1920.] 
Jenkinson.—Design of Laminated Spkings. 
25 
required to make the spring is as small as can be used, again the m top 
plates being the only disturbing factor in the approach to perfection. 
It may be pointed out as a curiosity that a spring of maximum strength 
and flexibility could also be arrived at by making each leaf full length and 
tapering each one for the whole length of the cantilever ; but, although 
the finished weight would be the same, this spring would cut into much more 
steel and would offer no advantages. 
In conclusion, one point should be cleared up. It has been objected 
that this refinement of design is of little moment, since the flexibility is 
increased only slightly and the strength not at all over the ordinary spring, 
in which more steel is used. In one sense this is quite true. The flexibility 
of the perfect spring is only 50 per cent, greater than the reductio ad 
absurdum one in which every plate is made full length. The strength also 
is not increased, if by “ strength ” we mean the ability of the plates to 
withstand the application of stresses above the elastic limit. The reason is 
that although the lower plates may be, and usually are, severely over¬ 
stressed, on the first application of the load they immediately relieve them¬ 
selves by taking a permanent set ; and it is impossible to load any single 
plate beyond the elastic limit unless all the plates are so loaded. But in 
practice the constant application of fluctuating loads considerably below the 
elastic-limit load will fracture a plate ; and those plates which are more heavily 
stressed do undoubtedly break before the others, even although it is granted 
that the static elastic limit is not exceeded. Further, this very act of relief 
means that the whole spring takes a permanent “ set,” and these successive 
“ sets ” are a source of expense and trouble in practice, and are the most 
vexatious feature of the ordinary spring in railway work, since they call 
for constant adjustment of the suspension gear to keep the vehicle level 
and at the correct height. Finally, the saving in spring steel effected by 
careful design is an item of considerable importance on any large output, 
and amply repays the time spent on this investigation. 
