Design is not quite the same activity as inventing. New design elements can come from scientific discoveries, from patient experimentation, from chance observations relevant to a current need. Examples of each are the transistor, the electric light bulb, the Bessemer converter and the Mannesmann tube-piercing method. In the case of the electric light bulb the design concept came first, derived from logical use of scientific principles, followed by tests on likely filament materials to find which had the most suitable combination of properties.
One of our first duties in design is to make things strong enough. Accepting this, the most efficient structure is that in which the material is strategically placed so as to give both the lightest structure for a given load and permissible stress level and also the stiffest structure for a given weight.
A major source of confusion for students is the expression ‘safety factor’. It may be of some use in a qualitative sense, indicating which factors in a problem tend towards safety and which towards danger of failure. The trouble starts when attempting to give it a numerical value. In design practice there is no single value for assessing safety margins, as can be seen from the examples below.
Most practical systems are required to be stable, at least dynamically. Consider a bicycle; it is statically unstable, yet a small child can learn to ride it. How is intrinsic stability obtained? There is a popular notion that the head angle confers stability, yet early bicycles had upright steering heads. A little thought shows that sloping heads actually tend to de-stabilise. Under gravity the load and earth try to come together. Relative to the bicycle, the ground point P as defined in figure 26 tries to come upwards from the lowest position at straight ahead, which it does by rotating the steering. This is easily confirmed in practice; the equilibrium point comes at a steering angle of 60 to 80° from straight. The stability comes from the trail. If the bicycle leans sideways, the ground-reaction has a lateral component steering the wheel towards the leaning side until centrifugal force restores balance. If the trail accidentally becomes negative, riding hands-off becomes very difficult. Some small-wheeled bicycles were designed on the basis of head angle and swept-forward forks, leaving too little trail.
A prime example of straight-line motion is the lathe bed whose straightness, if parallel with the axis of rotation, generates accurate cylinders. It is usually guided by three surfaces arranged in V-flat formation as in figure 31a, giving unique kinematic location so long as the loads are downwards. Where this is not certain, slides are often of dove-tail shape, with three fixed surfaces, the fourth being adjustable, as in figure 31b. If we provide four rigid surfaces, we are over-located as in figure 31c, which shows in exaggerated form the consequences of slight error such as may occur due to distortion, thermal expansion, etc. The three-surface location is free from this trouble; when the V-slide is symmetrical and wear is even on both sides, the wear causes no slackness and no horizontal error. Note that the cutter position is arranged so that wear of the slides or cutter deflection cause only small, second-order errors in the workpiece.
The designer must obviously know a great deal about materials, their properties and problems. The maker of the object also should be familiar with these; to call for materials new to the maker can produce problems of supply, identification, treatment and storage. So whenever possible the designer should check up on this point and possibly change one or the other, material or maker.
This chapter discusses both permanent and disconnectable joints. The most common permanent joints are made by welding, sometimes with a hot flame, more usually with an electric arc. The workpiece usually forms one of the electrodes; the other is guided by the welder and is either of tungsten, which is not intended to melt, or of filler material which forms part of the joint. Other methods use local heating within the joint region, sometimes combined with high pressure, the heat source being electrical resistance, friction, electron beam, laser light, impact or ultrasonic vibration.
This chapter discusses joints with rotary or linear motion, mostly with sliding or rolling friction. It is, however, convenient to start with a short section on flexure bearings, useful where small movements are involved. In instrument work, crossed-leaf springs form a friction-free and slack-free pivot for angular movements of a few degrees. The arrangement requires at least three springs for symmetry. Any loads which tend to compress the springs must be kept small to avoid buckling.
Vibration gives rise to two main types of design problems: one is that of isolating a body from random external inputs, the other, mounting a vibrating system so that as little vibration as possible is transmitted. The first is the transport (passenger or delicate cargo) problem; the second is the general machine or engine-mounting problem. A subsidiary problem is predicting and stopping odd component resonances. For the first case, particularly considering human cargo, there are some agreed standards of what inputs a person can accept for various lengths of time without undue fatigue. Unfortunately there are two sets of data in existence, sometimes confused with each other, one referring to drivers and passengers in vehicles, the other to people in buildings who have to stand, sit on relatively firm chairs or work on rigid tables. An internationally agreed set of standards for tolerable accelerations is given in Neale50, section D3, but specific body resonances are not included.
Designing to ease manufacture must be based on a knowledge of the processes in existence and more particularly on those available in the firm or by contract, in other words those likely to be used in practice, though the purchasing of plant for a new product is not necessarily ruled out.
