Engineering a cutting blade design for a robotic lawn mower

The scope of this project was the design and engineering of a cutting blade and guard design to fit on newly developed robotic lawn mower. At the start of the project a set of requirements were noted.

• 1. Blade and guard fits within existing frame with minimal modification.
• 2. A 7″ blade.
• 3. Blade accelerates to cutting speed within 2s.
• 4. Blade stops within 0.5s. (Ideally faster.)
• 5. Blade has linear speed of 7000 to 19,000 feet per minute.
• 6. The blade should generate enough lift to lift wet grass.
• 7. The blade should mulch grass clippings.
• 8. The guard should not accumulate excessive clippings.
• 9. Rocks and other debris are not thrown with excessive force and are contained.
• 10. Blade does not get tangled up in tall grass.
• 11. Cutting height can be manually controlled from 1″ to 4″.
• 12. We would prefer a quieter and more energy efficient system.
• 13. Blades should be very low cost to manufacture. I suggest injection molding using nylon with 50% glass fiber and not requiring any inserts. 1 month of heavy cutting would be an acceptable blade life. (More is great too.)
• The last significant requirement is that I want a blade that is as intrinsically safe as possible.

According to these requirements 3 concepts have been designed, engineered, developed and tested. Some of the requirements where conflicting with each other and had to be slightly changed to achieve the required results.

The final blade has a low mass moment of inertia, good cutting properties, low energy consumption and grass mulching capabilities. While the guard is protecting the blade and is strengthened in the areas where high stresses occur. Aerodynamic analyses and FEM analyses were used to optimize the blade and examine the mulching blade capabilities.

A non-disclosure agreement has been signed, which forbids me to explain or give any values of the designs.