harmonic balancer?

[bms1]

took my 96 markVIII in for brakes and a serpentine belt.the shop called me and said the harmonic balancer slipped.can anyone tell me what this actually means and why it has to have a new one?

[Towncar]

It creates a harmonic balance

[Towncar]

Actually, I can't see where it can "slip".
Some cars, way back when used a steel hub, then a steel pully ring sandwiched to the hub with a large rubber mass. In that case I can immagine a slip, where the outer pully ring may have slipped on the rubber mass.

However, I don't see that in the pictures of your "Crankshaft Damper" (harmonic balancer), it looks to me like a solid steel pully wheel.

This is weighted to offset the excentricities of the spinning crankshaft.

This Damper is "keyed" to the crank snout, maybe that's what the shop is talking about, the keyway got wasted??

Here's that section of the Service Manual
(need pdf viewer)

[Huer]

I've seen them where the key had been sheered off. I manage inventory for the yard I work for and I got spooked the other day when I was going around the inventoried motors on the racks and found a chevy 4.3 where when I'd try to turn it over by the pulley the flywheel wouldn't turn with it and thought that the crankshaft was broken. It's not a hard fix. Need a balencer for it?