Screw holding Loudspeaker in wood box is loosening - Spring Washer? 1

[LucasVidler]

I have a product in production that is basically a speaker in a wood box mounted with a single rear screw. The configuration is illustrated in the attachment.

Wood: MDF
Speaker Material: Mild Steel
Screw: M8 x 50 mm, Hex Head, Class 8.8, Zinc Plated, Torqued to 5kg-cm
Washer: 15mm OD, Zinc Plated Steel

After shipping we've had some speakers start to rattle, requiring the screw to be re-tightened. There are probably a few reasons for this: 1. The MDF cabinet shrinks and/or shifts due to temperature and humidity changes. 2. The thin foam sealing gasket under the speaker flange compresses further over time. 3. The MDF under the washer compresses further over time. I doubt that the screw turning out due to vibrations is a significant factor.

My first thought for a fix is a Belleville Spring Lock Washer between the screw head and the washer. However, I've read that some engineers don't find these very effective in applications with vibration. Supposedly loctite type compounds are the most effective. But, this rattling happens right out of the box after shipping. So, I think the primary mechanism in my situation is expansion and contraction, not torque on the screw due to long term vibrations. So, it seems like a spring washer is the best way to maintain tension on the screw. And I would like to avoid using loctite, if I can. Maybe I need both? Your thoughts?

EDIT: Reading a bit further it appears that Belleville Disc Springs are more resilient because they are made of actual spring steel. Belleville Spring Lock Washers tend to flatten out after compression. Also, the Disc Springs have specific load ratings. Given the 5kg-cm (0.35 ft-lb) torque on my screw there is about 60 lbs of tension. So, could choose a Disc Spring that compresses but doesn't fully flatten out. However, the Disc Springs are significantly more expensive than the Spring Lock Washers. Is a Disc Spring overkill?

 

[Compositepro]

Your picture is different than I expected from your description. I expected that the MDF was clamped between the bolt head and some rigid surface. But actually the bolt is pulling the speaker toward the back of the MDF speaker case. So you already have a Belleville washer in your product, but it is made of MDF and is called the back side of a speaker cabinet. MDF does not make a good spring. Have you ever seen an MDF bookcase after it is loaded with books? The shelves take a permanent sag.

A coil spring around the bolt might fix your problem, but the MDF case can still expand and contract or otherwise distort so that you lose clamping pressure holding the speaker in the case. Metal stiffeners for the back of the case would act as springs that do not creep. The standard design of screws on the rim of the speaker holding the speaker to the front of the case would be far more reliable.

 

[skeletron]

Strange design with the screw going right through the speaker magnet (?)

Having worked on dozens of guitar amplifiers and cabinets, I have always seen four screws around the front rim (ie. the metal base supporting the cone) attaching to a front baffle. Typically the screw head is on the front side, there is often a grip washer and then either a wingnut or simple nut on the other side. The screw fastens through the baffle which is almost always plywood (solid). There is often a slim piece of foam between the front rim and where it attaches to the baffle.

As always, this doesn't answer your question, but hopefully it sparks some idea to help get you around the issue that is occurring on your speaker cabinet.

 

[kingnero]

Can you place a tube around the bolt, inside the box?
But as long as the bolt goes through the MDF, there is no good solution as the MDF will deform after time, making the bolted connection lose preload.

Basically, the ratio of stiffness bolt/assembly is not good here.

 

[dik]

I did a pile of study about 50 years back in acoustics... as a lead in to studying light optics... but, have forgotten most of the stuff.

Most speakers are designed with the attachment being to the outer rim for acoustics; mounting should reflect the manner of the speaker design mounting. One of the neatest speaker 'enclosures?' I've seen was simply mounting the large speaker on 4 threaded rods, vertically, with a roll of corrugated cardboard, about 4' high, on the back (roll was vertical). The fine 'tiny holes' provided by the corrugations provided exceptional damping of the sound coming off the rear of the speaker. The front of the speaker faced the floor, and a simple ceramic 'bowl'shaped reflector directed the sound away from the floor.

A sleeved tube might work, but should have it bearing on a washer on the MDF on the inside of the enclosure. A shorter tube with a length nearly the thickness of the MDF could transfer the load through the MDF with the Belleville washer and the 'stretch' of the bolt providing the spring loaded resistance to keep it from loosening.

A larger washer and Bellville washer on the outside at the back may work. The flexural movement of the MDF 'slab' might complicate things a bit and limit the tension in which case the second tube might have to be nearly the thickness of the MDF. The fastener will be subject to a lot of vibration over the lifetime of the speaker (usually the front mounted anchorage is separated from the speaker by a 'rubber' membrane between the speaker and the outer rim).

I think the connection is poor and not a really good one. If in production, you may want to try a couple of 'mock ups' to come up with something that works better. You might want to discuss this with the speaker manufacturer.

Dik

 

[Tmoose]

Please provide pictures and details of all the joints in the MDF that form the enclosure.
Dado, butt, biscuit, etc, and how they are oriented relative to the force exerted by the 2 inch long screw.

 

[MintJulep]

This design is destined to fail. No way to fix it. Start over.

 

[LittleInch]

I think you will need to stiffen up that back panel by maybe a few ribs or similar.

Any spring washer type effect will just cause the back plate to bend over time.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[dik]

LI: "I think you will need to stiffen up that back panel by maybe a few ribs or similar.
Any spring washer type effect will just cause the back plate to bend over time."

That's why I suggested sleeving the bolt through the back... I think it needs a little more than a couple of ribs.

Dik

 

[Tmoose]

Hi dik,

I am unclear what the speaker end of the sleeve would bear against.

thanks,

Dan T

 

[kingnero]

Doesn't matter, as the MDF thickness would still be "unsleeved", and the stiffness ratio would still be out of the workable. I second MintJulep's suggestion.

 

[dik]

Tmoose... the long sleeve would bear against a washer on the inside of the cabinet back and the short sleeve would go through the hole in the bearing on the washer on the inside and a washer on the outside and a Belleville washer on top of this. The short sleeve would be slightly shorter than the thickness of the MDF. The bolt would connect the speaker to the back of the cabinet and the Belville washer and bolt tension would, hopefully, keep things in place. It's a messy connection and may not work in the long term.

Dik

 

[Tmoose]

Hi Dik,

"the long sleeve would bear against a washer on the inside of the cabinet back...."

What would the other end of the long sleeve bear against? The speaker?

 

[dik]

Yup...

Dik

 

[IFRs]

This is not going to succeed over time. Use glue or silicone caulking to hold the speaker into the hole, screw the bolt. Vibration, moisture and creep will make it sound bad ( fail ) unless bad sound is OK. You need to seal the driver into the enclosure to achieve consistent sound pressure. Using electrical analogies for the mechanical system ( driver, enclosure, port ) in combination with the electrical circuit diagram of the crossover ( and possibly of the connecting wires, amp and pre-amp, and an analogy of the room environment ) can lead you to a solution that predicts the frequency response and resonances that you need to dampen to level out the response curve.

I could be missing the boat entirely of course...whatever you do, have fun !!!

 

[dik]

IFRs: as I noted, "It's a messy connection and may not work in the long term."

The speaker 'enclosure' I noted using the corrugated cardboard worked perfectly and nearly mimicked the manner in which speakers are designed... and was simple and easy to build... nothing like the 300lb sand filled ones I built for my brother. I don't think either of us missed the cruise... and no virus...



Dik