Page 244 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

226 Major Process Equipment Maintenance and Repair

the smaller number of teeth. A rack is a gear with teeth spaced along a
straight line and is suitable for straight-line motion. Many kinds of gears
are in general use. For each application, the selection will vary depend-
ing on the factors involved. One basic rule of gearing is that to transmit
the same power, more torque is required as speed is reduced. The torque
is directly proportional to speed, and therefore, the input and output
torques for power transmission are directly proportional to the ratio if
efficiency is neglected.
Gears are usually used to change the speed of the driven equipment
from that of the driver, to alter the direction of power flow, or to change
rotation direction. In very few cases are the most efficient design speeds
of a driver and driven machine identical. Probably the most common ex-
ample we see of this is the modern automobile where we use gearing to
change both the speed and the direction of power flow from the engine to
the wheels. In this case, the reciprocating engine would be extremely
large should gearing not be available to change the speed.
Today’s best reason for using gearing is to conserve energy due to its
scarcity and high cost. In most cases, more efficient drivers and driven
machines can be used when a gear is available for a better speed match.
For instance, steam turbines operating at speeds available for reciprocat-
ing compressors would be very inefficient. In addition, the use of gears
enables a reduction in the size of driving and/or driven machines and
comparable conservation of materials since higher-speed machines tend
to be smaller than lower-speed ones for the same amount of work pro-
duced.
Other reasons for using a gear unit are to change the direction of power
flow and to change the direction of rotation between the driving and
driven machines. Were gears not available to perform all of these impor-
tant functions, designing and constructing compact, efficient machinery
systems would be virtually impossible. Gearing gives the engineer the
flexibility to make the machine system fit the job, not the other way
around. The power loss of 1.25 to 4.0 percent is a small price to pay for
the advantages obtained.

Gear Types

Some of the common gear types are listed below:
1. Spur-Cylindrical in form and operate on parallel axes (See Figure 6-
1). The teeth are straight and parallel to the axis.
2. HeZicd-Cylindrical in form and have helical teeth-teeth set at an
angle to the axis (See Figure 6-2).