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Computerized materials handling and facility layout design
Computers ind, Engng Vol. 17, Nos 1-4, pp. 55-60, 1989 Printed in Great Britain. All rights reserved
CO~FEERX~R~_
MATERIALS
0360-8352/89 $3.00+0.00 Copyright © 1989 Pergamon Press plc
HANDLING
AND FACILITY LAYOUT DESIGN
M. Reza Ziai and Dileep R. Sule Department
of Mechanical and Industrial Engineering Louisiana Tech University Ruston, LA 71272
can be made on an objective basis, a tentative design of the facility is usually made knowing what is to be produced, how much, what machines, how many departments and their sizes plus relative interactions between them. Using this layout arrangement, estimate of the interdepartmental production rates, number of different product types, and their corresponding characteristics, one will be able to design the least cost MH system. Results of the first part [14] of this two stage design problem has been used to produce the corresponding MH system. Two influential factors in the equipment selection process are the cost and utilization. These two factors are also compatible. Selection of a suitable equipment that depends on materials characteristics has not been explicitly developed [4, ii, 12] because of the difficulty to quantify them. The systematic approach proposed here, identifies and quantifies the primary factors while devising an algorithm for the selection of the most suitable equipment.
ABSTRACT
We are proposing a systematic approach to the design of materials handling (MH) system that includes an algorithm to select the most suitable equipment. Initially, a conveyor system is proposed while determining its associated cost. To seek improvements of the system in terms of cost and equipment utilization, the possibility of supplementing or replacing the conveyor with one or more fork-lift trucks (trucks) will be investigated. The final MH system configuration will consist of conveyors and trucks so that total system optimization will be achieved. The literature on MH design is very limited and often this problem is not directly coupled with the layout problem [4]. The model proposed uses the results of a tentatively designed layout and two important characteristics of the materials, namely size and weight. Facility in this paper refers to a manufacturing plant with low to medium production volume in a process oriented environment. Facility layout and materials handling are two highly interdependent problems. The location of departments, the corresponding distances and production rates dictate the cost of MH. On the other hand, MH costs between departments, influence the departmental arrangements [i0]. An iterative process is developed between the layout design and MH selection problems.
~i~ODOLOGY
Conveyors and fork-lift trucks are the two most common means of transporting materials throughout a facility. There are several types within each with corresponding costs. Selection of a type depends on the application, size and weight of the loads, and operating environment, among other things. In order to get realistic data and to learn how industry designs and selects its equipment, several dealers of these two MH equipment types, were interviewed and given a questionnaire. The Material Handling Institute [MHI, 6] and the Modern Material Handling magazine were also contacted to furnish information about MH equipment selection and case studies. As the result of the field survey and literature, review, the following approach for the design of the equipment is presented. CONVEYORS Used for
INTRODUCTION
Design of a new facility needs a great deal of data which can only be estimated. For example, the cost of transportation, and the production volume will not be known with certainty until the production has actually started and the order has been received. This information is estimated based on historical data and/or forecasts. A larger portion of the data is, therefore, subjective. This underlies the necessity of a design tool to investigate different production and MH scenarios. Before calculations 55
56
Proceedings of the l lth Annual Conference on Computers & Industrial Engineering
transporting, storing, and accumulating materials as single units, c o l l e c t e d units, and bulk, on fixed paths w h i c h m a y be horizontal, inclined, or vertical. Pree n g i n e e r e d p r o d u c t s are used in the i n d u s t r y to c o n f i g u r e the suitable c o n v e y o r system. This a p p r o a c h is u s e d in this paper too. For the p u r p o s e of this research, the f o l l o w i n g a s s u m p t i o n s are made: * C o n v e y o r s are u s e d only to transport materials, * O n l y h o r i z o n t a l and rectilinear m o v e m e n t s take place, * Belt or roller conveyors are used * R e v e r s i b l e d r i v e s and variable speed o p t i o n s may be used, * Loads w i l l be single items, * T r a n s f e r m e c h a n i s m s will be used to c o n n e c t c o n v e y o r s , * L o a d i n g / U n l o a d i n g are p e r f o r m e d m a n u a l l y or by robots where no shock is e x p e r i e n c e d by the conveyor. The time for l o a d i n g / u n l o a d i n g is ignored FORK-LIFT TRUCKS Very v e r s a t i l e e q u i p m e n t w h i c h can carry heavy loads on various paths h o r i z o n t a l l y or vertically. T h e y are mainly used for intermittent transportation. C a r r y i n g c a p a c i t y of the truck per trip m a y be increased g r e a t l y by p a l l e t i z i n g and stacking. No other d e v i c e is needed for loading and unloading. F o r k - l i f t trucks may be c l a s s i f i e d b a s e d on the following features: • C a p a c i t y - 2000 ibs to 80,000 ib, • N e e d of rider, • E l e c t r i c m o t o r or internal c o m b u s t i o n m o t o r (Gasoline,Diesel,LP) • T r a n s m i s s i o n - S t a n d a r d or automatic • Tires - C u s h i o n type used indoors p n e u m a t i c type for outdoors and rough indoors, • S i t - d o w n for wide aisles (greater than 12 feet) and s t a n d - u p used in n a r r o w aisles. Despite their attractiveness, f o r k - l i f t trucks should be used only where and when they are needed b e c a u s e of their h i g h o p e r a t i n g cost. In this research fork-lift trucks will be used under the following conditions: * Indoors, * Low capacity, up to 5000 ibs, A SYSTI~ATIC DEVELOPH~RT
APPP~ACHTO
MODRL
In o r d e r to decide the type and number of material handling equipments, a great deal of data is n e e d e d a b o u t the plant layout and the materials. One m i g h t start with the results of a c o m p u t e r layout routine such as C O M L A D [14] to establish the (I) number of departments, (2) i n t e r d e p a r t m e n t a l distances, and (3) product flow information which
includes the flow route and required d a i l y rate among the departments. Since the m o d e l is c o m p u t e r i z e d and interactive, the analyst (user) f u r n i s h e s the n e c e s s a r y information b a s e d on the e x p e c t e d values for the parameters. For example, knowing the w i d e s t item among several which may be c o n v e y e d b e t w e e n two departments and the heaviest one, a suitable c o n v e y o r is selected• The belt width of this c o n v e y o r must be at least as wide as the g i v e n w i d t h and its live load c a r r y i n g c a p a c i t y per foot and for its entire length must not be exceeded. To select the suitable truck one needs to k n o w the number of pallets and w e i g h t of e a c h w h i c h are to be transported on a given trip. A pallet loading a l g o r i t h m may be e m p l o y e d for a g l v e n size pallet to a r r a n g e as m a n y items as possible on that pallet. The resultant total w e i g h t per p a l l e t and knowing how many are stacked per trip will determine the required truck capacity• N u m b e r of items per trip and the s p e e d of the truck will tell us w h e t h e r that truck is capable of h a n d l i n g the r e q u i r e d daily rate. P r e - e n g i n e e r e d products will be used for the design of the MH equipment. A database is c r e a t e d w h i c h c o n t a i n s typical prices for the type of c o n v e y o r and truck used in the research. THE
COST
EVALUATION
ROUTINE
The i n t e g r a t e d approach w h i c h is used for the MH cost analysis i n c l u d e s the f o l l o w i n g factors: I n v e s t m e n t - Such as initial prlce, and i n s t a l l a t i o n charges, • F i x e d c h a r g e s - Depreciation, c a p i t a l i z a t i o n (interest), taxes, insurance, other, • V a r i a b l e cost - Labor, m a i n t e n a n c e , other, E q u i p m e n t cost is based on the total cost per hour, this includes the fixed cost and the variable cost. The total h o u r l y cost for c o n v e y o r includes: O p e r a t i n g cost (OC) m a i n t e n a n c e and fuel. F i x e d c o s t (FC) - Initial cost and cost of capital. The above cost is c o m p u t e d based on the economic life ( five is industry practice - otherwise, the analyst should specify), of the e q u i p m e n t and the number of o p e r a t i n g shifts (one o p e r a t i n g shift of 40 hours a w e e k for 50 working weeks amounts to 2,000 hours per year). Salvage value of c o n v e y o r s is u s u a l l y ignored since it is too minimal unless the a n a l y s t specifies a value.
Ziai and Sule: Computerized materials handling The t o t a l h o u r l y cost lift trucks is as follows: • O p e r a t i n g cost (OC) -
for
fork-
m a i n t e n a n c e , operator, and fuel • F i x e d cost (FC) - Initial cost plus the cost of capital. The same procedure as the c o n v e y o r s y s t e m ' s is used to compute the cost. The salvage value may be represented as percentage of the i n i t i a l cost (e.g. 20%). In o r d e r to specify the type of c o n v e y o r and s u b s e q u e n t l y its cost, the f o l l o w i n g p a r a m e t e r s are defined: - C a p a c i t y (max. live load (ibs) on the c o n v e y o r for the given length, S - S p e e d (feet per minute - fpm); 60 fpm w i l l be u s e d as the standard u n l e s s h i g h e r speeds are required in w h i c h case v a r i a b l e speed motor d r i v e s w i l l be used, L - C o n v e y o r length (feet - ft), D - D e p a r t m e n t a l d i s t a n c e (ft), CL- C l e a r a n c e b e t w e e n two loads (ft pallets, boxes or single items), P - M a x i m u m n u m b e r of items per b a l l e t for item type k, n - Number of items per pallet/box, r ~ - T o t a l rate (items/day) between ~ 4 p a r t m e n t i & j and j & i, M k- M a x i m u m n u m b e r of k items allowed on a g i v e n length of a conveyor• The m a x i m u m number of items on the c o n v e y o r k n o w i n g the weight of the h e a v i e s t items will determine the required live load capacity. The d i m e n s i o n s of the largest item will i n d i c a t e the w i d t h of the conveyor• U s i n g these d i m e n s i o n s and the weight of the item, one is able to determine how m a n y p o u n d s per foot will be placed on the conveyor• The required speed is also computed knowing the rate between the two departments, number of items ( i n c l u d i n g the clearance) on a given length of conveyor, and the interdepartmental distance. After all the above d a t a are calculated one is able to use the p r e - e n g i n e e r e d p r o d u c t tables to find out the cost of the conveyor. The daily u t i l i z a t i o n of the conveyor is also determined. If it exceeds the given value, then other solution alternatives will be followed as e x p l a i n e d in the s e l e c t i o n algorithm. The type of f o r k - l i f t truck and its cost will be based on the following parameters: A - The m a x i m u m size (base area) load, - The m a x i m u m load c a r r y i n g c a p a c i t y per trip, S - L o a d c a r r y i n g s p e e d (fpm), D - I n t e r d e p a r t m e n t a l d i s t a n c e (ft), ri5- T o t a l rate (items/day) between d ~ p a r t m e n t s i & j and j & i,
57
n - N u m b e r of items per pallet, m - N u m b e r of p a l l e t s stacked, R - N u m b e r of trips r e q u i r e d per p e r i o d (day). The above p a r a m e t e r s are used to c o m p u t e the r e q u i r e d c a p a c i t y of a truck and number. Cost is then determined using tables and the c u r r e n t i n d u s t r y cost estimates to o p e r a t e the trucks. In the selection a l g o r i t h m c e r t a i n number of trucks are a s s i g n e d a m o n g the departments r e q u i r i n g m i n i m u m cost.
A MATHEMATICALMODELFOREQUIPM~RT SRT~CTION An integer linear programming model is proposed to obtain the o p t i m u m s e l e c t i o n of conveyors and trucks (or any other two classes of m a t e r i a l h a n d l i n g systems). The cost coefficients are calculated as discussed previously. In effect, different types of conveyors and trucks are being considered here represented only by their c o r r e s p o n d i n g costs.
Minimize
2 N N Z = ~ E E Cki j Xki j k=l i=l j=l i%j j#i
S u b j e c t to XI(N_I,N) S 1 not to allow m o r e than one conveyor s y s t e m b e t w e e n two d e p a r t m e n t s (here 1 & 2) X2(N_I,N) S [ t/8] 8 Xk(i,j) { 0 , 1 } for k = 1,2 i = 1,2, .... N j = 1,2,...,N Where: Cki~ = total cost of equipment type k n ~ e d e d per day b e t w e e n d e p a r t m e n t s i and j, Xki 5 = n u m b e r of equipment type k n e e d e d ~ p e r day b e t w e e n d e p a r t m e n t s i and j, t = hours of o p e r a t i o n for f o r k - l i f t trucks, o b t a i n e d from the cost c a l c u l a t i o n section (the 8 stands for the eight hour d a i l y operation), N = number of departments. [t/8] = the largest integer value, This is a s i m p l i s t i c model w h i c h gives an i n i t i a l solution• The above f o r m u l a t i o n of the p r o b l e m is not able to h a n d l e all the different situations that may arise. For example, r e a s s i g n i n g trucks to other departments when they are only u t i l i z e d for instance, 50% of the time, or r e a s s i g n i n g the operator to other d u t i e s and k e e p i n g the truck idle, or the case w h e n a truck is
58
Proceedings of the 1lth Annual Conference on Computers & Industrial Engineering
r e q u i r e d in a d d i t i o n to a conveyor system to handle certain type of item. In order to solve the equipment s e l e c t i o n p r o b l e m in its entirety, the following algorithm has been devised which allows better u t i l i z a t i o n of the equipment.
e f f i c i e n t s o l u t i o n is sought. For any solution, each path may have any of the three possible configurations, (i) only a c o n v e y o r system, (2) o n l y a truck system, or
AN A L G O R I T H M TO SOLVE T H E M H EQUIPMENT SELECTION An a l g o r i t h m will be implemented to find the number of conveyors and f o r k - l i f t trucks for the MH system proposed in an efficient manner. Before we start the algorithm the following calculations and a s s u m p t i o n s are made:
PHASE I:
(a) O n l y one conveyor system is considered practical between each pair of d e p a r t m e n t s . (b) The time, utilization, and cost a n a l y s i s will be c a r r i e d out for a conveyor system as the initial (partial) solution. A table will be generated depicting the cost and u t i l i z a t i o n of this system. (c) A c o n s t a n t speed of S = 60 fpm w i l l be u s e d b a s e d on i n d u s t r y practice. This speed of the c o n v e y o r m a y be i n c r e a s e d to a c c o m m o d a t e the extra daily handling requirements. But the i n c r e a s e will not be b e y o n d TOPSPD = 280 fpm, for practical considerations. This will be one of the physical constraints on the c o n v e y o r system. (d) O n l y an integer number of trucks are assigned to handling tasks. It is the crux of the algorithm to assign the a v a i l a b l e time of the t r u c k to other routes in an e f f i c i e n t manner. One or more truck units are a l l o w e d to be used along with a conveyor if the e c o n o m i c s of the o p e r a t i o n s d i c t a t e it. (e) H i g h e r c a p a c i t y trucks will be u s e d to h a n d l e the extra MH task if needed. T h e s e c a p a c i t i e s will be from 2000 lbs to 5000 lbs in i n c r e m e n t s of I000 ibs. This is a suitable range since the h a n d l i n g tasks take place in an indoor environment. H i g h e r c a p a c i t y trucks u s u a l l y need extra wide (more than 12 feet) aisles. (f) A u t i l i z a t i o n of 8 = %85 will be a s s u m e d for each e q u i p m e n t to c o m p l y w i t h the i n d u s t r y practice. Therefore, one unit of e q u i p m e n t is r e c o m m e n d e d for e a c h ~ utilization. Use of e x c e s s time available on the equipment are discussed in the i m p r o v e d phase of the algorithm. STEPS OF THE ALGORITHM: There are two phases to the algorithm. The first one is the c o n s t r u c t i o n phase where an initial s o l u t i o n is produced, and a second phase called improvement where an
(3) c o m b i n a t i o n trucks systems.
of
a
conveyor
and
CONSTRUCTION OF AN INITIAL FEASIBLE SOLUTION (THE ASSIGNMENT ARRAY) S t e p I.l. Design a conveyor system for the e n t i r e task of MH using a c o n s t a n t speed of S while i d e n t i f y i n g the paths for w h i c h this system will not be feasible. Calculate the total cost and e q u i p m e n t utilization, this w i l l be the initial assignment. S t e p 1.2. Examine the daily h a n d l i n g rate b e t w e e n any pair of departments, assign the conveyor if the u t i l i z a t i o n is less than or equal to 9, and c o m p u t e the total daily cost. For routes w h e r e the c o n d i t i o n is not m e t go to step 1.3. Step 1.3. The constant speed c o n v e y o r s y s t e m has not been able to s a t i s f y the d a i l y rate. Consider two a l t e r n a t i v e s s e l e c t i n g the one w i t h the minimum cost. (i) keep the c o n s t a n t speed conveyor and utilize trucks to s a t i s f y the excess h a n d l i n g requirement. Assign one or more trucks until the daily requirements are satisfied. Go to phase If. (2) i n c r e a s e the speed of the conveyor as high as the TOPSPD if necessary. Compute the utilization, if it is g r e a t e r than ~ go to 1.4, otherwise go to 1.5. Step 1.4. Conveyor system is t h e r e f o r e not feasible, use trucks instead. There will be m u l t i p l e c h o i c e s d e p e n d i n g on the number of d i f f e r e n t c a p a c i t i e s of the trucks used. Add one or more trucks for the initial solution. Go to phase II. Step 1.5. This higher speed c o n v e y o r s y s t e m is feasible, assign c o n v e y o r to this route and compute the total d a i l y cost and utilization. Go to phase II. PHASE II: AN I M P R O V ~ f E N T A L G O R I T H M This is the improvement phase w h e r e the o v e r a l l cost is lowered and u t i l i z a t i o n is i n c r e a s e d if possible. This m a y be a c h i e v e d by i n t e r c h a n g i n g the systems, lowering the number of trucks and using equipment of higher p e r f o r m a n c e characteristics. This is a c o m b i n a t o r i a l p r o b l e m because of the i n t e r d e p e n d e n c i e s of the p r o b l e m components. The m a i n c o n t r i b u t o r to the c o m p l e x i t y is the
Ziai and Sule: Computerized materials handling
fact that trucks may be shared among routes. The problem is further complicated since higher capacity trucks may be used whose larger cost can be offset by increase in their handling capacities and thus reducing the use of conveyors and/or lower capacity trucks. There are many advantages and disadvantages for using each type of equipment. For instance, conveyors do not always occupy floor space but their path is fixed and they may not be shared by other routes even though their handling capacities are high. Another situation may occur when a higher capacity (H.C.) truck is able to handle twice as much as a lower capacity (L.C.) truck with an operating cost which is not necessarily twice as much. Figure 1 highlights this point by showing an expected cost vs capacity of different trucks. The main advantage of using the H.C. truck is lower number of trips and thus less total operating time but the disadvantage of it will be requirement of more floor space for the in-process inventory due to the larger size of the load. Therefore, there may be a limit for the handling capacity allowed or feasible as depicted in Figure 2.
Thus, increase in cost in one part due to a truck acquisition may be more than offset through the savings in other parts by sharing the trucks among many routes. The utilization is also increased by using up the available time on the present given number of trucks. Therefore, an overall integrated approach has to be taken. The steps below are not a comprehensive coverage of the pertinent factors. They merely highlight some of the major points that must be considered in order to achieve an efficient solution. Step II.l. For each of the routes in which trucks are used there will be some available time (minutes) on the truck. These times are added up for the same truck type. If the sum is less than the allotted operation time of the truck in a given shift (e.g. with %85 efficiency and 480 minutes in a shift, this time will be %85 X 480 = 408 min.), one truck will be assigned to all those routes for the given length of the previous available time while deleting one truck unit from the total number needed. If the sum is larger than or equal to the allotted time go to the next step. Step II.2. Two alternatives will be pursued, (a) consider using the same type truck, take the integer value of the ratio of the sum to the daily operating time, this will be the proposed number of truck units; (b) use a higher capacity truck. Compute the time it takes for this type to handle the extra MH requirements, now repeat part (a). Pick the lower cost truck type system.
O
U
u
I
I
!
I
Step
II.3. Arrange the conveyor systems according to descending order of their total cost.
I000 2000 3000 4000 5000 truck capacity (ib) Fig. i.
Possible cost behavior vs truck capacity
.~ o
~o
59
/
o~ O I
1000
Fig. 2.
i
2000
i
3000
,
4000
i
5000
truck capacity (ib) Constrained handling vs truck capaclty
Step II.4. Starting with the conveyor having the highest cost (in case of ties pick the one with the lower utilization), replace the conveyor system with the lowest capacity truck system. If total operating cost (TOC) of the truck is less than the conveyor's, implement this assignment, otherwise, keep the initial assignment. Mark the assignment array accordingly. Step ZI,5 Repeat step II.4 for the other conveyor systems in the ordered list. If more than one low capacity truck is needed, examine the use of the next higher capacity truck. Replace if lower TOC is realized. Step II.6 If for any route more than two of the higher capacity trucks are
60
Proceedings of the 1lth Annual Conference on Computers & Industrial Engineering
needed, examine the feasibility of one lower capacity truck instead of the second unit of the higher type. II.7. Repeat steps II.l and II.2 until no appreciable cost saving is realized (the difference value set by the user) Step
II.8. The routes have been checked for the possibility of being supplemented or replaced by trucks, a feasible efficient MH system has been suggested. Compute the total number of conveyors and trucks, their respective utilizations, and the overall hourly costs. Return to the layout problem. Using the new MH cost data, make the appropriate changes. For example, update the MH cost matrix and if trucks are to be used between departments aisle space must be allocated for their usage but with a conveyor system no such consideration needs to be given. Step
CONCLUSION
This interactive computerized model developer helps analyst design a facility where both the layout and the MH are jointly considered. Since a large portion of data should be estimated and there is a great deal of computations, this computerized approach should prove to be an effective tool in the overall design process. It must be noted, however, that the MH algorithm is under development, completion of it will be presented in a future publication.
BIBLIOGRAPHY
i. Allegri, T. H., MATERIALS HANDLING Principles and Practice, Van Nostrand Reinhold Co. Inc., New York, 1984. 2. Apple, J. M., Material Handling Systems Design, John Wiley & Sons, New York, 1972. 3. Automated Conveyor Systems, Inc. (Conveyor Catalog), West Memphis, AR 4. Hassan, M. M. D., G. L. Hogg, and D. R. Smith, "A Construction Algorithm and Assignment of Materials Handling Equipment," IDternat. J. Production Research, Vol. 23, No. 2, pp. 381-392. 5. Hyster Industrial Trucks, Shreveport, LA. 6. The Material Handling Inc., Charlotte, NC.
Institute,
7. Material Handling Handbook, Edition, Raymond A. Kulwiec, 1985.
2nd.
8. Modern Materials Handling, Cahners publishing Co., Boston,MA. 9. Southern Material Handling specialists, Shreveport, LA. i0. Sule, D. R., MANUFACTURING FACILITIES , Location, Planninq, and Design, PWS-KENT Publishing Boston, MA. 1988.
Co.
Ii. Tompkins, J. A., and J. A. White, Facilities Planning, John Wiley & Sons, New York, 1984. 12. Webster, D. B., and Reed, R., "A Material Handling System Selection Model," AIIE Trans., Vol. 3, No. 1 (1971), pp. 13-21. 13. Ziai, M. R. and D. R. Sule, "Facility Layout: An Interactive Approach," presented at the TIMS/ORSA national meeting in St. Louis, Missouri, October 1987. 14. Ziai, M. R., and D. R. Sule, "Microcomputer Facility Layout Design," Proceedings of the 10th Annual Computers and Industrial Engineering Conference, Dallas, Texas 1988. BIOGRAPHICAL SKETCH:
M. Reza Ziai - Doctoral student in Industrial Engineering current research areas: Computer Aided Facilities Layout and Materials Handling, Computer Aided Design and Graphics. Areas of interest: AI and fuzzy set theory. Teaching and research assistant for the past seven years, five papers published, two pending publication and one conference presentation. Member of Alpha Pi Mu, APICS, ASEE, ASQC, IEEE, and IIE. Dileep R. Sule Professor and coordinator of I.E. at Louisiana Tech University. He received his M.S. and Ph.D. from Texas A&M and has published in journals such as IJPR, AIIE Transactions, Computer & I.E. He is also the author of the book Manufacturing Facilities, Location, Planning and Design, publishers P.W.S. Kent.
CO~FEERX~R~_
MATERIALS
0360-8352/89 $3.00+0.00 Copyright © 1989 Pergamon Press plc
HANDLING
AND FACILITY LAYOUT DESIGN
M. Reza Ziai and Dileep R. Sule Department
of Mechanical and Industrial Engineering Louisiana Tech University Ruston, LA 71272
can be made on an objective basis, a tentative design of the facility is usually made knowing what is to be produced, how much, what machines, how many departments and their sizes plus relative interactions between them. Using this layout arrangement, estimate of the interdepartmental production rates, number of different product types, and their corresponding characteristics, one will be able to design the least cost MH system. Results of the first part [14] of this two stage design problem has been used to produce the corresponding MH system. Two influential factors in the equipment selection process are the cost and utilization. These two factors are also compatible. Selection of a suitable equipment that depends on materials characteristics has not been explicitly developed [4, ii, 12] because of the difficulty to quantify them. The systematic approach proposed here, identifies and quantifies the primary factors while devising an algorithm for the selection of the most suitable equipment.
ABSTRACT
We are proposing a systematic approach to the design of materials handling (MH) system that includes an algorithm to select the most suitable equipment. Initially, a conveyor system is proposed while determining its associated cost. To seek improvements of the system in terms of cost and equipment utilization, the possibility of supplementing or replacing the conveyor with one or more fork-lift trucks (trucks) will be investigated. The final MH system configuration will consist of conveyors and trucks so that total system optimization will be achieved. The literature on MH design is very limited and often this problem is not directly coupled with the layout problem [4]. The model proposed uses the results of a tentatively designed layout and two important characteristics of the materials, namely size and weight. Facility in this paper refers to a manufacturing plant with low to medium production volume in a process oriented environment. Facility layout and materials handling are two highly interdependent problems. The location of departments, the corresponding distances and production rates dictate the cost of MH. On the other hand, MH costs between departments, influence the departmental arrangements [i0]. An iterative process is developed between the layout design and MH selection problems.
~i~ODOLOGY
Conveyors and fork-lift trucks are the two most common means of transporting materials throughout a facility. There are several types within each with corresponding costs. Selection of a type depends on the application, size and weight of the loads, and operating environment, among other things. In order to get realistic data and to learn how industry designs and selects its equipment, several dealers of these two MH equipment types, were interviewed and given a questionnaire. The Material Handling Institute [MHI, 6] and the Modern Material Handling magazine were also contacted to furnish information about MH equipment selection and case studies. As the result of the field survey and literature, review, the following approach for the design of the equipment is presented. CONVEYORS Used for
INTRODUCTION
Design of a new facility needs a great deal of data which can only be estimated. For example, the cost of transportation, and the production volume will not be known with certainty until the production has actually started and the order has been received. This information is estimated based on historical data and/or forecasts. A larger portion of the data is, therefore, subjective. This underlies the necessity of a design tool to investigate different production and MH scenarios. Before calculations 55
56
Proceedings of the l lth Annual Conference on Computers & Industrial Engineering
transporting, storing, and accumulating materials as single units, c o l l e c t e d units, and bulk, on fixed paths w h i c h m a y be horizontal, inclined, or vertical. Pree n g i n e e r e d p r o d u c t s are used in the i n d u s t r y to c o n f i g u r e the suitable c o n v e y o r system. This a p p r o a c h is u s e d in this paper too. For the p u r p o s e of this research, the f o l l o w i n g a s s u m p t i o n s are made: * C o n v e y o r s are u s e d only to transport materials, * O n l y h o r i z o n t a l and rectilinear m o v e m e n t s take place, * Belt or roller conveyors are used * R e v e r s i b l e d r i v e s and variable speed o p t i o n s may be used, * Loads w i l l be single items, * T r a n s f e r m e c h a n i s m s will be used to c o n n e c t c o n v e y o r s , * L o a d i n g / U n l o a d i n g are p e r f o r m e d m a n u a l l y or by robots where no shock is e x p e r i e n c e d by the conveyor. The time for l o a d i n g / u n l o a d i n g is ignored FORK-LIFT TRUCKS Very v e r s a t i l e e q u i p m e n t w h i c h can carry heavy loads on various paths h o r i z o n t a l l y or vertically. T h e y are mainly used for intermittent transportation. C a r r y i n g c a p a c i t y of the truck per trip m a y be increased g r e a t l y by p a l l e t i z i n g and stacking. No other d e v i c e is needed for loading and unloading. F o r k - l i f t trucks may be c l a s s i f i e d b a s e d on the following features: • C a p a c i t y - 2000 ibs to 80,000 ib, • N e e d of rider, • E l e c t r i c m o t o r or internal c o m b u s t i o n m o t o r (Gasoline,Diesel,LP) • T r a n s m i s s i o n - S t a n d a r d or automatic • Tires - C u s h i o n type used indoors p n e u m a t i c type for outdoors and rough indoors, • S i t - d o w n for wide aisles (greater than 12 feet) and s t a n d - u p used in n a r r o w aisles. Despite their attractiveness, f o r k - l i f t trucks should be used only where and when they are needed b e c a u s e of their h i g h o p e r a t i n g cost. In this research fork-lift trucks will be used under the following conditions: * Indoors, * Low capacity, up to 5000 ibs, A SYSTI~ATIC DEVELOPH~RT
APPP~ACHTO
MODRL
In o r d e r to decide the type and number of material handling equipments, a great deal of data is n e e d e d a b o u t the plant layout and the materials. One m i g h t start with the results of a c o m p u t e r layout routine such as C O M L A D [14] to establish the (I) number of departments, (2) i n t e r d e p a r t m e n t a l distances, and (3) product flow information which
includes the flow route and required d a i l y rate among the departments. Since the m o d e l is c o m p u t e r i z e d and interactive, the analyst (user) f u r n i s h e s the n e c e s s a r y information b a s e d on the e x p e c t e d values for the parameters. For example, knowing the w i d e s t item among several which may be c o n v e y e d b e t w e e n two departments and the heaviest one, a suitable c o n v e y o r is selected• The belt width of this c o n v e y o r must be at least as wide as the g i v e n w i d t h and its live load c a r r y i n g c a p a c i t y per foot and for its entire length must not be exceeded. To select the suitable truck one needs to k n o w the number of pallets and w e i g h t of e a c h w h i c h are to be transported on a given trip. A pallet loading a l g o r i t h m may be e m p l o y e d for a g l v e n size pallet to a r r a n g e as m a n y items as possible on that pallet. The resultant total w e i g h t per p a l l e t and knowing how many are stacked per trip will determine the required truck capacity• N u m b e r of items per trip and the s p e e d of the truck will tell us w h e t h e r that truck is capable of h a n d l i n g the r e q u i r e d daily rate. P r e - e n g i n e e r e d products will be used for the design of the MH equipment. A database is c r e a t e d w h i c h c o n t a i n s typical prices for the type of c o n v e y o r and truck used in the research. THE
COST
EVALUATION
ROUTINE
The i n t e g r a t e d approach w h i c h is used for the MH cost analysis i n c l u d e s the f o l l o w i n g factors: I n v e s t m e n t - Such as initial prlce, and i n s t a l l a t i o n charges, • F i x e d c h a r g e s - Depreciation, c a p i t a l i z a t i o n (interest), taxes, insurance, other, • V a r i a b l e cost - Labor, m a i n t e n a n c e , other, E q u i p m e n t cost is based on the total cost per hour, this includes the fixed cost and the variable cost. The total h o u r l y cost for c o n v e y o r includes: O p e r a t i n g cost (OC) m a i n t e n a n c e and fuel. F i x e d c o s t (FC) - Initial cost and cost of capital. The above cost is c o m p u t e d based on the economic life ( five is industry practice - otherwise, the analyst should specify), of the e q u i p m e n t and the number of o p e r a t i n g shifts (one o p e r a t i n g shift of 40 hours a w e e k for 50 working weeks amounts to 2,000 hours per year). Salvage value of c o n v e y o r s is u s u a l l y ignored since it is too minimal unless the a n a l y s t specifies a value.
Ziai and Sule: Computerized materials handling The t o t a l h o u r l y cost lift trucks is as follows: • O p e r a t i n g cost (OC) -
for
fork-
m a i n t e n a n c e , operator, and fuel • F i x e d cost (FC) - Initial cost plus the cost of capital. The same procedure as the c o n v e y o r s y s t e m ' s is used to compute the cost. The salvage value may be represented as percentage of the i n i t i a l cost (e.g. 20%). In o r d e r to specify the type of c o n v e y o r and s u b s e q u e n t l y its cost, the f o l l o w i n g p a r a m e t e r s are defined: - C a p a c i t y (max. live load (ibs) on the c o n v e y o r for the given length, S - S p e e d (feet per minute - fpm); 60 fpm w i l l be u s e d as the standard u n l e s s h i g h e r speeds are required in w h i c h case v a r i a b l e speed motor d r i v e s w i l l be used, L - C o n v e y o r length (feet - ft), D - D e p a r t m e n t a l d i s t a n c e (ft), CL- C l e a r a n c e b e t w e e n two loads (ft pallets, boxes or single items), P - M a x i m u m n u m b e r of items per b a l l e t for item type k, n - Number of items per pallet/box, r ~ - T o t a l rate (items/day) between ~ 4 p a r t m e n t i & j and j & i, M k- M a x i m u m n u m b e r of k items allowed on a g i v e n length of a conveyor• The m a x i m u m number of items on the c o n v e y o r k n o w i n g the weight of the h e a v i e s t items will determine the required live load capacity. The d i m e n s i o n s of the largest item will i n d i c a t e the w i d t h of the conveyor• U s i n g these d i m e n s i o n s and the weight of the item, one is able to determine how m a n y p o u n d s per foot will be placed on the conveyor• The required speed is also computed knowing the rate between the two departments, number of items ( i n c l u d i n g the clearance) on a given length of conveyor, and the interdepartmental distance. After all the above d a t a are calculated one is able to use the p r e - e n g i n e e r e d p r o d u c t tables to find out the cost of the conveyor. The daily u t i l i z a t i o n of the conveyor is also determined. If it exceeds the given value, then other solution alternatives will be followed as e x p l a i n e d in the s e l e c t i o n algorithm. The type of f o r k - l i f t truck and its cost will be based on the following parameters: A - The m a x i m u m size (base area) load, - The m a x i m u m load c a r r y i n g c a p a c i t y per trip, S - L o a d c a r r y i n g s p e e d (fpm), D - I n t e r d e p a r t m e n t a l d i s t a n c e (ft), ri5- T o t a l rate (items/day) between d ~ p a r t m e n t s i & j and j & i,
57
n - N u m b e r of items per pallet, m - N u m b e r of p a l l e t s stacked, R - N u m b e r of trips r e q u i r e d per p e r i o d (day). The above p a r a m e t e r s are used to c o m p u t e the r e q u i r e d c a p a c i t y of a truck and number. Cost is then determined using tables and the c u r r e n t i n d u s t r y cost estimates to o p e r a t e the trucks. In the selection a l g o r i t h m c e r t a i n number of trucks are a s s i g n e d a m o n g the departments r e q u i r i n g m i n i m u m cost.
A MATHEMATICALMODELFOREQUIPM~RT SRT~CTION An integer linear programming model is proposed to obtain the o p t i m u m s e l e c t i o n of conveyors and trucks (or any other two classes of m a t e r i a l h a n d l i n g systems). The cost coefficients are calculated as discussed previously. In effect, different types of conveyors and trucks are being considered here represented only by their c o r r e s p o n d i n g costs.
Minimize
2 N N Z = ~ E E Cki j Xki j k=l i=l j=l i%j j#i
S u b j e c t to XI(N_I,N) S 1 not to allow m o r e than one conveyor s y s t e m b e t w e e n two d e p a r t m e n t s (here 1 & 2) X2(N_I,N) S [ t/8] 8 Xk(i,j) { 0 , 1 } for k = 1,2 i = 1,2, .... N j = 1,2,...,N Where: Cki~ = total cost of equipment type k n ~ e d e d per day b e t w e e n d e p a r t m e n t s i and j, Xki 5 = n u m b e r of equipment type k n e e d e d ~ p e r day b e t w e e n d e p a r t m e n t s i and j, t = hours of o p e r a t i o n for f o r k - l i f t trucks, o b t a i n e d from the cost c a l c u l a t i o n section (the 8 stands for the eight hour d a i l y operation), N = number of departments. [t/8] = the largest integer value, This is a s i m p l i s t i c model w h i c h gives an i n i t i a l solution• The above f o r m u l a t i o n of the p r o b l e m is not able to h a n d l e all the different situations that may arise. For example, r e a s s i g n i n g trucks to other departments when they are only u t i l i z e d for instance, 50% of the time, or r e a s s i g n i n g the operator to other d u t i e s and k e e p i n g the truck idle, or the case w h e n a truck is
58
Proceedings of the 1lth Annual Conference on Computers & Industrial Engineering
r e q u i r e d in a d d i t i o n to a conveyor system to handle certain type of item. In order to solve the equipment s e l e c t i o n p r o b l e m in its entirety, the following algorithm has been devised which allows better u t i l i z a t i o n of the equipment.
e f f i c i e n t s o l u t i o n is sought. For any solution, each path may have any of the three possible configurations, (i) only a c o n v e y o r system, (2) o n l y a truck system, or
AN A L G O R I T H M TO SOLVE T H E M H EQUIPMENT SELECTION An a l g o r i t h m will be implemented to find the number of conveyors and f o r k - l i f t trucks for the MH system proposed in an efficient manner. Before we start the algorithm the following calculations and a s s u m p t i o n s are made:
PHASE I:
(a) O n l y one conveyor system is considered practical between each pair of d e p a r t m e n t s . (b) The time, utilization, and cost a n a l y s i s will be c a r r i e d out for a conveyor system as the initial (partial) solution. A table will be generated depicting the cost and u t i l i z a t i o n of this system. (c) A c o n s t a n t speed of S = 60 fpm w i l l be u s e d b a s e d on i n d u s t r y practice. This speed of the c o n v e y o r m a y be i n c r e a s e d to a c c o m m o d a t e the extra daily handling requirements. But the i n c r e a s e will not be b e y o n d TOPSPD = 280 fpm, for practical considerations. This will be one of the physical constraints on the c o n v e y o r system. (d) O n l y an integer number of trucks are assigned to handling tasks. It is the crux of the algorithm to assign the a v a i l a b l e time of the t r u c k to other routes in an e f f i c i e n t manner. One or more truck units are a l l o w e d to be used along with a conveyor if the e c o n o m i c s of the o p e r a t i o n s d i c t a t e it. (e) H i g h e r c a p a c i t y trucks will be u s e d to h a n d l e the extra MH task if needed. T h e s e c a p a c i t i e s will be from 2000 lbs to 5000 lbs in i n c r e m e n t s of I000 ibs. This is a suitable range since the h a n d l i n g tasks take place in an indoor environment. H i g h e r c a p a c i t y trucks u s u a l l y need extra wide (more than 12 feet) aisles. (f) A u t i l i z a t i o n of 8 = %85 will be a s s u m e d for each e q u i p m e n t to c o m p l y w i t h the i n d u s t r y practice. Therefore, one unit of e q u i p m e n t is r e c o m m e n d e d for e a c h ~ utilization. Use of e x c e s s time available on the equipment are discussed in the i m p r o v e d phase of the algorithm. STEPS OF THE ALGORITHM: There are two phases to the algorithm. The first one is the c o n s t r u c t i o n phase where an initial s o l u t i o n is produced, and a second phase called improvement where an
(3) c o m b i n a t i o n trucks systems.
of
a
conveyor
and
CONSTRUCTION OF AN INITIAL FEASIBLE SOLUTION (THE ASSIGNMENT ARRAY) S t e p I.l. Design a conveyor system for the e n t i r e task of MH using a c o n s t a n t speed of S while i d e n t i f y i n g the paths for w h i c h this system will not be feasible. Calculate the total cost and e q u i p m e n t utilization, this w i l l be the initial assignment. S t e p 1.2. Examine the daily h a n d l i n g rate b e t w e e n any pair of departments, assign the conveyor if the u t i l i z a t i o n is less than or equal to 9, and c o m p u t e the total daily cost. For routes w h e r e the c o n d i t i o n is not m e t go to step 1.3. Step 1.3. The constant speed c o n v e y o r s y s t e m has not been able to s a t i s f y the d a i l y rate. Consider two a l t e r n a t i v e s s e l e c t i n g the one w i t h the minimum cost. (i) keep the c o n s t a n t speed conveyor and utilize trucks to s a t i s f y the excess h a n d l i n g requirement. Assign one or more trucks until the daily requirements are satisfied. Go to phase If. (2) i n c r e a s e the speed of the conveyor as high as the TOPSPD if necessary. Compute the utilization, if it is g r e a t e r than ~ go to 1.4, otherwise go to 1.5. Step 1.4. Conveyor system is t h e r e f o r e not feasible, use trucks instead. There will be m u l t i p l e c h o i c e s d e p e n d i n g on the number of d i f f e r e n t c a p a c i t i e s of the trucks used. Add one or more trucks for the initial solution. Go to phase II. Step 1.5. This higher speed c o n v e y o r s y s t e m is feasible, assign c o n v e y o r to this route and compute the total d a i l y cost and utilization. Go to phase II. PHASE II: AN I M P R O V ~ f E N T A L G O R I T H M This is the improvement phase w h e r e the o v e r a l l cost is lowered and u t i l i z a t i o n is i n c r e a s e d if possible. This m a y be a c h i e v e d by i n t e r c h a n g i n g the systems, lowering the number of trucks and using equipment of higher p e r f o r m a n c e characteristics. This is a c o m b i n a t o r i a l p r o b l e m because of the i n t e r d e p e n d e n c i e s of the p r o b l e m components. The m a i n c o n t r i b u t o r to the c o m p l e x i t y is the
Ziai and Sule: Computerized materials handling
fact that trucks may be shared among routes. The problem is further complicated since higher capacity trucks may be used whose larger cost can be offset by increase in their handling capacities and thus reducing the use of conveyors and/or lower capacity trucks. There are many advantages and disadvantages for using each type of equipment. For instance, conveyors do not always occupy floor space but their path is fixed and they may not be shared by other routes even though their handling capacities are high. Another situation may occur when a higher capacity (H.C.) truck is able to handle twice as much as a lower capacity (L.C.) truck with an operating cost which is not necessarily twice as much. Figure 1 highlights this point by showing an expected cost vs capacity of different trucks. The main advantage of using the H.C. truck is lower number of trips and thus less total operating time but the disadvantage of it will be requirement of more floor space for the in-process inventory due to the larger size of the load. Therefore, there may be a limit for the handling capacity allowed or feasible as depicted in Figure 2.
Thus, increase in cost in one part due to a truck acquisition may be more than offset through the savings in other parts by sharing the trucks among many routes. The utilization is also increased by using up the available time on the present given number of trucks. Therefore, an overall integrated approach has to be taken. The steps below are not a comprehensive coverage of the pertinent factors. They merely highlight some of the major points that must be considered in order to achieve an efficient solution. Step II.l. For each of the routes in which trucks are used there will be some available time (minutes) on the truck. These times are added up for the same truck type. If the sum is less than the allotted operation time of the truck in a given shift (e.g. with %85 efficiency and 480 minutes in a shift, this time will be %85 X 480 = 408 min.), one truck will be assigned to all those routes for the given length of the previous available time while deleting one truck unit from the total number needed. If the sum is larger than or equal to the allotted time go to the next step. Step II.2. Two alternatives will be pursued, (a) consider using the same type truck, take the integer value of the ratio of the sum to the daily operating time, this will be the proposed number of truck units; (b) use a higher capacity truck. Compute the time it takes for this type to handle the extra MH requirements, now repeat part (a). Pick the lower cost truck type system.
O
U
u
I
I
!
I
Step
II.3. Arrange the conveyor systems according to descending order of their total cost.
I000 2000 3000 4000 5000 truck capacity (ib) Fig. i.
Possible cost behavior vs truck capacity
.~ o
~o
59
/
o~ O I
1000
Fig. 2.
i
2000
i
3000
,
4000
i
5000
truck capacity (ib) Constrained handling vs truck capaclty
Step II.4. Starting with the conveyor having the highest cost (in case of ties pick the one with the lower utilization), replace the conveyor system with the lowest capacity truck system. If total operating cost (TOC) of the truck is less than the conveyor's, implement this assignment, otherwise, keep the initial assignment. Mark the assignment array accordingly. Step ZI,5 Repeat step II.4 for the other conveyor systems in the ordered list. If more than one low capacity truck is needed, examine the use of the next higher capacity truck. Replace if lower TOC is realized. Step II.6 If for any route more than two of the higher capacity trucks are
60
Proceedings of the 1lth Annual Conference on Computers & Industrial Engineering
needed, examine the feasibility of one lower capacity truck instead of the second unit of the higher type. II.7. Repeat steps II.l and II.2 until no appreciable cost saving is realized (the difference value set by the user) Step
II.8. The routes have been checked for the possibility of being supplemented or replaced by trucks, a feasible efficient MH system has been suggested. Compute the total number of conveyors and trucks, their respective utilizations, and the overall hourly costs. Return to the layout problem. Using the new MH cost data, make the appropriate changes. For example, update the MH cost matrix and if trucks are to be used between departments aisle space must be allocated for their usage but with a conveyor system no such consideration needs to be given. Step
CONCLUSION
This interactive computerized model developer helps analyst design a facility where both the layout and the MH are jointly considered. Since a large portion of data should be estimated and there is a great deal of computations, this computerized approach should prove to be an effective tool in the overall design process. It must be noted, however, that the MH algorithm is under development, completion of it will be presented in a future publication.
BIBLIOGRAPHY
i. Allegri, T. H., MATERIALS HANDLING Principles and Practice, Van Nostrand Reinhold Co. Inc., New York, 1984. 2. Apple, J. M., Material Handling Systems Design, John Wiley & Sons, New York, 1972. 3. Automated Conveyor Systems, Inc. (Conveyor Catalog), West Memphis, AR 4. Hassan, M. M. D., G. L. Hogg, and D. R. Smith, "A Construction Algorithm and Assignment of Materials Handling Equipment," IDternat. J. Production Research, Vol. 23, No. 2, pp. 381-392. 5. Hyster Industrial Trucks, Shreveport, LA. 6. The Material Handling Inc., Charlotte, NC.
Institute,
7. Material Handling Handbook, Edition, Raymond A. Kulwiec, 1985.
2nd.
8. Modern Materials Handling, Cahners publishing Co., Boston,MA. 9. Southern Material Handling specialists, Shreveport, LA. i0. Sule, D. R., MANUFACTURING FACILITIES , Location, Planninq, and Design, PWS-KENT Publishing Boston, MA. 1988.
Co.
Ii. Tompkins, J. A., and J. A. White, Facilities Planning, John Wiley & Sons, New York, 1984. 12. Webster, D. B., and Reed, R., "A Material Handling System Selection Model," AIIE Trans., Vol. 3, No. 1 (1971), pp. 13-21. 13. Ziai, M. R. and D. R. Sule, "Facility Layout: An Interactive Approach," presented at the TIMS/ORSA national meeting in St. Louis, Missouri, October 1987. 14. Ziai, M. R., and D. R. Sule, "Microcomputer Facility Layout Design," Proceedings of the 10th Annual Computers and Industrial Engineering Conference, Dallas, Texas 1988. BIOGRAPHICAL SKETCH:
M. Reza Ziai - Doctoral student in Industrial Engineering current research areas: Computer Aided Facilities Layout and Materials Handling, Computer Aided Design and Graphics. Areas of interest: AI and fuzzy set theory. Teaching and research assistant for the past seven years, five papers published, two pending publication and one conference presentation. Member of Alpha Pi Mu, APICS, ASEE, ASQC, IEEE, and IIE. Dileep R. Sule Professor and coordinator of I.E. at Louisiana Tech University. He received his M.S. and Ph.D. from Texas A&M and has published in journals such as IJPR, AIIE Transactions, Computer & I.E. He is also the author of the book Manufacturing Facilities, Location, Planning and Design, publishers P.W.S. Kent.