The acceleration phenomenon caused by gravity action in the process of mechanism or load descending is also a prominent problem because of the long travel of crane lifting, boom expansion and jib luffing. The speed limit of the truck crane usually adopts the following two methods.
(1) Series balancing valve in return circuit
The function of the balance valve can not only lock tightly, but also limit speed. When the mechanism or the load drop, the balance valve in the order valve can rely on the balance of the valve core, so that the hydraulic cylinder (hydraulic motor) of the return oil flow to maintain a stable state, so that the hydraulic cylinder (hydraulic motor) to maintain the speed of movement. In order to achieve a uniform descent speed, the design system should pay attention to two points: one is not to save costs, simply with the general unidirectional valve and external controlled internal leakage type sequential valve combined to replace the balance valve. This is because the balance valve is the combination of the check valve and the external control internal leakage type sequential valves, but the sequential valve has increased the double spring, damping small hole, such as the valve core damping device; second, the control circuit of the balance valve should be in series throttle valve, so that the valve spool action "hysteresis", not because of the small changes in external pressure to make the speed change. In the speed limiting scheme, the effect of using the balance valve is ideal and is the preferred method.
(2) Speed limit of manual reversing valve
In all the reversing valves, only the manual reversing valve at the same time, through the control valve opening size can be both a finite speed and throttling speed regulation effect. Therefore, when the vehicle crane mechanism or load in the downward acceleration phenomenon, the driver can reduce the manual reversing valve handle pull degree to reduce the valve opening of the method to solve. Of course, because the driver's hand jitter will make the manual reversing valve speed limit effect is not very ideal, however, because the speed of loading and unloading of the truck crane is often adjusted according to the concrete situation of the construction site, the movement of the mechanism is often discontinuous intermittent operation, so skilled driver manual directional control valve can still receive a certain speed limit effect.
In short, the crane hoist lifting, jib telescopic, boom luffing 3 mechanism has both locking and limited speed requirements, Qing when the use can meet both the lock and speed limit of the two requirements of the balance valve, and then mechanical braking.
The leg should adopt two-way hydraulic lock with two-way locking requirement.
Turntable slewing mechanism only need to rely on the manual reversing valve m type of short time locking and reversing valve itself speed limit function can be.