회사 소식 Crane Universal Joint: Key Component in Crane Power Transmission Systems

In the power transmission system of a crane, the universal joint (also known as the universal coupling) is a rather crucial component, specifically designed to address the issue of torque transmission between shaft systems of different axes. In practical applications, it can easily handle an Angle deviation of ±3° to ±30° between two transmission shafts. Even if there is an axial movement of 5 to 20 millimeters or a radial offset of 0.1 to 1 millimeter due to equipment operation, the power transmission will not be greatly affected.

Structurally speaking, it mainly consists of the driving shaft, the driven shaft and the cross shaft assembly, with needle roller bearings also installed on the cross shaft. If classified by load-bearing characteristics, there are two types: rigid and flexible. For instance, the cross-shaft type belongs to the rigid category. This type of rigid universal joint is quite powerful, capable of withstanding an instantaneous impact torque of up to 5,000 kN · m. Therefore, it is widely used in bridge and tower cranes, accounting for more than 80%.

The core role of universal joints in cranes is as follows

• Torque transmission connection

It is like a bridge, transmitting the torque of the crane drive device to the actuator. For instance, for a 25-ton truck crane, the universal joint of the hoisting mechanism must have a rated torque transmission of 1800N · m. Moreover, under 1.2 times the rated load, it should not undergo plastic deformation even after continuous operation for 30 minutes. This is related to the safety of hoisting operations.

• Compensate for axis deviation

When the crane is in operation, the extension and retraction of the boom will cause changes in the Angle of the transmission shaft system. For instance, as the telescopic arm increases from the shortest to the longest, the Angle between the axes of the universal joint of the luffing mechanism may change from 5° to 22°. At this point, the cross shaft bearing must maintain a rotational accuracy of 1,500 revolutions per minute, and the clearance must be controlled within 0.05 millimeters; otherwise, abnormal transmission sounds are likely to occur, and the vibration will also increase. However, for cranes equipped with high-quality universal shafts, transmission faults caused by shaft deviation can be reduced by more than 60%.

• Buffer vibration and shock

At the moment of lifting and starting, the power system will generate an impact load of 2.5 times the rated torque. If the flexible universal joint with rubber elastic elements is used, the vibration acceleration can be reduced from 15g to below 8g, which is measured by the acceleration sensor. In this way, the fatigue damage to the reducer gears (with a module generally ranging from 8 to 12 millimeters) and the drum bearings (commonly self-aligning roller bearings of type 23230) is much smaller, which can extend the service life of these key components by 30% to 50%.

• Adapted structural layout

The slewing mechanism of a crawler crane should be capable of continuous 360° rotation. The axial extension of its universal joint should be ±15 millimeters to compensate for the clearance error of the slewing bearing (with a diameter usually ranging from 2 to 4 meters). Without universal shafts, the installation alignment error of the transmission shaft system must be controlled within 0.02mm/m, which is basically impossible to achieve in practical work.

For universal shafts used in cranes, these technical parameters must meet the standards

• Material strength: The cross shaft is made of 40CrNiMoA alloy structural steel. After quenching and tempering treatment, its hardness can reach 28-32HRC, and its tensile strength is no less than 980MPa. This ensures that it will not break under 1.5 times the rated torque.
• Sealing performance: It is sealed together with a double-lip skeleton oil seal (lip pressure 0.3-0.5MPa) and an O-ring (temperature resistance -40 ℃ to 120℃), and the dust-proof grade can reach IP65. Even in an environment with a dust concentration of 10mg/m³, it will not leak after continuous operation for 500 hours.

• Lubrication requirements: Lithium-based grease (NLGI Grade 2) should be added every 200 hours of operation. The grease filling amount should be 1/3 to 1/2 of the bearing cavity volume. Otherwise, the lubrication will be poor and premature wear is likely to occur. When the wear exceeds 0.1 millimeters, it needs to be replaced.

These technical details ensure that the universal joint can operate reliably under heavy loads and complex working conditions, and also make it a key component in the crane's power system that can both transmit rigidly and adapt flexibly.