Dr J. McQuaid
The No 2 hammer, illustrated in the sketch, is a helve hammer.
The helve, or main beam of the hammer, rotates freely about the fixed pivot. The blow to the component being forged is delivered by the tilting action of the helve, as compared to the linear free fall of the tup of a drop hammer. A question that has often been posed is: 'How does the Wortley No 2 hammer compare in "size" with drop hammers?'.
Those familiar with drop forging are accustomed to sizing drop hammers as 1 ton or 5 ton or whatever. This measure of the size is simply the weight of the tup. The total weight of the helve of No 2 is about 6.4 tons. It is obvious, I think, that the hammer is not equivalent to a 6.4 ton drop hammer. A lot of the weight is concentrated near the pivot, a long way from the hammer face. on the other hand, the weight at the hammer face alone is only about 0.36 tons. Again, I think it is obvious that the hammer is not equivalent to a 0.36 ton drop hammer because some of the weight of the helve will contribute to the blow.
So what is the answer? Somewhere in between the two figures would seem a good guess, but what value in between? Let us look again at the explanation of the 'size' of a drop hammer. We would expect that two hammers of the same size would have the same forging capacity. In other words, the blow of each hammer would produce equal permanent deformation of the component being forged. The amount of deformation depends on the energy delivered by the blow. So drop hammers ought to be sized according to the energy delivered. This energy depends on the height of free fall of the tup as well as on its weight. In practice, the height of free fall does not differ very much from one drop hammer to another around 3ft is typical. So it is convenient, though not good engineering, to quote the size of a drop hammer as simply the weight of the tup. It is not surprising that we did not get very far in looking at the weights of different parts of a helve hammer to answer our question.
We are now able to rephrase our question more precisely as: 'What size of drop hammer will deliver the same energy at each blow as the Wortley No 2 hammer?,. In order to tackle this question, we need some knowledge of dynamics. I have done the analysis and taken measurements of the No 2 hammer in order to get the answer. I will confine myself to explaining in words what is involved rather than giving the equations. (I can supply these to anyone who is interested and a copy has been placed in the Society's archives.)
The first step is to convert the usual size convention for drop hammers into energy terms. A free fall height of 3ft is taken as typical. The energy in question is the kinetic energy, or energy of motion, at the moment of impact. This is equal to the work done by the force of gravity on the mass of the tup as the tup falls through 3 ft. The actual energy is simply the weight of the tup multiplied by 3 ft. Hence, for example, a 1 ton drop hammer delivers 3 ton ft of energy at each blow. (I will not complicate matters by distinguishing between mass and force. Readers with a grounding in these matters will understand the difference.) So much for the energy associated with a drop hammer of a given size. For a helve hammer, the relevant energy is the kinetic energy of the rotating mass, in other words the mass of the parts that are in motion at the moment of impact. The complication is that the different parts are moving at different linear velocities. A bit of the helve near the pivot will be moving more slowly than a bit near the free end. So its energy will be less and it will therefore contribute less to the total energy of the blow, which is what we want to find. It is like a flywheel effect; at a given speed, you can store more energy in a given bit of the flywheel the further you move it away from the axis of rotation. Hence, an effective flywheel has a thick rim and slender spokes. An effective helve hammer would have a lot of the weight at the free end and a slender helve, except of course that the forces generated by the impact would shatter the helve, just as the sudden arrest of a flywheel would shatter the spokes!
So it is not just the weight of the helve but also the way it is distributed that matters, as far as the total energy is concerned. This weight distribution effect has to be taken into account. It is done by finding the centre of gravity of the helve. This is the position along the helve at which you could put a knife-edge and the helve would balance on it if the fixed pivot was taken away. It is, of course, easier to calculate where it is than to carry out that experiment!
The analysis, which can be done in several ways, shows that the total energy of the helve is the total weight of the helve multiplied by the free fall height at the hammer face but also multiplied by the ratio of the distance from the fixed pivot to the Centre of Gravity, to the distance from the fixed pivot to the hammer face. This all fits in nicely with the picture presented earlier. If most of the weight is near the hammer face, the ratio is nearly one and the hammer is very like a drop hammer. If most of the weight is near the fixed pivot, the ratio is very small and the hammer is useless.
Figures can now be given. I have said earlier that the total weight of the helve is about 6.4 tons. The Centre of Gravity of the helve is at 24.2 inches from the fixed pivot and the overall length of the helve from the fixed pivot to the centre of the hammer face is 102.5 inches. The free fall height of the hammer face is about 13 inches. Remembering that the free fall height of the tup of a drop hammer is taken as typically 3 ft, the calculation gives the answer that the 'size' of the Wortley No 2 hammer is 0.55 tons.
The hammer is thus estimated to have a forging capacity equivalent to that of a 10 cwt or thereabouts drop hammer. I am told by those knowledgeable that this sounds about right, given the kind of work that the hammer was used for in its working life. An interesting facet of the design, pointed out to me by Gordon Parkinson, is that the weight distribution of the helve seems to be just about right to avoid a reaction at the fixed pivot as the blow is delivered at the hammer face. This would indicate either a remarkable grasp of engineering by the millwrights of long ago or careful adjustment of helve hammer design in the light of experience (or possibly just good luck). It is something worth exploring further and I shall return to it in a later issue.
The Cutting Edge - No.7 - 1991