1. INTRODUCTION Origin
The Lucas fuel injection system was
originally designed for Rolls Royce. Around the end of the war Lucas
designed a fuel injection system for the Merlin aero engine (Probably
its tank version, the Meteor and later tanks e.g. the Centurion, had
Lucas fuel injection instead of a large Zenith carburettor.) It was not
a direct fuel injection such as was fitted to German Daimler Benz aero
engines of World War 2 as this would have had lost the effect of charge
cooling. Lucas continued to design and build fuel systems for Rolls
Royce gas turbo aero engines and they still do.
The Lucas petrol injection system
however was designed for a particular application of one of the "B"
range military and commercial petrol engines produced by Crewe.
These engines, of 4, 6 and 8
cylinders were designed in the late 1930s for motor cars which might
have replaced the Phantom Ill, Wraith and Bentley Mk V if war had not
intervened. The post-war cars were powered by versions of the B60 and
B80, but a major application of these engines was, and is, powering
military combat vehicles.
One particular application of a B
range engine was a post-war German light tank. A tight spec. was put on
this engine. This led to developing a special cylinder head with
individual instead of twinned exhaust ports, and also to Lucas
developing the fuel injection system in place of the normal carburettors.
In finality the tank was not proceeded with, but the fuel system was
employed in racing and found its way into certain production Triumph
models.
The System
This system, which is fitted to the
Triumph 2.5 PI Mks 1 & 2 and Triumph TR5 & 6, is described in the
relevant workshop manuals.
The manuals give adequate information
for stripping and rebuilding the system, trouble shooting and setting up
both throttle butterflies and the pressure relief valve. They do not,
however, give sufficient quantitative information on the "correct"
settings for the metering unit camtrack and setting rings. A great deal
of useful quantitative information is, however, given in the Lucas
Service Training Centre manual entitled "Petrol Injection Mk ll." But
even this does not give a lot of background information that would be
useful in keeping the system operative in future years when factory
replacement became short.
Further information has thus been
sought from key personnel within the Lucas organization. The system was
designed by Harry Bottoms, who normally designs aero engine fuel
systems. It was developed by Jim Littlehales of the Engine Fuelling and
Controls Systems Development, commonly referred to in the past as the
Injection Lab. Both have contributed information, but the vast majority
has come from Jim Littlehales, whose patience with enthusiasts for the
system is saintly' it would have been impossible for him to have helped
more.
2. THE INLET MANIFOLDS
Original Design
When I first spoke to Harry Bottoms
in 1973 he was horrified to hear that 6 separate inlet pipes and
butterflies had been fitted. His original design called for only one or
at most two. Fitting six brings obvious synchronization problems.
Early Pattern Inlets fitted to the Mk 1,
and possibly early Mk 2's had individual tickover stop adjusters.
Throttles were operated by separate rods which operated the crank of
each inlet pair. These rods were actuated by a master shaft below the
inlets. These rods were adjustable for length with the result that
tickover and pickup were both easy to set up.
Later Pattern
Later systems, for some reason, work
on the basis of the first inlet pair butterfly spindle picking up the
second, and the second picking up the third through crank levers similar
to the systems with multiple SUs. Tickover stops are not fitted and it
is much more difficult to achieve a smooth pickup from tickover than
with the earlier system. This is because the first pair open before the
second and the second before the third once there is any wear on the
spindles. For reasons I have been unable to find, this later system, is
fitted with a double interlinking balance pipe between the three pairs
of inlet castings. Earlier systems only had one balance pipe and seemed
to work perfectly well.
3. INJECTORS
These are simple in design compared
with diesel injectors. They do not have to contend with the pressures
and temperatures of the combustion chamber as they are mounted in the
inlet manifold.
FIRST TYPE
The earliest pattern of at least
three designs fitted was designed by Jim Littlehales. It is illustrated
in the injection manual. This pattern has a nylon collar which is screw
threaded onto the injector barrel. The inner bore is tapered and honed.
Thus, when the injector itself is fitted, its "O" ring seal is
progressively compressed to form a seal. As far as Jim can remember, no
spring circlip was fitted on the nose to retain the injector insert. It
was unnecessary as the "O" ring effectively held the insert in place.
CAV made these early injectors. This pattern was dropped once the value
engineers got their hands on the system because the screw thread taper
bores were costly to make.
SECOND TYPE
A further pattern was introduced of
which Jim has little clear memory. In this, the injector insert appears
to have been inserted from the front end of the injection barrel, the
barrel then being swaged over to retain it. They appear to have had no
"O" ring seal and to have relied on metal to metal contact of ground
faces for sealing. The actual injector was identical to that in the
earlier unit except that there was no pip on the valve. The pip was the
original grinding centre.
LAST AND MOST COMMON TYPE
The third kind, and by far the most
common, looks on first sight to be identical to the second, except in
that the valve has the pip once more, like the original. These are
built, like the original, with an "C" ring seal, but in place of the
taper bore there is a stepped bore of two parallel diameters. When
replacing the internal "O" ring seal therefore it is necessary to use a
thick oil to prevent damage to the "O" ring when it meets the sharp edge
at the change in internal diameter. The injector insert is retained in
this design by a circular spring ring on the nose of the insert. As with
the second pattern, the nylon block is a press fit on the injector
barrel.
Dribbling
Whilst Jim Littlehales never
experienced leakage problems with the inner 'C' ring seals, this has not
been the experience of others who have run cars with this system for
many miles. The injector inserts tend to shuffle a little in the
injector barrels. Carbon gets between the barrel and the 'C' ring and
wears the ring until it is a 'D' section instead of an 'O'. It then
starts dribbling, but can be cured with a new 'O' ring once the bore has
been carefully cleaned.
Some of the third pattern were
manufactured with too short a thread engagement in the inner nylon
adjuster nut. They unthreaded themselves and the injector valve then
damaged below the valve seat. The nylon adjuster is an interference fit
on the thread and is adjusted with concentric Allen Keys on all three
types.
Injector Performance
Injectors should blow at between 47.5
and 50 psi. They are all right at anything above 45 up to 55. They
should give an even cone spray. There should be no leakage at all at a
pressure of 5 psi below their blow off setting. This is what Jim said
when I last saw him but I believe there was a production standard of
several drops per minute allowable leakage.
Rough running can be experienced if
injectors dribble although this has little effect on fuel consumption,
if any.
4. INJECTOR LEAD NON-RETURN VALVES
These are located at the junction of
each pipe with the metering unit. The valve looks to be of neoprene
although Jim describes it as hard rubber. They are designed with plenty
of clearance between the valve and the bore in which they are located.
In theory this allows the valve to process around on its seating. In
practice, however, they tend to stay on the same spot and continuous
operation tends to produce a circular groove on the valve face. They
then leak, allowing the residual pressure caused by the elasticity of
the fuel line to leak back into the metering unit. The next injection
stroke of the metering unit therefore spends much of its energy
re-expanding the pipe instead of forcing the correct quantity of fuel
out of the injector. Also, after running, heat from the engine can
vaporize the fuel in the pipe and it then takes a long time to reprime.
That cylinder runs "dead" until the line refills. The neoprene valve
face needs carefully rubbing flat again if leakage occurs.
5. METERING UNITS
Return Pipe
Early metering units had a "push on"
rubber connector to the fuel return pipe. Later ones had a screw on
connection. If the return pipe gets blocked, and this is not an uncommon
fault, pressure rises in the unit and forces the diaphragm between the
metering barrel and cambox towards the latter. This tends to prevent the
roller climbing to the maximum depression the fuel is in weak condition
up the camtrack. in consequence fuel consumption is increased. The pipe
can usually be unblocked either by using a foot pump or airline on the
return pipe, or by carefully pushing soft iron wire down it.
Golden Units
One cause of blockage can be copper
particles in the return pipe. This happens if the lead rich copper end
thrust washer on the metering barrel wears, as the swarf returns by the
pipe to the fuel tank. Bill Phelps came across this problem, which in
its extreme reduces the alignment of the barrel ports with the injector
lines. This reduces fuel delivery to the injectors. When I reported this
to Jim Littlehales he said that this problem did occur and that units so
affected were referred to as "golden units". Petrol has a low lubricity
and, he said, if the end washer did not sit exactly at right angles to
the bore (i.e. parallel to the face on which it sits) the petrol escapes
unevenly and wear sets in on the "high spot". Once this happens, the
process of wear is very rapid. It also has the effect of increasing the
clearance between the rollers, camtrack and piston stop, thus making the
unit go "fuel rich". A few thou can absolutely ruin fuel consumption.
Clearance between the metering barrel and its sleeve, incidentally is a
fraction of a thou, although subject to variation in manufacturing
tolerance . Tolerances are unusually fine for car components.
Lubrication
The shaft on which the rollers run
should be lubricated with a little moly green or moly additive. The
piston stop should not need lubricating. Jim Littlehales says they
should run in an oilite bush. However, I have seen signs of "pick up" on
the piston stop. Jim says that if lubricated at all, it should be with a
thin oil like 3 in 1.
Springs
The springs in the diaphragm capsule
are critical and must not be changed unless the capsule nuts and
camtrack are recalibrated.
A critical fault occurred in some
units. The high rate "second" springs were feather ended. In other words
the end coil, ground flat and therefore thin to give the spring a flat
end, did not touch the first full section coil. As a result the high
rate spring did not have sufficient rate until the feather end had
deflected enough to touch the next coil. The first part of the spring's
compression happened too easily. This has the result of putting a fuel
rich kink in the depression/fuel delivery working line right where you
don't want it - in the very middle of the normal driving range. A
possible cure would be a blob of Araldite to support the feather end on
the next coil. This fault is always worth looking for. Feather ends were
sporadic throughout production - not just an occasional batch problem.
Wear And Roller Pin & Cam Setting
Frettage corrosion can occur on the
pin if run dry. When this happens it increases the clearance between the
rollers and the piston stop. The cure is to readjust the camtrack to
.002" and .058" clearance at either end of its working range, using
feeler gauges. This is easily done with the capsule springs removed
using the mouth to suck on a pipe to raise the rollers to their to
position. Measurements should be made with the unit in its normal
operating attitude and the feeler gauges should be stroked downwards,
not upwards, to retain the correct positions given by gravity when there
is any "play" either in the nylon ball-joint or roller pin.
Camplates
These are hardened steel and do not
wear. However, the constant hammering of the metering shuttle can move
the camplate away from the rollers by rotating it about its fulcrum.
This sends the system fuel rich. Two setscrews hold the camplate in
position on the fulcrum arm. In some cases these are cross slotted
screws. Others are Phillips or Posidrive. The former can be adequately
tightened whereas the latter have proved unsatisfactory in my
experience. The centre tends to trepan out before the screw is tight
enough. When tightening these screws I apply Loctite then leave the unit
for 24 hours before using it again. On my Mk1 PI my fuel consumption was
around 19.7 on overall running - much of which was my 17 mile drives to
and from work - until I reset the camtrack. After resetting this went up
to 27.4.
Faults of the System
The final demise of the system was
the difficulty of meeting the emission control regulations especially at
the very low fuel delivery quantities at tickover and low throttle
openings. The system had by then already got itself a bad reputation. In
part this was because garages did not understand the system and could
not set it up correctly. It was also partly due to manufacturing
problems with the system. As developed the system was excellent but the
value engineers cheapened important bits of it. Furthermore, when
manufacture transferred from Lucas Aerospace to Lucas Automotive, the
criticality of manufacture was lost. It has been said that the people
building the all important fuel pump motors "thought they were building
windscreen wiper motors". Oil got under the commutator segments during
motor manufacture, with the result that the segments lifted and then the
motors failed. Pump shaft seals wore and allowed fuel to leak into the
motor. All these problems were overcome in due course, but rather late
in the day.
Emission Control
There is obviously a small (but
variable in manufacturing terms) clearance between the metering barrel
and its sleeve. As a result "inter port" leakage takes place, thus
increasing fuel consumption. The greater the clearance, the greater the
leakage. It can be as much as 20% more than the quantity metered at
small shuttle movements. In 1973 Lucas therefore introduced three
production standards - A, B and C. These are marked on the metering unit
if built after about July of that year.
The unit was run at 2.50 rpm rotor spud
and a 0.010 shuttle stroke. Using petrol, average measured quantities
per 1000 injections (1000 revolutions) had to be as follows:
A - 8.4 cc to 8.9 cc
B - 7.8 cc to 8.3 cc
C - 7.2 cc to 7.8 cc
All plus or minus 10% with petrol.
Green Top Units
At the end of the production period a
three spring capsule was introduced to attempt to get a little nearer to
the theoretical depression/delivery curve requirement. These units
had a green plastic cap on the adjuster nuts and were called "green top"
units. They were only a marginal improvement in attempting to meet
emission regulations and few were fitted to cars.
Springs
I have a metering unit with square
section springs. Jim Littlehales cannot recall any of these and is
suspicious that the springs are not original. He believes all springs
were manufactured from round section wire. He told me to check the cam
clearances at various depressions against the data in the manual to see
if the unit was properly set up.
Pressure Relief Valve Design
The pressure relief valve is a very
neat piece of design and was the work of Harry Bottoms. It is very well
finished, as are many parts of the injection system. The quality being
above normal motor practice and reminiscent of aero engine components
prior to World War 2. It does not, however, approach modern practice.
Function
The function of the valve is to
maintain fuel pressure at between 100 and 110 psi when the car is
running. However it is also designed to achieve one other thing. A
concern from the outset of the system design was the case of a "dry
tank". In this situation the system had to be capable of repriming
itself and the relief valve is designed to facilitate this. Early
systems (about 70) had a "well" or small tank alongside the pump. This
filled by gravity from the main fuel tank. If the main tank ran dry the
first fuel put in it would run down the well. The well was fitted with
an air vent pipe which connected to the top of the tank. It could have
vented the atmosphere above the fuel tank "full" level except that it is
both safer to close circuit back to the tank and also ensures that any
air passing down the pipe is filtered (see "filters"). This return pipe
allowed the air in the well to be displaced when the fuel flowed in,
the well thereby filling completely.
Its Operation
Supposing the system to be dry, the
pump will deliver air at a pressure of about 20 psi. At 20 psi the
relief valve begins to open. In doing so small bleed holes are
uncovered. These allow the air to escape through the valve and back to
the fuel tank. Once the air is displaced and fuel flows, fuel begins to
flow through the bleed holes. However, as it is denser than air, it
cannot escape as fast as the pump delivers it. The pressure then builds
up further and at 60 psi the valve moves further, cutting off the bleed
holes. No more bleed occurs until the full operating pressure of 100 -
110 psi is reached when the main valve opens. PRESSURE SHOULD NEVER
BE BELOW 100 psi at "full chat" on the road according to Jim
Littlehales or misfiring will occur.
FILTRATION
Cleanliness of fuel is essential with
the system. There is a full flow main filter between the fuel tank and
the fuel pump. There are two small coarse filters in the pump inlet and
outlet elbows. There is a small nylon filter in each injector barrel and
I think, one in the metering unit. Air entering the fuel tank to replace
burned petrol is also filtered. There is only one place that a filter is
needed but not fitted and that is on the fuel pump breather pipe.
Early systems had the well referred
to in the previous section. Petrol entering it was filtered through a
very tiny unit which according to the handbook, was to be changed at
something like 12,000 mile intervals. It was totally inadequate. A small
quantity of water in the fuel was all it needed to block the filter
partially. If the fuel level was low or even with plenty of fuel if you
turned left suddenly, the fuel pump found it easier to suck air down the
well's breather pipe than from the tank. The filter assumed gravity
would always force fuel through the filter down into the well. My engine
used to cut out on my 2.5 Mk1 on left hand corners for 2 - 3 seconds
while the pump screamed. Colleagues at Hucknell nearly rammed me behind
on several occasions when we were hurrying over from Derby to get to
work on time. I asked Jim Littlehales what to do. He told me to throw
away the well and fit a CAV diesel oil filter (which was similar to or
the same as fitted to 2.5 Mk 2 cars). He said it would pass nothing
above 2 microns. I did this, but retained the well although clipping the
vent pipe so that its maximum rate of passing air was greatly reduced. I
never had any more problems. Recently Jim has warned me that these CAV
elements are not flushed on production and that fibres can get into the
pump and cause damage. He suggests giving a new filter element a really
good wash out before fitting it I do, not believe any of the other
filters are important, except perhaps the air filter on the fuel tank,
and that is doubtful as the breather pipe is very long, goes downhill
all the way, and fuel does not burn quickly enough for air movement up
the pipe to be speedy enough to carry dust with it.
The Pump Reliability
The pump is the only part of the
system that can let you down completely and leave you stranded. They can
do so with absolutely no warning at all. I have had many fail on me.
As originally designed, it was a good
piece of equipment but the value engineers got their hands on it and the
troubles started. Slowly, their integrity was restored and late pumps
were generally very good.
Regardless of date of manufacture,
pumps vary greatly. Some have lasted me just over a year. One especially
selected for its low current consumption for me by Jim Littlehales ran
from the summer of 1975 to the autumn of 1978 when it aired. Cleaned, it
resumed work in the summer of 1979 and, at the time of writing, is going
as beautifully as ever. The one originally fitted to my 2.5 PI MK 2 ran
from 1 January 1975 till the summer of 1979, and the pump on Jim
Littlehales own car ran from something like 1972 or 3 until just after
my original MK 2 failed in the summer of 1979. Even then it didn't
really fail it occasionally skipped a beat. On examination it was found
that the carbon brushes had worn out completely and the copper rat tail
brush heads were lying on the commutator and supplying the current.
Early Failures
My first failure was of a shaft seal
on my MK 1. The pump was then 3½ years old. It did not leak fuel down
the vent but entrained air through the seal in some quantities. It thus
pumped an aerated mixture, especially when hot. This caused restarting
problems and the best solution was not to switch the engine off if
avoidable in hot weather.
My second failure followed about a
year later. The bearing pins of the pump gearwheels wore until the gears
started to cut into the pump casing centre (figure 8 aperture) plate.
Since then I have never had any real problem with the actual pump part
of the unit.
Later Failures
Recent failures have been of the
motor. In almost every case, stripping and carefully cleaning the
commutator restores the unit to continued good service. I have done this
many times and the pump can run, trouble free, for as long again.
The Motor Bearings
The rotating assembly is supported by
two spherically mounted oilite bushes, and positioned at one end by a
thin steel thrust washer bearing on one face against a bush and on the
other against a circlip on the motor shaft. The other end is positioned
by a nylon buffer on the end of the adjustable setscrew projecting from
the motor casing's end. I have never experienced pump bearing problems
either with bushes or the shaft.
Motor Rotating Assembly
These vary in current
consumption. Good ones absorb less than 5.8 A and poorer ones well over
6 A. Jim always prefers 6 A max. (5.8 for hot countries as heat causes
cavitation). Lucas manuals are a little ambitious in their claims.
Windings can burn out. It has happened to me once.
Commutators at one stage gave
mechanical trouble. Oil got below the segments during manufacture with
the result that sooner or later they came apart. This problem did not
last long. With time however, on all pumps the commutator gets dirty and
a certain amount of burning of the copper takes place, causing the pump
to stop working. I clean them by mounting the rotating assembly in an
electric drill to spin it then gently rubbing fine sandpaper on the
commutators. Once this shows as smooth and bright copper all the way
round again, I use fine, blunted sandpaper to polish the commutator and
finally Duraglit wrapped in a piece of old handkerchief. I then clean
out the gaps with a needle and finally clean with a hanky moistened with
"tric" or "carbontet". Wrap sellotape on the shaft before "chucking" in
the drill.
Brushes
Brushes give no trouble other than in
that they wear very slowly away. When reassembling a motor be careful
not to let the rat tail brush heads get caught behind the brush holders
as, with wear, the brushes will be restrained from moving for- ward and
maintaining contact with the commutator.
Loads Heat and/or petrol vapour can
harden the plastic covering. New leads can be made and soldered on to
the brush holders although the originals are welded on. Jim Littlehales
told me this.
Replacing Brushes
Someone once told me that the paxolin
brush, deck and brushes from a Lucas windscreen wiper motor will fit,
although only two of the three brushes are needed. I have never tried it
has not been necessary.
Heating
According to their current
consumption and state of wear, motors can get very hot. They normally
run too hot to touch comfortably. After a long run on a summer's day the
heat will sink from the motor into the pump on a car that has stopped.
Fuel then evaporates and the pump screams. The car often will not start
until the whole thing has cooled down. During running the fuel cools the
pump so no problem arises. The problem is accentuated in a hot climate.
Best cure courtesy Jim Littlehales is
to install a pump cooler. Wrap ¼" bundy pipe in a spiral round a mandrel
of slightly smaller diameter than the motor. Then spring the coil open a
bit and work it over the pump housing. Connect the bleed valve return
fuel flow to the copper pipe. It then provides fuel cooling, the warmed
fuel returning to the tank. This works well. A cruder cure is to slap a
rag soaked in water onto the motor in hot weather. It does work. Bill
Phelps suggested that mounting the pump vertically should help, motor up
and pump down, as heat rises.
Magnets
Their power varies but Jim
Littlehales does not see this affecting current consumption. They can
however crack, so if removing them to clean the motor housing, slide
them out with care. Replace them the right way round afterwards!
Noise
Some are quiet, some are noisy, some
start noisy and run quieter as they warm up. The noise dips and rises
with the flashes of the indicator, or application of brakes. This is
only the effect of variation in the current supply available. It is not
a cause for concern. Minimum noise requires ½ turn of end float on the
endfloat adjusting setscrew.
Vent Pipe
Between the motor housing and the
shaft seal which isolates the motor from the pump there is a vent pipe
which exhausts through the car floor onto the ground. It is intended to
dump any petrol that leaks past the shaft seal and prevent its entering
the motor. With a horizontally mounted motor as in the 2.5 PI saloon one
wonders if it picks up all the leaking fuel or whether some reaches the
motor. In the estate the pump is mounted vertically and this should
ensure that any leaking fuel is dumped, quite apart from being a better
way of keeping the motor's heat away from the pump.
The vent pipe is the one point in the
system where a filter should be fitted, but there is none. This is
because the pipe exhausts at one of the dirtiest and dustiest points
possible - right behind the rear wheel of the car. This would be fine if
no air ever moved up the pipe, but does. When the car is run, the motor
heats. The air inside expands and some is expelled. When the car stops,
the motor cools and air is sucked up inside the pipe. Quite apart from
this the shaft seal usually leaks a little - fuel out when stationary
and sometimes when running - air in when the engine is running in many
cases. Minute differences in the pump determine whether the seal is
subjected to pressure or a depression. Many pumps actually suck air in
past the seal and feed it into the system under pressure with the fuel.
This causes erosion when the air is humid.
My solution is to push a bit of
cellular plastic foam up the vent pipe and, from time to time, moisten
it with a squirt of WD40. It is easy to do and at least gives a measure
of filtration to the air.
Shaft Seal
The shaft seal as originally designed
was in Viton but value engineers changed this for a cheaper and less
satisfactory material. Pumps today again have the seal made in Viton and
it is very satisfactory. It is a standard proprietary seal made by
George Angur.
The seal runs with less interference
on the shaft than such a seal normally would. The pressures are not very
great and too much friction would cause undue heating. As it is, the
Viton seal is rather hard when cold and tends to leak a little until it
warms up. It warms quickly once the motor starts however, through
friction.
As stated elsewhere in this document,
the seal can allow air to pass into the pump or fuel to leak out. If
damp air is sucked in, corrosion takes place on the motor shaft and the
rust particles stick to the seal lip. They act as an abrasive and a
groove is worn in the shaft by the seal. The seal then becomes less
effective in its functioning. 1 have seen a number of pumps so affected.
The seal is to some degree lubricated - by petrol splashed at it by the
helices on the plastic drive coupling between the motor shaft and pump
drive gear. Provided the seal lip is at least wet, it is lubricated.
The Pump
The pump itself consists of three
brass plates. The rear one incorporates the inlet and outlet unions and
carries the bearing pins for the two pump gear-wheels. The centre plate
incorporates a figure of eight aperture to accommodate the two pump
gears. The front plate - thin on very early pumps but beefed up on later
ones as they distorted under the working pressure inside the pump -
abuts to the motor and has one aperture through Which the drive gear
passes to its coupling to the meter.
The pump is made to exceedingly tight
tolerances and these are critical.
Clearance Gear Faces
The centre plate is slightly thicker
than the gear wheels so that there is a running clearance between the
gearwheels and the top and bottom plates. The production tolerance on
total clearance for the gears was 0.0002" - 0.0008" (2/10 to 8/10 thou).
In practice Lucas aimed with great care to achieve 0.0004" (4110 thou)
on production build. Above 8/10 thou clearance the pump rapidly loses
flow and becomes useless. That is why the tolerance is so critical.
(1/10 thou extra clearance loses you 1 gallon/hour at 100 psi approx.)
Clearance Gear Teeth
The clearance peripherally between
the gear teeth and the figure of eight aperture is nominally 0.004" (4
thou). Again this is critical. Above that figure you lose one gallon per
hour pumping capacity at 100 psi for every extra thou of clearance.
Several thou, therefore, and you have zero flow.
There is considerable clearance between
the tips of the teeth on the gearwheels and the roots of the teeth on
the mating wheel. This is to prevent hydraulic locking occurring by fuel
being trapped between the teeth when they mesh.
Bearings
The gearwheels are bushed and run on
hardened and ground steel pins mounted in the bottom plate. The driven
gear pin is not provided with special means of lubrication but the drive
gear pin is. It is hollow and has a hole bored at right angles into it.
Fuel flows down the inside of the pin and lubricates the bush through
the side drilling.
The bushes in the gearwheels are of a
special carbon which has a copper content. The copper is to conduct the
heat away which carbon on its own would not. As far as Jim Littlehales
can remember, this is Morganite MY3D. The bush is fitted into the
gearwheel with virtually zero clearance. The clearance in fact is
0.00005 (½ a tenth of a thou). Inserting the bush normally gives the
slightest shaving to the bush.
Bearing clearance between the bush and
bearing pin is nominally 0.001 " (one thou) but this is not as critical
as the surface finish in the bush bore. There is only one way to get
this finish, and it can be done by hand. Use a soft mild steel bar and
have syntox (aluminium oxide) sprayed onto it. Then grind the syntox
parallel on a diamond wheel. Place the rod in the carbon bush and polish
it. It will give a mirror finish.
Bearing Wear
Bushes can pick up metal particles
and then they will cut up the bearing pins rapidly. I have only seen
this in one (my second ever) pump. Strangely, Jim Littlehales once saw a
pump in which the bush to bearing clearances had become massive. The
gears had cut their way into the centre plate creating a "pregnant"
figure of 8 aperture. It still worked and still pumped fuel.
Gear Corrosion
They don't.
Gear Manufacture
Gears are ground and through
hardened. Early gears were hobbed but this left striations on the teeth.
Grinding was found more satisfactory.
Wear Between Gear Facings and
Plates
In theory there is high pressure on
one side of the gears (c 100 psi) and low on the other so a slight
leakage occurs across the gear faces and lubricates them. In practice
the flow tends to set up on one face rather than both faces of the gear.
The other face can therefore run dry and cause friction, heat and wear.
In the early days of developing the Mk 1 system Jim Littlehales tried
just about everything to achieve lubrication of the gear faces. Finally,
in desperation, he inscribed a helical groove on a gear face with a
scriber. To his surprise the current consumption of the motor went down,
showing that lubrication had been effected. It became known as "Jim's
Groove" although its precise functioning was not fully understood. Any
loose fibres from the filter used to work their way down the groove and
form a blockage. In effect the blockage became a disc brake pad. Mk 2
systems do not use Jim's Groove.
Gears can cause grooving of the top or
bottom plate. 1 have seen both although Jim says they are common on the
pin plates below the drive gear. As Jim says, they are not very
important as they tend to look a lot worse than they really are.
Restoring Side Plate Clearance
If it is desired to remove the
grooves, the top and bottom plates can be restored but the centre figure
of 8 plate must not be touched as its thickness is so critical. use
brand new wet and dry paper on a completely flat surface - a measuring
table or piece of optically ground glass of size than the brass plate.
The wet and dry must have no kinks or creases.
After sanding you must rub the
surface with your fingers with paraffin and keep doing this until your
fingers remain clean. This will remove the embedded particles.
Lastly, place clean paper on your flat
surface and pour Brasso on it rub the plate on the paper |
