How much weight can the safety bars on a Cybex 5420 handle? The safety bars do not go all the way through the holes, they just catch the inside edge.
http://www.cybexintl.com/Products/strength/5420/intro.aspx
For example, if I am doing rack pulls and drop 300lbs onto the pins, how much stress is this causing.
What about on the economy power racks?
I guess that if rack Pulls damage the equipment, you would not find them in a commercial gym.
If you’re seriously concerned, you could probably call Cybex’s 800-number and ask directly. But really, if it’s in a commercial gym, you’d have to try to break it to cause any serious damage.
As long as the bars aren’t already showing signs of misuse or overstress (bending, dents, etc.), there’s probably not a lot you’ll do that can mess it up badly. So do your shrugs, rack pulls, and fatman pull-ups with an easy mind. 
I know this is pretty old, but I just got one of these racks and decided to test them a little. I put 4 45s on each side of a bar on the safety bars about 2 feet high. I lifted one side of the bar about 7 or 8 inches and dropped it on the catches and the catches did not flex one bit. Mine are even bent a bit! I cant even imagine what bent them. When I saw the catch design, I was a bit sketched but after doing that to them, Im pretty confident theyll hold up. Also, John Meadows you tube videos has one where he is squating 5 plates a side in this rack. If the safetys were weak, i doubt he would be trusting them.