Synthetic composite fuel metering membrane

A synthetic composite flexible diaphragm of a fabric of warp-knitted polyester fibers and an elastomeric polymer coating applied only to one side of the fabric and disposed in interstices between the fibers to provide when cured a barrier at least substantially impervious to liquid fuel.

FIELD OF THE INVENTION 
This invention relates to diaphragms and more particularly to an improved 
diaphragm and method of making diaphragms for carburetors and the like. 
BACKGROUND OF THE INVENTION 
Diaphragm carburetors generally have a flexible diaphragm disposed in a 
fuel chamber which opens to main and idling jets or orifices. Typically, 
the diaphragm divides the chamber into a wet chamber side which is 
supplied with fuel and is subjected to sub-atmospheric pressure during 
operation of the engine and a dry chamber side which may be subjected to 
atmospheric pressure. The diaphragm controls a fuel inlet valve disposed 
between a supply of fuel and the wet chamber side. As the engine draws 
fuel from the wet chamber side, the quantity of the fuel in the wet 
chamber will decrease, and the diaphragm will move against the bias of a 
spring to open the needle valve and allow fuel to enter the chamber. 
In operation, the diaphragm repeatedly opens and closes the inlet needle 
valve so that fuel can enter the diaphragm chamber in response to 
sub-atmospheric pressure in the throat of the carburetor. Accordingly, a 
certain amount of fuel can be maintained in the diaphragm chamber at a 
substantially constant pressure to supply fuel to the main and idling 
jets. The diaphragm must be extremely flexible because the valve must open 
and close rapidly with a small pressure differential which is typically 
one to two inches of water. It must also operate over a wide temperature 
range of about -40.degree. F. to 180.degree. F. 
For over 40 years the small engine industry has experienced many problems 
in the manufacture, in service use, and performance of fuel metering 
diaphragms. Currently most diaphragms are made of a rubber coated silk 
material. In use, the performance characteristics of these diaphragms 
change and deteriorate substantially. The inventors have discovered that 
the silk fibers absorb moisture from the atmosphere and/or the fuel, 
resulting in a diaphragm that changes its performance characteristics 
depending on the ambient weather conditions, such as temperature and 
humidity, and the moisture content of the fuel which severely limits the 
performance of the fuel metering diaphragms. Other materials, such as 
woven nylon, have also been found to be ineffective because they produce a 
diaphragm which is too thick and/or inflexible to adequately respond to 
small pressure changes. When in prolonged contact with liquid fuel, the 
coating also swells or increases in volume and deteriorates in hardness, 
tensile strength and ultimate elongation. 
Additionally, currently available manufacturing processes are unable to 
produce diaphragms having the dimensional tolerances and performance 
characteristics required. A two-ply rubber coating is used with one ply or 
layer being applied to both sides of a sheet of material from which the 
diaphragm is cut. This increases the rigidity of the material which, when 
coupled with the above mentioned problems of the material, results in a 
diaphragm that in incapable of consistently performing as required. 
Previously, the silk fabric with an uncured rubber coating on both sides 
was heated in an oven to fully cure the rubber and then cooled to room 
temperature. Thereafter, a stack or pile of several of the resulting flat 
composite sheets were placed in a multiple cavity mold and simultaneously 
molded under a force of 6-20 tons for a 54 cavity mold at a temperature of 
330.degree. to 375.degree. F. for 4 to 8 minutes to form a bellows or 
convolution in the diaphragm. Thereafter, the molded sheets were cooled 
and trimmed in a die and press to cut or blank out the individual 
diaphragms from each sheet. When in service in a carburetor, the 
convolution tends to become smaller, diminish or even disappear and thus 
is not permanent and degrades performance of the diaphragm. 
Since up to four cured two ply rubber coated sheets are molded at the same 
time to form the bellows, they are subjected to an uneven application of 
pressure and heat. This results in different performance properties for 
each diaphragm. 
SUMMARY OF THE INVENTION 
A fuel metering diaphragm embodying this invention has a warp knit fabric, 
preferably of polyester, with an elastomer coating applied to only one 
side of the fabric and disposed in the interstices of the fabric to 
provide a fuel resistant barrier. The elastomer is applied to only one 
side of a tensioned sheet of the fabric and one coated sheet at a time is 
molded under heat and pressure to dispose the elastomer in the interstices 
of the fabric, vulcanize the coating on the fabric and permanently form 
the desired contour of the diaphragm. 
Objects, features and advantages of this invention are to provide a 
diaphragm having greater moisture resistance, enhanced flexibility, 
improved consistency when mass produced, greatly increased in service 
useful life, flexibility and tensile strength, and of simple design and 
economical manufacture and assembly.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
As illustrated in FIGS. 1-3, a conventional fuel pump and carburetor 20 for 
a two cycle small engine has a main body 22 with a mixing passage 24 in 
which a throttle valve 26 is mounted upon a shaft 28 controlled by a lever 
30. A fuel pump 32 in the body receives fuel from a fuel inlet 34 and 
delivers it to a carburetor diaphragm chamber 36 through a diaphragm 
controlled inlet valve 38 in a conventional circuit including the fuel 
pump, the inlet valve, and a fuel metering diaphragm 40 embodying this 
invention. The carburetor is attached to the engine through mounting holes 
42. 
Small passages 44 opening into the mounting face of the carburetor (FIG. 2) 
receive engine crankcase pressure pulses for actuating a diaphragm 46 of 
the fuel pump received on the top of the carburetor body under a top plate 
or housing cap 47. 
Referring to FIG. 3, in use, the pump 32 delivers fuel to the metering 
valve assembly 38 through a chamber 50 with a filter screen 52 therein and 
a passage 54 terminating at a valve seat 56. The valve assembly has a 
needle valve 58 which is actuated by a lever arm 60 connected at one end 
62 to the valve, fulcrumed between its ends at 64 and having a control 
finger 66 actuated at its free end by the diaphragm 40. The diaphragm in 
cooperation with the body defines a fuel chamber 68 and in cooperation 
with a cover plate 69 defines a dry or air chamber 70 communicating with 
the atmosphere. The needle valve 58 is yieldably urged to its closed 
position bearing on the seat 56 by a coil spring 72 and is actuated to an 
open position by movement of the diaphragm 40. The coil spring is received 
in a pocket 73 in the body and bears on the finger 66 of the lever arm. 
Preferably, the diaphragm has a backing disc 74, and a plunger 76 with a 
shank extending through the disc, the diaphragm and a washer 78 which are 
staked together in assembled relationship. Fuel in chamber 68 is supplied 
to a main metering jet 80 and idle jets 82 through passages (not shown) 
communicating with the fuel chamber 68. If desired, needle valve 
assemblies 84 (only one of which is shown), can be provided to adjust the 
quantity of fuel supplied to the main jet and the idle jets. 
In use, as fuel is drawn from the chamber 68, the quantity of fuel therein 
will decrease and the differential pressure on the diaphragm will move the 
lever arm 60 against the bias of the spring 72 in a counter-clockwise 
direction (as viewed in FIG. 3), to open the valve 58 to allow fuel to 
enter the chamber 68. As the chamber fills with additional fuel, the 
diaphragm will tend to move the lever arm clockwise and close the needle 
valve to thereby regulate the pressure of the fuel within the chamber. 
When the engine is under load, and particularly when operating at full 
throttle, fuel flows rapidly into the chamber which causes the diaphragm 
to cycle or fluctuate quite rapidly. Thus, for the carburetor to function 
properly under a large variety of operating conditions, it is necessary 
that the diaphragm be very flexible and yet still be relatively strong and 
impervious to liquid fuel and not deteriorate in use due to repeated 
flexing and contact with the liquid fuel. 
The diaphragm 40 of the present invention, as seen in FIGS. 4 and 5, 
comprises a thin sheet of knit fabric 86 preferably of polyester which has 
been coated on only one side (preferably the fuel chamber 68 side) with an 
elastomer 88 and molded to form a typical bellows 90 with a convolute 
shape. Desirably, a sealing gasket 89 is attached to the fabric adjacent 
its periphery and outside of the convolution. 
One feature of the present invention is that the fabric 86 is preferably a 
warp knit fabric. A knit fabric is composed of a series of interconnecting 
loops which may be prepared in its simplest form from a single yarn. By 
means of a needle, a loop is formed in a yarn 91 and is cast off when the 
needle draws a second loop through the first loop. The process is then 
repeated, to form a chain of loops interconnecting with adjacent chains to 
form a fabric. FIG. 7 shows the basic structure of the warp knit fabric 86 
with interconnecting loops of yarn 91 with interstices 92 between the 
yarn. As viewed in FIG. 7, the lines of loops running left to right are 
called "courses"; and those running from top to bottom are called "wales". 
The knit fabric 86 is a plain warp knit fabric preferably of polyester 
fibers. Warp knit and weft knit are the basic types of knit fabrics. 
Warp-knit fabrics are manufactured by passing each end of the fibers 
through its own needle forming a loop which intersects with adjacent 
loops. Most warp-knits are made on a tricot machine, as is known in the 
knitting industry. The resulting fabric is one of a tight weave having 
great strength, flexibility and moisture resistance. 
Preferably, the fabric is a tricot warp knit fabric of Dacron.RTM. 
polyester fibers and has a nominal thickness of 0.004 to 0.010 and 
preferably 0.005 to 0.007 of an inch as determined by American Society for 
Testing Materials (ASTM) procedure D-1777, a weight of 0.5 to 3.0, 
desirably 0.6 to 1.0, and preferably about 0.75 ounces per square yard as 
determined by ASTM procedure D-3776 with the courses having 36 to 58, and 
preferably 46 threads per inch, and the wales having 30 to 40 and 
preferably 34 threads per inch, as determined by ASTM procedure D-3887. 
Preferably, this fabric has a transverse break strength of 10 to 20 pounds 
and a longitudinal break strength of 15 to 25 pounds as determined by the 
grab test method of ASTM procedure D-5034. Preferably, the fabric has a 
Mullen burst strength of 45 to 75 psi as determined by ASTM procedure D- 
3786. Preferably, the threads of the fabric have a maximum denier of 20. 
The tricot knit polyester fabric is coated on only one side with an 
elastomeric polymer, which is preferably a polyacrylonitrile butadiene 
rubber (NBR). Preferably, this polymer is a medium acrylonitrile butadiene 
copolymer that is compounded with various solvents, accelerators, 
nucleating agents, vulcanizing agents, plasticizers, release agents, etc., 
to optimize fuel resistance and flexibility. Preferably, the compounded 
NBR has a Money viscosity of 40 to 60, hardness on the Shore A scale of 30 
to 40 points under ASTM procedure D-2240, and under ASTM procedure D-412, 
a tensile strength of at least 1500 psi, an ultimate elongation of at 
least 800% and a Young's modulus of elasticity of at least 80 psi at 100% 
elongation. 
Vulcanized samples of the compounded NBR after being soaked in the 
specified fuel at room temperature for seventy hours preferably have the 
following properties: 
______________________________________ 
ASTM D471 
ASTM Test Reference Fuel 
Property Procedure B C 
______________________________________ 
Maximum Hardness 
D2240 10 10 
Decrease in PTS 
Maximum Tensile 
D412 60 80 
Strength Decrease in % 
Maximum Ultimate 
D412 35 50 
Elongation Decrease in % 
Maximum Volume D471 20 35 
Increase in % 
Maximum Dryout D471 15 20 
Volume decrease in % 
______________________________________ 
Other suitable materials are fabrics with warp knit synthetic fibers with 
high moisture resistance such as fibers of polyaramid or polyamide (Nylon) 
having similar physical properties to the preferred polyester tricot knit 
fabric and elastomeric polymer coatings of fluorosilicones or low 
temperature fluoroplastics or fluorocarbons (Viton.RTM. GFLT). 
The knit fabric is coated on only one side with the elastomeric polymer. 
While being coated, preferably the fabric is tensioned or stretched to a 
taut and generally planar condition. To facilitate tensioning the fabric, 
preferably about one inch of the material on each side is heated and fused 
to provide strips along the side edges for feeding and handling the 
fabric. This keeps the material from "necking down" when tension is 
applied to the fabric during the coating process and produces a relatively 
stress-free finished composite membrane material for molding. 
When a relatively small quantity of coated fabric is produced, a piece 94 
of fabric 86 may be tensioned or stretched to a taut and generally planar 
condition, as shown in FIG. 6, by stretching it over a carrier frame 96 
and wrapping the ends of the piece about the frame. After a piece of 
fabric is tensioned and framed, a coat or thin layer 98 of the compounded 
elastomer is applied to one side, preferably by using a doctor blade 100 
as shown in FIG. 8 to produce a composite sheet or membrane 102. The 
elastomer is applied relatively uniformly over only one surface or face 
104 of the fabric. In applying the compounded elastomer, only sufficient 
force is used to insure that the elastomer adheres to the fabric. 
Preferably, the wetting of the fabric by the elastomer is minimized so 
that the courses and wales of the fabric will not be adhesively "locked 
up" which reduces the overall flexibility of the finished composite 
membrane. The fabric, not the elastomer, is believed to be the primary 
contributor to the flexibility of the molded diaphragm. After coating and 
before vulcanizing, the coated composite fabric preferably has a nominal 
thickness of about 0.012 to 0.018 of an inch. 
To facilitate further handling, preferably before molding, the elastomer is 
allowed to develop a tacky condition by partially drying or slightly 
curing it, which may be accomplished by exposure at room temperature or if 
desired by heating to an elevated temperature the composite sheet 102. The 
composite sheet may be preheated by placing it in an oven 106, as shown in 
FIG. 9, for about 5 to 20 minutes with an oven air temperature of about 
150.degree. F. to 200.degree. F. 
Thereafter, the coated sheet 102 is removed from the carrier frame, dusted 
with a conventional mold releasing agent, and cut into blanks 108 of the 
appropriate size for the mold. For a mold 110 having 36 cavities laid out 
in a generally square (6.times.6) arrangement, these blanks are typically 
about 10".times.10". 
As shown in FIG. 10, one blank 108 at a time is placed in the open mold 110 
which is closed by a press 112 to apply pressure and heat to the composite 
blank to force the elastomer into the interstices 92 between the fibers of 
the knit fabric, to cure the elastomer and to produce the bellows or 
convolute shape in the finished diaphragm 40. The mold is preheated to 
about 310.degree. to 370.degree. F. and preferably to 340.degree. F. to 
360.degree. F. The composite blank has a residence time in the closed mold 
of about to 41/2 minutes and preferably 31/4 to 33/4 minutes, while being 
subjected to a molding force of 10 to 15 tons or about 220 to 330 pounds 
per square inch of the area of one face of the composite blank received in 
the mold. Preferably, to permit gases to escape from the mold, it is 
vented or it may be periodically rapidly slightly opened and then closed 
several times during the molding cycle. 
This molding forces the elastomer on one side through the interstices 92 of 
the fabric to the opposite side of the fabric and results in the molded 
composite sheet 108' of the fabric having an elastomeric coating which is 
only one-ply deep but substantially covers the fibers on both sides of the 
fabric. Typically, after the molding is completed, the sheet 108' of 
molded diaphragms has a nominal thickness of about 0.007 to 0.010 of an 
inch. 
The sheets 108' of molded diaphragms are cooled to room temperature and 
then cut and trimmed in a die 114 (FIG. 12) received in a reciprocating 
press 116 which cuts the individual diaphragms 40 from a single molded 
sheet 108' at a time. The individual finished diaphragms may then be 
packaged for shipment, installation and use. 
Desirably, but not necessarily, the sealing gaskets 89 are bonded to the 
coated blank 108 during the molding operation. Individual gaskets may be 
die cut from a sheet of high density cellulose fiber reinforced material 
coated on one side with a suitable bonding agent or adhesive. The gaskets 
are placed in the mold on the blank 108 preferably with the adhesive 
facing the fabric side of the blank. The gaskets may be located in the 
mold with suitable locating pins projecting into the bolt mounting holes 
through each gasket. A suitable cellulose gasket material is commercially 
available as NV-512 from Specialty Paperboard, Inc. of Beaver Falls, New 
York 13305. A suitable vulcanizable bonding agent is commercially 
available as Thixon.RTM. OSN-2 from Whittaker, Dayton Chemicals Division, 
P.O. Box 127, West Alexandria, Ohio 45381. 
FIG. 13 illustrates an apparatus 118 and method for coating with a 
compounded elastomer a relatively large quantity of a knit fabric. A thin 
sheet 120 of the compounded elastomer is produced by disposing a quantity 
of the compounded elastomer 122 between a pair of co-rotating cylindrical 
rollers 124 and 126. The sheet 120 is stripped from the roller 126 by a 
stripper bar 128 and passed along with a web 130 of the knit fabric 86 
between the nip of co-rotating cylindrical rollers 126 & 132 to calender 
the sheet of elastomer onto only one face 134 of the fabric. Typically, 
the sheet of elastomer has a nominal thickness of about 0.008 to 0.010 of 
an inch and the gap between the calendering rollers is adjusted to produce 
just enough force for the sheet of elastomer to adhere to the fabric. The 
web 130 of fabric is rolled on a reel 136 from which it is unwound as it 
passes between the calendering rollers. 
The web of coated fabric 130' is wound on a take-up reel 138 along with a 
polyethelene sheet 140 received on a reel 142 to prevent the layers of 
uncured elastomer from sticking together. If desired, the web of coated 
fabric 130' can be supported on one or more driven rollers 144 disposed 
between the take-up reel 138 and the calendering rollers. The relative 
speed of the reel 136 unwinding the web 130 of fabric, the calendering 
rollers 126 and 132, the support roller 144 and the take-up reel 138 can 
be varied and adjusted within predetermined limits to properly tension the 
web 130 of fabric for application of the sheet 120 of elastomer thereto by 
the calendering rollers 126 and 132. 
To facilitate substantially uniform tensioning of the web 130 of fabric, 
preferably each side edge is heated to provide a fused strip about 1" wide 
which is used in winding it on the reel 136, unwinding it, feeding it 
through the calender rollers, and winding it on the take-up reel 138. As 
previously indicated, these strips keep the fabric from necking down when 
tension is applied to the fabric during the coating process and minimize 
the stresses in the coated, molded and cured composite material of the 
finished diaphragms 40. 
Preferably, the polyethelene sheet 140 has a smooth surface which permits 
any residual stresses in the coated fabric web 130', due to it having been 
tensioned, to relax and dissipate before the coated fabric is molded. If 
desired, rolls of coated fabric 130' with the polyethylene sheet between 
the layer can be stored for several weeks before molding by refrigerating 
it at a relatively low temperature of about 30.degree. F. to 40.degree. F. 
to prevent curing of the compounded elastomer. 
When desired, the web of coated fabric 130' can be unrolled and cut into 
blanks of the appropriate size such as blanks 108, molded and trimmed in 
the same manner as previously described for blanks 108 to produce 
completely finished diaphragms 40. 
The resulting diaphragms 40 are about 30% to 50% thicker and yet 
substantially more flexible than the prior art diaphragms made with a silk 
fabric. The resulting diaphragms have dramatically improved moisture and 
fuel resistance, durability and in service useful life, improved and more 
stable properties and dimensions, and are of economical manufacture.