The Screw Drives in continuously variable transmissions (CVTs) can be divided into three main types based on their function and application:
- Power Transmission Screws
These screws are primarily working in transmitting power and typically require a large axial force to ensure self-locking capability during the transmission process. Common applications include screw jacks and screw presses. In these applications, the screw must withstand large forces. And ensure no slippage or reverse rotation, so the design of the screw usually incorporates high self-locking ability. - Guiding Screws
Guiding screws are working in transmit motion and require high precision, typically applied in systems requiring fine motion control, such as the feed systems of metal-cutting machine tools. The design of guiding screws emphasizes smooth, precise motion transmission while minimizing friction to maintain motion accuracy. - Adjusting Screws
Adjusting screws are used to adjust the relative position of the components within a CVT. A typical application is in the rolling mills where screw drives adjust the positions of the rollers. In these cases, the screw is responsible for adjusting and securing the relative positioning of parts to achieve the desired operational state.
Common Types and Features of Screw Drives:
- Trapezoidal Threads: The most commonly used type in screw drives due to their high load-bearing capacity and self-locking ability. They are prevalent in applications that require significant axial forces and self-locking, such as screw jacks and presses.
- Sawtooth Threads: Used for unidirectional power transmission applications, such as screw jacks, where the design ensures that power is transmitted only in one direction, preventing reverse motion.
- Square Threads: Used in applications where high precision is not crucial. They are easy to manufacture but have lower transmission efficiency.
Advantages and Disadvantages:
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Advantages: The screw drive in CVTs is simple in structure, easy to manufacture, low in cost, smooth in transmission, and easy to achieve self-locking. These features make it suitable for many industrial applications.
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Disadvantages: The system experiences high frictional resistance, leading to wear and lower transmission efficiency, typically in the range of 0.3-0.7, and sometimes below 0.5 when self-locking may occur.
This type of transmission system is suitable for situations with light loads, where high precision is not required, or in devices that need self-locking capabilities.