Crank shaft balancing

[Stevesr]

Why can't I put my crank shaft/flywheel/crank pully combo on a dynamic balancer and balance as close as possible then go back and weigh the pistons/rods/rings and make them all the same?
A guy here at work said you have to use something called "bob" weights which are bolted onto the crank journals and equal the weight of the piston assembly. Then spin on the balancer. Who is right?

SteveSr

[RacerX11]

RE: Crank shaft balancing

He is. To accurately dynamicaly balance the engine, you need to equalize the weights of the components (pistons, rods, rings, bearings, pins, etc.). Then use the weight of these to calculate the bob weight to use on the crank throws while balancing it. The counterweights of the crank are used to offset the motion of these components, so getting the weights right is critical.

Marty

[Stevesr]

It would be interesting to see what these bob weights look like.......the relationship of the weight to the centerline of the crank changes as the whole assembly rotates...........

SteveSr

[RacerX11]

Just do a search on Google for engine balancing....there are tons of sites explaining the process, and showing pictures of the aparatus used. The bob weights clamp onto the journals, so they don't move at all in relation to the crank centerline as the crank spins.

Marty