Multi-part piston for servomotor

A servomotor is actuated by a pressure medium and has a piston reciprocated in the cylinder, the piston being mounted by a piston rod that extends through a cylinder end wall. The piston has a piston member bearing against the cylinder and is mounted for radial movement relative to the piston rod by a pair of piston guide plates that extend on opposite sides of the piston member. The guide plates are mounted by the piston rod for axial movement therewith.

BACKGROUND OF THE INVENTION 
The invention relates to a servomotor actuated by a pressure medium, 
comprising a piston reciprocatable in a cylinder and a piston rod brought 
out at one side, the piston being movable relatively to the piston rod and 
its central portion being for this purpose arranged between annular faces 
on the piston rod. 
A known servomotor of this kind (U.S. Pat. No. 31 58 072) has a long 
stroke. The piston has a small diameter and a long axial length. By reason 
of these dimensions, it is well guided in the cylinder. Since the piston 
is secured to the piston rod with the interpositioning of elastic rings, 
it can move at will relative to the piston rod. There is therefore no 
danger of the piston jamming if the free end of the piston rod that 
carries the piston moves out of centre because the piston rod is guided in 
only one end wall of the cylinder. 
Under otherwise identical conditions, however, the small piston diameter 
requires a comparatively high actuating pressure and the length of the 
piston contributes considerably to the cylinder length, which is 
particularly significant in the case of servomotors with a short stroke. 
The invention is based on the problem of providing a servomotor of the 
aforementioned kind which, under otherwise identical prerequisites, can be 
driven at a lower pressure and made of a shorter length. 
SUMMARY OF THE INVENTION 
This problem is solved according to the invention in that the external 
diameter of the piston is a multiple of the axial length of its 
circumference and that the annular faces are so closely juxtaposed that 
they form a radial guide for the piston. 
This piston has a large diameter in comparison with its axial length. A 
predetermined actuating force is therefore achieved with a lower pressure. 
The cylinder length can also be kept shorter. In addition, the piston can 
be lighter in weight so that it is more rapidly actuatable. Such a piston 
is no longer guided by the co-operation of the circumferential surface and 
the cylinder. Instead, it has a considerable tendency to tilt which would 
also arise in the case of the known elastic clamping and lead to jamming. 
For this reason, a radial guide is provided for the piston. This is 
readily achieved by the annular faces because, by reason of the large 
piston diameter, a correspondingly large guide surface can be made 
available. Departures of the piston rod axis from the cylinder axis are 
therefore compensated by a radial displacement. One can therefore arrange 
the piston in the cylinder with very little play (a few .mu.), which leads 
to a correspondingly good seal. 
It is favourable if the central portion of the piston is formed by an 
annular plate which is overlapped by the annular faces over a radial width 
at least equal to twice the thickness of the annular plate. Because of the 
piston dimensions, such a guide face is readily possible to achieve. 
Preferably, the annular faces receive the annular plate therebetween with 
such a tight fit that it has play of less than 0.02 mm. This play is 
sufficient for a radial displacement of the annular plate but holds same 
exactly in the predetermined plane. 
The thickness of the annular plate is desirably less than the axial length 
of the circumferential surface. This results in a very light piston. 
In a preferred embodiment, the two annular faces are formed by two guide 
plates which are secured to the piston rod with the interpositioning of a 
spacer. Such a construction is easy to produce and assemble. 
In particular, the spacer may be made in one piece with one of the guide 
plates at the inner periphery thereof. It is therefore only necessary to 
assemble two parts. 
Preferably, the two guide plates are secured between a step of the piston 
rod and a beaded rim at the free end of the piston rod. This results in 
particularly simple manufacture and assembly. The beading forces are 
received by the spacer between the guide plates. 
It is also favourable if an O-ring is disposed between the two guide faces 
and surrounded and radially compressed by the central portion of the 
piston. This O-ring brings about a seal. In addition, its elasticity 
ensures that there is a certain frictional connection in the radial 
direction between the piston rod and the piston. The O-ring can abut 
against the piston rod or, better still, against the spacer between the 
guide plates. 
It is also possible to provide an annular expansion groove at the 
separating gap between the central portion and the guide face. This can be 
sufficient for sealing between the annular plate and the guide faces, 
especially with low pressure at which the inlet pressure is below 5 bar. 
Preferably, the annular plate forming the central portion is then provided 
with the at least one expansion groove. 
At least one expansion groove may also be provided at the circumferential 
surface of the piston. This serves sealing purposes at low pressures. At 
elevated pressures, a conventional sealing ring can also be used. 
It is favourable if damping rings for abutment of the piston are provided 
at both ends of the cylinder concentric with the piston rod. This leads to 
noise reduction and the piston is mechanically protected. Desirably, the 
damping rings are disposed in the region of the guide plates. The incident 
forces are therefore not taken up by the piston and the latter is not 
damaged.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The servomotor of FIG. 1 comprises a cylinder 1 closed at one end by a 
housing 2 and at the other by a cover 3, these parts being interconnected 
by screws 4. A piston rod 7 mounted in the housing 2 in bearings 5, 6 is 
brought out at one side and is reciprocatable in the direction of the 
arrow F. At the free end, it carries a piston 8 which subdivides the 
interior of the cylinder 1 into two pressure chambers 9, 10. The pressure 
chamber 9 can be connected to a control valve arrangement 13 by way of a 
passage 11 in the housing 2 and the pressure chamber 10 by way of a 
passage 12 in the piston rod 7, so that the pressure chamber 9 is 
connectable to a pressure medium discharge conduit 15 and vice versa. This 
leads to the axial reciprocating motion of the piston rod 7. 
The piston 8 consists of a circumferential portion 16 and a central portion 
17. The external diameter of the piston 8 is a multiple of, in this case 
eight times, the axial length of the circumferential surface 18. The 
central portion 17 is formed by a flat annular plate of which the 
thickness is still less than the axial length of the circumferential 
surface 18. Two annular faces 19, 20 formed on two annular plates 21, 22 
form a radial guide for the piston 8. They abut the end faces of the 
annular plate 17 with a close fit. Consequently, the piston can be 
displaced radially during its reciprocation but it cannot tilt. The one 
annular plate 22 is made in one piece with an annular spacer 23 at the 
inner circumference. The arrangement consisting of the annular plates 21, 
22 is secured between a step 24 of the piston rod 7 and a beaded rim 25 
provided at the free end of the piston rod. The force to be received 
during beading is transmitted by way of the spacer 23 onto the shoulder 
24. 
To provide a better seal between the two pressure chambers 9, 10, three 
relief grooves 26 extending in the circumferential direction are provided 
in the circumferential surface 18. In addition, one relief groove 27, 28 
is provided at each of the ends of the annular plate 17. 
A damping ring 29 of elastic material is applied to the housing 2 and a 
damping ring 30 to the cover 3. These co-operate with the guide plates 20, 
22 in order to damp noise. 
As may be seen from FIG. 1, circumferential (radial outer) portion 16 has 
inner and outer diameters that are substantially larger than the than the 
outer diameter of each of guide plates 21, 22 and of substantially the 
same axial thickness as the corresponding spacing of the remote annular 
surfaces of the guide plates. Further the outer diameter of the radial 
inner central portion 17 where it is joined to portion 16 is of a 
substantially larger diameter than the outer diameters of the guide plates 
21, 22 while the inner diameter of the piston central portion 17 is 
substantially less than the outer diameters of the radial outer parts of 
the guide plates. Further as may be seen the inner annular edge of portion 
17 is substantially annularly spaced from the spacer portion 23. 
The FIG. 2 embodiment has much the same construction. The only difference 
is that the piston 108 has no relief grooves. Instead, there is a sealing 
ring 31 which is accommodated axially between the guide plates 21, 22 and 
in the radial direction between the annular plate 117 and the spacer 23 in 
a manner such that it undergoes slight radial compression. This sealing 
ring assists the seal between the two pressure chambers 9, 10. It also 
ensures a frictional connection between the piston 108 and piston rod 7. 
Many alterations can be made to the illustrated embodiments without 
departing from the basic concept of the invention. Thus, the piston may 
have a thickness throughout corresponding to the axial length of the 
circumferential surface. The guide plates 21, 22 can be separately applied 
to the piston rod 7. The cylinder 1 may also be longer.