In the field of mechanical transmission, key connections, as a common circumferential fixing method between shafts and hubs, play a crucial role in transmitting torque and preventing relative rotation between the shaft and the hub. Flat keys and semicircular keys, as important members of the key connection family, leverage their unique structural features and performance advantages to fulfill key functions in various mechanical application scenarios. A thorough understanding of their usage scenarios and selection methods is of great significance for ensuring the stable operation and efficient performance of mechanical systems.
I. Structural Features of Flat Keys and Semicircular Keys
(A) Flat Keys
The cross-section of a flat key is rectangular, with both side surfaces serving as working surfaces. There is a certain gap between the upper surface of the flat key and the bottom surface of the hub keyway. This structure enables the flat key to transmit torque through the extrusion between the key and the side surfaces of the keyway during operation. Flat keys can be classified into ordinary flat keys, guide flat keys, and sliding keys according to their purposes. Ordinary flat keys are used for static connections where there is no relative axial movement between the shaft and the hub. Guide flat keys and sliding keys are applied to dynamic connections with relative axial movement between the shaft and the hub. Specifically, guide flat keys are fixed to the shaft with screws, allowing the hub to slide axially along the key, while sliding keys are fixed to the hub and move axially along the keyway on the shaft together with the hub.
(B) Semicircular Keys
The upper surface of a semicircular key is flat, the lower surface is semi-circular, and both side surfaces are parallel and serve as working surfaces. It can swing around the center of curvature of the arc at the bottom of the keyway on the shaft, adapting to the inclination of the bottom surface of the hub keyway. Thanks to this structural feature, semicircular keys are easy to install, especially suitable for connecting tapered shaft ends and hubs. However, due to the relatively deep keyway, they significantly weaken the strength of the shaft.
II. Usage Scenarios of Flat Keys and Semicircular Keys
(A) Usage Scenarios of Flat Keys
- Machine Tool Transmission Systems: Flat keys are widely used in the connection between the main shafts and transmission components such as gears and pulleys in various machine tools. For example, the main shaft of a lathe is connected to the chuck via an ordinary flat key to transmit torque, ensuring that the chuck can stably hold the workpiece and rotate at high speed with the main shaft. Similarly, flat keys are often used to connect the main shaft of a milling machine and the milling cutter disc, guaranteeing that the milling cutter disc can accurately transmit torque during the cutting process and achieve efficient machining.
- Speed Reducers: As essential components in mechanical transmission for reducing speed and increasing torque, speed reducers mostly adopt flat keys for the connection between internal shafts and gears, as well as between shafts and couplings. Ordinary flat keys are used to fix the relative positions of gears and shafts, enabling gears to accurately transmit torque. Guide flat keys are commonly applied to the connection between sliding gears and shafts to realize the axial movement of gears on the shaft, thereby changing the transmission ratio.
- Construction Machinery: In construction machinery such as excavators and loaders, flat keys play a vital role in connecting components like the engine crankshaft and the flywheel, as well as the transmission shaft and gears. These devices need to withstand large torques and impact loads during operation, and flat keys can reliably transmit torque to ensure the normal operation of the equipment.
(B) Usage Scenarios of Semicircular Keys
- Light Industry Machinery: In equipment of the light industry, such as textile machinery and food machinery, due to the relatively small working loads and less stringent requirements for shaft strength, semicircular keys are widely used because of their ease of installation. For instance, semicircular keys are used to connect the spindles and pulleys of textile machines, facilitating installation and disassembly. In food mixers, semicircular keys are often employed to connect the mixing shafts and mixing blades, ensuring that the mixing blades can rotate stably with the mixing shafts.
- Agricultural Machinery: In some small agricultural machinery, such as small tractors and harvesters, semicircular keys are commonly used for the connection between shafts and hubs. When these machines operate in the fields, the working environment is harsh. The swingable characteristic of semicircular keys can, to a certain extent, accommodate installation errors between the shaft and the hub, ensuring the reliability of the connection.
- Connection of Tapered Shaft Ends: When the shaft end is tapered, the semicircular key is an ideal connection method. For example, in the connection between the tapered holes of the main shafts of some machine tools and the centers, as well as between the tool shanks of milling cutters and the tapered holes of the main shafts, semicircular keys can closely fit the tapered shaft ends to achieve reliable torque transmission and centering.
III. Selection Key Points of Flat Keys and Semicircular Keys
(A) Selection Based on Load Size and Characteristics
- Static Load: For situations with static loads or small impact loads, both flat keys and semicircular keys can be selected. If less weakening of the shaft strength is required and the installation space permits, semicircular keys can be given priority due to their ease of installation. However, if higher requirements for shaft strength exist, flat keys should be chosen, and the key dimensions should be determined reasonably according to the load size.
- Dynamic Loads and Impact Loads: In the face of large dynamic loads and impact loads, flat keys have more advantages. Ordinary flat keys are suitable for situations where large torque needs to be transmitted and there is no relative axial movement between the shaft and the hub. Guide flat keys and sliding keys are applicable when there is relative axial movement and large torque needs to be transmitted. In such cases, the appropriate type of flat key should be selected according to the actual working conditions, and the key dimensions should be increased to enhance the strength and reliability of the connection.
(B) Selection Based on Shaft Diameter
The dimensions of keys are usually related to the shaft diameter, and national standards clearly specify the key dimensions corresponding to different shaft diameters. When selecting keys, relevant standards should be consulted according to the shaft diameter to choose appropriate key widths, key heights, and key lengths. Generally, as the shaft diameter increases, the key dimensions also increase accordingly to meet the requirements of torque transmission. For example, when the shaft diameter is small, flat keys or semicircular keys with smaller dimensions can be selected; when the shaft diameter is large, larger – sized flat keys are required, and it may be necessary to use multiple keys side by side to improve the bearing capacity.
(C) Selection Based on Installation and Dismantling Requirements
- Easy Installation and Dismantling: If high requirements are placed on the convenience of key installation and disassembly, semicircular keys are a better choice. Their swingable structure allows them to be easily installed in the keyway on the shaft, and they are also relatively easy to disassemble. In contrast, for flat keys, especially guide flat keys and sliding keys, tools may be required during the installation and disassembly process, which is relatively more complex.
- Fixing Requirements: For situations where a firm fixation between the shaft and the hub is required without any relative axial movement, ordinary flat keys can provide a reliable connection. When relative axial movement between the shaft and the hub is needed, guide flat keys or sliding keys should be selected.
(D) Considering the Impact on Shaft Strength
Since the keyway of the semicircular key is relatively deep, it significantly weakens the strength of the shaft. Therefore, it should be used with caution in scenarios where high requirements for shaft strength exist. The keyway of the flat key is relatively shallow, causing less weakening of the shaft strength, making it more suitable for situations with large loads and strict requirements for shaft strength. When selecting keys, factors such as the shaft material, diameter, and working load should be comprehensively considered to evaluate the impact of key connections on shaft strength and ensure the safety of the shaft during operation.
