The Triple Helix model has long provided a powerful lens for analysing university-industry-government interactions, yet its explanatory power is weakened when applied to sustainability transitions. Classical assumptions of goal congruence, linear knowledge transfer, and stable institutional complementarities fail to capture the plural values, deep uncertainties and adaptive governance demands of the sustainability agenda. This paper reframes the Triple Helix through a mechanism-based approach, specifying four recurrent processes--normative alignment, cross-sectoral learning, institutional embedding and collective reflexivity--that explain how collaborations generate sustainability-oriented outcomes. Drawing on institutional logics, organisational learning and reflexive governance, the framework integrates insights from mission-oriented and transformative innovation policy while avoiding the proliferation of additional 'helices'. Using illustrative cases from the EU's Climate-Neutral Cities Mission, China's wastewater recycling pilots, and the Ellen MacArthur Foundation's Circular Electronics Initiative, the paper demonstrates how mechanisms manifest differently across global innovation systems shaped by state capacity, market openness and civic accountability. The contribution is threefold: advancing a testable meso-level architecture for Triple Helix research, generating propositions for cross-national comparison, and providing operational tools for empirical analysis and policy design. Overall, this mechanism-based reframing advances Triple Helix theory by offering a portable, testable process grammar and by equipping researchers and policymakers with practical instruments--indicators, measurement strategies and design levers (e.g., standards, public procurement, reflexive evaluation)--to steer sustainability-oriented collaboration.