What is "normal" operating temperature for variable piston pump?

[WesRo]

Hi everyone, I have a (new) John Deere 35G/Hitachi, but that does not really matter because this should be a more general question.

What is the typical operating temperature for the hydraulic pump? and after operating for a while, how hot do the cylinders feel? And what is your ambient temperature?

For me, on a cool day (21C), the temperature is about 42C on the cylinders, and 44C on the tank return line (after the oil cooler). This temperature seems normal, however on a hot day (34C), the cylinder temperatures are like 68C, and the return line is about 75C, and the outside of the pump itself is a few degrees hotter. This seems way to hot, because the oil viscosity of vg46 oil is about 10mm2/s at 75C, thats below the minimum of 20mm2/s for my pump.

However, from what I read online, its normal for operating temperature to be 38C above ambient, but not sure how true this is...

The pump does not seem to have a pressure compensation, just has one unknown line going to the base of the pump thats connected to a solenoid valve from the pilot pressure, not sure what this does. I do notice the hydraulic movements gets slightly slower under load. What adjustments can I make to the pump?

If anyone has any information, please share your knowledge with me. The dealers/service does not have useful information regarding this.

Thanks
Wes

 

[Cujet]

Load dependent. Hard working hydraulic systems have considerable friction and often need coolers.

 

[DriveHard]

Pictures would help...of the pump ;-) Former hydraulic system design engineer here...

If it is indeed a variable piston pump, the pressure compensation function (PC) is built into the pump control...no hoses or lines needed.

If it is a load sense pump, a small -4 hose will communicate load pressure back to the pump. The variable pump will maintain a margin pressure 15-30 bar above the working pressure needed to move the load. This is NOT pilot pressure, but a line back to the pump from the valve, and it will be at the load pressure (HIGH).

The temps you are seeing are perfectly normal. If you are below require viscosity, I would increase the fluid viscosity based on the temps you are seeing.

If it is a load sense pump, and the load slows down, it sounds like it has a torque control. The torque the pump requires to turn is simply the product of the displacement of the pump times the pressure. So...as pressure increases to move the load, the maximum pump stroke will be limited to keep from killing the engine. If this is the control you have, then it is usually a mechanical system of springs and ramps that may or may not be able to be adjusted. I would need to know the manufacturer of the pump, and control type. This can also be done electronically...I actually hold the US patent on that design

Happy to help more, but I will need more details. Ideally a schematic.

 

[DriveHard]

Ah...that is a mini excavator. It might have a negative flow control which is a pressure dependent pump control...so as pressure increases, the flow will decrease, slowing the movement. Common control scheme for excavators.

 

[DriveHard]

https://www.deere.com/assets/pdfs/common/products/sync/DKAXG35-35G-compact-excavator.pdf

Looks like you have a negative flow control system. So you are correct, there is a pilot line back to the pump. Here is a good explanation of how that control system works. It is what we would call a "soft" control in that it is pressure dependent (slows down as pressure increases). That is usually considered a good trait for excavator controls.

Negative Flow Control vs Load Sensing - Which One is Better?

In this short post I talk about the main advantage of NFC hydraulic systems over the classic closed center LS systems

www.insanehydraulics.com

That article refers to it as stiffness.

 

[DriveHard]

10mm2/s at 75C, thats below the minimum of 20mm2/s for my pump.

The pumps similar to this that the company I used to work for had an upper temp limit of ~105c and lower viscosity requirements of 9cST continuous and intermittent rating of 6.4cST. A target of about 12cST should give the best balance of efficiency and life.

 

[Cujet]

On our Gulstream jets, the hydraulic system is cooled through an "S" series of tubes in the bottom of the fuel tank. This serves to heat the fuel a bit and prevent gelling. And keeps the hydraulic system happy.

 

[WesRo]

Thank you this is great information! I cant thank you enough, I will be reading all about the negative flow control system.

I wish I had a schematic, but I have a few pictures, the solenoid valve block you see is right after the pilot filter, those hoses get distributed to valves/spools, pilot pressure should be about 590psi regardless if the pilot is "enabled" or not. I dont know what that solenoid does, I checked it and it does not get voltage on eco or power mode.

The main pump is a Nachi PVD-1B-32CP-9AG5-5264A, I was not able to find specs for this exact model, just that its a constant horsepower control: https://www.nachi-fujikoshi.co.jp/eng/yua/pump/pvd01a.htm

I'm wondering how I can adjust the max power the pump will use...

Here is the solenoid valve block, I *think* all the hoses on top are connected to each other, there is a connection at the bottom we cant see, but that connects to "From valve" in the other images. Again, not sure what this valve does, or its internal connections.

As a side not, this pump has no case drain. Please tell me I'm wrong, but this is a bad design imo - without a case drain, it will have higher temperatures, and on a catastrophic pump failure, it will guarantee to also destroy all your spools, because the pump has no where to drain dirty oil on a failure, only to ingest it.

Thank you all again, I'm very grateful for you helping and teaching me more about this! I will also be looking at thicker oil, maybe like 68.

 

[WesRo]

Ah, so from that article that @DriveHard provided (thank you very much for that, I want to learn more about hydraulics!), it says:

"This is less efficient but offers a much more stable control"

I assume less efficient means more heat in this case, so knowing that I'm sure the pump/valve/cylinder manufactures will design it with higher temperature tolerance (I hope). And as you said the temperature is normal. Also this 46 weight oil is factory, and I believe the bottle says it works up to 40 or 50C ambient temperature, that would be about 78-88C hydraulic temperature, I guess the zinc free stuff can handle higher temperatures...

My hydraulic oil analysis shows a viscosity of 47 cSt at 40C, and 7.49 cSt at 100C, I guessing that might be ok for the temperatures I'm seeing, but I might want to switch to a thicker oil(?).

Also, I keep the hydraulic oil really clean, with the addition of a low flow pilot return filter, the fluid has a cleanliness of at least ISO 15/14/11. From what I found on the internet (for general pumps, not mine specifically), during a high temperature operation, and cleanliness of 19/17/14 is required, and a general cleanliness of 20/18/15 during normal operation. So my fluid is well within the very clean spec. So I hope the clean fluid will help maximize the life of the pump and spools. Not that I'm really going to be putting 12000 hours on it, but I just like to keep the hydraulics at operation at optimal. And I'd imagine lower temperatures = longer life for pump, spools and cylinders, especially the cylinders because they have a lot of rubber seals.

 

[DriveHard]

Just a guess, but without a schematic, it is just...well, a guess.

That valve block would be called an HIC (hydraulic integrated circuit). It appears to be your hydraulic pilot pressure on/off as well as slew motor brake release?

To save money, it can be done that the pump case drain vents into the inlet of the pump...meaning leakage from the kit goes to the case, then there is a passage for it to be sucked into the inlet again. This eliminates a leak path and a hose which saves money. If you shell a pump, having a case drain will do NOTHING to save your valves downstream. The only way to do that is an expensive pressure full flow filtration system $$$$, and almost never included in OEM equipment. It means slightly higher pump outlet pressure, but not a big deal. It might also drain through the aux. pad into the gear pump and go through that drain.

You have a single kit pump that uses half the pistons for one "service" and half for the other "service". This can reduce power consumption because one outlet can be operating at a lower pressure than the other. It does limit performance though, as it destrokes BOTH outlets when pressure on one side goes high.

The marketing material is false...that is a traditional torque control. It is set with a ramp and spring usually inside the pump control. You see on the graph, the flow is adjusted based on pressure. If it were a true power control, speed on the input shaft would be required, but it is not. Just a torque control.

This might be the print for your pump...

https://www.miniexcavator-hydraulic-pumps.com/pdf/14-NACHI-PVD-2B-40P-16G5-wb.pdf

Interesting, it appears that the pump outlet pressures on P1 and P2 are reduced when the pressure on the gear pump outlet P3 increases. So...if you know what P3 is doing and can reduce that pressure, it will increase what you can get out of P1 and P2.

Here is the 35U (Hitachi) guessing it is the same? Best version I could find...hard to see. It looks like there is no signal back to the pump, it is just operating open centered...meaning it is always at full stroke, and only destrokes when it hits the pressure curve. It doesn't look adjustable...but I bet if you turn that screw in on the end of the servo it will increase max pressure (where it says do not adjust in view A). I would put a pressure gauge on it though...you could blow things up pretty quick. I do not support doing this, as it will shorten life and might destroy your pump and machine...but if it were mine, I would be tempted to start there.

 

[DriveHard]

That screw on the end of the servo might also be the displacement limiter...so it might change the maximum flow you can get out of the pump as well...until you figured out for sure, I'm not sure I would mess with it.

Honestly if it were mine, I would have to tear it apart ;-)