Page 179 - Analysis and Design of Machine Elements

K – multiple-strand correction factor. Chain Drives 157
p
The ratings in design standards and handbooks are for single-strand chains. If
multiple-strand chains are used, the single-strand rating is multiplied by 1.7, 2.5 and
3.3 for double-, triple- and quadruple-strand chains, respectively [9, 12], to account for
the uneven load sharing among parallel strands.

7.4 Design of Chain Drives

7.4.1 Introduction
The design of a chain drive requires the selection of an appropriate roller chain and
sprockets to transmit a given power to meet specified design requirements. The
information provided includes the transmitted power P, the rotational speed of driving
sprocket n and driven sprocket n , or speed ratio i, the allowable speed fluctuation,
2
1
layout requirements, dimensional limitations or space constraints and operating
conditions.
After analysis and design, decisions must be made about primary design variables for
a chain drive, including the designation of chain, the number of links L and strands; the
p
number of teeth of driving and driven sprockets, their materials, structure and dimen-
sions; the centre distance a between sprockets; drive arrangements, forces acting on the
shafts; lubrication, mounting and tensioning methods and other details. Finally, draw-
ings of sprockets are provided.

7.4.2 Materials
The life of a roller chain depends to a great extent on materials, heat treatments and
manufacturing precision. Based on the failure modes discussed in Section 7.2.4, candi-
date materials for roller chains should have high strength and good resistance to fatigue,
wear and corrosion.
Low capacity chains are usually made of cast iron or carbon steel, while high capacity
chains require high-strength alloy steels. All parts, including bushings, pins, rollers and
link plates are heat treated by either carburizing, case hardening or through harden-
ing, as application requires, to improve strength, wear resistance and impact resistance
[5, 10].
The materials for sprockets should also have sufficient strength and wear resistance.
Low or medium carbon steels are selected with the surface hardened to 59–63 HRC.
Materials and surface treatments for small sprockets are usually better than large sprock-
ets, as small sprockets experience more cycles during operation.

7.4.3 Design Criteria
−1
For a medium- or high-speed chain drive, that is, v ≥ 0.6 m s , fatigue is a primary failure
mode candidate. According to power criteria, the selected chain should have sufficient
capacity to transmit the required power, that is
K P
p 0
≥ K P (7.23)
A
K z