The principle of boom extension and retraction in cranes

There are several types of telescoping mechanisms for crane booms: 1. Sequential telescoping mechanism – the various sections of the boom extend one after another in a specific order. 2. Synchronous extension mechanism – The various sections of the extension arm extend and retract at the same relative speed. 3. Independent telescoping mechanism – a mechanism in which each arm section can extend and retract independently. 4. Combined telescoping mechanism – When the telescoping arm has more than three sections, a mechanism that can use any two of the aforementioned telescoping methods simultaneously can be employed. The advantage of a sales-free fully hydraulic telescopic mechanism is that it allows for easy adjustment of the arm length, offers a variety of working arm lengths, and is highly practical. The downside is its high self-weight, which has a significant impact on the overall stability of the device. The pin less fully hydraulic telescopic mechanism comes in various configuration options: it can consist of multiple hydraulic cylinders combined with one level of rope arrangement, or it can involve a single hydraulic cylinder or multiple hydraulic cylinders along with two levels of rope arrangement. The feature of multiple hydraulic cylinders combined with a single level of rope arrangement is that the last telescopic arm uses steel wire ropes for extension, while the other telescopic arms utilize multiple cylinders or multiple single-cylinder units, or a combination of multiple cylinders and single cylinders, to achieve extension through hydraulic cylinders. Therefore, the cross-section of the end telescopic arm changes significantly, while the cross-sections of the other arm segments change less.

Rope array system: The rope array system is already well-established in China, and it is also a technology with a long history. The advantages of this technology are easy adjustment of the arm length, a wide variety of working arm lengths, the ability to carry loads while extending, and high practicality. The disadvantages are its heavy weight and the significant impact it has on the overall stability of the machine. It is now widely used in cranes with a capacity of less than 100 tons. Its principle is as shown in the diagram – it’s simply the principle of pulleys. For the telescoping mechanisms of crane booms with four or more sections, there are two types: multiple cylinders or multiple stages of cylinders combined with one rope arrangement, and a single cylinder or multiple cylinders combined with two rope arrangements. Some of Demag and Doiya’s products use a type of telescopic mechanism. The characteristic of this mechanism is that the last telescopic arm relies on wire ropes for extension, while the other telescopic arms use multi-stage cylinders or multiple single-stage cylinders, or a combination of multi-stage and single-stage cylinders, with extension achieved through hydraulic cylinders. Therefore, the cross-section of the end telescopic arm changes significantly, while the cross-sections of the other arm segments change less. In the past, Xuzhong, Puyuan, and Changqi have followed Liebherr’s technology for many years, generally using the second type of telescopic mechanism – one or two cylinders combined with rope arrangements to enable the extension and retraction of four- or five-section arms. This type of telescoping mechanism is used in China.,** but it is very difficult to handle the telescoping of boom structures with more than five sections. Manufacturers such as Bei Qi, Tai Qi, and Jin Zhong use a certain type of telescopic mechanism (multiple single-stage cylinders combined with one rope arrangement), but due to outdated technology, the inflow and outflow of oil for the second and third cylinders are achieved through hose reels. Today, most 5-joint crane models use a dual-cylinder dual-rope arrangement; typically, the 2nd joint extends independently, while the 3rd, 4th, and 5th joints extend synchronously ； The general single-cylinder double-rope arrangement of 4 sections extends and retracts synchronously in the sequence of 2, 3, 4. Its limitation is that the last one or two boom sections use wire ropes for extension, while the other boom sections employ hydraulic cylinders; as a result, the cross-sectional area of the last boom section changes significantly, which reduces the crane’s lifting capacity at large angles ； At the same time, for large-tonnage cranes, the requirements for steel cables are also very high, and it is very difficult to manufacture steel cables that meet these requirements. Although some Japanese companies have developed rope array technology to 6 sections or more, for medium and heavy-duty cranes, ordinary companies still give priority to the single-cylinder pin technology.

Single-cylinder pin system: The single-cylinder pin-type telescopic arm technology represents a typical integrated mechanical, electrical, and hydraulic system. Taking the German company Liebherr as a representative example, the actuator used for the extension and retraction of the telescopic arm consists of, as shown in the diagram, 1. a telescopic cylinder, 2. a pin-pulling mechanism, and 3. cylinder pins. To ensure the safety and reliability of the arm’s extension and retraction process, this system features an internal interlock mechanism – a spring-driven cylinder pin is installed on the telescopic cylinder to secure the arm in place, and only then is the connection between that arm segment and the other segments mechanically released. In this method, a certain section of the telescopic arm can have its connection to the other sections released only after the telescopic arm and the corresponding telescopic cylinder are locked together. Liebherr places the release mechanism above the telescopic mechanism; the advantage is its simple structure, strong self-locking capability, and ease of implementation ； For GROVE, Demag, and Tōno, the pull-out devices are placed on both sides of the telescopic mechanism, which makes the structural layout rather complex; high precision is required in processing and assembly, and it is relatively difficult to insert or remove the pins. The cylinder pins are all arranged on the side of the telescopic mechanism. A single-cylinder telescoping mechanism is required to have flexible operation, high reliability, fast response speed, and good interlocking; otherwise, it is difficult to achieve reliable extension and retraction of the boom. This technology uses a single cylinder, interlocked cylinder pins and arm pins, as well as **length-measuring electronic technology. Its advantages include low weight and minimal impact on the overall stability of the device; however, it presents challenges in terms of technical complexity, high costs, a limited range of available arm lengths, long extension times, and difficulties when changing the arm length. Currently, various single-cylinder pin-type truck cranes produced by domestic companies such as XCMG, Sany Heavy Industry, and Zoomlion have been launched on the market; these cranes use a pin-pulling mechanism similar to that of Liebherr, located above the telescopic mechanism.