B757-200 CREW WAS PUSHED BACK FROM THE GATE; SO HARD THAT THE TOW BAR BROKE.

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FLT EXPERIENCED A BROKEN TOW BAR ON PUSHBACK. BRAKES RELEASED AT XA37Z FROM GATE AT LAX. WE WERE CLRED TO PUSH BACK ONTO TXWY. WE WERE GIVEN CLRNC TO START ENGS DURING PUSHBACK FROM THE TUG OPERATOR AS WE LEFT THE RAMP AREA. THERE WERE A SERIES OF ERRATIC JERKS FELT. I LOOKED UP AND NOTICED WE WERE NOT ALIGNED WITH THE CTRLINE OF TXWY. IT APPEARED TO ME THE TUG OPERATOR WAS HAVING DIFFICULTY ALIGNING THE ACFT ON THE CTR OF THE TXWY. THE NOSE OF THE ACFT WAS POINTING S OF THE CTRLINE OF TXWY. THE TUG WAS ALIGNED ON THE L SIDE OF THE ACFT. IT APPEARED THE TUG DRIVER ATTEMPTED TO CORRECT THE ALIGNMENT. THERE WAS A VERY ROUGH BACK AND FORTH MOVEMENT. THE SEVERITY OF THIS MOVEMENT WAS ALARMING. THERE WAS A CRUNCHING NOISE. I QUERIED THE TUG OPERATOR; 'WHAT HAPPENED? DID SOMETHING STRIKE THE ACFT?' HE RESPONDED WITH 'SET BRAKES; THE TOW BAR BROKE.' I ASKED IF THE ACFT WAS OK. HE SAID IT WAS. I SAID 'ARE YOU SURE?' THERE WAS A PAUSE. THE TUG OPERATOR THEN SAID A MECH WAS ON THE WAY. A MECH CAME UP ON THE INTERCOM AND SAID THAT THERE WAS A PART OF THE TOW BAR STUCK ON THE NOSEWHEEL AND WE WOULD HAVE TO RETURN TO THE GATE. I ASKED MY FO TO CALL RAMP AND TELL THEM WE HAD TO RETURN TO THE GATE. THEY GAVE US A GATE ABOUT THE SAME TIME THE MECH SAID HE WANTED US AT GATE XX OR GATE XY (I THINK; BUT NOT SURE). I ASKED HIM TO STAND BY AND NOT DISCONNECT THE HEADSETS. WE COORDINATED; THEY SAID OK. I CALLED THE MECH ON THE INTERCOM AND TOLD HIM WE WERE READY TO TAXI IN AND ASKED HIM TO GIVE ME A SALUTE AND RELEASE FROM GUIDANCE ON THE L SIDE OF THE ACFT; WHICH HE DID. WE TAXIED IN. MAINT SAID THEY HAD TO REMOVE ONE OF THE NOSE GEAR WHEELS IN ORDER TO REMOVE THE TOW PART THAT WAS STUCK ON THE ACFT. THE REPAIR TOOK BEST PART OF AN HR. ONCE IT WAS FINISHED; WE DEPARTED FOR SFO WITHOUT FURTHER INCIDENT. IN MY OPINION; UNLESS THERE WAS A MECHANICAL PROB WITH THE TUG; THE CAUSE OF THIS INCIDENT CAN BE ATTRIBUTED TO A TUG OPERATOR ERROR AS OPPOSED TO A RANDOM FAILURE OF A TOW BAR.