High volume production systems

05/03/23 1Hareesha N G, Dept of Aero Engg, DSCE, Blore

Syllabus

05/03/23 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.05/03/23 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 Automated Production Lines

• 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.05/03/23 Hareesha N G, Dept of Aero Engg, DSCE, Blore 9

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 LineA 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 LineMultiple 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 workstations2)Segmented in-line – two or more straight line

segments, usually perpendicular to each other3)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 line2)It allows reorientation of the work piece to present different surfaces for machining3)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 configurationAlthough 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.05/03/23 Hareesha N G, Dept of Aero Engg, DSCE, Blore 25

Rotary Indexing Machine


Work Transport SystemsThere 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.05/03/23 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.05/03/23 Hareesha N G, Dept of Aero Engg, DSCE, Blore 31

Mechanized Work TransportThree 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.


• 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.05/03/23 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 available


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 receiving


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.


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

)( 1k )( 2k


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 line05/03/23 Hareesha N G, Dept of Aero Engg, DSCE, Blore 82

Applications of Automated Production Lines

• 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

Engineering

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