GB2320907A

GB2320907A - Hammer

Info

Publication number: GB2320907A
Application number: GB9727479A
Authority: GB
Inventors: Keith England
Original assignee: Individual
Current assignee: Individual
Priority date: 1997-01-06
Filing date: 1997-12-30
Publication date: 1998-07-08
Also published as: GB9727479D0; GB9700132D0

Abstract

A hammer comprises a shaft 4 having a longitudinal axis and hammer head 1 having a main body portion fixed to one end of said shaft which extends generally along a line transverse to the longitudinal axis of the shaft. A striking surface 3 at one end of the main body portion extends asymmetrically about said line, preferentially towards the end of the shaft remote from said one end. The centre of percussion CP intersects the striking surface 3 and this reduces the reaction felt by the hand of a user.

Description

HAMMER The conventional hammer comprises a shaft on which is fixed a hammer head. The head is generally symmetrical about an axis generally transverse to the longitudinal axis of the shaft. The hammer head is usually of much denser material than that of the shaft but even so the centre of percussion of the complete hammer is displaced towards the shaft from the transverse axis of the hammer head and may even lie outside the hammer head, displaced in the direction of the free end of the shaft (which is the end remote from the end at which the hammer head is fixed). The effect of this is that, when the hammer head strikes the workpiece, there is a reaction on the hand of the user by the shaft in the same direction as the force applied by the hammer head, so that only a fraction of the force applied by the user to the hammer is applied to the workpiece by the hammer head. This is inefficient and when the hammer is used to deliver repetitive blows, the reaction on the hand of the user may cause a repetitive strain injury.

The present invention overcomes this problem of efficiency by adapting the hammer head so that the width (measured in the plane defined by the head axis and the longitudinal axis of the shaft) of its striking surface extends asymmetrically about the axis of the head, extending preferentially towards the free end of the shaft, the invention being defined in claim 1. The striking face of the hammer is preferably plane; it preferably intersects the normal to the striking face at the point of impact through the centre of percussion of the hammer or the whole of the hammer head behind the striking face may cross the normal through the centre of percussion so that the striking face is completely contained on the side of the normal closer to the free end of the shaft. In the latter case, there is still a reaction on the hand of the user which may cause repetitive strain injury as mentioned above, but the reaction is in the opposite direction to the blow imparted by the striking face on the workpiece so that the resultant of the force applied by the hand of the user and of the reaction of the shaft on the hand of the user at the moment of impact provides a greater force of the striking face on the workpiece than if the workpiece had been struck on the line of the centre of percussion.

The invention is applicable to other hammer heads such as those with non-plane striking surfaces, including ball-pane hammers and chisel-head hammers. Hammers of all sizes are envisaged: tool-box hammers, sledge hammers (2-8 kg) and club hammers (1-2 kg) can all be adapted according to the invention. A rubber-covered sledge hammer used to drive fence posts can also be so adapted.

The hammer head may be two-sided, with a striking face at each end. At one end the hammer head may be conventional with the width of the striking face symmetrical about the transverse axis of the hammer head extending through the point of fixing to the shaft and at the other end the width of the striking face extending asymmetrically about the axis of the head, extending preferentially towards the free end of the shaft. The hammer can then be used in the conventional manner when desired and in particular when the object to be hammered is in a closely enclosed position which could not be reached by the non-symmetrical striking face at the other end of the hammer head, as well as being used with the asymmetrically arranged head according to the invention.

Examples of the invention will now be described with reference to the accompanying drawings, in which each of Figs. 1 to 4 is a side elevation of a hammer showing only the portion of the hammer shaft adjacent the hammer head.

In Fig. 1, the hammer head 1 extends along an axis generally at right angles to the length of the shaft 4.

The head is fixed on the shaft 4 in the conventional manner. At one end of the hammer head there is an approximately spherical surface which can be used for hammering a workpiece into a generally dish-shape, in the conventional manner. At the opposite end, the hammer head extends asymmetrically of the hammer head axis and ends in a plane striking surface extending generally at right angles to the axis and hence generally parallel to the length of the shaft. The striking surface intersects the axis of the head and extends a considerable distance towards the free end of the shaft beyond the extent of the central portion of the hammer head.

The centre of percussion of the hammer head is located a little (up to about 3 cm in conventional-sized hammers) beyond the edge of the central portion of the hammer head facing the free end of the shaft, but it can be seen that the normal to the striking surface through the centre of percussion intersects the striking surface at approximately its mid-point. If a workpiece is hammered by the striking surface at the region of this mid-point, the full force applied by the user to the handle will be transmitted to the workpiece without any reaction on the user's hand at the shaft which might have the effect of reducing the efficiency of the hammer blow. If the user can tolerate a reaction on the hand at the shaft, then the user can strike the workpiece at the region of the striking face on the side of its mid-point closer to the free end of the shaft, which will have the effect of multiplying the force the user applies to the handle at the point of impact with the workpiece.

In Fig. 2, a similar hammer head is shown except that the first end of the hammer head is a conventional claw and does not include a striking surface. The claw is used as a lever for extracting nails and is not used for hammering. Only its side surfaces are plane. In Fig. 2, the shaft is not exactly at right angles to the length of the hammer head, so that the striking surface is not exactly parallel to the length of the shaft, but the consideration of the efficiency of the applied forces is very similar. The main reason for having an angled shaft, as in Fig. 2, is to allow room for the user's knuckles between the shaft and the workpiece while keeping the centre of gravity near the striking surface.

Since the shaft centre-line should pass through the centre of gravity, this is best achieved when the shaft is at a non-right angle with respect to the head axis, since there is then a small distance between the shaft and the workpiece at the centre of gravity and a larger distance nearer the free end where the user's hand is located. This angled shaft could be used on the hammers of Figs. 1 and 3, if desired.

In Fig. 3, a hammer head with two plane striking surfaces is shown, one striking surface at each end of the hammer head. At the first end there is a striking surface 2 which is conventional and is wholly located on the side of the normal to the striking surface passing through the centre of percussion remote from the free end of the handle. Its width is symmetrical about the transverse axis of the hammer head. The opposite end of the hammer head extends in an S-shape to end in a striking surface 3 parallel to the striking surface 2 and intersecting the line through the centre of percussion closer to the free end of the handle. It is asymmetrical about the transverse axis of the hammer head. The S-shape extends across the normal to the line through the centre of percussion. Although in this case the striking surface 3 intersects the line through the centre of percussion, it could be entirely on the side of the line closer to the free end of the shaft. When the hammer is used to impact a workpiece using the striking surface 3, the force applied to the workpiece is equal to or greater than the force applied by the user to the handle depending on whether the point of impact is on the normal passing through the centre of percussion or whether it is closer to the free end of the shaft. Fig. 4 shows a club hammer very similar to the tool hammer of Fig. 3. The greater weight of the head 1 relative to the shaft 4 means that the line normal to the striking faces passing through the centre of percussion is contained within the head, whereas in Fig. 3 it is on the side of the head closer to the free end of the shaft. In Figure 4 the striking surface 3 is symmetrical about the centre of percussion, which means that a blow to an object located at the centre of the surface 3 will result in no adverse reaction to the holder of the shaft. In both Figures 3 and 4 the hammer head extends some distance beyond the end of the striking surface 3 remote from the free end of the shaft, so that it is not possible to use the striking surface 3 in a confined space where the workpiece is closer to surrounding walls than the distance between the remote end of the striking surface 3 and the remote end of the main portion of the hammer head. In such cases it is necessary to use the striking surface 2, using the hammer in the conventional mode which is less efficient.

Claims

1. A hammer comprising a shaft having a longitudinal axis, a hammer head having a main body portion fixed to said shaft adjacent one end thereof and extending generally along a line transverse to the longitudinal axis and a striking surface at one end of the main body portion, the striking surface extending asymmetrically about said line, preferentially towards the end of the shaft remote from said one end.

2. A hammer as claimed in claim 1 wherein said striking surface intersects a line transverse to said longitudinal axis through the centre of percussion of the hammer.

3. A hammer as claimed in claim 2 wherein said striking surface extends symmetrically about said line transverse to said longitudinal axis through the centre of percussion of the hammer.

4. A hammer as claimed in any one of claims 1 to 3 wherein the main body portion has a second striking surface at the end opposite the first-mentioned striking surface, the second striking surface extending symmetrically about a line transverse to the longitudinal axis through the point of fixing of the main body portion to the shaft.

5. A hammer substantially as hereinbefore described with reference to the accompanying drawings.