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arend-jan 29th September 2007 09:49 AM

DIY weighted diaphragm stretcher (pic)

The picture shows the weighted diaphragm stretcher being used on a diaphragm for a Quad ESL bass panel.

I finished the build of this tretcher a couple of days ago. The results are outstanding!

The hard part was sourcing all the components and finding solutions to some of the parts that were custom made on the original Quad stretcher. The most critical parts are the clamps that need to have a firm grip on the film but not tear it, and the belt wheels - you can't have much friction at this point.

It is quite a thrill to have a diaphragm tensioned while suspended in air. Everything should be nicely balanced, when you pull one of the cords softly the whole diaphragm moves in the same direction.

I build it to refurbish Quad ESL diaphragms. On the stretcher is a bass panel film ready to be coated. The weights are made from PVC plumming pipe filled with leadshot and can be opened and adjusted to experiment to get the right tension. Each weight consist of two parts - the primary weight is tuned for tensioning bass panel diaphragms and the second part can be attached to the bottom of the first to increase the tension for the treble panels.

In the middle is the stator table which can be adjusted in height with a car jack. I wanted to build something a little more sophisticated but it proved difficult to make a nice lift mechanism. The car jack get's the job done.

I've tried a lot of different tensioning methods and this is the best by far. Apart from some unique features that this type of stretcher offers, it also has excellent consistancy (the resonance frequency is stable within 1 Hz between panels) and it is a breeze to use. Tensioning a diaphragm takes only a couple of minutes.

-(daz84)- 29th September 2007 09:26 PM

That really is cool!!!Well done!I wondered how much you need to tension a diaphram?

arend-jan 29th September 2007 10:00 PM

Hi daz,

I'm not sure if I understand your question correctly, but if you're asking about the size of the weights then I'm sorry but I won't tell. Not in public anyway.

Thanks for the compliment!

SY 29th September 2007 11:15 PM

Will you whisper in my ear at ETF?;)

I_Forgot 29th September 2007 11:47 PM



arend-jan 30th September 2007 08:38 AM

SY: if you buy me a beer :D

SY 30th September 2007 11:32 AM

Consider it done. A nice Leffe.

Capaciti 1st October 2007 05:45 AM

Hi Arend-Jan,

outperforming !! Congratulations.

Its funny, you experienced the same issues i did. Especially the clamps are tricky. You are right, this is the best and the most reproducible way to stretch an ESL membrane.

But i wonder that you just do mechanical tensioning. Quad implemented a thermal treatment to the process and i do as well.


arend-jan 1st October 2007 07:54 AM


Originally posted by Capaciti
Hi Arend-Jan,

outperforming !! Congratulations.



Its funny, you experienced the same issues i did. Especially the clamps are tricky.

Cool, you build a stretcher too! I'd love to see it. Can you post a picture?


Quad implemented a thermal treatment to the process and i do as well

send me a PM.


SkySeeker 4th October 2007 04:33 PM

2 Attachment(s)
That's off-limits on my coolness scale!

I wonder if you could have done with just half the weights though. Of course the "other side" of a weight should then have been clamped with a spring between the clamp and the supporting frame.

Come to think of it, when fitting a primary mirror to a (newtonian) telescope, we (amateur telescope builders) use a "floating support" (a pressure equalizing mechanism) to ensure that the mirror is supported equally on all contact points.

Like this:

(About half way down the page)

To improve on your design, something similar could be done with a lever system like in the annex. That way you would be guaranteed that the force pulling on each clamp would be the same. At least if the distances between AB and AC are equal and between DB, EB, FC and GC. Same for the opposite side.
(It goes without saying that you would need it for all sides.)

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