SHAFT- Power transmission

A shaft is a rotating machine element, usually circular in cross section, which is used to transmit power from one part to another, or from a machine which produces power to a machine which absorbs power. The various members such as pulleys and gears are mounted on it.

A shaft is a rotating machine element, usually circular in cross section, which is used to transmit power from one part to another, or from a machine which produces power to a machine which absorbs power. The various members such as pulleys and gears are mounted on it.

As torque carriers, drive shafts are subject to torsion and shear stress, equivalent to the difference between the input torque and the load. They must therefore be strong enough to bear the stress, whilst avoiding too much additional weight that would in turn increase their inertia.

Historical Background

The term drive shaft first appeared during the mid19th century. In 1861 Storer's patent reissue for a planning and matching machine, the term is used to refer to the belt-driven shaft by which the machine is driven, the term is not used in original patent.
Another early use of the term occurs in the 1861 patent reissue for the Watkins and Bryson horse-drawn mowing machine. Here, the term refers to the shaft transmitting power from the machine's wheels to the gear train that works the cutting mechanism.

Terminology

·        In machinery, the general term “shaft” refers to a member, usually of circular cross-section, which supports gears, sprockets, wheels, rotors, etc., and which is subjected to torsion and to transverse or axial loads acting singly or in combination.

·        An “axle” is a non-rotating member that supports wheels, pulleys, and carries no torque.

·        A “spindle” is a short shaft. Terms such as line-shaft, head-shaft, stub shaft, transmission shaft, countershaft, and flexible shaft are names associated with special usage

 Steps of Shaft Design

·        Material selection

·        Geometric layout

·        Stress and strength

·        Static strength

·        Fatigue strength

·        Deflection and rigidity

·        Bending deflection

·        Torsional deflection

·        Vibration due to natural frequency

Possible Shaft Materials 

1.     Shafts can be made from low carbon, cold-drawn or hot-rolled steel,  

such as ANSI 1020-1050 steels.

2.     A good practice is to start with an inexpensive, low or medium carbon steel for the first time through the design calculations.

3.     Typical alloy steels for heat treatment include ANSI 1340-50, 3140-

50, 4140, 4340, 5140, and 8650. 

     4. Typical material choices for surface hardening include carburizing

          grades of ANSI 1020, 4320, 4820, and 8620.

5.     Cast iron may be specified if the production quantity is high, and the gears are to be integrally cast with the shaft.

          DESIGN OF SHAFTS SUBJECTED TO TWISTING MOMENT / TORQUE ONLY:

We have the general Torsion equation as   T / J = Ƭ / r - 

Where, T = Torsional moment / Twisting Moment / Torque - N-mm

            J = Polar Moment Inertia of cross sectional area about the axis of rotation

                  - mm⁴

            Ƭ = Torsional Shear stress of the shaft – MN / mm²

             r = Radius of the outer most fabric from the axis of the rotation

               = d/2, where d = dia. of the shaft.

               Also J = πd⁴ / 32

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