234 



HANDBOOK OF MECHANICAL DESIGN 



AUTOMATIC TIMERS 



^ 



- Make and break 

 confacfs 



■D 



i^ 



'Trip 



I 



Adjusfable 

 venf ^ 



^^ Spring 

 or 

 solenoid 



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2) Fixed 

 end 



IK 



Fig. 477.- 



■ Adjustable 

 veni 



,'Fluid 



yTo con lad 

 I h rough 

 spring or 

 solenoid 



-Dashpot principle. Simplest form consists of a 

 piston or plunger operating in oil, mercury, or air. Adjustable 

 small orifices or bleeders provide time adjustment. A by-pass 

 may be provided near the end of the piston travel for snap 

 action closing of the contact. Widely used because of its 

 simplicity and low cost. When air is used, changing clearances 

 caused by dust, gumming of lubricant, and leakage affect the 

 timing. If oil is used, the temperature will change oil viscosity 

 and affect the timing. Also subject to error because of clear- 

 ance changes from wear. 



^^1 



^Magnef 



f 



Weighl- 

 Pivoi 



Weighl 



Clearance 

 gap 



>i 



V^') 



~ Make and 

 break 

 conlacis 



Fig. 478. — Inertia mechanism. 

 Time delay is by virtue of the inertia 

 of two weights mounted on a pivoted 

 arm and the length of arc to be trav- 

 ersed before mechanical contact is 

 made. Tilted by gravity, this device 

 gives a relatively short interval and 

 becomes clumsy for long time intervals . 



^Moin air line fr 



^Valve 



Fig. 479. — Contactor works on back pressure from the main cylinder on the welder, pressure being assured 

 between the welding points before the welding contactor closes. When the back pressure has built up to a predeter- 

 mined value, the plunger moves upward at a definite rate of speed and the hardened cam closes the mam contacts. 

 After a predetermined time, the cam moves by the roller that it engages and the main contacts open. One adjustment 

 sets the back pressure at which the contactor plunger starts to move and therefore determines the lag in applymg the 

 current after pressure has been applied. A second adjustment changes the needle valve opening to the contactor air 

 cylinder and thus times the upstroke. This determines the welding time. A third adjustment varies the time of the 

 downstroke and is of importance only when used with a repeater. 



