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.
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Direction of rotation
Flexible shafts are classified in two functional groups: bidirectional and unidirectional. Bidirectional shafts transmit motion in both directions of rotation in applications such as remote valve controls, robotics, and aircraft actuators.
These shafts typically contain a large number of small-diameter wires wound over many layers. This configuration gives them nearly comparable torsional stiffness and torsional strength in both clockwise and counterclockwise directions.
A bidirectional shaft should transmit the same amount of power, as nearly as possible, in both directions without large variations in shaft twist or ultimate strength. The torsional stiffness ratio (TSR) gives a measure of this capability by comparing the torsional stiffness in the LOL direction with that in the TOL direction. A shaft must have a TSR value of at least 40% to be considered bidirectional. And, the best bidirectional shafts have values around 60 to 65%.
Unidirectional shafts are designed to operate in one direction of rotation only. They have fewer wires and layers, producing bending flexibility, torsional stiffness, and ultimate torsional strength values that are generally higher than those for bidirectional shafts, but only for the direction of operation they are designed for. If used in the other direction, performance drops significantly. Common applications include motor couplings, speedometer cables, and power tools.
Making a choice
Selecting a flexible shaft is fairly easy once you understand its basic operation and performance limitations. These shafts are generally available in diameters ranging from 0.030 to 1.625 in. (excluding any outer casing). And, they sometimes offer a choice of bending flexibility and torsional stiffness for each diameter. Bending flexibility, often the key parameter in choosing a shaft, is indicated by the “flexural rigidity ratio,” which compares flexibility to that of a solid shaft of equal diameter and material. Values are typically in the 400 to 700 range.
Before selecting a flexible shaft, answer the following questions:
• Does the application require bidirectional or unidirectional operation?
• In what direction will the shaft rotate, clockwise or counterclockwise, when viewed from the driving end? This determines the shaft construction required to ensure operation in the TOL direction (tightens the outer wires).
• What torque must the shaft carry? Torque-carrying capacities of flexible shafts range up to 5,000 lb-in. As an example, a 0.25-in. diameter shaft can carry a 60 lb-in. continuous torque. Shafts must be chosen for the maximum torque they will experience, which is usually the startup or stall torque rather than the running torque.
• What is the shaft speed? Those speeds under 100 rpm are considered to be in the category of low-speed manual operation, which permits higher operating torque, usually twice the rated dynamic operating load.
Shafts up to 0.188-in. diameter operate at speeds up to 20,000 rpm, depending on the application. As a rule-of-thumb for large shafts (0.25-in. diameter and over), the surface speed should not exceed 500 fpm. Thus, maximum rpm = 500 × 12/(π × diameter).
• How will the shaft ends be connected? There are many standard machined end fittings, including the oftenused “integral-form-squared” end, where the shaft ends are formed into square fittings that mate with square-shaped receptacles.
• What is the minimum bend radius the shaft must meet? Torque-carrying ability is reduced as the bend radius gets smaller.
• If torsional deflection is a concern, how much angular deflection can be tolerated? This deflection is expressed as the amount of twist per unit shaft length divided by the torque (deg/ft/lb-in.). Torsional stiffness and bending flexibility are inversely related. Therefore, achieving more torsional stiffness usually requires giving up some bending flexibility. Computer modeling techniques (see box), help shaft design engineers optimize the tradeoffs between these characteristics. This enables users to specify shafts that exactly meet their requirements while minimizing design time and reducing weight and cost.
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• How long is the shaft? An overly long shaft can cause excessive torsional deflection and decreased torque capacity. The required length can be closely approximated for many applications by using a formula supplied by the manufacturer.
• What are the environmental conditions? A hostile environment may require special shaft materials. For general use flexible shafts are manufactured from high-tensile carbon steel, generally the strongest material available. Other options include two grades of stainless steel for corrosion resistance and temperatures to 600 F, phosphor bronze for lowest magnetic properties, Monel for low magnetic properties and high corrosion resistance, and Inconel for temperatures up to 1,100 F.
• Is a casing required? There are three reasons for placing a shaft inside a casing. First, a casing limits the tendency of the shaft to helix or corkscrew, particularly when length exceeds 12 to 14 in. It also increases the torque-carrying ability of the shaft.
Second, the casing retains the lubricant that coats the shaft while protecting the shaft from dirt, corrosive chemicals, or impacts.
Finally, it shields the user from contact with a potentially dangerous highspeed shaft. In a power tool with a long shaft (up to 10 ft), for example, the casing acts as a shield as well as a handle with which to manipulate the cutting or abrading tool.
High-tech shaft designA new computer program enables engineers to optimize the physical properties of a flexible shaft to meet any design criteria. This program performs hundreds of thousands of simultaneous calculations in the optimization process. Previously, shafts were designed through an imprecise, expensive, and time-consuming trial-and-error method. This process left gaps in the range of possible designs and was not effective in improving the performance of existing shafts. The new program enabled the development of two optimized flexible shafts for bidirectional and unidirectional applications. These shafts perform, on average, 30 to 40% better in torsional stiffness, torsional strength, and bending flexibility than conventional standards. A selection of three grades of bending flexibility and torsional stiffness gives engineers more design flexibility, and, in some cases, enables them to specify smaller diameter shafts that weigh less and cost less. In creating the new program, researchers at S.S. White discovered previously unidentified parameters that play a key role in maximizing shaft performance. One of these is “axial stretch,” the tendency of a shaft to stretch or shrink when torque is applied. The program controls this parameter because: • Minimizing axial stretch improves torsional stiffness and torsional strength. • Axial stretch contributes to shaft helixing, or corkscrewing, at lower-than-expected torque loads. Helixing reduces the torsional stiffness of the shaft. • Shrinkage pulls the shaft from its fittings or causes axial loads, which, in turn, reduce operating life. |
Brian Parlato is the manager of new applications for S.S. White Technologies Inc., Piscataway, N.J.