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
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
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.