1. Introduction
In cutting-edge fields such as semiconductor manufacturing equipment, precision machine tools, aerospace actuation systems, and high-precision measuring instruments, the performance of the drive system directly determines the ultimate accuracy of the entire machine. These applications require couplings to transmit high torque while achieving absolute angular displacement synchronization (zero backlash), providing extremely high torsional stiffness to ensure rapid response, and compensating for minor installation misalignments. Metallic bellows couplings, through their single-piece flexible metal bellows, perfectly meet these functional
requirements.
However, the connection interface between the coupling and the drive/driven shaft is often the weakest link in the transmission chain. Traditional keyway connections introduce stress concentrations that weaken fatigue strength; setscrew fixation is prone to fretting wear and loosening. Locking Assembly technology, as an advanced keyless connection solution, achieves full-friction torque transmission by applying uniform radial pressure, fundamentally eliminating the aforementioned defects. Its integration with bellows couplings constitutes one of the ultimate transmission solutions for ultra-precision applications. This paper aims to provide a comprehensive engineering analysis of such couplings.
2. Structure and Working Principle of Bellows Couplings with Locking Assemblies
2.1 Basic Structure
This type of coupling primarily consists of two core components:
1.Metal Bellows: Typically manufactured from high-strength stainless steel (e.g., AISI 304, 316) via hydroforming or electrodeposition processes. Its thin-walled, concentric corrugated structure provides the necessary flexibility, allowing it to compensate for axial, radial, and angular misalignment while transmitting torque.
2.Hub with Integrated Locking Assembly: The hub bore incorporates a locking assembly structure. A typical locking assembly system consists of an inner ring, an outer ring, and a set of precision-designed tapered clamping rings and high-strength clamping bolts.
2.2 Working Principle
Torque Transmission & Misalignment Compensation: Torque is transmitted through the bellows itself. The elastic deformation of the bellows absorbs relative displacements (misalignment) from both ends.
Shaft-Hub Locking: Tightening the clamping bolts causes conical interaction, forcing the inner and outer rings of the locking assembly to undergo precise elastic deformation.
The Outer Ring expands radially, creating an interference fit with the coupling hub's bore.
The Inner Ring contracts radially, creating an interference fit with the transmission shaft surface.
This process generates extremely high and uniform surface pressure across the entire contact area between the shaft and the hub bore. The resulting immense static frictional torque transmits the full working torque. Its transmission capacity far exceeds that of a keyed connection of comparable size.
3. Core Advantages and Technical Superiority
1.Stress Concentration-Free Connection:
Completely eliminates the stress concentration effects caused by keyways, avoiding the associated reduction in the shaft's fatigue strength (often by 30% or more). This significantly enhances the reliability and service life of the shafting, particularly under high-cycle fatigue loads.
2.Absolute Zero Backlash and Ultra-High Torsional Stiffness:
Zero Backlash: The keyless, clearance-free connection guarantees absolute angular displacement synchronization between input and output shafts.
High Torsional Stiffness: Both the bellows and the locking assembly connection possess very high rigidity, ensuring extremely high system servo stiffness and excellent dynamic response characteristics, making them particularly suitable for high-frequency positioning applications.
3.Excellent Centering and Dynamic Balancing Performance:
The uniform contraction of the locking assembly ensures perfect concentricity between the shaft and the coupling. The symmetrical design and superior manufacturing processes allow for achieving very high dynamic balancing grades (up to G2.5 and above), meeting the requirements of high-speed applications.
4.Comprehensive Misalignment Compensation Capability:
The bellows effectively compensates for axial, radial, and angular misalignment, protecting bearings at both ends from additional loads.
5.Superior Repeatability and Maintenance Convenience:
Loosening the bolts releases the locking assembly, allowing the coupling to be easily dismounted. This process causes absolutely no damage to either the shaft or the coupling itself, enabling non-destructive repeated installation and adjustment, which greatly reduces maintenance costs and downtime.
4. Application Fields
Due to their unparalleled precision and reliability, these couplings are widely used in the following high-end fields:
Semiconductor Manufacturing: High-precision linear motor and ball screw drives in lithography scanners, wafer handling robots, and bonders.
Precision Machine Tools and Robotics: Direct servo drives in 5-axis machining centers, CNC grinding machines, and robot joints.
Aerospace: Flight control actuation systems, radar antenna drives, gyro-stabilized platforms.
Medical and Optical Equipment: Drives in medical imaging equipment (CT, MRI), precision optical scanning mirrors.
5. Selection, Installation, and Usage Considerations (Key Engineering Practices)
1.Precise Selection Calculations:
Torque and Speed: Selection must be based on maximum operating torque, peak starting torque, and maximum operating speed. Note that the allowable torque for bellows couplings is often limited by their torsional strength.
Misalignment Capacity: Select a model with appropriate compensation capacity based on the system's expected misalignment (thermal expansion, installation errors). Operation beyond specified limits is strictly prohibited.
Torsional Vibration Analysis: Although stiff, the coupling's torsional stiffness parameter must be included in the system's torsional vibration model for analysis to avoid resonance.
2.Stringent Installation Protocols:
Alignment Requirements: Although capable of compensating for misalignment, excellent initial alignment is a prerequisite for long life and optimal performance. Laser alignment tools must be used to keep residual misalignment within 50% of the values recommended in the product catalog.
Contact Surface Treatment:
The fitting surfaces of the shaft and hub bore must have strict tolerances (typically recommended h6/g6) and very low surface roughness (Ra < 0.8 μm).
Contact surfaces must be absolutely dry, clean, and free from any oil, grease, dust, or burrs. Minute contaminants can significantly reduce the coefficient of friction, leading to a drastic drop in torque transmission capacity.
Bolt Tightening Procedure:
A calibrated torque wrench or hydraulic bolt tensioner must be used.
Strictly follow the manufacturer's specified bolt tightening sequence, torque values, and steps (usually involving crosswise, uniform tightening in multiple steps). This is the only way to ensure the locking assembly generates uniform radial pressure.
3.Operation and Maintenance:
Regular Inspection: The torque of all clamping bolts must be checked and re-torqued if necessary after the first 24 hours of operation.
Preventive Maintenance: Regularly inspect the coupling surface for signs of abnormal wear, cracks, or corrosion. For high-speed applications, periodic dynamic balancing checks are recommended.
6. Conclusion and Outlook
Bellows couplings based on Locking Assembly technology represent the pinnacle of precision mechanical drive connection technology. They successfully integrate the technical advantages of keyless connections with the performance benefits of bellows couplings, providing an ultimate solution for achieving ultra-high precision, high dynamic response, and high reliability in drive systems.
For the drive system engineer, recognizing their performance potential and mastering their strict installation and application protocols is key to successful deployment. Although their initial cost is higher than traditional solutions, the value they bring in improving overall machine performance, extending equipment life, and reducing downtime losses makes their total life-cycle cost highly competitive.
Looking forward, with advancements in materials science (e.g., titanium alloy bellows), condition monitoring (integrated micro-sensors for preload monitoring), and digital assembly tools, such couplings will evolve towards being lighter, stronger, and smarter, continuously pushing the boundaries of high-end equipment performance.
