What are Super Tough bearings?
By Cory Mittleider on 12/29/2022
Super Tough bearings are where the Wind industry turns to improve life in the toughest applications such as gearboxes bearings and main shaft bearings. In this article we will explore what makes Super Tough bearings different from traditional through hard and case hardened bearings.
Tough Steel Family
NSK has several specialty bearing materials within their Tough Steel family. Having multiple material options available allows for the best material to be chosen based on the industry and failure modes found. For Wind applications Super Tough and Anti-White Structure Tough are used based on analysis of failed bearings from the field.
Steel Alloy
All the way to the raw material tough steel bearings are different. Super Tough and Anti-White Structure Tough bearings have specially formulated and tested steel alloys. Traditional bearings have well defined formulations that guide all bearing manufacturers, but alloys used in tough steel bearings are specially developed and refined by NSK. These formulations are not known by or available to other bearing manufacturers.
Heat Treatment
Categorically there are two types of heat treatments applied to bearings. The first and most common is a through hardened heat treatment where the entire cross-section of the bearing is the same hardness. The second is case hardened. You can read more about through hardened vs case hardened bearings here.
Visualization of through hard to case hard cylindrical roller bearing.
Super Tough bearings use a specialized case carbonitrided heat treatment. This heat treatment does result in a case hardened profile, but is notably different from more traditional case carburizing heat treatment. Case carburizing adds carbon to a low carbon steel base alloy while case carbonitriding adds both carbon and nitrogen to the part.
Case Carburizing
Carburizing diffuses carbon in to a low carbon steel alloy resulting in a case hardened profile.
Case Carbonitriding
Carbonitriding diffuses both carbon and nitrogen to a low or medium carbon steel alloy resulting in a case hardened profile.
The additional nitrogen diffused in the part during carbonitriding provides important benefits over traditional carburizing alone.
Super Tough package
Super Tough bearings are a combination of the specially formulated alloy and the upgraded case carbonitrided heat treatment. When these two things combine they provide many great features.
High surface hardness and tough durable core
Compressive residual stress
High retained austenite at the raceway surface
Improved carbide distribution
Can still have surface treatments and coatings applied such as; Black Oxide, DLC, surface finish improvements
Even in the most severe lubrication conditions Super Tough (Super-TF) bearings dramatically outlive through hard and case carburized bearings.
Super Tough (Super-TF) bearings offer a substantial increase in life when subjected to contaminated lubrication.
Specific to the White Etching Cracking (WEC) problems found in Wind gearboxes two tough steel family materials really shine. Super Tough bearings offer a great improvement against WEC but for the most severe WEC problems Anti-White Structure Tough (AWS-TF) is an even better choice. We will review AWS-TF further in another article.
The information and test results shown above demonstrate how Super Tough bearings are a great problem solver in Wind applications such as:
Gearbox Bearings in HSS and HSIS positions
Gearbox Bearings in Planet positions
Main Shaft Bearings
Please contact me with any questions, feedback, or additional conversation on bearing problems.
605-357-1076