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?