Cylinder/piston drive, especially for retightener systems in automatic safety belt winding devices

Cylinder/piston drive especially for re-tightening systems in automatic safety belt winding devices, with a cylinder in which a drive piston is movably guided, as well as with a pressure accumulator which can be connected to the interior of the cylinder. The drive piston 13 has in the region facing the connecting point of the pressure accumulator (cartridge 34) at least one reduced cross sectional pressure surface (annular 15, 27) and operates with a guide part (10,21) in the cylinder 1 conforming to the reduced cross sectional shape of the drive piston such that at the beginning of the pressure action, only part of the cross section of the piston is exposed to the pressure force.

CROSS-REFERENCE TO RELATED APPLICATION 
My application Ser. No. 307,882, filed on Oct. 2, 1981. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a cylinder/piston drive, especially for 
retightening systems in automatic safety belt winding devices, having a 
pressure accumulator which can be connected to the interior of the 
cylinder. 
2. Description of the Prior Art 
In cylinder/piston drives of the type mentioned above, especially in such 
drives for retightener systems in automatic safety belt winding devices, 
the retightening of the belt should take place in a very short time from 
the time of activation, occurring for instance in the event of a crash. 
Pyrotechnical gas generators are used as pressure accumulators for this 
purpose. However, a problem arises, in that at the beginning of the 
activation, i.e. immediately after the ignition of the pyrotechnical gas 
generator, a steeply rising peak force is developed in accordance with the 
expansion of the pressure gas which then drops off very rapidly. Thereby, 
due to the steep force-time characteristic, very large acceleration forces 
act on the piston, which usually has its entire cross sectional area 
exposed to the gas pressure, and these large forces act as well as on the 
tensioning means connected to the drive piston. Consequently the 
above-mentioned parts are heavily stressed mechanically and, due to the 
mass inertia, full utilization of the activated motion energy, i.e. 
conversion into the retightening motion and thereby a high efficiency 
cannot be obtained. The above-mentioned problems exist, of course, also in 
drive systems for other purposes in which a very high acceleration of the 
drive piston is desired. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide a cylinder/piston drive especially 
for retightening systems in automatic safety belt winding devices, having 
a drive piston in a cylinder, as well as a pressure accumulator which can 
be connected to the interior of the cylinder, and when activated 
discharges fluid under pressure into the cylinder to move the drive piston 
therein, whereby an extreme mechanical peak load on the drive piston as 
well as the parts connected thereto is avoided. 
With the foregoing and other objects in view, there is provided in 
accordance with the invention a cylinder/piston drive especially for 
retightening systems in automatic safety belt winding devices, comprising 
a cylinder, a movable drive piston in the cylinder, a pressure accumulator 
from which fluid under pressure will be released, adapted to be connected 
to the interior of the cylinder, an opening in the cylinder wall for flow 
of fluid under pressure from the pressure accumulator through said opening 
into the interior of the cylinder, a guide part in the cylinder with a 
protrusion which extends into the piston to initially reduce the cross 
sectional area of the piston exposed to the fluid under pressure and upon 
movement of the piston and withdrawal of the piston from the protrusion of 
the guide part to expose a greater cross sectional area of the piston to 
the fluid under pressure. 
Other features which are considered as characteristic for the invention are 
set forth in the appended claims. 
Although the invention is illustrated and described herein as embodied in a 
cylinder/piston drive, especially for retightener systems in automatic 
safety belt winding devices, it is nevertheless not intended to be limited 
to the details shown, since various modifications may be made therein 
without departing from the spirit of the invention and within the scope 
and range of equivalents of the claims.

DETAILED DESCRIPTION OF THE INVENTION 
In accordance with the invention, the drive piston 13 has in the region 
facing the connecting point of the pressure accumulator (cartridge 45) at 
least one cross section reducing pressure surface (annular 15; 27) and 
operates with a guide part (10, 21) in the cylinder 1 adapted to the 
reduced cross sectional shape in a manner that at the beginning of the 
pressure action, only part of the piston cross section is exposed to the 
pressure force. 
In this simple manner, the high initial forces which act on the drive 
piston in conventional systems are reduced. Among the advantages obtained 
are a simplification and cost reduction of the cylinder system and even a 
design in light-weight construction may be used. By virtue of the 
invention, only part of the cylinder surface is stressed at the beginning 
of the pressure action, and only after travelling through a predetermined 
travel distance of the piston, is the remaining piston cross section or 
further partial cross sections exposed to the pressure force, for instance 
gas pressure, until finally the entire piston cross section is exposed to 
the gas pressure. In this manner, a more uniform pressure stress is 
obtained overall at the drive piston and the subsequent parts as seen over 
the entire stroke of the piston, without impairing the operating capacity 
of the system. 
Advantageous details of the invention are seen from the embodiment examples 
which are shown in the drawings and will be described in the following. 
In FIGS. 1 and 2, cylinder/piston drives are shown with a straight or 
curved cylinder 1 which is closed off in the Figures at the bottom by a 
screw cap 2. Toward the top in the Figures, the cylinder 1 may be closed 
off by a rupture plate of a known type, not shown. Connected to the 
cylinder 1 is a connecting piece 3 which engages with a fastening means 4 
in a manner similar to that of a pipe clamp on the outside circumference 
of the cylinder 1 and is spaced substantially at right angles to the 
cylinder axis by means of a connecting extension 5 with a thread 6. In the 
connecting extension 5 is a connecting line 7 which is designed as a hole 
and leads into an opening 8 in the cylinder wall. As explained with the 
aid of the example according to FIGS. 3 and 4, the cylinder 1 is in 
communication with a pressure accumulator via the above-mentioned 
connecting piece 3, specifically for connecting extension 5. 
In the embodiment example according to FIG. 1, a cylindrical guide part 10 
is rigidly supported at the lower end in the interior of the cylinder 9. 
The guide part 10 has a guide post 11, the diameter of which is reduced 
and forms a step with an annular area 12 which is located underneath the 
opening 8 in the cylinder wall. A drive piston 13 has a drill hole section 
14 of length a. The inside diameter of the drill hole section 14 
corresponds to the outside diameter of the guide post 11. In the rest 
position, the drive piston 13 sits on the guide post 11 of the guide part 
10, and the guide post 11 is emersed into the drill hole section 14 of the 
drive piston 13. The annular area at the end side of the face of the drive 
piston 13 ends above the opening 8 and has a cross section which is only 
part of the total cross sectional area of the piston 13. Between the 
annular area 15 and the annular area 12 of the cylindrical guide post 10, 
a cavity 16 is formed which is in communication with the pressure 
accumulator via the connecting piece 3. The guide part 10 as well as the 
drive piston 13 have axial holes 17, 18, in which, for instance, a 
cable-like, flexible pulling means 19 is guided. The latter is connected 
via a pinched part 20 to the drive piston 13. If fluid under pressure from 
the pressure accumulator is released, only the reduced cross section or 
annular area 15 of the drive piston 12 is initially acted upon by the 
fluid under pressure until the drive piston has travelled the distance a, 
whereupon the drill hole section 14 is lifted off the guide post 11 and 
the entire cross section of the piston is exposed to the fluid under 
pressure. 
In the embodiment example according to FIG. 2, the cylinder/piston system 
has several steps with different cross sections. Here, the guide part 21 
has two guide posts 22 and 23 following each other in the axial direction, 
each with a reduced diameter. Accordingly, the drive piston 24 has two 
drill hole sections 25, 26 with the lengths b and c which are stepped with 
respect to the inside diameter. The drive piston 24 is again seated 
generally flush with its drill hole sections 25 and 26 on the stepped 
drive posts 22 and 23. If fluid under pressure from a pressure accumulator 
is released, only a small partial cross section in accordance with the 
ring surface 27 is acted upon initially by the fluid under pressure. After 
the distance b has been traversed, a further partial cross section 
according to the ring surface 28 is added to the above-mentioned partial 
cross-section, until finally, after traversing the distance c, the entire 
piston cross section is exposed to the pressure of the fluid. It is, of 
course, possible to provide even further steps in accordance with the 
intended application. It is not absolutely necessary that the guide part 
and the drive piston engage each other in the manner explained. A design 
of the guide part and the drive piston which varied from the embodiment 
examples may be employed, provided only a partial cross section, i.e. a 
reduced cross sectional area no matter of which form, is exposed to the 
fluid pressure at the beginning of the pressure exposure and a further 
partial cross section or the entire piston cross section is acted upon by 
the pressure only after traversing a predetermined distance. As indicated 
in FIGS. 1 and 2, the screw cap 2 and the hole 17 in the guide part 
adjacent the screw cap 2 in the Figures are expanded in funnel-fashion 
downward so that if the course of cable-like pulling means 19 is not in a 
straight line, the pulling means 19 will be subjected to friction forces 
as small as possible. 
In the embodiment example according to FIGS. 3 and 4, is an automatic 
safety belt winding device 29 with a rotatable belt shaft 30, which, on 
the side as shown in FIG. 4 is in connection with an automatically 
activated locking device behind a cover 31, and on the other side in also 
known manner is connected with a rewinding spring in the interior of a 
cover 32. Wound up on the belt shaft 30 is a flexible belt 33 which is 
brought to a belt lock, for instance via a deflection fitting, and is 
fastened to the belt lock. The belt shaft 30 has a shaft extension 35 
which extends beyond the cover 32 on the side of the rewinding spring the 
circumference of which is smooth or slightly roughened. The free end of 
the tensioning means is fastened and wound to a spool-like pulling means 
roll 36. The tensioning means roll 36 has a hole 37', the inside diameter 
of which is larger than the outside diameter of the shaft extension 35, 
whereby the tensioning means roll 36 can be placed on the cover 32 or on 
shear pins 37 arranged at the cover 32 and can be locked against rotation. 
Pivoted at the tensioning means roll 36 are three clamping levers 38 which 
are pushed by springs 39 against the shear pins 37. In this embodiment 
example a receiving block 42 is rigidly fastened to the U-shaped housing 
40 of the automatic device by means of screws 41. The receiving block 42 
has an opening 43 which can be narrowed by clamping screws 44 and which 
serves for holding a pressure accumulator in the form of a propulsion 
charge cartridge 45 with fuse 46 as well as for holding the associated 
cylinder/piston drive according to FIG. 1. The end of the cartridge 45 on 
the drive side, is closed off by a bursting or rupture plate 48 followed 
by a nozzle plate 49 which is narrowed in wedge-fashion. The cartridge 45 
can be screwed onto the connecting piece 3 of the cylinder/piston 
arrangement according to FIG. 1. In the event of a crash, the fuse 46 is 
activated and, thereby, the propulsion charge 50 is ignited automatically, 
followed by a rupture of the bursting plate 48. The drive piston 13 in 
FIG. 4 which is connected with the tensioning means 19 is driven via the 
cavity 16 upward in the manner explained. The tensioning means 19 exerts a 
torque on the tensioning means roll 36. At the beginning of the rotary 
motion of the tensioning means roll 36, clamping levers 38 are pushed 
against the circumferential surface of the shaft extension 35 and are 
clamped there. On further tension on the tensioning means 36 the shear 
pins 37 are sheared through, so that the tensioning means roll 36 is free 
to rotate, whereby the tensioning means roll 36 is coupled to the belt 
shaft 30 via the clamping levers 38, so that the belt shaft 30 also 
rotates and causes an instantaneous rewinding of the belt 33 about the 
body of the vehicle passenger. 
In the embodiment example according to FIG. 5, the cylinder/piston drive 
consists substantially of an L-shaped dimensionally stable, for instance, 
metallic end part 55 which has on the one hand a connecting piece 56 with 
a screw thread 57 and on the other hand, concentrically with the cylinder 
axis, an internal thread 58. A threaded sleeve 59 is screwed to the 
internal thread 58. Sleeve 59 has a fastening shoulder 60 with a saw-tooth 
shaped external hose nipple profile 61 surrounded by a flexible cylinder 
section 62 of a high-pressure hose. This cylinder section 62 consists of 
elastic, for instance, rubber-like material and has a wire armor 63. One 
end of section 62 is pushed onto the fastening shoulder 60 and is pressed 
on by a screw cap 64. A pressure accumulator in the form of a propulsion 
charged cartridge 65 is fastened to the connecting piece 56 by a screw 
connection. In the interior of the cylinder, a guide or a support part 66 
is arranged for a drive piston 67, the outside diameter of which 
corresponds to the inside diameter of the hose 62. The guide or support 
part 66 and the drive piston 67 have through holes for cable-like pulling 
means 68 which are secured above the drive piston 67 by a pinched or 
squeezed part 69. The operation of the described drive corresponds to that 
of FIGS. 1-4; however, the cylinder section 62 can be bent practically in 
any desired direction without impairment of the driving function to take 
into consideration the frequently unfavorable and cramped installation 
conditions in a motor vehicle. This substantially facilitates, in 
particular, the assembly of the drive in the interior of a hollow column 
of the motor vehicle body.