Question on piston oil pump for a shop press.

[Flynth]

I have a cheap Chinese double piston (hand and air driven) hydraulic pump for a shop press that looks like this:


The piston with the handle attached is larger, for quicker movement. The piston with the air cylinder is smaller, so higher pressures can be reached.

In general I'm happy with the pump, but I'm having a weird problem with the pump.

When I want to lower the ram and I use the air driven pump it barely moves the ram then it stops and doesn't move it at all. If I pump with the handle it is fine. Also, once it touches the pressed object and I want to increase the force the air pump works fine.

Here let me say the system has been bled of air, and I tried both with the vent open as well as closed (with vacuum forming in the oil sump). It is even worse with the vacuum in the oil sump. In fact when I use the hand pump and I lower the ram half way with vent closed it too stops working.

By "feel" I can determine the problem is caused by the oil not having enough time to fill the pump's piston before a working stroke occurs.


With vent open if I pump by hand I can simulate the same condition by attempting to pump really fast (the air cylinder does maybe 4 pumps per second when it's lowering the ram). If I do that. I can feel the pump's cylinder is only half full. If I wait longer it fills it fully between strokes. With vent closed and resulting vacuum in the sump it is much worse.

I thought this is an issue of oil viscosity as the system came with oil that looked like 30 weight and I've added hydraulic 46 weight oil. So to lower the viscosity I've also added enough 10 weight spindle oil for the oil to become even less viscous than initially. It didn't seem to make much difference I can tell. Also it is rather cold around here. Approximately 3C or 37F.

Does anyone else notice this issue with this pump? It is a bit silly the air cylinder that is supposed to speed up ram lowering only works for actual pressing, not ram lowering and one needs to pump by hand to lower the ram.

Perhaps I have air bubbles in the oil? Otherwise, I can't think of anything. This system has sat unused for quite a while, but it was filled with fresh oil before. I doubt there is any crud in there.

Unless anyone has any tips how to fix it I'll have to drain it and check the insides. As it will involve repeating the whole air bleeding cycle again. I'm not looking forward to it.

 

[Nutfarmer]

Sounds like a check valve hanging up

 

[benmychree]

The vent should always be open on any hydraulic system, vacuum is your enemy.

 

[Flynth]

The vent should always be open on any hydraulic system, vacuum is your enemy.

Thanks.

I've disassembled and reassembled the pump. Other than some slightly messed up threads I haven't found any crud, jelled oil etc. It all looked fine.

I filled the pump with some diesel to test the theory about the viscosity causing this and to clean further. With diesel the hand pump worked fine, the air pump didn't work at all :-(

So, after removing that diesel I filled it with pure 46 weight hydraulic oil( thicker than the original oil). With the ram disconnected I'm now getting a stream of oil from the air pump, but the amount is still a lot less than by pumping by hand. Also that feeling of the hand pump not filling fast enough with very fast pumping returned.

Sounds like a check valve hanging up

How would one go about fixing it? I disassembled all check valves. They all looked ok(I'm not sure how they would look if they were broken). Do you think I should adjust spring tension? Half of them are adjustable, half are not. The problem is I don't know which valve is for what for sure. I can only guess based on their location. Pity, there is no proper manual for this pump (as for most of Chinese equipment). :-(

 

[Firstram]

Confirm which port/valve is which with a flashlight or low pressure air.

 

[Flynth]

Confirm which port/valve is which with a flashlight or low pressure air.

Let's say I do find the valve that opens when the air pump is supposed to suck new oil in. What should I do with it? Lower, or raise the spring pressure?

I've cleaned them all already.

 

[Firstram]

The pump intake port shouldn’t have much, if any, spring pressure holding it closed. All of the jacks I’ve ever worked on only had springs on the pressure bypass circuit to prevent over pressure. It only takes a small piece of garbage to stop the ball from seating and allowing fluid to just move back into the reservoir. Was the pump full of shavings when you pulled it apart?

 

[Flynth]

The pump intake port shouldn’t have much, if any, spring pressure holding it closed. All of the jacks I’ve ever worked on only had springs on the pressure bypass circuit to prevent over pressure. It only takes a small piece of garbage to stop the ball from seating and allowing fluid to just move back into the reservoir. Was the pump full of shavings when you pulled it apart?

Interesting. I didn't consider it not closing being a problem, but it may be.

There are springs on all valves, but 3 have very thick springs (2mm, 80 thou wire) and all 3 have adjustable pressure by means of a threaded plunger with a Philips head. Then there is a rather weak closing cap for each. It looks like fine thread with very little sealing surface on the heads. Caps have Phillips head too so one can't really torque them.

2 additional valves have very thin (0.15mm,5 thou) wire and are not adjustable. One of those has a normal tapered spring, the other has its tapered spring that looks like someone pulled manually to increase reach or strength. Those valves are closed with caps that have big Allen heads and respective seats cut into the valve body. They were torqued so high by the factory they stripped the top of the threads. Also the threadform looks like buttress thread.

In addition there is the pressure release knob. It ends with a plunger that pushes a ball against it's seat. This is the only ball with no spring. By my counting this means there is an "extra" valve. (because there are 2 pumps, so 2 valves per pump are 4). Is it an overpressure relief valve? One of those 3 with thick springs is placed directly under the high pressure output. Perhaps?

So I'm thinking obviously thick springed are output valves, thin springs are in input valves, but if this is so, then why did they use large strong caps torqued so high as to strip some threads for input valves? It makes no sense!

The pumps didn't have any loose shavings in, but there were shavings stuck to ends of holes. Also valves seem to have some very persistently attached sand/grit at the very bottom of their seats just out of ball's way. I removed as much of it as I could, but it is as if glued in there. I was poking with an end of an Allen wrench to loosen it, but I didn't want to use too much force for fear of scratching the metal and the sand appears not to cause any problems as it is attached on the circumference around the ball seat with quite a bit of clearance to the ball.

I found the same grit/sand under the "adapter" on the end of the press ram. I had to use a big flathead screwdriver to remove it. It was really glued in place. I can only imagine how "well" those parts were stored before assembly to have this sand in there.

Then, as we're talking about manufacturing quality. Let me talk about those two "weak springe" valves with Allen heads torqued to thread breaking point. All other threads appear normal, but this one really looks like a buttress thread. Unfortunately the top coil of the thread is stripped. So if I get high pressure leaking there I'll have to redrill, retap and remake those caps bigger. I wonder if a buttress thread there is really necessary, or did it end up looking like this due to over tightening?

I think, I'll test which valve is which by removing them selectively while attempting to use the pump. Then I'll know which one to focus on.

 

[Firstram]

So I'm thinking obviously thick springed are output valves, thin springs are in input valves, but if this is so, then why did they use large strong caps torqued so high as to strip some threads for input valves? It makes no sense!

the heavy adjustable spring is set to bypass oil above a set pressure to limit force. I believe you have the other two sorted, put it back together and test it.