1. * GB780052 (A)
Description: GB780052 (A) ? 1957-07-31
Improvements in or relating to metal pistons
Description of GB780052 (A)
PATENT SPECIFICATION
Date of Application and filing Complete Specification: Aug. 16, 1955.
i 2, 111Application made in Netherlands on Aug. 18, 1954.
Complete Specification Published: July 31, 1957.
Index at acceptance:-Class 122(1)5 B7A2.
International Classification:-FO6j.
COMPLETE SPECIFICATION
Improvements in or relating to Metal Pistons We, N. V. PHILIPS'
GLOEILAMPENFABRIEKEN, a limited liability Company, organized and
established under the laws of the Kingdom of the Netherlands, of
Emmasingel 29, 5Eindhoven, Holland, do hereby declare the invention,
for which we pray that a patent may be granted to us, and the method
by which it is to be performed, to be particularly described in and by
the following statement:This invention relates to a metal piston
intended particularly for use in piston apparatus in which it is
desirable that the working medium should remain uncontaminated by
lubricant. Such apparatus may be, for example, hot-gas reciprocating
apparatus and compressors. Ahot-gas reciprocating apparatus should be
understood to mean herein a hot-gas engine, a refrigerator operating
on the reversed hot-gas engine principle or a heat pump.
A hot-gas reciprocating apparatus is defined as an apparatus
comprising two spaces which vary continuously in volume with a
substantially constant relative phase difference, one of them having a
lower mean temperature and the other a higher mean temperature, said
spaces being in communication with each other by way of a first
heat-ex30changer, a regenerator and a second heatexchanger and
containing a gas of invariable chemical composition which performs a
thermodynamic cycle. The heat-exchangers have different nomenclatures
according to the ap35paratus. Thus in a refrigerator the
heat-exchanger adjacent the first-mentioned space extracts heat from

2. the medium to be cooled and is termed "a freezer" whilst the second
heat-exchanger transfers heat from the gas and is termed "a cooler".
In a hot-gas engine, the heat-exchanger adjacent the firstmentioned
space extracts heat from the gas and is termed "a cooler" whilst the
second heat-exchanger supplies heat to the gas from 45a thermal source
and is termed "a heater".
[Price 3/61 The gas is substantially expanded in one space and
substantially compressed in the other space.
It is known to provide piston apparatus having a piston without a
piston ring, but 50 having a definite, though small clearance between
the piston and the cylinder. The cylinder and the piston can be made
of metal, so that the running surfaces are constituted by metal, but
this would in general require 55 abundant lubrication. In the case of
such lubrication it is likely that lubricant enters into the working
chamber of the apparatus which is undesirable for various reasons. In
the hot-gas reciprocating apparatus the lubricant may be drawn along
with the working medium to the heat-exchangers, in which it may be
deposited, so that the output of the machine is reduced. Moreover, the
regenerator may become choked. 65 With compressors it is also
undesirable for the compressed gas to be contaminated by lubricant. If
for example carbon dioxide is compressed to obtain solid carbon
dioxide, the lubricant will be left after the solid carbon dioxide has
evaporated, which may give rise to a disagreeable taste of a product
cooled by the solid carbon dioxide.
The whole piston could be made from a material which ensures a smooth
running 75 and requires no, or only little lubrication.
Such materials are, for example, wood or a polyethylene compound, for
example teflon, or a polyamide, for example nylon. However, these
materials have the disadvantage 8 that they have a higher expansion
coefficient than metals and they are, moreover, often sensitive to
atmospheric influences, so that the gap between the piston and the
cylinder could become inadmissibly small. 85 The invention has for its
object to mitigate this disadvantage.
According to the present invention a piston is provided with a layer
comprising a porous organic material in a thickness of not more 90
780,052 No. 23598/155.
780,052 than 2 millimeters preferably of 0.5 millimeters.
In the sense of the present application the term "porous organic
material" is used not only in case that this material is only of
organic character (for instance is constituted by wood or a fibre mass
impregnated with a heat resistant synthetic resin), but also in those
cases in which metal compounds are impregnated with a heat resistant
synthetic resin. It is important for the material to ensure a
satisfactorily smooth running.

3. The layer applied to the piston has preferably a thickness of at least
30",.
The piston may be constructed as a displacer for use in a hot-gas
reciprocating machine. A layer of organic material may be applied to
the piston, for example in a manner similar to that used in applying
veneer, for example by glueing. The layer may be finished, for example
by grinding to obtain the desired thickness.
The invention will now be described, by way of example, with reference
to the accompanying diagrammatic drawing.
Fig. 1 shows a part sectional elevation of a gas refrigerator
comprising a displacer, the running surface of which is provided with
a layer of organic material.
Fig. 2 shows, on an enlarged scale, a section through part of the
displacer of this machine.
The gas refrigerator shown in Fig. 1 is of the displacer type and
comprises a cylinder 1, in which a displacer 2 and a piston 3 are
adapted to reciprocate with a substantially constant relative phase
difference. The displacer 2 is coupled by means of a connecting rod
mechanism 4 with a crank of a crankshaft 5 and the piston 3 is coupled
by connecting rods 6 with other cranks of the same crank shaft. The
machine is driven by an electric motor 7. Owing to the reciprocating
movement of the displacer 2 the volume of a space 8 above the
displacer (the freezing space) is varied. This space communicates
through a freezer 9, a regenerator 10 and a cooler 11 with the cooled
space 12, which lies between the displacer and the piston.
Mainly expansion occurs in the cooled space 12. The heat supplied to
the working medium of the apparatus during the expansion may be
withdrawn from a medium to be cooled and this medium is supplied
through an aperture 13 to a condenser space 55 14, in which the
medium. for example air, can condense, after which the condensate is
collected in an annular duct 15 whence it passes from the refrigerator
through a duct 16, which comprises a liquid trap 17. 60 The displacer
2 comprises a cap 18 and a part 19, which hus a running surface of a
porous organic material. As is evident from Figs. 1 and 2, the
displacer is not fitted with piston rings, so that a narrow gap 20 is
left 65 between the wall of the displacer and the cylinder. The
difference in diameter between the cylinder and the displacer may be
for example 60". at room temperature.
A layer of wood 21, is applied to the metal 70 displacer body which
may, if desired, be impregnated with a lubricant. This layer has, in
this embodiment, a thickness of 0.5.- millimeter; in general, it has a
thickness of not more than 2 millimetres. 75
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