A Brief Analysis of Two Cases of Faults in Fully Hydraulic Wet Braking of Loaders

With the advancement of hydraulic technology, fully hydraulic wet braking technology has been gradually and widely used in domestic loaders. Generally speaking, fully hydraulic wet braking technology is divided into single-circuit and double-circuit types. This article will share with you two fault problems encountered in the design and application process of double-circuit fully hydraulic wet braking technology in loaders.

The working principle of the system

This system is composed of gear pumps, combined brake valves, accumulators, brake cylinders, handbrake solenoid valves, handbrake cylinders and other actuating mechanisms of the hydraulic pilot system such as steering. Its working principle is shown in Figure 1.

The combined brake valve is directly connected to the gear pump. The liquid is filled into the accumulator at the set flow rate through the throttle port, and the remaining flow is sent to other actuators (such as the steering gear in Figure 1) through port N. When the liquid filling pressure reaches the set pressure value of the liquid filling valve, the liquid filling valve switches positions, the pressure compensator changes positions, the liquid filling pressure is cut off, the liquid filling process is completed, and all the flow flows to port N to other mechanisms. During braking, the brake valve core is repeatedly operated (by stepping on the brake pedal), and the pressure oil in the accumulator is consumed. When the pressure of any accumulator is lower than the cut-off pressure by a certain set value, the filling valve reverses, the pressure compensator changes position, and the filling pressure is restored. The liquid is filled into the accumulator at the set flow rate through the differential throttle port, and the remaining flow rate flows to other actuators through port N. This cycle repeats itself, completing the entire process of fluid filling - braking - refilling. The reason why the liquid filling valve can flip back and forth under different pressures is that the areas affected by the controlled oil at both ends of the valve stem of the liquid filling valve are different.

Fault Issue 1: The steering gear fails to steer after the brake is applied

Fault phenomenon

During the road test and commissioning of a wheel loader applying this wet braking system, after the debugger continuously stepped on the brake, the steering gear became very heavy for a short time and even failed to rotate.

As the steering system of this loader adopts pilot control, it is analyzed that it might be caused by insufficient oil supply to the steering gear from the N port of the combined brake valve. Connect the pressure and flow meter to the P port of the combined brake valve. When the brake is continuously pressed and the steering gear becomes very heavy for a short time, observe the pressure and flow meter. It is found that the pressure at point P rises rapidly from the set pressure value of the pilot relief valve to the starting pressure value set by the filling valve, and then rises slightly and slowly until the set filling pressure is reached, and then the filling is completed, as shown in Figure 2. It was found that the flow rate at point P dropped rapidly and then decreased slightly and slowly until the liquid filling was completed and the flow rate recovered, as shown in Figure 3.

Analysis of the cause

Analysis of pressure

As shown in Figure 2, we can see that when the liquid filling begins, the position of the liquid filling valve switches, the pressure compensator changes position, and the pressure at point P switches from the state of supplying oil to the pilot oil line to the state of filling liquid to the accumulator. The pressure here should be high rather than low. So the pressure instantly rises to the starting value of the liquid filling. Then, the hydraulic oil passes through the fixed-difference throttle port and fills the accumulator with liquid at a stable flow rate. After a short period of time, the pressure set by the liquid filling valve is reached. The liquid filling valve switches its position, the pressure compensator changes its position, the liquid filling pressure is cut off, and the liquid filling process ends. The pressure returns to the overflow pressure value of the pilot oil circuit relief valve. From this, it can be known that the change of the pressure curve is normal.

Let's take a look at Figure 3 again to analyze the flow rate. At the beginning of the liquid filling, the liquid filling valve switches positions and the pressure compensator changes positions. The flow rate at point P is the output flow rate of the gear pump, and the flow rate at this point should not change much. However, in Figure 3, the flow rate drops sharply from 10.5 L/min to 6.3 L/min, and then decreases slightly as the liquid filling time progresses. After the liquid filling is completed, the flow rate recovers to approximately 10.5 L/min. From this, it is not difficult to see that the volumetric efficiency of the gear pump here has changed too much. Under normal circumstances, such a phenomenon is unlikely to occur in a gear pump. It should be caused by internal leakage damage of the gear pump.

Fault handling

The gear pump was replaced, and the fault phenomenon was eliminated. The flow rate change at point P was retested as shown in Figure 4.

Fault issue 2: Low-pressure alarm occurs after braking

Fault phenomenon

During the road test and commissioning of a wheel loader equipped with this wet braking system, every time the debuggers stepped on the brake, the brake system alarm would trigger a low-pressure alarm. The pressure gauge value of the brake system would drop sharply to below 90Bar in a short time when the brake was stepped on and then rise back to the normal value. Moreover, it was found that the braking distance increased and there was a tendency for the braking to be unresponsive.

Analysis of the cause

In conclusion, the possibility of damage to the pressure sensor and the pressure gauge of the braking system has been ruled out. The pressure sensor of this loader is located at port DS2. From the variation pattern of the pressure gauge, it can be known that the pressure change at the oil inlet and outlet of the accumulator should also be quite significant at this time. Generally, there are three reasons for this situation:

There is a leakage in the braking system pipeline.

2. The accumulator capacity does not match the braking system, and the accumulator capacity is too small

3. The pre-nitrogen filling pressure parameter of the accumulator is incorrect. The nitrogen filling pressure deviates seriously from the normal value. The nitrogen filling pressure is either too large or too small, or even worse

Eventually, the seal failed and the nitrogen inside the accumulator leaked out completely.

The first situation is relatively easy to check and rule out. If you pay attention and observe carefully, you can find it. This case does not have this phenomenon, so it can be ruled out.

The second situation is generally not very likely to occur either. The capacity selection of the accumulator depends on the braking pressure, displacement (the amount of brake oil used), and the number of emergency braking times after the power disappears. For systems with relatively short brake lines and not too much elastic deformation, the influence of the lines can be ignored. Make a calculation and correction. Such problems should not exist in this example.

The third situation is likely to occur frequently. First, we test the pre-nitrogen pressure of the accumulator. Before measuring the pressure, fully release the pressure first. The method of pressure release is: after the engine is turned off, repeatedly press the brake pedal several times. Then, the pre-nitrogen filling pressure of the accumulator was measured using the nitrogen filling pressure gauge of the accumulator. After testing, it was found that one of the two accumulators in the vehicle had normal pressure while the other had no pressure, indicating that the fault phenomenon was caused by the lack of pre-filling pressure in one accumulator. After checking that the sealing gasket of the accumulator's nitrogen filling port is in good condition, re-fill the nitrogen. After completion, tighten the nitrogen filling bolt of the accumulator's nitrogen filling port, conduct a test run, and the fault is eliminated.

Whether it is a single-circuit or dual-circuit system, the braking principle of a fully hydraulic wet brake is the same. When encountering such problems, one can draw inferences by analogy and conduct a careful analysis. The fault point is not difficult to find.