Due to the relatively long strokes of load lifting and lowering, boom extension and retraction, and boom luffing in mobile cranes, the acceleration phenomenon caused by the force of gravity during the downward movement of the mechanisms or loads is a rather prominent issue. To address this, mobile cranes typically employ the following two speed limiting methods.
Functions of the Balance Valve The balance valve serves not only to lock but also to limit the speed. When the mechanism or load is being lowered, the sequence valve within the balance valve can, by means of the balancing action of the valve spool, maintain a stable return oil flow rate of the hydraulic cylinder (hydraulic motor), thereby enabling the hydraulic cylinder (hydraulic motor) to move at a uniform speed.
Design Considerations for Achieving Uniform Descent Speed When designing the system to obtain a uniform descent speed, two points need to be noted. Po pierwsze, it is not advisable to simply replace the balance valve with a combination of a common one-way valve and an external control internal leakage type sequence valve just to save costs. Although the balance valve is also a combination of a one-way valve and an external control internal leakage type sequence valve, its sequence valve has additional devices such as double-layer springs and damping orifices to dampen the vibration of the valve spool. Secondly, the control oil circuit of the balance valve should be connected in series with a throttle valve. This is to make the movement of the sequence valve spool “sluggish” so that its speed will not change due to minor variations in external pressure. Among the speed limiting schemes, using a balance valve yields the most ideal results and is the preferred method.
In more detail, the balance valve plays a crucial role in ensuring the stable operation of the mobile crane during the downward movement of its components. When the mechanism or load starts to descend, the hydraulic fluid in the hydraulic cylinder (or hydraulic motor) needs to flow back to the reservoir. Without proper control, the force of gravity could cause the fluid to rush back too quickly, resulting in an uncontrolled acceleration of the descent. The balance valve intervenes in this process by precisely regulating the flow rate of the return oil.
The sequence valve within the balance valve is designed with specific mechanisms to achieve this regulation. The double-layer springs and damping orifices are carefully engineered to counteract any sudden changes in pressure and maintain a smooth and consistent movement of the valve spool. Ten, in turn, ensures that the return oil flow remains stable, allowing the hydraulic cylinder (or hydraulic motor) to move at a constant speed rather than accelerating uncontrollably.
The addition of a throttle valve in series with the control oil circuit of the balance valve further refines this speed control. By creating a certain resistance in the oil flow path, the throttle valve makes the response of the sequence valve spool less sensitive to minor pressure fluctuations. This means that even if there are small changes in the external pressure acting on the system, such as those caused by vibrations or slight variations in the load, the speed of the descent will remain relatively constant.
Overall, the use of a balance valve in the return oil circuit is a highly effective way to limit the speed of the mobile crane’s mechanisms and loads during the downward movement, providing both safety and precision in operation.
Unique Function of Manual Directional Control Valve among Directional Control Valves Among all types of directional control valves, only the manual directional control valve can, while changing the direction of the oil flow, also serve the functions of speed limiting and throttling speed regulation by controlling the opening size of the valve port.
How Manual Directional Control Valve Addresses the Downward Acceleration Issue When the mechanism or load of a mobile crane experiences a downward acceleration phenomenon, it can be addressed by the driver reducing the pulling extent of the manual directional control valve handle to decrease the opening size of the valve port. However, due to the shaking of the driver’s hand, the speed limiting effect of the manual directional control valve may not be entirely ideal. Nevertheless, since the loading and unloading operations of mobile cranes often need to be continuously adjusted according to the specific circumstances of the construction site, and the movement of the mechanisms is usually discontinuous and intermittent, a skilled driver can still achieve a certain speed limiting effect by operating the manual directional control valve.
To elaborate further, the manual directional control valve offers a practical solution for speed control in mobile cranes, albeit with some limitations. When the crane is in operation and the mechanism or load is descending, the speed can potentially increase rapidly due to gravity. The manual directional control valve allows the driver to intervene in this process by adjusting the opening of the valve port. By reducing the pulling force on the valve handle, the driver can effectively narrow the opening, thereby restricting the flow of hydraulic fluid and slowing down the movement of the mechanism or load.
However, the human factor comes into play as the driver’s hand may not be completely steady. This shaking can cause slight and inconsistent changes in the valve port opening, which may lead to less than perfect speed control. But considering the nature of mobile crane operations, where the work pace and requirements vary constantly depending on the construction site conditions, and the movements are often intermittent, the ability of a skilled driver to make on-the-fly adjustments using the manual directional control valve still proves to be of value. In many cases, it can prevent excessive speeds and maintain a reasonable level of control over the crane’s movements.
IV. Overall Considerations for Different Crane Mechanisms
For the Three Mechanisms of Load Lifting and Lowering, Boom Extension and Retraction, and Boom Luffing For these three mechanisms of the mobile crane, namely load lifting and lowering, boom extension and retraction, and boom luffing, there are requirements for both locking and speed limiting. In such cases, it is advisable to adopt a balance valve that can simultaneously meet these two requirements, along with the addition of mechanical braking. The balance valve ensures a stable and controlled movement during the operation of these mechanisms, preventing any uncontrolled acceleration or sudden movement. The mechanical braking provides an additional layer of safety, ensuring that the mechanisms can be firmly held in place when needed.
For the Outriggers The outriggers should employ a two-way hydraulic lock that can fulfill the requirement of two-way locking. This is crucial for maintaining the stability of the mobile crane during operation. When the crane is in use, the outriggers need to be firmly locked in place to provide a stable base. The two-way hydraulic lock ensures that the outriggers can be locked both when they are extended and when they are retracted, preventing any unwanted movement that could potentially tip the crane or affect its stability.
For the Turntable Slewing Mechanism The turntable slewing mechanism only requires the short-term locking function of the M-type neutral position of the manual directional control valve and the speed limiting function of the valve itself. During the rotation of the turntable, the manual directional control valve in its M-type neutral position can temporarily hold the mechanism in place, preventing any sudden or uncontrolled rotation. At the same time, the valve’s inherent speed limiting function helps to control the speed of the rotation, ensuring a smooth and safe operation of the turntable slewing mechanism.
Podsumowując, different mechanisms of the mobile crane have specific requirements for locking and speed limiting during operation. By carefully selecting and implementing the appropriate speed limiting methods and locking devices for each mechanism, such as using balance valves, manual directional control valves, two-way hydraulic locks, etc., the safety and efficiency of the mobile crane’s operation can be significantly enhanced. This ensures that the crane can perform its various tasks, including load handling, boom movement, and turntable rotation, in a controlled and stable manner, minimizing the risks associated with uncontrolled accelerations and movements.
