Theoretical background of method of springs recovering

During their life span, high load compression springs lose their resistance and their operating load is reduced. In the course of time, relaxation is observed. The method of compression springs recovering from hardened wire by low-temperature thermo-mechanical treatment and contact predeformation is presented. The results of experimental tests on recovering load bearing characteristics of the lot of springs according to a new technology are reported. Load bearing characteristics of recovered springs are theoretically specified. The difference between theoretical and experimental researches does not exceed 3.2 %. This method of springs parameterization is recommended to be used in the development of technology of springs recovering with the use of low-temperature thermo-mechanical hardening and contact predeformation.


Introduction
During their life span, high load compression springs used in modern technics requiring high load speeds and compact installation of the springs in the units lose their resistance, and their operating load is reduced. In the course of time, relaxation is observed. Relaxation of the power of valve springs reduces the power and efficiency of the engine. Decreased resistance of operating valve springs causes the increase of valve train wear [1,2]. Relaxation of automobiles' suspension springs is the reason for early fatigue of frame rails and the body of a car. Compression of lock valve springs in automobile cargo lift В-28 leads to an emergency situation with the cost of springs used in road and construction engineering reaching up 5000 rubles and higher and automobiles' suspension springs from 2000 to 3630 rubles per set depending on the model of the car. Thus, springs are expensive and they influence the efficiency and reliability of technics. That is why, the task of improvement and theoretical justification of technologies of springs recovering and hardening is urgent.

Results and Discussion
Automobile industry is one of the main consumers of springs from hard drawn spring wire [3]. To solve the task the method and devices for recovering compression springs from hardened wire (patented or hard drawn) with the use of low-temperature thermo-mechanical hardening and contact predeformation are worked out [4]. The method is carried out as follows. A heated spring is extended with the interval exceeding that of the completed spring, then it is released, hot and extended, and kept 2 in such state until it gets cold. Then peen hardening and spring contact predeformation with axle load within 10…300F 3 (F 3 -spring force before coils contact) are performed.
The operations of heated spring extension, releasing and cooling in an extended state are referred to the methods of thermo-mechanical treatment leading to the formation of specific structure and substructure of martensite and bainite. As a result, steel acquires high mechanical properties [5]. Due to the operation of pressing plastic spring, hardening occurs, and favorable stress state on the surface and inside the coils antagonistic to spring compression is created [6].
Typical representatives of high load and compact springs from hardened wire are inner valve springs of internal combustion engines of VAZ (figure 1) which were chosen for experimental research. Valve springs must have high fatigue resistance in the multicycle area and high relaxation and creep resistance at high temperature. Minor disturbances in the process of hardening or low quality metal for springs influence their performance qualities [7,8].   In spite of the fact that springs after extension on the mandrel with a 10 mm interval were released at 400 ºС, after removing from the mandrel the height of their operating part decreased on average from 45 to 42.53 mm [4]. It is connected with residual tension in springs after short-radius bending, arisen in the process of coiling and with residual torsional tension occurred with the spring being extended. In the releasing process residual tensions do not relax completely but only to 0.3σ y (σ yyield limit). If the releasing temperature is increased, steel strength and cyclic stress of the spring will be lowered [8].
Strength limit of spring wire of the 1 st class with the diameter of 2.7 mm according to GOST 9389-75 "Steel carbon spring wire" is 1900 МPа, yield limit is 1520 МPа. Taking into account the influence of residual tensions after coiling and extension releasing [7,8] tensions under which the residual deformation of the spring σ y , МPа: In further research, this tension should be considered as yield limit.
The influence of residual tensions on spring shortening when it is extended can be calculated: The calculations and experiment are presented for springs manufactured from hard drawn spring wire after coiling, cycling test and extension. For springs from patented wire, the results can be different. Since patented wire has residual microtension after drawing, its total level of plastic deformation is much higher, which influences the result of the researches of this kind.
Valve springs must correspond to geometric and load bearing characteristics. Besides, they must pass cyclic loading resistance test in number of 6×10 6 compression cycles from H 0 to H 2 with the frequency not less than 25 s -1 [4]. To perform compression cyclic test from H 0 to H 2 , recovered springs, according to the new technology, were installed on the resonant stand DV8-S2 by «Gejrg Reicherter». The tests were stopped after 10.5 × 10 6 cycles (table 2). This is 1.75, as much as the established norm for springs (6 × 10 6 cycles). All the springs passed the tests without intolerable compressions and failures [4]. The resource of the recovered springs proved to be not less than the new ones.
Theoretical research of strain-stress state of valve springs when hardened by load of 12400 N (40F 3 ) has been done according to the method expressed in the literature [9]. The parameters of an extended spring before predeformation: mean diameter of the spring D = 20.239 mm; height in a free state H 0 = 45.73 mm (reference size); height of the operating part of the spring H oper = 42.53 mm; full coils number i = 6.5; operating coils number i oper = 4.5; diameter of the cross section of the coil (of wire) of the spring d = 2.7 mm; material of the spring -wire 2.7 -70HGFA-III (70ХГФА-III), Specifications 14-4-1380-86 (Oteva 60); interval of operating spring coils t = 9.45 mm; modulus of elasticity of the first kind Е = 2.10×10 5 МPа; Poisson ratio µ = 0.3; transverse modulus of elasticity G = 8.077×10 4 МPа.
Half of the coil section of the spring with the diameter 2.7 mm is divided into 49 elements 1…49. Hardening stress and dependence stress in every element are defined as follows (1): torque data М (table 3, 4).
where ∆к -torsional spring coil increment, mm -1 ; х, y -coordinates of coil section point in which stresses and torques are defined, mm [9]; S -coil cross-section area, mm 2 ; Z -value, characterizing the depth of plastic hardening according to spring coil section.
where R -spring midradius, mm; α -angle of coils of the operating part of the spring, deg. In the process of recovering elastic core value of springs d el (table 5) does not fall outside the recommended value d el ≥ 0,5d [11]. The spring is in working order.

Conclusion
The new method of high load springs recovering from hardened wire has been presented. The results of experimental tests on recovering of load bearing characteristics F 2 , N, of inner valve springs have been reported. Load bearing characteristics of recovered springs (F 2min = 266.6 N, F 2max = 288.2 N) satisfy the requirements for design documentation (F 2 = 275.4±13.7 N). Recovered springs have passed cyclic tests without intolerable compressions and failures. Load bearing characteristics of recovered springs F 1 and F 2 , N have been theoretically specified. The difference between theoretical