In order to use the power of the engine more effectively, wheel loader maintenance companies generally choose constant power variable pumps. The so-called constant power variable pump is a bit more obvious. The product of the pump pressure and the pump flow rate is a constant. There will be the suffocation we often say. Therefore, as far as their design thinking is concerned, each designer must make the power of the entire hydraulic system infinitely close to the power of the engine but certainly not greater than the power of the engine.
In our common loaders, except Komatsu uses LS control, most of them use negative flow control. In recent years, some companies have introduced positive flow control, and in this way, positive flow has many advantages, so is the positive flow really so god? Let us take the Kawasaki K3V series as an example to analyze the hydraulic pump on the loader Ground control principle:
Constant power control of loader
The constant power control of the loader is divided into two parts: one is the power control inside the pump: it is based on the output pressure of the pump and the output pressure of the other pump (another pump) to control the displacement of the pump, When the pressure increases, the displacement of the pump decreases accordingly; when the pressure decreases, the displacement of the pump increases accordingly; if the system pressure is lower than the pilot pressure, the pilot pressure is introduced to control its displacement. It is for positive flow or negative flow. For this part, whether it is theoretically or structurally, there is no difference, that is to say, there is no difference between positive flow and negative flow in this part. This is the constant of the hydraulic pump. The main body of power control is not discussed here. The second is the control of the power of the pump by external signals: the external signal here refers to the pilot operating system, the main pressure system, the engine system, etc. are all related to the power control of the pump Induction of information. In the negative flow, it is the induction of the negative pressure signal and other signals. In the positive flow, it is the induction of the positive pressure signal and other signals. These two other signals are also no different. Pressure signal different.
We know that on the loader, the speed of each fulfillment element will change with the stroke of the operating handle.The hydraulic system will control its displacement according to this change.The difference between negative flow and positive flow is the signal of this change. Collect different negative flows of orientation:
In the hydraulic system with negative flow control, the pressure of the negative pressure signal is about 5MPa to 6MPa.This pressure is only used to generate the negative pressure signal; while in the hydraulic system with positive flow control, because there is no such equipment, his return pressure is only It is back pressure (generally around 0.5MPa), which reduces an unnecessary power loss, so that the positive flow loader must be more fuel-efficient than the negative flow loader when the same workload is completed.
The greater the handle stroke, the greater the corresponding secondary pilot pressure, and the greater the opening of the main spool controlled by the secondary pilot pressure. Correspondingly, the greater the opening of the main spool, the main oil circuit The more oil in the split fulfillment element, the faster the fulfillment element speed.The less oil that flows through the negative pressure signal generation device through the median position, the smaller the pressure value of the negative pressure signal; otherwise, if the handle stroke is smaller , The smaller the corresponding secondary pilot pressure will be, the smaller the opening of the main spool controlled by the secondary pilot pressure will be. The less oil there is, the slower the speed of performing the components, and the more oil that flows through the negative pressure signal generation equipment through the middle, the greater the pressure value of the negative pressure signal. To control its displacement. This is negative flow control. His signal collection point is the outlet of the main control valve in the main oil circuit.
Positive flow:
There is no negative pressure signal generation device on the main control valve of positive flow, and his signal is collected in the second pilot. The other parts are not different from negative flow.
Compared with the negative flow rate, the positive flow rate is why the operation sensitivity is good:
Because the signal collection point of the negative pressure control is at the outlet of the main excavation valve, this negative pressure signal will change only when the main control valve is activated, so that the displacement of the pump changes, which makes the control of the hydraulic pump lag permanently For the control of the main control valve.
In positive flow, because the pump's control signal is collected in the secondary pilot pressure, this pressure signal is sent to the hydraulic pump and the main control valve together, which is to make the actions of the two can be synchronized. This is "compared to negative flow. The main reason for good flow operation sensitivity.
