Introduction to Universal Couplings: Constant Speed vs. Non-Constant Speed

A universal coupling is a mechanical device designed to connect two shafts that intersect in the same plane, allowing for angular misalignment and enabling the angle to change as needed during operation. Universal couplings come in various types, with the main categories being constant speed (CV), non-constant speed, and standard constant-speed types. In some classifications, the “near-constant speed” coupling is considered equivalent to a constant-speed type, so the categorization only includes constant-speed and non-constant-speed types.

1. Constant Speed Universal Coupling (CV Joint)

A constant-speed universal coupling, often referred to as a CV joint, maintains a constant angular velocity ratio of 1 between the driving shaft and the driven shaft, regardless of the angle between them. This means that the rotational speed is steady, and the torque and motion are transmitted smoothly. The advantages of a constant-speed coupling include:

• Smooth Power Transmission: The driving and driven shafts rotate at the same speed, ensuring smooth and steady motion.
• High-Speed Capability: It can operate at higher rotational speeds compared to non-constant-speed couplings.

However, there are some drawbacks to constant-speed couplings:

• Complex Design: The design is more intricate, making it harder to manufacture.
• Higher Cost: The manufacturing complexity leads to higher costs.
• Limited Load Capacity: Despite the smooth operation, constant-speed couplings generally have lower load-bearing capacity.
• Design Restrictions: Due to the complex nature of the design, there are certain limitations regarding the installation and configuration of the coupling.

2. Non-Constant Speed Universal Coupling

Non-constant-speed universal couplings do not maintain a constant angular velocity ratio between the driving and driven shafts. As the angle between the shafts changes, the instantaneous angular velocity of the driven shaft fluctuates periodically, which can result in varying rotational speeds. The key features of non-constant-speed couplings are:

• Speed Variation: As the angle between the shafts increases, the variation in the driven shaft’s speed also increases. This causes periodic changes in the angular velocity.
• Dynamic Loads: The changing rotational speed of the driven shaft leads to additional dynamic loads, which can result in higher stress on the system.

Common types of non-constant-speed universal couplings include:

• Single Cross Universal Coupling: Often used in standard transmission systems, it can tolerate moderate angle changes but is not suitable for high-speed or precision applications.
• Single Ball-type Universal Coupling: Features a simple structure and is suitable for applications where small angle changes are required.
• Rubber Ring Universal Coupling: Typically used for lower power transmissions and lower rotational speeds.

Because of the periodic variations in the speed of the driven shaft, non-constant-speed couplings are generally not recommended for high-speed or high-precision applications. They are more appropriate for scenarios where the angular misalignment is moderate and the speed fluctuations can be tolerated.

3. Near-Constant Speed Universal Coupling

The near-constant-speed universal coupling is designed to operate with an angular velocity ratio that is very close to 1 under typical working conditions. While the instantaneous angular velocity may fluctuate slightly, the coupling behaves similarly to a constant-speed coupling, especially when the angle between the shafts remains relatively small. These couplings are often constructed using a double cross design to achieve near-constant speed characteristics.

Summary

• Constant Speed Universal Coupling: Provides smooth and stable speed transmission with a constant angular velocity ratio of 1. Suitable for high-speed applications but with complex design, high cost, and limited load capacity.
• Non-Constant Speed Universal Coupling: Allows for more significant angular misalignment but causes periodic fluctuations in the driven shaft’s speed. Not suitable for high-speed or high-precision applications due to dynamic load variations.
• Near-Constant Speed Universal Coupling: In certain conditions, these couplings approximate the performance of constant-speed types, often using a double cross design.

Each type of universal coupling has its strengths and limitations. The appropriate selection depends on the operational conditions, such as speed, precision requirements, and angular misalignment, of the system.