Hydraulic pump basics

[Quantumkev]

Forgive me for asking a stupid question, or if it has been asked and answered before please point me at the thread.

I am trying to understand how my hydraulic pump works….

I am on an old Ford 4500 industrial backhoe. The hydraulic pump is mounted at the front of the tractor, connected to the engine by a small drive shaft. This means that if the engine is running, then so is the hydraulic pump.

I understand the basic fundamentals. Simply put the pump can move a maximum volume of fluid. If the fluid can flow unrestricted, then the pump can never build pressure. In hydraulic systems that is not the case. Let’s assume a scenario where nothing is moving (no hydraulic cylinders being actuated). So basically as fluid is pumped, and it has no place to go, pressure starts to build.

This is where I am trying to figure out what happens next. Logically in my head I get to two possible scenarios.

Scenario one. - There is a pressure relief valve somewhere in the system (let’s say set to 3000 psi). So when I start the engine the hydraulic pump starts to pump, and then because there is no flow pressure starts to build until it hits that magic 3000psi, when the pressure relief valve starts to open, and flow starts occurring, but only enough flow to maintain the 3000psi system pressure.

Scenario two. - the pump rotor just spins in the fluid, but the fluid doesn’t go anywhere. So when I start the engine the hydraulic pump starts to pump, and then because there is no flow pressure starts to build. It continues to build and build until it basically hits the maximum pressure that the pump can deliver. Once it is at maximum pressure the pump rotor continues to spin in its own fluid, but obviously the pressure can not increase, and the pump is at maximum pressure.

Can someone help me understand which of these two scenarios it is? Or explain to me the third (real) scenario that I have not described here.

Thanks in advance!!!

 

[oldnslo]

Quantum
It depends on what style of pump you have in the system.
Most common for smaller tractors is fixed displacement gear pumps. This style system oil is moving whenever the pump is turning. These are called open loop circuits since oil is pulled from the reservoir and then flows through the valves in series back to tank. These also require open center valves so the flow passes through them to tank when no functions are active. These must have a system relief valve to protect the pump and system from excessive pressure.


Second style of pump is variable volume. This style pump changes displacement supplying flow on demand. These are also open loop circuits but the valves are closed center and plumbed in parallel. There is no flow until a function is activated. These pumps can have various types of controls from basic pressure to electronic proportional flow and pressure.

I suspect your 4500 has a fixed displacement gear pump.

 

[Doaaj50]

To build off of what oldnslo stated, here is a schematic of the two different types of systems.

The image is from this webpage that also contains some more text about the two different types of systems.
Open center vs Closed center systems

I also agree that most likely your backhoe has an open center system. I checked a few sites online for a replacement pump for your backhoe and all of them show a picture of a gear pump which would run in an open center configuration.

 

[rScotty]

Forgive me for asking a stupid question, or if it has been asked and answered before please point me at the thread.

I am trying to understand how my hydraulic pump works….

I am on an old Ford 4500 industrial backhoe. The hydraulic pump is mounted at the front of the tractor, connected to the engine by a small drive shaft. This means that if the engine is running, then so is the hydraulic pump.

I understand the basic fundamentals. Simply put the pump can move a maximum volume of fluid. If the fluid can flow unrestricted, then the pump can never build pressure. In hydraulic systems that is not the case. Let’s assume a scenario where nothing is moving (no hydraulic cylinders being actuated). So basically as fluid is pumped, and it has no place to go, pressure starts to build.

This is where I am trying to figure out what happens next. Logically in my head I get to two possible scenarios.

Scenario one. - There is a pressure relief valve somewhere in the system (let’s say set to 3000 psi). So when I start the engine the hydraulic pump starts to pump, and then because there is no flow pressure starts to build until it hits that magic 3000psi, when the pressure relief valve starts to open, and flow starts occurring, but only enough flow to maintain the 3000psi system pressure.

Scenario two. - the pump rotor just spins in the fluid, but the fluid doesn’t go anywhere. So when I start the engine the hydraulic pump starts to pump, and then because there is no flow pressure starts to build. It continues to build and build until it basically hits the maximum pressure that the pump can deliver. Once it is at maximum pressure the pump rotor continues to spin in its own fluid, but obviously the pressure can not increase, and the pump is at maximum pressure.

Can someone help me understand which of these two scenarios it is? Or explain to me the third (real) scenario that I have not described here.

Thanks in advance!!!

What has been posted in the answers so far is correct. Let me just add that the reason why neither of your two original Scenerios is correct is because in both you say that when the hydraulic pump begins to spin there is no flow. That is NOT correct.

In an open center circuit - which yours is - then once the engine starts and the pump begins to turn there is always flow. With the pump turning, if nothing is using the fluid it simply circulates. It flows through the open centers on all the valves and back to the reservoir to be sucked up, filtered, and pumped again in an endless loop.

To do some work with that flow, you move (open) a control lever that diverts some of the flow to the hydraulic cylinder. Now there is resistance and pressure will build - but only in the flow path that has been diverted. The rest of the fluid just circulates as beforel. If you leave the lever open once the hydraulic cylinder has moved its maximum amount or encountered maximum resistance then the relief valve will switch that small part of the flow back to the main flow stream - often the relief valve will make a noise when that happens. The noise is your signal to quit asking for what it cannot do.

Keep at it until your mental model is working. None of us got this the first time.

rScotty

BTW, there is another type of hydraulic system: THE CLOSED CENTER type.
But it is only slightly different. Lets leave it along for now. Mostly used in larger commercial machines and some medium to larger JD tractors.

 

[Quantumkev]

Thanks for all your replies so far. It make sense, but it drives me to this next question.

If I assume my tractor is open center valves, it all makes sense, that fluid is circulating through the pump, through the valves, back to the tank, then start the cycle again.

The bit I don’t understand now then is that the fluid is going to always want to take the shortest path, so looking at my backhoe only I have 6 valves, so 6 open center valves right? So when I actuate one of the valves, the fluid still flows through the open center of the other 5 valves, and if that total flow volume can be greater than the pump can deliver I can’t build any pressure. So in this scenario how does the pump ever build pressure?

Sorry for the basic questions, but I really want to understand this!

 

[Quantumkev]

Sorry, just re-read oldnslo’s post. If I understand correctly all the valves are in series, not parallel, so once one valve is actuated all of the flow is redirected there. So I can only operate one valve at a time then right?

 

[oldnslo]

Sorry, just re-read oldnslo’s post. If I understand correctly all the valves are in series, not parallel, so once one valve is actuated all of the flow is redirected there. So I can only operate one valve at a time then right?

Quantum
You can operate more than one function but you must feather the functions to regulate where the flow goes.

Example: on the backhoe to dig level trench you would have to feather crowd and dipper functions and possibly curl.

 

[rScotty]

Sorry, just re-read oldnslo’s post. If I understand correctly all the valves are in series, not parallel, so once one valve is actuated all of the flow is redirected there. So I can only operate one valve at a time then right?

That's right, but the explanation is not yet complete. Yes, the valves are in series (but there are tricks like proportional fluid paths)..... but for now assume they are all in series because that is simpler. What makes it work is that when you trigger a valve to divert flow, you are only diverting a little bit of the flow. The rest of the unused flow goes onwards through the open center and on to the next valve and the next to be used by those valves for their own cylinders.

What that means is you want as much flow as you can get if you are using multiple cylinders at the same time. But for use in the real world, you can usually get by simply by anticipating that if you try to do too many cylinders at once they will all go slower....based on resistance to flow.

Wanting as much flow as you can get would be nice, but it also means a big pump that is most of the time doing nothing but moving fluid, burning fluid, and making heat.

So this is where we come to the other game in town: the "Closed Center" hydraulics. Closed Centers use a variable flow pump (similar to a HST tranny) and a pressure containing reservoir in the circuit. With those advantages, Closed Center systems can use a smaller pump and still have flow when needed. But the systems are much more expensive. Not really needed for backhoe loader work.

rScotty

 

[LD1]

The 4500 is a nice sized old farm hoe.

Are you experiencing any issues with it? Or just trying to understand it? HAve you used the hoe before and understand what others are saying by feathering to control more than one function?

 

[Quantumkev]

I just bought the hoe, and it has sat for an unknown number of years so I have a number of problems… the good thing is the engine fires right up and appears to run ok. It is a four speed with power reversing and it SEEMS like it will pull in both directions. When the gear shifter is in neutral and I put the lever in either forward or reverse I hear the gears running pretty smoothly.

My first real problem though is that both the hoe and the bucket are in the down position, and buried a little bit, so the tractor won’t move, and I can’t get it loaded on a trailer to get back to my place.

That’s where this question thread started. I was trying to understand if when I ran the engine the whole hydraulic circuit tried to pump up to full pressure. I am fairly proficient in pneumatics, and assumed that hydraulics operated the same way that all the valves sat on a manifold, all at the same inlet pressure, and then all operated independently. These first few explanations have helped me a lot. Basically the fluid just circulates through all of the valves with the system at some nominal low pressure, then as a valve is actuated it is that action that cause pressure to build and work to be done. Thanks for all who commented and helped so far!!! But this moves me on to my next problem (or next two problems).

I need to raise the bucket and hoe.

I made a start on this. I have a farm jack that I got under the front end loader bucket and started to jack it up. It did two things. It started to lift the bucket, and also curl the bucket. I know it was doing this as I would Jack it a little bit, and then actuate the loader and bucket valves to release pressure. I would hear fluid flowing, so I knew the it was working, and when I released the jack it held in place. I got the bucket barely off the ground, maybe 2-4 inches, but now when I jack it I can still lift but if I actuate the valves I don’t hear any fluid flow, and when I lower the jack it moves back to the start position…. Why would it do that? Why would I be able to start lifting the bucket, but then not be able to get it to go any further?

 

[rScotty]

That's interesting. I'll bet when you got those 2 to 4", worked the valves, heard the fluid move, and saw it hold in the new position you thought it was going to be easy... I know I would have.

First of all, I don't know. But here are three guesses:
1. One is that the cylinder shafts were really corroded and where they began to enter the cylinder body they are now jammed in that bushing and won't move. The problem with this, is that hydraulic cylinder shafts have a wiper, don't corrode easily, and you probably cleaned them up a bit when you saw it might move.
OR

2. Your jacking force should have caused some fluid back through the return line to the hydraulic sump when you worked the valve. And it must have done so, after all you heard something happen. And now for some reason the return line is not accepting fluid. I don't like this idea because the one part of a hydraulic system that is always working has to be the return. Otherwise big danger. So returns are over designed. But it is simple to figure out which line is the return, and loosen the fitting to see if fluid sprays out.
OR

3. Maybe your jacking force cause fluid under pressure to leave from the compression side of the hydraulic cylinder, and for some reason the valve didn't do it's other job - which is to allow some fluid to go into the side of the cylinder closest to the ram and now you have a vacuum lock there. That's why you can lift it a little and it returns to where it was.... and why you cannot get the valve to move any more fluid.

I like this last idea the best. So put some force on it, loosen the hose fitting on the ram side of the cylinder to let some air in, and see if working the valve makes fluid flow now.

READ!
BTW, I'm going to assume that when the engine runs, the hydraulic pump turns, and and you have checked somehow to see that you are actually sucking fluid into the hydraulic pump and returning it to the sump. If not, lets back up right now and you should immediately think about what I'm sayng next.

Most hydraulic problems are one of three or four things, and all of them end up starving the suction side of the hydraulic pump. One is too little fluid, another is a cleanable screen clogged, or a clogged hydraulic suction fllter, or most common of all is an air leak in the suction hose between sump and pump. that air leak can be tiny and impossible to find. Cure is to replace hose.

You know it is returning properly when you can undo the return line and put it into a bucket, start the motor, and then quickly stop before you pump all your expensive hydraulic fluid all over the ground.
Or make up a section of clear vinyl hose - there shouldn't be any pressure - or nearly none - in the return flow....only flow.
Luck,
rScotty

 

[oldnslo]

Are you the same person that posted questions about what two valves do on a 4500? If yes those diverters might direct oil to either loader or back hoe.

Does engine or pump change sound when attempting to operate either loader or the hoe?

 

[Quantumkev]

I will be honest with you, when the bucket started lifting, and I heard the fluid moving when I released the valve, and did that a couple of times, YES, I did think it was going to be easy…. I should have known better.

To your point one. The shafts are a little pitted/corroded, but nowhere near enough to cause them to jam. Layer into this when I jack the loader I see the rams moving, so I know they are not jammed, and when I release the jack I see them move back. So we can rule out number one completely.

Before I get into two and three let me explain where my head is at. Because some people may have been thinking, if the engine starts and runs, why not just use the hydraulics to move the loader/hoe. Here is the back story on that.

It starts up really easy, and idles real well.

When I dip the hydraulic tank I show EMPTY. But I know I am not completely empty, just empty enough to not show on the dipstick.

I started and ran the engine a few times, no issue no problem. Then after I had messed with it, jacking the loader, and also one of the stabilizer feet on the hoe, and had heard oil returning to the system, I started it again. It was at this point I started blowing what I would describe as “snow”. One of the fittings that connected the main pump pressure outlet to the main hydraulic line running to the hoe was loose… what came out literally looked like snow. (Ruining a pair of jeans, and maybe my favorite boots in the process….) what I mean by snow is that it was white, and fairly light (like a clump of bubbles). I know milky white is bad, but I am not sure yet if my hydraulic fluid is milky white, or if when I moved the cylinders and put a little bit of fluid back in the tank it is barely enough to supply the pump and it just aerated the oil like crazy, then when it blew out the loose fitting it had a Venturi effect and aerated a little more so just looked like snow.

Layer into this that when the tractor is off, I have no oil leaks. But as soon as I start the tractor I start to get a drip from the hoe. Can’t tell where it is coming from, but it is either the dipper or bucket hoses (they all disappear together into the boom so it’s hard to tell where the leak is coming from). Layer into that that I am not operating any of the valves, so one of them must be passing fluid.

So my logic was to try and get the loader back to my place, so I can work on it with all the convenience of having all of my tools to hand, rather than working on it 45 minutes away.

I am starting to think maybe my better path is to try and fix the hydraulics where it sits, even if only just enough to get it loaded and Mack to my place. If the hydraulic fluid really is milky and white I guess the damage is already done, so use what I have to get it back to my place. Maybe throw in a gallon or two just to get everything moving, then drain and flush when I get it back to mine without wasting $150 on hydraulic fluid just to get it back to mine.

Any thoughts on that?

 

[jigs_n_fixtures]

The ”snow” is whipped up hydraulic fluid, because you are really low.

I’d add just enough hydraulic fluid to get things moving. And, I’d put some of the UV die in the fluid, to highlight leaks. Then when you get it home, keep it just full enough to keep trouble shooting. Then when the system is tight again, drain and replace the fluid.

If you have a pressure washer, clean the suspect areas for leaks, then cycle the hydraulics one at a time, and scan with the UV light. if you don’t have a pressure washer you can clean things up pretty fast with brake cleaner, but it may cost a lot to get everything clean. You could try running past a self serve car wash, and hosing off the hydraulics, and the engine/transmission.

You can add the UV die to both the engine, and the transmission oils and then you can scan it for leaks and tighten/replace/repair as necessary to get it as tight as you want.

You can also use one of the 4-ton porta-power type hand pumps to pressure check hydraulic lines for leaks. get one with the pressure gauge, and pump it up and watch the gauge. If it won’t come up to pressure, or the pressure bleeds off, that line has a leak.

 

[rScotty]

I will be honest with you, when the bucket started lifting, and I heard the fluid moving when I released the valve, and did that a couple of times, YES, I did think it was going to be easy…. I should have known better.

To your point one. The shafts are a little pitted/corroded, but nowhere near enough to cause them to jam. Layer into this when I jack the loader I see the rams moving, so I know they are not jammed, and when I release the jack I see them move back. So we can rule out number one completely.

Before I get into two and three let me explain where my head is at. Because some people may have been thinking, if the engine starts and runs, why not just use the hydraulics to move the loader/hoe. Here is the back story on that.

It starts up really easy, and idles real well.

When I dip the hydraulic tank I show EMPTY. But I know I am not completely empty, just empty enough to not show on the dipstick.

I started and ran the engine a few times, no issue no problem. Then after I had messed with it, jacking the loader, and also one of the stabilizer feet on the hoe, and had heard oil returning to the system, I started it again. It was at this point I started blowing what I would describe as “snow”. One of the fittings that connected the main pump pressure outlet to the main hydraulic line running to the hoe was loose… what came out literally looked like snow. (Ruining a pair of jeans, and maybe my favorite boots in the process….) what I mean by snow is that it was white, and fairly light (like a clump of bubbles). I know milky white is bad, but I am not sure yet if my hydraulic fluid is milky white, or if when I moved the cylinders and put a little bit of fluid back in the tank it is barely enough to supply the pump and it just aerated the oil like crazy, then when it blew out the loose fitting it had a Venturi effect and aerated a little more so just looked like snow.

Layer into this that when the tractor is off, I have no oil leaks. But as soon as I start the tractor I start to get a drip from the hoe. Can’t tell where it is coming from, but it is either the dipper or bucket hoses (they all disappear together into the boom so it’s hard to tell where the leak is coming from). Layer into that that I am not operating any of the valves, so one of them must be passing fluid.

So my logic was to try and get the loader back to my place, so I can work on it with all the convenience of having all of my tools to hand, rather than working on it 45 minutes away.

I am starting to think maybe my better path is to try and fix the hydraulics where it sits, even if only just enough to get it loaded and Mack to my place. If the hydraulic fluid really is milky and white I guess the damage is already done, so use what I have to get it back to my place. Maybe throw in a gallon or two just to get everything moving, then drain and flush when I get it back to mine without wasting $150 on hydraulic fluid just to get it back to mine.

Any thoughts on that?

I think you are on the right track. Lets talk about hydraulic fluid for a moment. The problem with hydraulic fluid is that there is no governing body.... no universally accepted requirements or even preferred chemistry. All the societal and governmental constraints like SAE and Oil Institute Standards you have in lubricating oils and even for fuels do not exist for hydraulic fluids. So they can be anything from alcohols to keytones to glycerins.... or mineral or vegetable. Some eat up seals. Some turn solid, and most are either miscible or react with water. Some wll burn, some put out fires.

Mixing with water is classic hydraulic fluid misbehavior. So that is what is probably causing your snow.... a weird hydraulic fluid mix plus some water - combined with who knows what and then a fascinating change of state caused by the pressure change from pumping. The "snow" may not ruin your boots but the "hydraulic fluid" component might. Snow!! Neat!!

Find some of the lightest viscosity and least expensive MOTOR OIL or Hydraulic Oil you can get. Either will work fine for short term to get home and troubleshoot. Drain as much of what you have out, and add enough new fluid to get it circulating and TO PROTECT THE PUMP VANES!!
I'd like that reservoir half full or better when the cylinders are full. It might even work then.
Cheap is fine. SAE 10 weight is plenty viscous. 0W-10 motor oil would be even better. The pump is very sensitive to suction cavitation pitting the metal inside the pump. Terminal for the pump..... But pumps are replaceable. Still, lets get what you can out of that one.

I've got a feeling this is going to be one of those that works out well. Hydraulics on those old tractors are easy to deal with. You've got engine and tranny - that's most of the problems solved right there.

rscotty

 

[rScotty]

Are you the same person that posted questions about what two valves do on a 4500? If yes those diverters might direct oil to either loader or back hoe.

Does engine or pump change sound when attempting to operate either loader or the hoe?

Right. It could certainly have a manual diverter valve so that the operator could choose either the loader or the backhoe - but not each at the same time. Massey used a similar thing.

So it is worth looking at to see if that is his problem. Number 4 on the list of probabilities. Good catch.

The manual diverter doesn't put the systems in parallel, they are considered as two independent series hydraulic circuits as you and I have been saying all along.

Now I admit that sometimes there will be a proportional power steerig valve in the circuit prior to the manual diverter and that Power steerimg proportional valve does steal a little flow for the power steering. Spoiler alert: That PSPV really is the definition of a parallel flow system....but suggest we ignore it for now because it is small quantity.... unless nothing else works. Hmmmm..... except it could be the Power Steering proportional valve if the fluid level is low. Normally the PS has priority on any fluid for safety reasons.

Still, his best bet is to get that old fluid out and half full or better of some kind of fluid so that he can do some decent diagnostics. Then probably either you or I or several other TBNers could get him going.

rScotty

 

[LD1]

Id start by getting the hydraulic oil to proper level

 

[pmsmechanic]

Id start by getting the hydraulic oil to proper level

First drain out as much of the old "oil" as possible. My guess is that it's mostly water by now.

 

[davedj1]

Undo the hoses for the crowd boom and jack it up then chain it up , should hold till you get it home.
Same for the outriggers if they are down.
It will also work for the bucket.

 

[LD1]

First drain out as much of the old "oil" as possible. My guess is that it's mostly water by now.

Certainly worth a shot.

Just don't have enough info....like when the last time it ran? But from the sounds of it, it ain't got anything in it other than the dab purged out of the cylinders by manually jacking the loader