If it is fan rated it is fine.
And I highly doubt that thing actually weighs 40#.

How is the box attached to the ceiling joists? As stated earlier if the box is fan rated it should be fine but the manner of mounting is also critical. How is it attached to the joist?

Thanks for your input! I know, the fan hasn't arrived yet, but I doubt that it's 40lbs too...although the CS person I spoke with said the shipping box is 60lbs!?!?!

The box is attached to the ceiling joists with 2 extra screws on top of the 2 nails that came with it. It's a Carlon SuperBlue 1-Gang Ceiling-Fan Box with Nails from Home Depot.

Thanks for your confidence! I'm really hoping it works. I love the fan and hope that it holds!

oops! my mom was logged in last night on my computer and didn't log out! (we are both relatively new to GW and obsessed!) I was showing her how to use it and didn't log her out (shows how much I still need to learn!). My post above as ellen is supposed to be from me!

Thank you all for your advice! I feel confident now that we'll be just fine!

"Carlon SuperBlue 1-Gang Ceiling-Fan Box with Nails"

"Listed for fixture support up to 50 lb. and fan support up to 35 lb."

http://www.homedepot.com/buy/electrical/electrical-boxes-conduit-fittings/carlon/ceiling-fan-box-with-nails-65278.html

Only listed for 35 pounds in fan applications.

Fans create vibration, even if they are perfectly balanced.

I much prefer the RACO 936 old work brace.

It is often available with a metal box and self locking nuts to hold the box to the brace.

It expands to grab the joists on both sides of the opening and dig into them.

A U-bolt and self locking nuts are then used to fasten the box to the brace.

Here is a link that might be useful: RACO old work fan box with brace

The problem is not the box but how it is mounted.

Looking at HD, that box is only rated for a 35lb fan.

You need something stronger. There are a couple of ways to do this, depending on whether your ceiling is open to the joists, or accessible from above, or not.

If open, you can use a 2x4 nailed in between the joists. The box then will be attached to the underside, so leave enough clearance for the box and sheetrock.

If closed, then there is a device out there that I've used several times. It is a steel rod-like thing that expands. The ends have spikes that, when expanded, drive into the joists to hold it. It screws together and fits up into the ceiling through the hole for the box. Once up there, you 'unscrew' it which lengthens it until it firmly grips the joists. You then attach a metal box to it with the hardware provided (U-clamp and aircraft nuts).

Here is a link that might be useful: HD westinghouse fan box

This is similar to what I was talking about.

Here is a link that might be useful: westinghouse

Do I need to find a new fan or is there a way we can reinforce this to hold the weight?

There's a way to reinforce anything, the question is just how far you're willing to go to do so...
Have a laugh

Amazing what a couple of lag screws and washers will do.

I find those box mount brackets a pain. I had issues with a ceiling fan til I got a standard metal - don't use plastic, they get brittle with age - screwed through side into truss (this helps you get it into the right position for the drywall) then cut a 2x4" piece to cross brace between the trusses, screwed it into place, then screwed the box up onto that, so it's held top and side. Fan is rock steady now. I really like that design, it's cool.

I wouldn't mount a ceiling fan any other way now.

Electricians I've spoken to agree, in their own place they'd use plastic boxes for switches and wall outlets - less risk of the screws shorting on the sides - but metal light boxes only. My house was built in the '80's and all the plastic light fixture boxes are breaking away from their nail mounts.

"I wouldn't mount a ceiling fan any other way now. "

The old work fan brace and boxes (all metal and can be installed through a box size hole) are about as easy and solid as it gets.

The box can be positioned anywhere on the length of the brace that runs perpendicular to the joists, and uses self locking nuts to attach the box to the brace.
It has stand offs on each end of the brace that position it above the drywall so the box edge is flush with the face.

Fastest install for fan and chandelier boxes I have used.