Seal, scraper, and guide for double-acting piston

A double-acting piston and cylinder assembly has a piston which in turn has an outer piston surface centered on an axis, a radially outwardly open piston groove and a pair of axially oppositely facing piston ends. The cylinder has an inner surface centered on the axis and spaced radially outside the outer piston surface. A tubularly annular element received in the piston groove has an outer surface centered on the axis and bearing radially outwardly on the cylinder so the piston is guided in the cylinder by the outer element surface. The annular element has an element groove open radially inwardly to the piston between the end element surfaces and having a pair of axially spaced element-groove flanks, a pair of seal lips axially level with the element groove and having juxtaposed seal-lip ends lying when unstressed radially outside the outer element surface but normally bearing against the cylinder surface, and radially inner web surfaces exposed in the element groove, and respective end scraper lips having respective outer scraper-lip ends extending axially beyond the piston ends and lying when unstressed radially outside the outer element surface but normally bearing radially outward on the cylinder surface. The assembly also has an elastomeric ring in the element groove bearing radially inward on the piston and radially outward on the inner lip surfaces. Thus the ring urges the webs elastically outward against the cylinder surface.

FIELD OF THE INVENTION 
The present invention relates to a double-acting piston and cylinder 
assembly. More particularly this invention concerns a seal, scraper, and 
guide for a double-acting piston. 
BACKGROUND OF THE INVENTION 
A piston normally has a cylindrical outer surface that is perfectly 
complementary and coaxial with the cylindrical inner surface of a cylinder 
in which the piston moves axially, with or without rotation of the piston 
about the axis. Obviously it is essential that no fluid flow be possible 
around the piston, that is axially between the piston outer surface and 
cylinder inner surface. Furthermore it is always essential that the piston 
remain perfectly square and centered within the cylinder. 
To this end the piston is normally provided with a seal that prevents 
leakage, and with a scraper that prevents particles or the like that have 
somehow come to adhere to the cylinder wall from coming into contact with 
the fragile and accurately dimensioned seal. Furthermore either the piston 
itself is very accurately dimensioned to guide itself in the cylinder, or 
special guides are provided on the piston to keep it perfectly centered. 
A double-acting piston must, of course, be provided with two seals and 
scrapers. The seals are normally constituted as soft elastomeric glands of 
U-section with one leg bearing inwardly on the base of the respective 
piston groove and another leg bearing outwardly on the inner cylinder 
surface. Such multiplication of parts even further complicates the 
structure. 
The piston must therefore normally be built with a plurality of grooves or 
steps formed to hold the guides, seals, and scrapers. When no separate 
guide is provided the piston must be accurately machined to fit precisely 
in the cylinder bore. Obviously such construction is expensive. 
Furthermore the labor entailed in servicing any of these wear-prone parts 
is usually so excessive that the mechanic normally also replaces all of 
the others while the piston is out. 
OBJECTS OF THE INVENTION 
It is therefore an object of the present invention to provide an improved 
piston and cylinder assembly. 
Another object is to provide an improved seal, scraper, and guide for a 
double-acting piston separating two chambers in a cylinder. 
A further object is the provision of such an assembly which is relatively 
easy to service. 
SUMMARY OF THE INVENTION 
These objects are attained according to the instant invention in a 
double-acting piston and cylinder assembly whose piston has an outer 
piston surface centered on an axis, a radially outwardly open piston 
groove having a piston-groove base lying radially inside the outer piston 
surface and a pair of piston-groove flanks transverse to the axis, and a 
pair of axially oppositely facing piston ends. The cylinder has an inner 
cylinder surface centered on the axis and spaced radially outside the 
outer piston surface. A tubularly annular element received in the piston 
groove has an outer element surface centered on the axis and bearing 
radially outwardly on the cylinder surface. Thus the piston is guided in 
the cylinder by the outer element surface. In addition the annular element 
has respective end element surfaces confronting the piston-groove flanks, 
an element groove open radially inwardly to the piston groove base between 
the end element surfaces and having a pair of axially spaced 
element-groove flanks, a pair of seal lips axially level with the element 
groove and having juxtaposed seal-lip ends lying in the absence of the 
cylinder radially outside the outer element surface but normally bearing 
against the cylinder surface, and radially inner web surfaces exposed in 
the element groove, and respective end scraper lips having respective 
outer scraper-lip ends extending axially beyond the piston ends and lying 
in the absence of the cylinder radially outside the outer element surface 
but normally bearing radially outward on the cylinder surface. The 
assembly also has an elastomeric ring in the element groove bearing 
radially inward on the piston groove base and radially outward on the 
inner lip surfaces. Thus the ring urges the webs elastically outward 
against the cylinder surface. 
The system according to the instant invention therefore is a single 
subassembly fitted into the piston groove to perform the functions of 
sealing, scraping, and guiding. The unit is normally stretched to fit over 
the piston, snapping into place in the piston groove, and is thus simply 
mounted in place. For repair it is removed, normally by simply cutting it 
out, and in one simple operation can be replaced with a new subassembly. 
The manufacturer of a double-acting ram or the like incorporating the 
subassembly according to the instant invention need not machine the piston 
to exact external dimensions, and need merely form it with a single large 
groove to receive the subassembly, which is normally supplied by a 
subcontractor. Thus manufacturing costs can be held quite low. 
According to another feature of this invention the seal lips have 
respective webs generally axially level with the element-groove flanks and 
axially flanking the seal-lip ends. These seal lips are radially 
substantially thinner at these webs than at the respective seal-lip ends. 
Thus, unlike the prior-art arrangements, the seal lip gets thicker towards 
its outer business end. The thin hinge-like web allows it to flex, even 
when this element is made of relatively hard material. Such use of hard 
material further avoids the prior-art problem of the seal lip sticking to 
and gumming up on the cylinder wall. In effect the webs are relatively 
long and serve to guide the seal lips, not simply to prevent it from 
getting caught between the piston and cylinder. 
According to another feature of the present invention the element is formed 
between the seal-lip ends with a thin hinge unitarily interconnecting the 
seal-lips and forming a small radially outwardly open groove therebetween. 
The system according to the instant invention therefore compensates for the 
so-called memory loss or plastic deformation common in virtually any 
material subject over a long term to some kind of deformation, even within 
its range of elastic deformation. This plastic deformation is most noticed 
in the soft elastomeric ring that urges the seal lips out against the 
cylinder inner wall. Normally in the prior-art systems a single spring 
ring is provided for urging the seal into contact with the surface to be 
sealed. Thus the force that largely determines the effectiveness of the 
seal derives from a single source. According to the instant invention, 
however, this force has three sources: the elastomeric ring, the hinges or 
webs of the outer edges of the seal lips of the seal lips, and the hinge 
interconnecting the seal lips. Obviously this means that the sealing 
action will be more effective and will be more likely to have a long 
service life. 
It is possible according to this invention for the element to be formed 
with an outwardly projecting ridge constituting both of said seal-lip ends 
and radially outwardly engaging the cylinder surface. In such an 
arrangement there is therefore another element acting as seal, which 
obviously will give an even better sealing action. 
In accordance with another feature of this invention, the seal-lip ends 
have rounded radially inner corners turned toward each other and 
engageable with the ring. This prevents the seal lips from biting into and 
damaging the normally relatively soft ring. 
It is also possible according to this invention for the element to be 
formed of a pair of parts each having one respective end element surface, 
element-groove flank, seal lip, and scraper lip. In such a system the two 
parts can be perfectly complementary, so that manufacturing costs are 
maintained very low. The ring of such an arrangement may have a radially 
outwardly projecting ridge engaged between the seal-lip ends with the 
cylinder surface. 
The ring according to this invention is normally axially slightly shorter 
than the element groove. Similarly the element is axially slightly shorter 
than the piston groove. Such construction makes the element even easier to 
manufacture, and allows a little lost motion in the piston action to 
greatly reduce wear. 
The element of the assembly according to the invention further has a pair 
of relatively thin axial extensions having inner extension surfaces 
engaging the piston surface, outer extension surfaces engaging the 
cylinder surface, and outer ends carrying the respective scraper lips. 
These extensions themselves constitute the guide in part, for a long guide 
surface that effectively prevents the piston from canting in the cylinder 
bore.

SPECIFIC DESCRIPTION 
As seen in FIG. 1 a generally tubular element 1 has a pair of relatively 
thick main portions 6 flanking a rectangular-section groove 2 in which is 
received an elastomeric rectangular-section elastomeric ring 3, with two 
seal lips 4 overlying the ring. The outer ends of the main portions 6 are 
formed with extensions 5. 
A double-acting piston 8 is formed with an outwardly open groove having a 
cylindrical base or floor 7 and a pair of end flanks 9 extending 
perpendicular to the axis A on which the piston 8 and element 1 are 
centered. The element 1 has end sufaces 10 which confront but are spaced 
slightly from the respective surfaces 9. The extensions 5 are formed with 
scraper lips 11. 
As better seen in FIG. 2 the groove 2 has a depth 12 equal to the radial 
height of the ring 3 and an axial width 13 which is slightly greater than 
the corresponding dimension 14 of the ring 3. Thus this ring 3 can move 
axially limitedly in the groove 2, as the body 1 can move axially in the 
piston groove 7,9. 
The seal lips 4 have outer ends forming sharp edges 15 and are connected to 
the respective sections 6 by means of webs or hinges 16 which are 
substantially thinner than the webs at the edges 15. In addition the two 
lips 4 are connected together at their confronting outer ends by an 
integral web or hinge 17 which is even thinner than the webs 16. 
FIG. 3 shows how when the assembly is inserted into a cylinder having a 
cylindrical inner wall 18 centered on the axis A the lips 4 are pressed in 
as shown by arrow 19 from their FIG. 2 position. This action brings rear 
or inner surfaces 20 of the lips 4 to bear on the ring 3 and compresses 
this ring 3 somewhat, bringing to bear on the wall 18 forces 22 which are 
greater toward the edge 15. The rear surfaces 20 are rounded about radii 
21 at their outer ends to prevent the lips 4 from biting into and damaging 
the ring 3. The forces 22 net to a force 23 which is effective for good 
sealing action against the cylinder wall 18. 
It is possible, as shown in dashed lines in FIG. 2, for the center of the 
body 1 to be formed with a ridge 24 between the two lips 4. In such a case 
the normally flat and planar outer surfaces 25 of the lips 4 are made 
outwardly concave as shown at 26. Otherwise, as indicated in FIG. 3 the 
two lips 4 form at the hinge a small outwardly open groove 27 which 
normally is filled with hydraulic liquid to lubricate the assembly and 
ensure good sealing action. 
The extension 5 have, as shown in FIG. 4, a radial dimension 28 which is 
somewhat smaller than the radial height 29 of the surfaces 9 and 10, so 
that the body 1 is effectively blocked against extrusion out from between 
the piston 8 and the cylinder wall 18. The inner surface of the body 1, at 
these extensions 5, lies against the outer piston surface 30 for excellent 
guiding of the piston 8 on the surface 18. 
The lips 11 have sharp outer edges 32 that, like the edges 15 of the lips 
4, are in line contact with the wall 18 when the assembly is installed. A 
relatively thick web or neck 31 is provided for each of these lips 11, so 
that the inwardly effective force, as shown by arrow 33, will be resisted 
with a force sufficient to ensure good scraping action at the line/edge 
32. Any particles scraped off the wall 18 will therefore by moved radially 
inward to the outwardly convex frustoconical piston end 34. 
It is possible for the body 1 to be formed as seen in FIG. 5 of two parts 
1a and 1b, separated by a space 36 between the outer ends of the two lips 
4 which therefore form groove halves 2a and 2b in which a single ring 3 is 
received. The inner ends of the lips 4 may be pointed as indicated at 35. 
Finally FIG. 6 shows how in such a two-part assembly the ring 3 may be 
formed with an outwardly projecting ridge 37 that extends out through the 
gap 36 so that it can contact the cylinder wall 18 when installed. This 
ridge 37 has a flank angle 39 formed with a perpendicular to the axis A, 
which is also parallel to the lip end in the unassembled position, that is 
such that when the lip 4 is depressed inwardly when assembled, as shown by 
the equal angle 40, the lip 4 will not bite into and damage the ring 3. 
This ring 3 is also shorter than the groove 2a, 2b by a distance 41 that 
is equal to a gap 42 left between the ridge 37 and the gap 36. Thus the 
ring 3 can shift slightly axially without damage. It has been proven that 
a small degree of lost motion greatly increases the service life of such 
an arrangement. 
The system according to the instant invention therefore incorporates the 
functions of sealing, guiding, and scraping in a single unit that is 
fitted to the piston by being stretched over it and snapped in place in 
the piston groove. The arrangement can be produced by a manufacturer 
specializing in seals and the like and can be installed by even a 
relatively unskilled mechanic.