Flexible shafts transmit rotary motion between two components that are not aligned. They are flexible enough to bend around obstacles, yet stiff enough to transmit motion and light loads. They can even make 180-deg turns, Figure 1.

First used to drive dental drills in the 1870s, flexible shafts later replaced the drive links in automobile speedometers, a task they still perform today.

Commonly used in drive-shaft applications, these shafts eliminate tight installation tolerances and difficult assembly procedures normally required with solid shafts. Typical applications include pump drives, power-seat mechanisms in automobiles, lawn string trimmers, and conveyor drives, Figure 2.

Another common use is for machine control. In a typical application, a highspeed printing press that produces telephone books has 12 large rollers that need on-the-fly adjustment. Originally, the roller adjustment screws were located less than ¼ in. from fast spinning gears, and were only accessible through a maze of wires and pipes. A skilled technician made the adjustment, holding his breath each time he precariously lined up the long-bladed screwdriver with the adjustment screw.

Then the company installed flexible shafts for controlling the adjustments. Now, the process takes much less time and eliminates the risk of mangling a screwdriver in the gears. It also enables more accuracy because the adjustment knob is located where the operator can better see the results as he makes the adjustment.

Reasons to go flexible

A flexible shaft offers several advantages:

• Precludes the need for precision alignment that solid shafts and other drive components require. This saves precision machining of housings and bearings, and reduces installation costs.

• Provides greater design freedom by offering more positioning options for the motor, driving mechanism, and driven components.

• Offers more efficiency, 90 to 95%, than some traditional drive components.

• Accommodates offsets of 180 deg or more, whereas U-joints can handle about 30 deg, and flexible couplings, 5 deg.

• Offers a threeto- one weight advantage over other transmission alternatives.

• Naturally absorbs shock and dampens vibration that could harm connected equipment.

• Enables driving and driven components to move freely relative to each other during operation. An example is a stationary motor attached to a flexible shaft and casing assembly (up to 10 ft long) that has a grinding or cutting tool attached.

Construction

A flexible shaft is built by wrapping several layers of spring-grade wire around a mandrel, Figure 3. Each successive layer is wound onto the shaft at an opposing pitch angle. End fittings are then applied for attachment to the connected machines. Engineers vary the wire diameters and the number of wires per layer to produce different bending flexibilities and torsional stiffness, thus, balancing the tradeoffs between these conflicting requirements.

The result is a series of wire layers or coil springs, Figure 4. When a torsional load is applied to the shaft, half of these layers, or springs, try to expand as they unwind, while the alternate layers, above and below them, try to contract as they are wound tighter. This action, in which layers squeeze against each other under load, gives the shaft torsional stiffness.

When torque is applied in a direction that causes the shaft’s outer layer to expand or loosen, there is no other layer to resist it, and it will expand. For this reason, the loosen-outer-layer (LOL) direction of operation provides the lowest shaft stiffness and poorest performance. Conversely, the tighten-outer-layer (TOL) direction provides the highest stiffness, hence the best performance.

Continue on Page 2