DRIVES AND CONTROLS 



229 



Reversing 

 coni roller 



Mofor, 



^Poihof lap 



Fig. 459. — Motor-operated press with safety 

 control requiring operator to use botli hands 

 to start press. In starting, if either "close" 

 button is released, the motor stops. To guard 

 against blocking in one close button, the con- 

 trol is wired so that both close buttons must be 

 fully released or press will not operate. Limit 

 switches are used. 



Limif swiich ' 

 operated by 

 bridge 



^^■' Limii switch operated 

 by trolley 



Fig. 460. — In a machine for polishing telescope mirrors, an 

 elliptical motion of the polishing lap is sometimes required. 

 Controls are arranged to reverse bridge motor at center of 

 trolley motion and to reverse trolley motor at center of bridge 

 motion. 



'Sfyeet ^^^^^> 



J Tab/e ^^^^ 



^X) 



■limif switcti 



Fig. 461. — On a sheet catcher, the table must 

 reverse and return the sheet as soon as it passes 

 through the rolls. Since the length of the sheet 

 varies, the sheet itself is used to operate the limit 

 switch which reverses the table. This application 

 requires specially designed motors and exceptional 

 ruggedness in the control equipment. 



