3.15

Skip to 0 minutes and 9 secondsThis washing line folds down against the wall when it's not in use, to save space. A new model is being designed. It will be bigger and stronger. Your task is to design some of the connections. You will specify the wall anchors that secure the assembly to the wall at A and B, and the pin connection at C. This is an engineering drawing of the proposed design. It is in third angle projection, so you need to rotate your head 90 degrees to the left to see the end view in its usual orientation. Dimensions are in millimeters. Here is the end view in a more familiar orientation, showing the wall anchors and the pin that you will specify.

Skip to 1 minute and 3 secondsWhat load should we design for? Here is one possible set. The weight of the frame is straightforward, once you know the material. This was calculated for steel rectangular hollow section. The weight of the strut can be found in a similar way. The weight of wet washing was less clear. This is estimated for a washing machine load of eight kilograms dry, but soaked by rain. The child hanging off a corner is based on life experience. While there will be a warning against hanging of the frame, something like it might still happen and we'll aim to make the frame strong enough to take it. This assumes that the wall won't fail. You might like to explore this in the discussion.

Skip to 2 minutes and 0 secondsThe washing line is clearly three dimensional. So how can we use our two dimensional analysis? Well, we can use symmetry to find the loads on each side frame, half each, and then add a child on the end, assuming that the child hanging off the corner is the worst case. This might be a topic for the discussion. To find the pin force, we'll need to reveal it with an FBD. Pause the video, and sketch what you think is needed.

Skip to 2 minutes and 46 secondsHere's the answer. A is represented as a pin joint with two components. C is a pin joint, too, another two components. The other arrows are known loads. But we've got four unknowns, and only three equations. That's our three equilibrium equations. We need another one. We'll try an FBD of this strut. Pause the video and see if you can sketch it.

Skip to 3 minutes and 33 secondsHere it is. Notice that by Newton's third law, the forces at C are equal and opposite to the ones on the previous diagram. How does this help? If we take some of the moments about A on the first diagram, we will get an equation with only one unknown. That is, Cy, the vertical force at C. This is a common trick, choosing a moment axis that eliminates the forces acting there. It's 0, because of a zero momentum. If we now move to the strut and take moments about B, we will eliminate the forces at B from our equation. And we'll get an equation with only Cx as an unknown. We just found Cy.

Skip to 4 minutes and 29 secondsNow we can combine our two components to find the total force at C on the pin, in the side frame. We can use horizontal equilibrium to find the pull out force on the top wall anchor. That's force Ax.

Skip to 4 minutes and 48 secondsAnd vertical equilibrium gives us the vertical force, Ay.

Skip to 4 minutes and 54 secondsNow have a go yourself at implementing this analysis, and put the numbers in. The screen should give you all the information you need. Why not pause the video and try it?

Skip to 5 minutes and 19 secondsHere are the answers.

Skip to 5 minutes and 23 secondsNow we'll select a wall anchor. For resisting pull out, size 8 millimeter looks suitable. Typically, manufacturers of folding clothes line specify about 10 millimeters, which will allow higher loads and give a greater margin on the quality of the wall.

Skip to 5 minutes and 46 secondsThe shear load at the wall is complicated by the wall plate, but it is well within the capability of the wall anchors, as can be seen by comparing the total vertical load with the capacity of even one anchor. The bottom anchor at B is more lightly loaded, but we'll make it the same size as the one at A for convenience. You still have to decide if the wall is stable enough. You don't want to pull it over. Now, we'll look at selecting a component for the pin. You need to consider the various ways the pin joint could fail. We'll consider shear. The two diagrams are cross sections through a pinned joint.

Skip to 6 minutes and 35 secondsThe compressive load causes the pin to be chopped through in two places. This is double shear. The maximum permissible load on the pin is given by the maximum allowable shear stress. Shear stress is shear load over area. Units are Pascals, which is Newtons per square metre, or kiloPascals, or even MegaPascals. So we'll need to decide on a material strength and a factor of safety. We have included a factor of safety and our permitted stress, so we don't need to include it again. With this material, it looks like a 4 millimetre pin would do. The pin on a similar strut in a typical rotary clothesline is about this, but you would need to check for other considerations.

Skip to 7 minutes and 30 secondsYou might choose a lower strength bolt of greater diameter. Can you think of other ways the pin joint could fail? To finish this task, post your ideas on the discussion.

Design a hinge pin for a folding clothes line Part 1

This sets the scene for your design task. You will design a hinge pin and wall anchors for a folding washing line.

The video then leads you through the process with opportunities to do calculations yourself involving Free-Body Diagrams and equilibrium in two dimensions. In the end you will select items from a list.

It might help if you download the design specification in the Downloads section below in case you want to refer to it as you go.

If you just watch the video it will take about 8 minutes. If you take the opportunity to do calculations it will take longer; it’s hard to say how much longer because it depends on so many factors, but allow a total of 30 minutes.

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This video is from the free online course:

Through Engineers' Eyes: Engineering Mechanics by Experiment, Analysis and Design

UNSW Sydney