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;

  1. Slight changes in Geometry?
  2. Differences in temperature that alter the material stiffness?
  3. Or, raising/lowering the coefficient of friction by using alternative materials?

By using a combination of CAD, FEA, further Kinematic Analysis and material property research, HD were able to virtually demonstrate the impact of each variable. With these results, HD then concluded which variable had the largest impact to determine which of the 3 factors the device was most sensitive to. The findings were documented and delivered in a sensitivity study which gave some insight into the overall robustness of the design.


When it comes to moving parts, geometry is critical. For example, a loose tolerance on a CAM profile could have detrimental effects on mechanical performance – This was a similar case for the mechanism in question. There was a connecting face that influenced the force requirement and overall feel of the device when in use. If this face was oversized, the force would increase, and if undersized the force would decrease.

Design engineer using CAD and FEA to improve an inhalation medical device design robustness with sensitivity analysis

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.


Here at Haughton Design, we help clients develop new products and medical devices effectively and efficiently. Please get in touch to discuss how we can help with your new product or medical device development project.

Danny Morris - Senior Medical Device Design Development Engineer & Health & Safety Manager at Haughton Design Danny Morris 27 July 2022


Get in Touch with Danny Morris

Senior Design Development Engineer

Danny graduated from Aston University with a BSc degree in Industrial Product Design. Prior to HD, Danny worked in engineering roles, designing products varying from plastic vehicle bodies to sheet metal fabrication, gaining valuable experience of manufacture in the production environment. Within HD, Danny is an advocate for developing skills and capabilities within the team to broaden HD’s services. Danny also assists with project management activities and is our Mental Health First Aider.

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