GB2270494A

GB2270494A - Portable circular saw with damping

Info

Publication number: GB2270494A
Application number: GB9317315A
Authority: GB
Inventors: Andreas Hoelderlin
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1992-09-12
Filing date: 1993-08-20
Publication date: 1994-03-16
Anticipated expiration: 2013-08-20
Also published as: GB9317315D0; DE4230614A1; GB2270494B

Abstract

The sawing machine comprises a circular saw blade (10) clamped rotation-symmetrically on a drive shaft (11) and has a saw blade guide (12), which acts by means of guide elements (13) at a radial distance from the drive shaft (11) against both sides of the saw blade (10). The guide elements (13) bear in low-friction contact against both faces of the saw blade (10) in order to improve the vibration behaviour of the saw blade (10), against a damping device (16) which is active in the axial direction of the saw blade (10). The damping of the damping device is set in dependence on the amplitude of the axial vibrations of the saw blade (10). <IMAGE>

Description

A Portable circular sawing machine

Prior Art

The invention derives f rom a portable circular sawing machine of the generic type def ined in the preamble of Claim 1.

In the case of a known portable circular sawing machine of this type (US Patent Specification 4 136 590), the saw blade guide exhibits a holder, fastened conventionally to the machine housing, having an annular head, in which there is inserted a carrier having guide surfaces which are configured on opposite sides. The carrier is held loosely, axially movably and tiltably in the annular head and is stabilised in a position aligned flush with the circular saw blade. For this purpose, a pair of annular lip seals are provided, which are disposed at a distance apart on the periphery of the carrier and press against the annular head. The pressure of the annular sealing lips is increased by air being forced in between the sealing lips, which air eventually streams out through ducts opening out onto the guide surfaces. This air forms a stable slip film between the guide surface and the saw blade, which is also employed for the cooling of the saw blade.

Advantages of the invention The portable circular sawing machine according to the invention. having the characterising features of Claim 1, has the advantage that the vibration behaviour of the saw blade when sawing is decisively improved. The damping, acting in the direction of vibration, of the damping device is increased whenever the axial vibration amplitude of the saw blade is large, reduced whenever the vibration amplitude is small. The friction, connected to the damping, between the saw blade and the saw blade guide is thereby kept as low as possible in all operating situations, so that no unnecessary high friction losses are incurred.

As a result of the measures listed in the further claims, advantageous refinements of and improvements to the portable circular sawing machine specified in Claim 1 are possible.

A technically particularly simple control of the damping in the damping device is obtained if, according to a preferred embodiment of the invention. an electroviscous damping fluid is used in the damping device and its viscosity electrically altered in dependence on the axial vibration amplitude of the saw blade. The use of electroviscous (also known as rheoelectrical) damping fluids has already been proposed, for example, for motor vehicle shock absorbers (DE 37 09 447 C2). The damping characteristics of such motor vehicle shock absorbers are very largely determined by the electrically easily controllable damping behaviour of the rheoelectrical damping fluid, the viscosity of which can be altered very rapidly and reversibly under the influence of a sufficiently strong high-voltage field, to be more precise from the liquid state in the absence of current up to, with increasing field strength, the plastic or even solid state. Electrical direct-current fields are just as suitable for this as alternating-current fields. Since, in this case, only comparatively small electric currents flow through the rheoelectrical damping fluid, the energy-related control costs are correspondingly low.

The voltage of the high-voltage field to which the rheoelectrical damping f luid is exposed is controlled, according to a further embodiment-of the invention, by a vibration sensor which detects the axial vibrations of the saw blade. As the vibration sensor, a resistively, capacitively, optoelectrically or magnetically working displacement transducer can herewith be used, the vibration sensor preferably being disposed diametrically to the guide elements for the saw blade at approximately the same radial distance as these on the saw blade.

According to a preferred illustrative embodiment of the invention, the damping device exhibits a damping cylinder filled with the electroviscoug or rheoelectrical damping fluid and a therein axially displaceable damping piston, which divides the damping cylinder into two damping chambers interconnected by choke gaps. The damping piston is rigidly connected to a piston rod, which is guided outwards, in a liquid-tight manner, through the one damping chamber and supports, at the end side, a holder which receives the guide elements under low friction.

In order to compensate for the change in cubic capacity in the damping chambers upon the intrusion and reemergence of the piston rod, according to a f urther embodiment of the invention there is provided in the damping cylinder an axially displaceable compensating piston, which is liquid-tight in relation to the cylinder wall and which, with its one piston side, limits the other damping chamber and is supported resiliently against the damping cylinder.

According to a preferred embodiment of the invention, for the lowfriction realisation of the guide elements, these are conf igured as balls which are received in a ball bearing and roll against the two faces of the saw blade.

4 Drawing The invention is explained in greater detail, with reference to an illustrative embodiment represented in the drawing, in the following description. The drawing herewith shows, in diagrammatic representation, a saw blade of a portable circular sawing machine with saw blade guide and damping device.

Description of the illustrative embodiment

Of the portable circular sawing machine, only the circular saw blade 10 is represented in the drawing in longitudinal section, which citcular saw blade is clamped rotation-symmetrically on a drive shaft indicated by 11. The saw blade 10 is guided close to its outer periphery in a saw blade guide 13 by the f act that bearing upon each face of the saw blade 10, in low-friction contact, is a guide element 13. The guide elements 13 are here configured as balls 14, which are received under low friction in a ball holder 15 and roll against both faces of the saw blade 10 at a radial distance from the periphery of the saw blade, which periphery is somewhat larger than the tooth depth of the saw blade 10. The ball holder 15 is fastened to a damping device 16, the damping extent of which is set in dependence on the amplitude of the axial vibrations of the saw blade 10.

More particularly, the damping device 16 exhibits a damping cylinder 18, which is closed off by two end walls 25, 30 and in which there is guided, in an axially displaceable manner, a damping piston 19 and a compensating piston 20. The compensating piston 20 is sealed in a liquid-tight manner, by a ring seal 21, against the inner wall of the damping cylinder 18 and divides the interior of the damping cylinder 18 into an operating chamber 22 and a compensation chamber 23. The operating chamber 22 is filled with an electroviscous damping fluid 17, also known, inter alia, as - 5 rheoelectrical damping f luid, whilst the compensation chamber 23 is linked to the atmosphere by a compensation bore 24 in the end wall 25 of the damping cylinder 18. The compensating piston 20 limits with its one piston side, together with the end wall 30 of the damping cylinder 18, the operating chamber 22 and is supported by its other piston side, by means of a helical compression spring 38, against the end wall 25 of the damping cylinder 18.

The damping piston 19 is disposed in the operating chamber 22 and divides this into two damping chambers 26 and 27. These two damping chambers 26, 27 are interconnected by an annular 'choke gap 28 left between the outer periphery of the damping piston 19 and the inner wall of the damping cylinder 18. The damping piston 19 is fastened to the end of a piston rod 29, which is guided through a bore 31 in the end wall 30 of the damping cylinder 18 and supports, on the end side, the ball holder 15. The piston rods [sic] 29 is guided in an axially displaceable manner in the bore 31 and is sealed in a liquid-tight manner against the bore wall.

In order to detect axial vibrations of the circular saw blade 10, a vibration sensor 32 is fastened to or close to its one face, to be more precise at the same radial distance from the drive shaft 11 as the balls 14 in the ball holder 15. The vibration sensor 32, which can be a resistively, capacitively, optoelectrically or magnetically working displacement transducer, emits at its output an electric signal, the intensity of which is proportional to the amplitude of the axial vibrations of the saw blade 10. This electric signal is fed via an electric connecting lead 33 to a voltage generator 34, the one output terminal 36 of which is connected to the damping cylinder 18 and the other output terminal 37 of which is connected to an electrode 35 protruding into the electroviscous damping fluid 17. The voltage generator 34 generates between its output terminals 36, 37 a high- voltage field, the intensity of which is proportional to the output signal of the vibration sensor 32.

As a result of the damping device 16, the axial vibrations of the saw blade 10 are damped. According to the vibration amplitude, the damping piston 19 is displaced, by means of the piston rod 29, from the ball holder 15. The electroviscous damping fluid 17 is thereupon forced through the choke gap 28, the throughput of fluid and hence the damping force being dependent upon the viscosity of the damping f luid 17. The vibration sensor 32 measures the amplitude of the axial vibrations of the saw blade 10 and delivers a corresponding output signal to the voltage generator 34. The higher the vibration amplitude, the greater is the output signal of the vibration sensor 32 and the higher the voltage set by the voltage generator 34 at its output terminals 36, 37. As the voltage rises, the viscosity of the damping fluid 17 increa ses. A higher viscosity signifies a higher vibration damping, since a greater force is required in order to move the damping piston 19 in the operating chamber 22. Large axial vibrations of the saw blade 10 are thus heavily damped and lesser axial vibrations of the saw blade 10 are only slightly damped. In the event of a small damping, the friction between the balls 14 and the saw blade 10 is slight, in the event of a large damping it is heavy. Since the degree or extent of damping of the damping device 16 is always matched to the instantaneous vibration behaviour of the saw blade 10, the friction is kept as low as possible in all operating states of the saw blade 10.

Upon the displacement of the damping piston 19, the piston rod 29 intrudes to a greater or lesser extent into the operating chamber 22 and thereby alters the cubic capacity of the operating chamber 22. In the same way. a thermal expansion of the electroviscous damping fluid 17 gives rise to a change in cubic capacity in the operating chamber 22. This change in cubic capacity is compensated for by the compensating piston 20, which is thereby displaced to a greater or lesser extent against the pressure of the helical compression spring 38.

Claims

1. Portable circular sawing machine having a circular saw blade (10) clamped rotation- symmetrically on a drive shaft (11) and having a saw blade guide (12), which acts by means of guide elements (13) at a radial distance from the drive shaft (11) against both sides of the saw blade (10), characterised in that the guide elements (13) bear in low-friction contact against both. faces of the saw blade (10) and are held against a damping device (16) which is active in the axial direction of the saw blade (10) the damping extent of which damping device is set in dependence on the amplitude of the axial vibrations of the saw blade (10).

2. Machine according to Claim 1, characterised in that the damping device exhibits an electroviscous (rheoelectrical) damping f luid, the viscosity of which is electrically altered in dependence on the axial vibration amplitude of the saw blade (10).

3. Machine according to Claim 2. characterised in that the electroviscous damping fluid (17) is exposed to an electrical voltage f ield, the voltage of which is controlled by a vibration sensor (32) which detects the axial vibrations of the saw blade (10).

4. Machine according to Claim 3, characterised in that the vibration sensor (32) is a resistively, capacitively, optoelectrically or magnetically working displacement transducer.

5. Machine according to Claim 3 or 4, characterised in that the vibration sensor (32) is disposed diametrically to the guide elements (13) at approximately the same radial distance as these f rom the drive shaft (11).

6. Machine according to one of Claims 2 - 5, characterised in that the damping device (16) exhibits a damping cylinder (18) filled with the electroviscous damping fluid (17) and a therein displaceable damping piston (19), which divides the damping cylinder (18) into two damping chambers (26, 27) interconnected by at least one choke gap (28) and is firmly connected to a holder (15) which receives the guide elements (13) under low friction.

7. Machine according to Claim 6, characterised in that the damping piston (19) is rigidly connected to a piston rod (29), which is guided outwards, in a liquid-tight manner, through the one damping chamber (26) and supports, at the end side, the holder (15) with the guide elements (13).

8. Machine according to Claim 7. characterised in that in the damping cylinder (18) there is disposed an axially displaceable compensating piston (20), which is liquid-tight in relation to the cylinder wall and which, with its one piston side, limits the other damping chamber (27) and, with its other piston side, is supported resiliently against the damping cylinder (18).

9. Machine according to one of Claims 1 - 8, characterised in that the guide elements (13) are configured as balls (14) which are received in a ball holder (15) and roll against the two faces of the saw blade (10).

10. A portable circular sawing machine substantially as herein described with reference to the accompanying drawing.

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