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Diagnosing Snap-Fit Problems
8 Diagnosing Snap-Fit Problems
The goal of careful design is always to prevent problems from occurring in the first place but snap-fits do sometimes fail. They also have their share of assembly and usage problems. A full understanding of the various failure modes and their relationship to the most likely root causes can help one more quickly diagnose and solve problems. This, of course, minimizes the cost and time impact of fixing a problem but it also helps ensure that the proposed changes will indeed fix the problem. Nothing is worse than making changes to a product and finding that the problem still exists or has even gotten worse. Accurate diagnosis is particularly valuable during product development when prototype testing may indicate the need for improvements, yet time and cost constraints limit the available options.
8.1
Introduction
The root causes of many snap-fit problems are at the attachment level. Yet, many times, the first attempts to fix the problems are at the feature level, thus are doomed to failure or to cost much more than they should. When evaluating any snap-fit problem, even feature failure or damage, first verify that all attachment level requirements have been satisfied. If not, address them before attempting a feature level fix [1]. First, it is important to define ‘‘problem’’ because the term includes much more than simply feature breakage. A snap-fit problem is identified by the following symptoms:
Difficult assembly Short-term feature failure or damage Long-term feature failure or damage Part distortion or damage Part loosening and=or squeaks or rattles Unintended part release Service difficulty Customer complaints about ease of operation
It is important to remember that these are symptoms of a problem; they are not the root cause of the problem. Simply treating the symptom may not fix the real problem or it may create other problems in the attachment. Many of the above symptoms can have both attachment and feature level root causes. Sometimes, the root cause turns out to be a combination of several shortcomings. Figure 8.1 reflects the author’s personal experience in trouble-shooting snap-fit problems in products. Note the high incidence of causes related to attachment level issues and the high frequency of multiple root causes for problems.
256
Diagnosing Snap-Fit Problems
(a) Incidence of multiple root causes
[Refs. on p. 265]
Relative frequency
Applications with only one root cause Applications with two root causes Applications with three or more root causes
(b) Feature level vs. attachment level root causes Feature level
Relative frequency
Feature strength (retention) Feature behavior (assembly) Material properties
Attachment level
Installation options Constraint violations Enhancements missing
Figure 8.1 General trends in snap-fit problems
8.1.1
Rules for Diagnosing Snap-Fit Problems
Do not mistake a symptom for a root cause. This is important in any problem-solving effort. Always remember that the root cause of many, if not most, snap-fit problems is at the attachment level, not the feature level. Corollary to this rule is to also remember that many attachment level problems are related to improper constraint. Resolve all attachment level causes of a problem before attempting any feature level fixes. Recall that some problems are a combination of both feature and attachment level causes. Always try the easiest fixes first. Be aware that most feature level changes in a snap-fit will have multiple effects. A change to fix one problem is very likely to change other behaviors and may create new problems.
8.1.2
Mistakes in the Development Process
Tracing back through the development process can sometimes give clues as to the root cause of a problem. Most problems, whether they are attachment or feature level, are the result of mistakes made during the development process. Some of the more common mistakes are:
8.2 Attachment Level Diagnosis
257
Improper constraint in the attachment, characterized by: Over-constraint where features ‘‘fighting’’ each other can cause breakage during assembly or in service due to thermal expansion. Over-stress due to residual assembly forces can cause long-term failure. Under-constraint where: Features are carrying the wrong loads or excessive loads. Weak or compliant parts are expected to provide a rigid base for lock or locator features. Failure to fully consider material properties, including: Incomplete material property data available. Failure to consider plastic creep and thermal effects. Failure to anticipate assembly variables such as: Incompatibilities involving engage direction, assembly motion, feature positioning and feature style. Designed-in assembly frustration and difficult assembly. Failure to anticipate all possible end-use conditions, including: Failure to consider disassembly and service. Failure to consider all loads including unexpected or improper but possible load conditions, (such as dropping or striking a product). Failure to consider customer usage.
8.2
Attachment Level Diagnosis
Attachment level problems are often independent of the lock. In other words, they would be occurring regardless of the locking features style used. Understanding the key requirements of constraint, compatibility and robustness can help one recognize and resolve many attachment level problems. Remember too that certain enhancements are required for every application. Always verify the presence of these four enhancements, if any are missing, problems are likely. They are:
Guides Clearance Operator feedback Process-friendly
The four most common symptoms related to attachment level problems are:
Difficult assembly Parts distorted Feature damage Loose parts
For each of these symptoms, the most likely root causes are listed below.
258
Diagnosing Snap-Fit Problems
8.2.1
Most Likely Causes of Difficult Assembly
Over-constraint Assembly motion and constraint feature incompatibility Basic shape and assembly motion incompatibility Access and basic shape incompatibility Access and assembly motion incompatibility Parts warped Simultaneous engagement of several features No guide or clearance enhancements No operator feedback and=or feedback interference Mating part is hard to hold or handle
8.2.2
[Refs. on p. 265]
Most Likely Causes of Distorted Parts
Parts warped when made Distorted in assembly Feature tolerances and position robustness Over-constraint Compliant (flexible) parts, often panels are not constrained at enough points
8.2.3
Most Likely Causes of Feature Damage
Feature damage does not necessarily indicate a feature problem. This is one of the most common errors in diagnosis. Many times feature damage is a symptom, not the root cause.
Over-constraint Under-constraint Incompatibility between features and assembly motion Long-term creep or yield Damaged during assembly (see Difficult Assembly) Damaged during shipping and handling Poor processing, not process-friendly Abuse in usage Abuse or damage during service=removal Missing guide or clearance enhancements
8.2 Attachment Level Diagnosis
8.2.4
259
Most Likely Causes of Loose Parts
Feature damage (see above) Weak feature mounting area(s) on mating and base parts Difficult assembly (see above) Under-constraint Compliant parts do not provide a strong base for the constraint features
8.3
Feature Level Diagnosis
Only after all attachment level root causes are either fixed or ruled out can we begin to consider feature level root causes for the problem. Sometimes, new parts must be produced that reflect all the attachment level fixes before feature level causes can be identified. Recall the panel-to-opening-application example in Section 4. Obviously, however, it is desirable to identify and make feature level changes before new parts are made. If the problem is indeed a feature problem, simple changes to the feature dimensions may be possible. These are the easiest changes to make. If they do not fix the problem, then more difficult changes to the lock feature style or to the lock pair are indicated. The most common feature level problems are:
High assembly force, see Table 8.1 for recommended fixes. High feature strain or feature damage during assembly or disassembly, see Table 8.2. Low retention strength, and lock damage under loads, see Table 8.3. High separation force, see Table 8.4.
In Tables 8.1 through 8.4, the fixes are listed from top to bottom beginning with those that are easier to implement and moving down through the changes that are more difficult or costly. Ease of implementation was based on the following reasonings:
Changes to the lock retention mechanism are generally the easiest. Changes to the lock deflection mechanism are generally more difficult. Changes to the attachment system are generally the most difficult.
The recommended changes and the predicted interactions in these tables are written primarily with the cantilever beam style lock in mind. However, many of the changes apply to all lock styles. We know that fixing one feature problem may create another. For example, making a cantilever hook lock stronger to solve a problem with low retention strength may increase the assembly force and may also increase the strain in the hook. If the assembly force becomes too high or strain is excessive, a new set of problems will surface. Whenever a change is proposed, it is important to understand these interactions to avoid creating other problems.
Deflection mech.
Deflection mech.
2
2
Locking system
Locking system
Locking system
Locking system
3
3
3
3
3
Change part material
Change lock style
Make base area more flexible (Q-factor)
Decrease mating feature stiffness (increase deflection)
Redesign for a tip assembly motion
Design for sequential lock engagement
Decouple insertion and retention behaviors
Reduce beam width at end by tapering
Reduce beam width overall
Reduce beam thickness at end by tapering
Reduce beam thickness overall
Make beam longer
Make retention face shallower (decrease deflection)
Add dwell surface to catch
Add contour to insertion face
Reduce insertion face angle
Recommended change
—
—
reduce
reduce
—
—
reduce
—
—
reduce
reduce
reduce
reduce
—
—
—
Feature strain or damage during assembly or disassembly
—
—
worse
worse
—
—
improved
worse
worse
worse
worse
worse
worse
—
—
—
Retention strength or lock damage under loads
—
—
reduce
reduce
—
—
reduce
reduce
reduce
reduce
reduce
reduce
reduce
—
—
—
Separation force
Reducing high assembly force may also have these effects:
1
1
3
3
1
1
4
2
2
3
3
3
3
1
1
1
þ
?
?
1
1
0
0
0
1
1
1
1
1
1
0
0
0
Interactions Diagnosing Snap-Fit Problems
A ‘‘—’’ in the effects column indicates either no effect or effect cannot be predicted.
Locking system
Locking system
3
Locking system
Deflection mech.
2
3
Retention mech.
1
Deflection mech.
Retention mech.
1
Deflection mech.
Retention mech.
1
2
Retention mech.
1
2
Make change to
Ease
Table 8.1 Feature Level Solutions for High Assembly Force
260 [Refs. on p. 265]
Locking system
Locking system
Locking system
3
3
Change part material
Change lock style
Add guard enhancement feature
Make base area more flexible (Q-factor)
Decrease mating feature stiffness
Add visual enhancement feature
Add guidance enhancement feature
Decouple insertion and retention behaviors
Verify part design is process-friendly
Increase beam thickness at base by tapering
Reduce beam thickness at end by tapering
Reduce beam thickness overall
Make beam longer
Make retention face shallower (decrease deflection)
Verify part manufacturing process is correct
Recommended change
—
—
increase
reduce
reduce
—
—
reduce
—
increase
reduce
reduce
reduce
reduce
—
Assembly force
A ‘‘—’’ in the effects column indicates either no effect or effect can not be predicted.
Locking system
Locking system
3
3
Locking system
3
3
Deflection mech.
2
Locking system
Deflection mech.
2
3
Deflection mech.
2
Locking system
Deflection mech.
2
Locking system
Retention mech.
1
3
Process
1
3
Make change to
Ease
—
—
—
worse
worse
—
—
improved
—
improved
worse
worse
worse
worse
—
Retention strength or lock damage under loads
—
—
increase
reduce
reduce
—
—
reduce
—
increase
reduce
reduce
reduce
reduce
—
Separation force
Reducing high feature strain may also have these effects:
Table 8.2 Feature Level Solutions for High Feature Strain or Damage During Assembly or Disassembly
1
1
1
3
3
1
1
4
?
2
3
3
3
3
?
þ
?
?
2
1
1
0
0
0
?
2
1
1
1
1
?
Interactions
8.2 Attachment Level Diagnosis
261
Deflection mech.
Deflection mech.
2
2
Locking system
Locking system
Locking system
Locking system
Locking system
3
3
3
3
3
Change material
Change lock style
Make base area less flexible (Q-factor)
Increase mating feature stiffness
Add retainer enhancement feature
Add more lock features
Reorient lock to carry less load
Decouple insertion and retention behavior
Increase beam width overall
Increase beam thickness overall
Make beam shorter
Increase beam width at base by tapering
Increase beam thickness at base by tapering
Make retention face deeper (increase deflection)
Add contour to the retention face
Increase retention face angle
Load beam closer to neutral axis
Recommended change
—
—
increase
increase
—
—
—
reduce
increase
increase
increase
reduce
increase
—
—
—
Feature strain or damage during assembly or disassembly
—
—
increase
increase
increase
increase
—
reduce
increase
increase
increase
increase
increase
increase
—
—
—
Assembly force
—
—
increase
increase
increase
increase
—
reduce
increase
increase
increase
increase
increase
increase
increase
increase
—
Separation force
1
1
1
1
1
1
1
4
1
1
1
1
2
1
1
1
1
þ
?
?
3
3
2
2
0
0
3
3
3
2
2
3
1
1
0
Interactions
Diagnosing Snap-Fit Problems
A ‘‘—’’ in the effects column indicates either no effect or effect can not be predicted.
Locking system
Locking system
3
3
Locking system
Deflection mech.
2
3
Retention mech.
1
Deflection mech.
Retention mech.
1
Deflection mech.
Retention mech.
1
2
Retention mech.
1
2
Make change to
Ease
Changes to fix low retention strength or lock damage under load may also have these effects:
Table 8.3 Feature Level Solutions for Low Retention Strength or Lock Damage Under Load
262 [Refs. on p. 265]
Locking system
Locking system
Locking system
Locking system
3
3
3
3
Decouple insertion and retention
reduce reduce
—
Change material
Change lock style —
—
Make base area more flexible (Q-factor) reduce
Decrease mating feature stiffness (increase deflection)
Add assist enhancement feature
A ‘‘—’’ in the effects column indicates either no effect or effect can not be predicted.
Locking system
Locking system
3
3
—
Deflection mech. Reduce beam width overall
Deflection mech. Reduce beam width at end by tapering —
2
reduce
reduce
reduce
2
Deflection mech. Reduce beam thickness overall
Deflection mech. Reduce beam thickness at end by tapering
2
Deflection mech. Make beam longer
2
2
Retention mech. Reduce retention face angle
1
—
Retention mech. Make retention face shallower (decrease reduce deflection)
Feature strain or damage during assembly or disassembly
—
—
reduce
reduce
—
reduce
reduce
reduce
reduce
reduce
reduce
—
reduce
Assembly force
—
—
worse
worse
—
improved
worse
worse
worse
worse
worse
worse
worse
Retention strength or lock damage under loads
Reducing High Separation Force may also have these effects:
1
Ease Make change to Recommended change
Table 8.4 Feature Level Solutions for High Separation Force
?
?
3
3
1
4
2
2
3
3
3
1
3
þ
?
?
1
1
0
0
1
1
1
1
1
1
1
Interactions
8.2 Attachment Level Diagnosis
263
264
Diagnosing Snap-Fit Problems
Within each ease-of-change group in the tables, the suggested changes are ranked by the number of additional positive or negative interaction they may have on the attachment. Usually a negative effect is simply an incremental shift in a particular characteristic. A negative effect does not guarantee a new problem, just a movement toward a condition that will increase the likelihood of a problem. For each of the four types of problems addressed by the tables, these interactions were developed in terms of the other three problems.
8.4
Summary
This chapter described an attachment level approach to diagnosing and fixing the most common snap-fit problems. Problems were first defined as a broad range of situations including, but not limited to, the more obvious ones involving snap-fit feature damage and failure. Most importantly, an approach of addressing systemic causes before attempting feature level fixes is explained. The snap-fit diagnostic process is summarized in Fig. 8.2.
8.4.1
Important Points in Chapter 8
Do not mistake a symptom for a root cause. The root cause of many, if not most, snap-fit problems is at the attachment level, not the feature level.
Review all attachment level causes and solutions. Snap-fit problem is identified.
• Difficult assembly • Parts distorted • Feature damage
Evaluate attachment level changes.
• Loose parts
Review all feature level causes and solutions: • High assembly force If problem is not resolved.
• High strain or damage during assembly or disassembly • Low retention strength or lock damage under loads • High separation force
Figure 8.2 The diagnostic process for snap-fits
Verify performance after feature level changes.
8.4 Summary
265
Do not assume that a feature failure has a feature level root cause. Many attachment level problems result from improper constraint. Resolve all attachment level causes of a problem before attempting any feature level fixes. Some problems will be combination of both feature and attachment level causes. Always try the easiest fixes first. Most feature level changes in a snap-fit will have multiple effects. A change to fix one problem is very likely to change other behaviors and may create new problems.
Reference 1. Bonenberger, P.R., (1999), Solving Common Problems in Snap-Fit Designs, Western Plastics Expo, Jan, 1999, Long Beach, CA.
The goal of careful design is always to prevent problems from occurring in the first place but snap-fits do sometimes fail. They also have their share of assembly and usage problems. A full understanding of the various failure modes and their relationship to the most likely root causes can help one more quickly diagnose and solve problems. This, of course, minimizes the cost and time impact of fixing a problem but it also helps ensure that the proposed changes will indeed fix the problem. Nothing is worse than making changes to a product and finding that the problem still exists or has even gotten worse. Accurate diagnosis is particularly valuable during product development when prototype testing may indicate the need for improvements, yet time and cost constraints limit the available options.
8.1
Introduction
The root causes of many snap-fit problems are at the attachment level. Yet, many times, the first attempts to fix the problems are at the feature level, thus are doomed to failure or to cost much more than they should. When evaluating any snap-fit problem, even feature failure or damage, first verify that all attachment level requirements have been satisfied. If not, address them before attempting a feature level fix [1]. First, it is important to define ‘‘problem’’ because the term includes much more than simply feature breakage. A snap-fit problem is identified by the following symptoms:
Difficult assembly Short-term feature failure or damage Long-term feature failure or damage Part distortion or damage Part loosening and=or squeaks or rattles Unintended part release Service difficulty Customer complaints about ease of operation
It is important to remember that these are symptoms of a problem; they are not the root cause of the problem. Simply treating the symptom may not fix the real problem or it may create other problems in the attachment. Many of the above symptoms can have both attachment and feature level root causes. Sometimes, the root cause turns out to be a combination of several shortcomings. Figure 8.1 reflects the author’s personal experience in trouble-shooting snap-fit problems in products. Note the high incidence of causes related to attachment level issues and the high frequency of multiple root causes for problems.
256
Diagnosing Snap-Fit Problems
(a) Incidence of multiple root causes
[Refs. on p. 265]
Relative frequency
Applications with only one root cause Applications with two root causes Applications with three or more root causes
(b) Feature level vs. attachment level root causes Feature level
Relative frequency
Feature strength (retention) Feature behavior (assembly) Material properties
Attachment level
Installation options Constraint violations Enhancements missing
Figure 8.1 General trends in snap-fit problems
8.1.1
Rules for Diagnosing Snap-Fit Problems
Do not mistake a symptom for a root cause. This is important in any problem-solving effort. Always remember that the root cause of many, if not most, snap-fit problems is at the attachment level, not the feature level. Corollary to this rule is to also remember that many attachment level problems are related to improper constraint. Resolve all attachment level causes of a problem before attempting any feature level fixes. Recall that some problems are a combination of both feature and attachment level causes. Always try the easiest fixes first. Be aware that most feature level changes in a snap-fit will have multiple effects. A change to fix one problem is very likely to change other behaviors and may create new problems.
8.1.2
Mistakes in the Development Process
Tracing back through the development process can sometimes give clues as to the root cause of a problem. Most problems, whether they are attachment or feature level, are the result of mistakes made during the development process. Some of the more common mistakes are:
8.2 Attachment Level Diagnosis
257
Improper constraint in the attachment, characterized by: Over-constraint where features ‘‘fighting’’ each other can cause breakage during assembly or in service due to thermal expansion. Over-stress due to residual assembly forces can cause long-term failure. Under-constraint where: Features are carrying the wrong loads or excessive loads. Weak or compliant parts are expected to provide a rigid base for lock or locator features. Failure to fully consider material properties, including: Incomplete material property data available. Failure to consider plastic creep and thermal effects. Failure to anticipate assembly variables such as: Incompatibilities involving engage direction, assembly motion, feature positioning and feature style. Designed-in assembly frustration and difficult assembly. Failure to anticipate all possible end-use conditions, including: Failure to consider disassembly and service. Failure to consider all loads including unexpected or improper but possible load conditions, (such as dropping or striking a product). Failure to consider customer usage.
8.2
Attachment Level Diagnosis
Attachment level problems are often independent of the lock. In other words, they would be occurring regardless of the locking features style used. Understanding the key requirements of constraint, compatibility and robustness can help one recognize and resolve many attachment level problems. Remember too that certain enhancements are required for every application. Always verify the presence of these four enhancements, if any are missing, problems are likely. They are:
Guides Clearance Operator feedback Process-friendly
The four most common symptoms related to attachment level problems are:
Difficult assembly Parts distorted Feature damage Loose parts
For each of these symptoms, the most likely root causes are listed below.
258
Diagnosing Snap-Fit Problems
8.2.1
Most Likely Causes of Difficult Assembly
Over-constraint Assembly motion and constraint feature incompatibility Basic shape and assembly motion incompatibility Access and basic shape incompatibility Access and assembly motion incompatibility Parts warped Simultaneous engagement of several features No guide or clearance enhancements No operator feedback and=or feedback interference Mating part is hard to hold or handle
8.2.2
[Refs. on p. 265]
Most Likely Causes of Distorted Parts
Parts warped when made Distorted in assembly Feature tolerances and position robustness Over-constraint Compliant (flexible) parts, often panels are not constrained at enough points
8.2.3
Most Likely Causes of Feature Damage
Feature damage does not necessarily indicate a feature problem. This is one of the most common errors in diagnosis. Many times feature damage is a symptom, not the root cause.
Over-constraint Under-constraint Incompatibility between features and assembly motion Long-term creep or yield Damaged during assembly (see Difficult Assembly) Damaged during shipping and handling Poor processing, not process-friendly Abuse in usage Abuse or damage during service=removal Missing guide or clearance enhancements
8.2 Attachment Level Diagnosis
8.2.4
259
Most Likely Causes of Loose Parts
Feature damage (see above) Weak feature mounting area(s) on mating and base parts Difficult assembly (see above) Under-constraint Compliant parts do not provide a strong base for the constraint features
8.3
Feature Level Diagnosis
Only after all attachment level root causes are either fixed or ruled out can we begin to consider feature level root causes for the problem. Sometimes, new parts must be produced that reflect all the attachment level fixes before feature level causes can be identified. Recall the panel-to-opening-application example in Section 4. Obviously, however, it is desirable to identify and make feature level changes before new parts are made. If the problem is indeed a feature problem, simple changes to the feature dimensions may be possible. These are the easiest changes to make. If they do not fix the problem, then more difficult changes to the lock feature style or to the lock pair are indicated. The most common feature level problems are:
High assembly force, see Table 8.1 for recommended fixes. High feature strain or feature damage during assembly or disassembly, see Table 8.2. Low retention strength, and lock damage under loads, see Table 8.3. High separation force, see Table 8.4.
In Tables 8.1 through 8.4, the fixes are listed from top to bottom beginning with those that are easier to implement and moving down through the changes that are more difficult or costly. Ease of implementation was based on the following reasonings:
Changes to the lock retention mechanism are generally the easiest. Changes to the lock deflection mechanism are generally more difficult. Changes to the attachment system are generally the most difficult.
The recommended changes and the predicted interactions in these tables are written primarily with the cantilever beam style lock in mind. However, many of the changes apply to all lock styles. We know that fixing one feature problem may create another. For example, making a cantilever hook lock stronger to solve a problem with low retention strength may increase the assembly force and may also increase the strain in the hook. If the assembly force becomes too high or strain is excessive, a new set of problems will surface. Whenever a change is proposed, it is important to understand these interactions to avoid creating other problems.
Deflection mech.
Deflection mech.
2
2
Locking system
Locking system
Locking system
Locking system
3
3
3
3
3
Change part material
Change lock style
Make base area more flexible (Q-factor)
Decrease mating feature stiffness (increase deflection)
Redesign for a tip assembly motion
Design for sequential lock engagement
Decouple insertion and retention behaviors
Reduce beam width at end by tapering
Reduce beam width overall
Reduce beam thickness at end by tapering
Reduce beam thickness overall
Make beam longer
Make retention face shallower (decrease deflection)
Add dwell surface to catch
Add contour to insertion face
Reduce insertion face angle
Recommended change
—
—
reduce
reduce
—
—
reduce
—
—
reduce
reduce
reduce
reduce
—
—
—
Feature strain or damage during assembly or disassembly
—
—
worse
worse
—
—
improved
worse
worse
worse
worse
worse
worse
—
—
—
Retention strength or lock damage under loads
—
—
reduce
reduce
—
—
reduce
reduce
reduce
reduce
reduce
reduce
reduce
—
—
—
Separation force
Reducing high assembly force may also have these effects:
1
1
3
3
1
1
4
2
2
3
3
3
3
1
1
1
þ
?
?
1
1
0
0
0
1
1
1
1
1
1
0
0
0
Interactions Diagnosing Snap-Fit Problems
A ‘‘—’’ in the effects column indicates either no effect or effect cannot be predicted.
Locking system
Locking system
3
Locking system
Deflection mech.
2
3
Retention mech.
1
Deflection mech.
Retention mech.
1
Deflection mech.
Retention mech.
1
2
Retention mech.
1
2
Make change to
Ease
Table 8.1 Feature Level Solutions for High Assembly Force
260 [Refs. on p. 265]
Locking system
Locking system
Locking system
3
3
Change part material
Change lock style
Add guard enhancement feature
Make base area more flexible (Q-factor)
Decrease mating feature stiffness
Add visual enhancement feature
Add guidance enhancement feature
Decouple insertion and retention behaviors
Verify part design is process-friendly
Increase beam thickness at base by tapering
Reduce beam thickness at end by tapering
Reduce beam thickness overall
Make beam longer
Make retention face shallower (decrease deflection)
Verify part manufacturing process is correct
Recommended change
—
—
increase
reduce
reduce
—
—
reduce
—
increase
reduce
reduce
reduce
reduce
—
Assembly force
A ‘‘—’’ in the effects column indicates either no effect or effect can not be predicted.
Locking system
Locking system
3
3
Locking system
3
3
Deflection mech.
2
Locking system
Deflection mech.
2
3
Deflection mech.
2
Locking system
Deflection mech.
2
Locking system
Retention mech.
1
3
Process
1
3
Make change to
Ease
—
—
—
worse
worse
—
—
improved
—
improved
worse
worse
worse
worse
—
Retention strength or lock damage under loads
—
—
increase
reduce
reduce
—
—
reduce
—
increase
reduce
reduce
reduce
reduce
—
Separation force
Reducing high feature strain may also have these effects:
Table 8.2 Feature Level Solutions for High Feature Strain or Damage During Assembly or Disassembly
1
1
1
3
3
1
1
4
?
2
3
3
3
3
?
þ
?
?
2
1
1
0
0
0
?
2
1
1
1
1
?
Interactions
8.2 Attachment Level Diagnosis
261
Deflection mech.
Deflection mech.
2
2
Locking system
Locking system
Locking system
Locking system
Locking system
3
3
3
3
3
Change material
Change lock style
Make base area less flexible (Q-factor)
Increase mating feature stiffness
Add retainer enhancement feature
Add more lock features
Reorient lock to carry less load
Decouple insertion and retention behavior
Increase beam width overall
Increase beam thickness overall
Make beam shorter
Increase beam width at base by tapering
Increase beam thickness at base by tapering
Make retention face deeper (increase deflection)
Add contour to the retention face
Increase retention face angle
Load beam closer to neutral axis
Recommended change
—
—
increase
increase
—
—
—
reduce
increase
increase
increase
reduce
increase
—
—
—
Feature strain or damage during assembly or disassembly
—
—
increase
increase
increase
increase
—
reduce
increase
increase
increase
increase
increase
increase
—
—
—
Assembly force
—
—
increase
increase
increase
increase
—
reduce
increase
increase
increase
increase
increase
increase
increase
increase
—
Separation force
1
1
1
1
1
1
1
4
1
1
1
1
2
1
1
1
1
þ
?
?
3
3
2
2
0
0
3
3
3
2
2
3
1
1
0
Interactions
Diagnosing Snap-Fit Problems
A ‘‘—’’ in the effects column indicates either no effect or effect can not be predicted.
Locking system
Locking system
3
3
Locking system
Deflection mech.
2
3
Retention mech.
1
Deflection mech.
Retention mech.
1
Deflection mech.
Retention mech.
1
2
Retention mech.
1
2
Make change to
Ease
Changes to fix low retention strength or lock damage under load may also have these effects:
Table 8.3 Feature Level Solutions for Low Retention Strength or Lock Damage Under Load
262 [Refs. on p. 265]
Locking system
Locking system
Locking system
Locking system
3
3
3
3
Decouple insertion and retention
reduce reduce
—
Change material
Change lock style —
—
Make base area more flexible (Q-factor) reduce
Decrease mating feature stiffness (increase deflection)
Add assist enhancement feature
A ‘‘—’’ in the effects column indicates either no effect or effect can not be predicted.
Locking system
Locking system
3
3
—
Deflection mech. Reduce beam width overall
Deflection mech. Reduce beam width at end by tapering —
2
reduce
reduce
reduce
2
Deflection mech. Reduce beam thickness overall
Deflection mech. Reduce beam thickness at end by tapering
2
Deflection mech. Make beam longer
2
2
Retention mech. Reduce retention face angle
1
—
Retention mech. Make retention face shallower (decrease reduce deflection)
Feature strain or damage during assembly or disassembly
—
—
reduce
reduce
—
reduce
reduce
reduce
reduce
reduce
reduce
—
reduce
Assembly force
—
—
worse
worse
—
improved
worse
worse
worse
worse
worse
worse
worse
Retention strength or lock damage under loads
Reducing High Separation Force may also have these effects:
1
Ease Make change to Recommended change
Table 8.4 Feature Level Solutions for High Separation Force
?
?
3
3
1
4
2
2
3
3
3
1
3
þ
?
?
1
1
0
0
1
1
1
1
1
1
1
Interactions
8.2 Attachment Level Diagnosis
263
264
Diagnosing Snap-Fit Problems
Within each ease-of-change group in the tables, the suggested changes are ranked by the number of additional positive or negative interaction they may have on the attachment. Usually a negative effect is simply an incremental shift in a particular characteristic. A negative effect does not guarantee a new problem, just a movement toward a condition that will increase the likelihood of a problem. For each of the four types of problems addressed by the tables, these interactions were developed in terms of the other three problems.
8.4
Summary
This chapter described an attachment level approach to diagnosing and fixing the most common snap-fit problems. Problems were first defined as a broad range of situations including, but not limited to, the more obvious ones involving snap-fit feature damage and failure. Most importantly, an approach of addressing systemic causes before attempting feature level fixes is explained. The snap-fit diagnostic process is summarized in Fig. 8.2.
8.4.1
Important Points in Chapter 8
Do not mistake a symptom for a root cause. The root cause of many, if not most, snap-fit problems is at the attachment level, not the feature level.
Review all attachment level causes and solutions. Snap-fit problem is identified.
• Difficult assembly • Parts distorted • Feature damage
Evaluate attachment level changes.
• Loose parts
Review all feature level causes and solutions: • High assembly force If problem is not resolved.
• High strain or damage during assembly or disassembly • Low retention strength or lock damage under loads • High separation force
Figure 8.2 The diagnostic process for snap-fits
Verify performance after feature level changes.
8.4 Summary
265
Do not assume that a feature failure has a feature level root cause. Many attachment level problems result from improper constraint. Resolve all attachment level causes of a problem before attempting any feature level fixes. Some problems will be combination of both feature and attachment level causes. Always try the easiest fixes first. Most feature level changes in a snap-fit will have multiple effects. A change to fix one problem is very likely to change other behaviors and may create new problems.
Reference 1. Bonenberger, P.R., (1999), Solving Common Problems in Snap-Fit Designs, Western Plastics Expo, Jan, 1999, Long Beach, CA.