Weekend Mechanical Engineer Jobs in California (NOW HIRING)

About Mainspring
Mainspring Energy manufactures and delivers fuel-flexible, low-emissions local power solutions that rapidly add new capacity and deliver reliable, affordable, and sustainable electric power. The company began commercial shipments of its Mainspring Linear Generators in 2020 and today has hundreds of megawatts in advanced development and field operations for leading Fortune 500 companies, data centers, and utilities. Mainspring also partners with global energy leaders including AEP, NextEra Energy Resources, Schneider Electric, and more.
The core values that ground our work, guide our decisions, and connect us together:

• Pragmatic Optimism
• Excellence without Ego
• Proactive Collaboration

About the Role
Mainspring is seeking a Mechanical Engineer to own defined-scope mechanical engineering tasks, workstreams, or moderate-complexity projects for our Linear Generator platform. In this role, you will design mechanical hardware, support analysis and testing, drive assigned work through validation and release, and work cross-functionally with engineering, manufacturing, quality, supply chain, test, and service teams.
This role is intended for engineers who have developed practical mechanical engineering experience through multiple design, analysis, test, release, or implementation cycles. A strong candidate should be able to independently own defined-scope engineering work, apply sound mechanical fundamentals, identify common risks, communicate clearly, and follow through on deliverables with support available for ambiguity, complex tradeoffs, or higher-risk decisions.
Candidates who are still primarily learning engineering fundamentals, company processes, and task-level execution may be better aligned to an Associate Mechanical Engineer role, while candidates who have already demonstrated independent ownership of significant projects, subsystems, or technical workstreams may be better aligned to a Senior Mechanical Engineer role.
Responsibilities

• Own defined-scope mechanical engineering tasks, workstreams, or moderate-complexity projects with clear objectives and some ambiguity in execution.
• Design moderately complex parts, assemblies, fixtures, or subsystems while considering function, interfaces, manufacturability, serviceability, cost, reliability, and safety.
• Use engineering calculations, tolerance reviews, simulations, experiments, test data, and hands-on troubleshooting to validate designs and guide improvements.
• Develop and execute test plans, interpret results, document findings, and use evidence to support design, validation, and release decisions.
• Track deliverables, manage dependencies within your scope, communicate blockers early, and follow through on design, documentation, validation, and release activities.
• Work cross-functionally with engineering, manufacturing, quality, supply chain, test, reliability, suppliers, and service teams to resolve issues and move projects forward.

Qualifications

• BS or MS in Mechanical Engineering or a similar engineering field, with demonstrated experience contributing to mechanical design work beyond purely academic projects.
• Experience designing, analyzing, testing, and iterating mechanical hardware through multiple project phases or design cycles.
• Ability to independently own defined-scope engineering work with clear deliverables, while seeking guidance for ambiguous, complex, or high-risk decisions.
• Strong mechanical fundamentals, including practical experience with parts, assemblies, fixtures, mechanisms, structures, thermal systems, fluid systems, electromechanical packaging, or other relevant mechanical systems.
• Demonstrated ability to use analysis, testing, data, and engineering judgment to evaluate designs, compare options, and recommend practical next steps.
• Clear communication and execution skills, including the ability to document assumptions, decisions, risks, test results, and project status for technical and cross-functional stakeholders.

Preferred Qualifications

• Experience supporting hardware from concept through prototype, validation, release, production, or field use.
• Experience with CAD tools such as SolidWorks or similar, including part design, assembly design, drawings, and design documentation.
• Experience with PLM systems, drawing release, engineering change processes, supplier qualification, production support, or field implementation.
• Experience with structured problem-solving, root cause analysis, failure analysis, design of experiments, reliability testing, or risk-based validation planning.
• Experience working with suppliers, contract manufacturers, production teams, quality teams, or field service organizations.
• Experience with energy systems, engines, generators, compressors, pumps, thermal systems, fluid systems, structures, high-cycle fatigue environments, precision alignment systems, or industrial equipment.

$119,000 - $140,000 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!
Does your experience not meet all of our posted requirements? Studies have shown that some people are less likely to apply to positions unless they meet every listed requirement. At Mainspring, we are committed to building a diverse, inclusive, flexible, and collaborative environment, so if you want to help us transition the world to clean and affordable electricity, and don't meet all posted requirements for a particular role, we'd still love to hear from you. Mainspring can sometimes be flexible enough to shift responsibilities for the right person, or otherwise identify open or upcoming roles that may better fit your professional background.
In more traditional words, Mainspring Energy, Inc is an Equal Opportunity Employer. All qualified applicants will receive consideration for employment without regard to race, color, religion, sex, sexual orientation, gender identity, national origin, or protected veteran status and will not be discriminated against on the basis of disability.
This compensation and benefits information is based on Mainspring Energy's estimate as of the date of publication and may be modified in the future. We generally do not negotiate on salary once we have made an offer. The level of pay within the range will depend on a variety of job-related factors that may include location, relevant prior experience and/or education, or particular skills and expertise. New hires joining the company tend to be paid within the starting base pay range noted above, with opportunities to increase pay over time based on development of additional skills, competencies, and company-specific knowledge.
We may use artificial intelligence (AI) tools to support parts of the hiring process, such as reviewing applications, analyzing resumes, or assessing responses and identifying potential inconsistencies or verification signals in application materials based on available information. These tools assist our recruitment team but do not replace human judgment. Final hiring decisions are ultimately made by humans. If you would like more information about how your data is processed, please contact us.