Stiffness Testing Fixture

• To further quantify mattress cover performance, I designed and constructed a low-cost solution to measure stiffness.

One of the first test fixtures I built in my current role at Eight Sleep, a sleep fitness company building smart mattress covers, was a fixture to measure stiffness. The stiffness of a mattress, also known as how “firm” it is, is perceived as how hard or soft the surface feels as you lay on it. Since we needed to make a product that was comfortable with customers regardless of what their mattress preferences were, we needed to measure the firmness, or stiffness, of our product on a variety of mattresses.

This is done at many mattress manufactures by measuring the force-deflection of the mattress, that is, how much force it takes to depress the surface x amount, and calculating the stiffness based on the slope of that curve.

We could not install any heavy machinery in our traditional office space, so we needed a somewhat modular, moveable fixture that could be easily assembled. It needed to be able to measure mattresses of all sizes in any position, and remain rigid up to test loads of 250 lbF. I opted to make an 8020 frame to support the load of the testing, and design custom plates to act as trusses, to ensure that the 9’ span wouldn’t deflect over the course of the measurement and effect the data.

The final structure, making up an 8’x9’ area, was made out of 8020 beams, waterjet aluminum plates, and off-the-shelf test equipment. The feet supporting the main body can be retracted so that the entire fixture can be moved on casters.