THE SPRING CLUTCH 



739 



Straight pieces of the stainless-steel ribbon were given the same series 

 of heat-treatments as the springs. Young's modulus was determined 

 for each of these samples and a bending test was also applied to de- 

 termine whether the wire had been permanently annealed. No ap- 

 preciable effect was noted. The proportional limit and the ultimate 

 tensile strength of this ribbon at room temperature as measured on a 

 tensile-testing machine were 41,600 and 252,000 psi, respectively. It 

 is thus seen that except with the high-temperature anneals the residual 

 stress alone exceeds the proportional limit and any additional stress 

 will cause a permanent deformation. 



It is also possible to obtain a favorable residual-stress distribution, 

 that is, an initial compression on the inner and a tension on the outer 

 fibers. If the released spring having the stress distribution shown in 

 Fig. 7(c) is expanded until considerable plastic flow takes place on the 

 inner and outer fibers the stress distribution of Fig. 9(a) is obtained, 

 which on release results in the residual-stress distribution as shown in 

 Fig. 9(b). 



(b) SPRING 

 RELEASED 



(C) SPRING ON 

 CLUTCH ARBOR 



(d) CLUTCH 

 UNDER LOAD 



■* h *■ •* h *■ •* h " 



Fig. 9 — -Stress conditions in an expanded clutch spring. 



To verify the validity of these arguments three springs of stainless- 

 steel ribbon with different preliminary treatments and one of heat- 

 treated phosphor bronze were tested for backlash as a function of 

 previous loading. The backlash angle was measured from a point of 

 slipping in the free direction to the point in the holding. direction at 

 which it would sustain a load of 0.05 in. -lb. An initial load of 0.5 in. -lb. 

 was then applied and removed and the backlash measured as before. 

 This was repeated for various loads up to the breaking point of the 

 spring. The results are shown in Fig. 10. In the case of the untreated 

 stainless steel the backlash began to increase immediately. Its higher 

 initial value was probably due to the unavoidable stressing occasioned 



