Tool driving or impacting – Impacting devices – Hammer head driven by pulsating fluid pressure
Reexamination Certificate
2000-02-09
2001-05-08
Smith, Scott A. (Department: 3721)
Tool driving or impacting
Impacting devices
Hammer head driven by pulsating fluid pressure
C173S109000, C173S212000
Reexamination Certificate
active
06227309
ABSTRACT:
BACKGROUND OF INVENTION
This invention relates to rotary hammers, and, in particular to hammers that incorporate an air cushion hammering mechanism.
Such hammers will normally include a tool holder that can hold a hammer bit or chisel bit for acting on a workpiece, and an air cushion hammering mechanism which comprises a piston and a beat piece that are slidably located in a cylinder so that reciprocation of the piston in the cylinder will cause the beat piece to hit a bit located in the tool holder. Such hammers may, however, be employed in more than one mode. For example a hammer may be capable of being employed in a hammer only or so-called “chiselling” mode in which the piston reciprocates within the cylinder in order to cause the beat piece to hit the bit without any rotation of the tool, or alternatively a drilling only mode in which the cylinder may form part of a spindle connected to the tool holder and is caused to rotate about the piston, thereby causing the bit inserted in the tool holder to rotate. The hammer may also be capable of being employed in a combination rotary hammer mode in which the piston reciprocates within the cylinder causing the beat piece to hit the bit while at the same time the cylinder rotates about the axis of the piston, thereby causing the bit to rotate.
As an example, one such hammer is described in WO 98/47670. This hammer has a drive motor that is arranged with its armature shaft at right angles to the axis of the hammer spindle, and has a single switching mechanism that can switch the hammer between pure rotation, pure chiselling and combination rotation and chiselling modes. The armature shaft of an electric motor is coupled to a drive shaft on which is mounted one end of a crank arm that causes the piston to reciprocate within a horizontally oriented cylinder when the drive shaft rotates. The piston is linked to a ram also located in the cylinder by means of an air gap so that reciprocation of the piston causes the ram to reciprocate and to hit a beat piece located forward of the ram, thereby causing the beat piece to impact the rear end of the bit that is inserted in the tool holder. The mode of operation may be changed by means of a switch into a rotary mode in which the piston crank is decoupled from the drive shaft, and instead the cylinder is caused to rotate about the piston, ram and beat piece, thereby causing the bit to rotate in the tool holder. By moving the switch to a third position, the piston can be caused to reciprocate while the cylinder rotates, thereby putting the bit into rotary hammering or chiselling mode.
The cost of such a hammer and also the vibration caused by the reciprocating parts could be reduced if they were formed from a plastics material instead of metal. However, forming the piston from such a material will cause problems with regard to heat generation in the piston during operation, due to the much lower thermal conductivity of plastics materials than that of metals.
SUMMARY OF INVENTION
The present invention is characterised in that the piston is formed from a plastics material and is sealed in the cylinder by means of an annular seal that is located within an annular groove in the piston, the annular seal having an inner diameter that is greater than the diameter of the radially outwardly directed surface of the groove, and the groove having an axial dimension that is greater than that of the annular seal, so that when the cylinder rotates about the piston (without reciprocation of the piston) the seal will rotate with the cylinder.
We have found that if a conventionally sealed piston were replaced with one that is formed from a plastics material it would operate quite satisfactorily in pure chiselling mode in which the piston reciprocates in the cylinder without rotation of the cylinder, but when the hammer is operated in rotational mode, friction between the seal in the piston and the surfaces of the cylinder and the surfaces of the locating groove in the piston causes undue temperature rises in the seal and/or the piston and degradation thereof, this temperature rise being caused at least partly by the fact that the plastics material of the piston will have a much lower heat conductivity that of the metals that have hitherto been used. In accordance with the present invention, however, the seal is in the form of a “floating” seal. This seal is not in contact with the bottom of the groove in the piston, and, even if it is in contact with the side walls of the piston groove during pure rotational mode, does not impart a force against the piston groove walls. Thus, in pure rotational mode, the seal will rotate with the cylinder but will not generate significant frictional heat in the piston. When the hammer is used in chiselling mode, in which the piston reciprocates but the cylinder does not rotate, any frictional heating will be generated between the seal and the metallic cylinder, and will be dissipated by the cylinder.
Preferably the axial dimension of the groove is up to 0.5mm greater than that of the seal, and more preferably from 0.1 to 0.3mm greater than that of the seal. The outer diameter of the seal should be greater than that of the piston, but preferably by no more than 1 mm in order to reduce deformation of the seal.
The hammer may be one that is capable, as mentioned above, of operating in combination rotary hammer mode in which the cylinder rotates about the piston while at the same time the piston reciprocates within the cylinder. In this case, the annular seal is preferably arranged so that it will rotate about the piston but at a slower rate than the cylinder. This may be achieved simply by the appropriate size of the seal with respect to the piston. In this case, some heating of the piston will occur due to friction between the seal and the side walls of the piston groove, but this will be limited because the speed of the seal with respect to the piston groove walls is reduced, and because the seal will alternately engage opposite sides of the groove as the direction of reciprocation of the piston alternates.
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Henke Andreas Werner
Lauterwald Martin
Ruethers Bruno Meinholf
Stanke Thomas Michael
Black & Decker Inc.
Leary Michael P.
Shapiro Bruce S.
Smith Scott A.
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