The handling of the fault where the transfer case of a concrete pump truck does not switch

A brief introduction to the switching principle of the transfer case of a concrete pump truck

Concrete pump trucks are modified from the chassis of heavy-duty trucks. They are equipped with motion and power transmission devices, pumping and mixing devices, material distribution devices, and some other auxiliary devices on the chassis. The power of the concrete pump truck is transmitted to the hydraulic pump set or the rear axle through the power transfer case. The hydraulic pump pushes the piston to drive the concrete pump to work. Then, the concrete is transported to a certain height and distance by using the fabric rods and delivery pipes on the pump truck. The engine of a concrete pump truck is not only used to drive the pump truck but also to drive working devices such as the pumping mechanism, mixing mechanism and distribution mechanism.

The power for each working device of the concrete pump truck comes from the car engine. When the concrete pump truck is in operation, the power of the car engine is transmitted to the transfer case through the gearbox, and then switched by the transfer case and transmitted to each hydraulic pump or the rear axle of the chassis. When the driver issues the command to switch to the pumping position, the pneumatic solenoid valve controls the cylinder on the transfer case to push the fork, which then pushes the separation gear to switch to the pumping position and simultaneously cuts off the power supply to the rear axle, putting the vehicle in a stationary state. Conversely, switch to the driving position.

2. Fault Phenomena and fault analysis

Our company has a BSF36.09 pump truck produced by Putzmeister Machinery (Shanghai) Co., LTD. Once, after the construction site was completed and the work was about to end, a sudden fault occurred in the entire hydraulic system of the vehicle, where there was no pressure at all. It seemed that the output power of the transfer case was in the driving position. When the driver issued the command to switch to the pumping position, it was seen that the end of the fork rod could be retracted. However, the separation gear inside the transfer case does not leave the driving position, so this gear will not switch to the pumping position either. Although the boom has been retracted and the three outriggers have been lifted, the outriggers have not been fully retracted yet. The most headache-inducing problem is that there is still one front outrigger that is fully supported on the ground, with about a quarter of the vehicle's weight pressing down on it, with a gravity of nearly 10 tons on this outrigger. If this problem is not solved, even if the pump truck can run, it will not be able to drive back to the company from the construction site.

The handling of the outrigger problem of the pump truck

By analyzing the principle of the hydraulic system of the pump truck's outriggers, it was found that each outrigger hydraulic cylinder of the pump truck is protected by a bidirectional brake valve. Even if the oil supply pipe is removed, the hydraulic oil in the roeless cavity of the cylinder cannot be discharged. Moreover, the bidirectional brake valve is connected to the oil inlet of the roeless cavity of the hydraulic cylinder. Without removing the bidirectional brake valve, it cannot be disassembled, and thus the outriggers cannot be retracted. The only solution is to work on the bidirectional brake valve. The schematic diagram of the hydraulic system of the pump truck's outriggers is shown in Figure 1.

There is a large nut on the bidirectional brake valve on the outrigger (the structure of this bidirectional brake valve is not detailed in the spare parts manual). It is analyzed that the bidirectional brake valve is composed of two controllable check valves, and this nut is one of them. When someone tries to loosen the nut with a wrench, hydraulic oil seeped out. When it was loosened to three or four turns, the controllable check valve was ejected. Fortunately, the operator was well-prepared and managed to avoid it, causing no personal injury. Finally, use a crowbar or similar tool to retract this outrigger and reinforce it with iron wire. Use a similar method to retract and close the other three outriggers.

To summarize the experience in handling this malfunction, the outrigger arm should be lifted with a jack to release the oil pressure in the cylinder, and then the bidirectional brake valve should be removed. This way, there will be no danger.

4. Methods for repairing transfer cases

Along the axis. Use a hand chain hoist to lift the pump set with the main pump, remove the pump set, and then take off the boom and outrigger pumps. After removing the transfer case and disassembling it, it was found that the copper alloy fork was broken.