Hydraulic cylinder and assembly method thereof

A hydraulic cylinder with which it is possible to promote cost reduction and weight reduction by reducing the number of parts, ensure that a sealing function is fully performed and also make possible a reduction in size comprises a cylinder tube 4, a piston 1 slidably installed inside this cylinder tube 4, a rod 6 connected to the piston, a bearing 9 supporting the rod 6, a tubular cylinder head 10 mounted in the front end of the cylinder tube and seals 12 and 13 mounted in contact with the rod 6 on an inner circumferential surface of the cylinder head 10; a cylindrical recess 19 is formed in the front end of the cylinder tube 4, the radius R of this cylindrical recess 19 is made larger by a predetermined dimension than the radius r of a piston sliding surface formed on the cylinder tube 4, a screw thread 19a is formed in this cylindrical recess 19, the cylinder head 10 is screwed into this cylindrical recess 19 and the bearing 9 supporting the rod 6 is mounted on an inner circumferential surface of this cylinder head 10.

BACKGROUND OF THE INVENTION 
This invention relates to a hydraulic cylinder used mainly in fork lifts 
and the like, and particularly to a cylinder head structure thereof. 
In an example of a conventional hydraulic cylinder shown in FIG. 3, a seal 
2 and a bearing 3 are mounted on an outer circumferential surface of a 
piston 1 and this piston 1 is slidably installed inside a cylinder tube 4. 
A rod 6 is connected to one end of this piston 1 by way of a stopper 5. 
When the piston 1 and the rod 6 have been installed in this way, a bearing 
holder 7 is inserted into the front end of the cylinder tube 4. A flange 
part 8 is formed on one end of this bearing holder 7, and this flange part 
8 is caused to abut with the front end of the cylinder tube 4. 
A bearing 9 is fitted to an inner circumferential surface of the bearing 
holder 7, and the rod 6 is supported by this bearing 9. 
Also, a cylinder head 10 is mounted on the front end of the cylinder tube 4 
in the following way. That is, a screw thread 4a is formed on the outer 
circumferential surface of the front end of the cylinder tube 4, this 
front end of the cylinder tube 4 is brought to face one of the open ends 
of the tubular cylinder head 10 and the two are screwed together. At this 
time, the flange part 8 is made to abut with a step part 11 formed on the 
inner surface of the cylinder head 10 and secures the bearing holder 7. 
On the inner circumferential surface of the cylinder head 10 at the other 
open end thereof there is mounted a dust seal 12 for preventing the entry 
of extraneous matter from outside and a seal 13 for sealing the sliding of 
the rod 6 is mounted on the inner side of this. An O-ring 14 is provided 
adjacent to the threaded parts of the cylinder tube 4 and the cylinder 
head 10. 
In this hydraulic cylinder, the inside of the cylinder tube 4 is divided by 
the piston 1 into a rod side chamber 15 and a bottom side chamber 16. A 
fluid is supplied to and discharged from the bottom side chamber 16 
through a supply and discharge port 17. The rod side chamber 15 is charged 
with a fluid for lubrication and connected to a tank not shown in the 
drawings by a supply and discharge pipe 18. 
In this hydraulic cylinder, when fluid is supplied through the supply and 
discharge port 17 into the bottom side chamber 16, the piston 1 moves into 
the rod side chamber 15 while discharging fluid in the rod side chamber 15 
and as a result the rod 6 is extended. 
When fluid is discharged through the supply and discharge port 17 from the 
bottom side chamber 16, the piston 1 moves into the bottom side chamber 16 
and as a result the rod 6 is contracted. 
However, in the conventional hydraulic cylinder described above, because 
the cylinder head 10 and the bearing holder 7 are different members, there 
are correspondingly numerous parts and this increases its cost also its 
weight. 
Also, because the cylinder head 10 and the bearing holder 7 are different 
members, it is necessary to align their centers completely. If the centers 
of the cylinder head 10 and the bearing holder 7 are not aligned, the 
centers of the bearing 9 and the seals 12 and 13 also become misaligned 
and because the interferences of the seals 12 and 13 consequently become 
uneven in the circumferential direction of the rod 6 they cannot fully 
perform their sealing function and this results in fluid leakage and the 
entry of extraneous matter from outside. 
Also, because the cylinder head 10 is screwed onto the outside of the front 
end of the cylinder tube 4, that part is enlarged to the diameter of the 
cylinder head 10. Consequently, when for example this hydraulic cylinder 
is used as a master cylinder of a fork lift, the fork lift operator's 
field of vision is impaired by an amount corresponding to this 
enlargement. 
SUMMARY OF THE INVENTION 
Accordingly, an object of this invention is to provide a hydraulic cylinder 
with which it is possible to promote cost reduction and weight reduction 
by reducing the number of parts, ensure that the sealing function is fully 
performed and also make possible a reduction in size. 
A first aspect of the invention presupposes a hydraulic cylinder comprising 
a cylinder tube, a piston slidably mounted inside this cylinder tube, a 
rod connected to the piston, a bearing for supporting the rod, a tubular 
cylinder head mounted on the front end of the cylinder tube and a sealing 
member mounted in contact with the rod on an inner circumferential surface 
of the cylinder head. 
A cylindrical recess is formed in the front end of the cylinder tube and 
the diameter thereof is made larger than the diameter of a piston sliding 
surface formed on the cylinder tube whereby a 0.3 mm to 1.5 mm step part 
is formed between the cylindrical recess and the piston sliding surface, 
the cylinder head is screwed into a screw part formed in this cylindrical 
recess and the bearing supporting the rod is mounted on an inner 
circumferential surface of this cylinder head. 
A second aspect of the invention is a method of assembling a hydraulic 
cylinder according to the first aspect of the invention wherein a jig 
having a tubular part of a wall thickness substantially the same as the 
0.3 mm to 1.5 mm step formed between the piston sliding surface and the 
cylindrical recess is inserted into the cylindrical recess and the piston 
is passed through the inside of this tubular part into the cylinder tube. 
In the first aspect of the invention, because the cylinder head is mounted 
in a cylindrical recess in the front end of the cylinder tube, the 
cylinder head does not increase the diameter of this part of the hydraulic 
cylinder. Therefore, the hydraulic cylinder can be made small. 
Furthermore, because the bearing supporting the rod is mounted on an inner 
circumferential surface of the cylinder head, it is not necessary to 
provide a bearing holder. Also, because the sealing members and the 
bearing are mounted on the same member, the cylinder head, the centers of 
the sealing members and the bearing can be aligned at all times. 
In the second aspect of the invention, the wall thickness of the tubular 
part of the jig is made substantially the same as the step formed between 
the cylindrical recess and the piston sliding surface. As a result, when 
the tubular part of this jig is inserted into the cylindrical recess 
formed in the cylinder tube, the inner circumferential surface of the 
tubular part and the piston sliding surface of the cylinder can be made 
substantially aligned. 
In these first and second aspects of the invention, the step formed between 
the piston sliding surface and the cylindrical recess is made 0.3 mm to 
1.5 mm; the reason for this is as follows: 
When this step is made large, the wall thickness of the tubular part of the 
jig is also made large to match this. When this wall thickness is made 
large, because the strength of the tubular part increases, even if the 
piston inclines inside the tubular part the inner surface of the tubular 
part is not dented. 
It has been confirmed experimentally that the tubular part can be prevented 
from deforming in this way by the wall thickness of the tubular part being 
made 0.3 mm or more. 
However, when the size of the step is set too large, machining of the 
cylindrical recess in the cylinder tube is hampered. That is, making the 
step large means making the cutting depth in the radial direction of the 
cylindrical recess deep, and the deeper this cutting depth is made the 
more cutting steps are necessary. 
The depth of the cylindrical recess in the cylinder tube which can be cut 
in one step is generally 1.5 mm, and in this invention the upper limit of 
the step is made 1.5 mm for this reason. 
If the step is set in the range 0.3 mm to 1.5 mm in this way it is possible 
to fully prevent deformation of the tubular part of the jig and 
furthermore it is possible to form the cylindrical recess with a cutting 
step consisting of only one feed of a lathe.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The preferred embodiment of the invention shown in FIGS. 1 and 2 is the 
conventional example described above with the cylinder head changed. 
Because the rest of the construction of this preferred embodiment is 
substantially the same as that of the conventional example, the same 
constituent elements have been given the same reference numerals and a 
detailed description thereof will be omitted. 
A piston 1 is slidably installed in a cylinder tube 4, and a cylindrical 
recess 19 having a radius R slightly larger than the radius r of the 
sliding surface of the piston 1 is formed in the front end of the cylinder 
tube 4. A screw thread 19a is formed in this cylindrical recess 19. 
Also, a tubular cylinder head 20 has an external diameter substantially the 
same as the diameter of the cylindrical recess 19, and a thread 20a is 
formed on an outer circumferential surface thereof. A dust seal 12 and a 
seal 13 are mounted on the inner circumferential surface of this cylinder 
head 20, as in the conventional example described above, and a bearing 9 
is also fitted to this surface. 
Next, a method of assembling this hydraulic cylinder will be described. 
First, as shown in FIG. 2, a jig 21 is inserted into the cylindrical recess 
19. The jig 21 has a tubular part 21a, and the wall thickness of this 
tubular part 21a is substantially the same as the step x formed between 
the piston sliding surface of the cylinder tube 4 and the cylindrical 
recess 19. As a result, when this tubular part 21a is inserted into the 
cylindrical recess 19, the inner surface of the tubular part 21a and the 
piston sliding surface are substantially aligned. 
If the piston 1 and the rod 6 are passed into the cylinder tube 4 through 
the inside of the tubular part 21a, it is possible to avoid the outer 
circumferential surface of the piston 1 and the seal 2 being scratched by 
the thread 19a. 
The step x formed between the piston sliding surface of the cylinder tube 4 
and the cylindrical recess 19 is set to 0.3 mm to 1.5 mm. 
Next, the effects of the hydraulic cylinder of this preferred embodiment 
will be described. 
In this hydraulic cylinder, the cylinder head 20 is screwed into 
cylindrical recess 19 formed in the front end of the cylinder tube 4. 
Also, the bearing 9 is fitted to the inner circumferential surface of this 
cylinder head 20 together with sealing members such as the dust seal 12 
and the seal 13. As a result, it is not necessary to provide a bearing 
holder 7 as it is in the conventional example described above and the 
number of parts can be correspondingly reduced. If the number of parts can 
be reduced, cost reduction and weight reduction can be achieved. 
Also, because the bearing 9 is mounted together with the dust seal 12 and 
the seal 13 on a single member, the cylinder head 20, the centers of these 
seals 12 and 13 and the bearing 9 do not become misaligned. If the centers 
of the seals 12 and 13 and the bearing 9 are aligned, because the 
interferences of the seals 12 and 13 are kept uniform in the 
circumferential direction of the rod 6, they can perform their sealing 
function well and leakage of fluid and entry of extraneous matter from 
outside can be surely prevented. 
Furthermore, because the cylinder head 20 is inserted into the inside of 
the front end of the cylinder tube 4 it does not increase the diameter of 
this part and the hydraulic cylinder can be made smaller. Consequently, 
when for example this hydraulic cylinder is used as a master cylinder of a 
fork lift, the field of vision of the operator of the fork lift is 
impaired less by an amount corresponding to this decrease in size. 
The step formed between the piston sliding surface of the cylinder tube 4 
and the cylindrical recess 19 is made 0.3 mm to 1.5 mm for the following 
reasons: 
When this step x is made large, the wall thickness of the tubular part 21a 
of the jig 21 is also made large to match this. When this wall thickness 
is made large, because the strength of the tubular part 21a increases, 
even if the piston 1 inclines inside the tubular part 21a the inner 
surface of the tubular part 21a is not dented. 
It has been confirmed experimentally that the tubular part 21a can be 
prevented from deforming in this way by the wall thickness thereof being 
made 0.3 mm or more. 
However, when the size of the step x is set too large, machining of the 
cylindrical recess 19 in the cylinder tube 4 is hampered. That is, making 
the step x large means making the cutting depth in the radial direction of 
the cylindrical recess 19 must be made deep, and the deeper this cutting 
depth is made the more cutting steps are necessary. 
The depth of the cylindrical recess of the cylinder tube which can be cut 
in one step is generally 1.5 mm, and in this invention the upper limit of 
the step is made 1.5 mm for this reason. 
If the step is set in the range 0.3 mm to 1.5 mm in this way it is possible 
to fully prevent deformation of the tubular part of the jig and also it is 
possible to form the cylindrical recess with a cutting step consisting of 
only one feed of a lathe. 
According to the first aspect of the invention, because a bearing holder is 
not necessary the number of parts can be correspondingly reduced and it is 
possible to achieve cost reduction and weight reduction of the hydraulic 
cylinder. 
Furthermore, because the centers of the seal members and the bearing are 
aligned, the interferences of the sealing members can be kept uniform in 
the circumferential direction of the rod. As a result, the sealing members 
can fully perform their sealing function and leakage of fluid and entry of 
extraneous matter from outside can be surely prevented. 
Also, the diameter of the cylinder head does not enlarge that part of the 
hydraulic cylinder and consequently it is possible to reduce the size of 
the hydraulic cylinder. 
According to the second aspect of the invention, it is possible to make the 
inner surface of the tubular part and the piston sliding surface of the 
cylinder tube substantially aligned. If in this state the piston is 
inserted into the cylinder tube through the inside of the tubular part, 
the outer circumferential surface of the piston and seals mounted on this 
outer circumferential surface are not scratched by the screw thread formed 
on the cylindrical recess. 
Furthermore, in these first and second aspects of the invention, because 
the step is set in the range 0.3 mm to 1.5 mm, it is possible to prevent 
the tubular part from being dented and deforming and also the number of 
machining steps for forming the cylindrical recess does not increase.