Lesson 2 - Materials

Designing for disassembly has several benefits. It can make it easier for your product to be repaired or upgraded, thereby prolonging its useful life. It can also help ensure your product is recycled and enable whole components to be reused. In fact, the degree to which your product can be disassembled easily often determines how the product will end its life.

Tactics for Designing for Dissassembly and Recycling

Designing for disassembly involves some straightforward tactics. For example:

· The fewer parts you use, the fewer parts there are to take apart.

· As with parts, the fewer fasteners you use, the better.

· Common and similar fasteners that require only a few standard tools will help to simplify and speed disassembly.

· Screws are faster to unfasten than nuts and bolts.

· Glues should be avoided.

· Building disassembly instructions into the product will help users understand how to take it apart.

For a more complete list of strategies, see the Design for Product Lifetime Quick Reference Guide.

To help ensure your product is recycled responsibly, you should design it so that an e-waste recycler can easily process its parts. The recycler should be able to remove the valuable metals and plastics without letting any toxins escape, and extract components that shouldn’t be shredded.

Using the same line of thinking, you can also design your products so that whole components can be reused rather than just materials. This is known as remanufacturing.

Digital Prototyping Can Help You Design for Disassembly and Recycling

Autodesk Inventor Publisher can help you visualize and communicate how designs are assembled and disassembled. It can be used to create interactive documentation to ensure your design can be properly repaired, upgraded, and disposed of. See an example of how the Stanford Bloom Laptop team used Publisher to help communicate the design of their laptop which is easy to disassemble and recycle.

Products like electronics have components that can fail, or need to be upgraded, well before the rest of the product needs to be replaced. Millions of pounds of electronics are scrapped every year. Repair and upgrade can address this e-waste problem by extending your product’s useful life and slowing down the rate of disposal.

Repair and upgrade is easier when your product’s components are easier to disassemble and access. It’s most important to allow easy access to the components most likely to break or need maintenance. This isn’t a new idea: until recently, almost everything was designed to be repaired. Materials and energy are used to make each thing we manufacture, and so making things last longer has a real environmental benefit.

No matter how easy your product is to repair, it’s hard to keep it from becoming obsolete as new technologies roll out. You can intervene by designing your products to be easy to upgrade. One key approach is to use a modular product architecture that makes it easy to swap out components. This gives users flexibility and allows for product extensions you can’t even see coming yet.

Clear instructions are very helpful, but it’s also important to think about how someone could make sense of your design without detailed instructions.

d. Design for Repair and Upgrade

Products like electronics have components that can fail, or need to be upgraded, well before the rest of the product needs to be replaced. Millions of pounds of electronics are scrapped every year. Repair and upgrade can address this e-waste problem by extending your product’s useful life and slowing down the rate of disposal.

Repair and upgrade is easier when your product’s components are easier to disassemble and access. It’s most important to allow easy access to the components most likely to break or need maintenance. This isn’t a new idea: until recently, almost everything was designed to be repaired. Materials and energy are used to make each thing we manufacture, and so making things last longer has a real environmental benefit.

No matter how easy your product is to repair, it’s hard to keep it from becoming obsolete as new technologies roll out. You can intervene by designing your products to be easy to upgrade. One key approach is to use a modular product architecture that makes it easy to swap out components. This gives users flexibility and allows for product extensions you can’t even see coming yet.

Clear instructions are very helpful, but it’s also important to think about how someone could make sense of your design without detailed instructions.

Digital Prototyping for Design for Repair and Upgrade

The most important things for designing for repair and upgrade is a clear design intent and an understanding of user behavior. But once you have a good design, Autodesk Inventor Publisher can help you communicate to your users how to repair, service, or upgrade your product. See an example of how the Stanford Bloom Laptop team used publisher to help communicate the design of their disassemble and recyclable laptop.

Every pound of material that you save in your product saves much more waste and material upstream. Material Inputs and Ecological Rucksacks are closely related concepts that provide a tangible short-hand for understanding this larger ecological impact of the products and materials around us, and can help understand the importance of lightweighting.

The answer: it’s been lightweighted. And you can do this too, beginning with a few simple strategies: Hollow parts and thinner walls, spot reinforcement, reinforcing posts and ribs, corrugation, trusses, and gussets.

Here’s how the beam is being stressed. The arrows show the amount of force at different points.

As you move toward the center, there’s less and less force. In the middle there’s no force at all. So there is no need for material there.

But how much material can you remove without the beam bending too much under the load? Here’s a simplified form of the equation.

Bending increases as the CUBE of the length between supports. It decreases linearly with the beam’s width and with the CUBE of the beam’s height.

So the length between supports and the height of the beam make HUGE differences.

Now if you hollow out a solid beam it bends more, but you can play with length and height to avoid the extra bending. The math for a hollow beam is the same as a solid beam with the hollow space subtracted out.

Since the length and height variables are cubed, they make a much bigger difference than the hollowness, so you can get the same stiffness using much less material.

For example, if you make the beam 50% taller, you can make it 80% hollow, reducing materials by 70% and still get a 20% stiffer beam! Higher performance with less material.

You can use tools like Autodesk Inventor to test the impact of these types of design changes. They have the math built-in.

That’s one of the reasons why, in spots like these, the tubes are welded to solid plates that are much stronger.

If the part were injection-molded or cast, it’d be easier to design it to be thicker in some places and thinner in others. Either way, reinforcing a few spots like this with thick material lets you use much less material elsewhere.

When designing a product, use your engineering intuition to find these points of stress and determine if they are tension, compression, torsion, shear, or a combination. Tools like Autodesk’s Inventor and Algor can help.

Posts, Ribs & Corrugation:

But what if your product is already hollow, like a laptop or the body of a car?

Well, you can still lightweight it by making the walls thinner.

Of course, this will reduce its strength and stiffness. But remember the bending equation? The top of your hollow part is like that beam we were talking about.

Bending increases as the cube of the reduction in height, or thickness. But bending also decreases as cube of the unsupported span decreases.

Instead, you can strategically place ribs, not connected to anything at all, to reinforce the surface. The thickness, or height, of the rib makes the surface much stronger there. You can even lightweight the ribs, making them hollow!

Ribs are great, and pretty easy to add to injection molded or extruded parts. But if you're manufacturing from bent sheets of metal or plastic, extra wall thickness means extra parts with extra welds or fasteners.

There’s another way to approximate solid ribs in a bent sheet: corrugation. By folding the material, you stiffen it just like with a post or rib. That’s what you see in lightweight soup cans and cardboard.

Trusses:

This is great, but we can go even further by replacing solid walls altogether with trusses, which are arrays of thinner beams.

Look again at the beam we hollowed out earlier. What happens if we hollow out the walls too? We reduce material use enormously. But right off, you can see we’ll need to put in posts to avoid long unsupported spans.

Also, this extra hollowing has made the part much less able to resist shear forces--which push parallel to the face of the object. We can think of the edge that’s resisting the shear force as a separate beam, with the top end unsupported.

To support it, we can anchor a diagonal beam to the opposite corner. Similar to what we did with the post.

With this diagonal beam, the shear force effectively becomes a compressive force, directly fighting the strength of the material.

Adding another diagonal beam connecting the opposite corners also helps by resisting that shear force in pure tension and also by resisting shear forces from the other side.

A diamond-frame bike is a truss, where the tubes are resisting a number of forces from different directions. The individual components usually experience stress from the rider’s weight as compression or tension, rather than bending.

Trusses are usually made by welding or fastening smaller components together. But they can be made in other ways. Corrugated cardboard is a truss in two dimensions.

You'll see trusses in bridges, roofs, the Eifel Tower, any structures that need to be stiff but light.

On this mountain bike, a gusset is welded into this joint for when impacts on the front wheel exert shear and bending on the frame.

Conclusion:

The kinds of strategies we've just looked at: hollow parts, spot reinforcement, posts/ ribs and corrugation, trusses and gussets have allowed great leaps forward in lightweighting.

Combined with advancements in material science, these have allowed bicycle designers to reduce the weight of their products by more than half in the last hundred and fifty years, at the same time making them much stronger.

Designing sustainably means doing this everywhere, reducing materials use and improving performance. PDF File: LW1_VideoScript.pdf (view)

f. Lines of Force and Stress Concentration

To keep your products strong, even as they get lighter, you'll need to learn how to follow lines of force and avoid stress concentrations.

Following Lines of Force

Following lines of force is a nice non-mathematical way of finding where you can remove material. You can even just do it on paper before you get into your detailed design.

To start, think about how external forces will be acting on your design, draw them as lines, and then create designs that remove material in a way that matches these lines. That's all there is to it. Let me explain how.

Have you ever noticed that folding bicycles often weigh as much or more than normal bicycles, even though they're smaller?

They often have wider and thicker tubes than a diamond frame bike because their frame geometries are less optimized to where forces are being exerted. If you draw the force lines, you can see this.

It doesn't matter what manufacturing method or material you're using. Drawing these lines of force helps you see simple ways to strengthen your product.

Following Lines of Force (within a part)

Now let's visualize how lines of force will travel within a specific part.

Here's a picture of a corner in a flat plate under tension, gripped at the top and bottom. Drawing the lines of force shows that the corner of the part isn't experiencing any stress. So you could cut it off in a nice smooth curve following the force lines, saving weight without reducing strength.

Drawing lines of force will also show you where stress concentrates. And that's good because avoiding stress concentrations is our next key strategy.

Avoiding Stress Concentrations

Stress concentrations are locations where forces go from being well-distributed to being focused. These are usually the first places to break, so avoiding them or reinforcing them will let you use less material in your parts

Here's a drawing of a solid beam that's being pulled on, with arrows going through it to show the force lines -- they're evenly distributed, so the stress is low everywhere.

Now let's get fancy here and see what happens if we cut out a hole. The area in the middle can't support any forces, so they have to go around. The force lines are denser in the material around the hole, and the stresses there are higher. This will certainly be where the part breaks, so those regions need to be reinforced, while you could make the part thinner elsewhere.

Here's the same part with the same stresses, shown in Inventor's stress analysis environment. The stress concentrations are color-coded, showing the same results as when we drew force lines, but quantified so you can see if it's too much.

Stress concentrations around holes are why you should be careful when you connect flat parts together with bolts or rivets. Ever wondered why you use washers with bolts? Part of the reason is to help distribute stress and add reinforcing material, so the rest of the material can be thinner.

You can also distribute the load more evenly with a lot of small bolts or rivets rather than a few big ones. You can see this on airplanes and on the Golden Gate Bridge.

Now, if the hole you've made is a flat notch, the stresses become even higher. Stress concentrations are determined by both the size of the hole and its radius, so the sharper the curve, the more it concentrates stress.

The math for this is difficult, but using finite element analysis in Autodesk Inventor or Algor can help you find the location and magnitude of these stress concentrations.

Some manufacturing methods can cause notches in a part. Like welding plates.

A weld joint that doesn't go all the way through the material is like having a notch cut out of it. A nice fat weld joint with deep penetration is better, because it both decreases the size of the notch, and also increases the effective radius of the joint.

That's right, joints and corners also cause stress concentrations, not just holes.

You could deal with these stress concentrations in two ways: either by reinforcing, or by rounding the corners.

It again depends on your manufacturing method. With a steel tube frame, you can't round the corners much, so you should reinforce with lugs or thick weld beads.

Curves are easy to make with injection-molded or forged parts, or extruded parts in 2 dimensions. They can even be done in sheet metal, through processes like hydroforming or deep-drawing.

This [aluminum can] is actually an amazing feat of lightweighting that uses deep-drawing to turn a flat metal sheet into a highly optimized curved form.

With a carbon fiber bike, you have even more control over how the material is sculpted, so you can make the joints gentler curves. Remember, the bigger the radius the less the stress concentrates. Nature avoids corners too. Look at bones or trees -- they usually have gradual curves and radiused corners.

Swoopy curves to make your products look more like nature, that's nice aesthetics. But sweeping curves to avoid stress concentrations, so you can make products both strong and lightweight, now that's hot!

g. Tensegrity

By now, you're probably seeing things everywhere that have been lightweighted.

We've talked a lot about a bike's frame, but what about the wheel? Did you ever wonder why bike wheels have thin metal spokes? Another lightweighting strategy. But how does it work? Well, let's find out.

So far, we've seen how stress can be the enemy when trying to lightweight. Too much stress, too little material, you've got problems. But not all stresses are created equal. Sometimes turning compressive stress into tensile stress can help you reduce material use enormously.

Buckminster Fuller called the strategy of using tension for structural integrity - tensegrity. It's great when working with materials whose strength in tension is similar to their strength in compression.

Anything that's resisting compression forces is resisting buckling. Compression can buckle your product long before the strength of the materials fail. The longer and skinnier your parts are - like spokes - the more likely they are to buckle and the more you could lightweight them by using tension instead of compression.

How big a difference can it make?

In this sculpture, the rods and cables are under the exact same amount of stress, but the much lighter cables are in tension while the rods are in compression.

Here's a simplified version of the math for buckling strength: The stress at which a column buckles is inversely related to the SQUARE of how slender the column is, so making a column three times more slender makes it nine times more likely to buckle. By contrast, a part under tension doesn't need to worry about this buckling equation; if it fails, it will fail at the yield strength of the material.

We can't exactly say how likely ANY column is to buckle under compression before it breaks in tension, because that depends on the material's elasticity.

Take aluminum, which is more elastic than steel. A cylindrical, aluminum rod that's 30 times taller than it is wide (like a thick spoke in a wheel) has about 2 and a half times more strength in tension than it does in compression. So an aluminum cable in tension could use 60% less material than a rod in compression.

but modern bicycles use thin steel spokes that are being pulled on rather than pushed on.

The weight of the rider at the axle is pulling down on the top of the wheel. So tension is being substituted for compression. If the spokes were being pushed on, they would certainly buckle, because they are 150 times as long as they are wide.

How do you make things that use tensegrity? It's easy, you can do it with almost any manufacturing method. That's why you'll see tensegrity all around you if you look for it. Old sailing ships used ropes between their masts to create tensegrity trusses, so they could use the biggest sails and capture the most force from the wind without snapping.

Suspension bridges like the Golden Gate Bridge also do the same thing, with the deck of the bridge hanging from cables off of the tall spars.

What limits tensegrity is mostly the materials you use. It's great when working with materials that have good strength in tension. But brittle materials like glass or ceramics--they don't work so well.

Sometimes manufacturing methods can make a good material too brittle for tensegrity. Casting can do this, so cast metal doesn't work as well as if it were rolled or forged.

But other manufacturing processes improve the tensile strength of your materials, like drawing wire and braiding the strands into a cable, or tempering glass.

So that's how the spokes work. But something else on the wheel is using tensegrity too. Can you tell what?

Bicycle Tires

The tire. Rubber doesn't resist buckling well, obviously, but the air pressure inside the tire pushes outwards in all directions. The weight of the bike squashing the tube and tire against the ground creates a tension stress on the inflated walls of the tire. This is how a thin sheet of rubber can support your whole weight as you bounce over bumps.

All inflated and fluid-pressurized structures work this way. The trick is finding a material that's rigid enough and won't leak.

Aluminum cans are actually a good example. Their internal fluid pressure adds to their structural integrity and allows them to be stacked. And aluminum is a material that's both rigid and doesn't leak. But sometimes you'll have to combine plastic, rubber, or other materials. That's why the innertubes of bikes are separate from the tires and why car tires have steel belts embedded in the rubber.