So I replaced my power steering pump, because the old one had play in the pulley.

Put on a new belt, a few days later, it snapped under acceleration on the highway... booo.

So I checked the alignment again, it seems good.
After searching, I will check the surfaces of the pulleys to see if they are damaged.

Other than that, what can cause a new belt to snap so soon?
How tight should it be on?

Thanks again...

The tightness is a little tricky to measure unless you have one of those gauges. I do not have air conditioning and I keep it tight enough so if you grab the middle of the belt with 2 fingers you can just about turn it 180 degrees.

2016 WRX STi Limited (current)
2001 A4 1.8tqms (donated)
1991 GVR4 #1933 (sold)
old.dsmregistry.com/detail.php?carid=218 (dismantled)

Too loose it will slip and squeal. Too tight and you damage the bearings for the pulley.
If you push straight down on it with a decent amount of pressure, you should have about 1/4- 1/2" play.

__________________________________________________________
-Tony <--- NOT asian.
'06 Wicked White EVO IX

new belts will stretch a little so you need to re-adjust the tension after the car has been running for a few minutes

dsm/mitsubishi service manuals https://www.mediafire.com/folder/5k14qlaulb9s1/dsm_mitsubishi

service manuals(my whole collection) https://www.mediafire.com/folder/ac51jwwa3g1n1/automotive_manuals

I wish i had the belt so I could examine it... then I'd know if i broke it, or threw the belt.

What if the belt didn't snap and was thrown off?

If you're throwing belts, either the tensioner is bent/loose, your accessories aren't bolted down, or your crankshaft pulley isn't tightened down all the way with an impact wrench. If you have a UDP or rebuilt engine, the latter is most likely the problem.