Page 320 - Intro Predictive Maintenance

Failure-Mode Analysis 311

Figure 14–23 Typical universal-type jackshaft.

Lubrication Problems
Proper lubrication is essential for all jackshafts and spindles. A critical failure point
for spindles (see Figure 14–22) is in the mounting pod that provides the connection
between the driver and driven machine components. Mounting pods generally use
either a spade-and-slipper or a splined mechanical connector. In both cases, regular
application of suitable grease is essential for prolonged operation. Without proper
lubrication, the mating points between the spindle’s mounting pod and the machine-
train components impact each time the torsional power varies between the primary
driver and driven component of the machine-train. The resulting mechanical damage
can cause these critical drive components to fail.

In universal-type jackshafts like the one illustrated in Figure 14–23, improper lubri-
cation results in nonuniform power transmission. The absence of a uniform grease
film causes the pivot points within the universal joints to bind and restrict smooth
power transmission.

The typical result of poor lubrication, which results in an increase in mechanical loose-
ness, is an increase of those vibration frequencies associated with the rotational speed.
In the case of gear-type spindles (Figure 14–22), both the fundamental (1¥) and second
harmonic (2¥) will increase. Because the resulting forces generated by the spindle are
similar to angular misalignment, the axial energy generated by the spindle will also
increase significantly.

The universal-coupling configuration used by jackshafts (Figure 14–23) generates an
elevated vibration frequency at the fourth (4¥) harmonic of its true rotational speed.
The binding that occurs as the double pivot points move through a complete rotation
causes this failure mode.

Fatigue
Spindles and jackshafts are designed to transmit torsional power between a driver and
driven unit that are not in the same plane or that have a radical variation in torsional
power. Typically, both conditions are present when these flexible drives are used.

Both the jackshaft and spindle are designed to absorb transient increases or decreases
in torsional power caused by twisting. In effect, the shaft or tube used in these designs