Physics question, looking for ideal angle.

Replies to This Discussion

Permalink Reply by Piotr on August 14, 2013 at 7:51am

I would say it doesn't matter if the screws are placed between the wires and there is the same distance between the mounting brackets in options 1 and 2.

Permalink Reply by Jon Mirtschin on August 14, 2013 at 8:12am

There's also physical/practical considerations as well as mathematically balancing the forces.  Drilling on an angle is not so easy, and if they already exist cranking the clothes line might be adverse to the longevity of it.

Have you made up the brackets yourself?  I'm sure they are designed to withstand the bending (unless you're hanging abnormal washing) and inclination won't affect the forces on the anchor bolts etc.

Permalink Reply by David Rutten on August 14, 2013 at 11:15am

It's standard metal brackets, with wooden blocks on top where I'll drill the holes for the lines. My main worry is the forces that are transferred from the brackets into the wall. 

It may well be that the effect of angling the brackets is non-existent or insignificant, I just imagine that if I were holding the lines I'd like to be able to lean back. I thought the same might apply to fixed brackets.

--

David Rutten

david@mcneel.com

Tirol, Austria

Permalink Reply by Jon Mirtschin on August 14, 2013 at 11:21am

It will have some effect, but not likely to the impact of an anchor working or not (unless you're really trying to use something with a small safety margin).

Permalink Reply by Frane Zilic on August 14, 2013 at 9:43pm

I am not an expert but here it goes anyway.

From a theoretical point I think option 2 is better. The lines with clothes will exert a diagonal force on the bracket. The angle? I guess it depends on the weight of your clothes (do you wear a high tech second skin or a sweater made of woolly mammoth fur) and it also depends on the initial tension and elasticity of your lines. The ideal situation would be to have the force exerted in the same plane of the bracket.

To get to an optimal solution you shoud also know 

The number and position of the bolts on the bracket

The geometry of the bracket.

Option 3 (impractical I think)

Theoretically again, you could hang the bracket upside down form a single strong bolt at the upper end, allowing them to pivot, so that they can adapt to the direction of the force. 

In practice they wouldn't pivot and/or the corner of the bracket would scratch your wall... If you could put a small strong wheel in the corner of the bracket it could work, but that is another project/design. 

 

Permalink Reply by Frane Zilic on August 15, 2013 at 12:50pm

Option 3 would look like this but with more tension on the wires

Permalink Reply by Emilio Morello on August 15, 2013 at 2:11am

I would add a stick/bar at the end of the brackets (at the side far from the wall),

it would prevent the brackets from (trying to) rotate and stress the wall with higher forces.

Only the weight would remain, and the moment around the horizontal axis, but the brackets's

design is fine for that.

The bar would have to be rigid and light enough though, and the brackets not to be too far away from each other ... in that case another bracket in the middle would help.

Obviously I decline any responsibility about the aesthetic of the proposed solution ...  ;)

Permalink Reply by pb on August 15, 2013 at 1:16pm

A cable structure works (in a good approximation) like this:

If the load q (in kN/m), the horizontal length l (in m) and the sag f (in m) is given, than the horizontal force H = q * l^2/(8* f) and the vertical load V = q * l/2 and the sum-vector of H and V points in the tangent direction of the cable (as zilic mentioned before).

For example:

The wight of the wet fabric q = 4 kg/m == 0.04 kN/m, l = 2 m and you choose f ~ 0.20 m, then

H = 0.04 * 2^2/(8*0.2)= 0.10 kN and V = 0.04 kN

If you reduce f to f ~ 0.05 m (5 cm) then H will become 4 times more > H = 0.40 kN, V = 0.04 kN

The formular is a very good approximation, but it does`nt include the elongation of the cable itself. If you want to be exact, you have to add df (due to the elongation of the cable under load) to f (the sag of the cable without load) and, of course, you have to add the wight of the cable itself to the wet fabric load too.

pbau