aiar. unit ii. transfer lines

Transfer Lines

The manufacturing systems considered in this unit are used for high production of parts that require multiple processing operations. Each processing operation is performed at a workstation and workstations are physically integrated by means of a mechanized material handling system to form an automated production line. Machining e.g. milling, drilling and similar rotating cutter operations are common processing operations performed on these production lines. These are called automated production lines or transfer lines or transfer machines.

Transfer Lines are appropriate only under the following conditions:

• High production demand.• Stable product design• Long product life• Multiple operations

Transfer Lines

When the application satisfies these conditions, automated production lines

provide the following benefits:

• Low direct labor content.

• Low product cost because cost of fixed equipment is spread over many units.

• High production rates.

• Production lead time (the time between beginning of production and

completion of a finished unit) and work in process are minimized.

• Factory floor space is minimized.

Fundamentals of Automated Production Lines

An automated production line consists of multiple workstations that are linked together by a work handling system that transfers parts from one station to the next, as depicted in figure.

Automated production line (Transfer line)

System Configurations

Although the first figure shows the flow of work to be in straight

line, the work flow can actually take several different forms. We

classify them as follows:

1. In-line

2. Segmented in-line, and

3. Rotary


There are a number of reasons for designing a production line in these configuration rather than in a pure straight line, including• Available floor space

may limit the length of a line.

• It allows reorientationof workpiece to present different surfaces for machining, and

• The rectangular layoutprovides for return of

work holding fixtures to the front of the line for reuse.


Compared to in-line and segmented in-line configurations, rotary indexing systems are commonly limited to smaller work parts and fewer work stations and they cannot readily accommodate buffer storage capacity. On the positive side, the rotary systems usually involves a less expensive piece of equipment and typically requires less floor space.

Workpart Transfer Mechanisms

The workpart transfer system moves parts between stations on the production line. Transfer mechanisms used on automated production lines are usually either synchronous or asynchronous.

The transfer mechanisms are divided into two categories:• Linear transfer systems and• Rotary indexing mechanisms

Linear transfer systems: These include powered roller conveyors, belt conveyors, chain driven conveyors and cart-on-track conveyors.


walking beam transfer systems


Rotary Indexing Mechanism

1. Geneva Mechanism

Where Ѳ is angle of rotation of worktable during indexing (degrees of rotation), and ns = number of slots in the Geneva.


Rotary Indexing Mechanism

2. Cam Drive

Storage Buffers

A storage buffer in a production line is a location where parts can be collected and temporarily stored before proceeding to subsequent (downstream) workstations.

There are number of reasons why storage buffers are used on automated production lines. The reasons include:

• To reduce the effect of station breakdown:• To provide a bank of parts to supply the line• To provide a place to put the output of the line• To allow for curing time or other delay• To smooth cycle time variation

Control of the Production Line

A storage buffer in a production line is a location where parts can be collected and

temporarily stored before proceeding to subsequent (downstream) workstations.

Basic control functions:

• Sequence control

• Safety monitoring

• Quality control

Line Controllers

For many years the traditional equipment used to control the sequence of steps on

automated production lines were electromechanical relays. Since the 1970s,

programmable logic controllers (PLCs) have been used as the controllers in new

installations. More recently, personal computers (PCs) are being used to

accomplish the control functions to operate automated production lines. In

addition to being more reliable, computer control offers the following benefits:

• Opportunity to improve and upgrade the control software, such as adding

specific functions not anticipated in the original system design.

• Recording of data on process performance, equipment reliability, and product

quality for subsequent analysis.

• Diagnostic routines to expedite maintenance and repair when the breakdowns

occur and to reduce the duration of downtime incidents.

• Automatic generation of preventive maintenance schedules indicating when

certain preventive maintenance actions should be performed. This helps to

reduce the frequency of downtime occurrences.

• Provides a more convenient human-machine interface between the operator and

the automated line.

Analysis of Transfer Lines

Analysis of Transfer Lines with no Internal Storage

Basic Terminology and Performance Measure Assumptions• The workstations perform processing operations such as machining, not

assembly.• Processing times at each stations are constant, though not necessarily equal.• Synchronous transfer of lines and• No internal storage buffers.

Ideal cycle time Tc of the production line, which is the processing time for slowest station on the line plus transfer time.

Tc = Max [Tsi] + Tr

where Tc = ideal cycle time in min. Tsi = the processing time at station i in min, and Tr = transfer time in min.We use the Max [Tsi] because the longest processing time establishes the pace of the production line. Remaining stations with lower processing times must wait for the slowest station. Therefore, these other stations will experience idle time.


In the operation of a transfer line, random breakdowns and planned stoppages cause downtime on the line. Common reasons for downtime on an automated production line are listed below:

• Tool failure at workstations• Tool adjustments at workstations• Scheduled tool changes.• Limit switch or other electrical malfunctions• Mechanical failure of a workstations• Mechanical failure of the transfer system.• Stock-outs of starting work units• Insufficient space for completed workpiece• Preventive maintenance on the line• Worker breaks


When the line stops, it is down a certain average time for each downtime occurrence. These downtime occurrences cause the actual average production cycle time of the line to be longer than the ideal cycle time. We can formulate the following expression for the actual average production time Tp:

Tp = Tc + F Tdwhere F = downtime frequency, line stops / cycle, and Td = downtime per line stop, min.One of the important measures of performance on an automated transfer line is production rate, which can be computed as the reciprocal of Tp:

where Rp = actual average production rate (pc/min), and Tp is the actual average production time. The ideal production rate is given by:

where Rc = ideal production rate (pc/min). It is customary to express production rates on automated production lines as hourly rates (multiply Rp and Rc by 60)


In context of automated production lines, line efficiency refers to proportion of uptime on the production line and is really a measure of reliability more than efficiency. Nevertheless, this is the terminology of production lines. Line efficiency can be calculated as below:

where E = the proportion of uptime on the production line, and the other terms have been previously defined.An alternative measure of performance is the proportion of downtime on the line, which is given by

where D = proportion of dwntime on the line. It is obvious that


An important measure of performance of an automated production line is the cost per unit produced. This piece cost includes the cost of the starting work blank, the cost of time on the production line, and the cost of tooling that is consumed (e.g. cutting tools on a machining line). The piece cost can be expressed as the sum of these three factors:

Where = cost per piece Rs./pc= cost of starting work material Rs./pc.= cost per minute to operate the line Rs./min= average production time per piece, min/pc.= cost of tooling per piece Rs./pc

Analysis of Transfer Lines With Storage Bufferss

In an automated production line with no internal parts

storage, the workstations are interdependent. When one

station breaks down, all other stations on the line are

affected, either immediately or by the end of a few cycles of

operation. The other stations will be forced to stop for one of

two reasons:

1. Starving of stations or

2. Blocking of stations.

Limits of Storage Buffer Effectiveness

Two extreme cases of storage buffer effectiveness can be identified:

1. no buffer storage capacity at all and

2. infinite capacity storage buffer.

In the case of no storage capacity, the production line acts as one stage. When

a station breaks down, the entire line stops, This is the case of a production line

with no internal storage analyzed earlier. The efficiency of the line is given by:

E0 as the efficiency of a line with zero storage buffer capacity

Limits of Storage Buffer Effectiveness

The opposite extreme is the case where buffer zones of infinite capacity are installed

between every pair of stages.

E∞ = minimum {Ek}

E0 < Eb < E∞

Analysis of a Two-Stage Transfer line

The overall line efficiency for the two-stage line can be expressed:

Eb = E0 + D1h(b)E2

where

Eb = overall line efficiency for a two-stage line with buffer capacity b;

E0 = line efficiency for the same line with no internal storage; and

(D1h(b)E2) represents the improvement in efficiency that results from having a

storage buffer with b > 0.

Analysis of a Two-Stage Transfer line

Determination of h(b)

Assumptions and definitions:

1. Td1 = Td2 = Td

2. Tc1 = Tc2 = Tc

3. F1 = downtime frequency for stage 1, and

4. F2 is downtime frequency for stage 2

Define

Given buffer capacity b, define B and L as follows:

Where B is the largest integer satisfying the relation:

and L represents the leftover units, the amount by which b exceeds

Determination of h(b)

There are two cases:

Case 1 r = 1.0,

Case 2 r ≠ 1.0,

aiar. unit ii. transfer lines

Engineering