Russell Davison

CASE STUDY - THE DESIGN OF A HYBRID FLEXIBLE ASSEMBLY SYSTEM FOR SPEEDOMETERS

The case study describes how a hybrid flexible assembly system was designed for the assembly of a mechanical drag cup speedometer. This type of speedometer is the most widely used today and its design has not changed over the last 50 years. If there is already a heavy investment in capital equipment for the manufacture of the individual parts then it is not economical to re-design the product for automatic assembly.

The input shaft of the speedometer carries a permanent magnet. The flexible drive shaft from the engine drives the input shaft, thus setting up a rotating magnetic field. A metallic cup is situated in this field and is continuously connected to the pointer. As the input shaft rotates, a torque is produced at the spindle, which is proportional to the speed of the input shaft. The spindle is free to rotate and yet is restrained by a delicate hairspring. The spring rate is chosen to be linear over the range of the spindle angular deflection, thus providing a pointer movement that is proportional to the input shaft speed. The hairspring returns the pointer to zero when the vehicle is at rest. A series of gears from the input shaft convert the rotation of the flexible drive shaft to a rotation of the odometer wheels. Gear ratios typically vary from 600:1 to 2000:1.

There are 25 parts used in the assembly of the speedometer and more than 50 product styles can be obtained by a variation in the design of six parts. These are the dial, second worm gear, third worm gear, odometer sub-assembly, hairspring and pointer sub-assembly. The total annual production volume for all the styles is in excess of one million units. An individual style may be required in volumes between 200 and 200,000 per year. Clearly, these volumes require an assembly system which has flexibility to handle such large demand fluctuations.

The speedometer consists of four sub-assemblies and twelve parts. The dial sub-assembly has three parts, the first worm sub-assembly has six parts, the speed cup sub-assembly has two parts and the frame sub-assembly has two parts. Each sub-assembly is a self-contained unit and does not require any holding of the parts for stability between workstations.

Synchronous assembly machines are most economical for the high volume assembly of a small number of parts. Each sub-assembly contains six or less parts, making them most suitable for this method of assembly.

A rotary indexing machine for the FRAME SUB-ASSEMBLY is used for the assembly of two components. There are eight workstations on this machine to allow for non-value adding operations in addition to the direct insertion process. The handling difficulty level of the bearing means that it is presented by a specially designed feeder. It is impregnated with oil and this doesn’t allow the part to be handled by a conventional vibratory feeder. The frame cannot be handled by an automatic feeder because it is large and has no symmetry about any axis. The complex shape of the frame means that it cannot be magazined and it is, therefore, palletised. A robot places the frames onto the machine because they are picked from several hundred pallet locations.

The rotary indexing machine for the SPEED CUP SUB-ASSEMBLY uses a simple pressing operation to secure the speed cup to the spindle. There are four workstations for; the assembly of the spindle to the fixture, the speed cup to the fixture, the pressing of the speed cup onto the spindle and an output station. Both parts are fed by vibratory bowl feeders and inserted by dedicated workheads.

The FIRST WORM SUB-ASSEMBLY consists of six components, all of which are fed by vibratory bowl feeders. The indexing machine uses ten dedicated workstations to complete the sub-assembly. The first worm shaft is burnished before final assembly. This operation is executed after the rotary indexing machine, on a free-transfer line. Two burnishing stations are used, in parallel, to achieve the cycle time. The free transfer line also provides a buffer stock of completed sub-assemblies before the final assembly line.

The rotary indexing machine for the DIAL SUB-ASSEMBLY assembles three parts. Only the pointer stop can be automatically fed and so the dial and label use special feeding methods. Different designs of dials are used to create product variety. However, only the print face and diameter of the dial are variable and the dial is picked from a magazine, on the reverse face, by a dedicated workhead. The label is applied by a conventional labelling device.

All sub-assembly indexing machines are linked to the final assembly machine by free-transfer lines, for overall system efficiency. This also creates space for auxiliary operations to be carried out on the sub-assemblies before final assembly. The speed cup sub-assembly is dynamically balanced before final assembly, and this is done with the aid of two robots. The programmability of a robot is required for the 'decision making' operations of this process. Feedback from the balancing machine determines whether the sub-assembly has to be balanced more than once or, in the case of it being excessively out of balance, it is rejected.

There are twenty six workstations used for the FINAL ASSEMBLY of the speedometer, making it necessary to use a free-transfer linear machine to allow buffer stocks to be created between each workstation, to maintain high system efficiency. Of the twelve parts used during final assembly; seven parts are handled by conventional vibratory bowl feeders, two parts by multiple vibratory feeders, one part by pallet, one part by manual handling and the remaining part by actual manufacture on the assembly line.

The parts which are fed by vibratory feeders are small components with either useable symmetry or definite asymmetry. These are inserted into the part-built assembly by dedicated workheads.

The two parts to be handled by multiple vibratory feeders are the second worm gear and the third worm gear. These parts are changed to produce the various gear ratios used to create different product styles. The disruption to production, during product changeover, is minimised by using a group of vibratory feeders which deliver one particular second or third worm. The pick up point of the workhead is thus quickly changed to the output of a particular feeder for the assembly of a different style.

The jewel--plate sub-assembly is a large and delicate part which cannot be fed by an automatic feeder. It can, however, be palletised. A robot picks up the jewel-plate sub-assembly from the pallet and inserts it into the part-built assembly. The operation is relatively complex and an operator has been retained at this station to assist the robot when difficulties arise.

The hairspring is a delicate part that can’t be handled by an automatic feeder. The insertion process is also difficult because the end of the spring is welded to a stub on the jewel plate. This part is assembled manually by two workers in parallel, because of these difficulties.

The second worm gear retaining pin is manufactured from wire and it is most cost effective to manufacture this part on the final assembly line by a guillotining operation. The bending of the pin is carried out simultaneously to the part being inserted and secured.

CONCLUSIONS

1) Product re-design for ease of assembly creates worthwhile savings in assembly costs. However, particularly for large products, these cost savings must be offset against the additional tooling modification costs for the manufacture of re-designed components.

2 ) When assembling a product which has :
a) Many parts
b) Many variants in the product family
c) A large annual production volume
d) Many common sub-assemblies

a hybrid flexible assembly system is required and it will combine manual, automatic and robotic assembly methods.

3) Sub-assemblies, having a fixed content, are always best assembled on dedicated automatic assembly machines.

4 ) Variable content sub-assemblies are most economically assembled using either
a) Assembly robots
b) Flexible free-transfer machines

5) Transfer between sub-assembly production units and final assembly need large buffers to de-couple these two activities and reduce downtime.

I originally published this article under the title, “Changes in Assembly Work Environments” in the book “Programmable Assembly”, ISBN 0.903608.65.0.

The role of the modern assembly worker is very different now from that of 3 generations ago. Improvements in parts quality consistency has eliminated the previously required skill of the apprentice trained fitter. A new breed of unskilled assembly workers has been created, through the division of labour, to carry out repetitive and mundane tasks. However, many companies use assembly automation if it can be economically justified, and after the product has been re-designed for automatic assembly.

The development of modern assembly techniques is discussed, together with future trends in manual and automatic assembly. Emphasis is given to the changing needs of the people directly involved in these assembly operations.

Introduction

There has been a rapid increase in living standards in the developed nations throughout the previous century, mostly due to the application of technology to manufacturing. The mass production of goods has made many items available at economic prices. Homemakers now have a multitude of labour saving devices to reduce the amount of time spent on household chores. This has enabled many homemakers to work in factories which produce these goods. Assembly workers can master a simple assembly task and repeat it for more than 1000 times per day; every day. Working with other people on an assembly line can create a sense of cooperation within a joint effort.

However, there has been criticism of the assembly line technique. It is argued that the repetitive work is boring and tedious and that workers no longer gain satisfaction from doing their job. Workers never see the finished product and the continual repetition of movements creates boredom. Industrial unrest in high volume manufacturing companies has been associated with the job dissatisfaction of assembly line workers. Manufacturers now realise that the economic benefits of the division of labour have to be judged alongside the sociological and psychological disadvantages.

The use of assembly automation during product manufacture eliminates worker dissatisfaction with repetitive work, since most of the mundane tasks are done by machines. Workers are then used to fill magazines/feeders and to maintain the equipment. The reduced labour content often creates a cost reduction in the finished goods. The culmination of this desirable process is an increase in leisure time, through a reduction in the working week. Emphasis must then be placed on how people are to spend their leisure time. This should be the subject of major reform in our training establishments.

Technology

Technology is the systematic knowledge of the industrial arts. Industrial engineers have been applying technology to the workplace for over two centuries. Manufacturing systems analysed by method and time studies have been improved by the division of labour, automation and robotics. Large productivity improvements have been achieved by applying technology to manufacturing processes. From the mechanisation of flour production to the robotic assembly of vehicles, process costs have been reduced. The application of technology to the motor industry has resulted in vast increases in productivity.

Method study is concerned with the dissection of a complex operation into it’s single constituent parts, which are then systematically analysed. The method study engineer synthesises the complete operation using components which optimise factors such as symmetry and the rhythm of movement.

The time study engineer measures the time taken to carry out an operation. The analysis is carried out in a systematic manner and it makes this form of study suitable only for simple and repetitive tasks. Often, time study exposes inefficient operations and these can then be analysed using method study.

It was the use of both method and time studies that led to the wide-scale use of the division of labour and the creation of the assembly line concept. Workers grouped on lines achieve productivity levels many times greater than single operatives making the entire product.

Automation has also produced large productivity increases by replacing men with machines. In highly automated manufacturing plants, the operator controls and supervises the process. The main power olders in future societies will not be capitalists or socialists, but people who possess expert technological skills. In this way, power will be passed to the techno-structure.

Automation

Automation in the manufacturing industries covers a whole range of electrical and mechanical equipment. In the field of automatic assembly, devices are used for automatic feeding and insertion. In addition, work transfer is by conveyor or rotating table. The type of system used for the assembly of a product is dependent upon many factors. The local cost of labour affects the economic justification of using automation to replace that labour. The frequency of design changes and the number of product styles dictate how flexible the equipment needs to be. The market life of the product influences the amortisation period of the capital investment. Finally, the annual product volume determines the required cycle time.

In addition to the above economic considerations, another reason for employing automatic assembly may be one of necessity. In certain areas, where labour is scarce, the use of automatic assembly is imperative. Certain operations may be hazardous or they must take place in dangerous working conditions. For example, the handling of toxic chemicals or working in extreme temperature conditions may exclude the use of manual workers. A further reason may be associated with the scheduling of the assembly operations: better control over production can be achieved with automation and product quality will be more consistent.

The assembly operation consists of the two basic activities of handling and insertion. If a product is to be assembled automatically then thought has to be given to the economics of these activities. The automatic feeding of simple parts is usually carried out using a vibratory bowl feeder. Components in bulk random orientation are placed into the feeder and the parts are presented to the workhead in an ordered manner. Difficult parts may be fed by special feeders, hoppers or by magazines. The insertion process is defined as being the action where one part is assembled to another part, or group of parts. High speed operations, where the same parts are inserted for long periods of time, are normally effected by standard pick-and place units. Difficult operations, involving the assembly of a number of different parts with different operations may require assembly robots. The flexibility of the robot is created by using computer programs to control the robot arm movements. The difference between a robot and a pick-and-place is that the path of the robot arm is not restricted by mechanical means, whereas pick-and-place units rely upon mechanical stops to determine the path they follow.

Division of labour

The division of labour is the process whereby one complex operation is broken down into a number of simpler tasks. These single tasks are carried out using a series of people, each doing one task. In this manner, a complex task performed by one worker is replaced by a number of workers operating in series. This allows operations to be carried out simultaneously, instead of the single operator having to complete one task before commencing another, different task. Unskilled workers can then be used to carry out these simple operations and they soon become efficient at the particular task.

Assembly systems

An assembly method can be classified into one of six types, and most systems may contain a number of different methods.

The traditional form of assembly is manual and, for high volume production, the workers are arranged on an assembly line. Other forms of manual assembly include a single worker assembling a complete product and groups of workers assembling a portion of the product.

When the range of products is more limited, a manual assisted method can be used, whereby workers are assisted by mechanical devices, such as parts feeders. The feeders present the parts to the assembly worker in an ordered manner. The assembly time is reduced by eliminating the time taken to separate the parts from bulk random orientation.

The third form of assembly uses automatic indexing assembly machines. A rotary or in-line machine has a number of workstations with automatic feeders which supply components to workheads for assembly of the part to the fixture, or part-built assembly. The workstations are 'special-purpose' and are dedicated to the assembly of one product only. Production volumes need to be high for the economic justification of these machines. Component quality must also be high to avoid excessive workstation downtime, caused by jamming, etc.