Chapter 3 Gear material and Heat treatment
3.1 Selecting gear material
When load to flank of gear is excessive, wearing off (pitting) of flank may occur easily. It is necessary to select material with greater strength of surface durability therefore the case hardening steel is recommended due to higher
hardness.
The impact to flank of gear during operations causes damage to the gear tooth, therefore it is necessary to selectsteel with higher bending strength. Selected material should be able to apply induction hardening after quenching
and annealing treatment. Such selection emphasizes on core hardness instead of surface strength.
Take note of the following while selecting material with manufacturing expense and productivity in mind.
1)	For necessary strength for gear, select the material character by emphasizing on either Surface durability or Bending strength. Generally, ideal material selected for gear tooth should be tough and hard to withstand damaged by
the load.
2) Suitable material for machining.	Pitting occur easily in free cutting steel even after surface treatment is applied to gear. This material is unsuitablefor gear even though it has good machinability.
3)	Material which is easy to apply heat treatment and little deformation. Even if deformed after applying heat treatment, amount should be stable.
4) Material should be economical and easily obtained.
Table 1 shows the common gear materials used for transferring power.
Table 1. Iron and steel materials used for gear
Name of Standard

JIS numberMaterials

Gray iron casting

G5501

FC200, 250, 300, 350

Spheroidal graphite iron casting(1)G5502

FCD400, 450, 500

Carbon steel forging for general use

G5101

SC410, 450, 480

High tensile strength carbon steel castingand Low alloy steel casting for structural
purposes

G5111

SCC3A, 3B
SCCrM1, 3, SCNCrM2

Carbon steel for machine structural use

G4051

S38C ～ 58C, S09CK, S15CK, S20CK

Nickel chromium steel

G4102

SNC631, 836, 415, 815

Nickel chromium molybdenum steels

G4103

SNCM625, 630, 439, 447 SNCM220, 415,420, 616, 815

Chromium steel

G4104

SCr415, 420

Chromium molybdenum steel

G4105

SCM435, 440, 415, 420, 421, 822

Aluminium chromium molybdenum steels

G4202

SACM645

Stainless steel bars

G4303

SUS304, 440C

Note (1) This material includes Ductile Cast Iron and meehanite
Remarks. For Case hardening, it is common to use SCM415 or SNCM415. SNC815 and SNCM815 are suitable for Spiral bevel
gear. Please refer to Table 5 (Pg. 58) for Load, Material and its heat treatment.

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Characteristics of Polyacetal
Recently, industries prefer to use various engineering plastics for machinery elements. We would like tointroduce you to our commercialized KG-Polyacetal
gears (one of the engineering plastics). Note that gear
strength and heat resistance should be taken into
consideration when comparing with metal gears.
There are 2 types of Polyacetal, uniformed formaldehyde and copolymerizated ethylene oxide. Theformer is called Acetal - homopolymer, the latter iscalled Acetal - copolymer. Usage condition: meanload and mean speed or less is recommended.
Polyacetal has following features. Please refer to thebelow.
• Physical characteristics - Thermoplastic resin. Usedfor extensive mass production. Polyacetal has excellent physical characteristics compared to all otherresins.

• Wear characteristics - Regarding Wear proof, Polyacetal is excellent next to Polyamide due to littleabsorbency.
• P olyacetal has tendency for minute dimensionalchanges due to minute absorption. It has excellent fluidity and has less remainded strain for molditems.
• Chemical character - There will be no damages to
the Polyacetal properties even after soaking it in organic solvent with inorganic drug without mineralacids for 6 months. However, use of phenol is notadvisable. Polyacetal is extremely resistant against
erosive Alkali. It will not be damaged by industrial
lubricating oils, motor-oil, break-oil and even contact to copper material.
• Heat resistant - Polyacetal has excellent heat resistant features.

Table 2. Properties of Polyacetal
Specific gravity

Testing methods

Units

ASTMD-792-Water absorption (soaked for 24 hour)		
(60% RH)

ASTMD-570%Tensile strength (yield point)

ASTMD-638

N/mm2

Numerical value
1.41
0.22
0.16
61

Tensile elongation (breaking point)

ASTMD-638%40

Modulus of elasticity in tension

ASTMD-638

N/mm22,830

Flexural strength

ASTMD-790

N/mm289Flexural modulus

ASTMD-790

N/mm22,590

Compressive strength (Deformation of 10%)

ASTMD-695

N/mm2103

Shear strength

ASTMD-732

N/mm255

Izod impact value (with notch)

ASTMD-256

J/m

74

Rockwell hardness

ASTMD-785

M scale78

R scale119

ASTMD-1044
Westover style friction testingmachine

mg/100 cycle

14-0.13-0.35

DSC analysis temperature 10ºC/min℃

165

Deflection temperature under load (182.4 N/cm2)		
(45.1N/cm2)

ASTMD-648℃

Coefficient of linear expansion-25 ～ +25℃

× 10-5/℃

Taper abrasion (1kg.CS17 wheel)
Coefficient of dynamic friction (for steel)Poisson's ratio
Melting point

Combustion property

110158
9UL94-

HB

Dielectric constant (102 ～ 106Hz)

ASTMD-150-

3.7

Dielectric dissipation factor (102Hz)		
(106Hz)ASTMD-150-

Surface resistance

ASTMD-257

Ω1.0 × 1016

Volume characteristic resistance

ASTMD-257

Ω･m

1.0 × 1012

0.001
0.007

The above properties are for reference only. They are not covered under warranty.
Extract from Nippon Polypenco Co,. Ltd - Polypenco Acetal catalogue.

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Features of Polyacetal gear
Strength of plastic gear compared with metal gear
excluding external factors is １／６ to 1/9. However, it isnecessary to take factors like temperature, humidity
and others into considerations.
Table 3. C ircumferential speed and Limitation of frictional speed
Lubrication

Without
lubricating oil

Lubricating
oil

Circumferential
speed for Spur andBevel gears

m/s

612

Frictional speed forWorm gear pair

m/s

12.5

Backlash for plastic gear
Plastic material has extremely smaller thermal
conductivity and larger thermal expansion factor
compared with metals. Plastic gear pair has highertendency to change dimension compared with metalgear. Therefore KG has intentionally fabricated wider
backlash plastic gears as compared with metal gears.

Lowest usage temperature limitation -38°C

Combination of gear materials
The combination of materials for plastic gear pair, assuming combination between Polyacetal, metal material factor is 1.0. When combining two Polyacetals,
material factor is 0.75. Therefore gear strength for
Polyacetal gear pair becomes 75%.

56

We believe that engagement between Polyacetal andmetal gears are best combination.
However, note that maximum surface roughness 6S
at flank for metal gear is advised to prevent wear forplastic gears.