Truck-mounted cranes are essential pieces of equipment in various industries, playing a crucial role in lifting and transporting heavy loads. However, during the operation of these cranes, one common issue that operators may encounter is jitter in the boom during telescoping. This jitter not only affects the stability of the lifted cargo but also poses potential safety hazards. In this article, we will explore the several major causes and solutions for this problem in detail.
When the boom of a truck-mounted crane is telescoping, it is not uncommon to experience jitter. This phenomenon is particularly noticeable when the boom is fully extended or reaches the highest position through upward luffing, and then retraction or downward luffing operations are performed. At these times, the jitter can become even more severe, making it extremely difficult to control the lifted cargo and potentially leading to accidents.
One of the main causes of boom jitter is the blockage of the damping hole in the balance valve. The balance valve is installed in the telescoping circuit to ensure the stability of the boom during telescoping and to prevent load impact. It consists of a check valve and a relief valve, with a damping hole provided on the relief valve spool. This damping hole plays a crucial role in controlling the flow of hydraulic fluid and maintaining the stability of the system. However, if the damping hole becomes blocked, the balance valve loses its ability to function properly. As a result, the flow of hydraulic fluid becomes unregulated, causing sudden pressure changes and leading to jitter during boom telescoping. To address this issue, the balance valve needs to be disassembled and cleaned thoroughly to unclog the damping hole.
Another possible cause of boom jitter is the presence of air in the hydraulic cylinder. If the liquid level in the hydraulic oil tank is too low or there is a leak in the hydraulic circuit, air can enter the system. Once air is present in the hydraulic circuit, it mixes with the hydraulic oil, increasing the elasticity of the oil and reducing the rigidity of the hydraulic transmission. This can cause the telescopic cylinder to experience impact and crawling, resulting in jitter during boom telescoping. To troubleshoot this problem, it is necessary to identify the fault point where air is entering the hydraulic circuit. This could involve checking for leaks in the hoses, fittings, or seals. Once the leak is located and repaired, the air in the hydraulic circuit needs to be expelled. This can be done by bleeding the system, which typically involves operating the hydraulic controls in a specific sequence to allow the air to escape.
Excessive friction during boom telescoping can also lead to jitter. If the boom slider is severely worn, for example, it can cause the boom to tilt downward when extended and increase the resistance when retracting. This uneven movement and increased resistance can cause the system pressure to fluctuate suddenly, resulting in hydraulic shock and jitter. To solve this problem, the slider with wear exceeding the specified value should be replaced. Additionally, the gap between the boom and the slider should be adjusted to meet the specified value. It is important to note that the slider gap should be adjusted without load so that it has an appropriate upward deflection value after extension. This upward deflection can compensate for the downward deflection of the boom when it is under load, helping to prevent jitter during telescoping.
The loosening of the telescoping wire rope of the boom is another factor that can cause jitter. To determine if the wire rope is the cause of the problem, a simple test can be performed. Remove the telescoping wire rope and rely solely on the telescopic cylinder to drive the boom’s telescoping. Observe whether the boom telescoping is smooth and free from jitter. If there is no jitter in this case, reinstall the telescoping wire rope and conduct a telescoping test. If jitter occurs, it can be concluded that the telescoping wire rope is loose. To address this issue, the length and tension of the telescopic boom wire rope should be checked and adjusted to meet the requirements. Additionally, the lubrication condition of the boom extension and retraction pulleys and the wear condition of the pulley bushings should be inspected. If necessary, lubricating grease should be added or the pulley bushings replaced.
The machining dimensions of the telescopic cylinder can also have an impact on boom jitter. If the geometric tolerances such as the roundness and cylindricity of the inner diameter of the cylinder barrel and the coaxiality between the guide sleeve and the piston are out of tolerance during the machining process, it will increase the movement resistance of the telescopic cylinder. This uneven movement can cause jitter during boom telescoping. To solve this problem, the telescopic cylinder parts need to be processed strictly according to the drawings, ensuring that the dimensional tolerances and geometric tolerances are within the allowable range.
Similarly, insufficient manufacturing precision of the boom can also lead to jitter. When manufacturing the boom, if the dimensional tolerances and geometric tolerances do not meet the drawing requirements, it can result in uneven gap sizes between the boom sections. This unevenness can cause the boom to be tight in some areas and loose in others during telescoping, leading to jitter. To address this issue, the boom needs to be processed strictly according to the drawings, with the dimensional tolerances and geometric tolerances controlled within the allowable range.
Poor lubrication of the boom slider and slideway is another common cause of boom jitter. The nylon slider and slideway installed inside the boom require regular lubrication to reduce friction and ensure smooth movement. If they are not properly lubricated, the friction between the slider and the slideway can increase, causing the system pressure to fluctuate and resulting in jitter. To prevent this, the lubrication condition of the slider and slideway should be regularly checked, and lubricating grease should be applied in a timely manner.
Finally, an unreasonable boom design can also contribute to jitter during telescoping. The boom is typically telescoped using a combination of the hydraulic cylinder, wire rope pulley block, and a sliding trolley installed at the head of the hydraulic cylinder. When the boom is fully extended, the sliding trolley exits from the horsehead boom (the thinnest section of the boom), and when retracting, it enters the horsehead boom. If there are no guide plates or if the guide plates are not properly designed on the left, right, and bottom surfaces at the rear end of the horsehead boom, the sliding trolley can impact the rear end of the horsehead boom when entering and exiting. This impact can cause the boom to vibrate violently and may also result in the lifting wire rope hitting the basic boom. To solve this problem, when designing and processing the boom, guide plates should be installed at the rear end of the horsehead boom, and their position should be carefully calculated to ensure that the hydraulic cylinder and the sliding trolley can enter and exit the horsehead boom smoothly, avoiding violent jitter.
For example, in a construction site, a truck-mounted crane is being used to lift heavy building materials. During the operation, the operator notices significant jitter in the boom when telescoping. By following the troubleshooting steps outlined above, the operator can systematically identify the cause of the jitter and take the appropriate measures to resolve the issue. This not only ensures the safety of the operation but also helps to maintain the efficiency and productivity of the construction project.
In conclusion, understanding the causes and solutions for jitter during the telescoping of truck-mounted crane booms is essential for crane operators and maintenance personnel. By regularly inspecting and maintaining the crane’s components, ensuring proper lubrication, and addressing any design or manufacturing issues, it is possible to minimize the occurrence of boom jitter and ensure the safe and efficient operation of the crane.
