Reducing Bedding Dispenser DefectsReducing Bedding Dispenser Defects

Center for Comparative Medicine, Massachusetts General Hospital, Charlestown, MA

BackgroundCages being processed through a tunnel washer then through an automated bedding dispenser were ending up without bedding. A process flow analysis was performed to ascertain problems, look for root causes of those problems, and establish countermeasures. Throughput of mouse cage bottoms was analyzed starting with their exit from the clean side of a tunnel washer and continuing through an automated bedding fill station (Figure 1). Cages move off the tunnel washer conveyor belt bottom-side up (Figure 2), slide down a short stainless steel ramp (Figure 3) to a mechanical inverter (Figure 4) where they are flipped (Figure 5), drop onto conveyor rollers of the bedding dispenser (Figures 6) and pass through a cascading stream of hardwood chip bedding (Figure 7) to fill the cages).

1

Figure 6

Figure 1

Figure 3

Variables observed included tunnel washer belt speed (adjustable), bedding dispensing rate (adjustable), cage orientation on the tunnel washer belt that affects cage flip behavior, and how cages landed onto the bedding fill station rollers. Problems identified included catch points along route through the tunnel washer, poor alignment of elements, bent cage inverter feed tray, inconsistent cage inversion, and cages bouncing on hard plastic bedding fill station rollers, all of which contributed to cages not positioned to retain the intended amount of bedding (Figure 8).

Materials and Methods Results and DiscussionAny cage arriving at the bedding cascade other than top-side up was recorded as a defect event. Initially, it was determined at the outset that at ~10% of cages (50 out of a run of 500) were defective, i.e., completing the filling cycle without bedding and requiring their manual transfer to the bedding fill station entrance. After all countermeasures were implemented, the resultant defect rate dropped to 1.0% (Table 1), a 90% reduction.

Figure 9

Figure 10 Figure 11

Tunnel Washer conveyor belt

RampMechanical inverter

Bedding Dispenser

Figure 4

Figure 5

Figure 2

Figure 7

Figure 8

Countermeasures included 1) aligning the bedding fill station to the tunnel washer, 2) straightening the cage inverter feed tray, 3) standardizing how cages are placed on the tunnel washer belt, 4) controlling the flip process, and 5) cushioning the first 4 bedding fill station rollers where the flipped cages land. Common materials from home improvement outlets were used, such as installing a plastic curtain over the mechanical inverter (Figure 9) and wrapping the first four transport rollers of the bedding fill station initially with sheet foam insulation and duct tape (Figure 10) and finally with foam pipe insulation and spray adhesive (Figure 11).

Table 1

Run Batch size Defects Defect rateA 500 3 0.6%B 250 1 0.4%C 380 5 1.3%D 160 5 3.1%E 310 2 0.6%

totals 1600 16 1.0%

After all countermeasures were implemented, it was calculated that almost 700 labor hours (1/3 FTE) of re-work were avoided on an annual basis. This translates to $13,000 in yearly savings in combined wages and benefits, compared to less than a one-time investment of ~$1,000 in time and materials to generate throughput data and modify transport rollers on the bedding fill station. This exercise demonstrates how conducting a process-flow analysis of machine operations can highlight inefficiencies that may lead to substantial productivity improvements via simple solutions.