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Welcome to this months newsletter! In this issue you'll find:

• An article by Leonardo Chwif of Simulate Tecnologia de Simulação on Assembly Lines with Returnable Pallets
• Some Hints and Tips on using SIMUL8
• and Are you an R7?

Assembly Lines with Returnable Pallets in SIMUL8 By Leonardo Chwif

Why Assembly Lines with Returnable Pallets?

Assembly lines with returnable pallets (ALRP) have an important manufacturing system frequently adopted in plants. One of their major advantages is to better control the flow of products and its position to perform a transportation or work, since they have, in majority of cases, independent movement in each station. The figure below shows a single "O" Shape Assembly Line with Returnable Pallets. Zone 1 refers to loaded pallets (pallets with products) and zone 2 is the return zone. In this case there is one loading point A (La) and three unloading points (Ua, Ub and Uc). There are lots of other possible configurations of ALRP´s.

The More Pallets the Better?

One of the most important question in the dimensioning of ALRP is the ideal number of pallets. Contrary to common sense more is not the better. Generally speaking the productivity of a ALRP where the independent variable is the number of pallets follows the figure below:

Of course a low number of pallets in the system leads to a production shortage. In fact you can see from the graph, there is an optimum number of pallets that should be used in the system. If we are dealing with simple "O" shape ALRP, the ideal number of pallets could be given by the approximation:

p = l/(c*s) + m

where
p = recommended number of pallets
l = length of the loop in feet
c = cycle time of slowest station (seconds)
s = speed of the conveyors (ft/sec)
m = number of machines (not stations)

This formula however is only an approximation. It doesn't take into account all the system details. In the case of more complex lines there is no rule of thumb and the answer to determine the ideal number of palettes is: Simulation!

To show how effective simulation is in this case, I developed a model of ALRP system from an automobile industry. Before simulation the proposed number of pallets was 600. By doing the simulation, I found that only around 400 pallets were indeed needed. In this case since the cost of each pallet is USD 650,00, simulation saved US $ 13,000,000!

How to model ALRP´s systems in SIMUL8?

One way to build ALRP is to use SIMUL8´s conveyor functionalities. However to gain more control and independence I prefer to model each ALRP segment as a Work Center (WC). So an ALRP will be a set of WCs connected in series. If the ALRP segment is only a transportation segment, the WC time will be the time that a pallet takes from the center of one segment to the center of the next segment. If the ALRP segment process also the product, then the WC time will be the sum of the processing time and the transporting time. The major inconvenience of this approach is that you cannot separate the transportation and processing busy time statistic from the Work Center. If a WC is say 80% utilized, this is the sum of transportation and processing time. The figure below show a "O" shape ALRP with 10 segments built by WC´s. Of course the graphic of each segment should be altered to resemble real segments.

There are two issues to model. First is how we create the products that will flow though the line. The easiest way is to create a Work Entry Point object and a queue and link it to the loading station. Then in the Work Entry Point Properties use a very high number for inter-arrival times (much more than the simulation time) and check the button "First at Start Time". By doing this you guarantee that just only one entity will enter the model. But what if "I need 10 pallets?" The "magic" is to set the Batching property to the overall number of pallets in system (that can be numerically fixed or associated with a variable - in our example Num_pallet), so the Work Entry Point will generate the exactly number of pallets given by the Batching Value. In this case, it is also important to set up the correct Warm up time because there is a time lag to let the system accommodate.

The second issue is how to change between loaded pallet and empty pallet. This is easy: after the unloading station just change the Work Item Image to an empty pallet and after the loading station change it to a loaded pallet graphic. It is advisable that each work center displays the pallet figure, so just check the Work Item box within Work Center Graphics properties.

Case Study

The objective of this simulation study is to determine exactly the ideal number of pallets in a ALRP System of a Home Appliance Manufacturing Company. In this case the shape of the system is not so common because there is a division of flow (because of duplications stations). This ALRP System and the flow are depicted below. Point A and B is respectively the loading and unloading station.

Full Size Image

This simulation study enables me to determine the ideal number of pallets in the system, reinforcing the saying that "more is not better". In fact due to internal line priority logic, strange behavior occurs if the number of pallets is high. But what was the ideal number of pallets? I will not tell you but I will let you experiment yourself with the model. Please Download the model here.

Conclusions

ALRP´s are important manufacturing systems. As with any other manufacturing system it is important that they are well dimensioned to maximize productivity while reducing investment cost. In this article we saw how an ALRP system is easily and straightforward modelled in SIMUL8. Simulation allow us to determine the ideal number of pallet in any concept of ALRP´s with different sizes and complexity. In this case the plug-in OptQuest could be easily used to determine the ideal number of pallets.

For further information please contact Leonardo at Leonardo@simulate.com.br

Hints and Tips

• Did you know you can change the spreadsheet opened with the button interactively by pressing CTRL when you press it. This will open up the information store where you should select the spreadsheet you wan to view. Then the next time you click the button the spreadsheet you choose will open automatically.
• If you're using the simulation time variable in Visual Logic and want to convert it to different time units you can do this easily using the mathematical functions DAY, HOUR and MINUTE provided with SIMUL8. You can access these functions through the fomula editor in SIMUL8.

If you any questions about these tips or anything else to do with SIMUL8 email support@SIMUL8.com we'd be happy to help you out. (Remember technical support is free!)

Are You an R7?

SIMUL8 R11 is due to be released this summer. Upgrades are only available to those who have a SIMUL8 Version that is no older than 2 previous versions. This is to encourage users to use the most up to date SIMUL8 and to reward those that do. We skipped R8 of SIMUL8 so that means that R7 users are about to lose the chance to qualify for the upgrade.

However our kindly CEO has agreed that if you upgrade to R10 in April then you will qualify for the upgrade to R11 for free, Check out Why R10? to see all the great features made available in R10. Watch this space for what's going to be available in R11!

To qualify for this offer you will need to purchase the upgrade on-line or contact us. Please quote offer code ZX2451.

If you have any questions generated from, or comments about this newsletter then email Newsletter@SIMUL8.com