GB2332355A

GB2332355A - Rotary cutters

Info

Publication number: GB2332355A
Application number: GB9726548A
Authority: GB
Inventors: Andrew Quigley; Adrian Coussins
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1997-12-16
Filing date: 1997-12-16
Publication date: 1999-06-23
Also published as: GB9726548D0; GB2332355A9; FR2772227A1; DE19857601A1

Abstract

A rotary cutting mechanism is disclosed wherein a rotating blade (12) is retained on a power-driven shaft (11) by a screw-threaded retaining member (14), and wherein the mechanism includes ratchet means (20)(Fig.5) permitting relative rotary movement between the blade and the drive shaft in one direction when the shaft is being driven and the blade is obstructed but preventing relative rotary movement in the opposite direction when the shaft is braked. In a preferred embodiment the mechanism is applied to the blade of a rotary lawnmower.

Description

1 ROTARY CUTTERS 2332355 This invention relates to a rotary cutter in

which a rotating blade is secured at one end of a motor-driven shaft by a screw-threaded coupling. Such a cutter is used, for example, in a rotary lawnmower when the cutter blade is retained on the motor shaft by a screw-threaded retaining member.

It) For safety reasons, it is desirable that the stopping time of the rotating blade should be as short as possible. However, when the motor is braked, there is a tendency for the blade to continue rotating relative to the drive shaft under its own inertia. This can produce a frictional force on the screw-threaded retaining member and, in extreme cases, particularly when the blade is braked with a stopping time of less than 0.5 seconds, can lead to a loosening of the screw-threaded coupling.

To prevent this happening the blade may include a pair of holes which receive respective locating pins depending from the surface of a member rotatable with the drive shaft and against which the blade is clamped. This will prevent any relative movement between the blade and the rotating shaft. A disadvantage of this arrangement, however, is that the pins may shear if the rotating blade strikes a heavy 20 object and is prevented from rotating while the motor is driving the drive shaft. Moreover, a user might not be aware that the pins had sheared.

According to the present invention this problem is overcome by providing the cutter assembly with means permitting relative rotary movement between the blade and the drive shaft in one direction when the motor is driving the drive shaft but preventing relative rotary movement in the opposite direction when the motor is braked. By arranging that the permitted relative movement is in a direction that will apply a tightening force to the screw-threaded coupling, the risk of undesirable loosening of the coupling is substantially eliminated.

tP The said means is preferably secured to a surface of a member rotatable with the drive shaft and against which the blade is clamped by the screwthreaded retaining member.

2 The means may include a catch that engages the blade if there is relative movement in the non-permitted direction but which is deflected out of the path of the blade in response to relative movement in the permitted direction.

In one particular embodiment, a series of such catches are angularly spaced around a circle to form a ratchet device.

By way of example only, a rotary cutter embodying the invention will now be described with reference to the accompanying drawings in which:

Figure 1 is a schematic drawing of a rotary lawnmower having a motordriven cutter blade; Figure 2 is a cross-section view through a fan which, in use, is mounted on the motor drive shaft of the lawnmower above the cutter blade; Figure 3 is a plan view of the cutter blade; Figure 4 is a cross-section view of the end nut which holds the cutter blade on the 20 drive shaft; Figure 5 is a plan view of a ratchet device which, in use, is secured to the underside of the fan of Fig. 2; Figure 6 is a section C-C on Fig. 5 with the ratchet barbs in a flattened state; Figure 7 is a part section B-B on Fig. 5; and Figure 8 is a part section A-A on Fig. 5.

Referring to these figures, a rotary lawnmower 10 has an electric motor (not shown) enclosed within a housing 11, and a cranked cutter blade 12 secured to a motor drive 3 shaft 15 for rotation about a substantially vertical axis. 'I'lle blade 12 (Fig. 3) includes a central opening 13 for receiving the drive shaft 11 and is retained on the shaft by an end nut or bolt 14 (Fig. 4) which includes an internal screw thread 15 for engaging an external thread formed around the drive shaft. The central portion of the blade around the opening 13 is clamped between the bolt 14 and the underside of a fan 19 (Fig. 2) which is also mounted on the drive shaft 11 for rotation therewith. The fan 19 consists of a circular disc 16 with angularly spaced upstanding radial blades 17 extending outwardly from a central hub 26.

The direction of rotation and the threaded coupling are so arranged that, when the motor is first started or the rotating blade meets heavy resistance, any rotation of the drive shaft 11 relative to the blade 12 will produce a fricitional force on the bolt 14 tending to tighten the screw-threaded coupling between the bolt 14 and the drive shaft 11. However, when the motor is braked and the shaft ceases rotating, the inertia of 15 the blade could apply a frictional force to the bolt in the opposite direction tending to loosen the screw-threaded coupling if the blade were allowed to rotate relative to the bolt. With a braking time of less than 0.5 seconds, this loosening force could be substantial.

To overcome this problem, a ratchet device 20 as illustrated in Figs. 5-8 is inserted between the blade 12 and the underside of the fan 19.

The ratchet device 20 is formed from 0.25mrn stainless steel hard rolled strip. It consists of a disc 21 having a central opening 22 for receiving the motor drive shaft 11, an inner circle of six generally rectangular projecting barbs 23 cut out from the disc, and an outer circle of sixteen generally circular projecting barbs 24 also cut out from the disc. The barbs 23 project upwardly from the disc, and the barbs 24 project downwardly.

The inner barbs 23 are designed to press-fit into respective apertures 25 formed in the hub 26 of the fan 19 on the underside thereof. In this manner, the ratchet device is secured to the fan.

4 The outer barbs 24 cooperate with a pair of diametrically opposed holes 27 formed in the central portion of the blade 12. "Ilie barbs 24 in effect form a series of resilient catches or teeth which are progressively flattened in response to relative movement between the blade 12 and the fan 19 in one direction but which prevent relative movement in the opposite direction by engaging the holes 27. In this manner the ratchet device allows relative movement that produces a tightening of the screw coupling between the end bolt 14 and the drive shaft 15 but prevents relative movement that would otherwise produce a loosening of the coupling.

i

Claims

1. A rotary cutting mechanism wherein a rotating blade is retained on a power5 driven shaft by a screw-threaded retaining member, and wherein the mechanism includes ratchet means permitting relative rotary movement between the blade and the drive shaft in one direction when the shaft is being driven and the blade is obstructed but preventing relative rotary movement in the opposite direction when the shaft is braked.

2. A mechanism according to claim 1 wherein the blade is clamped between a surface of the screw-threaded retaining member and a surface of a work member rotatable with the drive shaft.

33. A rotary cutting mechanism according to claim 2 wherein the ratchet means is interposed between the blade and the said surface of the work member 4. A rotary cutting mechanism according to claim J31 wherein the ratchet means includes at least one tooth adapted to engage the blade to prevent relative rotary movement in the non-permitted direction, the tooth being deflected out of the path of the blade in response to relative rotary movement in the permitted direction.

5. A rotary cutting mechanism according to claim 4 wherein a plurality of the said teeth are angularly spaced around the wds of the drive shaft to form a ratchet.

6. A rotary cutting mechanism according to claim 5 wherein the ratchet means comprises a disc having projections on one side for locating in respective recesses of the said surface of the work member, and having additional projections on the opposite side to provide the said teeth.

7. A rotary cutting mechanism according to claim 6 wherein a central portion of 6 the blade includes a plurality of apertures angularly spaced around the axis of the drive shaft for engaging the teeth of the ratchet to prevent relative rotary movement in the non-permitted direction.

8. A rotary lawnmower including a rotary cutter mechanism according to any one of the preceding claims.

9. A rotary lawnmower according to claim 8 as dependent on any one of the claims 2-7 wherein the said work member comprises a fan.