Unable to loosen nut on chain drive

My garage door opener hums briefly and stop. According to the troubleshooting, need to release the tension of the chain.
Upon check, the chain is very tight close to the motor (marked with yellow arrow on the screenshot). I need to loose the outer nut (marked with red arrow on the screenshot) by turning it counterclockwise. However that nut is extreme hard that I am not able to loose at all. I am using 1/2 inch wrench (screenshot attached too). I've tried to spread WD-40 on it couple times but not helping. Looks it is distort a little bit and stuck somehow? Any suggestions on how to unscrew the stuck nut?

thank you!

 

Put a wench on both nuts and loosen them both at the same time.

 

Like Tolyn said..... use two wrenches and turn them in the opposite direction.
However.... the nut that the red arrow is pointing to looks rounded off.
You may need to use Vise Grips on that nut and the 1/2" wrench on the other nut.

Make sure the wrench fits snugly as there is a chance the nut could be metric.

 

Yes the red arrow side has rounded off a little bit due to my multiple tries. The other side can be easily loosen. Could you recommend a good Vise Grips that fit this job?

It is Chamberlain 1/2 HP Chain Drive Garage Door Opener-HD200D, I found the manual http://embed.widencdn.net/pdf/plus/c...9.pdf?u=gsn58x, based on the tools needed, seems the nut is either 7/16 or 9/16? I might be using wrong size wrench which caused the rounded off.

 

Basic 7" Vise Grip would work fine.
If the 1/2" fits loose... it definitely won't be 9/16".

 

There is also a master link in the chain just to the left of the yellow arrow. You could remove that instead.

 

Vice grips will make the nut chewed up and unusable. (It's still the right tool to use in this context, just alerting you that the nut will be mangled.) When you go to reassemble that tensioner for troubleshooting you'll want a proper (unmangled) nut, and correctly sized wrenches so you can make adjustments.

You might also buy an adjustable wrench, which will let you size the wrench to the nut. Home Depot or Lowes will sell you one nut at a time: bring the mangled nut so you can get the right size.

 

I was able to find two wrenches one 13mm and one 1/2in and loosed both nuts at the same time. Thanks all for the advice!

I didn't change the outer nut this time, I was afraid that I may not able to take it out and replace a new one with myself alone on the ladder. But will try to get new but and replace it.

Remove the master link also seems difficult to me, because that part of the chain is tight, and I am afraid of not able to connect it back.

If the nut is too light and the above way doesn't work, will impact driver useful? Seems the space is too tight there, not sure if there is such flexible extension socket that will fit in this case?

 

Proper sized wrenches are the best thing to use.
Impact driver would not be a good choice.
You should take the fitting apart and get replacement nuts.

Use a thin piece of wire or strong string to hold the chain in place when taking the coupling apart.

 

Thanks for the tip, I'll get the nut replaced.