Assurer la longévité des cercles d'oscillation des pelles hydrauliques : Conception, transport, installation et maintenance

Introduction

Swing circles, also known as slewing bearings, play a crucial role in the operation of various machines, particularly excavators. They allow the cab to rotate relative to the lower machine frame, facilitating smooth and efficient work operations. However, these components are subject to intense stress, including high-frequency cyclic loading, tilting moments, and considerable inertial, axial, and radial forces. To maximize the useful life of swing circles and prevent issues like seizing and broken teeth, it is essential to adhere to specific guidelines during their design, transport, installation, and maintenance.

Understanding Swing Circles in Excavators

Swing circles are essential components in machines where the cab needs to rotate relative to the lower machine frame, such as tracked and wheeled excavators. These bearings endure significant stress and require meticulous care to avoid rapid wear and tear.

Components of Swing Circles

Swing circles are large and consist of four main components:

• Lower Toothed Ring: This component is attached to the lower machine body and includes teeth that engage with the transmission pinion to facilitate rotation.
• Upper Ring: This ring, which lacks teeth, is connected to the cab frame.
• Rolling Elements: These can be balls arranged in one or two bands or rollers, allowing smooth rotational movement.
• Spacers: These components maintain the proper spacing between the rolling elements.

Both the upper and lower rings have grooves, or races, designed to hold the rolling elements and spacers securely. The swing circle’s toothed ring is fitted to the lower machine body with screw connectors, while the upper ring is connected to the cab frame. This arrangement allows the two frames to rotate relative to each other, with a reduction gear system controlling the rotary movement. The reduction gear is fitted to the cab frame and features a transmission pinion aligned with the bearing’s teeth to drive the movement.

Common Causes of Swing Circle Breakage and Replacement

Tooth Breakage

The most frequent cause of swing circle replacement is the breakage of one or more teeth on the lower ring. This issue can render the operation impossible as the number of broken teeth tends to increase rapidly. The primary causes of tooth breakage include:

• Deterioration or Failure of the Drive Gear Braking System: When the braking system is ineffective, the inversion of torque during changes in the cab’s movement direction causes the teeth to flex beyond their design specifications. This risk is heightened when the shovel is loaded, and the rotation speed is high.

Bearing Seizing

Another common problem is bearing seizing. Despite the high loads to which swing circles are subjected, their relatively small transverse cross-section makes them deformable if not adequately supported. Important factors to consider include:

• Rigidity of the Connecting Structure: Ensuring that the connecting structure is sufficiently rigid helps prevent deformation.
• Flatness of Support Surfaces: Adequately flat support surfaces are crucial to prevent seizing.
• Fixing System: A robust fixing system prevents deformation under heavy loading.

Failing to address these factors not only risks bearing seizing but also causes permanent damage to the rolling races.

Preventing Damage to Swing Circles

To extend the useful life of swing circles, it is essential to follow best practices from construction through maintenance.

Materials and Heat Treatments

Choosing the right materials and heat treatments is critical for strong, durable swing circles, especially for the teeth’s lifespan. Typically, two types of steel are used:

• 42CrMo4: Hardened with a tensile strength of 90-100 kgf/mm² for internal rings.
• C45: Hardened with a tensile strength of 70-80 kgf/mm² for external rings.

Surface heat treatment is also necessary for the most heavily loaded areas of the bearing, including the rolling element races and the teeth of the internal ring. Induction hardening increases the resistance of the teeth to bending and pitting on the contact surfaces. Similarly, the rolling races are treated to counter wear from the continuous transit of moving parts.

Transport and Storage

Proper transport and storage are vital to prevent damage to swing circles. Key considerations include:

• Transporting Horizontally: Enclose the swing circle in rigid casing and transport it horizontally to avoid impact and radial damage.
• Indoor Storage: Store the casing and bearing indoors, protected from atmospheric agents that could cause corrosion.
• Careful Unpacking: When unpacking, take care not to damage the gaskets and degrease the bearing with commercial chloride-free thinners, as chloride is harmful to the surfaces.

Installation and Maintenance Guidelines

Following specific guidelines during installation and maintenance can significantly extend the lifespan of swing circles.

Installation

• Ensure Rigidity: The connecting structure must be sufficiently rigid to support the cercle de balançoire without deformation.
• Flat Support Surfaces: Ensure the support surfaces are adequately flat to prevent bearing seizing.
• Proper Fixing: Use a robust fixing system to prevent deformation under heavy loading.

Maintenance

• Inspections de routine : Conduct regular inspections to identify and address issues early.
• Lubrication and Greasing: Properly lubricate all moving parts with recommended grease and lubricants to reduce friction and wear.
• Filter Maintenance: Maintain and replace hydraulic filters according to schedule to prevent contaminants from circulating and causing damage.

Conclusion

Swing circles are vital components in excavators, enabling smooth and efficient rotation of the cab relative to the lower machine frame. However, they are subject to intense stress, making proper design, transport, installation, and maintenance crucial for their longevity. By following best practices and addressing common issues such as tooth breakage and bearing seizing, operators can significantly extend the lifespan of swing circles, ensuring reliable and efficient operation of their machinery.

Q&R

Q: How often should I inspect my swing circle?

A : Regular inspections should be conducted every 250 hours of operation, with more detailed inspections every 1,000 hours. This helps identify and address issues early, preventing major failures.

Q: What type of grease should I use for lubricating swing circles?

A : Use the manufacturer-recommended grease and lubricants specifically designed for heavy-duty machinery to ensure optimal performance and longevity.

Q: What are the signs of a failing swing circle?

A : Common signs include unusual noises, difficulty in rotation, visible wear or damage to the teeth, and fluid leakage. Early detection and repair can prevent major failures.

Q: How can I prevent tooth breakage in swing circles?

A : Ensure the drive gear braking system is effective and routinely inspected. Avoid overloading the shovel and high-speed rotations to minimize the risk of tooth breakage.

Q: Why is surface heat treatment important for swing circles?

A : Surface heat treatment, such as induction hardening, increases the resistance of the teeth and rolling races to bending, pitting, and wear, significantly enhancing the durability of swing circles.