What's the purpose of "thrust balance" in Turbo Fan Engine?

happyroof

I'd like to know about the purpose of the Thrust balance system in commercial
Turbo Fan Engine. Can you tell me the major role of thrust balance system?
Also, the relations between thrust balance system and compressor / main bearing.
thank you in advance

FredGarvin

Thrust balance is a way to ensure the loading on the thrust supporting bearings in an engine stay relatively constant over the operating range of the engine. If not constant, then at least an effort to limit its maximum value. This usually allows for a smaller, less load capacity bearing to be used in applications that would usually require something very large and thus add cost and weight.

There are correlations for sizing the bearing and thrust balance, but it is a process that is really somewhat iterative and relies a lot on the aerodynamics of a particular core and the design constraints for a particular engine. In the end, you are balancing the aerodynamic forces by increasing/decreasing balance piston area and the design constraints, i.e. size, speed, cost, of your thrust loaded bearings. There is a lot of trade offs and experience in this aspect. There is no "plug and chug" equations to do this.