Which loctite for magnet shift?

turkeyaddict

Hi All,

Happy to have found this forum!

I have a pair of 7bs that I use in my wood shop. I recognize that some of you just gasped, but I spend a lot of time in that shop. The drivers are MW6500 with the Peerless tweeters, so I'd really like to fix these.

Probably due to the temperature flux we see in Michigan, one of the speakers just locked up and developed the dreaded shifted magnet. Lucky me, when I took the other speaker apart to look at that driver number, that magnet also shifted. There was no scratchiness when it locked, and both voice cones seem fine. The SN are in the 42- range and the DOM is printed as '81.

The DIY instructions on this forum and AK are relatively straightforward, and I say 'thanks' to all that have contributed over the past 10+ years. I've already got the PVC coupler and making a jig is kinda second nature for a woodworker, but I'm a bit confused about 'which' of the red loctites is recommended. I searched the forum (and AK) and can't find this info listed anywhere.

Are you using a threadlocker (like 241) or an adhesive (like 609)? Or an epoxy like this one sold by Simply Speakers: https://www.simplyspeakers.com/speaker-repair-adhesive-mi-1500.html ?

I'm assuming everyone is using 609, which is anaerobic and low viscosity, but want to make sure before I monkey with these further.

Thanks in advance!

Chris

boston1450

I have used LocTite PowerGrab with great results. The one you listed MIGHT work - others may chime in. I put on gloves & used my finger to make it look smooth & they came out great - with no issues for years now... I have never used the jig - but many here have. Wish you the best

pitdogg2

On the exterior the powergrab works. I believe in this case he will need to remove the magnets to clean and he will need anaerobic glue to re-glue the magnets.
I'm not sure which red loctite you will need. Maybe @lawdogg or others can chime in who've done this.

turkeyaddict

I read that many have used the PowerGrab around the outside of the 3 pieces after setting the pole piece correctly - are you implying that you used PowerGrab to attach the magnet to the basket and the magnet to the cover/pole piece? Compared with the amount of adhesive used by Polk, PowerGrab seems like a very thick adhesive to adhere the cover piece to the magnet...

pitdogg2

turkeyaddict wrote: »

I read that many have used the PowerGrab around the outside of the 3 pieces after setting the pole piece.

Correct

turkeyaddict wrote: »

You used powergrab to attach the magnet to the basket and the magnet to the cover/pole piece?

No he @boston1450 did not.

turkeyaddict wrote: »

Compared with the amount of adhesive used by Polk, PowerGrad seems like a very thick adhesive...

Yes you need a very thin adhesive that will still dry in the absence of oxygen. You also need to use very little just enough as too much will get into the voice coil and then you have bigger problems.

Gardenstater

Don't use power grab in an anaerobic bonding condition like between the magnet and the pole pieces.
It is also way too high in viscosity. Either Loctite 271 (Red) or 609 (Green) should be perfect, with 10min work times and 24 hour cure time for the Red and 6 hours for the Green. Epoxy should be good as well, since it cures chemically. I would go with either of the Loctites since they are close fitting smooth parts. Then do a fillet of epoxy when everything tests out good after a week or so of playing them. I would use something like JB-Weld or West Systems G-Flex.

It is hard to say which is stronger Loctite 609 or 271 because they market the 271 as a thread locker and the 609 for bonding cylindrical surfaces like electric motor rotors to shafts. They don't give the strength data in the same terms to allow comparisons. I'd guess that the 609 may be stronger.

turkeyaddict

Thanks for the quick replies! 609 advertises a sheer strength of 3000psi after fully cured, I suspect they would blow well before reaching vibrations high enough to reach that threshold. New bottle of Loctite 609 ordered off Amazon Prime...

xschop

Ive been using Blue 242 when magnet separation is a must for resetting. On the perimeters, when magnet shift has not occurred, I've recently been using Super Glue Gel with excellent results.

Gardenstater

turkeyaddict wrote: »

Thanks for the quick replies! 609 advertises a sheer strength of 3000psi after fully cured, I suspect they would blow well before reaching vibrations high enough to reach that threshold. New bottle of Loctite 609 ordered off Amazon Prime...

**NERD ALERT** **NERD ALERT** **NERD ALERT**

They give, for the 609 (Green) a sheer strength of 2300 psi for steel

For the 271 (Red) with steel 3/8 - 16 bolt and nut they give breakaway torque of 150 to 300 lb in.
If you do the math the surface area of the threads is 0.48 sq in. and pitch dia. is 0.334 in.
That gives a sheer strength of 1900 psi min and 3700 max. avg of 2800 psi.

For the 242 (Blue) they give breakaway torque of 70 to 150 lb in.
That gives a sheer strength of 900 psi min and 1900 max. avg of 1400 psi.

boston1450

I'm a nerd. I'm offended 😂

turkeyaddict

I’m a physiologist with physics training - so I appreciate the geek out - thanks!

I may be wrong, but I don’t think you can directly compare rotational and linear forces.

I’m not doubting the 271 is strong (I have a tube in my shop), and I’m sure it would probably work. However, locking the threads on bolts up to 1 inch in diameter on a rotational axis is (to me) hard to compare with the forces needed to hold/separate two 2 large smooth surfaces joined together on the same/parallel axis.

Gardenstater

**SUPER NERD ALERT** **SUPER NERD ALERT** **SUPER NERD ALERT**


Strictly speaking, if the manufacturer were to compare the strength of the 3, they would have to use the same testing methods. That would've been nice if they had done that, especially since they market the Green for use in closely fitting (even press fit) cylindrical surfaces, and even rotational situations like an electric motor rotor on a shaft.

What I did is the only way to theoretically compare with the data we have.

If someone wants to do a test with a precision torque wrench and 3/8-16 nut and bolts of the same class of thread fit that would be one way to go. They specify max. gap filling of 0.006" for the Green however and in some loose fitting bolts and nuts you could have more than that so you'd have to keep the class of thread fit close.

At least you know that the Green is definitely going to be pretty strong. Probably the Green and Red are comparable but the Green is lower viscosity than the other 2 (110 to 140cP vs 400 to 600cP for the Red and 900 to 1400cP for the Blue). That may be good because it will make it harder for you to put too much on and have it go into the voice coil, as long as you don't apply it too close to the VC. Also, the final bond thickness may be a tad less and maybe better Magnetic Flux Density (?) resulting in the driver.

pitdogg2

The green is lower viscosity because it is used more in a "wicking" environment. From my understanding it will not dry out in the open, Someone here (IIRC it was the guy from Mexico) who used it to glue his magnets that had NOT shifted. He went around the perimeter with the green and allowed it to wick into the two pieces and then sealed it up after 24-48 hrs with either powergrab or JB weld epoxy mix. My machinist uses the green on parts he doesn't want to take apart and uses the Red on those parts he can take apart.

Gardenstater

I can't help but wonder (no offense ) if one could use the 609 Green on magnets that have shifted, but without separating the parts; just by utilizing the wicking action you describe. If you went over it later with a fillet of JBWeld or G-flex it just might make repairs easier, along with the jig. Maybe best to remove the dust cap if doing this method, to see what direction it is off initially.

Then you can do the @xschop phase plug mod

xschop

Red 271 for the phase-plugs is nerdy enough. Has to be an anaerobic compound of some kind.

Astrl55

Gardenstater wrote: »

**SUPER NERD ALERT** **SUPER NERD ALERT** **SUPER NERD ALERT**

Probably the Green and Red are comparable but the Green is lower viscosity than the other 2 (110 to 140cP vs 400 to 600cP for the Red and 900 to 1400cP for the Blue). That may be good because it will make it harder for you to put too much on and have it go into the voice coil, as long as you don't apply it too close to the VC.

Don't see how the viscosity of a substance dictates how easy/hard it is to apply "too much" (or too little)

More importantly -its lower viscosity makes it easier for it to flow into undesirable areas.

Gardenstater

You don't? When you apply a bead or drop of something to a horizontal surface gravity acts upon it and its final thickness depends upon the viscosity. If it is a relatively high viscosity giving a thick layer and then you clamp it (or allow the magnet to do the clamping), it then has to flow out somewhere. The more you have the more flows out for the most part, although like I said, final bond thickness may be less with the thinner stuff.

If you spread it out, like with a brush, it is still going to be thicker or thinner depending upon the viscosity.

The devil's in the details and not everything is totally cut and dried on first glance...

If you have the thinner stuff and a little flows out, it is less likely to bridge the gap and bond to the VC. Think water compared to molasses.

Astrl55

Sorry - your condescending answer didn't clear up a thing.

In your post, which I referenced, you made two inaccurate claims:

1. the viscosity of a substance doesn't prevent someone from applying too much or too little;

2. The low viscosity (thinner) substance has a greater chance of finding its way to the voice coil than does a thicker (high viscosity) substance.

Gardenstater

Sorry I don't know any other way to explain it. How much adhesive you apply, volumetrically, has to be right so that with the final bond thickness you do not have any excess.

If you do have a *little* excess you would much prefer that material to be something thin like water, than thick like molasses because the clearance to the voice coil is very small and you don't want it to get stuck. Also these Loctite materials do not tend to cure in the presence of air, unlike epoxy which cures chemically, so that is another level of safety.

Astrl55

I appreciate your explanation.

Kryten

In the thread
https://forum.polkaudio.com/discussion/comment/2530539#Comment_2530539
Loctite 290 is discussed because it does wick and has adhesive properties.

I agree that using something that is too thin, less viscous, could lead to wicking way down into the voice coil and should be avoided. We can't always control the direction excess material flows.

turkeyaddict

Assuming you apply the same volumes of a thick and thin substance...

If you apply a thin adhesive that flows well, I think anyone that uses adhesives or finishes typically is better at 'guestimating' the final amount to use. For me, it's largely because (1) I typically use less because (2) the thin adhesive flows quicker/better over the surface. I know I typically use less which limits or eliminates squeeze out. On the other hand, the volume to use for a thicker substance is harder to guestimate because it is often difficult to determine the direction and/or amount of actual 'squeeze out' - in this case, the direction being the worse unknown variable.

In my experience with different wood glues, I've found the volume to use for glue-ups with a thicker polyurethane based glue (that foam) is more difficult to guestimate than a PVA style glue (like Titebond).

I appreciate the discussion, but I'm not worried about using too much Loctite, largely because using just enough to bond seems to be preferable. The 3-piece unit will be secured with thin beads of either PowerGrab or an epoxy as a secondary bonding agent on the outside, which should ensure everything stays tight. Right?

Gardenstater

I knew as a fellow woodworker, you'd grock what I was saying

I wouldn't use PowerGrab but epoxy is great. One member did an impact test and the PowerGrab failed spectacularly. I think if you want the best I'd use G-Flex because different coefficients of thermal expansion between steel and ferrite is definitely a factor for longevity of the bond. G-Flex has a better chance of keeping its grip IMO.

pitdogg2

The 3-piece unit will be secured with thin beads of either PowerGrab or an epoxy as a secondary bonding agent on the outside, which should ensure everything stays tight. Right?

Yes in theory that is correct. When folks had to repair because the bond had broke and VC was under magnetic pressure you could see areas on the magnet that had no anaerobic adhesive. FYI when the speakers are built NEW there is no magnetic properties yet. They are just Ferrite rings and a pole piece that are glued together. They're magnetized by induction AFTER they are glued together.

Gardenstater

Kryten wrote: »

In the thread
https://forum.polkaudio.com/discussion/comment/2530539#Comment_2530539
Loctite 290 is discussed because it does wick and has adhesive properties.

I agree that using something that is too thin, less viscous, could lead to wicking way down into the voice coil and should be avoided. We can't always control the direction excess material flows.

This woke company (kidding) Loctite has too much diversity!

I checked the viscosity on that wicking stuff (Green 290) and it is only 20 to 55cP.

Breakaway torque on a M10 bolt and nut (of course they couldn't stick with 3/8-16 !) is only 90 lb in. vs 70 to 150 for 241 Blue and 150 to 300 for 272 Red, although slightly different threads.

Make it stop (beat ya to it F1 )

turkeyaddict

Gardenstater wrote: »

I knew as a fellow woodworker, you'd grock what I was saying

...G-Flex has a better chance of keeping its grip IMO.

Yes sir, and I have experience with the West System products You can bounce a good-sized hammer off any of their epoxy and not cause a breech or damage. Awesome...

F1nut

West Systems epoxy is bomb proof.

lawdogg

I use regular ol' 2-part epoxy (off the shelf Gorilla Glue brand at the moment) and it's more than sufficient. In practice it does wick to a degree as I apply the final adjustments in the re-centering and it's basically impossible to get back apart once it cures (well, impossible to get it back apart without destroying something, as far as I've been able to figure out).

I've had powergrab shift on me pretty easily (that was me noted above and quoted below, it was a highly scientific experiment of throwing glued magnets/pole piece assemblies on the garage floor and seeing what happened). (The 2-part epoxy didn't budge but the magnet itself chipped.)

(As an ME, I appreciate the nerding out though!)

Gardenstater wrote: »

I knew as a fellow woodworker, you'd grock what I was saying

I wouldn't use PowerGrab but epoxy is great. One member did an impact test and the PowerGrab failed spectacularly. I think if you want the best I'd use G-Flex because different coefficients of thermal expansion between steel and ferrite is definitely a factor for longevity of the bond. G-Flex has a better chance of keeping its grip IMO.

xschop

Powergrab works only up to about 2 ft. gravity drop. 😒

lawdogg

I should clarify that I don't put any adhesive between the faces, only around the perimeters. (Some does wick inside, but not much.)

I've found that adhesive applied anything other than PERFECTLY even between the faces can cause the angle of the voice coil movement relative to the pole piece to shift from perfectly normal, which makes it impossible to go back together correctly ... the voice coil can no longer move up and down in perfect alignment with the pole piece axis.

That was a (really) annoying lesson early on in my re-centering learning curve.

Gardenstater

Thanks for the clarification. I was wondering about epoxy between the faces, and the feasibility of that.....