Abstract:
Device for absorbing kinetic energy of a moving body includes a plastically deformable helical spring having a stack of convolutions, the inner surfaces of which delimit a passage. At least a part of the convolutions can be pulled successively through the passage. Due to the resulting deformation of the convolutions, energy can be absorbed.

Description:
The invention relates to a device for absorbing kinetic energy of a moving body having a tension member that comprises a plastically deformable helical spring having a stack of convolutions, the inner surface of which delimits a passage. 
     BACKGROUND OF THE INVENTION 
     Devices in which energy is absorbed by stretching a tension member against its material and form resistance are used in many areas of application, such as e.g. fall brakes in mountaineering or assembly work in the construction trade, as so-called “shock absorbers” for safety belts in automotive engineering, or as brakes in dynamically stressed supporting structures such as safety nets, rope barriers and the like. 
     Plastically deformable tension members are preferred here because with predominantly elastic deformability, as provided e.g. by non-overstretched springs made of spring steel or rubber straps, the kinetic energy absorbed was only temporarily stored and then the majority of it was returned to the braked body, which would set the latter in motion again. 
     With energy absorption a constant development of the braking force over the whole braking path is desirable because in this way the moving body is braked with constant negative acceleration and so is subjected to forces which remain uniform. 
     However, when subjected to tension, tension members in the form of elastically or plastically deformable helical springs do not have constant spring forces over the range of the spring. With a helical spring the spring force rises as the range of the spring increases because all of the convolutions are stressed simultaneously by the latter stretching with simultaneous reduction of the convolution diameter and increase in the pitch, and so constantly increase their resistance to further stretching. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is the object of the present invention to specify a device having a tension member which can be stretched over the bigger part of the whole braking path with the most constant possible braking force. 
     A device that achieves these objects includes a stack of convolutions having a first part including at least one convolution the inner surface of which defines a passage, and a second part including at least one convolution that each have a diameter smaller than a diameter of the convolution(s) in the first part and a pitch larger than a pitch of the convolution(s) in the first part arising from deformation of the convolution(s) in the second part relative to the convolution(s) in the first part, with the convolution(s) in the second part of the stack of convolutions being at least partially situated in the passage defined by the first part of the stack of convolutions. The dependent claims specify preferred embodiments of the device according to the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is further explained by means of exemplary embodiments with reference to figures. These show as follows: 
         FIG. 1  is a longitudinal section through a device according to the invention in the stressed state; 
         FIG. 2  is the section I-I through the device from  FIG. 1 ; 
         FIG. 3  is a braking force/braking path diagram; and 
         FIG. 4  is a longitudinal section of a further variant of a device according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a device  1  according to the invention after partial subjection to an external force  15  which acts upon a tension member  2 , and so brings about a braking force  16  in the opposite direction to the force  15 . The tension member  2  is in the form of a helical spring with a number of 360° convolutions  9   a ,  9   b ,  9   c .  9   a  designates the part of the convolutions which are stacked on top of one another and have not yet been subjected to the force  15 .  9   b  designates the part of the stressed convolutions which extends within the non-stressed convolutions  9   a , whereas  9   c  is the part of the stressed convolutions which projects out of the lower end of the non-stressed convolutions  9 a according to  FIG. 1 . In the following, this lower end of the stack of convolutions  9   a  is also called the support end of the stack, whereas the free end designates the upper end of the stack of convolutions  9   c  according to  FIG. 1 . 
     The helical spring is formed from a wire with the wire diameter  3  and is plastically deformable. In  FIG. 1  the deformed tension member  2  is shown after stretching by the braking distance  14 . 
     The stack of 360° convolutions  9   a  is closed by a protective hood  8 . The support end of this stack is surrounded by a centring ring  5  and rests against a supporting plate  6  connected securely to the latter. Said supporting plate is provided with a through-hole  7  the diameter of which corresponds at least to the internal diameter of the convolutions  9   a . 
     The part of the convolutions  9   b  which is progressively plastically deformed adjoins the stacked part of the convolutions  9   a  of the tension member  2 , and this is followed by the part of the convolutions  9   c  which are not deformed any further. 
     The non-stressed convolutions  9   a  are stacked with the convolution diameter  10   a  and the pitch  11   a . The stack of convolutions  9   a  with the end of the tension member  2   b  is supported on the supporting plate  6  and is positioned over the through-hole  7  by means of the centring ring  5 . 
     The centring ring  5  and the supporting plate  6  form stop means which guarantee that the stack of convolutions  9   a  retains its position when the convolutions  9   b  are deformed. In particular, by providing the centring ring  5  a stop surface is created which prevents displacement of the stack laterally to its direction of extension, and so that passage is always aligned with the through-hole  7 . 
     An external force  15  acting at the start of the tension member  2   a  and which originates from the tension which is caused by the movement of the body to be braked brings about a braking force  16  in the opposite direction to the force  15 . By means of the convolutions  9   a  being pulled successively through the passage, delimited by the inner surface of the stack of convolutions  9   a  and the through-hole  7  of the support plate  6 , the convolutions  9   a  are deformed via the intermediate form  9   a  into convolutions  9   c  with the smaller convolution diameter  10   c  and the greater pitch  11   c . The intermediate form  9   a  has a convolution diameter  10   b  and a pitch  11   b . The deformation of the tension member  2  takes place such that the respective convolution diameter  10   a,    10   b ,  10   c  decreases while the respective pitch  11   a ,  11   b ,  11   c  increases. 
     When deformed the convolution  9   b  slides over the convolution  9   a  lying beneath it, the contact point  12  located between them progressively changing so that the new surface of the convolution  9   b  always slides over the new surface of the convolution  9   a  lying below it. When the convolutions  9   a  are pulled through the passage, delimited by the inner surface of the stack of convolutions  9   a  and the through-hole  7  of the supporting plate  6 , the convolutions  9   b  slide along the surfaces of the convolutions  9   a , the contact points  13  located between the latter progressively changing so that the new surface of the convolution  9   a  always slides over the surface of the stack of convolutions  9   a . 
     The portion of sliding friction on the braking force (designated as  16  in  FIG. 3 ) resulting from the portions at the contact points  12  and  13  adopts an approximately constant value in addition to the main portion of plastic deformation of the convolutions  9   b  over the whole braking path (designated as  14  in  FIG. 3 ) so that the braking force  16  as a whole remains uniform over the braking path  14 . 
     The centring ring  5  has an axial length which is smaller than the length of the stack of convolutions  9   a  and preferably only surrounds a few of the convolutions  9   a  on the support end of the stack. Therefore, the free end of the latter is not surrounded by the centring ring  5 , by means of which additional friction between the tension member  2  and the centring ring  5  is avoided when the respective outermost convolution  9   a  at the free end of the stack starts to deform. 
     In order to reduce the sliding friction at the contact points  12  and  13 , the gliding properties and the wear resistance of the surface of the wire or wires can be improved e.g. by salt bath nitrocarburizing (e.g. according to the Tenifer QPQ method, QPQ standing for Quench/Polish/Quench). Other measures for surface treatment are also conceivable in order to reduce the sliding friction. Under certain circumstances e.g. polishing the surface of the wire is sufficient. 
     In order to facilitate connection of the tension member  2   a  to the body to be braked provision can already be made when producing the device  1  such that the start  2   a  of the tension member  2  is pulled through the stack of windings  9   a  or at least projects into the latter. Furthermore, the start  2   a  can be provided with an appropriate attachment device which serves, e.g. to attach a rope. 
       FIG. 2  shows the section I-I through the device of  FIG. 1  with the protective hood  8  which surrounds the stacked convolutions  9   a  with the convolution diameter  10   a  and which are successively pulled as a plastically deformed convolution  9   b  with the convolution diameter  10   b  through the passage delimited by the inner surface of the convolutions  9   a  and the through-hole  7 . 
       FIG. 3  shows the development of a braking force/braking path diagram when the device  1  according to  FIG. 1  is stressed. As can be seen, over a very short braking path  14  the braking force  16  reaches its final value which remains practically uniform over the rest of the braking path  14 . The area  17  below the curve corresponds to the absorbed energy  17  resulting from the braking force  16  and the braking path  14  when the device  1  according to  FIG. 1  is stressed. 
       FIG. 4  shows a further variant of the device  1 ′ according to the invention. Stop means are provided here in the form of a casing  5 ′ which is designed to taper by forming a step. The stacked convolutions  9   a  rest against the step of the casing  5 ′, whereas the deforming convolution  9   b  projects through the smaller hole in the casing  5 ′ the diameter  7  of which is chosen to correspond to the variant according to  FIG. 1 . The tapered end of the casing  5 ′ engages in a hole formed in a carrier  6 ′. 
     The casing  5 ′ serves on the one hand as a support for the tension member  2 , and on the other hand as a side stop which prevents the tension member  2  from swerving to the side when subjected to stress. The casing  5 ′ can be produced from one piece. 
     The carrier  6 ′ can be e.g. an already existing prop or the like in which a hole is drilled for fitting the device  1 ′, and then the tapered end of the casing  5 ′ is inserted. 
     The device according to the invention can be used in many different ways, e.g. as a fall brake, as used in mountaineering or assembly work in the construction trade, as a “shock absorber” for safety belts in automotive engineering and/or as a brake in dynamically stressed supporting structures such as safety nets, rope barriers and the like. 
     The device  1 ,  1 ′ is designed to correspond to the application. The range of braking forces e.g. for fall protection for people comes within the range of two to three kN with braking path lengths of from a few decimeters to one to two meters. With dynamically stressed supporting structures a braking force of up to 200 kN or greater may be required with braking path lengths of a number of meters. 
     Tension members are preferably made of round wires with a diameter in the range of a few millimeters to a few centimeters and which have tensile strengths of 500 to 2000 N/mm 2 , wires which also have a high degree of ductility being particularly preferred. Among others, steel wire is suitable as a tension member. 
     The above description makes numerous modifications accessible to the person skilled in the art without straying from the scope of protection of the invention defined by the claims. 
     Instead of a solid profile, a hollow profile such as e.g. a thick-walled steel pipe can also be used as a tension member  2 . 
     It is also conceivable to use as a tension member  2  a twisted steel pipe through the interior of which a rope is guided which acts as the primary tension member and thereby transfers the external forces  15  onto the convolutions ( 9   a,    9   b,    9   c ) and so mobilises the braking force  16 . The external force  15  is thereby transferred via the rope after the braking. 
     Furthermore, the stack of non-stressed convolutions  9   a  does not necessarily need to have, as shown in the figures, a circular cylindrical external form. Other forms are also conceivable, e.g. those in which the stack becomes constantly wider or narrower towards one and/or the other end.