212 



CHAPTER XI 



Fig. 



Fig. 115. This arrangement was followed by American firms until recently. 

 At the present time the hydraulic is almost always placed in the top cap, 

 and is designed with regard to accessibility. Such an arrangement is shown 



in Fig. lib : .4 is the top cap of a mill, in which 

 is formed the aperture B ; the top of this aper- 

 ture is closed by an easily removable " plug 

 cover " ; the form shown employs an interrupted 

 screw thread, and is known as the breech block 

 type. By means of a quarter turn the cover 

 may be lifted from the cap. C is the fluid 

 chamber, filled by a pipe in communication 

 with the accummulator. D is the ram bearing 

 on the upper brass, E, of the top roller. F shows 

 the U-cup leathers forming the hydraulic joint. 

 Various other devices are employed to make a 

 tight joint in the plug cover. Accessibility may 

 also be obtained by inserting a distance piece 

 longer than the ram between it and the top 

 brass, so that if the distance piece be slid out the 

 ram falls, and may also be removed by sliding. 



In modern practice the pressures exerted 

 reach up to 500 tons in a seven-foot mill, and 

 correspondingly less in smaller plants. 



An irregularity in the use of hydraulics is the 

 unequal pressure on either side of the mill caused 

 by the thrust of the pinions when these are in- 

 stalled on one side only. This may be compensated for by making the 

 rams of unequal size, or by employing independent accumulators for 

 either side, less pressure being applied at the pinion end than at the 

 other end. 



Another pressure-regulating device which has been and is very extensively 

 employed is the " Toggle gear " of Hudson (13102 of 1887), shown in Fig. 

 117. The toggles act between the 

 top roll cap and a yoke connecting 

 the upper ends of the king bolts. 

 The toggles are connected by hori- 

 zontal bars, upon which the su- 

 perior or inferior ends of the 

 toggles play. 



This bar also carries the con- 

 volute spring, constrained and 

 controlled by the nuts. When at 

 rest the toggles assume a vertical 

 position, becoming forced out- 

 wards against the pressure of the 

 spring when the top roll lifts. The 

 actual pressure exerted is con- 

 trolled by the compression of the 

 springs, and increases with the 

 lift of the roller. This device 

 eliminates trouble with the hy- 

 draulic leathers. 



O 

 O 



o 



o 



Fig. 116 



