Abstract:
A hydraulic device having a nut part with a threaded hole which has an internal screw thread with a defined nominal diameter and with a defined, uniform pitch, and having a screw part which has an external screw thread with the same nominal diameter as the threaded hole and with a defined, uniform pitch, which is screwed into the threaded hole. (In hydraulic devices, screw parts, in particular nozzle bodies, are generally adhesively bonded in place, in order to prevent them from becoming detached. Adhesive bonding has certain drawbacks, since the screw threads have to be free of grease, and in the case of nozzles there is a risk of blockages, adhesive can enter the hydraulic circuit and also process reliability is not ensured during application.) There is provided a slight difference between the pitch of the internal screw thread of the nut part and the pitch of the external screw thread of the screw part. This slight difference in pitch while the screw part is being screwed in leads to an elastic deformation of a plurality of thread turns, with the result that the screw part is secured so that it cannot become detached.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
   The invention is based on a hydraulic device, e.g. on a hydraulic valve or a hydraulic pump, the hydraulic device having a nut part, e.g. a housing or a control piston, with a threaded hole which has an internal screw thread with a defined nominal diameter and with a defined, uniform pitch, and having a screw part, which has an external screw thread with the same nominal diameter as the threaded hole and with a defined, uniform pitch, which is screwed into the threaded hole. 
   It is known, for example from the journal “Industrieanzeiger”, Edition 83/88, pp. 24 to 25, to secure a screw part in a threaded hole by adhesive bonding. To do this, the threads have to be free of grease, dirt and moisture. In hydraulic devices, in which the screw part is very often a nozzle or a closure screw, which generally has to be exchanged or released and refitted from time to time, the measures required in order to secure the screw part used in an exchange of this type are very laborious, since it is difficult to keep the threads free of grease or to remove grease and hydraulic oil which have penetrated into the gap between the threads while the hydraulic device is operating. A further drawback is that when the screw part is being screwed into the threaded hole adhesive can flow away or be shaved off and passes into the hydraulic circuit in which the hydraulic device is located. This contaminates the hydraulic fluid which is used. Particularly if the screw part is a nozzle, there is a risk of the nozzle bore becoming blocked by applied adhesive. Overall, securing a screw by adhesive bonding is laborious, not a very reliable process and is also not clean. 
   SUMMARY OF THE INVENTION 
   The invention is therefore based on the object of developing a hydraulic device having the features described in the introductory-mentioned paragraph such that the screw part is secured in the threaded hole in a simple manner without the use of adhesive. 
   In a hydraulic device in accordance with the introductory-mentioned paragraph, the desired object is achieved, according to the invention, wherein there is a slight difference between the pitch of the internal screw thread of the nut part and the pitch of the external screw thread of the screw part. The difference is selected taking account of the thread turns which engage in one another after the screw part has been screwed in, in such a way that individual turns are deformed not permanently but rather elastically. These elastically deformed turns cause the external and internal screw threads to be elastically clamped together, so that the screw part is secured by the forces exerted by the elastically deformed turns. 
   Although it is known, for example, from U.S. Pat. No. 4,266,590, 2,870,668 or 1,922,689 to provide the nut part of a threaded connection with a different pitch than the screw part, in a hydraulic device this particular type of screw connection has not hitherto been employed despite the considerable inadequacies of the conventional adhesive-bonding processes. 
   Advantageous configurations of a hydraulic device according to the invention can be found herein. 
   For example, as has already been indicated, securing a screw by using different pitches of the internal screw thread and the external screw thread is highly advantageous in particular wherein the screw-in part is a nozzle body in which there is a risk of a blockage if adhesive is used. 
   In accordance with a feature of the invention the difference between the pitch of the nut part and the pitch of the screw-in part is preferably in the range from 15% to 10% of a mean formed from the two pitches. 
   Nozzles have hitherto been made primarily from brass, a material in which it is relatively easy to bore holes with a very small diameter of between 0.5 and 1.5 mm. However, brass is relatively inelastic. Moreover, especially with the different pitches between the internal screw and the external screw thread according to the invention used to clamp the screw threads together, particles may be shaved off, pass into the hydraulic fluid and contaminate the latter. Therefore, in another particularly advantageous configuration of a hydraulic device according to the invention, the screw-in part is made of a steel, in particular of a free-machining steel. This steel has a high elasticity and can also be machined easily. 
   Hitherto, screw parts which have a screw shank, which bears the external screw thread, and a screw head and have been screwed into the nut part until the head part comes to bear against the nut part, have been used for hydraulic devices. If, in accordance with the invention, the screw threads are provided with different pitches, thread turns are quickly plastically deformed beyond the elasticity limit if the screw part is rotated further beyond the time when contact is made between the screw head and the nut part. This causes the securing of the screw to be lost. Therefore, according to still another feature of the invention it is provided that, to limit the screwing-in movement of the screw-in part, one of the two screw threads has a run-out section in which the depth of the thread groove decreases continuously. As soon as the mating screw thread engages in the run-out section of the screw thread, the torque required to screw in the screw increases greatly without permanent deformation at least of the thread turns located outside the run-out section. In order on the one hand not to give the fitter the feeling that he has to screw in the screw part so far that a screw head bears against the nut part and, on the other hand, in order nevertheless to be able to provide the screw part with a larger screw head, it is provided, according to yet still another feature of the invention the head, starting from the shank, has a diameter which increases over a defined axial length. According to still another feature this increase preferably takes place in the form of a truncated cone. 
   In another particularly preferred configuration, the internal screw thread of the nut part and the external screw thread of the screw-in part, in particular the external screw thread of a nozzle, are metric screw threads with a diameter of 4 mm, the pitch of the first thread, preferably the pitch of the internal screw thread, is 0.7 mm, and the pitch of the second screw thread differs by 0.05 mm from the pitch of the first screw thread, and the engagement length between the two screw threads is approximately 4 mm. According to still a further feature of the invention it is preferable for the screw-in part to be hardened on its surface and, according to another preferable feature the screw-in part can be provided with an oxidation-resistant layer which protects against rust. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     An exemplary embodiment of a hydraulic device according to the invention, which is designed as a pilot-control pressure control valve, is illustrated in the drawings. The invention will now be explained in more detail with reference to the figures in these drawings, in which: 
       FIG. 1  shows an excerpt from the main stage of the pressure control valve according to the invention, in which a nozzle as screw part is screwed into the housing as nut part, 
       FIG. 2  shows an enlarged illustration of the region from  FIG. 1  in which the nozzle is located, 
       FIG. 3  shows an excerpt from  FIG. 2  which has been enlarged still further and clearly illustrates the engagement between the screw threads, and 
       FIG. 4  shows an alternative to the design of the run-out section of the screw thread of the nozzle which can be seen in FIG.  3 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   In the pressure control valve shown, a bore in a cast iron housing  10  securely holds a sleeve  11  in which a main cone  12  is guided and on which a seat  13  for the main cone is formed. The main cone  12  is loaded toward the valve seat  12  by a relatively weak compression spring  14  which is located in a rear control space  15 , which is filled with control oil. The housing (not shown in more detail) of the pilot valve  16 , only the circuit symbol of which is illustrated, is seated on the housing  10 . Any design details of the pressure control valve which are not shown further can be found in the applicant&#39;s data sheet RD 25 802/01.99. 
   The pressure which is to be controlled is present in an entry passage  17  to the housing  10  and at that end side of the main cone  12  which faces this passage. When the cone lifts off the seat  13 , pressurized liquid can flow from the entry passage  17 , through the end-side opening and through radial bores in the sleeve  11 , into an exit passage  18  of the housing  10  and, from there, to a tank. 
   Passages which are part of a control-oil flow path also run in the housing  10 . A control bore  19 , which leads radially from the entry passage  17 , into which a control-oil nozzle  20  is screwed and which merges eccentrically into a larger transverse bore  22 , which is closed off with respect to the outside by a stopper  21 , lies in the inlet to the pilot valve  16 . In turn, a bore  23 , which runs parallel to the axis of the sleeve  11 , leads from the transverse bore  22  to the entry to the pilot valve  16 . The control space  15  is also connected to this entry via a line  24 . A line, in which a bore  25  of the housing  10  running parallel to the sleeve  11  also belongs, leads from the exit of the pilot valve into the exit  18  of the housing  10 . 
   Therefore, when the pilot valve  16  is closed, the same pressure prevails above the control-oil nozzle  20  in the control space  15  behind the main cone  12  as in the entry  17 . The spring  14  therefore holds the main cone  12  closed. If the pressure in the entry  17  rises to the value set at the pilot valve  16 , the latter opens and control oil can flow out of the control space  15  via the pilot valve  16  into the exit  18 . The pressure in the entry  17  still rises slightly by the pressure equivalent of the compression spring  14  and is then held at this value by a corresponding opening cross section between the sleeve  11  and the main cone  12 . A control-oil stream, which is determined by the opening cross section of the nozzle and the pressure equivalent of the pressure spring  14 , which is in the region of a few bar, flows via the control-oil nozzle  20 . 
   The nozzle  20  is screwed into the bore  19 . For this purpose, this bore is provided over a certain distance, from the larger transverse bore  22 , with a metric internal screw thread which has a nominal diameter of 4 mm and a pitch of 0.70 mm. In short, the person skilled in the art would say that the internal screw thread is an M 4 ×0.70 thread. 
   The nozzle or, more specifically, the nozzle body  20  has a screw shank  31  and a screw head  32 , the diameter of which, starting from the screw shank, initially increases linearly, in the manner of a truncated cone, with an included angle of 30°, and then remains constant over a short section. Therefore, at the screw head  32  it is possible to distinguish between a frustoconical section  33  and a cylindrical section  34 . On the screw shank, the nozzle body  20  bears an external screw thread  35  which, like the internal screw thread  30 , is an M 4  thread, the pitch of which, however, is not 0.70 mm, but rather 0.75 mm, i.e. slightly greater than the pitch of the internal screw thread  30 . As is clearly apparent, the screw shank and screw head merge into one another without a recess between them. Accordingly, the screw thread  35  also does not end in a recess. Rather, at a short distance from the head  32  the thread groove becomes gradually less deep and ultimately ends completely in the shank  31 . This is achieved as a result of the fact that, during the cutting of the thread, the cutting tool is drawn back in the radial direction as it continues to rotate and as the axial movement of the nozzle body continues.  FIG. 3  shows how the thread groove  36  has become shallower close to the head  32 ; while the cutting tool is being pulled out, the speed of axial movement of the nozzle body is the same as during the cutting of the screw thread ahead of the run-out section, so that even in the run-out section there is the same distance between the thread turns as in the regular part of the screw thread. The thread end is then not pointed but rather flat in the run-out section, as can be seen at  37  from FIG.  3 . 
   It is also possible, during cutting of the screw thread in the run-out section, to reduce the speed of the axial movement of the nozzle body  20 . In this case, as can be seen from  FIG. 4 , the thread end  37  remains pointed, but the axial distance between two thread turns is reduced in the run-out section. 
   The result of the difference between the pitch of the internal screw thread  30  and of the external screw thread  35  is that when the nozzle body  20 , which can be referred to as the screw part, is being screwed into the bore  19 , which can be referred to as the threaded hole, the front thread turn only bears against a location on a turn of the internal screw thread if a certain pressure is exerted on the nozzle body  20  while it is being screwed in. Without this pressure, the rear thread turn of the nozzle body  20  which is in engagement with the internal screw thread  30  bears against a location on the internal screw thread  30 . When the nozzle body  20  has been screwed far enough into the bore  19 , a further thread turn comes into contact—albeit by means of the opposite flank—with a flank of the internal screw thread  30 . This state is illustrated in FIG.  3 . It can be seen that the thread turn of the nozzle body  20  which has been screwed in furthest is in contact with a corresponding turn of the internal screw thread  30  by means of the inwardly facing flank, and the last turn of the nozzle body  20  which is in engagement bears against a corresponding turn of the internal screw thread  30  by means of the outwardly facing flank. If the nozzle body  20  is then rotated further, the turns which are in contact with one another are elastically deformed, and the nozzle body  20  and the housing  10  are clamped together, during which process further thread turns can come into contact with one another. With the thread dimensions and pitches which have been given, this takes place at a thread length of approximately 4 mm. As soon as the situation shown in  FIG. 3  is in existence, or just before this time or just after this time, the internal screw thread  30  passes into the run-out section of the external screw thread  35 , so that when the nozzle body  20  is rotated further, the torque required rises considerably and the rotation of the nozzle body is deliberately ended before regular thread turns which engage with one another are permanently deformed beyond the elasticity limits of the materials. In the run-out section there is a certain permanent deformation of the thread turns, which additionally contributes to securing the screw part in the threaded hole. 
   The particular shape of the head  32 , the greater diameter of which is required in order to enable a conical widening  41 , which influences the flow characteristics of the control oil, to adjoin the actual nozzle bore  40  and to allow introduction of the slot  42 , does not allow the fitter to feel that he has to screw in the nozzle body  20  all the way to an axial stop and then tighten it with a high torque, which would entail the risk of permanent deformation to the thread turns. 
   The nozzle body  20  is made of a free-machining steel which has good elastic properties yet can nevertheless be machined easily in order in particular to be able to drill the very small nozzle bore  40 . At its surface, the nozzle body  20  is hardened, in particular by carbonitriding, so that there is a low risk of particles being shaved off when the thread turns are being screwed in and possibly passing into the hydraulic circuit. Moreover, the nozzle body  20  is provided with an oxidation-esistant layer on its surface.