Patent Publication Number: US-10774410-B2

Title: Method of manufacturing a spring with improved thermal stabilization

Description:
FOREIGN PRIORITY 
     This application claims priority to European Patent Application No. 17192902.9 filed Sep. 25, 2017, the entire contents of which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     This disclosure relates to the field of thermal stabilization of springs that may be used in high temperature applications. The disclosure relates to methods that, in some instances may modify the characteristics of springs that may be used for high temperature pressure relief valves. The disclosure also relates to the manufacture of pressure relieve valve springs. The disclosure also relates to such springs produced via these methods, as well as other components that may benefit from such spring characteristics modification. 
     BACKGROUND 
     As is known in the art, high bleed temperature—pressure regulating pneumatic valves are commonly used for many A/C or other heavy duty industrial applications. As A/C application example, environmental control systems (ECS) often comprise valves and wing/engine lip anti-ice valves (ATVs) and the pressure regulation function of these valves is usually performed by means of a pressure relief valve (PRV). The purpose of the PRV is to establish the desired pressure set in a reference chamber (this reference pressure will be thus sensed by a sleeve piston or other mobile elements able to limit the pressure downstream of the main pneumatic valve). 
     The simplest concept of PRV is constituted by a plunger that is pushed against its seat by a spring. The spring preload is adjusted to reach the desired pressure set-point and when the pressure inside the reference chamber (which is continuously feed by a control orifice) reaches the PRV set-point (i.e. the force on the plunger seat overcomes the spring preload), the plunger displaces, thereby venting the control orifice flow. In this way, the desired reference pressure is established. 
     It is therefore clear that such PRVs heavily rely on the correct functioning of the spring element. First of all, the spring geometry (mainly in terms of spring faces parallelism) has to be tightly controlled in order to minimize transverse force to the plunger (which in turn causing friction and thus hysteresis on the reference pressure value with respect to upstream bleed pressure variation). Second of all, the spring preload, as well as the spring stiffness should not vary over time in order to guarantee a constant pressure set-point. The control of the combination of these two requirements (i.e. load stability together with tight dimensional control) is particularly challenging considering the high temperature the PRV is exposed to (engine bleed up to 700° C., PRV spring temperature up to 500° C.). Considering these temperatures, PRV springs are currently typically manufactured from Inconel® X750 or other suitable materials. 
     There is therefore a need to find an improved method of manufacture of these springs, and indeed to provide an improved method of thermally stabilising a material that may be used in this way. 
     SUMMARY 
     A method for manufacturing a spring is described herein that comprises forming the spring from a material; heat treating the spring; performing a first machining step to the ends of the spring; subjecting the spring to a first stress relief heat treatment; performing a second machining step to the ends of the spring and subjecting the spring to a second stress relief heat treatment step. 
     In some of the examples described herein, the first and second machining steps may comprise grinding, or machine grinding the ends or end-coils of the spring. 
     In some of the examples described herein, the second machining step may be a finer machining step than the first machining step to produce a less coarse surface of the spring ends. 
     In some of the examples described herein, the material may be a precipitation hardenable Nickel-Chromium alloy with high strength temperatures and high oxidation resistance. 
     In some examples, the material may be Inconel® X750. Other materials may also be used with this method, however. 
     In some of the examples described herein, and particularly wherein the material is Inconel® X750, the step of heat treating the spring may comprise heat treating the spring according to condition C, AMS 5699. 
     In some of the examples described herein, the first stress relief heat treatment may comprise compressing the spring to a length that is reduced compared to the spring&#39;s original uncompressed length, via the application of a load and whilst also applying heat. 
     In some of the examples described herein, the load and heat applied during the stress relief heat treatment step(s) are representative of the most extreme operative conditions of the spring when in use. 
     In some of the examples described herein, the same load and temperature conditions may be used for both the first and second machining steps. 
     Any of the methods described herein may be used to manufacture a spring. The spring may also be used in a pressure relief valve. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments will now be described by way of example only, with reference to the accompanying drawings. 
         FIG. 1  is a flow diagram of a method of manufacturing a spring using thermal stabilisation. 
         FIG. 2  depicts a perspective view of a spring positioned within a pressure relief valve. 
     
    
    
     DETAILED DESCRIPTION 
     Although the examples described herein with reference to the drawings may be used for, and are described relating to, the manufacture of an Inconel® X750 spring for a high temperature pressure relief valve spring, the improved spring manufacturing techniques described herein may also be used with, or for, any other type of suitable material, spring size, and/or use. The examples described herein with reference to the drawings should therefore not be limited to the specific Inconel® X750 spring described below, or its features and/or properties. For example, the material used to form the spring may be another precipitation hardenable Nickel-Chromium alloy with high strength temperatures and high oxidation resistance. Other materials may also be used that are not nickel-chromium alloys. 
     For reference purposes only, the examples described below involved the formation and modification of a one type of Inconel® X750 spring that had a free length of 17.34 mm, a wire diameter of 1.9 mm, an outer diameter of 13.8 mm, a stiffness of 19.29 N/mm, a reference assembly load of  25 N, a reference assembly working length of 16 mm, faces perpendicularity (with respect to spring axis) of 0.15 mm, faces planarity of 0.2 mm and a face roughness 0.8 μm. 
       FIG. 2  depicts a spring  200  that is installed within a pressure relief valve  230 . Although this  FIG. 2  depicts an example of anti-ice valve  240 , i.e. a pressure regulating and shut-off valve spring  200 , the examples of improved springs described herein could of course also be used in other assemblies and are not limited to this specific relief valve or anti-ice valve arrangement. Such anti-ice valves are known in the art. Indeed, the proposed manufacturing methodology can be applied to springs installed for any application where constant load and precise spring geometry are required. 
     A new and improved method  100  for manufacturing a spring  200  (e.g. for use in a high temperature pressure relief valve) will now be described with reference to the figures. This new manufacturing method relieves the stress that may be induced during spring end-coil grinding operations, resulting in the guarantee of tight geometric characteristics during the service life of the spring. 
     The method  100  comprises the steps of first forming  105  the spring  200  from a suitable material. Any conventional methods of forming a spring  200 , as are known in the art, may be used. The next method step comprises heat treating  110  the formed spring  200  according to the requirements of that particular material. The heat treatment is performed as prescribed by the applicable material specification. For example, for Inconel® X750; condition C, this is performed according to AMS 5699, as is known in the art. No load is applied during this step. This heat treatment step should be performed prior to the step of machining  120  the ends, or end-coils  210  of the spring  200 . 
     The next step is therefore the machining  120  of the ends, or end-coils  210   a ,  210   b  of the spring  200 . In some examples, this may comprise the grinding of the end-coils  210   a,b  using a grinding machine. In some examples, the dimensional tolerances of the spring  200  after this stage may optionally then be checked  125  to confirm that they are approximately three times the dimensional tolerances of the finished item. 
     The spring  200  is then subjected to a first stress relief heat treatment  130 . In this step  130  the spring is compressed to a reduced length via the application of a load. This load should be representative of the most severe operative conditions that the spring  200  is likely to encounter when in use within the valve. During this step  130 , the oven temperature should be representative also of the temperature that the spring  200  would be operating under when in use. For example, in one specific example, i.e. in the case of the Inconel® X750 spring described above, the heat treatment may be compressed from a free length of 17.34 mm to a length of approximately 16 mm at a temperature of 530° C. for 24 hours. 
     Following this step, and after the removal of the heat and load, a second machining step  140  is then performed, wherein the end-coils  210   a, b  of the spring are again machined, for example, via grinding. This second machining step  140  is finer than the first machining step  120  so that the coil-ends  210   a, b  are not as coarse. 
     After this second machining step  120 , in some of the examples described herein, the dimensional tolerances of the spring  200  may optionally also be checked  145  to see if they are the same as for the finished spring. Following this, or following the second machining step  140 , a second stress relief heat treatment step  150  is performed. The same load and temperature conditions are used as for the first machining step  120  described above; however, due to the steps performed so far, the spring may compress further under the same load than during step  130  and so the spring  200  may be compressed using the same load so that it now contracts to a length of 15.5 mm when heated to 530° C. for 24 hours. Following on from these steps, the method may then either end  160 , or optionally the spring  200  may be checked to make sure the dimensional tolerances of the spring  200  are correct  160 , before the method  200  then ends  160 . 
     This manufacturing technique provides numerous benefits over known methods. For example, the spring produced via this method meets the PRV performance requirements in that no hysteresis occurs. The spring also has improved reliability, in that it has a constant pressure set-point throughout its entire working life. It also deals with the issues discussed earlier in the background section of the present disclosure. 
     Typically, if the second, fine machining step  140  is not performed, then it may not be possible to guarantee repetitive dimensional control (i.e. it would not be possible to guarantee spring faces parallelism). On the other hand, if the step of performing the second stress relief heat treatment  150  does not occur, the spring load may tend to diminish after in-service high temperature exposure (since the end-coils relieve the stress induced during the last machining operation).