**2. Case study: Reliability design of a refrigerator drawer and handle system**

Figure 2 shows a refrigerator with the newly designed drawer and handle system and its parts. In the field, the refrigerator drawer and handle system had been failing, causing consumers to replace their refrigerators (Figure 3). The specific causes of failures of the refrigerator drawers during operation were repetitive stress and/or the consumer improper usage. Field data indicated that the damaged products had structural design flaws, including sharp corner angles and weak ribs that resulted in stress risers in high stress areas.

A consumer stores food in a refrigerator to have convenient access to fresh food. Putting food in the refrigerator drawer involves opening the drawer to store or takeout food, closing the drawer by force. Depending on the consumer usage conditions, the drawer and handle parts receive repetitive mechanical loads when the consumer opens and closes the drawer.

Figure 4 shows the functional design concept of the drawer and handle system. The stress due to the weight load of the food is concentrated on the handle and support slide rail of the drawer. Thus, the drawer must be designed to endure these repetitive stresses.

The force balance around the drawer and handle system cans be expressed as:

$$F\_{draw} = \mu \mathbf{W}\_{load} \tag{11}$$

The Reliability Design and Its Direct Effect on the Energy Efficiency 231

(a) Parameter diagram of drawer and handle system

(b) Design concept of mechanical drawer and handle system

Because the stress of the drawer and handle system depends on the food weight, the life-

*<sup>f</sup> draw load T AS AF A W*

*SF WW AF SF WW* μ

The normal ranges of the operating conditions for the drawer system and handle were 0 to 50℃ ambient temperature, 0 to 85% relative humidity and 0.2 to 0.24G vibration. The normal

() ( ) ( ) *nn n*

11 1 1 00 0 0

 === = 

μ

μ−− − == = (12)

2 22

(13)

**Figure 4.** Functional design concept of the drawer and handle system

where A is constant. Thus, the acceleration factor (*AF*) can be derived as

*n*

stress model (LS model) can be modified as follows:

**3. Laboratory experiments** 

**Figure 2.** Refrigerator and drawer assembly. (a) French refrigerator (b) Mechanical parts of the drawer: handle ①, drawer ②, slide rail ③, and pocket box ④

**Figure 3.** A damaged product after use

(b) Design concept of mechanical drawer and handle system

**Figure 4.** Functional design concept of the drawer and handle system

Because the stress of the drawer and handle system depends on the food weight, the lifestress model (LS model) can be modified as follows:

$$\mathbf{T}\_f = A \begin{pmatrix} \mathbf{S} \end{pmatrix}^{-n} = A \begin{pmatrix} F\_{dran} \end{pmatrix}^{-n} = A \begin{pmatrix} \mu \mathbf{W}\_{load} \end{pmatrix}^{-n} \tag{12}$$

where A is constant. Thus, the acceleration factor (*AF*) can be derived as

$$AF = \left(\frac{S\_1}{S\_0}\right)^n = \left(\frac{F\_1}{F\_0}\right)^2 = \left(\frac{\mu \mathcal{W}\_1}{\mu \mathcal{W}\_0}\right)^2 = \left(\frac{\mathcal{W}\_1}{\mathcal{W}\_0}\right)^2\tag{13}$$

#### **3. Laboratory experiments**

230 Energy Efficiency – The Innovative Ways for Smart Energy, the Future Towards Modern Utilities

(a) (b)

handle ①, drawer ②, slide rail ③, and pocket box ④

**Figure 3.** A damaged product after use

**Figure 2.** Refrigerator and drawer assembly. (a) French refrigerator (b) Mechanical parts of the drawer:

The normal ranges of the operating conditions for the drawer system and handle were 0 to 50℃ ambient temperature, 0 to 85% relative humidity and 0.2 to 0.24G vibration. The normal

number of operating cycles for one day was approximately 5; the worst case was 9. Under the worst case, the objective drawer open/close cycles for ten years would be 32,850 cycles (Table 3).

The Reliability Design and Its Direct Effect on the Energy Efficiency 233

Figure 5 shows ALT equipment and duty cycles for the repetitive food weight force, *Fdraw* . For the ALT experiments, the control panel on top of the testing equipment started and stopped the drawer during the mission cycles. The food load, *F*, was controlled by the accelerated weight load in the drawer storage. When a button on the control panel was

Figures 6(a) and 6(b) show the failed product from the field and the 1st accelerated life testing, respectively. The failure sites in the field and the first ALT occurred at the drawer handle as a result of high concentrated stress. Figure 7 shows a graphical analysis of the ALT results and field data on a Weibull plot. For the shape parameter, the estimated value on the chart was 2.0. For the final design, the shape parameter was determined to be 3.1. These methodologies were valid for pinpointing the weak design responsible for failures in

(a) Failed product in field (b) Failed sample in first accelerated life testing

**Figure 6.** Failed products in field and first ALT

pushed, mechanical arms and hands pushed and pulled the drawer.

**4. Parametric ALTs with corrective action plans** 

the field and 1st ALT.


**Table 3.** Operating number of a drawer

For the worst case, the food weight force on the handle of the drawer was 0.34 *kN*. The applied food weight force for the ALT was 0.68 *kN*. With a quotient, *n*, of 2, the total *AF* was approximately 4.0 using equation (13).

The parameter design criterion of the newly designed drawer can be more than the target life of *B1* = 10 years. Assuming the shape parameter β was 2.0 and *x* was 0.01, the test cycles and test sample numbers calculated in Equation (7) were 67,000 cycles and 3 units, respectively. The ALT was designed to ensure a *B1* life of 10 years with about a 60% level of confidence that it would fail less than once during 67,000 cycles.

(a) ALT equipment and controller

(b) Duty cycles of repetitive food weight force on the drawer **Figure 5.** ALT equipment and duty cycles.

Figure 5 shows ALT equipment and duty cycles for the repetitive food weight force, *Fdraw* . For the ALT experiments, the control panel on top of the testing equipment started and stopped the drawer during the mission cycles. The food load, *F*, was controlled by the accelerated weight load in the drawer storage. When a button on the control panel was pushed, mechanical arms and hands pushed and pulled the drawer.
