Midterm II Flashcards
(46 cards)
What is the Variability Law? describe how its effects and what types of variability there are.
Variability degrades performance. If any kind of variability (process, flow, batching) is increased something has to give.
- inventory will build up
- Throughput will decline
- lead time will grow
How does variability and utilization interact?
- more variable system = possibility for congestion and effect multiply
- utilization effects are nonlinear
- importance of bottleneck management
high utilization results in more congestion and the variability is based upon the machine.
How to reduce variability with a buffer?
must have of these: Inventory Capacity Time or you are stuck with: long cycle times and high inventory levels, wasted capacity, long throughput, long lead times/poor customer service.
What are the difference between flexible buffers and fixed buffers?
Flexibly buffers are more effective. You can use capacity, inventory and time in more than one way to reduce amount of total buffering required.
How is material conserved in a system?
- what flows in will flow out as good product or scrap.
- Flow propagates and non-bottlenecks can become major problems
What is the capacity law? how does it work?
its best to plan reduce release rates before a “system blows up”. All plants will releases at an average rate less than average capacity.
How does CT with overtime compare to regular CT?
CT overtime is less than CT w/0
When is variability most disruptive along a line? why?
high process variability at front of the line propagates downstream. The trend to reduce variability at front of the line than back end.
How does cycle time and utilization interact?
as U approaches 1, then WIP or CT approaches infinity. System performance is highly sensitive to release rates at high utilization.
What is the utilization law?
if a station increases utilization w/o making any other changes, avg WIP & CT will increase in a highly nonliner fashion.
What kind of batching is there? what are the differences?
1) Process: serial batching and parallel.
- related to length of set up
- longer the set up, larger the lot size required for same capacity as move.
2) Move (transfer)
- smaller move batch = shorter CT and more material handling
How does a process batch affect capacity?
As a batch size increase:
- queue decreases
- CAPACITY increases.
- Wait to batch increases
- wait in time for batch increases
How to reduce variability of batches?
1) reduce set up times = small and efficient batch sizes
2) CT reduction for batch size >1
How does CT and move batching interact?
CT increases proportionally to size of move batch.
choosing move batch decreases measure CT because there isn’t a large wait time for batching and unbatching.
What is the law of assembly ?
the performance of assembly station is degraded by increasing any of the following?
a) # of components being assembled
b) variability of component arrivals
c) lack of coordination between component arrivals
How does matching become an important source of delay in assemby sytems?
lack of synchronization caused by variablity can cause significant build up of WIP & delay assembling components.
How to reduce variability of assembly systems?
1) Reduce CT
a) queue: reduce utilization (arrival/process rates) and variability (failures.setups).
b) batch: reduce delay at stations (optimize batch size) and between them (reduce move batching).
c) batch: lack of synchronization (improve coordination and reduce # of components)
2) Increase TH
a) reduce blocking / starving (add a buffer)
b) increase capacity (add equipment or increase operation time).
What are the sources of variability?
1) Process: set ups, random outages, quality problems
2) Flow: way work is moved or released.
What does variability measure?
CV of effective process times and CV of interarrival times
What are the components of Process Variability?
failures, setups, deflate/inflate of capacity, long infrequent disruptions are worse than short frequent ones.
What are the consequences of variability?
congestion, propagation, variability and utilization interact, pooled variability is less destructive.
How is CV key measure of item variability? what are some examples?
use this ratio to make comparisons to variability in process time and flow.
EX) Tortoise and the Hare: long infrequent shortages inflate CV more
EX) Milk Cartons: small carton is more variable
How does variability propagate in low or high utilization?
Low utilization: flow variability determined from arrival.
IN CV = OUT CV
High utilization: flow variability determined by station process time.
STATION CV = OUT CV
What is the largest component of cycle time? what is its cause?
Waiting time: high utilization levels and high levels of variability increase cycle time.
