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-   -   Alpair 7.3 plastic ring cracked. (http://www.diyaudio.com/forums/markaudio/227996-alpair-7-3-plastic-ring-cracked.html)

bqc 17th January 2013 03:58 AM

Alpair 7.3 plastic ring cracked.
 
1 Attachment(s)
So I was tightening down the supplied screws and one of them cause the ring to crack as shown in the picture. Does that happen often? Should I not have use the supplied screws? It seems that the screw heads were a bit larger than the hole or something because it cracks even before the driver is completely mounted firm against the baffle.

markaudio 17th January 2013 10:29 AM

Quote:

Originally Posted by bqc (Post 3329390)
So I was tightening down the supplied screws and one of them cause the ring to crack as shown in the picture. Does that happen often? Should I not have use the supplied screws? It seems that the screw heads were a bit larger than the hole or something because it cracks even before the driver is completely mounted firm against the baffle.

Hi bgc,
The screws we supply are custom made, with smaller heads than the frame hole recesses. We only make and supply 1 size of screw for the Alpair 7.

From your description of events and looking at the pic, the screws weren't properly centred within the frame plus heavy over-tightening caused the failures. With pressed steel and composite frames, care must be taken to properly align the the mounting holes; And gentle tightening is all thats required.

I suggest carefully undo the drivers, get some suitable A/B bond or super-glue and apply it to the cracks. Assuming you've kept the front covers, bond them on for added strength.

Then carefully double check the screw hole alignments. Make sure their centred to the holes in the driver's frame. Drill small 2mm pilot holes in the box should you wish to turn the drivers to make new mounting on the box. Place the driver in position and use a 4-mm drill or punch bit inside the frame holes to mark the pilot hole positions.

Don't murder the screws when tightening down on to the frame. Be gentle and use a hand tool for this process. You'll get to feel when the screws is taken the frame tight on the baffle.

Make 100% sure your baffle mounting surface if flat.

With luck, you'll save the drivers and live to play another day.

Cheers
Mark

ChrisMmm 24th December 2014 09:37 PM

This thread is now quite old but I found another, more recent one, with the same issue documented. Unfortunately it was "closed" so I am posting here.

I also have experienced the same cracking with a pair of 7.3 drivers. Now I take full responsibility for the damage, I have mounted the drivers probably 6 times to change damping and finally move to an OB. I fully accept that I almost certainly overtightened at some stage and probably did not align the holes with great accuracy. However I do believe these drivers need a bit more care than their metal framed counterparts.

Anyway my solution, when I finally mounted them in OBs, was to route the rebate as tightly fitting as possible then screw the drivers down quite lightly. The fact that they are crossed over around 200hz means that cone travel is not large but they still sound stunning as ever. Better than ever actually in OB and with the crossover.

In a sense I am glad to see a few others have had the same problem and I will be taking much greater care in future (with any driver mounting in fact). These are awesome drivers and I will continue to buy and use them in future (already have some 10.3s in FHXL cabs).

paul burchell 25th December 2014 11:56 PM

I would suggest not finish tightening any screws until all the screws are in far enough for the screw head to clear the edges of the recess. This leaves a little wiggle room against imperfect alignment of the bored holes.

ChrisMmm 26th December 2014 12:41 AM

Quote:

Originally Posted by paul burchell (Post 4163508)
I would suggest not finish tightening any screws until all the screws are in far enough for the screw head to clear the edges of the recess. This leaves a little wiggle room against imperfect alignment of the bored holes.

Good point. I always tighten in stages and in opposing pairs so as not to overtighten any one screw. Still managed to damage the driver tho!

I future I would precut the screw threads by screwing them in and out a few times before installing the driver. maybe even hardening the thread you create by adding a few drops of thin superglue, letting it set and then reworking the threads with the screws.

That way you will feel the resistance as you put pressure on the driver and very lightly tighten.

gaston senac 26th December 2014 03:56 AM

Quote:

Originally Posted by ChrisMmm (Post 4163523)
I always tighten in stages and in opposing pairs so as not to overtighten any one screw.

Yes, it should be done as in motorbickes or cars mechanics, as Chris pointed out. But never tighten a screw until bottom unless you have the other screws well positioned and 3/4 tightened.
I hope you get the idea, if not, see a youtube video how it's done in mechanics (engine cover).
Other thing to take care for is to have a smooth surface and not let the plastic frame lay down in an uneven surface.
Hope it helps.
Cheers.

Bob Brines 26th December 2014 02:01 PM

I have never seen an Alpair frame crack radially at the bolt holes. I can't imagine how that happens. Typically, over torquing the screws breaks out a pie shaped piece on the back side of the hole starting at the level of the flat inside the hole. Yes, I have done it myself being a bit over zealous with a power driver. The fix for my error is simple -- epoxy the broken piece back in. For the OP's cracks, I doubt that they are causing any problem other than cosmetic unless the backs of the holes are also cracked, in which case the previous sentence.

Bob

ChrisMmm 26th December 2014 08:51 PM

Quote:

Originally Posted by Bob Brines (Post 4163951)
I have never seen an Alpair frame crack radially at the bolt holes. I can't imagine how that happens. Typically, over torquing the screws breaks out a pie shaped piece on the back side of the hole starting at the level of the flat inside the hole. Yes, I have done it myself being a bit over zealous with a power driver. The fix for my error is simple -- epoxy the broken piece back in. For the OP's cracks, I doubt that they are causing any problem other than cosmetic unless the backs of the holes are also cracked, in which case the previous sentence.

Bob

I noticed the "pies' first but wasn't sure what it was. After a few more removal and re-fittings a full crack developed. Should have taken more notice when I first saw the minor crack. However all is well and sounding fine.

vinylkid58 26th December 2014 09:05 PM

Quote:

Originally Posted by Bob Brines (Post 4163951)
I have never seen an Alpair frame crack radially at the bolt holes. I can't imagine how that happens.

Not paying attention to the screw hole locations, so the screw head's aren't concentric to the plastic frame holes?

jeff

Bob Brines 26th December 2014 09:47 PM

Could be. I always use a self-centering punch to mark the holes, then drill the pilot holes at the drill press.

Bob


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