An example of a simple bell crank that transfers force from the actuator to the arm at 90º from the actuator. More complex bell cranks have significant transitions from thick to thin sections and tight tolerance features like hole locations, concentricity, and parallelism between holes.
The bell crank for the B-1 aircraft has been identified by the Air Force as a challenging component for metal additive manufacturing (AM) fabrication. The Air Force Sustainment Center has difficulty obtaining this part using conventional fabrication technologies such as casting or machining. The bell crank has numerous thick-to-thin transitions and multiple angular features which makes machining from the plate difficult and time-consuming. The original component is cast. Although specific cost and time data are not available (but are to be determined at the beginning of this project), it is assumed that casting tooling production involves a substantial lead time with high tooling costs, particularly considering the complex part geometry.
This project aimed to improve the ability of the Air Logistics Complex (ALC) to rapidly find replacements for challenging parts required for legacy aircraft. Specifically, the effort focused on manufacturing a flight-critical bell crank for the B-1B aircraft which includes features of varying thicknesses and complex geometries. Challenges, solutions, and opportunities identified through the AM fabrication of the bell crank have significant applications in supplying legacy parts to the Department of Defense (DoD) supply chain. The project sought to provide a baseline for AM execution in a tooling-based sustainment community.
The project plan was to fabricate a significant number of bell cranks and coupons on two metal laser powder bed fusion system types (EOS and 3D Systems), measuring and recording each layer of the structure during the build. Metrology was executed and recorded during each post-processing step (e.g. stress relief, build plate removal, anneal, hot isostatic pressing, heat treatment, machining), thus populating and recording the digital thread of the component. Repeatability, process robustness, and the ability to specify requirements that are not machine-specific in a technical data package were evaluated with multiple AM systems.
Nineteen usable bell cranks were fabricated by laser powder bed fusion additive manufacturing and evaluated by proof testing. Fabrication was conducted across four different machines at three different locations (Pennsylvania State University Applied Research Laboratory, 3D Systems, Oerlikon, and Youngstown State University) and two different machine vendors (EOS M290 and 3D Systems ProX 320). All bell cranks were stress-relieved, HIP, heat treated, and machined. Machined surfaces consisted only of critical, high-tolerance, and mating features.
All 19 were subjected to a calculated proof load, followed by an ultimate load of 1.5 times the proof load. No bell cranks experienced failure during the proof or ultimate testing, and no cracking or defects were visually or acoustically noted post-test.
The proof load values calculated at the onset of the project were accomplished by using the stress-intensity factors for an A357 casting alloy since the fatigue properties for the alloy used (AlSi10Mg) had not yet been developed. Near the conclusion of the project, the fatigue properties for AlSi10Mg became available, and the resulting critical stress-intensity factor was found to be lower than that for an A357 casting alloy. Therefore, the proof loads that were calculated are conservatively higher using A357 data and provide an additional factor of safety utilizing the proof load derived from the stress-intensity factors of AlSi10Mg.
Other Project Participants
- Pennsylvania State University – Applied Research Lab
- M-7 Technologies
- Youngstown Business Incubator
- Boeing Company
- Lockheed Martin
- U.S. Department of Defense