faulty battery box

The purpose of a Failure Modes Effects Analysis (FMEA) is to identify how a product or process could fail to satisfy customer requirements and expectations, ensuring that action is taken to mitigate the effects of failure so that the impact on users is reduced to an acceptable
level.

We recently identified a quality problem with the Fisher Price Musical Gym. Like many toys this requires a couple of AA batteries to power the audio output - but fitting them proved difficult due to a manufacturing defect shown below:

In this case, although the problem is revealed by a simple visual inspection - the defect passed through the production process and reached the end user, who not surprisingly rejected the product as 'not fit for purpose'.

But how would an FMEA prevent this type of customer complaint?

  • In this design the two batteries are fitted 'in series'. and each battery is located by two electrical contacts, a spring providing the negative contact pushes the battery into contact with the positive terminal formed by a  pressed plate.
  • The battery terminals are located in cavities in the battery housing, that are symmetrical about center line.

This basic design creates the possibility of errors in the production process, when the terminal are incorrectly fitted, with at least four possible 'error states' listed below:-

  1. As shown in the picture, a spring (negative terminal) may be fitted in place of the required positive terminal (a double negative installation), or
  2. Two positive terminals may be installed, creating a double positive installation, or
  3. Or the bridging contact, (on the Right of the picture) could be installed the wrong way round.

In the Design FMEA, we could identify the relevant functional requirement and failure modes shown below:-

Sample D-FMEA Data
FMEA DataSample data for this problemComment
ComponentBattery Box Assembly, with ContactsThis describes the unit under review
Functional RequirementContains two AA batteries with electrical contactsThis is the purpose of the parts in the system design
Potential Failure ModeStandard Batteries do not fit in battery boxThis describes the loss of function
Effect of FailureUser can not install batteries, or use productThis describes the user's experience resulting from the loss of function
Severity rating ( S )8 on a 1-10 rating scale8=Loss of primary function, 9 or 10 would indicate a risk of injury
Cause of failureWrong contacts installed during assemblyThis is the cause 'as observed' or known to the FMEA team
Occurrence (O)5 on a 1-10 rating scaleData is required to estimate how often the cause actually occurs.
Method of PreventionNoneNo design features prevent this error being made
Method of DetectionNone - visual inspection by userNo design features  prevent a defective part being supplied or used
Detection Rating (D)10 on a 1-10 rating scaleThere are no effective design controls so there is a significant risk of customer complaints, when process errors occur

 Risk Priority Number (RPN)

S x O x D

 400The high RPN score focuses attention on the risk of customer complaints if no action is taken to eliminate the causes of failure, by changing the design or controlling the production process.

Recommendations

Modify the design of the electrical contacts so that it is not possible to install the wrong terminals, or install the correct terminals the wrong way round. 

Unless the design is modified to prevent the contacts being installed incorrectly, there will be a continuing risk of process errors resulting in customer complaints. Although some of the faulty parts may be found by inspection, there is a risk that some will reach the customer - as happened in this case.

However, once the potential 'process errors' have been recognized, action can be taken to modify the design so that the terminals can only be fitted correctly.

Similar conclusions could have been reached using a Process FMEA, because there are no effective controls that prevent or detect the 'process error' - and error proofing the design avoids the need for inspections and rework to protect the customer from defects.

When correctly applied, the FMEA process eliminates customer complaints, safety defects and product recalls by identifying the need for design changes and process controls - before the customer finds the problem for you.

Contact Phil Stunell now for more information or to arrange FMEA Training