TYPES OF CYCLIC LOADING

TYPES OF CYCLIC LOADING

             Machine components are subjected to external force or load.  The external load acting on the component is either static or dynamic.  The dynamic load is further classified into cyclic and impact loads.  Static load is one: as a load which does not vary in magnitude or direction with respect to time, after it has been applied.

             Dynamic load is a load which varies in magnitude and direction w.r.t time, after it has been applied.

             There are two types of dynamic loads.

             Cyclic load

             Impact load

Cyclic Loads:

             This is a load, which when applied, varies in magnitude in a repetitive cyclic manner; either completely reversing itself from tension to compression or oscillating about some mean value.  In this case, the pattern of load variation w.r.to time is repeated again and again.

Examples of cyclic loads are

             -          Force induced in gear teeth

             -          Loads induced in a rotating shaft subjected to B.M.

There are three types of mathematical models are of cyclic loads.

             -          Fluctuating OR alternating load

             -          Repeated loads

             -          Reversed loads

Stress – time relationship corresponding to these three types of loads are given below.

(a) Fluctuating stresses

(b) Repeated stresses

(c) Reversed stresses

             The fluctuating or alternative load varies in a sinusoidal manner with respect to time.  It has some mean value as well as amplitude value.  It fluctuates between two limits – maximum and minimum load.  The load can be tensile compressive or partially tensile.

             The repeated load varies in a sinusoidal manner w.r.to time but varies from zero to some maximum value.  The minimum value (load) is zero in this case and therefore, amplitude load and mean loads are equal.

             The reversed load varies in a sinusoidal manner w.r.to time, but it has zero mean load.  In this case, half portion of the cycle consists of tensile load and remaining half of compressive load.  There is complete reversal from tension to compression between these two halves and therefore, mean load is zero.

             s_max     -           Maximum stress

             s_min     -           Minimum stress

             s_m         -           Mean stress

             s_a        -           Stress Amplitude

             s_m       -           ½ (s_max + s_min)

             s_a        -           ½ (s_max – s_min)

In the analysis of fluctuating stresses, tensile stress is considered as +ve, while compressive stresses are –ve.

References

1. Mechanical Engg. Design                               –          Joseph Shigley
2. Machine Design                                              –          Mubeen
3. Machine Design                                              –          Black
4. Machine Design                                              –          R. K. Jain
5. Machine Design an integral approach             –          Norton, Pearson
6. Machine Design data hand book                                –          Lingayah Vol I.
7. Elements of Machine Design                         –          Pandya & Shah