RUDDER TRIALS, U. S. S. STERETT. 



321 



APPENDIX III. 



caubration of dynamometer springs. 

 Table of Springs Tested . 



The springs used in the dynamometer were steel safety-valve springs 

 purchased from the Crosby Steam Gage and Valve Company of Boston. 

 The considerations which governed the choice of springs were : — 



1. The maximum capacity, roughly estimated in advance by applying 

 Joessel's formula. 



2. The maximum ordinate for the force diagrams, for which if inches 

 to 2 inches was fixed upon as a reasonable height; this corresponds to a 

 compression of the spring equal to one-fourth the ordinate (about ^ inch), 

 owing to the fourfold purchase which was used to transmit the motion of 

 the dynamometer to the stress-recording pen. 



3. A permissible compression of one inch in the spring before closing. 

 To prevent the possibility of exceeding the capacity of the springs, 



a limit stop was secured to the dynamometer table, which restricted the 

 compression of the springs to f inch. 



The dynamometer and springs were calibrated on the 150,000-lb. rope- 

 testing machine in the Mechanical Engineering Laboratory of the Massa- 

 chusetts Institute of Technology. A wooden platform, covered with a 

 f-inch greased steel plate, was built up on the ways of the testing machine 

 between the straining heads. The dynamometer and supporting table rested 

 on this platform and at such a height that the pull exerted by the straining 

 heads should act along the axis of the dynamometer chain grips. A length 



*This corresponds to an ordinate of 2 inches on force'diagram. 



