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4.1. Hinges


How To Invent (Almost) Anything > 4. Applied Simple Science > 4.1. Hinges

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We pass through many doors every day but seldom, if ever, pause to look at that wonderful little device that holds the door onto the doorframe. There are many varieties of hinge, using different materials and with many surprising functions.
Let us consider the function or purpose or the hinge. A function of ‘enabling the door to open’ is not really enough to stimulate inventive thought, so if we chunk down into the detail we can separate out the main functions of the hinge.

First, the door is closed, and so one function of the hinge is to hold it closed, and in a security-conscious situation (such as your front door), we may want it to withstand a ferocious physical assault. The door is a seal between two environments and you do not want the wind whistling around the hinges, so these must be a part of the seal.

When the door is being opened, it should not be stiff and the hinges should allow the door to start moving without having to give it an initial shove. The door should not collapse at any stage, so the hinge must support both its weight plus any additional forced placed on it (such as children swinging on the door handle).

As the door moves, it should move smoothly, neither racing away from the person opening it, nor being hard to push. It should also avoid any obstacles such as a carpet. At the end of its travel, we do not want the door to hit anything, like the wall or a table behind it, and it should slow down and stop as easily as it started on its journey.

Opening the door too far should also not result in the hinges being torn from their mounting point. When the door is open, it may be desirable for it to stay where it is put or it may be preferable for it to automatically close. Closing should be as smooth as the opening, and the door should fit snugly into its frame with the catch hitting the strike-plate in exactly the right position so the door fully closes to an exact fit.

And if that is not enough material for invention, an exposed hinge may also have an aesthetic function such that it is pleasing to the eye and matches well with the handle, catch and surrounding furniture.

Fig. 4.1 Some functions of hinges

Having identified what it is supposed to do, we now start the design stage, asking how it should be done. If you want a really radical design then you forget about how you do it now and explore the most abstract level of wish. You might start from the concept of a seal for the doorway which avoids obstacles and consequently invent a rail for the door to run along, with a concertina-type door, or nested sliding doors.

We can think about the hinges in terms of the forces involved. In hanging the door, there is a shear force across the hinges. The door will try to pull out the top hinge, which must hence be well attached to the door frame, and push in the bottom hinge, which thus might be a lighter attachment. Other factors that involve forces were found in the discussion about function. These include the additional weight on the door, security implications and the forces of opening, closing, starting and stopping the door.

The reality for most doors is that you have a lot of compromises. You find that someone fitting the door cannot get the hinges lined up perfectly as they are screwed maybe into wood and the wood gives way a bit so as the door is fitted it sags a bit. In practice, it works best if only one hinge carries the weight of the door. If two or more carry weight then the forces as the door swings tend to work against each other and it is difficult to open. The function of carrying weight tends to be best for the lowest hinge, with other hinges just holding the door neatly away from the frame.

We can also consider the friction involved. As well as the friction in the hinge, we can consider the what is happening in the catch, or even the rubbing of the door against the carpet. Rising-butt hinges that lift as they open have already been invented. How about using electromagnetism in some way? How about creating charges on the carpet and the bottom of the door that repel one another, so the carpet flattens itself as the door opens? Or perhaps a similar charge on the leading edge of the door and an opposite charge on the trailing edge that lifts the pile of the carpet back up again (Wow: we have just invented a device for restoring old and tired carpets! This is how inventions sometimes happen–whilst you are inventing for one purpose, you discover something better for another domain).

Notice how we are looking at the overall problem, not just the actual hinges. Let us return to the hinges themselves. The lower, weight-carrying hinge could use a small type of ball-race or ball and socket joint to carry the weight smoothly, reducing friction. The upper hinges can be of lighter construction as they are just holding the door in place. They may also have some vertical play in them as we do not want the hinges to fight one another.

We can take inspiration from other places, including our good friend, Mother Nature. Nature does not use metal (it requires too high a temperature to create) but it is full of hinges. Natural hinges are made of flexible substances, often fibrous to give them extra strength. But a small bendable hinge would not hold the weight of the door very well. So how about having one long flexible hinge the length of the door? You could even include it in the design of the door, moulding it as a single unit.
We can also look at the fastening method. With a combined door and hinge we could have a long plate that screws to the wooden frame (although we could reinvent this too). There could be then be a simple hook-and-snap mechanism that pulls the door exactly in to fit the frame, as in Fig. 4.2.

Fig. 4.2 A new door hinge?

These ideas are unproven! We have been illustrating ways of thinking here and not designing a finished product. Beyond this thinking process there is often a long process of try-outs and finding ways of economically manufacturing the product, let alone marketing and selling it to the sceptical public.

We can see, however, that it is not too difficult to generate good ideas by simply asking ourselves what functions we want the hinges to carry out, what forces are working for and against us and what type of friction is involved in different places.


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