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Design light and stiff
A crucial adage for mechanical design is: Design light and stiff products
1. Why light?
Light is easy to explain: use as little material as possible.
2. But why stiff?
Very often the maximum allowed deformations dominate product design and not the maximum allowed stresses. The higher the stiffness, the smaller the deformations caused by loads. Thus given a maximum allowed deformation, less material is needed if the material has a higher stiffness. But here is a catch, when a material with a lower density is chosen very often also the material stiffness decreases, see the material Elasticity page.
So although a material with a lower density is used the total mass of the product might still increase because the specific material stiffness (E/ρ) decreases more!
Example lower material density, higher product weight: This may happen when a low carbon Steel is replaced by an Aluminum alloy, for instance for a car [Ashby2013].
E_steel |
210[GPa] |
rho_steel |
7900[kg/m^3] |
E_steel/rho_steel |
210/7900=0.0265 [GPa*m^3/kg] |
E_Al |
70[GPa] |
rho_Al |
2900[kg/m^3] |
E_Al/rho_Al |
70/2900=0.024[GPa*m^3/kg] |
In this case, choosing Aluminum yields a higher mass compared to choosing Steel, while both yield the same product stiffness.
3. Why light and stiff?
There is another reason to design light and stiff, which has to do with vibrations. Remember the equation for the eigenfrequency, see the attachment (in which k = product stiffness, m = product mass) [Young2014].
The higher the stiffness and the lower the mass, the higher the eigenfrequency $f_0$. Why is this eigenfrequency so important? When a mechanical or industrial design product is excited with a vibrational load in the eigenfrequency the part starts to vibrate heftily, which will ultimately destroy the part due to material [fatigue](Fatigue). The higher this eigenfrequency, the lower the chance that the part will be excited in this eigenfrequency, because most of the exciting vibrations enter the product by ”contact” with the environment and these ”contact” vibrations are in the lower frequency ranges.
But be aware of:
- vibrations, especially internal, caused by actuators!
- shock loads, caused for instance by play between parts. Shocks contain a broad spectrum of vibration frequencies, thus likely to contain eigenfrequencies of the product.
Take home message: Design light and stiff products.