Pressure cylinder

A pressure cylinder has a piston actuated by a force-input rod. An enlarged head of the rod is engaged by a washer and the assembly retained within the cylinder by a cup-shaped retainer press-fitted into the cylinder open end. An outer surface of the retainer is flush with the adjacent axially outermost surface of the cylinder and engages a support, to which the cylinder is fixed, in order to transmit to the support retraction forces sustained by the rod.

This invention relates to a pressure cylinder within which pressure is 
generated by movement of a piston under the action of a force input rod, 
such a cylinder being primarily useful as a master cylinder for actuating 
a vehicle brake or clutch hydraulic system. 
The force input rod in such a cylinder can be required to sustain 
significant loads arising principally from an external return spring 
normally associated with a driver-operated pedal from which actuating 
force is applied to the rod. Additional force can arise from one or more 
piston return springs acting within the cylinder. In some conventional 
arrangements, such return spring loads are reacted against a circlip or 
similar device engaged, for example, in a groove formed in the cylinder 
internal wall and whilst this arrangement is generally satisfactory in 
use, the machining of the cylinder groove and provision of a relatively 
substantial circlip, usually of spring steel, adds significantly to the 
manufacturing costs of the cylinder. 
An object of the invention is to provide a pressure cylinder incorporating 
an improved rod retention means of simple and convenient form. 
According to the invention, a pressure cylinder comprises a cylinder body 
containing a piston movable in one direction by a generally co-axial force 
input rod, and rod retention means carried by the cylinder and presenting 
an outwardly facing surface so arranged that, when the cylinder is mounted 
on a fixed support, said surface faces a surface of the support, whereby 
forces sustained, in use, by the rod in a direction opposed to the 
actuating force input direction are reacted via said retention means on 
the fixed support. 
Preferably, the retention means has a first cylindrical portion engaged 
within an open end of the cylinder, and a second portion of reduced 
diameter projecting axially outwardly of the cylinder and extending, in 
use, through an opening in said support, a shoulder between said portions 
providing said surface of the retention means. 
Said second portion may conveniently be fixed to a protective boot for the 
rod, said boot preferably being moulded on to said second portion. By this 
means, the need for a separate boot retention means is avoided.

Referring to FIG. 1, this shows part of a master cylinder having a cast 
cylinder body 1 containing a piston, part of which is shown at 2, the 
cylinder having a radial flange 3 serving to mount the cylinder on a fixed 
support 4, such as a vehicle bulkhead, for example. The cylinder is 
secured in position by bolts, one of which is seen at 5, passing through 
the flange and support and engaged by nuts 6. 
The piston is actuated by a force input rod 7 provided with an enlarged 
head 8 which engages a retaining washer 9, itself contained within a 
retention device in the form of a cup 10 which has a cylindrical portion 
11 press-fitted into the open end of the cylinder and surrounding the 
washer 9. A further smaller diameter cylindrical portion 12 of the device 
10 is joined to the portion 11 by a radially directed portion 13 which 
lies behind and retains the washer 9. The portion 12 is secured to a boot 
14 surrounding the rod 7 to prevent the ingress of foreign material along 
the rod 7. The boot may be moulded on to the cylindrical part 12 of the 
retention device 10 so as to form a permanent assembly therewith. A 
portion 10A of the retainer is bent radially inwardly and serves to key 
the device 10 to the boot. The outwardly facing radial surface 15 of the 
device 10 is arranged to be flush with the axially outer surface of the 
support flange 3 of the cylinder for the purpose to be described. 
Prior to assembly of the rod in the cylinder body, the washer 9 is passed 
along the rod from the free end thereof to a position adjacent the head 8. 
The device 10 and attached boot are then passed along the rod in similar 
manner until the device engages the washer. This sub-assembly can then be 
mounted in the cylinder body by press-fitting the device 10 into the open 
end of the body and this retains the rod assembly with sufficient security 
for transit of the cylinder prior to it being finally secured in its 
position of use on the support 4. When it is desired to secure the 
cylinder in such position, the rod and boot, together with the smaller 
diameter cylindrical portion 12 of the retention device are passed from 
the left through an opening 4A in the support and bolts 5 are then used to 
bolt the cylinder body to the support. It will be seen that, once the 
cylinder is so secured, the surface 15 of the retainer 10 is engaged 
against the fixed bulkhead 4 and any return loads exerted on the push rod 
by external pedal return springs and/or internal piston return springs are 
reacted via the retainer 10 onto the fixed bulkhead. This arrangement is 
advantageous in that it enables the strength of the existing bulkhead to 
be used in order to react, via the relatively flimsy retainer 10, 
substantial forces arising as aforesaid on the force input rod. 
FIG. 2 shows the invention applied to a fabricated master cylinder in which 
the cylinder body 1 is a drawn metal tube. The open end of the tube is 
flared at 1A to receive the large cylindrical portion 11 of a retainer 10 
similar to that of FIG. 1 and the remaining details of construction and 
assembly of the device 10 are similar to those described above except that 
the boot 14 is of the type through which the rod 7 slides and is shaped to 
form a rod wiper 14A. 
It will be seen that the invention provides a pressure cylinder in which 
the inherent strength of the mounting support is used to react the 
substantial rod return forces and enables a relatively simple and 
inexpensive rod retainer to be used, such retainer being also 
advantageously employed as a boot securing means. The embodiments 
described are particularly advantageous in that they enable the rod 7 and 
its retainer to be assembled in the cylinder body 1 and the assembled 
cylinder to be subsequently installed by an assembler working entirely 
from one side of the mounting support 4. Assembly of the cylinder on to a 
vehicle is thus facilitated and the cylinder assembly may be readily 
stored and transported prior to use in a vehicle. 
Various modifications to the shape and disposition of the components 
described may be made within the scope of the present invention. It would 
be possible, for example, to interpose a thrust insert of hardened 
material between the surface 15 of the device 10 and the support 4 so that 
the device 10 no longer directly engages the support and would be inset 
within the cylinder to accommodate the insert. In a further alternative 
arrangement, the cup-shaped retainer 10 may be dispensed with and the 
washer 9 arranged to abut directly against the support 4 for transmission 
of said forces sustained by the rod.