Page 49 - Analysis and Design of Machine Elements

Table 2.1 Service factors for power transmission elements, K [2, 3]. Strength of Machine Elements 27
A
Power sources Driven machines
Uniform Light impact Moderate impact Heavy impact
Agitators, Machine Reciprocating Mining
light conveyors, tools, compressors machines,
centrifugal gear pump, heavy-duty heavy
compressors mixers conveyors machines
Uniform Electric motor, 1.00∼1.10 1.25∼1.35 1.50∼1.60 ≥1.75
steam or gas
turbines of uniform
speed and low
torques
Light Electric motor, 1.10∼1.25 1.35∼1.50 1.60∼1.75 ≥1.85
impact steam or gas
turbines, hydro
motor of high
and variable torque
Moderate Multi-cylinder 1.25∼1.50 1.50∼1.75 1.75∼2.00 ≥2.00
impact internal combustion
engines
Heavy Single-cylinder ≥1.50 ≥1.75 ≥2.00 ≥2.25
impact internal combustion
engines

torques and moments. To describe a load, information about its magnitude, direction,
point of action, duration, frequency and the number of cycles is required. The analysis
and design of machine elements involve extensive stress or deflection analysis on the
premise of correct load determination.
The accurate determination of realistic operating loads is often a difficult and chal-
lenging task in machine element design. When power is transmitted from a mover to
the executive components in a machine, nearly all machine elements participate in the
activity, either by transmitting power, like gears, or by supporting power transmitting
elements, like shafts and bearings. The dynamic effects due to impact or vibration during
operation, together with multiple force sources on an element, such as contact, fric-
tion, gravity or thermal expansion, make the precise determination of loads even more
complicated [1]. Once the loads are acquired, equilibrium analyses combined with vec-
torial approaches are used to reduce the complex loads into basic loads of tension, shear,
moment and so on.
According to the loading history, the load applied to an element could be static, fluc-
tuating, shock or impact and random loads. An ideal static load is applied slowly and
is never removed or is removed infrequently. Gravity load is a typical example of static
load. The load magnitude, direction and point of action of a static load do not change
with time, or change very slowly. A static load can be an axial tension or compression,
a shear load, a bending load, a torsional load or any combination of these. Although
in engineering practice relatively few elements are subjected to pure static loads, static
loads are still the fundamentals to the sizing of machine elements.