Putting heads

14.4.5 Putting heads

⊲ 14.1⊳ Although a set of golf clubs may only contain one

putter, it is typically used for 40–50% of the strokes where m is the weight of the arrow and k is the ‘virtual in a game. The dynamic demands are less than those

weight’ of the particular bow. The constant term k for ‘woods’ and ‘irons’, consequently putter designs

treats energy losses in the bow as an extra burden on have tended to be less innovative. Design parameters

the driven arrow, travelling with the same velocity. include ‘sweet spot’, weight distribution, bending stiff- ness, etc., as previously. Alloys used include stainless

arrow mass. Thus, for a certain yew bow ⊲k D 23.5 g⊳, steel (17Cr–4Ni), manganese ‘bronze’ (Cu–Zn–Mn)

increasing the arrow weight from 23.5 g to 70.5 g and beryllium–copper (Cu–2Be).

increased the efficiency of the bow from 50% to 75%. Bow materials are often compared in terms of spe- cific modulus of rupture and specific modulus of elas- ticity. Thus, for wooden bows, timbers which combine

14.5 Archery bows and arrows

erally preferred as they provide lightness, the necessary

14.5.1 The longbow

resistance to bending stresses and a capacity to store energy. The fine-grained hardwoods ash and wych elm

For centuries, archery bows have combined design meet these criteria. Although nominally a softwood, skill with knowledge of material properties. From

yew was favoured for longbows, its very fine grain the evidence of many well-preserved yew longbows

giving remarkable bending strength. Strain energy per retrieved in 1982 from the wreck of the Tudor war-

unit volume can be derived from the stress v. strain ship ‘Mary Rose’, which sank in Portsmouth harbour

diagram and expressed as 0.5ε 2 E . It follows that max- (1545), we know that the original seasoned stave was

imizing strain ε (below the elastic limit) is an effective shaped in such a way as to locate sapwood on the outer

way of maximizing stored energy. convex ‘back’ of the bow and darker heartwood at the

There are two main types of modern bow, the stan- concave ‘belly’ surface. When the bow was braced

dard recurve (Olympic) bow and the more complex and drawn, this natural composite arrangement gave

compound bow. In contrast to the D-section of the the greatest resistance to the corresponding tensile and compressive stresses. Considerable force, estimated to

3 Skeletal remains of an archer, taken from the same

be in the order of 36–72 kgf (80–160 lbf), was needed shipwreck, indicate that a lifetime of drawing the longbow produced permanent physical deformation. This powerful

weapon was developed in conflicts in the Welsh Marches; Vitreloy or Liquidmetal, developed at Caltech, now

its ability to penetrate plate armour had both military and produced by Howmet Corp. Greenwich, CT-06830, USA.

social significance.

412 Modern Physical Metallurgy and Materials Engineering

Figure 14.4 Modern competition bow, compound-type: laminated upper limb (wood, gfrp, cfrp) and CNC-machined central riser (Al–Mg–Si alloy 6082) (courtesy of Merlin Bows, Loughborough, U.K.) .

Tudor longbow, limb sections of a recurve bow are wide, flat and thin, giving resistance to twisting. The energy efficiency of a compound bow (Figure 14.4) is twice that of the longbow and, as a consequence, can propel an arrow much faster, at velocities of 90 m s or more. Pulley cams at the ends of the two limbs sus- tain the load at full draw during the sighting period of 10 s or more. The most powerful compound bows use light alloys for the mid-section (‘riser’ or grip) e.g. forged Al alloy, diecast Mg alloy. Laminated wood is used for some bow grips, e.g. maple plus rosewood. The two limbs are very often laminated in construction and are much less susceptible to temperature change and humidity than wood alone. Many different material combinations are used for laminae, e.g. CFRP, wood, GRP, foam, etc. For instance, in one type of compos- ite bow limb, facing and backing strips of GRP are joined to each side of a thin core strip of maple with epoxy adhesive. Tubular alloy steel limbs have been superseded as they were prone to internal aqueous cor- rosion: sudden fracture of a drawn bow (or its string) can be extremely dangerous.