Page 177 - Analysis and Design of Machine Elements

where the acceleration is Chain Drives 155
dv d 2
a = = (R cos )=−R sin (7.19)
c 1 1 1 1
dt dt
∘
180
when =± , incorporating Eq. (7.2), the maximum acceleration is
z 1
∘ 2
180 p
1
2
a =∓R sin =∓ (7.20)
cmax 1 1
z 2
1
Therefore, low angular velocity, small pitch or large number of teeth, as well as light
mass and small angular acceleration favour the reduction of dynamic forces.
7.2.4 Potential Failure Modes
As a chain drive in operation, the tensile force in the chain fluctuates from the tight side
tension to the slack side tension, and back, for each cycle. Fatigue, therefore, becomes a
primary potential failure mode for roller chains. Fatigue may occur on link plates, rollers
and bushings or sprocket tooth surfaces. Fatigue fracture may disrupt power transmis-
sion totally, often with catastrophic consequences. Fatigue strength is thus the basis for
chain ratings and essential for chain design.
Since rollers and bushings, bushings and pins are slip fitted, respectively, each time a
link articulates over a sprocket tooth, wear may occur in the chain and on the sprockets.
Abrasive wear, adhesive wear and fretting wear may be potential failure modes. As wear
progresses slowly, the pitch of each pin link increases gradually. When the wear on pins,
bushings, rollers and teeth produces sufficient dimensional change, rollers may climb
high on the sprocket teeth or even skip off from them.
When a roller chain comes into meshing with a sprocket, impact occurs between the
roller and sprocket teeth. Hertz contact stress also generates at the interface between
the chain roller and sprocket tooth. At heavy loads and high speeds, the intensity of
impact and contact stress increases dramatically, resulting in local welding at contact
surfaces. As the chain drive rotates continuously, the welded surfaces are forced to sep-
arate, resulting in excessive wear or galling.
−1
When a low speed chain drive (v < 0.6 m s ) carries heavy static loads, links are likely
deform plastically or even break due to insufficient tensile strength.

7.3 Power Transmission Capacities

7.3.1 Limiting Power Curves
The power performance of a chain drive is limited by potential failure modes. Normally,
three modes of failure are considered in determining the power rating of a chain drive
for its power transmission capacity, as shown in Figure 7.7. At low to moderate speeds,
the transmitted power is limited by fatigue failure of link plates. At higher speeds, roller
and bushing fatigue governs power transmission capacity of a chain drive. At even higher
speeds and heavier loads, lubrication breakdown may cause instantaneous local welding
and galling between pins and bushings, resulting in excessive wear and thus limiting the
transmitted power and the maximum speed of a chain drive [10].