Kawasaki’s K3V and K5V pumps are found in a number of different mobile hydraulic machines being used by a variety of manufacturers. Whilst they appear simple in terms of connection, when it comes to diagnosis there is so many things that could potentially trip you up. The pump we are going to look at is the K3VG112. This particular pump setup is basically 2 variable displacement pumps bolted together and driven by a shaft that passes through the entire body of the pump casing. Whilst there are differing displacements where the main internals aren’t too different, the variety of control can be very different.
The pump in question is one from a 2008 JCB JS220 which will be a case study when I’ve finished writing it all up!
A hurdle we face when diagnosing mobile hydraulic machinery is determining whether it is the hydraulic system or the prime mover at fault. By prime mover I mean the source of power and this could be an electric motor or an engine. The majority of hydraulic pumps are directly connected to the prime mover meaning they are constantly linked. If either an engine or hydraulic fault occurs, it will have a direct effect on the other.
This was especially true with the JS220 that has resulted in this article as the next challenge is knowing how these machines are supposed to work. JCB are pretty good with their manuals and there is a lot of useful content but trying to understand the pump control for this variant of K3V pump was not easy. Hopefully though this will help highlight some of the key areas to focus on when dealing with these pumps along with captures from PicoScope that highlight further the internal operation.
Above is a partially redrawn schematic from the K3V pump fitted to the JS220 we were working on. The same components are located on the rear pump which is why I’ve focused on just the front pump in this example.
Let’s start with what happens then during start up of the machine. As with most variable displacement pumps the pump starts on maximum displacement which, without any hydraulic pressure, is carried out by the spring on the proportional spool valve at F. This proportional valve is mechanical linked to the swash plate piston and as the spring holds the spool over to the left, in this position any oil present in the large end of this piston, B, is allowed to drain away to tank.
With the assistance of the pilot pump, additional pressure is applied to the swash plate piston at G where it continues to hold the pump on max displacement. Some oil will make its way to the spool at E but there is not enough force available to overcome the two springs and the swash plate piston.
As oil passes through the main control valve, MCV, the spools all have open centers in the neutral position, which means oil can flow freely where the oil meets a restrictor and some oil is allowed to pass through to tank. However, this creates back pressure which is regulated by a direct acting relief valve, K, set to around 40 Bar.
This pressure is felt back at the pump outlet but also through the port PT1. This means that there is extra pressure available at the negative control piston at I. As this pressure now acts against the spring and as back pressure builds up at the pump outlet, which is also seen at chamber E along with the pilot servo pressure, the spring on the proportional valve, F, starts to move. As it does so, pump pressure is now allowed to enter the large end of the swash plate control piston at B and as this end has a larger surface area to that of G, the pump is moved to minimum displacement but with minimal pressure.
Pressure Regulation with service active
When a service is selected, the open centres of the main control valve are no longer allowing oil to flow through. This prevents the oil entering the orifice creating the back-pressure signal to move the control piston, A, using chambers I and E. As such, the pressure at I drops and is forced back by the spring which in turn allows the spring at F to move the spool valve back resulting in the oil at B to drain back to tank. If we now follow the pump schematic below, we can see that the pump outlet is connected to the regulator via a check valve. This also has a path to the small end of the swash plate piston, G, putting the pump to max displacement. If there is a restriction to the pump outlet flow, caused by the use of a service this will begin to increase the pressure. Remember pressure is resistance to flow.
Let’s for now ignore some of the additional regulation for pump control and just observe it as a pressure compensated variable displacement pump. There is great example of how these function from Carl Dyke over at Lunch Box Sessions who have some amazing free YouTube videos with interactive schematics that helps when trying to understand what is going on inside these complex systems. Here is a link to their website - https://www.lunchboxsessions.com/materi ... mps-lesson with a preview of their content and a link to the YouTube video - https://youtu.be/jcCG6yJw1FY.
With a pressure regulated pump it does exactly that, the pump is regulated by pressure. When we talk about regulating the pump what we mean is adjusting the flow rate based on the pressure, so as the pressure increases, the flow rate is reduced. Let’s not forget that hydraulic pumps produce flow and it is only restrictions in the system that create pressure.
The pump is held in max displacement even though the pressure can be seen at E but it is not high enough to overcome the spring holding it back. This spring can be different but most of the literature I’ve found suggest it is set around 200 Bar for this particular machine. As the pressure continues to rise and is over 200 Bar, the pressure in chamber E starts to move the spool valve back towards minimum displacement, allowing pump pressure to be seen at the large end of the swash plate piston at B. Due to the surface areas, this now moves the pump to a minimum displacement proportionally to that of the system pressure. The higher the pressure then then less flow the pump produces.
With this particular pump configuration there is also the additional chamber at D. This is a connection to the rear pump outlet which means any pressure felt at the rear pump outlet can have an effect on the front pump’s displacement by adding pressure to force the swash plate control piston, A, to minimum displacement. It is worth noting that the front pump is also connected to the rear pump in the same way. When testing these pumps ensure you are aware of this as it will affect your flow readings.
Pressure compensated pumps are extremely common as it limits the need to regulate the pressure by using direct acting relief valves. Whilst a vital part of any hydraulic system, relief valves are extremely inefficient for pressure regulation as they have to pass oil flow and any oil that passes through an orifice without doing work, generates heat. By changing the flow rate based on the pressure means we can control the pressure without the relief valve opening. This is a much more efficient way of regulating the pressure within a hydraulic system.
Max Flow Cut
Pretty straight forward to understand what it does, as it’s exactly what it says, it cuts the maximum flow. Flow makes it go after all and there are times in operation when you want things to move a little slower, such as lifting. It does this by sending a pressure signal generated by the pilot pump to port H which is linked to both pumps meaning when it is active, it has the same effect on the front and rear pump. You’ll notice on the diagram the pilot pump sends oil to an 8-spool manifold which in this machine contains an electrical solenoid valve. When this valve is opened, it will allow a 40 Bar pressure signal back to the pump. When the pressure arrives at chamber H on the pump, it acts against the spring of spool valve F and forces the swash plate piston to move off from maximum displacement. However, it can’t force the swash plate all the way over to minimum displacement which means the flow rate is only reduced, not cut all together. For this machine this has the effect of reducing the flow rate by around 60% and is only activated when slow speed travel or lift, L, mode is selected by the operator. Flow makes it go after all! The more flow you have the faster things can move.
Below is capture with PicoScope showing the results of the flow rate and whilst a service is being operated and L mode has been selected. When in L mode, max flow cut is automatically applied. From our technical information we know that this pump should be set to produce 215 l/min at its maximum displacement.
In L mode we can measure the flow rate at 144.9 l/min which when doing the math, works out to be around 60-70% of the pumps maximum flow rating. By observing and graphing flow rates means we have a better idea of what is happening with pump and qualifying the expected results and the actual.
Pressure Reducing Valve (or Horsepower Control Valve)
Last, but no means least, is the Pressure Reducing Valve, J, sometimes called the Horsepower control valve. This is linked to both pumps and pressure is supplied by the Pilot pump.
The PRV, J, is an electromechanical device that uses a PWM signal to control the amount of current needed to determine the valves position. This valve is a held in the open position by a spring which allows the full pressure signal from the pilot pump to reach chamber C. When the solenoid is active, and depending on the current driving the valve closed, this pressure signal will change proportionally to the commanded current.
The current is controlled by the selection of different modes. This current varies from machine to machine and you should refer to the manual in order to determine the correct values. In this machine there are 3 modes to choose from.
In this mode the maximum current available for the valve is sent in order to close the valve and prevent the pressure reaching chamber C. This ensures that the pump will be delivering its maximum flow when required. The current for this machine can be around 400 to 500 mA. One note to mention here is that the engine speed is typically set to max RPM to deliver max RPM.
E mode is for economy. Engine speed is reduced and the PRV is allowed to partially open to offer some reduction in flow rate. Current is around half of that in A mode.
L (+P) Mode
L mode is for lifting and can also have a P mode which is used for Precision movements. Current is set to 0 mA and so the full pressure signal can be sent to chamber C. This ensures the pump is held off maximum displacement and so reducing the amount of flow when a service is selected. Engine speed will also be reduced again and lower than that when in E mode.
In the above capture we have taken one capture to show what happens to the flow rates and engine speed as the modes are changed. To graph the engine speed, we have used an optical pick up at the flywheel to detect 1 pulse per revolution. The math channel will then read as follows crank(A, 1) which will now show the engine speed in RPM.
Please note that the results above were carried out when performing a max flow test. In order to carry out this test we had to take away the regulation from the PT1 port, I, where the 40 bar pressure signal from the MCV helps to put the pump on minimum displacement when it is in the neutral state. This is a test procedure which can be done following the manufacturers technical information.
With the pump in A mode, we know that the pump should be set to max displacement with no effect from the PRV (or horsepower control valve) as the solenoid will be provided with approx. 400 -500mA in order to keep it closed. This will cause the pump to stay on max displacement as the pressure on the small end of the swash plate piston, G, and the spring at F is enough to stop the pump being moved back to minimum displacement. As you can see in the ruler overview, the pump is producing 215 lpm of flow and the engine speed is set to it’s max of 2000 RPM.
In L Mode, the current being applied to the PRV is no 0mA. This will allow the valve to stay open and so provide pilot pump pressure to be applied to chamber C. This will move the spool valve over slightly and in doing so move the pump off of maximum displacement. This is can be seen in in the flow rate as in this mode it has dropped 173 lpm. We also see that the engine speed has dropped as expected reading the technical information on the mode changes.
E Mode, does apply some current the PRV which will mean it allows some of the pilot pump pressure to be applied to Chamber C. Whilst this will still move the pump off maximum displacement, it doesn’t move it as much as L Mode. This again can be reflected by the flow rate as we measure around 194 lpm which is more than L Mode but less then A mode.
It is worth noting that in A mode, the machine can also control the current being applied to the PSV depending on the torque demand of the hydraulic system. Should the torque start to increase too much which starts to reduce the engine speed, the ECU will change the current to allow more pressure to move to swash plate piston over towards minimum displacement. This can be reflected in the capture below –
On the right-hand side of the graph, we can see the current in Channel B. Using the rulers, we aim approx. in the middle to give us the average of this signal which from the ruler legend is 439mA. This would put us in A Mode. You’ll notice the engine speed start to drop, indicated by the black math channel, and then the current suddenly switches to 200mA and then as the engine speed recovers the current goes back up to 439mA. This control is there to prevent stalling by reducing the flow rate and therefore reducing the load on the engine.
Please remember this is my interpretation of how this pump works. I may have things wrong and if so please do speak up. We all want to learn and in my eyes every days is a school day.
I hope it helps in some way.
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