1. Brake lock
The gap seal is often used between the valve core and the valve body of the reversing valve of the truck crane. Under the dual action of its own gravity and the gravity of the lifting load, the high-pressure oil often passes through the gap between the reversing valve spool and the valve body during braking. The small gap leaks slowly, and the hydraulic cylinder or hydraulic motor of each mechanism cannot be effectively locked, so it is very easy to cause accidents such as lowering of the lifting weight, retraction of the boom, increase in amplitude, and sinking of the outrigger. Normal work can lead to serious accidents such as vehicle overturning and casualties. There are at least the following 4 safety measures to eliminate hidden dangers.
(1) Connect hydraulic locks in series in the oil return circuit
The reverse oil circuit of the hydraulic control check valve can only be connected when the control oil circuit inputs a certain pressure of oil. It is a switch element that can realize on-off control of the oil circuit, and is usually called a hydraulic lock. Reversely connect the hydraulic lock to the oil return circuit of the hydraulic cylinder (hydraulic motor), and connect its control oil circuit to the oil inlet circuit of the hydraulic cylinder (hydraulic motor), so that the oil return circuit can be kept in the braking state. Because the control oil circuit is cut off due to the lack of pressure oil, it has a proper locking effect on the hydraulic cylinder (hydraulic motor).
The locking effect of the hydraulic lock depends on the shape of the spool and the structure of the valve.
If the valve core is of ball valve type or slide valve type, its leakage will still be inevitable, that is, the oil may leak through the groove caused by the impact of the valve core or the gap between the valve core and the valve body. Therefore, the hydraulic lock valve core used in the truck crane should choose the cone core with good sealing performance, which can ensure that the oil does not leak even if it is used for a long time.
According to the characteristics of the working conditions of the truck crane, the IY-F type high pressure hydraulic lock with an unloading valve core should be selected.
For the outrigger oil circuit, there are two locking requirements: support locking and suspension locking. According to different oil circuits, each leg can use only one hydraulic control check valve, a one-way hydraulic lock that can only play a one-way locking function, or a two-way hydraulic lock formed by cross-connecting two hydraulic control check valves. Lock. The latter scheme is generally used. Each outrigger locking device must be set independently.
(2) Connect the balance valve in series in the oil return circuit
The balance valve is essentially a combination valve composed of a one-way valve and an external control and internal leakage sequence valve. Its installation position and locking principle in the truck crane are exactly the same as that of the hydraulic lock, that is, it is installed in the oil return circuit. Cut off the oil return circuit after braking. If there are no other special requirements for the oil circuit, the hydraulic lock with relatively low price should be used as much as possible.
(3) Improve the sealing of hydraulic components
After installing the hydraulic lock or balance valve in the oil circuit, it may not be able to completely guarantee that the mechanism can be locked after braking. The tightness of the hydraulic cylinder (hydraulic motor) is also a key factor restricting the locking effect.
The oil leakage of hydraulic components is divided into two types: internal leakage and external leakage. The leakage in the hydraulic cylinder is not easy to detect, which will cause the three mechanisms of the outrigger retraction, the boom telescopic and the luffing mechanism to be unable to be locked and sinking. Therefore, to improve the sealing effect of the hydraulic cylinder, the key is to solve the internal leakage problem that occurs inside it.
Generally speaking, the amount of oil leakage inside the hydraulic cylinder of the truck crane should be strictly limited. During safety inspection, it is stipulated that the moving distance of the piston under rated pressure should not exceed 0.5min within 10min. If it exceeds this value, it means that the sealing performance is not good, and a new Y-ring should be replaced in time.
(4) Lifting and lifting hydraulic motor externally holding brake
It is difficult to detect the internal leakage of hydraulic cylinders and hydraulic motors for a long time. After the seals are worn, leakage will occur unconsciously. In this way, if the locking is firm, there is only one way to install the brake, and it can only be used on the hydraulic motor that lifts and lowers the hoisting weight. Others such as telescopic and luffing of the boom cannot be used.
The brake acting on the lifting mechanism is actually a small single-rod single-acting hydraulic cylinder, the brake is normally closed, and the braking force depends on the spring force in the cylinder. In order to obtain the ideal locking effect, it is of course hope that the larger the spring force, the better, but the excessive spring force will increase the energy consumption of the brake release, so the external brake is an auxiliary means of its locking scheme.
2. Movement speed limit
Due to the long strokes of the lifting and lowering of the crane, the telescoping of the boom and the luffing of the boom, the acceleration phenomenon caused by the action of gravity during the downward process of the mechanism or load is also a relatively prominent problem. The speed limit of truck cranes usually adopts the following two methods.
(1) Connect the balance valve in series in the oil return circuit
The function of the balance valve can not only lock, but also limit the speed. When the mechanism or load drops, the sequence valve in the balance valve can keep the oil return flow of the hydraulic cylinder (hydraulic motor) in a stable state with the help of the balancing effect of the spool, so that the hydraulic cylinder (hydraulic motor) can maintain a uniform speed movement. In order to obtain a uniform descending speed, two points should be paid attention to when designing the system: one is not to simply replace the balance valve with a parallel combination of a general one-way valve and an externally controlled internal leakage sequence valve in order to save costs. This is because although the balance valve is also a combination of a one-way valve and an externally controlled internal leakage sequence valve, its sequence valve has been added with double-layer springs, damping holes and other devices to dampen the valve core; the second is the control of the balance valve. The oil circuit should be connected in series with the throttle valve, so that the spool of the sequence valve is "hysteretic", and its speed will not change due to small changes in external pressure. In the speed limit scheme, the effect of using the balance valve is the most ideal and the preferred method.
(2) Speed limit with manual reversing valve
Among all the reversing valves, only the manual reversing valve can control both the speed limit and the throttle speed regulation by controlling the size of the valve opening while reversing. Therefore, when the downward acceleration of the truck crane mechanism or the load occurs, it can be solved by reducing the pulling degree of the manual reversing valve handle by the driver to reduce the opening degree of the valve port. Of course, the speed limit effect of the manual reversing valve is not ideal due to the shaking of the driver's hand, but because the loading and unloading speed of the truck crane often needs to be continuously adjusted according to the specific conditions of the construction site, the movement of the mechanism is often discontinuous. Intermittent operation, so skilled drivers can still receive a certain speed limit effect by manipulating the manual reversing valve.
In a word, for the three mechanisms of lifting and lowering, boom telescopic, and boom luffing of truck cranes, which have both locking and speed-limiting requirements, a balance valve that can meet both locking and speed-limiting requirements should be used, and a mechanical brake should be used. .
