SHAFT SELECTION BASIC INFORMATION AND TUTORIALS


To select the proper shaft, determine the following items:
● Torque requirement
● Shaft radius of curvature
● Length of path between driving and driven systems
● Operating speed
● Acceptable torsional deflection or backlash

Determining the torque requirement on a shaft may not be easy. Remote-control shaft requirements can be determined by attaching a lever of known length to the device to be turned and pulling the end of the lever with a spring scale. Use the highest torque as the required torque.

Measuring torque on a power shaft is more difficult because the requirement is dynamic. The best way is to instrument-load with a torque cell and measure the torque under operating conditions. Component manufacturers usually have these data available. A shaft efficiency of about 90 percent should be attainable for most applications; therefore, increasing the output torque by about 10 percent leads to the input torque.

The shaft path should then be examined.A drawing should be made that shows the path in a true view. The MOR should be determined from this drawing. A full-scale prototype can be used. The length of the shaft can also be determined at this time.

For power shafts, the operating speed is generally given. For remote-control shafts, in which the speed is essentially zero, the amount of torsional deflection acceptable between the turning device and the turned device is the important consideration.

Used intelligently, flexible shafts are the product designer’s allies. They are more flexible than universal joints; they are more versatile than gear systems because they are totally unaffected by the exact angle or offset necessary; finally, flexible shafts offer an inherent shock absorption capability, ease of installation, and maintenance unmatched by other forms of rotary-motion transmission.

However, flexible shafts do have to be treated carefully if they are to provide the service life built into them. They cannot be bent completely out of shape and must be used at radii equal to or greater than the MOR specified by the manufacturer.

They should be secured approximately every 18 in (46 cm) to prevent the possibility of helixing. They must be lightly lubricated as preventive maintenance at regular intervals. Replacement must match the original design because a remote-control shaft is not interchangeable with a power shaft.

The type of service expected must be clearly specified. Two differently built shafts, even of the same diameter and length, are not interchangeable. Above all, flexible shafts must be designed for the task they are to perform. For this reason, early consultation between the user’s engineering department and the shaft manufacturer is highly recommended.

Follow these simple, basic suggestions, and the design flexibility of a rotary-motion flexible shaft will be applied to utmost advantage.

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