Sprocket teeth may be hardened by the flame or induction hardening methods.
The results obtained are very similar, the choice of method depending on equipment availability, batch sizes, sprocket size (pitch) and product geometry (bore size, holes in the heat affected zone and keyways).
Hardening teeth substantially increases sprocket life and is recommended for any long term transmission application particularly where abrasion is an issue.
Degree of hardness
This is initially determined by the material used to manufacture the sprocket but hardness levels may be reduced by subsequent tempering to meet specified levels.
The majority of sprockets are made from K1042 plate which contains 0.45% carbon. The as-hardened hardness of this material is 45-55 Rockwell “C” (HRc) and it may be tempered back to any specified hardness level below this.
Example: If the application requires the sprocket to wear preferentially to the chain the hardness level specified for the sprocket would be 5 HRc points lower than the chain. A typical sprocket hardness specified for this type of application is 35-40 HRc.
Occasionally 4140 or 4340 is the specified material. This requires particular care when hardening. An oil or polymer quenchant is used and sprockets must be tempered after hardening.
1.5 – 2.0 mm is typical however deeper cases may be obtained for special applications.
The critical area to harden is the working face. This varies depending on the sprocket type however typically it is the concave area of the tooth where the chain links or rollers contact the tooth. The root of the tooth is theoretically not subject to wear and does not need hardening however it is usually hardened as part of either process (flame or induction). When a conveyor sprocket has extended pitch line clearance or relief in this area it is not necessary to harden this section of the tooth.
In many cases sprockets may be reversed to extend their life or run backwards as part of the operating cycle. In these cases the full profile would normally be hardened.
Sprocket types and typical hardened area
• Transmission sprockets – machined profile
• Conveyor sprockets – working faces & pitch line clearance
• Silent chain sprockets – pitch line
• Redler sprockets – working face
• Block link chain – working faces