School me on Pressure Gauges

[Richard]

I just rebuilt one (of three) hydraulic motors. Had a small leak where the shaft comes out at the bottom. (this is on a hydraulically powered flexwing 15' mower)

Got things back together, cuts much nicer. Plumbed in a single pressure gauge (no time for the other two yet) The breakaway PSI is 1500 PSI so I have two 3,000 PSI gauges and one 5,000 (for the center blade which pops off at 2,500 PSI)

The two 1,500 PSI MOTORS are fed by the same hose. Pressure hose goes from pump to motor 1, out of motor 1 into motor 2 and back to dump tank.
Motor 1 has the drain if that matters.

So I only had one of two gauges setup at the time (I need to insert the "T" into motor 2 and haven't done it yet)

I didn't realize just how wild the pressure would jump around. It would coast around 500 PSI then suddenly I'd run over some thicker grass and it would spike to say 2,500 then drop back down..... much too quick for me under the circumstances, to get a good feel for what are the blades doing, can I hear the relief at the blades (they can be heard at times if they're under stress)

I presumed if the pressure spiked over say, 1,500, then the gauge would tend to peak at 1,500 and not go much over. I'm now guessing that how quickly it spikes verses the reaction time of the relief opening up might be an issue.

I've made a manifold out of wood to set above the steering wheel and have mounted all three gauges there. (still need to insert the "T" in one line)... but watching a single gauge was an eyeball workout. Will it smooth out a bit when I get the other gauge on the other motor in the same circuit?

Should it be able to spike to say, 3,000 (very briefly) as I hit a clump of say 4' high grass/weeds? This is what surprised me the most.... were the transient peaks that were way over the 1,500 range only to have it calm down to 500/1,000 if I got into normal stuff.

Never really having done this before, I'm just trying to better understand how to interpret what I'm seeing since it's so volatile. If it was a steady-state, I think it would be easier to see a pattern.

 

[Richard]

Another queston if I may...

Two motors are on one circuit. Given I have no clue where the reliefs are currently set, here's my question....

If one of them happens to be "reasonably" correct and the other one is way out of wack...

I presume the out of wack unit would be weak so blade performance would be poor.

Would that situation, impact the OTHER blade if it happened to be reasonably accurate?

Maybe reworded.... am I wasting my time looking at anything until I get BOTH gauges connected so I can see them working together since they're on the same circuit?

 

[oldnslo]

Do you know how the relief valve is plumbed on motor one. Does it just pass oil from inlet to outlet of that motor? If yes then what you are seeing is the combined work load of both motors since on most relief valves tank pressure is directly additive to the set point.

Are your gauges liquid filled or dry? If dry they respond much quicker to spikes vs liquid filled so you would potentially see higher pressures from spikes.

 

[rScotty]

Richard, what is it that you are hoping to learn from the pressure gauges?

Lets run through some basics. I like to begin by assuming that pressure and flow rate are constants from the source hydraulic pump - that the source pump is 100% efficient and that the RPM is stable. That isn't true, but at least it is simple and gets a basic mental picture in place

Then more pressure in the system always means that something is restricting the flow rate. It cannot be otherwise, and pressure rise is just a measure of how much the flow is restricted.

So as the cutter motor slows down due to load, the exhaust port on the cutter motor is open for less time and therefore less fluid can pass through the motor in any given time period. That reduction in flow causes the pressure to go up. It's important to keep in mind that a change in pressure is indistinguishable from a change in flow restriction. We could have gotten the same increase in pressre by simply closing down a return valve. Ultimately flow goes to nothing and when it does then pressure tells us everything. But in a system where the fluid is simply being slowed down - like in a fast flowing cutter motor - just looking at pressure alone limits the reliability of the information we get when just looking at pressure.
I hope that makes sense....

And if you think about that, you discover that there is something sort of counter-intuitive that happens when you only look at pressure. Because then your highest horsepower to the cutter motor will occur when the flow rate is maximized....and that happens when the both restriction and pressure are at a minimum.... It's something to think on.....

BTW, I think Those spikes that you see are real. A glycerin damped gauge will slow down the gauge reading enough to keep your eyeballs from bouncing, but some fluctuation in horsepower is still there.
What the spikes show is that the system is not storing enough energy to hold RPM constant. The mower is calling out for a larger flywheel. One way to have a larger "hydraulic flywheel" is to move to a Closed Hydraulic system like Deere uses on their larger tractors.

Keep on it,
rScotty

 

[Fixastuff]

On my HDD drill I have three pressure gauges. You are supposed to slow then stop pushing when the pressure gets to 3k psi. In soil it will push fine with almost no psi on the gauge. If in cobble rocks the gauge jumps all over the place.

For rotation you stop at 3600psi. In soft dirt it spin without the gauge hardly moving. In cobbles or roots it jumps all over the place.

Don’t know if that helps but here is the info I have.

 

[Richard]

Do you know how the relief valve is plumbed on motor one. Does it just pass oil from inlet to outlet of that motor? If yes then what you are seeing is the combined work load of both motors since on most relief valves tank pressure is directly additive to the set point.

Are your gauges liquid filled or dry? If dry they respond much quicker to spikes vs liquid filled so you would potentially see higher pressures from spikes.

Pressure hose comes from tank to the first motor. The "T" and gauge are at this T, just prior to motor. Exit hose goes to next motor and just before entering motor, will be the second T on this circuit. The third has its own hose/drain but it will also be plumbed in just prior to motor.

Gauges are all filled.

 

[Richard]

what is it that you are hoping to learn from the pressure gauges?

To be blunt, I'm trying to make sure the relief valves are close to where they are supposed to be.

Short story made longer.... The first set of blades (first motor in the circuit) started to cut "most" of the grass but, would leave a lot of it laying down as it bogged down. I had replaced the pressure reliefs when I had rebuilt the motors so presumed the relief needed adjustment. I turned in in.....I turned it out.... I royally screwed up the adjustable setting. Thinking the other might impact the first, I messed with it and now, BOTH reliefs are out of whack.

I think in total, I checked "locally" (both here near Knoxville, Maryville and then an hour away in Crossville with SEVEN different hydraulic shops and nobody had the ability to adjust them back to where they should be.

I asked someone if I might be able to dial them in better if I had (these) gauges on there, he said yes.... so here we are.

As this process got started, I had raised a wing for some reason.... noticed some dampness at the center and realized that though small, it was leaking and that could certainly impact the blade performance.

Ordered three seal kits and decided to rebuilt just this first one. Now it's back to cutting "almost" like a monster but the second set of blades can now leave grass on the side instead of cutting it (mind you this is 4-5 feet tall grass & weeds that haven't been cut in a year so some of that laying down isn't going to surprise me)

Still.... with the rebuilt motor, (just one) the performance is now improved but I know the reliefs are.... well....let me reword that.... I DO NOT know where the reliefs are as I have no reference. Hence, the pressure gauges to try to see what's going on and help me dial them in.

If it doesn't work, then I've had my fun tinkering and spending money on something that might not work for me.

It's dawned on me it might have been net cheaper to simply buy two more reliefs and be done with them.

 

[Richard]

If it matters, I think the mower has a 75 HP minimum tractor size and I'm using a IH 1066 which I think has 125 at the PTO. (got it primarily so it could STOP the mower going down some of our hills)

 

[rScotty]

To be blunt, I'm trying to make sure the relief valves are close to where they are supposed to be.

Short story made longer.... The first set of blades (first motor in the circuit) started to cut "most" of the grass but, would leave a lot of it laying down as it bogged down. I had replaced the pressure reliefs when I had rebuilt the motors so presumed the relief needed adjustment. I turned in in.....I turned it out.... I royally screwed up the adjustable setting. Thinking the other might impact the first, I messed with it and now, BOTH reliefs are out of whack.

I think in total, I checked "locally" (both here near Knoxville, Maryville and then an hour away in Crossville with SEVEN different hydraulic shops and nobody had the ability to adjust them back to where they should be.

I asked someone if I might be able to dial them in better if I had (these) gauges on there, he said yes.... so here we are.

As this process got started, I had raised a wing for some reason.... noticed some dampness at the center and realized that though small, it was leaking and that could certainly impact the blade performance.

Ordered three seal kits and decided to rebuilt just this first one. Now it's back to cutting "almost" like a monster but the second set of blades can now leave grass on the side instead of cutting it (mind you this is 4-5 feet tall grass & weeds that haven't been cut in a year so some of that laying down isn't going to surprise me)

Still.... with the rebuilt motor, (just one) the performance is now improved but I know the reliefs are.... well....let me reword that.... I DO NOT know where the reliefs are as I have no reference. Hence, the pressure gauges to try to see what's going on and help me dial them in.

If it doesn't work, then I've had my fun tinkering and spending money on something that might not work for me.

It's dawned on me it might have been net cheaper to simply buy two more reliefs and be done with them.

A relief valve is basically there to keep pressures under a safe level The relief valve actuating pressure should be set far enough above the system pressure that the relief valve never actuates.
like a smoke alarm, a relief valve is doing fine if it never does anything at all.

You can easily set them yourself with a "pop off" type relief valve setting tool. You don't even need the mower. Google the tool online. Basically it is a hand pump and a pressure gauge with a fitting for the relief valve. There is a pressure gauge built in. When new they are about the cost of a good torque wrench. I'd get a manual pump up type with a 3600 to 5000 psi max range.
rScotty

 

[WranglerX]

Maybe (or maybe not) something like this video may shed some light on procedures and specifications....