In this article we will discuss about:- 1. Introduction to Transfer Lines 2. Objectives of Transfer Lines 3. Methods 4. Control 5. Performance.
Introduction to Transfer Lines:
The first step in factory automation was to automate the transportation of parts between various machines/ stations. An automated flow line or transfer line consists of several machines/work-stations linked by work handling devices to transfer lines parts automatically. The raw work parts enter from one end and are processed sequentially at various stations and finished part comes out of other end of automated flow line.
The various workstations in transfer line may be for processing, assembly, inspection and sorting. Buffer storage zones may be incorporated in between stations wherever required.
Objectives of Transfer Lines:
The various objectives of transfer lines are:
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i. High production rate,
ii. Reduced labour cost,
iii. Reduced work-in- process,
iv. To minimise transportation distance and time,
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v. Achieve specialisation and integration of operations.
The flow pattern of operations may be either in-line type or rotary type depending on the application. The latter type is commonly limited to smaller work pieces and to fewer stations. The flexibility in rotary type configuration is less but it occupies less factory floor space. The in-line configuration is suited for larger work pieces needing larger number of workstations.
Methods of Transfer Lines:
The transporting device (transfer mechanism) not only moves the part from one station to other, but also orients and locates the parts in correct position for processing at each station.
Various methods used are:
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(i) Continuous Transfer:
In this method, the parts move continuously at constant speed and work-head moves at same speed along with part and completes its job during motion of the part. Such a system is used in bottling plant, packaging, etc.
(ii) Intermittent Transfer:
The parts are transported with discontinuous motion. The workstations are fixed in position and parts are moved between stations. All parts are transported at the same time. (Synchronous transfer). Such systems are used in machining operation, press working operations, and mechanised assembly.
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(iii) Asynchronous Transfer:
It allows part to move to next station only when the processing on the current station is over. Each part thus moves independently of others. This system offers greater flexibility. This system permits in process storage and also compensates for line balancing problems where there are significant differences in processing times between stations. For longer operation, parallel stations can be used.
The commonly used linear transfer mechanisms are:
(i) Walking beam systems,
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(ii) Powered roller conveyor systems, and
(iii) Chain drive conveyor systems.
The commonly used rotary transfer mechanisms are:
(i) Rack and pinion,
(ii) Ratchet and pawl, and
(iii) Geneva mechanism.
Control of Transfer Lines:
Sequence control of the complete automated flow transfer line to coordinate the sequence of actions of the transfer system and its workstations is achieved by a programmable logic controller in which the sequence can be easily configured and can be modified as and when some changes are made in the line. Controls also ensure that the system does not operate in an unsafe or hazardous conditions. Various sensors and limit switches may be installed for this purpose.
The control system also monitors the quality and rejects the intolerable parts. It also decides whether there is need to stop complete line or one faulty part produced at any work station is earmarked and line allowed to work.
However if same defect is produced every time at one station, then complete line has to be stopped and corrective action taken. Diagnostic features are added to aid in identifying the location and cause of the trouble to the operators so that corrective measures can be taken.
Performance of Transfer Lines:
The systems aspect of designing and running the automated flow line and its reliability are of great concern, since just one failure at one workstation could mean stopping of complete line and huge loss of money. Thus the analysis of flow line using a systems approach to find answer for following considerations is essential.
(i) Effect of the number of workstations on the performance of the line.
(ii) Improvement effected by using one or more buffer storage zones.
(iii) Effect of component quality on the operation of an automated assembly machine.
(iv) Effect of use of manual workstations on performance of line.
(v) Whether the line is balanced, if not, to achieve same by spreading the total work load as evenly as possible among the stations in the line.
Flow line performance can be analysed by following measures:
(i) Average production rate
(ii) Proportion of time the line is operating (line efficiency)
(iii) Cost per item produced on the line.
Assembly Systems:
Assembly involves the process of joining two or more separate parts to produce a subassembly.
The process of assembly may involve:
(i) Mechanical fastening (use of threaded fasteners, rivets, crimping, press fits, snap fits, sewing and stitching, etc.
(ii) Joining methods which include welding, brazing, and soldering.
(iii) Adhesive bonding—using an adhesive material (thermoplastic or epoxies) to join components together.
Assembly processes could be either manual single- station assembly, or manual assembly line, or automated assembly line. In the first type, the assembly work is accomplished on the product at a single workplace. This method is used for complex product produced in small quantities. Manual assembly lines consist of multiple workstations in which the assembly work is accomplished as the product is passed from station to station along the line.
At each workstation one or more operators perform a portion of total assembly work on the product, and the final completed work comes out from last station. This approach is used in high production situations where the entire assembly work can be broken down into small tasks, permitting specialisation of labour. In automatic assembly, mechanised and automated devices like robots are used to perform the various functions in an assembly line.