Improvements in Pistons.
Year: 1897

page_text: Date of Application, 17th-July, 1897
Complete Specification Left, 17th May, 1898-Accepted, 2nd July, 1898
BUIFICATION.
Improvements in Pistons.
We, SAMUEL ROBINSON and ERNEST MORTON MOLYNEUX, both of St. Mary's
Works, Newport, Salop, Engineers, do hereby declare the nature of this invention
to be as follows:-
This invention relates to pistons for steam engines, gas engines, pumps and
the like, and it has for its object to provide an improved construction of piston
adapted to prolong the "life," and to increase the efficiency of the piston rings
and of the cylinder in which the piston is designed to work, by obviating or
lessening the friction between the said piston rings and the cylinder.
It is a fact well known to mechanical engineers that the great friction which
occurs between the piston or piston rings of pistons as usually constructed and
the cylinder in which said pistons work to and fro, is the cause of considerable
loss and waste of power and consequent expense arising from an increased con-
sumption of coal for the same output; and this is mainly due to two mutually
reacting causes, viz;-first, excessive local pressure of the piston rings against
the walls of the cylinder whereby the initially straight and parallel walls of the
cylinder become unequally worn with the result that the cylinder wears oval and
the piston gets out of alignment; and secondly excessive tightness of the piston
rings against the adjacent surfaces of the piston block and cover whereby the
said rings are unable to conform to the cylinder walls and thus produce the
drawback just referred to. In fact it is generally a case of choosing one of two
evils; viz. either loss of power through excessive friction of the piston caused
by undue tightness of the piston rings, or waste of steam due to a loosely fitting
and consequently leaky piston.
Now according to our invention we obviate the unnecessary friction between
the piston rings and the cylinder by a new construction of piston rings and piston
block, these parts being arranged together and on the piston rod in a novel way
and so as to prevent escape of steam."
We also provide the improved piston with devices for automatically centreing
the piston rings so as to render the piston self-centreing with relation to the
cylinder.
To this end we form the piston block with a sleeve-like extension having a
central hole provided with screw threads to fit a corresponding screw thread on
the piston rod, the piston block being screwed up tight on the piston rod, pre-
ferably against a shoulder formed on the latter.
We form the piston cover with a central hole tapped with screw threads to
fit the screw thread on the piston rod, and we also form the said piston cover with
an annular extension provided with a female screw thread to engage with the screw
threaded extension of the piston block.
We make the screw thread on the piston rod of a certain pitch, that is to say,
with a certain number of threads to the inch, and we make the male screw thread
on the piston block of a different pitch, that is to say, with a different number of
threads to the inch. By this means on screwing the piston block and the piston
CD
07

page_text: cover on the piston rod, the difference in pitch of the two screw-threads will act
to lock the several parts together. Additional security against coming loose
may be provided if desired, by driving a pin, key, or the like, through the piston
block and the piston rod; or one, two or more set screws may be employed with
this object.
Between the piston block and piston cover we arrange an annular part herein-
after called the centreing ring, approximately of rectangular shape in cross section
but having an outward circumferential bulge or prominence of wedge shape in
cross section.. Upon this centreing ring we arrange the piston rings proper one
on each side of the aforesaid wedge shaped bulge, each ring being cut away at the
place where it is made to fit the inclined side of the bulge. Each piston ring
has four acting steam-jointing surfaces, viz against the cylinder wall, against
the piston cover or piston block, as the case may be, and against the main portion
and the bulge of the centreing ring. The centreing ring is kept in proper central
position by means of springs fixed to or bearing on the annular extension of the
piston cover. These springs have for object to balance the weight of the centre-
ing ring and the piston rings.
A piston constructed as above described will always be maintained in a central
position in perfect parallelism with the walls of the cylinder.
The piston
rings will have only that degree of spring and consequently pressure which is
necessary to prevent escape of steam past the piston, whilst the arrangement of
the centreing ring which floats as it were upon the supporting springs will act
to keep the piston and piston rings perfectly central and prevent what is known
as the wabbling or reeling" of the piston with its consequent friction against
and wear of the cylinder.
We, SAMUEL ROBINSON and ERNEST MORTON MOLYNEUX, both of St. Mary's
Works, Newport, Salop, Engineers, do hereby declare the nature of this invention
and in what manner the same is to be performed, to be particularly described and
ascertained in and by the following statement:―
This invention relates to pistons for steam engines, gas engines, pumps and
the like, and it has for its object to provide an improved construction of piston
adapted to prolong the "life," and to increase the efficiency of the piston rings
and also of the cylinder in which the piston is designed to work, by obviating or
lessening the friction between the said piston rings and the cylinder, in which said
piston and rings work to and fro.
It is a fact well known to mechanical engineers that the great friction which
occurs between the piston or piston rings of pistons as usually constructed and
the cylinders in which said pistons work to and fro, is the cause of considerable
loss and waste of power and consequent expense arising from an increased con-
sumption of coal for the same output; and this is mainly due to two mutually
reacting causes, viz:-first, excessive local pressure of the piston rings against
the walls of the cylinder whereby the initially straight and parallel walls of the
cylinder become unequally worn with the result that the cylinder wears oval and
the piston gets out of alignment; and secondly excessive tightness of the piston
rings against the adjacent surfaces of the piston block and cover whereby the

page_text: said rings are unable to conform to the cylinder walls and thus produce the
drawback just referred to. In fact it is generally a case of choosing one of two
evils; vizeither loss of power through excessive friction of the piston caused
by undue tightness of the piston rings, or waste of steam due to a loosely fitting
and consequently leaky piston.
Now according to the present invention it is proposed to obviate the unnecessary
friction between the piston rings and the cylinder by a new construction of piston
rings and piston block, these parts being arranged together and on the piston rod
in a novel way and so as to prevent escape of steam.
We also provide the improved piston with devices for automatically centering
the piston rings so as to render the piston self-centering with relation to the
cylinder.
"
In some instances, the piston rings have been mounted upon springs, involving
as before explained excessive pressure due to the expansion of the rings, but, if
this expansion could be properly controlled it could be utilized to prolong the
lifetime of the rings.
Now the object of this invention is to adjust the said expansion in accordance
with the wear and tear of the rings.
In other instances the rings have been mounted on the piston block in grooves
provided therein, so that the piston when put in the cylinder has the appearance
of a solid mass of metal, with the rings projecting a shade above the piston block.
So far as the piston block and piston rings are concerned, the internal parts of
the rings bear tight against the block and the external parts of the rings
bear tight against the cylinder walls, thus, the extreme amount of expansion
contained in the piston rings is spent very rapidly, not only through the
tightness of the internal and external parts of the rings, but also by reason of the
weight of the piston block resting on the piston rings while in motion, because the
weight of the piston and rod resting on the rings almost permanently tends. to
prevent the rings expanding even to the extent for which provision is made.
It is proposed and intended by this invention to remedy these drawbacks by
introducing devices for automatically centering the piston rings, so as to render
the piston self-centering with relation to the cylinder and independent of the
piston block and thereby obviating the friction upon the piston rings due to the
piston block and piston rod resting on the piston rings while moving to and
fro in the cylinder.
To this end we form the piston block with a sleeve like extension, having a
central hole provided with screw threads, to fit a corresponding screw thread on
the piston rod, the piston block being screwed up tight on the piston rod, pre-
ferably against a shoulder formed on the latter.
We form the piston cover with a central hole tapped with screw threads to
fit the screw thread on the piston rod, and we also form the said piston cover with
an annular extension provided with a female screw thread to engage with the screw
threaded extension of the piston block.
We make the screw thread on the piston rod of a certain pitch, that is to say,
with a certain number of threads to the inch, and we preferably make the male
screw thread on the piston block of a different pitch, that is to say, with a different
number of threads to the inch. By this means on screwing the piston block and
the piston cover on the piston rod, the difference in pitch of the two screw
threads will act to lock the several parts together. Additional security against
coming loose may be provided if desired, by a locking strap or bar fixed in a slot
in the piston rod and fixed by means of a bolt at each end to the piston block.
Between the piston block and the piston cover there is arranged a ring, herein-
after called the centering ring. This ring which forms a seat for the piston rings
is approximately of rectangular shape in cross section but has an outward cir-
cumferential bulge of wedge shape in cross section.
Upon this centering ring are arranged the piston rings proper, one on each
side of the aforesaid wedge shaped bulge, each ring being cut away at the place

page_text: where it is made to fit the inclined side of the bulge. Each piston ring has four
acting fluid or steam-jointing surfaces, viz:-against the cylinder wall, against
the piston cover or piston block, as the case may be, and against the main portion
and the bulge of the centering ring.
The piston rings are cut in the ordinary way so as to expand tight against the
cylinder walls. Although the width of the cut is hardly perceptible, the rings
increase slightly in diameter, and in consequence, the centering ring would not
fit the internal part of the piston ring and steam would escape.
To meet this difficulty, it is proposed and intended to also cut the centering ring
so as to allow it to expand proportionately with the piston ring, but it is necessary
to make it clearly understood, that provision is made on this centering ring to
control the necessary expansion thereof, because, if this expansion of the centering
ring was not controllable and independent of the piston block, the conditions of
this principle of piston with regard to excessive friction, would be the same as all
other pistons in use.
But in order to remedy this, a dovetail lug is provided on the inside of the
contering ring and is fitted into a dovetail recess formed in the outside of a loose
ring which is slipped on the body of the piston cover or piston block and is capable
of swivelling to and fro so as to conform to any inequalities that may exist in
the cylinder walls.
By this swivelling movement we are enabled to obtain more effectively a
uniform elasticity in the combined rings around the cylinder_walls.
Whilst the centering ring is not cut it cannot expand. But by cutting the
centering ring through at its dovetail part and then thinning the metal on the
sides of the dovetail the centering ring will be allowed to expand to the
necessary amount to put sufficient pressure on the piston rings internally, and
to produce a fluid or steam-tight joint between itself and the piston rings,
and so obviate excessive friction between the piston rings and the cylinder,
and also as herein before stated, obviating the binding of the rings, due to the
weight of the piston and its rod whilst moving to and fro.
We will now proceed to describe our invention with reference to the accompany-
ing illustrative drawings in which:-
Figure 1 is a central longitudinal section; Fig. 2 a central cross section, and
Figure 3 a side elevation of a piston constructed according to this invention.
Figures 4, 4 to 8, 8* are detail views illustrating the several parts of our im-
proved piston separately in longitudinal section and in side elevation respectively.
In constructing our improved piston we form the piston block 1 with a sleeve
like extension 2 having a central hole provided with screw threads 3 to fit a
corresponding screw thread 4 on the piston rod 5, the piston block 1 being screwed
up tight on the pistón rod, preferably against a shoulder G formed on the latter.
We form the piston cover 7 with a central hole tapped with screw threads 8
to fit the screw thread 4 on the piston rod 5, and we also form the said piston
cover 7 with an annular extension 9 provided with a female scrow thread 10 to
engage with a male screw thread 11 formed on the extension 2 of the piston
block 1.
We make the screw thread 4 on the piston rod 5 of a certain pitch, that is to
say, with a certain number of threads to the inch, and we make the male screw
thread 11 on the piston block of a different pitch, that is to say, with a different
number of threads to the inch, the pitch of the female screw thread 10 being
made the same as that of the male screw thread 11. By this means on screwing
the piston block 1 and the piston cover 7 on the piston rod 5, the difference in
pitch of the two screw-threads will act to lock the several parts together.
Additional security against coming loose may be provided if desired, by slotting
the end of the piston rod, and fitting in such slot a strap or bar 12 which is
fixed at each end by means of a screw stud 12 to the piston block.
Between the piston block 1 and piston cover 7 we arrange a ring 13 hereinafter
called the centering ring, approximately of rectangular shape in cross section but
5

page_text: having an outward circumferential bulge or prominence 14 of wedge shape in
cross section. Upon this centering ring 13 we arrange the piston rings proper 15
and 16 one on each side of the aforesaid wedge shaped bulge 14 each ring being
cut away at the place where it is made to fit the inclined side of the bulge. Each
piston ring which is split in the ordinary way has four acting fluid or steam-
jointing surfaces, viz against the cylinder wall, against the piston cover 7 (in
the case of the ring 15) or against the piston block 1 (in the case of the ring 16)
and against the main portion and the bulge of the centering ring 13. The
centering ring 13 is suspended in proper central position by means of a dovetail
part 13 formed on its inner side and fitted in a corresponding dovetail recess 17*
formed in the outer side of a loose ring 17 slipped on the body of the piston
cover. The centering ring 13 is split through its dovetail portion as shown.
The loose ring 17 is not split.
A piston constructed as above described will always be maintained in a central
position in perfect parallelism with the walls of the cylinder. The piston rings 15,
16 will have only that degree of spring and consequently pressure which is
necessary to prevent escape of fluid or steam past the piston, whilst the arrangement
of the centering ring 13 which floats as it were centrally by means of its sus-
pending retaining loose ring 17 will act to keep the piston and piston rings per-
fectly central and prevent what is known as the wabbling or reeling "of the
piston with its consequent friction against and wear of the cylinder.
Having now particularly described and ascertained the nature of our said inven-
tion and in what manner the same is to be performed, we declare that what we
claim is:
-
1. In pistons for steam engines, gas engines, pumps and the like, the com-
bination with a piston rod, of a piston block formed with an extension and
screwing on the piston rod, and a piston cover formed with an annular extension
screwing on the extension of the piston block, said piston cover screwing also on
the piston rod by means of screw thread of different pitch to the screw thread.
on the extension of the piston block, for the purpose of locking the parts together,
substantially as described and shown.
2. In pistons for steam engines, gas engines, pumps and the like, the com-
bination of piston rings, each having four fluid-tight jointing surfaces, and a
floating centering split ring formed with an outward circumferential wedge-
shaped budge and with a dovetail part fitting in a dovetail recess in a suspending
retaining ring mounted loosely on an annular inner extension of the piston cover
or piston block constructed, arranged and operating, substantially as described
and shown.
3. The improved piston for steam engines, gas engines, pumps and the like,
constructed, arranged and operating, substantially as described and shown.
Dated this 17th day of May 1808
J. C. MEWBURN & ELLIS,
-
55, and 56, Chancery Lane, London, Chartered Patent Agents.