400 



schlink: vaeiance of measuring instruments 



sis loop of the rhomboidal form of which the two horizontal 

 sides correspond to the geometrical clearance in the bearing. 

 This type of hysteresis loop may be eliminated from consideration 

 since instrument mechanisms almost invariably comprise an 

 element to provide force-closure of the linkwork, so that the 

 journals tend to remain in contact with the same general faces 

 of the bearings. 



In point of fact, the hysteresis loop of an instrument is normally 

 lenticular, this form arising from the progressive relative rolling 

 and slipping of journal within the bearing, permitting the point 

 of contact to occur at successive zones of the interior of the 

 bearing, and permitting at the same time divagation of the 



FJS 



Fig. 2. Hysteresis loop of dial indicator of screw-train type, having high 

 multiplication; an instrument much used for direct reading of small displace- 

 ments and other linear magnitudes. 



center of relative motion from the geometric center of the bear- 

 ing. This action is suggested in figure 3. The effect of this 

 action in producing variance arises in the modification of the 

 instrument magnification or leverage ratio which it permits. 

 In the case of instruments comprising spring force-resisting ele- 

 ments, a similar loop arising from inelastic actions of the spring 

 will be combined in the calibration curve with that due to 

 backlash. 



When the calibration curve fails to form a completely closed 

 loop due to incomplete reversion of the parts of the mechanism 

 the residual deflection may be termed the set. 



Clearance between engaging teeth of gearing introduces back- 

 lash effects of the same general nature as those outlined above. 



