There are three primary types of hydraulic pumps. These are the main ones that you’ll see in any given hydraulic system. First you have a gear pump, a vane pump and the piston pump. We’re going to start with the gear pump.
With a basic gear pump, it is the kind of pump you’ll see on any given mobile machine. You can see it off of a PTO, you can see it on a log splitter. Typically, a log splitter would have a two-stage pump, but that will be left out of this discussion.
It’s pretty basic. Some have a steel housing, some smaller pumps (lower pressure) can have aluminum housing and steel ends. Your prime mover, whether it be electric, motor or a gas engine will rotate the shaft, which is your primary inlet shaft.
Inside the pump, spur gears mesh together; as you rotate the inlet shaft, it has no choice but to rotate the secondary set of gears that are below it. Fluid is sucked in, and as we know sucked in means being pushed from the atmosphere by increasing displacement inside the cavity. What happens is the fluid gets trapped between the spur gears and the walls of the housing. It gets carried around the outside, so it happens in two locations, both sides. It gets carried around the outside and then hits the pressure side of the pump where it gets fed into your system.
Here are the advantages and disadvantages of a gear pump: One advantage is they’re inexpensive, so a gear pump, you can get some down and dirty ones anywhere from maybe 50 to $100, anywhere up to a few hundred on the upper end. Those are ones that would have actual roller bearings and large displacement, all steel construction. Even then, $300 is pretty inexpensive for any hydraulic pump.
They are also contamination-resistant. What that means is part of our examination can be trapped or passed through a gear pump without doing a whole lot of damage. Even if you were to have a high contamination system, the amount of damage that can be done to it isn’t significant enough that it would stop pumping. Even a worn pump, gear pump, is able to pump whereas some other pump designs, as they wear or as they are damaged by contamination they pretty much become useless, so in that sense gear pumps are pretty contamination-resistant.
They’re also capable of high speed. This is kind of a requirement, because a lot of prime movers in the mobile industry are either gas engines—you might have a small engine such as a little 10 horse Honda or something like that that is running 3,600 rpm. Often, you will have a gear pump able to run up to 4,000 rpm in that case. The rotation can be easily changed.
Gear pumps are reliable. In that sense, I mean that because they’re contamination resistant, they can go for a long time. You might see Grandpa’s log splitter has been running in the backyard for 30 years or so. That’s because it’s hard to destroy a gear pump.
They do eventually go. Sometimes it’s from catastrophic failure, but when it’s wear related failure, it can take years and years. Another advantage is they can be stacked together. You could have two, three or four gear pumps that are all stacked in tandem with one input shaft.
This has some advantages of creative use of displacement or it just allows you to save some space, so rather than having two different sources and two different prime movers, you can stack two pumps together, two, three, four and have different parts of a circuit be fed from different pumps. The disadvantage of gear pumps is they are inefficient. Even a brand new gear pump—and not talking about crescent pumps or internal gear pumps—is inefficient.
Right off the factory floor, you’re lucky to get 80% efficiency. Efficiency in a pump is the amount of fluid by volume that is being pumped compared to theoretical volume. If you were to do the math and figure out the theoretical volume of a pump and it’s supposed to give you 10 gallons per minute. Even with a brand new gear pump, you’re lucky to be 80% efficient. As they wear, even though they can still be reliable and they can still run and give you some flow and pressure, that can drop to 40%, 50%, 60% or around that range.
If you’re trying to create a hydraulic system that is taking best user input power, a gear pump isn’t always your best choice. It’s not your first choice anyway. It’s your most economical choice. They can also be noisy, another disadvantage. By noisy, I don’t necessarily mean that they have the highest volume in decibels. Sometimes they do, but it all depends on the application. Sometimes, they can have a really annoying noise, the frequency and harmonics of a gear pump can be pretty annoying sometimes. You can spot the sound of a noisy gear pump a mile away.
As I mentioned, they quickly become less efficient with wear and higher pressure. Because of the nature of the clearances and the way a gear pump is designed, as you increase pressure too high in one of these pumps more is just lost to leakage. The same thing as the pumps wear.
Now we’re going to talk about vane pumps. You might notice on a lot of pumps that the suction port is larger. That’s to reduce restriction and reduce vacuum and reduce gravitation. With this particular pump design, the suction and pressure ports can be rotated. This allows you to have ideal suction and pressure configurations. For example, if you wanted to have this pump mounted below a reservoir, you’d have a large suction plumbing coming straight down into it, but it wouldn’t make any sense for the pressure to be going straight up into the reservoir as well.
On a variable displacement pump, whether it’s vane or whatever, there’s essentially a relief valve that controls the control piston. The internals of a variable displacement vane pump, you can see the example where I have this blue cam ring. Within it you have the rotating group that has the vanes and the input shaft. This particular design, what we have here with the compensators, what they do is they control the pressure that is added to this little control piston.
This one right here is, sorry. This one here would be the, yes, the bias piston. What’s happening is that it’s always … Sorry, the control piston. I was right the first time. This one right here is always trying to push on the cam ring to make it go full displacement. As it tries to go full displacement and you restrict it downstream, pressure will rise and cause this cam ring to spin in a neutral circle.
If you imagine that if you were to move the cam ring over this way as pressure increases, that there would be no increase and decreasing of displacement. It would just kind of spin in a circle and idle and nothing would really happen. The higher you set your compensator, the more it’s trying to force full displacement and the more downstream pressure has to rise before you reduce flow.
Pressure compensation is a way of trying to achieve the most amount of flow with any given pressure. The pump is always trying to put out a particular pressure. The more you resist that it will just reduce flow. There are advantages to this such as controlling flow, but also maintaining [inaudible 20:23] flow control or through a valve. It’s also more efficient as well. Rather than just always giving you full flow and full pressure, this will just give you the pressure you want and cut back flow to compensate for a higher pressure.
The advantages and disadvantages of a vane pump is they are quiet. They’re very quiet actually so in most industrial applications where high pressure and speed aren’t really necessary you will see a vane pump. Especially ones where there are humans around to be annoyed or not annoyed by a pump, these are a good choice.
They are relatively contamination resistant. Luckily they can be easily repaired, so should contamination become a problem in them, they can be fixed quite easily. They are relatively inexpensive, especially the fixed vane pump. As you go through the costing of a pump, you have your cheap gear pumps and you can have an inexpensive fixed vane pump which would kind of come in at the high end of a gear pump and then go up in price from there.
Variable displacement, vane pumps can be a little bit pricier but they don’t tend to reach into the cost of a piston pump until you get some really fancy ones. Like I mentioned earlier, they’re also easily repairable. Some of the disadvantages of a vane pump are low speed, so they don’t go terribly fast. One of the reasons is that as you spin a vane pump fast the vanes, because of centrifugal force, kind of get pushed out too deep into the cam ring. They can wear more quickly.
Anyway, vane pumps can also get very expensive. Some of the top brands they have some pretty fancy designs. They can be really complex to set up and run, but still, like I said, not as expensive as most piston pumps. Also one of the disadvantages that they have low pressure capacity. Like I said, because of the nature of the vanes and how they get pushed into the cam ring, it’s often difficult to get a lot of pressure out of these. Not more than two or 3,000 psi.
Now we’re onto piston pumps. Piston pumps is a familiar closed-loop type pump. Three types of piston pumps. Here’s the axial piston pump, the bent axis piston pump and the radial piston pump. All three of these are nice designs and fairly common, each to the piston pump family. The axial piston pump, so that is a design where the piston, so if you look inside the pump, we’ll open it up here in a second, what they do is they move back and forth on the same axis as the input shaft.
They will spin around in a circle going back and forth about the axis of a pump easily explaining why it’s called the axial piston pump. These can be had in fixed displacements, just like in a gear pump or a vane pump. This particular one is a pressure compensated load-sensing pump. What we have here is we have instead of a bias piston, this one actually has a bias spring. What the spring here is doing, it has a lot of force. What it’s trying to do is always push this swash plate, this is this plate right here that can rotate on an angle to either increase or decrease displacement of the pistons.
You can imagine if this angle is relatively flat that as these pistons travel back and forth they wouldn’t go in and out at all or very much. The volume would be reduced. If this was a severe angle, so if it was angled across this way, you can imagine that the pistons would travel a far distance. They would go in and out very far, so it would increase and decrease displacement at a higher volume.
The bias piston tries to do its best to always have full displacement and how displacement decreases is with the control piston right here. Similar to the one that was in the vane pumps. This one here is controlled by the relief valve in the compensator up here. Whatever pressure you have this set to is the pressure that this thing wants to push backwards on the swash plate to try to get it to go to a zero angle. When it’s at zero angle it would be essentially on what’s called standby pressure, meaning it’s not pumping at all.
It’s trying to create a little bit of pressure but has nowhere to go so it just kind of idles. As pressure increases, so if you were to increase or decrease the pressure on this compensator here, it increases or decreases the force which this control piston pushes on the swash plate. At a lower pressure you can imagine that if you increase system pressure, it’s really easy to shut off flow coming from this pump. If you raise the pressure on this compensator to resist this pump and then subsequently have it go on standby.
I’m not going to talk about the load-sensing function of a pump. It’s a conversation that takes a lot of explanation. I’m sure some of you may be confused by the idea of pressure compensation to begin with. We have the bent axis piston pump. This one is pretty straight forward of why it’s called bent axis. You can see that the input shaft comes in a one five and the rotating group and the ports will be on an angle to the input shaft.
We’ll go back and explain this one. The reason you would have a bent axis like this is in the case of a motor, what it allows you to have something be driven and inside the input shaft side there’s a really large bearing that supports it. If you were to have this pulley-driven, so if you imagine that the forces on a pulley were being driven from below, the angle that is on the bent axis really resists those side note forces that are put onto a shaft.
If you were to have a pump that required for some reason because of space or design to be run by say a chain or a pulley, a bent axis piston pump is the way to go. One of the other advantages of these regardless of, because of the bearing and then the way it’s supported, is that they’re good for really high pressure and also good for very high speeds. These things can rotate very quickly and they can also withstand very high pressures.
Hydraulic pumps, this is the radial piston pump. I always compare it to an airplane engine. What you have inside of a radial piston pump is a shaft with a cam. You can imagine as this camshaft rotates, so because it’s off-center as it rotates, what it does as it pushes on these pistons, which are each inside one of these cylinders, and as it goes in a circle radial to the shaft each of these pistons moves in and out.
Depending on the length, that dictates the displacement and therefore the flow. One of the advantages of these pistons is that this case they’re spring bias but they’re easily changeable. You can imagine you take off this particular, this valve cover right here and you could easily replace the entire piston assembly.
Advantages to the piston pumps, regardless of configuration they’re available in high to very high pressure capacity. You’re hard to find a piston pump rated for anything less than 4,000 psi, but some applications such as some radial piston pumps are good for up to 10,000 psi and in some cases even more. Some designs are good for higher PM. This would be the axial piston pump or the bent axis piston pump.
Because of the wobbly nature of a radial piston pump with the camshaft and with the distance of the pistons moving about the outside of the pump, they’re not as high speed rated. Good for high pressure and very efficient, but not good for high speed. Most piston pumps are very efficient, so whereas a gear pump might be 80% efficient, a vane pump could be 85% efficient. Most piston pumps are 90% efficient or more. There are some designs of radial piston pump because of their actual soft seal technology or other things that are good for 95% efficiency or higher. This means that if you have 10 gallons a minute of theoretical displacement from your pump, they could be putting up upwards of 9.5 gallons a minute of actual output flow.
I should also mention that when it comes to efficiency it is the flow that is sacrificed as a result of efficiency and not pressure. Pressure can be created as long as there’s enough molecules to stuff downstream to overcome your resistance. Volume, so flow is what is sacrificed. I should also note that inefficiency, anything not used to create actual work is wasted as heat so if you have a 10 horsepower system and you have a gear pump that is 80% efficient, you would have 2 horsepower, that’s 20% of the energy being wasted as pure heat or as in my example with a radial piston pump being 95% efficient, you would have only .5 a horsepower being wasted as heat and any application requiring an efficient, economical and green pump, can’t go wrong with a piston pump.
Also advantage of the piston pumps is that there are myriad configurations of compensation, so load-sensing, they’re torque-limiting, horsepower-limiting. There are so many advanced controls that you can get for a piston pump. The sky’s the limit.
Disadvantages are they are expensive. It can cost a lot of money. You’re lucky to have any piston pump cost less than say $1,200 and that would be even for a small one. There are some economical lines. Some people make some cheaper piston pumps that are not as high pressure rated or not as efficient, but pumps can easily go up to five, six, $10,000 or more for some pump designs.
They are contamination sensitive, so this means that for the tighter tolerances that allow them to be efficient particle sizes can’t be so big as to be stuck between piston and wall clearances to be moving across the swash plate clearances. All the areas in a pump with a tighter tolerance are more sensitive to contamination and as well because they are higher pressure, this means they can be forced through these clearances with more energy.
Whereas a lower pressure system, even if it was with a gear pump, I’m sorry, with a piston pump, you could go with a poor filtration, but when it comes to high pressure very sensitive to contamination and you should be using only the best filtration systems for high pressure piston pumps. As well, piston pumps are most expensive to repair. The rotating groups and components that are involved with them can be 40, 50, 60% of the cost of an entire new pump especially when it comes to the cost of rebuilding them and testing them after they’re done as well.
We talked about gear pumps. Once again, they are inexpensive, contamination resistant, capable of high speed. The rotation is easily changed. They’re reliable. Lost of gear pumps that are 20, 30, 40 years old that are still on machinery today. They can be stacked together in tandem so multiple pumps all for one system or one prime mover.
However, they can be inefficient, noisy and quickly become less efficient with wear and high pressure. The vane pumps, they are quiet ones of the bunch, relatively contamination resistant. They are fixed displacement pumps, can be inexpensive, they’re easily repairable in most cases. This particular example we’re looking at right here is one of the ones that is not inexpensive to repair.
Yeah, they can be tricky to set up. Anyway, this is an exception. One of the disadvantages are vane pumps are capable of only low speed. You can’t spin them too fast. Usually 1,200 to 1,800 rpm is the max. Variable piston pumps can get expensive. The one we’re looking at now is, although you can get some other brands that are less. Low pressure capacity. Typically, 2,000 or 3,000 psi is the upper range for a vane pump.
Then piston pumps, this new here being an axial piston pump, variable displacement. They are good for high to very high pressure capacity. Some of them are good for high speed, especially the axial designs and the bent axis designs. They are extremely efficient and myriad configurations of compensation are available, but they can be expensive and they are contamination sensitive. Use only the best filters, kidney loops, pressure filters on these systems. They’re also the most expensive to repair.