High volume production systems

4/2/2014 1Hareesha N G, Dept of Aero Engg, DSCE, Blore

Syllabus
4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 2

Automated Production Lines
 Automated production lines are used for high production of parts
that require multiple processing operations.
 Each processing operation is performed at a workstation, and the
stations are physically integrated by means of a mechanized work
transport system to form an automated production line.
 Machining (milling, drilling, and similar rotating cutter operations)
is a common process performed on these production lines, in
which case the term transfer line or transfer machine is used.
 Other applications of automated production lines include robotic
spot welding in automobile final assembly plants, sheet metal
press working, and electroplating of metals.
 Automated production lines require a significant capital
investment. They are examples of fixed automation, and it is
generally difficult to alter the sequence and content of the
processing operations once the line is built.4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 3

• High production of parts requiring multiple
processing operations
• Fixed automation
• Applications:
– Machining transfer lines
– Robotic spot welding lines
– Sheet metal stamping
– Electroplating of metals
– Electronics assembly
Features and Applications of
Automated transfer lines

Where to Use
Automated Production Lines?
• High product demand
– Requires large production quantities
• Stable product design
– Difficult to change the sequence and content of
processing operations once the line is built
• Long product life
– At least several years
• Multiple operations required on product
– The different operations are assigned to different
workstations in the line

Benefits of
• Low amount of direct labor
• Low product cost
-because cost of fixed equipment is spread over many units.
• High production rates.
• Manufacturing 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 Line
 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 .

Fundamentals of Automated Production Line
A raw workpart enters one end of the line, and
the processing steps are performed sequentially
as the part progresses forward.
 The line may include inspection stations to
perform intermediate quality checks.
 Manual stations may also be located along the
line to perform certain operations that are
difficult or uneconomical to automate.
Each station performs a different operation, so
that the sum total of all the operations is required
to complete one unit of work.

Fundamentals of Automated Production Line
Multiple parts are processed simultaneously on
the line, one part at each workstation.
 In the simplest form of production line, the
number of parts on the line at any moment is
equal to the number of workstations, as indicated
in the figure.
In more complicated lines, provision is made for
temporary parts storage between stations, in
which case there is on average more than one
part per station.

System Configurations
Depending upon the workflow, the automated
transfer lines are classified as below.
1) In-line (straight line) arrangement of
workstations
2) Segmented in-line – two or more straight line
segments, usually perpendicular to each other
3) Rotary indexing machine (e.g., dial indexing
machine)

In-line (straight line) arrangement of
workstations
This configuration is common for machining big work pieces, such
as automotive engine blocks, engine heads and transmission cases.
Because these parts require a large number of operations, a
production line with many stations is needed.
The in-line configuration can accommodate a large number of
stations.
 In-line systems can also be designed with integrated storage
buffers along the flow path.

Segmented In-Line Configurations
L-shaped layout
U-shaped layout
Rectangular configuration

Segmented in-line arrangement of
workstations
The segmented in-line configuration consists of two or
more straight-line transfer sections, where the segments
are usually perpendicular to each other.
There are a number of reasons for designing a production
line in these configurations rather than in a pure straight
line, including:
1) Available floor space may limit the length of the line
2) It allows reorientation of the work piece to present
different surfaces for machining
3) The rectangular layout provides for return of work
holding fixtures to the front of the line for reuse.

Two Machining Transfer Lines
Figure: Line drawing of two machining transfer lines: At bottom right, the first is a 12-
station segmented in-line configuration that uses pallet fixtures to locate the work
parts. The return loop brings the pallets back to the front of the line. The second
transfer line (upper left) is a seven-station in-line configuration. The manual station
between the lines is used to reorient the parts.

Rotary configuration
The work parts are attached to fixtures around
the periphery of a circular worktable, and the
table is indexed (rotated in fixed angular
amounts) to present the parts to workstations
for processing.
A typical arrangement is illustrated in Figure .
The worktable is often referred to as a dial,
and the equipment is called a dial indexing
machine, or simply, indexing machine.

Rotary configuration
Although the rotary configuration does not seem to
belong to the class of production systems called
"lines," their operation is nevertheless very similar.
Compared with the in-line and segmented in-line
configurations, rotary indexing systems are
commonly limited to smaller work parts and fewer
workstations
This configuration cannot accommodate buffer
storage capacity.
 The rotary system usually involves a less expensive
piece of equipment and typically requires less floor
space.4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 25

Rotary Indexing Machine

Work Transport Systems
There are two basic ways to accomplish the
movement of work units along a manual
assembly line:
(1) manually or
(2) by a mechanized system.

Manual Methods of Work Transport
• In manual work transport, the units of product are passed
from station-to-station by hand.
• Two problems result from this mode of operation are
starving and blocking.
• Starving is the situation in which the assembly operator
has completed the assigned task on the current work unit,
but the next unit has not yet arrived at the station. The
worker is thus starved for work.
• When a station is blocked, it means that, operator has
completed the assigned task on the current work unit but
cannot pass the unit to the downstream station because
that worker is not yet ready to receive it. The operator is
therefore blocked from working.4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 30

• To mitigate the effects of these problems, storage buffers
are sometimes used between stations.
• The work units made at each station are collected in
batches and then moved to the next station. In other
cases, work units are moved individually along a flat table
or unpowered conveyor. When the task is finished at each
station, the worker simply pushes the unit toward the
downstream station.
• Space is often allowed for one or more work units in front
of each workstation. Hence, starving and blocking are
minimized.
• It can result in significant work-in-process
• Workers are un-paced in lines that rely on manual
transport methods, and production rates tend to be lower.4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 31

Mechanized Work Transport
Three major categories of work transport systems
in production lines are:
(a) continuous transport,
(b) synchronous transport, and
(c) asynchronous transport.
These are illustrated schematically in Figure.

continuous transport, synchronous transport,
asynchronous transport

continuous transport system
• A continuous transport system uses a
continuously moving conveyor that operates at
constant velocity, as in Figure (a). This method is
common on manual assembly lines.
• The conveyor usually runs the entire length of the
line. However, if the line is very long, such as the
case of an automobile final assembly plant, it is
divided into segments with a separate conveyor
for each segment.
• Examples of this kind are overhead trolley
conveyor, Belt conveyor, Roller conveyor, Drag
chain conveyor.4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 34

• Continuous transport can be implemented in two ways:
(1) Work units are fixed to the conveyor, and (2) work
units are removable from the conveyor.
• In the first case, the product is large and heavy (e.g.,
automobile, washing machine) and cannot be removed
from the conveyor. The worker must therefore walk
along with the product at the speed of the conveyor to
accomplish the assigned task.
• In the case where work units are small and lightweight,
they can be removed from the conveyor for the physical
convenience of the operator at each station.
• Another convenience for the worker is that the assigned
task at the station does not need to be completed within
a fixed cycle time.4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 35

Overhead Trolley Conveyor
• A trolley is a wheeled
carriage running on an
overhead track from which
loads can be suspended
• Trolleys are connected and
moved by a chain or cable
that forms a complete loop
• Often used to move parts
and assemblies between
major production areas

Belt Conveyor
• Continuous loop with
forward path to move
loads
• Belt is made of
reinforced elastomer
• Support slider or rollers
used to support forward
loop
• Two common forms:
– Flat belt (shown)
– V-shaped for bulk
materials
(Support frame not shown)

Roller Conveyor
• Pathway consists of a
series of rollers that are
perpendicular to
direction of travel
• Loads must possess a flat
bottom to span several
rollers
• Powered rollers rotate to
drive the loads forward
• Un-powered roller
conveyors also available4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 46

Skate-Wheel Conveyor
• Similar in operation to
roller conveyor but use
skate wheels instead of
rollers
• Lighter weight and
unpowered
• Sometimes built as
portable units that can
be used for loading and
unloading truck trailers
in shipping and
receiving4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 52

synchronous transport systems
• In synchronous transport systems, all work units are moved
simultaneously between stations with a quick, discontinuous
motion, and then positioned at their respective stations.
Depicted in Figure (b), this type of system is also known as
intermittent transport, which describes the motion
experienced by the work units.
• Synchronous transport is not common for manual lines, due
to the requirement that the task must be completed within a
certain time limit. This can result in incomplete units and
excessive stress on the assembly workers.
• Despite its disadvantages for manual assembly lines,
synchronous transport is often ideal for automated
production lines.
• Examples of this kind are Walking beam transport equipment and Rotary indexing mechanisms.

asynchronous transport system
• In an asynchronous transport system, a work
unit leaves a given station when the assigned
task has been completed and the worker
releases the unit.
• Work units move independently rather than
synchronously as in Figure (c).
• Examples of this kind are Power-and-free
overhead conveyor, Cart-on-track conveyor,
Powered roller conveyors, automated guided
vehicle system, Monorail systems, and Chain-
driven carousel systems.4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 57

Workpart Transfer Mechanisms
• Linear transfer systems:
– Continuous motion – not common for
automated systems
– Synchronous motion – intermittent motion, all
parts move simultaneously
– Asynchronous motion – intermittent motion,
parts move independently
• Rotary indexing mechanisms:
– Geneva mechanism
– Others

Belt-Driven Linear Transfer System
Side view of chain or steel belt-driven conveyor (over and
under type) for linear transfer using work carriers

• Figure illustrates the possible application of a
chain or belt driven conveyor to provide
continuous or intermittent movement of parts
between stations.
• Either a chain or flexible steel belt is used to
transport parts using work carriers attached to
the conveyor.
• The chain is driven by pulleys in either an "over-
and-under" configuration, in which the pulleys
turn about a horizontal axis, or an "around-the
corner“ configuration, in which the pulleys rotate
about a vertical axis.

Walking Beam Transfer System

• Many transfer lines utilize various walking beam transfer
systems, in which the parts are synchronously lifted up
from their respective stations by a transfer beam and
moved one position ahead to the next station. The
transfer beam then lowers the parts into nests that
position them for processing at their stations. The beam
then retracts to make ready for the next transfer cycle.
The action sequence is depicted in Figure.
(1) work parts at station positions on fixed station beam
(2) transfer beam is raised to lift work-parts from nests
(3) Elevated transfer beam moves parts to next station positions.
(4) Transfer beam lowers to drop work parts into nests at new
station positions. Transfer beam then retracts to original position
shown in (1).

Geneva Mechanism with Six Slots

• The Geneva mechanism uses a continuously rotating driver to index the
table through a partial rotation, as illustrated in Figure.
• If the driven member has six slots for a six-station dial indexing table,
each turn of the driver results in 1/6 rotation of the worktable, or 60o.
• The driver only causes motion of the table through a portion of its own
rotation. For a six-slotted Geneva, 120° of driver rotation is used to index
the table. The remaining 240° of driver rotation is dwell time for the
table, during which the processing operation must be completed on the
work unit.
In general,
Where θ= angle of rotation of worktable during indexing (degrees of
rotation), and ns = number of slots in the Geneva.
• The angle of driver rotation during indexing = 2θ , and the angle of driver
rotation during which the work table experiences dwell time is (360-2θ).
• Geneva mechanisms usually have four, five, six, or eight slots, which
establishes the maximum number of workstation positions that can be
placed around the periphery of the table.

Given the rotational speed of the driver, we can determine total cycle
time as:
Where Tc = cycle time (min), and N = rotational speed of driver (rev/min).
Of the total cycle time, the dwell time, or available operation time
per cycle, is given by:
Where Ts = available service or processing time or dwell time (min),
and the other terms are defined above.
Similarly, the indexing time is given by:
Where Tr - indexing time (min).

Cam Mechanism to Drive Dial Indexing Table
•Various forms of cam drive mechanisms, are used to provide an accurate and
reliable method of indexing a rotary dial table.
•Although a relatively expensive drive mechanism, its advantage is that the
cam can be designed to provide a variety of velocity and dwell characteristics.

Ratchet and pawl mechanism
• A ratchet is a device that allows linear or rotary motion
in only one direction, while preventing motion in the
opposite direction.
• Ratchets are used in many other mechanisms, including
clocks, jacks, and hoists.
• Ratchets consist of a gearwheel (marked with a "b" in the
diagram to the left) or linear rack with teeth, and a
pivoting spring loaded finger called a pawl (marked with
an "a" in that same diagram) that engages the teeth.

• Either the teeth, or the pawl, are slanted at an angle,
so that when the teeth are moving in one direction,
the pawl slides up and over each tooth in turn, with
the spring forcing it back with a 'click' into the
depression before the next tooth.
• When the teeth are moving in the other direction,
the angle of the pawl causes it to catch against a
tooth and stop further motion in that direction.
• Because the ratchet's teeth can only stop 'backward'
motion at discrete points, a ratchet does allow a
limited amount of 'backward' motion, or backlash, to
a maximum of the spacing between its teeth.

Rack and pinion mechanism
• A rack and pinion is a pair of gears which convert rotational motion into linear
motion.
• The circular pinion engages teeth on a flat bar - the rack. Rotational motion
applied to the pinion will cause the rack to move to the side, up to the limit of
its travel.
• The rack and pinion arrangement is commonly found in the steering mechanism
of cars or other wheeled, steered vehicles.
• This arrangement provides a lesser mechanical advantage than other
mechanisms such as recirculating ball.

Storage Buffers in Production
Lines
A location in the sequence of workstations
where parts can be collected and temporarily
stored before proceeding to subsequent
downstream stations
• Reasons for using storage buffers:
– To reduce effect of station breakdowns
– To provide a bank of parts to supply the line
– To provide a place to put the output of the line
– To allow curing time or other required delay
– To smooth cycle time variations
– To store parts between stages with different
production rates

Storage Buffer
Storage buffer between two stages of a production line

Storage Buffer
)( 1
k )( 2
k

Control Functions in an
Automated Production Line
• Sequence control
– To coordinate the sequence of actions of the
transfer system and workstations
• Safety monitoring
– To avoid hazardous operation for workers and
equipment
• Quality control
– To detect and possibly reject defective work units
produced on the line

Applications of
• Transfer lines for machining
– Synchronous or asynchronous workpart transport
– Transport with or without pallet fixtures, depending
on part geometry
– Various monitoring and control features available
• Rotary transfer machines for machining
– Variations include center column machine and
trunnion machine

High volume production systems

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