1. Introduction

In modern industrial drive systems, such as those in fans, pumps, compressors, and general industrial machinery, diaphragm couplings are critical components connecting prime movers to driven machines. They utilize the elastic deformation of metallic diaphragm packs to absorb axial, radial, and angular deviations caused by installation errors and thermal expansion. Simultaneously, their metal-to-metal contact enables zero-backlash torque transmission, ensuring drive accuracy and efficiency.

The keyway connection is a mechanical coupling method that transmits torque through the shear force on the key's sides and the compressive stress on the contact surfaces between the key, the shaft keyway, and the hub keyway. It has a long history, mature technology, low cost, and a high degree of standardization. Consequently, despite the emergence of more advanced technologies like shrink disc connections, keyway-type diaphragm couplings remain significant in the market for medium and low-power, standard-speed industrial applications due to their economy and applicability. However, the stress concentration effects introduced by the keyway and its impact on system dynamic characteristics demand thorough attention from drive engineers. This paper aims to provide a comprehensive and objective technical evaluation of keyway-type diaphragm couplings.

2. Structure and Working Principle of Keyway Diaphragm Couplings

2.1 Basic Structure

A keyway-type diaphragm coupling primarily consists of three parts:

1.Diaphragm Pack(s): The core flexible element for misalignment compensation and torque transmission, comprising multiple layers of high-strength stainless steel laminations.

2.Hub: Typically a forged steel or alloy steel component with a keyway machined in its bore. A key provides the circumferential fixation to the shaft for torque transmission.

3.Spool Piece: Used to connect two diaphragm packs, forming a double-cardanic coupling to provide greater compensation capacity and suitability for longer shaft distances.

2.2 Single vs. Multiple Diaphragm Pack Structures

Single-Cardanic Type: One diaphragm pack directly connects two keyed hubs. Compact structure, lower cost, suitable for applications with relatively good alignment and lower compensation needs.

Double-Cardanic Type: Two diaphragm packs are connected by a spool piece. This structure better isolates misalignment between the input and output sides, offers superior compensation capacity, and is suitable for long drivelines, applications with potential foundation settlement, or significant thermal expansion.

2.3 Keyway Connection Working Principle

The torque path is: Shaft → Key → Hub → Diaphragm Pack → Opposite Hub → Key → Driven Shaft. Torque T is transmitted via shear force F_s on the key and compressive stress σ_c on the contact surfaces. Its transmission capacity depends on the key material's shear strength and the allowable compressive stress of the working surfaces.

3. Core Advantages and Application Fields

3.1Core Advantages

1.Mature Technology, Low Cost: Design, manufacturing, and machining processes are standardized, resulting in significantly lower initial procurement costs compared to couplings using technologies like shrink discs.

2.Stable Load Capacity: For static and quasi-static loads, its torque transmission capacity is based on well-established calculation theories, and its reliability is proven by long-term industrial validation.

3.Ease of Assembly/Disassembly: Given known keyway dimensions, disassembly and reassembly are relatively straightforward, requiring no specialized hydraulic tools.

4.Retention of Inherent Diaphragm Coupling Advantages: Similarly possesses benefits like zero backlash, high torsional stiffness, misalignment compensation, and maintenance-free operation (no lubrication).

3.2 Application Fields

Keyway-type diaphragm couplings are primarily suited for cost-sensitive, stable operating conditions within medium power and speed ranges:

General Industrial Machinery: Fans, water pumps, belt conveyor drives, gearbox connections.

Traditional Manufacturing: Injection molding machines, extruders, auxiliary drive lines in metal rolling.

Medium and Low-Speed Power Transmission: Some marine auxiliary power systems, diesel generator sets.

4. Inherent Limitations of Keyway Connections and Usage Considerations (Key Engineering Practices)

The defects of keyway connections define their application boundaries. The following precautions must be strictly adhered to:

4.1 Stress Concentration and Fatigue Strength Reduction

Core Issue: The presence of a keyway creates a sharp notch on both the shaft and the hub, leading to severe stress concentration and significantly reducing their fatigue strength (the fatigue strength reduction factor can reach 2-3). Under alternating torque loads, fatigue cracks readily initiate at the keyway ends, potentially leading to fatigue fracture of the shaft or hub.

Design Countermeasures:

Use Woodruff keys or implement a large radius at the ends of rectangular keyways to mitigate stress concentration.

Perform detailed fatigue strength verification, selecting a sufficient safety factor (typically higher than for keyless designs).

Prefer keyless connections (e.g., shrink discs) for heavy-duty, high-speed, or frequently start-stop applications.

4.2 Dynamic Balancing Challenges

Core Issue: The presence of the key and keyway disrupts the mass symmetry of the rotating assembly. Especially at high speeds, even after dynamic balancing, unpredictable residual unbalance can occur due to uncertainties in the assembly clearance between the key and keyway, potentially causing vibration.

Design Countermeasures:

Couplings must undergo two-plane dynamic balancing (G6.3 grade or higher, per ISO 1940-1) before leaving the factory.

Using a sled runner key or permanently fixing the key in the keyway (e.g., with adhesive) is recommended to prevent it from loosening or shifting at high speed, which affects balance.

4.3 Alignment Accuracy and Fretting Wear

Core Issue: Necessary fitting clearance exists between the key and keyway (e.g., H9/d10). This clearance can cause microscopic relative motion during start-up, reversal, or torque fluctuations, known as fretting wear. Long-term fretting wear exacerbates the clearance, generates oxidative wear debris, further accelerates wear, and ultimately leads to connection loosening, noise, and failure.

Design Countermeasures:

Strive to improve keyway machining accuracy and fit precision to minimize initial clearance.

A tapered key connection can be considered, but it introduces additional radial forces and must be evaluated carefully.

Regularly inspect for signs of torque transmission slippage.

4.4 Installation Accuracy Requirements

Although diaphragms compensate for misalignment, the keyway connection itself still imposes relatively high requirements on initial alignment accuracy. Significant angular misalignment can subject the key to additional bending moments, accelerating its failure. Laser alignment tools should still be used during installation to keep misalignment within the manufacturer's permissible limits.

4.5 Key Assembly Specifications

The key must fit snugly against the sides of the keyway. A clearance must exist between the top of the key and the bottom of the hub keyway. Forcefully hammering the key during assembly is strictly prohibited to avoid damaging the key and keyway, which alters the fit characteristics.

5. Conclusion and Outlook

The keyway-type diaphragm coupling is an important product resulting from techno-economic trade-offs. It successfully combines the flexible compensation advantages of diaphragm couplings with the economy of keyway connections, possessing strong vitality and market value in specific mid-to-low-end industrial applications.

However, engineers must be acutely aware of its inherent technical limitations, particularly the critical impact of stress concentration on fatigue strength and the challenges of dynamic balancing at high speeds. During the selection decision process, a comprehensive life-cycle cost analysis should be performed: while the keyway type has a lower initial cost, its potential for higher maintenance costs, shorter service life, and higher failure risk may outweigh this advantage in the long term.

For applications demanding high reliability, long life, high speed, and ultimate performance, keyless diaphragm couplings (e.g., shrink disc type) are the superior choice. In the future, with improvements in manufacturing precision and the use of new anti-fatigue materials, the performance boundaries of keyway couplings might be somewhat expanded. However, their fundamental limitations dictate that they will, for the foreseeable future, stably serve their specific market segment. The responsibility of the drive systems engineer is to accurately grasp their characteristics and apply them to the most suitable scenarios.