Tolerance Stack Analysis for Medical Device Design

Design for Manufacturing (DFM) ensures that there is effective communication between the designer and the manufacturer of a medical device, say a hip implant. Therefore, the designer needs to mention the GD&T control symbols in his drawings. Here is the process flow for achieving the tolerance stack analysis for an implant or any other medical device.

Approach to Tolerance Stack Analysis (Source: HCL Tech)

Types of Tolerances

Here are the widely used tolerances:

  1. Form Tolerance: Straightness, Flatness
  2. Profile Tolerance: Profile of a line or profile of a surface
  3. Orientation: Perpecdicularity, Parallelism, angularity
  4. Location: Position and tolerance

Tolerance Stack Analysis

Tolerance stack-up analysis is required to ensure the the geometerical fit of different mechanical parts.

Tolerance analysis help us to determine the Worst case minimum dimension, Worst case maximum dimension, Root of Sum of Squares (RSS) Minimum dimension, RSS maximum dimension and sensitivity analysis of the dimension.

The following video is a great explanation of the tolerance analysis.

Why is there a variation between the designed part and manufactured part?

Sources of error:

  1. Tooling: Variation in locating surfaces using a fixture, tool wear
  2. Materials: Non-uniformity in material properties and internal stress relaxation.
  3. Operator: Inconsistent setups and other manual errors
  4. Environment: Temperature, humidity etc.

Design Process of an Implant or Medical Device

The following flow chart is an exhaustive process highlighting the various stages of the design like design input (stakeholder analysis, user requirements), followed by concept design, design verification, manufacture, validation and design transfer.

Source: Aitchison et al.

Risk Management Steps

Schematic of risk management process

According to ISO 14971, risk management of medical devices involves 4 steps:

  1. Risk Analysis
  2. Risk Evaluation
  3. Risk Control
  4. Residual Risk Evaluation

Risk Assessment = Risk Analysis + Risk Evaluation

ISO 14971 dictates the responsibility of the management for implementing the risk management process. According to ISO 14971, the management should provide the adequate resources and assign the qualified personnel for risk management.

Moreover, the top management is responsible for defining and determining the criteria for risk acceptability.

I will solve a case study involving the design of an exoskeleton for lower limb rehabilitation and document the risk management steps for this.

1. Risk Analysis

These are the three tools typically used in Risk Analysis:

  1. Hazard analysis
  2. FMEA
  3. Fault Tree Analysis (FTA)

1.1. Hazard Analysis

Hazards are potential damage to the user or the environment because of the use of medical devices. ISO 14971 Annex C contains examples of hazards associated with use of medical devices.

Hazard analysis
Hazards stemming from the use of medical devices

1.2. FMEA

This is a tutorial to help you with FMEA and CAPA in medical device design.

Using only FMEA doesn’t meet the quality requirements as per ISO 14971. This is because FMEA assumes that risks are involved when the device fails. However, there are potential risks of the medical device even without failures.

Part Function of partPotential Failure ModeSeverityCause of Failure ModeOccurrenceDetection Methods or Design ControlsDetectionRPNCAPA
Series Elastic Actuator (SEA)Transmit force to the jointsInconsistent torque7Lack of planarity between the slots5Peer review, literature data for torque required270Increase the moment arms
Moment arm of jointProvide moment to the rotating partsImpact loading, yielding, fracture8Repeated loading, stress concentration7Finite Element Analysis (FEA)5280Increase the part thickness, optimize the design

According to ISO 14971, manufacturers have the freedom to decide upon the threshold for risk acceptability. Only non-acceptable risks have to be integrated into the overall risk-benefit analysis.

If you are working on a group project, just the way I was working in my internship, it is up to the design team to assign the scores. The relative RPN scores are important. The part with the highest RPN score gets the most attention.

3. Risk Control

The risk assessment step is followed by the risk control step. Here is an example of the risk control measures as per ISO 14971.


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