Home project -- spring-arm Monitor stand (solidworks)

[Cutlynx]

hello, first i would like to say, even through i enjoy working with SolidWorks, I am not an engineer. so any more technical stuff may go over my head, and everyone who would help me even through that disadvantage have my eternal gratitude.

my home project was made because of whole monitor stand market dissatisfaction. i want monitor stand with much more wider range to offer at least theese two setings:

so this is my model so far (whole model is in spring-arm.zip). i tried to put it into simulation with spring but it violently exploded

i thought to put in there some holed plates like this to just hopefully find the right way after its build.

my idea is to just connect it with spring and wire-tensor and figure out the right pull on run.
but I am quite nervous ill won't manage it. so I am asking you for any questions what would be your idea how to pull this off.

 

[Nidum]

Search on ' Anglepoise lamp mechanism '

 

[Cutlynx]

ok, I've found this. http://eprints.lancs.ac.uk/20295/1/20295.pdf its going most deeply into the subject and actually offers some equations.
through i cannot say it did go without a problems. unfortunately it doesn't offer spring arm i need. i found the most identical one through its missing upper second arm
and in equation there is "radii to the centres of the gravity" like what does that even mean? there isn't any help inside how to even get it

on wikipedia there is Tension springs within the arms model which fits me better, but doesn't offer any physics behind it

 

[Nidum]

Try this site instead

 

[Cutlynx]

thats the site i had inspire myself to make the model how it looks now. but it doesent offer any equations or at least better understanding for actually building it. i rechecked all outgoing links from that site and none of them provide any more information.

i don't believe myself that much to build only on illustration made of lines without thorough description. that's the reason I am here

 

[Nidum]

Try doing a simple diagram showing what forces are acting in the basic mechanism . Forget about the finer details of the design pro tem .

 

[Cutlynx]

oh thanks nidum! i think i get it what needs to be done. it will take me some time, i forgot like even the basics from physics

 

[Cutlynx]

ok. this is just my theory how to calculate it. there will be probably be trash, so please correct me if i get something wrong

1st picture is most folded arm with shortest lever. both springs should be only prestressed at this configuration. springs need to be at position to have length L¹ and deliver F¹ (that will be calculated later)

2nd picture is most stretched arm. both springs should be loaded at this configuration. springs need to be at position to have length L² and deliver F²

so i need to figure out what is the best position for the springs to to be in both scenarios at required length, and to deliver required force
(actually second arm doesn't have that much lever difference so it may be possible to calculate it with just force2 ?)
____________________________________________________________________________________

ill start with 2nd picture with 2nd arm and currently won't calculate weight of the construction itself.

so I've been thinking if i should use torque or lever. i couldn't find a way do do anything more with torque so i had tried a lever and hope it apllies.

so i tried to abuse a leaver system for this and I am not really sure if its apliable

(W·sinβ · X)/L = F · (here i should somehow put α, but I am not really sure how)

ok, i can't really think of more, or any other solution. so I am just going to sit in corner and cry in shame when this gets thrown our of table.

 

[Nidum]

Your inset picture of the lever with the red triangle pivot shows thinking in the right direction .

Can you devise a practical way to use springs instead of the weight ?

 

[Nidum]

You may need to deliberately introduce some friction to ensure that your stand is stable in all configurations .

 

[Cutlynx]

Ok. I made a mistake at the lever picture so here i post repaired version

While the X part of lever reaches up to the weight, L part of lever reaches only up to the force that is applied in form of spring. the way of force is only at 5,4 degrees in this scenario so most of the force is not used in the direction we need to.

you said earlier you wanted diagram of all the forces acting in the arm, and by last picture you didn't seem to be entirely satisfied. so i created some to maybe point at some flaws in my logic.

First i created this picture that represents the inner tension of the construction itself. the last part creates small lever by itself that can't rotate in any axis so it creates tension by pulling all upper parts and pushes on all lower parts

This is the weight distribution of arms. The red arm starts from pivot point B and have to hold weight from the monitor.
The blue arm is a bit trickier. Its pivot is point A and needs to hold the red arm along with the weight.
This means if the joints in red arm would be stuck the blue arm would move to the right, even if its currently angled to the left.
So the leverage of blue arm includes the red arm as well.

Which brings me to situation I've found:

This is representation of how much can upper arm move while lower arm stands still. This creates difference on leverage on the lower arm to which spring can't be adjusted.
As i described in post earlier force can be changed with the length of spring, but when the lower arm is not moving on its own i can't see a way that spring can compensate for this.

i guess this is why you advised me in last post to create some friction.

This is my new middle joint i created. made of square tubes. Its rigid, cheap and easy to make joint. So first I am trying to create most simple solution to friction.
If there isn't enough friction tighten the screws some more!

Currently there are M10 screws in my blueprints, so they could withstand any pressure i put on them.
But will it create enough friction? I have no idea.

Can you devise a practical way to use springs instead of the weight ?

Im not sure what you mean by that. You want me to try other types of spring-arm mechanisms?

Especially in this type of arm the spring is there to compensate for the weight

 

[Tom.G]

Search for magnifying lamp in your favorite search engine. Google even shows some illustrtations.

You will notice they have adjustable friction at joints. I recommend you use knobs for friction adjustment, not just nuts. They need periodic re-adjusting.
Also, the joints will need interior support at the friction adjusting points.

 

[Cutlynx]

ok, i tried to find a appropriate handle but non are in length i need. i need 80mm long one and maximum i could find is 70mm
http://www.aliexpress.com/item/10mm...97782.html?spm=2114.10010108.1000014.2.PdjinS.

if i put simple screw on one end and on other one female handle, the screw would probably turn with the handle right?

and if i understand the interior support should be put there, because the square tubes could deform under the pressure like in picture above?

i could incorporate this thingy at the end of the arms to increase the friction. but the making of the project would get uncomfortably harder

 

[Tom.G]

if i put simple screw on one end and on other one female handle, the screw would probably turn with the handle right?

Probably not with a lockwasher under the screwhead. Or you could use a carriage bolt, although then you need a square hole. Also the handles I'm familiar with have a threaded brass insert that is also the clamping surface. That should reduce the thread friction a little bit.

For the internal support, I was thinking spacers (piece of tubing) to snugly fit from side-to-side in the square tubing to support the walls.

A long handle will require a large clearance to the monitor. How about using a fluted knob style instead?edit: added last sentence

 

[Cutlynx]

well i used the handle so i can easily tighten the screw a lot more if needed. i didn't paid much mind into it because there is a lot of space aside of the last joint.

the spacers can be a bit difficult. i didn't mention that but my workshop is a bit limited for this project. all i have is jewellery equipment, which kinda isn't meant for such project as this. by doing this project i will have to scratch-draw the blueprint on the metal and drill it and file it by hand. so if i would like to do exact hole for spacer, it would take me a lot of work. so i try here to do a really easy to make model, but it there is no other way of course I am going to do whenever it takes. i really want this project to be build and working

how about just soldering squared metal plate to the end of the squared tube? it would give it some more structure integrity. i could probably solder some scrap metal in the middle of the other side of screw hole.

 

[Tom.G]

and if i understand the interior support should be put there

Yes, in each of the tubes. The tubing and hole size don't have to be a perfect match. The tubing must be large enough to get the bolt thru yet small enough that it supports the nut or handle.

how about just soldering squared metal plate to the end of the squared tube?

Well that would help but it seems like more work than a spacer.

You could put knobs/handles on both ends of the threaded rod. Then you don't have to worry about the bolt head turning.

Maybe we should invite some mechanical folks to evaluate the torque and clamping force needed on the joints (I'm electronic w/ mechanical I've picked up over the years).

 

[Cutlynx]

Well that would help but it seems like more work than a spacer.

ok when i will start building it, i will first try the spacer and see how much work it will take

You could put knobs/handles on both ends of the threaded rod. Then you don't have to worry about the bolt head turning.

but the problem is i didnt find any male knobs that would have needed length. so first i will try just classical screw and if it won't work, i will go with carriage bolt.

Maybe we should invite some mechanical folks to evaluate the torque and clamping force needed on the joints

i don't really think its needed. like my half-assed project isn't really that spectacular to invite every specialist from this forum to discuss the best way to recreate a spring-arm
like if someone else wants to help, i don't mind, but definitely don't want to actively seek other people for this minuscule project
but you both have my big gratitude Tom.G and Nidum for helping me up to now.

what i believe is needed now is to somehow calculate the force of spring with the weight and then figure out how much friction will have to be there to handle the excessive weight under the various arm-configurations. i need some more simple equation how to solve the forces that are running there and and then i can figure out by trial and error what is the best point to put spring there
(by that i mean i would put a random points for spring into the bluprint and calculate it, replace the points again and again until i find the best position)im really sorry I am asking you to figure out the equation for me, but I am unable to figure it out by myself as you can see in my previous posts. and i cannot really progress without it

 

[Nidum]

The springs fitted as shown are almost parallel to the arms so they can never be very effective .

You need to devise an improved geometry . This probably means putting the springs external to the arms .

Think about which direction you would need to pull the arms by hand so as to provide an effective balancing force for the monitor weight and then try to draw the force diagram mentioned earlier .

Have another look at my post #9 .

 

[Tom.G]

...i didnt find any male knobs that would have needed length.

I was thinking two female knobs with a threaded rod between them (or a bolt with the head cut off.)

 

[arydberg]

Buy a bag of elastic bands. Make weak springs by attaching 3 or so rubber bands together. Use a bundle of these strings of 3 until the force is what you want. Then measure the elastic bands and match it with a coil spring.

 

[Cutlynx]

Think about which direction you would need to pull the arms by hand so as to provide an effective balancing force for the monitor weight and then try to draw the force diagram mentioned earlier .

ok ill work on that. it might take me some time to calculate that

I was thinking two female knobs with a threaded rod between them (or a bolt with the head cut off.)

this genius idea is awesome. already looked it up, and it will same me lot of bucks (the female ones are much cheaper)

Buy a bag of elastic bands. Make weak springs by attaching 3 or so rubber bands together. Use a bundle of these strings of 3 until the force is what you want. Then measure the elastic bands and match it with a coil spring.

thank you for replying. i might use that idea as last resort when if i manage to screw the maths. but use non-elastic rope and some hanging weight. through if i manage to screw it in first place there's large possibility i won't find the right spring for the forces at certain leghts i need.