190 Chapter 7
Pitch Circle.
The pitch circle is the circle with the pitch diameter, the line (circle) of contact between two meshing gears.
Working Depth.
The working depth is the greatest depth to which a tooth of one gear extends into the tooth space of an-
other gear.
Rack.
A rack may be compared to a spur gear that has been straightened out. The linear pitch of the rack teeth
must equal the circular pitch of the mating gear.
Helical Springs.
The helical spring, in which wire is wrapped in a coil that resembles a screw thread, is probably the most
commonly used mechanical spring. There are three classifications of helical springs: compression, ex-
tension, and torsion. Drawings seldom show a true presentation of the helical shape; instead, they usu-
ally show springs with straight lines. Figure 7.22A shows several methods of spring representation in-
cluding both helical and straight-line drawings. Figure 7.22B shows the use of typical helical spring
nomenclature.
A spring can be designed to carry, pull, or push loads. Twisted helical (torsion) springs are used in
engine starters and hinges. Helical tension and compression springs have numerous uses, notably au-
tomobile suspension systems, gun-recoil mechanisms, and closing.
In manufacturing coiled helical springs, the spring wire is wrapped around a mandrel that serves as
a support; when seamless tubing is being extruded, a long mandrel forms the internal diameter.
Helical springs, often called spiral springs, are probably the most common type of spring. They may be
used in compression, extension, tension, or torsion. A spring used in compression tends to shorten in ac-
tion, while a tension spring lengthens in action. Torsion springs, which transmit a twist instead of a direct
pull, operate by a coiling or an uncoiling action. In addition to straight helical springs, cone, double-cone,
keg, and volute springs are classified as helical. These types of springs are usually used in compression.
7.5 RECOMMENDED PRACTICE.
With industrial drawings and blueprint reading, as with other types of graphical representation, it is al-
ways advisable to follow the best industry practices current at the time:
- Draw one part to one sheet of paper. If this is not practical, apply enough spacing between parts
and draw all parts using the same scale. Otherwise, the scale should be clearly noted under
each part’s drawing. Standard parts such as bolts, nuts, pins, and bearings do not require de-
tail drawings. - Always use decimals.
- Lettering should be all in capitals.
- Select a front view that best describes the part.