Page 48 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
26
To ensure machine elements work safely throughout design life, it is imperative to
analyse the strength under service loads, and predict potential failures at early design
stage well before the machine is built. Strength analysis requires evaluating each
possible failure mode by analysing loads and stresses so as to design an element strong
enough against failure. Since loads relate to strength via stress analysis, in this chapter,
we start with a brief introduction to loads and stresses, followed by a concise review of
static strength due to static loads and a detailed discussion of fatigue strength due to
cyclic loads. Contact strength is also covered in this chapter.

2.1 Fluctuating Loads and Stresses

2.1.1 Service Factors and Design Loads
In machines, power is transmitted from power sources, like motors, engines and so on,
to executive components. Under stable working condition, nominal loads are applied
on the elements. However, the inherent characteristics of primary movers, driven
machines and their interactions may cause the actual loads on the machine elements
greater than the nominal loads. A service or application factor is introduced to account
for this effect. The actual loads, or design loads, are the product of a service factor
and nominal loads.
Loads on a driven machine are generally defined in terms of torque or power. The
design power is obtained by multiplying a service factor with the nominal power to be

transmitted. Service factors are usually selected within the range of 1–5 [1], depending
on the power sources, operating conditions and duration of service. Large service factors
allow for a high degree of safety to account for uncertainties in design analysis, material
properties, manufacturing tolerances, operating environments and so on.
Table 2.1 lists the range of service factors for power transmission elements under var-
ious operating conditions. Uniform, light impact, moderate impact, and heavy impact
power sources are represented by electric or hydraulic motors, steam engines, multi-
cylinder engines and four or less cylinder engines, respectively. Agitator and light-duty
conveyors; machine tools and uniformly loaded conveyors; reciprocating compressors
and heavy-duty conveyors and rock crushers and heavy machines are examples rep-
resenting uniform, light impact, moderate impact and heavy impact driven machines.
Small service factors are usually for relative smooth operation, while large service factors
for vibratory, shock and pulsation operation. Therefore, the design of gears and cou-
plings selects greater values, while the design of belt drives or worm gearings can select
smaller values. Besides, long duration of service requires greater values of service factor.
The service factors listed in Table 2.1 are established after considerable field experi-
ence in a particular application and can be used with reasonable accuracy. For specific
elements, the precise value of service factor can be found from design handbooks or
manufacturer’s catalogues [4, 5].

2.1.2 Types of Loads
A load is any external force, torque or moment applied to a machine element. The basic
forms of load include tension, compression, shear, torsional shear and friction, as well as