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Analysis of the Causes of Load-Carrying Failure in the Connecting-Rod Structure of a Double-Arm Rock-Drilling Rig
Release time:
2026-11-16
Source:
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Summary:
The base at the bottom of the boom of a hyperbolic-arm rock drilling rig incorporates multiple linkage structures with numerous joints. Under conditions where lateral forces are applied, deformation tends to be relatively large, making instability and vibration failures more likely. Additionally, these linkage structures can experience structural failure when subjected to heavy loads. Such load-bearing failures are typically not caused by a single factor; rather, they result from the combined effects of structural design, wear of connecting components, load distribution, and operating conditions. The following analysis examines these issues from several key perspectives.
I. Inherent Weaknesses in Structural Design
Traditional linear linkage mechanisms often lack sufficient stability when subjected to uneven forces or load distributions, prone to distortion and stress concentrations that reduce the service life and reliability of supporting structures and may even lead to structural failure. Although the linkage structure of the double-curved-arm trolley improves the range of motion of the boom, the series arrangement of multiple links and joints lengthens the force transmission path, increasing additional bending moments and torsional torques generated during force transfer. Compared with a single‑arm straight‑boom configuration, when the double‑curved arm experiences lateral loads, the coordinated deformation required of the links on both sides can subject local nodes to combined stresses far exceeding design values.
II. Wear of pins and hinge points leads to accumulated clearance
All brackets and connecting rods of the drilling boom are joined by expansion pins. Wear is a common cause of damage to these pins; the longer the pin bushings are in service, the more severe the wear becomes, creating gaps between the bracket and connecting rod joints and reducing the positioning accuracy of the articulated boom. If the pin and pin hole do not achieve effective interference fit, the drilling boom will become unstable and shake violently during rock drilling, generating substantial impact loads that frequently lead to pin fractures. Copper bushings are installed at the small-arm hinge point, on the connecting tie rods, and in the eye‑bolt holes at both ends of the angle‑adjusting hydraulic cylinder. As the cylinder extends and retracts to adjust the boom’s attitude, the pin undergoes continuous small‑angle reciprocating oscillation, allowing dust to readily enter the clearance between the pin and the copper bushings, resulting in pronounced abrasive wear. Once such clearances develop, the load distribution shifts from uniform to concentrated impacts, accelerating damage to the connecting‑rod structure.
III. Load Concentration and Stress Concentration
Simulation analysis of the boom system of an articulated (knuckle‑boom) rock‑drilling vehicle shows that, in the fully extended configuration, the stress distributions differ significantly among three typical operating conditions: roof drilling, side‑wall drilling, and face drilling. Loads in the linkage assembly are often concentrated at a small number of hinge points; unless these loads are dispersed by adding hinges or by optimizing the link cross‑sections, local stresses may exceed the material’s load‑bearing capacity. In addition, when the swing arm is actuated by a single hydraulic cylinder, the large torsional moment generated during operation can cause the two sides of the boom to shift laterally, increasing the load on one side of the linkage.
IV. Fatigue Accumulation and Welding Defects
During drilling, the boom of a rock‑drilling jumbo is subjected to high‑frequency impacts and cyclic loads. If defects such as lack of fusion, porosity, or undercutting are present in the weld zone of the connecting rod, they can serve as initiation sites for fatigue cracks. Boom welding imposes strict requirements on process parameters; the joint gap, root face, welding current, and travel speed must all be maintained within appropriate ranges, otherwise joint quality cannot be ensured. Simulation analysis reveals that locations with low fatigue life in the drill boom typically occur in stress‑concentration zones, such as the edges of hydraulic cylinder hinge holes; under extreme operating conditions, the calculated fatigue life may be only a few tens of hours. This indicates that, under specific load spectra, the fatigue margin of the connecting‑rod structure may be inadequate.
V. Operating Conditions and Maintenance Factors
Overloading, prolonged rock drilling with the boom fully extended, and forced drilling under lateral tilt conditions all increase stress on the connecting rod. Meanwhile, inadequate lubrication causes dry friction between the pin and bronze bushing, dramatically increasing the wear rate. If the resulting clearance is not promptly corrected by re-tightening the expander bolt nut, the damage will further propagate.
In summary, the load-carrying failure of the connecting‑rod structure in a double‑curved‑arm rock‑drilling rig results from the combined effects of insufficient inherent structural stiffness, clearance accumulation due to pin wear, load and stress concentrations, fatigue and welding defects, and improper operation and maintenance. During troubleshooting, first inspect the fit clearances between pins and bronze bushings at all hinge points, and check the connecting‑rod welds for visible cracks; then assess whether insufficient stiffness is present by observing the boom’s behavior under lateral loading conditions. In routine operation, regularly monitor pin wear, tighten expansion pins to the specified torque, and ensure proper lubrication at all lubrication points to retard the degradation of the connecting‑rod structure’s performance.
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