About the Role

Mainspring is seeking a Principal Mechanical Engineer to serve as a company-level technical authority in a critical mechanical engineering domain for our Linear Generator platform. In this role, you will shape long-term technical direction, define or strongly influence architecture and design principles, guide high-consequence technical decisions, and help the organization solve problems that are novel, ambiguous, and central to long-term product and business success.

This role is intended for engineers who have already demonstrated broad technical leadership across multiple teams, systems, or major program areas and are now ready to create durable company-level technical leverage. A strong candidate should be able to operate with exceptional autonomy, advise senior leadership, anticipate long-term technical and business risks, and create principles, standards, methods, or decision frameworks that improve how Mainspring approaches major classes of engineering problems.

Candidates who primarily shape technical direction across multiple teams or system areas may be better aligned to a Senior Staff Mechanical Engineer role. Principal-level candidates should bring rare technical judgment in a company-critical domain and demonstrate impact that extends beyond individual projects, teams, or near-term program execution

Responsibilities

• Lead company-critical technical direction in a mechanical engineering domain with high ambiguity, high consequence, and broad cross-functional impact.
• Define or strongly influence technical architecture, design principles, standards, methods, and long-term product direction that affect product capability, reliability, cost, scalability, manufacturability, serviceability, and business outcomes.
• Solve the organization's most difficult and unprecedented engineering problems, especially where existing methods, assumptions, or design approaches are insufficient.
• Advise senior leadership on technical decisions with long-term product, customer, operational, financial, or business consequences.
• Define evidence and learning strategies for high-consequence decisions, including the right mix of analysis, testing, modeling, field data, production learning, external expertise, and engineering judgment.
• Create durable organizational leverage through technical frameworks, review practices, architecture decisions, standards, methods, or risk approaches that improve how teams execute engineering work.

Qualifications

• BS, MS, or PhD in Mechanical Engineering or a similar engineering field, with a distinguished track record of solving high-consequence mechanical engineering problems in a company-critical domain.
• Demonstrated ability to shape technical direction across major systems, platforms, product areas, or organizations, not only within a single project or team.
• Exceptional mechanical engineering judgment, including the ability to balance product capability, reliability, cost, scalability, manufacturability, serviceability, safety, customer needs, and business priorities.
• Proven ability to solve novel, ambiguous, cross-functional problems where requirements, ownership, evidence, methods, or tradeoffs are not initially clear.
• Deep experience using analysis, testing, modeling, field learning, production learning, and engineering judgment to guide long-term technical strategy and high-consequence decisions.
• Strong executive-level communication and influence skills, including the ability to align senior leaders and cross-functional stakeholders around technical strategy, risks, tradeoffs, and recommended direction.

Preferred Qualifications

• Experience serving as a company-level technical authority, principal engineer, architect, or equivalent senior technical leader in a critical hardware domain.
• Experience developing hardware for energy systems, engines, generators, compressors, pumps, thermal systems, fluid systems, structures, high-cycle fatigue environments, precision alignment systems, electromechanical systems, or industrial equipment.
• Experience defining architecture, design principles, validation strategies, technical standards, risk frameworks, or engineering methods that improved execution across multiple teams or product generations.
• Experience influencing long-term product strategy, platform architecture, reliability strategy, production scalability, field performance, cost reduction, or serviceability at an organizational level.
• Experience advising senior leadership on technical investments, product tradeoffs, risk posture, technical debt, or long-term engineering strategy.
• Experience developing other senior technical leaders through design reviews, technical strategy work, decision frameworks, and high-quality engineering judgment.

$210,000 - $247,000 a year

This position is onsite at our Menlo Park HQ. The salary will be adjusted to reflect local market conditions based on employee location as well as the experience of the employee. Along with the base salary, Mainspring offers pre-IPO stock options + benefits.

Before you apply, a quick note on your resume
We care about the substance of your work and want to make sure we learn as much as we can about your design experience. To help us evaluate your experience accurately, please ensure your resume includes:

• What you actually built or worked on - name the real product, subsystem, or hardware (not just "components" or "systems"). "Redesigned the bracket mounting the compressor to reduce vibration failures" tells us far more than "designed mechanical components."
• What you personally did, separate from what your team did. If you contributed to a group project, tell us your specific role and contribution.
• The engineering fundamentals you applied - tie your skills (CAD, FEA, GD&T, thermal/fluid analysis, testing, etc.) to the actual problem you used them on, rather than listing them as standalone skills.
• What happened as a result - did it pass validation/testing, resolve a failure, ship to production, and if so, at what volume or scale (prototype quantity, low-rate production, high-volume manufacturing)? This helps us understand whether your experience is with one-off/prototype hardware or product that shipped at scale.

One more thing: please don't feel obligated to keep your resume to one page. If accurately describing your relevant projects and contributions takes more space, that's completely fine - we'd rather see the detail than have you compress or omit meaningful work to fit a page limit.

Thank You!