Piston, method of making the piston and piston and cylinder unit comprising the piston

A piston for a door closer comprises metal cups on opposite ends of a stem. A plastics ring is a snap-fit in one of the cups and protrudes from that cup to seal against the wall of a cylinder. A groove in the side wall of the cup for receiving a rib on the ring is formed by pressing a substantially flat blank before the blank is formed into the cup.

BACKGROUND TO THE INVENTION 
From one aspect, the present invention relates to a method of making a 
piston. It is known to form a piston by combining with a piston body a 
relatively flexible ring which is seated in a groove formed in the 
external surface of the piston body. The groove is machined in the piston 
body after the piston body has been formed. 
SUMMARY OF THE INVENTION 
From one aspect, the present invention relating to a method of making a 
piston wherein a pressing operation is performed on substantially flat 
metal stock to form a recess in the stock at one face thereof, a cup is 
formed by drawing or pressing a portion of the stock which includes the 
recess, the recess being incorporated in a side wall of the cup at the 
inside of the cup, there is formed a ring which is resiliently deformable, 
relative to the cup, the ring having, when unstressed, a first end portion 
with an outside diameter exceeding the outside diameter of the cup and a 
second end portion having an outward projection and wherein the second end 
portion of the ring is inserted into the cup to seat the projection in the 
recess of the cup and leave the first end portion of the ring protruding 
from the cup. 
The cup may have an imperforate bottom. Alternatively, there may be at 
least one opening at the bottom of the cup to permit flow of fluid through 
the piston. Furthermore, the cup may be attached to a stem of the piston. 
The drawing or pressing operation preferably completes formation of the cup 
from the stock. Forming of the recess by a pressing operation performed on 
the flat stock enables machining operations on the cup to be avoided. 
The ring is preferably a snap-fit in the cup. 
After the ring has been inserted into the cup, the piston is preferably 
confined within a cylindrical wall which constrains the first end portion 
of the ring to a circular shape and thereby prevents the deformation of 
the ring which would enable the projection to escape from the recess of 
the cup. It will be understood that, if the first end portion of the ring 
is not confined and is therefore free to adopt an approximately elliptical 
shape, the ring may be deformed sufficiently to withdraw the projection 
from the recess. 
According to a further aspect of the invention, there is provided a piston 
comprising a metal cup and a ring which is resiliently deformable relative 
to the cup, wherein a first portion of the ring protrudes from the cup, a 
second portion of the ring lies inside the cup and a projection on the 
second portion lies in a recess in a side wall of the cup to retain the 
second portion in the cup, wherein the cup is formed by a pressing or 
drawing operation from a flat blank and the recess is pressed into the 
blank before the blank is formed into the cup. 
The piston is preferably confined within a cylinder in which the piston is 
contained, the cylinder confining the ring to prevent such deformation of 
the ring as would permit the projection to leave the recess.

DESCRIPTION OF PREFERRED EMBODIMENT 
The particular piston represented in FIG. 1 is a piston of a door closer. 
The piston may, for example, be used in a door closer generally as 
described and illustrated in GB2230050. The piston comprises an elongated 
stem 11 and respective cups 12 and 13 attached to the stem at opposite 
ends thereof. The cups 12 and 13 may be identical with each other and only 
the cup 12 will be described in detail. 
The cup 12 of the piston includes a substantially cylindrical side wall 14 
and a bottom wall 15 in which there is formed an aperture 16 to permit 
flow of damping fluid through the piston. There is associated with the cup 
12 a valve 24 for controlling flow of fluid through the aperture 16 as in 
known door closers. The bottom wall 15 may include an annular rib. 
The piston further comprises a circular ring 17 which is resiliently 
deformable relative to the cup 12. The cups 12 and 13 and the stem 11 are 
typically formed of metal, preferably steel. The ring 17 may be formed of 
a plastics material, for example a thermo-plastic. A first end portion 18 
of the ring lies outside the cup 12 and, in the unstressed condition, has 
an outside diameter which is slightly greater than the outside diameter of 
the cup. A second end portion 19 of the ring lies inside the cup and 
engages the side wall 14. 
The cup and the ring have a complimentary recess and projection for 
retaining the ring in the cup. In the example illustrated, the recess is 
formed in the side wall 14 at the inside of the cup and the projection is 
formed on the second end portion 19 of the ring. Also in the example 
illustrated, both the recess and the projection are annular. 
The ring 17 is conveniently formed by moulding a suitable plastics material 
to the required finished size and shape. 
The cup 12 is formed from substantially flat strip metal stock. A pressing 
operation is performed on a length of the stock to produce the blank 
illustrated in FIG. 3. During this pressing operation, material is cut 
away from the center of the blank to form the aperture 16, an annular 
depression 20 is formed in the blank at a face which will be the internal 
face of the bottom wall 15 and an annular recess 21 is formed in that part 
of the blank which will become the side wall 14 of the cup. The recess 21 
is spaced further from the aperture 16 than is the depression 20 and is at 
the same face of the blank as is the depression 20. 
The blank illustrated in FIG. 3 is subjected to a further operation in 
which scrap is severed from the periphery of the blank and the blank is 
drawn or pressed to form the cup 12. This operation completes formation of 
the cup. It will be noted that the cup is produced by two pressing 
operations. These may be performed by successive strokes of the same 
press, the stock being advanced between tools in the press from a first 
station where the depression 20 and the recess 21 are formed to a second 
station in which the cup is formed. The recess 21 is formed without 
cutting away material from the stock. 
The cup may be secured to the stem 11 by welding. There is at the outside 
of the bottom wall 15 of the cup an annular rib corresponding to the 
depression 20 and the cup may be welded at this rib to the stem. 
After the cups 12 and 13 have been welded to the stem 11, the moulded ring 
17 is fitted into the cup 12. Generally, only one of the cups will be 
fitted with a ring but a further ring identical with the ring 17 may be 
fitted to the cup 13, if required. 
On the second end portion 19 of the ring, there is provided an outwardly 
projecting rib 22 which, in the assembled piston, seats in the recess 21. 
During assembly of the ring with the cup 12, the ring is deformed from a 
circular shape to an approximately elliptical shape after the second end 
portion 19 at one side of the ring has been inserted into the cup. This 
facilitates movement of the rib 22 at the opposite side of the ring over 
the internal surface of the side wall 14 to the recess 21. Such 
deformation of the ring causes the first end portion 18 to project further 
beyond the side wall 14 in radially outward directions at two positions. 
This is possible, because the first end portion 18 is not confined. 
After the ring 17 has been fitted to the cup 12, the piston is introduced 
into a cylinder represented at 23 in FIG. 2. The cup 12 is received within 
the cylinder with a slight clearance. The exact amount of clearance 
depends upon the exact dimensions of the components of the door closer, 
these being within prescribed tolerance limits. The internal diameter of 
the cylinder is slightly less than the outside diameter of the first end 
portion 18 of the ring, when the ring is in an unstressed condition. 
Accordingly, the ring bears on the cylinder in a manner to seal the piston 
with respect to the cylinder. Furthermore, the cylinder constrains the 
first end portion 18 of the ring to a circular shape. Because the first 
end portion cannot deform to an elliptical shape, the rib 22 cannot escape 
from the recess 21 and the ring is retained in assembled relation with the 
cup 12. 
In the unstressed condition of the ring 17, the diameter of the projecting 
rib 22 is slightly greater than the diameter of the internal surface of 
the side wall 14 of the cup. The diameter of the rib 22 in the unstressed 
ring may be substantially equal to or may be somewhat greater than the 
diameter of the recess 21 of the completed cup. As shown in FIG. 2, the 
axial dimension of the rib 22 is substantially equal to the corresponding 
dimension of the recess 21. Accordingly, in the assembled piston, the 
recess 21 is completely occupied by the rib 22. It will be noted that each 
of the cups 12 and 13 is circular, as viewed along the axis of the piston; 
whereas the stem 11 is relatively narrow and has a rectangular transverse 
cross section in a plane perpendicular to the axis of the piston.