1st to 3rd mechanic
 

R1, no outs. B1 lays down sac bunt. R1 running on the pitch. BU takes play at first, R1 never stops running and heads to 3rd. Is PU supposed to take R1 into 3rd - just as he would on a base hit to the outfield? If not, why not?
thanks.

I always come about half way down the line when I am the PU to watch for INT at 2nd base on a DP anyway, so if R1 continues I tell my partner "I've got third if he goes" in this situation. I watch the play at first for swipe tag/pulled foot, and then move into the cut if the throw and R1 are both going to 3rd.

There are mechanics that state that the BU has all plays on the bases when the ball stays on the infield, but as long as the PU communicates with his partner, the play can be covered better by the PU.

The BU has responsibility for all bases with runners on with a ball hit in the infield. PU is 1st base extended watching swipe tag at first and anticipating possible play at plate.

In our Association, PU always has runner at 3rd, with R1 only. We use the mechanics described by SanDiegoSteve.

Also, with R1 and R3, PU has R3 into Home, then covers R1 at 3rd, if possible. Lots of communication required between the umpires.

This is the way we do it here also. I hate it when R3 slows down to nearly a dead stop knowing he has home in the bag, and I'm moving up the line alternating looking at where the ball is and waiting for the dufus runner to touch the plate. Makes me want to kick him in the seat of his britches!:D

Some mechanics give this play to PU, some give it to BU. Just as long as the two of them know who is going to have the play and follow through on that, either is fine.

The BU has the throw at third unless the PU calls them off. This is something that should be discussed before the game and you can gauge what your partner is willing or not willing to do.

Peace