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LCD monitor mount - physics question :-)

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LCD monitor mount - physics question :-)

Postby mozart999uk » Mon Mar 20, 2017 11:16 am

So............

I have a new sit/stand desk which is lovely! I have my 24" dell monitors, mounted like this.

https://goo.gl/photos/GkfcVHW3JVf8DHRm9

I'm concerned the two monitor mounts in the middle will cause the desk to bow over time.



I'm going to brace the desk underneath with some timberboard and a metal bookcase strip but......

Would I be better reversing the mounts so that the legs are out towards the edges rather than in the middle?

I can't get my head round the physics as to which would be better. Does the downward pull of the monitors on the arms counteract the force / weight of the legs in the middle?

If it helps, the mounts weigh 3.8kg each and the monitors weigh 6.2kg each

The mounts are these ones:

http://cablemountain.com/epages/eshop186554.sf/en_GB/?ObjectPath=/Shops/eshop186554/Products/%22Vertical%20Desk%20Arm%22

Thanks in advance!
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Re: LCD monitor mount - physics question :-)

Postby Music Wolf » Mon Mar 20, 2017 11:43 am

I would suggest to get a table leg (Ikea are cheap but B&Q also sell them) http://www.ikea.com/gb/en/products/desks/table-tops-legs/table-legs-trestles/ and position in at the back of the desk between the monitors.
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Re: LCD monitor mount - physics question :-)

Postby mozart999uk » Mon Mar 20, 2017 12:09 pm

Thanks for the reply and the suggestion

Unfortunately it's a height adjustable sit / stand desk so that wouldn't work :-(
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Re: LCD monitor mount - physics question :-)

Postby James Perrett » Mon Mar 20, 2017 12:22 pm

I'd suggest that your idea of spreading the load by adding bracing is the best. I'd probably clamp something like plywood both above and below the desktop.
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Re: LCD monitor mount - physics question :-)

Postby A Mole » Mon Mar 20, 2017 12:31 pm

mozart999uk wrote:I can't get my head round the physics as to which would be better. Does the downward pull of the monitors on the arms counteract the force / weight of the legs in the middle?

However you look at it, the full weight of the monitors is being transmitted though the foot of the stand into the desk. There may be a tendency to rotate as you suggest, but this can't make the monitors appear "lighter" to the desk.

You would therefore be better to put the stands on the outside, nearer to the desk legs.
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Re: LCD monitor mount - physics question :-)

Postby Music Wolf » Mon Mar 20, 2017 12:43 pm

The torque on the stand mount was something that concerned me (its as though the bracket would want to 'twist off' the portion of the desk within the clamp) but, having looked at the details of the stand, it would appear that the arm is fixed so the same rotational forces apply regardless as to how you position the screen.

Stands towards the outside reduces the bending moment but, applied maths aside, I'd be shoving an extra leg n there rather than trying to strengthen the desk with a bracket.
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Re: LCD monitor mount - physics question :-)

Postby Music Wolf » Mon Mar 20, 2017 12:45 pm

mozart999uk wrote:Thanks for the reply and the suggestion

Unfortunately it's a height adjustable sit / stand desk so that wouldn't work :-(

Just spotted that. Shove an extra leg in there anyway and make it a sit / kneel desk :D
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Re: LCD monitor mount - physics question :-)

Postby mozart999uk » Mon Mar 20, 2017 1:34 pm

Thanks all. Really useful. Mounts to the outside and sitting / kneeling it is :tongue:

BTW, I can't recommend these type of desks enough. It takes some working out with cables and bits but just the joy of spending a bit of my day standing and working is well worth it.

I got mine from this place.....very helpful peeps

https://heightadjustabledesks.com/
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Re: LCD monitor mount - physics question :-)

Postby mashedmitten » Mon Mar 20, 2017 1:55 pm

How thick is the desktop and what's it made of?

The downward force is moot compared to the rotational force the canter lever of the two monitors is exerting, like trying to snap a cookie in half.

All you'd need is a piece of flat stock or angle iron, depending on mounts, steel or aluminum, say a foot by 6" by 1/4" under-mounted where the monitors clamp on.

A piece of wood wide enough for the clamps, screwed and glued underneath running the length of the top would work as long as if it were real wood the top was made of, the grain of the support should run the same way. Wood should be applied and glue let dry before re-installing brackets.
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Re: LCD monitor mount - physics question :-)

Postby mozart999uk » Mon Mar 20, 2017 2:06 pm

Thanks for your reply.

The top is Melamine covered chipboard, 1" thick.
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Re: LCD monitor mount - physics question :-)

Postby mashedmitten » Mon Mar 20, 2017 2:16 pm

mozart999uk wrote:Thanks for your reply.

The top is Melamine covered chipboard, 1" thick.

I'm thinking a 1"x3 or 4" piece of wood glued and screwed the length of the top underneath would be more than enough. A piece of angle aluminium fitted the same way would also work.
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Re: LCD monitor mount - physics question :-)

Postby mozart999uk » Mon Mar 20, 2017 2:26 pm

Thanks that's really useful.

I've got a piece that's 3/4 " by 8" and then I was thinking of screwing one of these

http://www.wickes.co.uk/Wickes-Single-Slot-Upright-Bracket-White-1000mm/p/165149

under the additional piece of wood.

Do you think that would work?

And would you leave the "legs" of the mount in the middle or re-position them to the edges as others have suggested?
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Re: LCD monitor mount - physics question :-)

Postby Martin Walker » Mon Mar 20, 2017 2:43 pm

mozart999uk wrote:Thanks that's really useful.

I've got a piece that's 3/4 " by 8" and then I was thinking of screwing one of these

http://www.wickes.co.uk/Wickes-Single-Slot-Upright-Bracket-White-1000mm/p/165149

under the additional piece of wood.

Do you think that would work?

I think that should work admirably mozart999uk!

Just bolt it in every few inches along the bracket and your desk should stay perfectly flat.


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Re: LCD monitor mount - physics question :-)

Postby mozart999uk » Mon Mar 20, 2017 2:49 pm

Martin Walker wrote:
I think that should work admirably mozart999uk!

Just bolt it in every few inches along the bracket and your desk should stay perfectly flat.


Martin

Thanks dear chap.

And hi from the dusty end of our lovely county :-)
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Re: LCD monitor mount - physics question :-)

Postby MarkPAman » Mon Mar 20, 2017 2:58 pm

I'd screw & glue something like this along the back/under the desk - it shouldn't be visible from the front.

If your desk is rigid enough not to bend, the position of the brackets is irrelevant.
If it does bend, then spreading them apart reduces the load on a single point.
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Re: LCD monitor mount - physics question :-)

Postby mashedmitten » Mon Mar 20, 2017 3:03 pm

mozart999uk wrote:Thanks that's really useful.

I've got a piece that's 3/4 " by 8" and then I was thinking of screwing one of these

http://www.wickes.co.uk/Wickes-Single-Slot-Upright-Bracket-White-1000mm/p/165149

under the additional piece of wood.

Do you think that would work?

And would you leave the "legs" of the mount in the middle or re-position them to the edges as others have suggested?

Yes, as Martin says, that would be fine. I'd use construction adhesive in addition to the screws as particle board is less susceptible to expansion/ contraction than real wood so no issues there. You want to screw through just the metal touching the underside of the desk, not through the entire tube. Doing so would want to crush the tube under pressure, thus the glue being the main attachment medium. It'd rip the board before pulling a screw.

You say this as well as a piece of wood. What I'd do is put the wood to re-enforce the spot the clamps attach. I'd make it a foot or so long and just wide enough to fit the clamps. Again, glued and screwed. Just forward of this(closer to front of desk) is where I'd run the metal as I said earlier. You want to do this with no weight on the surface, flipping it upside down on a flat surface and applying would be the best, not moving it until glue is set. Remember what's underneath the work area, don't drill holes or drive screws through the desk and into whatever's beneath like floor or table, etc... .

I wouldn't move the mounts. They're in a good spot to bolster in one go rather than having to re-enforce two places.
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Re: LCD monitor mount - physics question :-)

Postby mashedmitten » Mon Mar 20, 2017 3:12 pm

Oh, on the metal piece, I'd sand the finish off the glue side and use construction adhesive, not wood glue, to assure adhesion.
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Re: LCD monitor mount - physics question :-)

Postby mozart999uk » Tue Mar 21, 2017 12:43 pm

Thanks everyone. Really helpful tips!

I shall press on!
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Re: LCD monitor mount - physics question :-)

Postby Dynamic Mike » Wed Mar 22, 2017 12:24 am

I'd leave the mounts where they are in the photo, drill two holes an inch or so beneath the monitors & use a long bolt between them. That will counteract the rotational forces & leave the desk fully functional. The vertical forces through the mounts will be minimal & shared between the clamps anyway.
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Re: LCD monitor mount - physics question :-)

Postby Music Wolf » Wed Mar 22, 2017 7:57 am

Dynamic Mike wrote:The vertical forces through the mounts will be minimal & shared between the clamps anyway.

the vertical force we be equal to the weight of two clamps + 2 monitors (=20kg according to the post) sitting in the middle of the desk (the clamps are close enough together in the picture). That's 44lb in old money (just over 3 st in Brexit land) which is a fair bit of weight, however..................

The numbers don't feel right. I just Googled a Dell 24" monitor and found this;

- Unboxed: 3.24 kg
- Boxed: 6.83 kg

So, unless you are leaving the monitor in the box :D (I also think that our clamps are more like 2.5kg each), I don't actually think that you have a problem, especially if you move the clamps outwards.
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Re: LCD monitor mount - physics question :-)

Postby mozart999uk » Wed Mar 22, 2017 9:41 am

Music Wolf wrote:
Dynamic Mike wrote:The vertical forces through the mounts will be minimal & shared between the clamps anyway.

the vertical force we be equal to the weight of two clamps + 2 monitors (=20kg according to the post) sitting in the middle of the desk (the clamps are close enough together in the picture). That's 44lb in old money (just over 3 st in Brexit land) which is a fair bit of weight, however..................

The numbers don't feel right. I just Googled a Dell 24" monitor and found this;

- Unboxed: 3.24 kg
- Boxed: 6.83 kg

So, unless you are leaving the monitor in the box :D (I also think that our clamps are more like 2.5kg each), I don't actually think that you have a problem, especially if you move the clamps outwards.

You are quite right. I got the monitor figure completely wrong :-) They are u2412m's and weigh 3.9kg each.

The figure of 3.8kg each for the clamps I got via email from Cable mountain themselves so I assume it's correct :-) They do feel really heavy.
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Re: LCD monitor mount - physics question :-)

Postby Dynamic Mike » Thu Mar 23, 2017 12:31 am

Music Wolf wrote:
Dynamic Mike wrote:The vertical forces through the mounts will be minimal & shared between the clamps anyway.

the vertical force we be equal to the weight of two clamps + 2 monitors (=20kg according to the post) sitting in the middle of the desk (the clamps are close enough together in the picture). That's 44lb in old money (just over 3 st in Brexit land) which is a fair bit of weight, however..................
I presume the original (boxed) figures included the PSU's, so now we're looking at a combined weight of around 2st 5lb carried across 2 mounts. We've got 1" of chipboard, which is thicker than most standard flooring sheets so the total weight isn't going to be much of a problem. However as the weight of the monitors in the photo is offset, the leverage will introduce a moment of force the point at which each of the clamps attach to the desk. Bolting the mounts together would eliminate the torsional force from the offset weights.
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Re: LCD monitor mount - physics question :-)

Postby mashedmitten » Thu Mar 23, 2017 1:02 am

1" MDF sounds heavy duty, but in this application, where the span is wide, over time it could sag from the weight. The way this one is made to raise and lower, any bracing underneath is most likely in the middle.

A floor has joists 16" on center, maybe 24" in older construction and the edges of any under-layment fall on the joists.

Here, the edge is unsupported so the monitors want to fall away from one another, but also tilt backwards. The plate and brace combats both.
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