218 



STRENGTH OF MATERIALS 



both of Philadelphia, Pennsylvania. The machines as now con- 

 structed for ordinary testing purposes consist of a platform scales 

 with the usual means of measuring loads, and a screw press operated 

 by an outside source for applying the loads. Fig. 145 is a machine 

 of 100,000 Ib. capacity, built by Olsen & Co., and may be taken 

 as a type. The four upright pieces A With the base B upon which 

 they rest form the platform of the scales. This platform rests upon 

 knife-edges C attached to a system of levers D which terminate 

 finally in a graduated lever E (the scalebeam) provided with a 

 movable poise. Each lever is supported by knife-edges resting upon 

 hardened steel plates. The screw press in this case is seen in the 



FIG. 144 



four screws F with their movable crosshead G. The upper cross- 

 head H is attached to the four upright pieces and i.s a part of the 

 scale platform. 



167. Tension tests. If a piece is to be tested in tension, one end 

 is attached to the upper crosshead and the other end to the Imver. 

 The turning of the screws, due to the driving mechanism on the 

 other side of the machine, causes the lower crosshead to move down- 

 ward, thus bringing pressure to bear on the upper crosshead. Fn>m 

 here it is transmitted to the base and thence to the levers, and is 

 measured by movement of the poise on the graduated scalebeam. 

 Machines of 20,000 Ik, 30,000 Ib., 50,000 Ib., 100,000 Ib., 200,000 Ib., 

 and 300,000 Ib. capacity are manufactured, as well as a great many 

 machines for making special tension tests. In the larger testing 

 machines the upper head is usually adjustable so as to accommodate 

 specimens of various lengths, but in the smaller machines the upper 

 head is fixed. 



