The telescopic boom truck mounted crane is used in small and medium tonnage industrial handling industries, gas transport, building construction, plant and mine cargo transport, etc. Compared with similar tonnage truck mounted cranes, it has a superior price-performance ratio and a wide range of functions. For a telescopic boom truck mounted crane, one of the main parts that determine the performance of the product is the boom telescopic system, and this article will talk about the boom telescopic cylinder.
The boom telescopic cylinder mainly pushes the boom to a telescope, and also bears the gravity of the load to enhance the stability of the boom when lifting. According to the tonnage of the crane, the boom of the truck-mounted crane generally has 2 to 6 jibs, the greater the tonnage, the greater the working height and amplitude requirements, the greater the number of jib sections, and the number of jib sections directly affects the way of jib telescoping.
Boom telescopic mechanism
There are three types of telescopic jibs for telescopic boom cranes: synchronous telescopic, sequential telescopic, and sequential plus synchronous telescopic. Simultaneous telescoping means that each section of the boom is telescoped simultaneously at the same stroke ratio by a single cylinder driven by a wire rope; Sequential telescoping means that multiple cylinders are used to directly drive each section of the arm to complete the telescoping action in a certain order. The order of extending the arm is generally outside first and then inside, while the order of retracting the arm is the opposite, namely inside first and then outside.
The boom of the sequential telescopic mechanism is lighter than the synchronous telescopic boom for the same lifting performance because the position of the dangerous section changes more when the latter is working and cannot be made into a variable section structure. Sequential plus synchronous telescoping, which means a combination of synchronous and sequential telescoping, can greatly simplify the internal structure of the boom, improve the reliability of the telescoping mechanism and provide more scope for optimizing the boom section.
Boom telescopic cylinders
Depending on the telescopic mechanism, different types of boom telescopic cylinders have emerged.
- Single-stage telescopic cylinders
Suitable for synchronous telescopic systems with a piston rod connected to the basic arm and cylinder barrel connected to the two-section arm. The piston rod is designed as a hollow rod with an internal oil guide tube, eliminating the need for an external hose and simplifying the internal structure of the boom. The piston rod is fed with oil during movement and the cylinder barrel expands and retracts to drive the two-section arm. The pulley mounted on the cylinder pulley pulls the rest of the boom through the wire rope to complete the synchronous movement of the boom. At present this type of cylinder application is very common. As the cylinder not only needs to bear the axial force of the gravity of the boom and the load and the friction force when telescoping but also the force of the telescopic boom wire rope on the cylinder, the number of jib sections on the cylinder has a great impact on the force, usually, more than 4 jibs need to consider the use of the multi-stage sequential telescopic cylinder.
- Secondary sequential telescopic cylinder
For sequential telescopic or sequential plus synchronous telescopic systems, the primary cylinder piston rod is connected to the basic arm and the cylinder barrel is connected to the two-section arm; the secondary cylinder piston rod is connected to the primary cylinder barrel and the cylinder barrel is connected to the three-section arm.The piston rod is designed as a hollow rod with an internal oil guide tube, with only 1 steel pipe and 2 hoses between the 2 cylinders, which reduces the leakage of hydraulic oil in the boom.
- Three-stage sequential telescopic cylinders
Suitable for sequential telescopic or sequential plus synchronous telescopic systems, where the primary cylinder piston rod is connected to the basic arm and the cylinder barrel is connected to the two-section arm.The piston rod of the secondary cylinder is connected to the cylinder barrel of the primary cylinder, which is connected to the three-section arm; the piston rod of the tertiary cylinder is connected to the cylinder barrel of the secondary cylinder, which is connected to the four-section arm.
Control method for secondary sequential telescopic cylinders
As the working principle of the three-stage sequential telescopic cylinder is the same as the two-stage sequential telescopic, the main analysis of the two-stage sequential telescopic.
Sequential control of the secondary sequential telescopic cylinder can be achieved in a variety of ways, including individual control of the duplex control valve; area difference control; one-way sequential valve control; electromagnetic reversing valve control; and mechanical touch control.
The use of duplex valves and solenoid operated reversing valve control is significantly more expensive and is not presented.
- The expansion and contraction sequence is controlled by the difference in area between the two cylinders, i.e. the area of the first extension cylinder is larger than the second extension, and the area of the first retraction cylinder is larger than the second retraction.Due to the limitation of the volume of the cylinders in the boom, the cross-section of the 2 cylinders will not be very different in practice and therefore cannot be fully guaranteed to extend and retract in sequence, which may result in chaotic movements.
- The principle of one-way sequence valve control is to add two one-way sequence valves between the telescopic oil circuit of the two cylinders, and to control the sequence by adjusting the opening pressure difference between the two cylinders.Operate the reversing valve to feed the A1 port of the first stage telescopic cylinder, the pressure oil reaches the rodless cavity of the first stage telescopic cylinder and the one-way sequence valve inlet through the piston rod.As the one-way sequence valve is equipped with a certain opening pressure, so the first class cylinder is extended first, with the rod chamber return oil through the B1 port back to the directional valve.When the first cylinder reaches the end of the stroke, the pressure increases until the one-way sequence valve is opened, at this time the pressure oil reaches the second cylinder rodless cavity through the A2 port, so that the second cylinder extends, the rod cavity return oil through the B2, B1 port back to the multi-way directional valve. Retraction action and extension principle is the same.The greater the differential pressure of the one-way sequence valve, the better the reliability of the sequence opening and the greater the pressure loss. 2 cylinders can be designed to the same specifications, and the internal structure is relatively simple, the one-way sequence valve is installed directly on the pipeline, the structure is more compact.