Markaudio Alpair 7 - problem with cracked plastic basket - diyAudio
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Old 29th August 2014, 07:17 AM   #1
spekr is offline spekr  United States
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Default Markaudio Alpair 7 - problem with cracked plastic basket

Hi,

I had a cracked basket around the screw hole in one of my Markaudio Alpair 7.3 although I didn't over-tighten the frame (driver was screwed in by hand and the mounting surface was flat). Please see the picture attached.

There was some helpful discussion in the Markaudio forum how to handle these obviously very fragile drivers and how to fix cracks but that thread was deleted by Markaudio.
As there seemd to be others with the same problem I thought it would be helpful to re-post some information about this issue and how to repair the damage.

Mounting the driver:

"Make sure you drilled pilot holes and pre-thread them prior to installation. Nip up the installation screws to feel a slight tightness. Eyeball the flatness of the frame front. If you're starting to visually distort the frame, you're applying too much torque. I don't recommend using a power driver. Do it be hand, you'll feel the increase in resistance when close to tightening." - mark audio

"1. Drill pilot holes the full diameter of the screw shank.
2. Run a screw through each hole before mounting the driver
3. With the driver in place, torque each screw just up to the point where the seal starts to compress and the driver frame starts of visibly move. You will feel the resistance in the screw driver at this point.
The worse thing you can do is run the screws in with a power driver without the benefit of pilot holes." - Bob Brines

Repairing cracket drivers:

"With the driver removed, turn it face down (DON'T touch the cone as you do this). Gently peal back the seal and apply a small amount of suitable plastic epoxy glue on the crack. Ideally, apply the glue to the inside of the frame where it can't be seen.
1- Test the glue on a non-critical are of the frame first. Check that it doesn't melt the composite.
2 - Apply a small volume of glue only.
Allow to harden and re-install." - markaudio

"I advise using an epoxy as its the industry standard adhesive formula for the strongest bond. Please check the glue makers instructions for material compatibility. Here's an example:
Epoxy Repair: Plastic - UniBond" - markaudio
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File Type: jpg IMG_4948.JPG (171.9 KB, 277 views)
 
Old 29th August 2014, 09:05 AM   #2
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All very useful no doubt, but 'obviously very fragile' is a trifle exaggerated. I've had many pairs through here & never had a problem with frames over-stressing. There don't appear to be large numbers of people reporting failures either, so I think it's safe to conclude

a/ There are not in fact very many failures, and

b/ You either did not install it correctly, or possibly were unfortunate & got a frame where a weakness existed. Such things can occur in all mechanical appliances.

Since the frames are a polymer type, some mechanical sympathy / common sense is required when mounting to the baffle -opposite screws / bolts and gradual tightening (not over-torqueing) is needed. This is not difficult, since it really should apply to all drivers (or anything akin -light-switch fittings, socket covers &c.) in any case.
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Last edited by Scottmoose; 29th August 2014 at 09:10 AM.
 
Old 29th August 2014, 09:22 AM   #3
spekr is offline spekr  United States
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Quote:
Originally Posted by Scottmoose View Post
All very useful no doubt, but 'obviously very fragile' is a trifle exaggerated. I've had many pairs through here & never had a problem with frames over-stressing. There don't appear to be large numbers of people reporting failures either, so I think it's safe to conclude

a/ There are not in fact very many failures, and
That's good to know but on the other hand most people probably never check once the speaker is mounted.

Quote:
Originally Posted by Scottmoose View Post
b/ You either did not install it correctly, or possibly were unfortunate & got a frame where a weakness existed. Such things can occur in all mechanical appliances.
All I can say is that I did not over-tighten and the mounting surface was flat. I have a couple of other drivers made out of plastic and they are all ok. The other Alpair is also ok. It's just this one driver and one screw hole.

Quote:
Originally Posted by Scottmoose View Post
Since the frames are a polymer type, some mechanical sympathy / common sense is required when mounting to the baffle -opposite screws / bolts and gradual tightening (not over-torqueing) is needed. This is not difficult, since it really should apply to all drivers (or anything akin -light-switch fittings, socket covers &c.) in any case.
I agree. Guess the risk of cracking comes with the use of plastic as a basket material. A metal basket is probably more forgiving. The surround smooths out any small bending. Of course it depends on the type of metal used. Some might crack easily too.
 
Old 29th August 2014, 11:10 AM   #4
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Quote:
Originally Posted by spekr View Post
That's good to know but on the other hand most people probably never check once the speaker is mounted.
Come on, I know a small crack around a screw-hole must be annoying, but that is not called for. What evidence have you to say that there is a widespread frame cracking problems with these drivers (or any others using a polymer frame for that matter -the very popular Vifa models are an obvious example)? Even a couple of dozen examples wouldn't be statistically significant when you think of the thousands of units out there with polymer frames from various manufacturers. If there was a widespread problem, we'd know about it. And so would the manufacturers. And they'd do something about it at a rate of knots for obvious reasons.


Quote:
I agree. Guess the risk of cracking comes with the use of plastic as a basket material. A metal basket is probably more forgiving. The surround smooths out any small bending. Of course it depends on the type of metal used. Some might crack easily too.
To an extent that can be true, but it varies significantly depending on the polymer type employed, the design of the frame itself and a host of other factors. Very hard polymers can be brittle, softer types less so. Since there appears to be no evidence to suggest there are widespread problems with the MA, Vifa etc. units, they appear to have struck a good balance on this front to date.

Re metal, it can be more forgiving on this score, but it's not devoid of potential problems either. Again, it depends on metal type, frame design, production method &c. Some cast types can let go for instance. I've not had a failure myself, but I've seen a handful of examples equivalent to the shot above, or with a crack over the frame itself. These things happen, and on the whole, the industry doesn't seem to do a bad job in this regard (leaving aside opinions / preferences on the merits of various frame materials aside). Speaking in general terms, any kind of over-tightening when mounting a drive unit is best avoided since it can warp the frame, which may then force the moving components out of alignment. You find this may particularly affect cheaper stamped types, but it's not exclusive to them alone. And so on. Clark Blumenstein for e.g. makes quite a deal on his site over the care with which the drivers on his commercial models are mounted / tightened (mostly Fostex based). Fair play. The pioneers of audio were big on decoupling drivers from baffles (granted not primarily to avoid failures but for acoustics -still, it does avoid some stresses) . Feastrex (goodness knows what's happening there since apparently their designer / manufacturer has left the company) liked rear-mounting via a long bolt & no direct affixing to the baffle via the front of the frame (same). And so on. It's an interesting area.
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Last edited by Scottmoose; 29th August 2014 at 11:24 AM.
 
Old 29th August 2014, 11:40 AM   #5
zman01 is offline zman01  Bangladesh
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spekr, Scott,

Might I suggest that a slightly misaligned screw hole can be another potential reason behind frame stress? The screw doesn't go in straight enough and can put extra pressure on the frame while being screwed in. For such a case you probably would not need to over-tighten too much.

Hope you have been able to repair the crack and have things up and running!
 
Old 29th August 2014, 12:21 PM   #6
spekr is offline spekr  United States
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Quote:
Originally Posted by Scottmoose View Post
What evidence have you to say that there is a widespread frame cracking problems with these drivers
I never said that.
 
Old 29th August 2014, 12:40 PM   #7
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Quote:
Originally Posted by zman01 View Post
spekr, Scott,

Might I suggest that a slightly misaligned screw hole can be another potential reason behind frame stress? The screw doesn't go in straight enough and can put extra pressure on the frame while being screwed in. For such a case you probably would not need to over-tighten too much.

Hope you have been able to repair the crack and have things up and running!
That's a possibility; it's very easily done (especially for the likes of me ) & not particularly obvious when trying to identify the source of problems. I wouldn't 100% rule out a minor flaw in the frame itself -these things can happen on occasion, though they tend to be very rare in reasonable production scales. That's why all those TVs, toasters, cars, drive units etc. are so reliable. Anyway, hopefully the small crack in the OPs driver is now fixed -the first post suggests it is, which is good.
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Last edited by Scottmoose; 29th August 2014 at 12:47 PM.
 
Old 29th August 2014, 12:45 PM   #8
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Hi Guys,
Please take a closer look at Spkr's pic of the frame damage. Note both the frame front face and its rear edge are uniform. There is no material-shape deformation on this component. Manufacturing defects on these components are visually obvious. The plastic composite deforms (goes out of shape) and out-of-tolerance units get rejected during quality control. Fail rates are extremely low averaging 3 frames per thousand. The proof is in the public domain. There are few references to failures over the years against the hundreds of thousands of these frames in use.

What can be observed from Spkrs pic is an irregular crack, typical of a stress load fracture. This can only be caused by excessive force being applied. Possibly the pilot hole could have been misaligned, although an end user can normally feel the additional tightness when screwing down. In any event, to force a relatively elastic composite material to break in this manner is usually down to the installation method.

It is fair to say that more complex moulded frames require installation care. Pressed steel frames can be delicate, as their bending resistance is nominal. Once taken out of alignment, they rarely spring back into shape. Die-cast has more compressive strength but its brittle fracture tolerance is low, once cracked its gone. Essentially, most smaller frames will usually be weaker than larger substantial units.

The best advice when installing any small driver (under 6.5") is to take time to correctly align pre-thredded holes and go easy when screwing down. Don't use power drivers. Use a hand tool to feel the increase in tension as the screw is close to the nip-up stage. Nip up means "nip-up". Don't be tempted to switch into Brutus mode. I freely admit to cracking a few myself (die cast and mine) over the years, in haste mostly between testing.

I had hoped to successfully resolve this situation with Spkr, but it relies on accepting a degree of responsibility for one's actions; Rather than posting full-force with a default finger-pointing, blame game agenda. Even so, the initial thread in the Markaudio section remained sufficiently open to give Spk time to receive help in the way of a simple frame fix, despite his efforts to damage the driver's following.

Thanks
Mark

Last edited by markaudio; 29th August 2014 at 12:54 PM. Reason: additional info
 
Old 29th August 2014, 04:38 PM   #9
spekr is offline spekr  United States
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Mark, "posting full-force", "finger-pointing" and "blame game agenda"? That's a rather distorted depiction of what I ever did and said. I just stated the facts, didn't blame anyone, didn't point fingers. You follow me here, edited my posts and even deleted a whole thread. Let it go. Everybody is entitled to have an opinion. Even me.
 
Old 29th August 2014, 05:26 PM   #10
godfrey is offline godfrey  South Africa
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Reminds me of this.
 

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