Stroke cushioning apparatus for hydraulic cylinders

A stroke cushioning apparatus for a hydraulic cylinder provided at rod side stroke end and/or head side stroke end. The rod side stroke end cushioning apparatus comprises a sleeve mounted on a piston rod and a cushion ring mounted on the inner face of a housing defining a passage which accommodates the sleeve when a piston assembly is moved to its rod side stroke end. The cushion ring is allowed to move slightly in an axial direction. When the piston assembly is moved towards its rod side stroke end and the sleeve enters into the passage, flow of fluid is restricted between the cushion ring and the sleeve thus effecting a cushioning action to the piston assembly. The head side stroke end cushioning apparatus comprises a plunger mounted to the piston rod and wall of a passage formed at the head end of the cylinder. The plunger is mounted to the piston rod by means of a plurality of balls with allowing slight movements of the plunger both axially and radially. When the piston assembly is moved towards its head side stroke end and the plunger enters into the passage, flow of fluid is restricted between the plunger and the wall of the passage thus effecting a cushioning action to the piston assembly.

BACKGROUND OF THE INVENTION 
This invention relates to a stroke cushioning means for a hydraulic 
cylinder adapted to slow down and stop a piston at the stroke ends 
thereof. 
There has heretofore been employed a rod-side stroke cushioning means 
wherein a sleeve is fitted from outside to a portion of the piston rod 
near the piston and a cushion ring is fitted on the side of one end of the 
cylinder to restrict the fluid flow between the cushion ring and the 
sleeve. In the above-mentioned prior art cushion ring, since it is 
difficult to align the center line of the cushion ring through which the 
sleeve is moved forwards and backwards with that of the hydraulic 
cylinder, it is necessary to fit the cushion ring inside the cylinder in 
such a manner that it can be moved to some degree and then secure the 
cushion ring by means of a snap ring. Such snap ring has been 
disadvantageous in that since it has a notch formed in part of the 
circumferential portion thereof, noise tends to generate in the notch when 
part of the fluid under pressure confined therein is discharged into the 
passage thereby damaging the snap ring quickly. 
Whilst, there has heretofore been employed a cylinder-head end stroke 
cushioning means wherein a plunger is fitted to the leading end of a 
piston rod to obtain a cushioning effect. This type of plunger has, 
however, been disadvantageous in that because it is difficult to align the 
center line of the passage in which the plunger is moved forwards and 
backwards with that of the cylinder, it is necessary to fit the plunger to 
the piston rod so that the plunger can be moved relative to the piston rod 
to some degree thereby complicating the structure of the component parts. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a stroke 
cushioning apparatus for a hydraulic cylinder which can overcome the above 
noted problems. 
Another object of the present invention is to provide a rod end stroke 
cushioning apparatus which is simple in construction yet can provide a 
reliable cushioing function. 
A further object of the present invention is to provide a head end stroke 
cushioning apparatus which is also simple in construction yet can provide 
a reliable cushioning function. 
In accordance with an aspect of the present invention, there is provided a 
stroke cushioning apparatus for a hydraulic cylinder, comprising: a 
housing; a piston assembly slidably mounted within said housing, said 
piston assembly including a rod and a piston mounted on said rod and 
defining at least one fluid chamber in said housing; a sleeve fixedly 
mounted on said rod; an axial passage communicating with the fluid chamber 
and outside of said housing, said axial passage being adapted to 
accommodate said sleeve therein when said piston assembly is moved to its 
rod side stroke end; a land formed in said housing within said axial bore; 
a stopper plate fixedly mounted to said housing; and a cushion ring 
mounted to said housing between said land and said stopper plate, said 
cushion ring being allowed to move slightly in an axial direction and the 
inner diameter of which is made slightly larger than the outer diameter of 
said sleeve whereby flow of fluid is restricted between said sleeve and 
said cushion ring when said piston assembly is moved to its rod side 
stroke end thereby effecting cushioning action to the movement of said 
piston assembly. 
According to another aspect of the present invention, there is provided a 
stroke cushioning apparatus for a hydraulic cylinder, comprising: a 
housing; a piston assembly slidably mounted within said housing, said 
piston assembly including a rod and a piston mounted on said rod and 
defining at least one fluid chamber in said housing; a plunger mounted to 
said rod; means for mounting said plunger to said rod with allowing slight 
movements of said plunger both axially and radially; and an axial passage 
communicating with the fluid chamber and outside of said housing, said 
axial passage being adapted to accommodate said plunger therein when said 
piston assembly is moved to its head side stroke end, the diameter of said 
passage being made slightly larger than that of said plunger whereby flow 
of fluid is restricted between said plunger and wall of said passage when 
said piston assembly is moved to its head side stroke end thereby 
effecting cushioning action to the movement of said piston assembly. 
The above and other objects, features and advantages of the present 
invention will be readily apparent from the following description taken in 
conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention will now be described in detail below by way of 
example only with reference to the accompanying drawings. In FIG. 1, 
reference numeral 1 denotes a housing or cylinder barrel in which is 
slidably disposed a piston 4 which is connected to a piston rod 2 by means 
of a nut 3. The piston rod 2 is formed at its one end with a recess 5 in 
which the base end or left-hand end of a plunger 6 is loosely fitted. The 
base end of the plunger 6 has an annular groove 8 having approximately 
V-shaped section formed in the outer peripheral surface thereof at a 
location opposite to an annular groove 7 formed in the inner peripheral 
surface of the recess 5. Accommodated between the annular groove 8 and the 
annular groove 7 of the recess 5 are a plurality of balls 9. The 
above-mentioned balls 9 are made, for example, of steel and are arranged 
to be inserted or taken out through an insertion hole 10 opening to the 
outer peripheral surface of the piston rod 2 so as to communicate with the 
annular groove 7. The insertion hole 10 is normally closed by means of a 
plug 11. The above-mentioned plunger 6 has a plurality of axially 
extending grooves 12 formed in the outer peripheral surface thereof, each 
of the grooves 12 being formed to have a depth which increases gradually 
towards the leading end of the plunger. 
The other end of the piston rod 2 extends through the end of the cylinder 
barrel 1 and projects outside thereof. The piston rod 2 has a sleeve 15 
fitted to the outer peripheral surface thereof and on the side of the 
piston 4 so that one end thereof abuts against the piston. The sleeve 15 
has a plurality of axially extending grooves 16 formed in the outer 
peripheral surface thereof and each having a depth which increases 
gradually towards the leading end thereof. The leading end of the sleeve 
15 is arranged to be introduced in a passage 17 formed in the end of the 
cylinder barrel 1 when the piston 4 has reached its stroke end on the side 
of the piston rod. The above-mentioned passage 17 has an inside diameter 
which is larger than the outside diameter of the sleeve 15, and its one 
end is opened into the inside of the cylinder barrel 1. Formed in the 
passage 17 is a land 18 to which a cushion ring 19 is loosely fitted from 
the inside of the cylinder barrel 1. The cushion ring 19 has an inside 
diameter which is a little bit larger than the outside diameter of the 
sleeve 15. The cushion ring 19 has rounded inner peripheral edges and is 
secured by means of a stop plate 20 which is fixedly secured to the end of 
the opening of the passage 17 between the land 18 and the stop plate 20. 
The above-mentioned stop plate 20 has a hole 21 formed in the central part 
thereof which is sufficiently larger than the outside diameter of the 
sleeve 15 and is fixedly secured to the inner end of the cylinder barrel 1 
by means of bolts 22. The stop plate 20 has also a hole 25 formed in the 
part thereof which can be aligned with a passage 24 communicating with a 
fixed orifice 23. The fixed orifice 23 serves to restrict part of the 
fluid within the cylinder barrel 1 and allow it to flow into the passage 
17. 
Referring to FIG. 2, there is shown the piston 4 which has been moved to 
the head end of the cylinder and stopped there. The leading end of the 
plunger 6 is inserted into a small-diameter passage 27 which extends from 
a large-diameter passage 26 and is open to the outside of the cylinder 
barrel 1. 
At the stroke end, the plunger 6 is arranged to be inserted into the 
small-diameter passage 27 with a small clearance .delta. therebetween. 
Thus, when the piston 4 has reached near the rod side stroke end, the 
leading end of the sleeve 15 is inserted into the passage 17 so that the 
fluid under pressure on the side of the piston rod being discharged into 
the passage 17 is restricted suddenly so as to decelerate the piston 4. 
With the advancement of the sleeve 15 within the passage 17 the sectional 
area of each of the grooves 16 at the cushion ring 19 is reduced gradually 
so that the piston 4 is decelerated further and then is stopped when the 
end face of the piston strikes against the stop plate 20. 
Reversely, when the piston 4 has reached the head side stroke end, the 
leading end of the plunger 6 is inserted into the small-diameter passage 
27 and subsequently the nut 3 is introduced into the large-diameter 
passage 26. As a result, the fluid under pressure which is confined within 
the large-diameter passage 26 will be throttled or restricted by the 
plurality of grooves 12 of the plunger 6 and discharged into the 
small-diameter passage 27 so as to decelerate the movement of the piston 
4. With further advancement of the plunger 6 into the small-diameter 
passage 27, the sectional area of each of the grooves 12 at the inlet 
portion of the small-diameter passage 27 is reduced so as to increase the 
piston deceleration effect thereby slowing down the piston 4. The piston 4 
is stopped when its end face is allowed to strike against the opposite end 
face of the cylinder barrel 1. 
As described in detail hereinabove, according to the present invention, 
since the cushion ring is movably disposed as a rod-end stroke cushioning 
means and is arranged to be held by the stop plate fixedly secured to the 
inner end face of the cylinder barrel, there is no possibility of 
generating noise due to partially increased fluid flow and thereby to 
cause earlier damage of stopper means as in the case of snap rings. 
Further, if and when a fixed restrictor is provided in combination to 
allow part of the fluid to pass therethrough, it is possible to adjust 
finely the deceleration speed of the piston only by replacing the fixed 
restrictor with ones having different bore sizes. 
Still further, as a head-end stroke cushioning means, the base end of the 
plunger is movably engaged with the leading end of the piston rod through 
a plurality of balls located in the circumferential groove formed by the 
recesses in the plunger and the piston rod so that the plunger can be 
readily fitted to the piston rod with some allowance for movement. 
Besides, the plunger can be removed readily from the piston rod by taking 
the balls out so that maintenance of the component parts can be made 
easily and the structure thereof can be simplified. 
It is to be understood that the foregoing description is merely 
illustrative of a preferred embodiment of the present invention, and that 
the scope of the invention is not to be limited thereto, but is to be 
determined by the scope of the appended claims.