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foreign gear designers. This multiplicity of equations underlines that gear strength and durability is not
an exact engineering  science, but rather is empirical and experience dependent. Also, the user should
be aware that most gear equations and empirical results pertain to coarse pitch gears. The literature
offers much less about tine pitch instrument gearing.

Computer programs — The AGMA design equations involving various parameters are defined with
specific detail in the standard. Several of these equation terms are subject to design modification,
but are complexly derived. Examples are geometry factors (I & J) which are alterable by profile 
modifications. Many computer programs have been generated which efficiently handle these complex 
calculations.

In addition  to strength and durabtity design, software exists for the entire gear and gear train 
design including the selection of gear type, pitch, geometry and materials. Programs are purchasable 
from a  number of universities and software houses.

14.0 GEAR MATERIALS

In order for gears to achieve their intended performance, life and reliability, the selection of a suitable
gear material is very important. Often not all design requirements are compatible. High load capacity
requires a tough, hard material which is difficult to machine; whereas high precision favors materials
that are easy to machine and, therefore, have lower strength and hardness ratings. Light weight and
small size favors light non-ferrous materials, while high capacity requires the opposite. Thus, tradeoffs and compromise  are required to achieve an optimum design.
Gear materials vary widely, ranging from ferrous metals, through the many non-ferrous and light-weight metals, to the various plastics. The gear designer and user faces a myriad of choices. The
final slection should be based upon an understanding of material properties and application requirements.

14.1 Ferrous Metals

Despite the introduction of many new exotic metals and plastics with impressive characteristics, ferrous metals are still the most widely used far gears, because they offer high strength, response
to heat treatment and low cost. Cast iron and steel, carbon steels and alloy steels are in common use.

14.1.1 Cast Iron is widely used for large gears where it is advantageous to save machining costs
by molding the gear blank. Cast steels also offer this advantage together with higher tensile and yield
strengths, but cast iron is superior under dynamic conditions, providing excellent internal damping
properties. 

14.1.2 Steels are divided into two main divisions: plain carbon and alloy. The carbon steels offer
low cost reasonably easy machining and ability to be hardened. A major disadvantage is the lack of
resistance to corrosion.
When elements other than carbon are added to the iron, the steel is termed "alloy steel". These
cover a wide range from low-grade types to special high alloys offering exceptionally high strengths.
Stainless steels  are contained within this large category. Alloy steels offer a wide range of heat
treatment properties that makes the category of alloy steels the most versatile.
Stainless Steels are divided into two types: the so called 300 series true stainless steels, which
resist nearly all corrosive conditions; and the 400 series, which although not truly stainless, offer less
corrosion resistance only in certain environments (such as certain acids and salt water) and are
otherwise considered stainless. The further significant distinction between the two series is that the
300 series generally are much more difficult to machine, non-magnetic and non-heat-treatable,
although somewhat responsive to cold working. The 400 series are magnetic, almost every alloy is

                                         T91