Does the application of a heat treatment process to harden a tooth profile surface so as to extend service life, solve a gear wear issue?

Let’s take a look at the factors involved in causing gear tooth wear and how to ensure gears have a long service life.

As gear tooth profiles wear down the clearance between meshing teeth or the backlash will increase. This creates inaccurate gear and mechanism positioning, vibration, shock loads and noisy operation.

Worn gears can lead to machine failure and production downtime. Regular inspection can highlight problems before failure occurs and correct maintenance will help minimise wear rates.

 

A gear expert will be able to determine the precise cause of wear or failure by careful inspection of the gear and the particular pattern of damage. Recognisable modes of failure include; abrasion, adhesion, corrosion, polishing, scuffing, cracking, fracture, pitting and fatigue.

The Common Causes of Wear on Gear Tooth Profiles Are Listed Below.

• Incorrect lubrication

Ensure correct type, grade and quantity of lubricant is supplied to the gears. This information will be provided with original documentation for the equipment. Alternatively, a lubrication expert will be able to provide recommendations.

• Incorrect gear meshing clearance.

Highly accurate gear centres position and meshing clearance must be achieved during design, manufacture and assembly of machinery and maintained during operation.

• Mating gear misalignment.

Highly accurate alignment of gears with respect to parallelism and lateral offset is critical for function and reliability in service.

• Gear material has insufficient strength and/or hardness.

Service life will deteriorate quickly if gears do not possess sufficient strength and/or hardness for an application. Gears are supplied in many different materials, grades and treatments and should be correctly specified during the design and drawing generation stage of a gear transmission arrangement.

• High contact stresses.

Underestimating the loads to be transmitted through gears will result in premature failure because the gear module and/or gear face width selection will be inadequate for the application.

• Excessive service temperature is adversely affecting gear material properties.

Prevent strength reduction of gear material by using appropriately designed heat shielding. Alternatively, select a gear material which is better suited to high temperature applications.

• Gear and tooth profile specification and/or manufacturing errors.

Gear bore and tooth profile geometry as well as surface finish must be accurately specified and precisely manufactured to ensure function and reliability in service.

• Debris ingress between meshing gears.

Prevent dirt, debris and dust etc from entering meshing gears by thoughtful design of surrounding components and/or using appropriately designed guarding. A recirculating lubrication system with filter may help in some applications.

• Metal corrosion due to water or chemical attack.

Prevent corrosive fluid from contacting gears by using an appropriately designed shield or sealed housing. Alternatively, select a gear material which can resist corrosive attack.

• Metal fatigue.

Failure of components subject to prolonged and cyclic stresses can be delayed by correct selection of gear size, material and elimination of defects within the gear.

Conclusion

To solve a problem involving premature gear wear on an existing industrial application, it is best to investigate every possible cause and solve only the issue(s) creating the wear.

Poor reliability of components within machinery could have been created at any one of many stages. For example, hasty design, errors during manufacturing, poor assembly, abuse during operation or lack of maintenance.

Therefore, the customer’s consideration of applying a heat treatment process to harden the tooth profile surface and therefore extend service life is a good idea. Although, for this solution to be effective all other potential sources of wear should have been considered and either fixed or verified to be not the cause.

 

One method to harden the tooth profile surface of a steel gear involves heating the material to a high specific temperature and duration with the addition of carbon at the surface and then followed by a rapid cooling.

A point to note is that only particular grades of steel can be hardened by this process.

A gear manufactured from a higher strength and hardness grade of steel may also prove to be adequate in certain applications.