This invention relates to plastics part and a method of making same,
particularly as a bearing cage, thrust washer and/or an annular shaft seal.
Bearing cages are conventionally used in ball and roller bearings
to retain and separate rolling elements (rollers, needles or balls) in predetermined
relative positions. The bearing cages lie between inner and outer bearing races.
In certain applications, bearing cages are exposed to relatively
difficult conditions that limit the useful life of the bearing cages or require
the use of complex and expensive construction. One such application is in the needle
bearing assembly for a crank of a two-cycle engine such as, for example, an outboard
In such application, the needle bearing assembly is exposed to high
loads generated by combustion pressure, inertia of the parts, and centrifugal force.
This application is so severe that a special high precision bearing cage made of
silver-plated steel, is generally used. The silver is believed to act as a lubricant,
at least during a break-in period of the bearing.
There are numerous disadvantages to the above described bearing cage.
The high precision requires numerous steps for manufacturing, such as piercing,
heat treating, grinding, and chemical finishing. The initial cost of tooling is
very high, and the chemical by-products produced during manufacture result in
environmental problems of disposal. In addition, because of the many operations
which must be performed, the time for manufacture of such bearing cages is long.
Thus, a larger than desired inventory of work in process must be maintained. The
larger number of operations, combined with the required precision, may result in
high labour costs or relatively high scrap rate. And, the required silver substantially
adds to the cost.
In H. Saechtling, "Kunststoff Taschenbuch", 24th edition of 1989,
published by Carl Hanser Verlag München Wien, there are disclosures at, e.g., pages
117 to 134 of a figure (3.43) which gives processing temperature ranges for different
plastics, including PEEK, with a similar range of injection temperatures to those
temperatures specified in claim 1 of the present invention, whilst Table 3.8 gives
guidance values for the injection moulding of plastics. Page 317 of that document
also suggests that a high crystallinity improves the wear resistance.
Reference is also made to A. Olschewski, "Hochtemperaturkunststoffe
für Wälzlagerkäfige", Kugellager-Zeitschrift 228, volume 61 (1987) at pages 13
to 16, which discusses properties and the use of PEEK as a high temperature plastic
for a bearing cage.
According to one aspect of the present invention, there is provided
a method for producing a part made of a plastics resin, the method comprising heating
a mould to a moulding temperature; heating said plastics resin to an injection
temperature; and injecting said plastics resin into said mould, wherein said plastics
resin is a polyetheretherketone, said moulding temperature is from about 250 to
270 degrees F (121 to 132° C) and said injection temperature is from about 700
to about 800 degrees Fahrenheit (about 371 to about 427° C), so as to produce
an amorphous surface layer covering a crystalline inner body of a substantial portion
of said part, said amorphous layer being arranged to provide a wear surface for
improving the ability of said part to withstand friction.
According to a second aspect of the present invention, there is provided
a method for producing a finished part comprised of a plastics resin, the method
comprising moulding an intermediate part having a crystalline structure; heating
at least a portion of a surface of said crystalline intermediate part; and quenching
said at least a portion of a surface of said crystalline intermediate part to
form an amorphous surface layer upon a crystalline inner body, said amorphous outer
layer being such as to provide a wear surface for improving the ability of said
plastics part to withstand friction.
For a better understanding of the invention and to show how the same
may be carried into effect, reference will now be made, by way of example, to the
accompanying drawings, in which:-
- Figure 1 is a sectional view of a portion of a two-cycle engine showing the
location of a crank pin upon which a bearing cage of the present invention may
- Figure 2 is a perspective view of half of a bearing cage; and
- Figure 3 is a cross-sectional view of a rail of the bearing cage of Figure
2, as indicated by the line 3-3.
Referring first to Figure 1, the illustrated portion of a two-cycle
engine 10 includes a cylinder 12 in which reciprocates a piston 14. A connecting
rod 16 transmits the reciprocating motion of piston 14 to crank pin 18 eccentrically
disposed on a crank 20. A drive shaft, not shown, is rigidly affixed to the crank
20. The reciprocating motion of piston 14 is thus converted to desired rotary motion
of the drive shaft through the action of the connecting rod 16 and crank 20.
A needle bearing assembly 24 (not shown in detail in Figure 1) provides
rolling contact between crank pin 18 and connecting rod 16. As noted in the earlier
description of the prior art, the needle bearing assembly 24 is subjected to severe
forces in this location. As previously stated, bearing cages in that application
are conventionally made of silver-plated steel.
A bearing cage of needle bearing assembly 24 includes a mating to
become a crystalline, even on its outer surface.
The applicants have found that wear is most likely on the surfaces
of rails 30 and 32. Little if any wear problem occurs in the bars 34 since these
contact only the very smooth surfaces of the bearing needles (not shown). For that
reason, it is preferable to employ a single gate in the centre of one of bars
34. The surface of this bar 34, in the vicinity of the gate, becomes crystalline.
However, rails 30 and 32 are sufficiently distant from the gate in this bar 34
to be formed with an outer layer 46 of amorphous plastics.
The exact thickness of the outer layer 46 for best operation has
not yet been determined. It is believed that a thickness of a few thousandths of
an inch (1 mil = 25.40 µm) is satisfactory. The thickness of outer layer 46 can
be changed at will by varying moulding parameters. For example, one or more of
the following changes in parameters increase the thickness of the amorphous layer;
reducing the temperature of the mould, decreasing the injection speed, and decreasing
the injection pressure immediately before closing off the gate. Changing one or
more of these parameters in the opposite direction reduces the thickness of the
Service tests in a two-cycle engine appear to indicate that use of
the present bearing cage also increases the life of the entire two-cycle engine.
This may result from the fact that cage half 28 is only one-third the weight of
its silver-plated steel counterpart, thus reducing the inertial and centrifugal
loading. Besides reducing loading on parts immediately adjacent to the bearing
cage, the reduced weight may reduce engine vibration sufficiently to increase the
life of components elsewhere in the two-cycle engine.
The above description is directed to an environment of a bearing
cage in a two-cycle engine. However, the invention should not be considered limited
either to a bearing cage or to a two-cycle engine. Certain thrust washers and annular
shaft seals, for example, are fabricated of. silver-plated steel to reduce frictional
wear. It is believed that the present plastics moulding technique may be advantageously
employed to produce a thrust washer or annular shaft seal, as well as other parts.
The applicants believe that the amorphous outer layer on the cage
half 28 tends to act as a lubricant or as a sacrificial break-in surface, and
tends to smear out over or otherwise conform to mating surfaces in needle bearing
assembly 24 during break-in. With the mating surfaces thus smoothed and lubricated,
friction is believed to be reduced sufficiently to give a substantial improvement
in the life of the bearing cage.
The moulding cage half 28 using the above non-standard moulding parameters
is only one way to achieve the present structure. For example, the cage half 28
may be moulded using conventional parameters to produce an intermediate product
entirely of crystalline plastics. Then, the surface may be melted and rapidly
quenched to produce the amorphous surface layer. Alternatively, the output of an
industrial laser may be directed against the surfaces of the bearing cage half
28 to melt a thin outer layer. Then the part may be plunged into a cooling bath,
or be rapidly cooled by liquid spray or air stream.
A further way in which the part may be formed includes producing
a crystalline intermediate product which is then dipped into a heated bath for
a sufficient time to melt the outer layer. Then, the part is withdrawn from the
bath and cooled by one of the above cooling techniques. For example, the part
may be plunged into a bath of molten lead or tin for a short time, then withdrawn
It is believed that other crystalline plastics may be employed. The
techniques for forming the parts will vary with the particular crystalline plastics
employed. It is believed that the present invention may employ nylon or polyacetal
plastics, among others, either with or without suitable fillers. Another possibility
would be polyetheretherketone. One skilled in the art, in light of the present
specification, would be fully enabled to establish required parameters for fabrication
or parts using such materials.
It will be appreciated that the present invention provides for example
a bearing cage moulded of plastics resin and having a significantly improved life,
even in difficult applications such as the needle bearing assembly for a crank
of a two-cycle engine. In extended tests, the useful life of the present bearing
cage has exceeded the normally expected life.