The following is an unedited transcript from a recent Fluid Power World Webinar: Load Sense Hydraulics Simplified. FPW Associate Editor Mike Santora moderates with the presenter, Carl Dyke.
Mike Santora: Hello and thank you, everyone, for attending today’s webinar, Load Sense Hydraulics Simplified brought to you by Fluid Power World magazine, Ametek, and Higginson. We would like to thank our presenter, Carl Dyke, for being here today. I’m Mike Santora Associate Editor for Fluid Power World Magazine and I’ll be your moderator.
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Just a couple of house-keeping details before we get started, you will see several boxes on your desktop, all of which can be moved around to suit your preferences. Initially, the Q&A box is at the lower left; this is where you will enter your questions for the Q&A session. Another box to note is the additional resources, initially at the lower right-hand corner of your desktop. These resources are for your information needs. We also have a tweet box right on the desktop, so feel free to tweet any interesting points right from there. There’s a list of hashtags for you to use as well.
Carl Dyke is the founder and chief educational officer of CD Industrial Group, a company focused on fluid power education for technicians. Carl’s company created the eLearning site LunchBoxSessions.com. Carl is an industrial mechanic whose career began as a boy in the lumber and logging industry. He spends much of his time each year in the classroom and on the shop and factory floor helping technicians with their hands-on skills development. And so, without further ado, here’s Carl Dyke.
Carl Dyke: Thank you, Mike. It was good to meet you and the entire Fluid Power World team at the recent Fluid Power Technology Conference there in Milwaukee. That was a great time. Thanks for hosting and moderating this morning, Mike.
Load sense hydraulic is a deep topic to try and cover inside of a one-hour session. There are so many design and engineering perimeters for manufacturers of machinery that integrate a load sense type of system. In this particular webinar, we’re going to try and stick to the basics and illustrate them as best we can for practical purposes.
A load sense system, if you think about the typical pressure compensated pump, the type of pump that might only have three hoses, the type of pump that has a high-pressure cutoff only, that’s the type of pump that can match the flow demand that occurs as you open a throttling valve, a proportional directional valve. But as we’re going to find out, a load sense system adds a fourth hose, a control hose to that pump. It allows the pump also to match the pressure needs and the pressure demands. We’re going to find out more about that as we go.
We’re going to start with a video. We’ve prepared a video to help simplify the understanding of some of the most important features and components in a load sense system. The video’s about 15 minutes long. It’s narrated. In the very last three minutes, the climax, if you will, that’s where we’re going to get into some of the most key features of what happens in a load sense system as it’s multi-functioning.
CD: All right, well I hope that your opportunity to fly through a load sense system in 3D helps a little bit with what is the sometimes slightly dryer, more obscure terminology that goes with load sense systems. Why is a load sense system so attractive? Primarily because of the opportunity to save input energy, which also translates in so many circumstances into a reduction of heat, but how is that done? Well, for a moment, let’s go backward to a time of simpler hydraulic systems, systems that are still in place for some simpler machinery, consider the gear pump and the relief valve.
A gear pump is always pumping at its maximum flow. If we’re using a proportional valve to meter the flow rate to an actuator, then we’re always operating this hydraulic system at maximum energy consumption. What you see here on a cut-away model with the gear pump and a relief valve, you will also see that the proportional valve is only set to half open in order to control speed as desired to the cylinder. Well, if that’s the case, our gear pump is pumping at full volume, of course, but also we are pumping over the relief valve, the unused fluid that is not desired at the cylinder. Therefore, we’re also pumping at maximum pressure.
The energy consumption is what you see by the horsepower line, which is the dark blue diagonal, all the way across the red zone. That’s a picture of our input energy. Well right now, we’re running with an enormous amount of input energy where, at the business end of the hydraulic system over there on the right past the hydraulic cylinder, we can see the energy requirement for flow and pressure at the cylinder is actually quite low. The horsepower line in green shows energy, or the horsepower output that we actually need to move the load; so a very inefficient system. I think you get the picture there.
Then, enter the pressure compensated pump, the type of pump that has only one setting, a high-pressure cutoff point. Well, in this type of system, this pump still operates at its maximum pressure. It’s matching the flow output to the demand at the proportional valve, but we’re still operating at maximum pressure. This is still a lot better than the gear pump. Gear pumps in some systems aren’t more than 10% energy efficient, where the pressure compensated pump arrangement can possibly make it possible to get to 25% energy efficiency.
Here we see an energy picture for the pressure compensated pump. What we can see that’s changed is while the flow rates of the pump now matches the demand through the proportional valve, we see that we’re still pumping at maximum pressure, but at least that diagonal dark blue line is now shorter by at least a percentage, and those energy savings are welcomed; both from in terms of cost of input energy, and potential for building up undesirable heat in the system.
As we bring in the load sense pump, which has an additional compensator, some manufacturers refer to that second adjustment as the flow compensator. But I think really what you saw in the video, and what you’ll see as we continue here, is that, that flow compensator, that load sense adjustment is just another pressure compensator with a softer spring setting. We also bring in pressure feedback on a special signal line, as we’ll see in the next screen, that ties in from the load at the cylinder. What we’re really doing is we’re setting pressure compensation on the fly. There’s a big energy savings potential associated with that.
In this image here, that thinner, narrower, yellow horizontal line you see it going from the proportional valve back to the pump’s compensator controller, that’s our load sense signal line. That’s our opportunity to instruct the pump about what’s happening at the load in the cylinder. We could have that load sense signal line coming right from the cylinder, but it’s generally more convenient to have it pick up either the A or the B work port lines from the cylinder just inside the directional valve, as shown here.
Here’s our energy savings curve when we look at where the big potential is for energy savings and heat reduction in a load sense system. What we’re seeing now is that this system not only matches the flow rate requirement, the pressure compensated pump did that. But the load sense system is also matching the pressure issues, the pressure requirements in the hydraulic cylinder, and producing a pump outlet pressure that’s only slightly higher than what’s needed at any one time. Look how short that diagonal dark blue line is now. There is the big savings. The opportunity here to go from pressure compensated pump systems, which might reach 25% energy efficiency, right up to potentially 60% energy efficiency for a load sense system. My source on that statistic came from a parker, a pump division manual that I was looking at to verify yesterday; and just refresh on that.
Looking at a slightly simpler model for a moment, getting away from a fancy proportional valve. What we’re really talking about for controlling the speed of an actuator is some kind of metering device. Here we’ve just installed a very simple needle valve, just to simplify matters for a moment. While these are stills from the simulations in our lunchbox sessions, stills are all we can work with on these screens.
If you imagine for a moment that I click on the green plus button, which very gently just adds one brick to the stack on top of the cylinder; which then might perhaps increase our load pressure from 900 PSI, that’s the gauge on the right, up to 1200 PSI as we add that one brick. That pressure value is transmitted along the load sense signal line up the yellow vertical and across horizontally to the left to the pump’s controller; where that 1200 PSI would be added to the 300 PSI perhaps spring setting in the load sense compensator, to then change the left gauge reading before the flow … before the metering valve. Increasing that one to 1500 PSI, and thereby always maintaining a 300 PSI pressure drop as seen on the bottom gauge, our delta P gauge. The type of gauge that has two pressure inputs to it.
What we find out is that no matter what happens to our load at the cylinder, adding or removing bricks as a way of seeing pressure changes, our pump is constantly changing its maximum output pressure and holding a value that allows us to have steady flow through the flow controlling device without needing to work at very high pressures. Lots of energy savings there.
You might be thinking for a minute, what happens during shock load? Well yeah, that’s where things start to get a little bit more interesting. What happens if that third brick drops from the sky, let’s say, instead of being added gently? It hits down hard on the cylinder as its lifting up. Well, that might induce a pressure like what we’re showing on the right-hand gauge at the moment, 1500 PSI.
Well, if our left side gauge was still at 1200 PSI, the momentary pressure compensation value from the pump. Well, now we have a reverse delta B of negative 300 PSI. As you could imagine, for a very brief moment in time, this cylinder might grind to a halt as the new load sense pressure is transmitted to the pump, which can happen a little slowly. In some cases with shock pressure, there may be a shudder or a brief interruption in the cylinder’s motion, and enter in discussions about exactly where should the margin pressure be set on a load sense pump. Well, we’ll come back to that one in a moment.
Let’s go to the next level, one that we looked at in the video where things get most interesting. That is when we have a multifunction load sense system, where we’ve got more than one parallel application. In this case, enter in a very small little element inside the load sense system, often just a very small ball bearing, the shuttle valve; sometimes called ball resolvers. One in each section of a multi-section valve bank, and also pressure compensators in each valve section. Why are they needed?
Here is a schematic depiction of a simplified two section load sense system. On the left, you’ll see our blue return line through a cooler and filtration to tank. In the middle, we’ll see our load sense pump with its controlling compensator shown in full detail. Then over to the right, a two-section valve bank with, in each case, a hydraulic motor. We put some brake shoe brick stackers over top of those motor symbols so that we’ll be able to imagine different pressure loadings for each section.
Let’s zoom in a little bit and move that return to tank line off to the left. Now that we’re just looking at … the pump on the left, and the load sense capable two section valve crank on the right take a moment to notice down low in the middle, a line marked ‘LS.’ Leaving the valve bank in orange, and it is our load sense signal line transmitting up to port X on the pump compensator, top left. Also notice the common pump line in red entering in through the port marked ‘P’ on the lower left side of the valve bank, and moving along the bottom of the valve bank supplying two applications. Two valve sections in parallel.
The interesting features that we’re drawing some attention to here, and you saw them in 3D in the video as well in green. We see circled the load sense shuttle valves, the ball resolvers. You saw what tiny little parts those were in 3D in the video, and highlighted in purple, our pressure compensators, also very necessary. There’s a reminder of the components that we were talking about in the video.
Adjustment procedures is an area where things get kind of interesting. There’s some different terminology that can be applied here. I think one of the things that’s really important, I’m going to jump to the bottom bullet point first. That is to check with the manufacturer of your pump, or check with the manufacturer of your machine, and find out what they recommend as the best and safest procedure. Make sure you have training and guidance the first time that you’re making these adjustments to keep safe around potentially hazardous pressurized hydraulic systems.
Typically, that pressure compensator adjustment, that pressure cutoff, the one with the stiffer spring, that one will be adjusted first at the highest pressure. To make that adjustment, that may require that the load sense adjustment be tightened right down to the bottom so that it doesn’t react, or it might be recommended that the load sense signal line coming into that compensator be connected somehow to the pump’s main outlet during that first setting. Again, check with your pump manufacturer for that.
Then, the load sense adjustment is set to the required standby pressure, which is going to be a low value, often in the range of around 300 PSI. That’s the standby pressure that would be present on the main outlet of the pump when the system is in neutral; meaning that there’s really no consumption flow path through the machine, that the valve handles have been released. We’re in a neutral state.
Alternately, some manufacturers of machinery want the adjustment process to involve the measurement of pressure differential across a valve section from the inlet of the valve section to an outlet to, say, a hydraulic motor while oil is flowing. Those are the two typical procedures there. Again, we’re moving through this very briefly. It’s very difficult to cover all facets of load sense hydraulics inside of a one hour session, but hopefully this is giving you a sense of some key issues.
On the maintenance side of things and troubleshooting side, what we find in our travels is that the margin or standby pressure setting has to be set correctly. If it’s set too high, if we widen that red margin that you see there, that’s going to cause heating in the hydraulic system that wasn’t there before. That’s a waste of energy as well, which could be electricity or it could be diesel fuel. If it’s set too low, that may allow for some momentary stalling or shuddering in hydraulic cylinder action. For reasons tied to the example I was showing you earlier, where we had a shock loader, perhaps a sudden increase in pressure loading at the actuator.
In our travels over 20 years, so many problems that we end up helping to troubleshoot that tied back to contaminated flue, a flue that’s contaminated specifically with solid particles. What we haven’t shown in schematic form is that in many cases, … but I think you saw it in the video. In many cases, there is a damping orifice in the load sense signal line. The reason that orifice is often installed is so that momentary, very short lived changes in pressure loading at the hydraulic cylinder or motor, if they’re going to be very short lived changes in increases in pressure, we may not want to tell the pump’s compensator about those pressure changes; at least not tell the pump fully about them. Because by the time the pump may upstroke to react, the pressure loading at the cylinder may have gone away. We can end up into an osculation cycle where this is some unevenness in cylinder speed produced by the slow feedback that occurs as a pressure shockwave travels through the load sense signal line.
Quite often, there’s a load sense damping orifice, and that’s a very small opening. Again, doesn’t take much contaminants in there to now cause a slowdown at actuators when the work becomes harder. In other words, higher pressure.
Again with contaminated particle, contaminated fluid, a plugged bleed down orifice may be a problem. What’s the bleed down orifice? Well, all load sense systems typically have this. It’s either in the pump’s compensator, or if it’s not there, if you don’t find it in the pump’s compensator, the bleed down orifice might be in the load sense valve bank itself. Basically, why it has to be there is when you let go of the valve handles, when we say, ‘Hey, we don’t need our hydraulic cylinders or hydraulic motors to move for a period of time, that’s the time to really save input energy and allow the pump to idle down to that low standby pressure.’ If that bleed down orifice becomes plugged, then the pump’s outlet pressure may not drop down to that low standby pressure. We’ll be wasting input energy and building up unnecessary heat. …
Yes, the contaminated … again with particle contaminants, we have one example from this model of valve bank sitting in our shop where another component coming apart in the system had shed some aluminum particles. It caused the pressure compensator to become completely stuck and jammed in place. What happens in this case, if the pressure compensator can’t move, is that we’ll often see a speed up or slow down at the lower pressure actuators on the valve bank when the heavier loaded valve sections experience their pressure changes.
That’s an example or a fairly brief tour through what is a load sense system, and how does it function, and some of the trouble shooting issues that you might bump into. Again, many of the images you saw in here are simulations from our LunchboxSessions.com, where you can interact with them fully.
I think that’s it from my end, and I think from here, Mike probably wants to take back over. I’ll let him moderate the question and answer period.
MS: Okay, thanks, Carl. Yeah, so we’re going to move on to some questions. The first we have coming in for you, Carl, is what is the most common load sense margin pressure value?
CD: Well, yeah, I’m not sure. I haven’t seen all of the load sense machines out there in the world that there are to see. There are some machines that are set fairly tight, may even have a steel load sense signal line on them. We’ve seen the odd one or two where the load sense value is down as low as 150 PSI or 200. In that type of machine, there’s an expectation that there really won’t be any shock loading at the cylinders, bumped into some machines set quite a bit higher. A lot of machine designers seem to sort of hone in around the 300 to 350 PSI range, seems to work out well in terms of being able to deliver consistent flow rates through most proportional valve banks. If I’m asked to say what I’ve seen from my experience about 300 to 350 PSI is the most common.
MS: Okay. So now Carl, why is the diameter of the load sense feedback hose so small?
CD: Yeah, right. Sure, on many machines, the load sense signal line could be a dash six, a three-eighths, or even a dash four, or a quarter inch load sense signal line. Keep in mind that if that load sense damping orifice is in there, you will have seen it in the 3D video there just where the load sense line left the multi-section valving. That orifice is quite a bit smaller yet. It could be an O-6-0 or down to O-3-0. Considerably smaller yet.
The physical size of the load sense line, in many respects, is actually quite large. Keep in mind that the load sense line is really not carrying any noticeable amount of flow. Yes, there is flow through the bleed down orifice when the valve handles are in neutral. There is a continuous flow, but it’s very, very small. The load sense signal line is really there just to transmit a pressure value, which can move as a shock wave through the fluid, whether it’s flowing or not.
In some regards, the load sense line is actually quite large. The only time where I hear people running into any significant problems is where the load sense line might be 100 [inaudible 00:42:27] or longer on some oil field drilling machinery, and when it’s operating in arctic conditions, that can be a problem.
MS: Okay, so here’s a question that I’ve actually had myself, because I hear both terms used. Is there a difference between margin pressure and standby pressure?
CD: In most cases, the answer to that is no. Certainly, the answer is no in terms of load sense system ideals. But there is the odd machine that we bump into where all of a sudden, there is a difference where the margin pressure may be referring to the setting of that softer spring, perhaps let’s say 300 PSI, that softer spring and the compensator. Due to the way the machine works when it’s running, or perhaps when the engine is at high idol, the machine manufacturer may have allowed for a signal higher than zero PSI to be transmitted back to the load sense compensator on the pump.
They may be allowing for a 50 or a 100 PSI signal to keep the system in a higher state of readiness for the valves when they open. In that case, we’ll see that the standby pressure, which you might measure on the pump’s outlet at that moment, might actually be 50 or 100 PSI higher than the margin pressure you set earlier, which was just the spring setting itself.
MS: Okay, Carl. I have another question I want to throw at you. It looks like somebody has asked is there a difference between post and pre-compensated valves?
CD: Oh yeah, okay. The pressure compensators in the valve banks themselves, I think is what’s being asked here. Yes, there is a bit of difference. There have been other webinars on that. We followed some other valve bank manufacturers. We’re not the designers or manufacturers of any valve banks, but yes, we do notice that some mobile machinery manufacturers choose to go with the pressure compensators directly on the A and B work ports as we’re traveling out to the hydraulic cylinders, as opposed to our example where the pressure compensator was coming in before the P port on the spool.
If I understand correctly and if my memory serves, I believe that in many cases, you get a more accurate … and faster acting compensation for extremes of changes in pressure loading at the cylinders when the pressure compensators are post spools. I think there’s a couple of arguments, couple of schools of thought there, and different valve bank manufacturers argue for the benefits of one versus the other.
MS: Okay, Carl. Well, that was fantastic. Thank you so much, a great webinar. I think everybody got a lot of great content. I just wanted to remind everybody on the webinar right now that if you do have any additional questions, you can refer them to the speaker bio box containing contact information for either Carl or myself; Just contact us.
Lastly, of course, I want to thank everyone for attending this webinar from Fluid Power World. This presentation will be emailed to everyone later today, and will also be available at FluidPowerWorld.com.
CD: Thank you.