Tool driving or impacting – Impacting devices – Hammer head driven by pulsating fluid pressure
Reexamination Certificate
2000-02-09
2001-05-29
Smith, Scott A. (Department: 3721)
Tool driving or impacting
Impacting devices
Hammer head driven by pulsating fluid pressure
C173S109000
Reexamination Certificate
active
06237699
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 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 various reciprocating parts of the hammer, and in particular the ram, must be sealed within the cylinder against pressure differences occurring on different sides thereof while at the same time being able to move within the cylinder. This has, however, been the cause of a number of problems: Generally such seals have been formed as an annulus of an elastomeric material located within a groove, which annulus sits slightly proud of the surface of the ram in order to seal the ram within the cylinder. In addition, the seal and/or the inner surface of the cylinder must be provided with a coating of grease in order to maintain the seal. However, reciprocation of the ram within the cylinder will gradually wipe the grease away from the region of the seal and so reduce the sealing effect of the seal and increase frictional heating of the seal in the cylinder due to the reciprocation of the ram.
SUMMARY OF INVENTION
The present invention is characterised in that the ram has an annular recess in its peripheral surface at the end thereof nearest the piston, in which recess is located an annular seal, the annular seal having an annular seating flap lying on the radially outwardly directed surface of the recess and an annular sealing flap that is joined to the seating flap and extends between the radially outwardly directed surface of the recess and the inner wall of the cylinder.
The use of such a form of seal on the ram has the advantage that it is possible for the seal to act without the necessity of any grease which can be lost from the sealing area by reciprocation of the ram in the cylinder. The sealing flap is preferably joined to the seating flap at the forward end thereof (in the direction of use of the hammer) and preferably extends between the radially outwardly directed surface of the recess and the inner wall of the cylinder at an acute angle to the seating flap. Such an arrangement means that an overpressure in the region between the piston and the ram will tend to force the sealing flap against the inner wall of the cylinder and so improve the sealing effect of the annular seal. Such an overpressure is caused by forward movement of the piston toward the ram and can be very high since it is this that drives the ram to impact the beat piece. When the piston moves rearwardly, i.e. away from the ram, a partial vacuum is formed in the region between the piston and the ram which causes the ram to move rearwardly toward the piston. The difference in pressure between the forward and rear ends of the ram when the ram moves rearwardly must also be sealed by the ram, but in this instance the pressure difference attempts to move the sealing flap away from the inner wall of the cylinder rather than toward it. Such a pressure difference, however, is never more than one bar, and so the sealing flap can be designed to have sufficient resistance to deformation to withstand such a pressure.
Preferably the seating flap extends axially along the ram by substantially the same distance as the sealing flap. This will reduce the possibility of the seating flap being forced out of position by deformation of the seal caused by any pressure acting on the sealing flap.
The provision of an annular recess in the rear of the ram will, in the absence of any other modifications, increase the volume of the region between the piston and the ram when the two are at their closest separation, and hence reduce the air pressure in the region between them. This would have the effect of reducing the transmission of force from the piston and the ram, and so the piston preferably has a face that is oriented toward the ram that has a shape that complements the profile of the end of the ram that is directed toward the piston. For example, and preferably, the piston has a ridge that extends around the periphery of the face that is directed toward the ram and which can extend into the recess in the ram when the piston and the ram reach the point of closest approach.
REFERENCES:
patent: 2081040 (1937-05-01), King
patent: 3791149 (1974-02-01), Strecker et al.
patent: 4192391 (1980-03-01), Kastreuz et al.
patent: 4442906 (1984-04-01), Simpson
patent: 4537264 (1985-08-01), Schmid et al.
patent: 4602689 (1986-07-01), Wanner
patent: 5873418 (1999-02-01), Arakawa et al.
patent: 5975217 (1999-11-01), Frenzel et al.
patent: 0564217 (1993-10-01), None
Plietsch Reinhard
Ruethers Bruno Meinholf
Wache Robert Bernhard
Black & Decker Inc.
Leary Michael P.
Shapiro Bruce S.
Smith Scott A.
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