DFMEA Block Diagram

Quick reference

• 00:04 Hi I'm Ray Sheen.
• 00:06 One of the techniques that's often used when completing the Design FMEA
• 00:10 is a block diagram.
• 00:11 This diagram is used to decompose the product and
• 00:14 simplify the definition of the functions used in the analysis.
• 00:19 So what is a block diagram?
• 00:22 This technique helps us to determine the functions
• 00:24 that are performed by each part or component in the subsystem.
• 00:28 Once those functions are known, the failure modes can be quickly identified.
• 00:33 There are many representations for the block diagram.
• 00:35 If your organization has a preferred method, use it.
• 00:38 One of the most popular is the exploded view of the bill of materials.
• 00:43 Regardless of which visual representation you use,
• 00:46 make sure you have included all the parts and
• 00:48 components including software programs that are part of the product.
• 00:53 Large systems lead to large block diagrams and very large Design FMEAs.
• 00:58 For that reason many times the block diagram is decomposed into the main
• 01:02 subsystems and many organization prepare the Design FMEAs at the part
• 01:07 of component level rather than at the sub-system, or full system level.
• 01:11 Again if your organization has a preferred approach, use it.
• 01:15 Otherwise the level of decomposition is a judgement call.
• 01:19 I'm going to use a ball point pen as my example in this lesson, and
• 01:23 in the remaining lessons that apply to the Design FMEA.
• 01:27 This diagram is a representation of the typical ball point pen.
• 01:32 On the next slide we'll do a decomposition of this pen into a block diagram.
• 01:37 I'll be using the exploded bill of materials approach to this block diagram.
• 01:41 We start with the highest level of assembly, the pen.
• 01:44 The pen is then decomposed into three subsystems.
• 01:48 The top barrel assembly, the bottom barrel and the ink refill assembly.
• 01:52 Each of these are further decomposed to the part level.
• 01:55 The top barrel assembly has four parts.
• 01:58 The top barrel, the plunger,
• 02:00 the back spring that provides the spring pressure for the plunger and
• 02:04 the clip that is used to attach the pen to a shirt or jacket pocket.
• 02:08 The bottom barrel is already at the part level.
• 02:10 Now at the bottom barrel, had to have a decal added to state that this is
• 02:14 from the Mom's Diner, we would have had two parts.
• 02:17 But in our case, for at least right now, we'll assume that the phrase,
• 02:20 Mom's Diner, is etched into this barrel.
• 02:23 The ink refill assembly also has four parts.
• 02:26 There is the ink tube, the actual ink, the front spring, and
• 02:30 the ball that is enclosed within the end of the tube.
• 02:33 We could have considered the ink tube as an assembly, and
• 02:36 just had two items at this level.
• 02:38 The ink tube assembly and the front spring.
• 02:41 Then we would need a third level of decomposition to break out the tube,
• 02:45 ink and ball.
• 02:47 But for simplicity, we just did two levels of decomposition.
• 02:50 Again, a judgement call.
• 02:52 With this block diagram, we can then determine the function of each item.
• 02:57 One of our sources of functions is the product specification.
• 03:01 The specs highlight the system function and this will cascade to component and
• 03:05 part functions.
• 03:06 But in addition, we'll create an interface matrix to help us understand the functions
• 03:11 associated with the interactions between the components and the assemblies.
• 03:15 The specification lists the functional requirements with targets for performance
• 03:20 and allowable variance or tolerance on those performance requirements.
• 03:24 The interface matrix starts with the function of each part of the component and
• 03:27 then shows the relationship between those items with respect to the functions.
• 03:31 It also includes provisions for environmental variation or
• 03:35 noise factors that can impact the functional performance.
• 03:38 Let's focus on the product specification first.
• 03:41 For each requirement in the spec, you first determine which product part or
• 03:45 components are involved in meeting that requirement.
• 03:48 Then you clarify the function they need to perform as part of fulfilling
• 03:51 the specification requirement.
• 03:53 So let’s work with our pen example.
• 03:55 Here is the specification for the pen, these requirements for ink storage and
• 03:59 dispensing which it typical performance requirements from this type of product.
• 04:03 Also there are requirements for the user handle and the clipping to the Pocket.
• 04:08 These are the users interface requirements.
• 04:11 Now let's take a look at the first one, it's storing ink in the pen.
• 04:14 There are two parts that are directly involved, the ink tube and the ball.
• 04:17 The ink tube's function is to contain the ink, the ball's function,
• 04:22 with respect to this part requirement, is to prevent leaks.
• 04:26 Now let's look at something that is probably not familiar.
• 04:28 The function interface matrix.
• 04:30 The matrix identifies in what ways the different parts interface with each other
• 04:35 and the external interfaces.
• 04:36 The top part shows the interface between assemblies.
• 04:39 You can also take that to a lower level and show the interfaces between parts.
• 04:44 As you can see in the case of our pen the interfaces are very simple.
• 04:47 They are merely ones of locating each part with respect to each other and
• 04:52 providing retention or support.
• 04:54 In this section you only need to note the functions for
• 04:57 those parts that actually touch, or interface, with each other.
• 05:01 The more complex interfaces are the external interfaces.
• 05:04 This is where it's important to have someone on the team with knowledge of the customer
• 05:09 use or application.
• 05:11 All the interactions that occur between a pen and its surrounding are listed.
• 05:14 And the functions performed by the sub assemblies during those interactions
• 05:18 are also identified.
• 05:20 So in our example, one interface is with the paper or writing material.
• 05:25 The only part of the pen that should touch that is the ball of the pen and
• 05:29 the interface function is to maintain a point contact.
• 05:32 Another interface is with the user's hand.
• 05:35 In this case, it is the top and bottom barrel are the interface,
• 05:39 not the refill assembly.
• 05:40 And the interface is both holding the pen and the feel of the pen.
• 05:45 We describe the storage interface using a shirt pocket.
• 05:48 If there was a different storage interface such as with a note pad,
• 05:52 we could describe that one also.
• 05:55 The last two interfaces are with the environment, not the user.
• 05:58 The FMEA team member who knows the customer application has indicated that
• 06:03 these two environmental factors can create failures for pens.
• 06:07 It is quiet possible that some of the interface functions will be redundant with
• 06:12 the product specification function and this is okay.
• 06:15 They will only be listed once in the FMEA form but
• 06:17 we want to make sure that we have captured them.
• 06:18 Also some parts may not have any external functions.
• 06:25 Okay, with the block diagram we have a good understanding of how
• 06:28 the product works.
• 06:29 And we can use the specifications and
• 06:32 interface matrix to identify each function.
• 06:34 That's the first column on your Design FMEA form.

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