What Is a Flexible Shaft?

A flexible shaft is a shaft whose operating speed is higher than a first critical speed and lower than a second critical speed. There are also shafts above the second critical speed. From the perspective of reducing vibration, such shafts should be used when higher working speed is required. The design margin needs to consider the safety margin. Generally, the operating speed is controlled to be> 1.3 times the first critical speed, and <70% of the second critical speed. When the flexible shaft rises, it must reach and pass the first critical speed. Due to the existence of various damping such as surrounding media, support bearings, and internal friction of the material, if the speed increases quickly, the shaft deflection is too late to reach the maximum. Beyond the first critical speed, the operation tends to be balanced. ^[1]

A flexible shaft is a rotary motion device used to transfer between two objects in an unfixed manner. It consists of steel wire rope or coil. The flexible shaft is flexible but has a certain torsional stiffness. The flexible shaft can be without any covering and can be bent but not rotated. It may transmit a considerable amount of power, or it can only perform motion with very low power.

Flexible shafts are commonly used in plumbers. Flexible shaft is a common accessory of hand-held rotary tools, and an integral part of rotary tools with remote motors, called "flexible shaft tools". Flexible shaft tools are often used in the dental, jewelry industry and other industries.

Flexible shafts are divided into three types: winding type, universal joint type and spring type.

Wire-wound flexible shaft: It consists of a steel wire (or steel belt) shaft, a hose and a shaft joint. The steel wire shaft is wound from 4 to 10 layers of high carbon steel wire. The outermost layer is generally left-handed. The torque is mainly borne by this layer. The rotation of the shaft is reasonable (forward rotation); It can only bear about 15% of the forward torque. If the outer layer is wound tightly with a thick steel wire, the torque to withstand can be increased by 70 to 80%. The structure of the steel belt shaft is similar to that of the steel wire shaft. The hose is mostly rolled from a formed steel strip, which is mainly used to store lubricant and dust. The steel wire joint is used to connect the two shafts, and the hose joint is used to connect the bearing parts of the two shafts. The joint is integrated with the shaft and pipe by welding or threaded connection, and there are many types of joints.

Gimbal-type flexible shaft: It is composed of many shaft joints, including tube shaft joints and column fork shaft joints, which can adapt to the bending of the shaft. The more the number of joints, the greater the flexibility.

Spring-type flexible shaft: It is a long cylindrical coil spring, which is equipped with various tools at one end for different operations. This kind of shaft can be used without a hose ^[2]

The advantages of the flexible shaft are convenient installation, compact structure, movement and power transmission over a long distance, and easy movement. It is mostly used for high speed and low torque transmission, such as driving portable sand blasters and grinders, automobile tachometers, valves and switches that activate certain devices. However, the torsional stiffness of the flexible shaft is small. For example, when the direction changes and the torque fluctuation is large, the unevenness of the rotation speed of the driven end is also large. Under normal circumstances, the power transmitted by the flexible shaft does not exceed 5.5 kilowatts; when there is no bending, the torque does not exceed 50 N · m, and the speed can reach 20,000 rpm. When used, the speed depends on the shaft diameter and the bending radius during operation. The joints must be fastened before use, and regularly cleaned and lubricated.