. I am assuming this pressure rise was just something that happened to jar the check ball loose enough to work like it is supposed to. That is what is so strange about this. Like was mentioned earlier, the pressure is not constant in a bend, and the point where the pressure would increase enough to bend a right angle in the piece was in an area where pressure would not be at a maximum, at the start of the bend, and at the point where the punch is coming close to coining at the bottom extreme of the bend. So it is still somewhat of a headscratcher, but the pump being the culprit makes the most sense.

Dale

Could it be that you are deflecting the ram when attempting to bend metal, but the ram remains in perfect alignment when deadheading the die? If so, I'd bet that the piston seals are leaking due to the small deflection, but not when everything is lined up properly.

I finally found the problem

This was a problem with my hyd brake press that had pressure problems. Since this thread was started, I have convinced myself that it was the original two stage pump that was not shifting to the high pressure side, but a brand new pump only made it more puzzling in that it would build to 1200 psi, compared to 600 on the original? So this past week I have been putting a pressure gauge in all the places I could, and found no consistent problems with anything. So I started dis-assembling so I could install a manual hyd valve instead of the solenoid controlled valve. During dis-assembly I needed to use the hydraulic shear which is pressured by the same pump, but flow controlled by a selector valve. I had this valve shifted to the right function, but when I turned on the motor, oil came shooting out ot the port (that goes to the brake) that I had previously disconnected. So, to make a long story even longer, further testing found that the selector valve was letting pressure leak past its seals, not allowing full pressure to develop at the work ports. Last thing I would have ever suspected, but sure am glad to have finally found the problem!

Dale Nelson