PSI/RPM curve for BX 25

[Luke'sScreenName]

Lifting something to heavy and my loader will go into relief at idle, increasing RPM's makes little to no difference in lift power.

Something to think about,
Going into relief does not mean no more pressure can develop. There is resistance in that relief valve and more flow through the valve increases the resistance. (However, your relief valve certainly could be different).

If you get a chance, put a gauge on a loader line and see for yourself.

In real world use I see the difference. I might be running at 2200psi and try to lift something and be unable. Often if I bump the RPMs to 2500 or 2800 suddenly I can make the lift.
:confused3: :drink:

 

[SPYDERLK]

My MX4800 is the same way. Always makes me chuckle when someone regurgitates the TBN dogma that RPMs don't dictate hydraulic force.

Yet your own numbers show the dogma; if you know to subtract its simplism.

Here is the data points for my MX.

View attachment 478609

And a few hundred PSI make a much bigger real-world difference than one might think....

Pump leakage/internal bypass.

Something to think about,
Going into relief does not mean no more pressure can develop. There is resistance in that relief valve and more flow through the valve increases the resistance. (However, your relief valve certainly could be different).

If you get a chance, put a gauge on a loader line and see for yourself.

In real world use I see the difference. I might be running at 2200psi and try to lift something and be unable. Often if I bump the RPMs to 2500 or 2800 suddenly I can make the lift.
:confused3: :drink:

And ^^ this. ... I have a full time pressure gauge on the pump pressure line on the 7520. All I have to do is deadhead on curl, or any other convenient action, and top system pressure is shown.
-- Pressures cool; 3000@900 rpm idle/3100@2500rpm. Pressures hot; 2500PSI@idle, 3050PSI@2500rpm. ... It is a 17GPM pump with system relief set @ 3000PSI with the system ~ warm and at 1500 RPM.

... The hot figures show that internal pump leakage limits the system to below relief pressure at idle. The big pump overides its leakage pretty quick as RPM rises. I dont have a hot curve but I know that theres always 3KSI available at 1500RPM.

 

[Luke'sScreenName]

"Yet your own numbers show the dogma; if you know to subtract its simplism"

Spyderlk, can you expound? I truly don't understand and am probably missing something.
---
As for the rest of what you said- I'm a little on the simple side. Do you generally agree with my point or disagree?

Also, would I be correct that the temperature difference is primarily related to viscosity and this would effect internal pump leakage AS WELL AS relief valve resistance?

Again, I am simple but this is an interesting conversation to me but may be 'old hat' to others. Thanks for your patience.

 

[SPYDERLK]

Here is the data points for my MX.

View attachment 478609

And a few hundred PSI make a much bigger real-world difference than one might think....

"Yet your own numbers show the dogma; if you know to subtract its simplism"

Spyderlk, can you expound? I truly don't understand and am probably missing something.
---
As for the rest of what you said- I'm a little on the simple side. Do you generally agree with my point or disagree?

Also, would I be correct that the temperature difference is primarily related to viscosity and this would effect internal pump leakage AS WELL AS relief valve resistance?

Again, I am simple but this is an interesting conversation to me but may be 'old hat' to others. Thanks for your patience.

No problem. I welcome the questions. ... Yes I generally agree. Your posts hint at some of the complicating factors that prevent the [actually true] dogma from seeming so.

Altho hyd pumps are nominally positive displacement there are internal clearances that allow some leakage of output side fluid back to the suction side. A slow turning pump at high pressure can leak all its "output" back to the suction side and therefore provide no flow only pressure. Hot fluid/ low viscosity exacerbates this. When sped up the pump overcomes the leakage to the point that pressure rises until the tractor relief valve cracks and limits further pressure rise by dumping fliud to sump. As you say pressure can rise a bit more with speed and that will depend on design and size of the relief valve and the flow volume and the viscosity of the fluid.

From your curve Ill guess that:

Your pump is in pretty good shape,
Your relief valve is set at ~2500PSI,
Your relief is undersized for the full flow your pump provides above 2000RPM,
You did the test warm not hot​

Clues to this come from the high escalation of pressure above 2000RPM and your relatively high low speed pressure - a little below relief so not cool, but not much below relief so not hot. The rather quick pressure rise above 2000 RPM pretty much convinces me the relief valve is overwhelmed by the flow. I would expect less rise with hot fluid.

Comparing to the figures on mine where I know the pumps rated flow, relief pressure, and the temperatures of test I can guess that:

My pump is in pretty good shape based on its low speed pressure hot vs cold. Its delivering no flow when deadheaded hot though - but resumes the capability to crack the relief a few hundred rpm faster,
The relief is well sized for the pump flow because it limits pressure pretty close to 3000 regardless of fluid temperature.​

More descriptive info ... Posts 10, 15 etc:

http://www.tractorbynet.com/forums/owning-operating/288691-another-newbie-question-fel-general.html

 

[Luke'sScreenName]

Very insightful. The tractor was at "operating" temp. Of course what is "hot" is relative.

Im trying to imagine a modern tractor where there is not resistance resulting in increased working pressure above relief.

In my case, this increased pressure adds several hundred pounds of increased lift or break-out force. So for either of the modern tractors with loaders I have owned the statement that "increasing RPMs makes little to no difference in lift power" is false. (and I suspect it is also false for most tractors)

I've read some "evidence-based" estimates on TBN where one PSI being equal to one pound of lift capacity is a good place to start.

Where is my logic off? I'm open to being wrong here.

 

[SPYDERLK]

Very insightful. The tractor was at "operating" temp. Of course what is "hot" is relative.

Im trying to imagine a modern tractor where there is not resistance resulting in increased working pressure above relief.

In my case, this increased pressure adds several hundred pounds of increased lift or break-out force. So for either of the modern tractors with loaders I have owned the statement that "increasing RPMs makes little to no difference in lift power" is false. (and I suspect it is also false for most tractors)

I've read some "evidence-based" estimates on TBN where one PSI being equal to one pound of lift capacity is a good place to start.

Where is my logic off? I'm open to being wrong here.

For me "hot" is a lot of loader work at ~ 2KRPM prior to measurement - manure pile work is a good high continuous load with lots of manipulation. The valve gets too hot to touch more than a second w/o a burn. Its convenient to have the gauge plumbed in.

There is virtually always more, but depending on PRV design and its size in relation to pump capacity the gain can be almost negligible. With the 7520 for example, I have found stumps I can barely lift off the ground at 2000rpm - then revved to 2500 and gotten only another foot higher.

This is governed by loader geometry and cylinder size. ... You may find a tractor size/loader size combination that accidentally follows that rule. The 7520, a larger platform is rated 3900pounds [at the pins I think] to full height with 2500PSI. This appears quite conservative based on a lift force test I did with bucket on, reported at the end of the following post quoted from another thread. 5100 pounds center bucket at 1 ft height - but at 3000PSI vs 2500
.

I have had the opportunity to do measurements at points on a boompole using a Dillon force gage. I used the 7520 as the test platform and turned hyd pressure down to 1000PSI so I wouldnt bend the boompole. I did 2 setups:
1]

Used a chain in place of the toplink to get the lift ratio as close to 1 as possible. Best I could do was 3/4 - - 24" lift at the eyes and 32" at the end loop of pole 108" out from the eyes.​

2]

Used the toplink adjusted to max length. The best lift ratio I could get at the pole end was 0.61​

The lift forces I measured in the 1st case were 1710# at the eyes, 1160 @ 56", and 980 @ 108".

The lift forces in the second case were 1700, 1200, 880 respectively.

,,,I believe the anomalies shown are due to my neglect in assuring that the lift was always done at the same eye height. The force available at the eyes varies some with height.

I did one more experiment in case 2 by successively shortening the toplink 3 turns at a time. Here I did take care to hold eye height pretty close to constant. The force measurements at 108"were 880, 850, 800, 760, 725.

While I had the force gage I took some loader measurements too. I found that it took 700PSI to get the loader to rise with the Tilt-tatch and bucket -- and 800PSI to get it to full height. I then turned the pressure up to 3000 as I normally keep it set. Center bucket force was 5100# at 1 foot lift height. ... Quite a surprise since this is 26" forward of the pins.​

 

[Luke'sScreenName]

Good stuff! I followed that thread closely. I might need to permanently mount a pressure gauge just for the fun of it.

You are correct that the fluid would not have been that hot. Maybe just barely too hot to hold onto a metal fitting.

I wonder how linear the temperature - viscosity curve is? I bet the expensive stuff at least has a 'flatter' responce.

 

[SPYDERLK]

Good stuff! I followed that thread closely. I might need to permanently mount a pressure gauge just for the fun of it.

You are correct that the fluid would not have been that hot. Maybe just barely too hot to hold onto a metal fitting.

I wonder how linear the temperature - viscosity curve is? I bet the expensive stuff at least has a 'flatter' responce.

I use Kubotas SUDT. Better in the cold and probably a little in the warm too. Pulling 17GPM thru the filter in the hyd suction line has me looking for good low temperature flow qualities.

If you go full time gauge use glycerin filed and use a critical orifice at the pump end of the hose feeding the gauge. That will prevent danger from a hose failure or a gauge gusher should the bourdon tube fail. I used a set screw threaded into the hose fitting - oil spirals thru the threads. Limits hose flow to a few drops per second.