Having completed kinematic analysis for a mechanism within an inhalation device, HD were approached by an existing client to improve their drug delivery device’s design robustness by using Sensitivity Analysis. Design Development Engineer, Danny, discusses how we achieved this and what benefits it brought.
The initial study focused on reducing the amount of force required to operate the mechanism within the device. However, this next phase of work was required to determine what variable factors would cause the most significant impact on the device’s operation. I.e., what would affect the device the most;
To give a precise estimate for the change in force, the original kinematic study was re-purposed, and the CAD model of the connecting face was replaced with a slightly smaller version as well as a slightly larger version. These geometry changes represented the upper and lower limit of the tolerances required to suit manufacture.
Temperature vs Stiffness
As this device was to be used in a wide range of environments, the effects of temperature changes on the mechanism were critical. As the device is plastic, an increase in temperature reduces material stiffness and a decrease in temperature increases it. To determine the altered stiffness’s, an FEA model was created with varying stress/strain curves to replicate the change in temperature. This gave an estimated change in stiffness at the upper and lower temperatures which was inputted again into the kinematic analysis model.
Changing the Coefficient of Friction by Replacing Materials
Lastly, HD wanted to determine the effects of varying friction and how this would impact the device. If the mechanism transpired to be highly sensitive to friction, then there was the option to swap out the Mould material with a different grade. By physically testing different material combinations, an upper and lower coefficient of friction was calculated. These new friction coefficients were then fed into the kinematic analysis and the study was re-run.
Having collated the data from these different studies, HD were able to compare them against the original benchmark study and determine which variable the design was most sensitive to. With this information, the client was able to specify materials and tolerances accordingly.
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