BACKGROUND ART A trimming knife unit is made up of a trimming knife in the form of a handle with a circular blade housing at one end and a circular blade mounted within the housing for rotation while driven. The trimming knife is powered by a motor (usually electric or pneumatic) mounted remotely from the trimming knife and which is linked to the knife by a flexible drive. A unit of this conventional type is briefly described hereafter in connection with Figures 1 and 2 of the accompanying drawing.

DISCLOSURE OF INVENTION According to a first aspect of the present invention there is provided a trimming knife made up of a handle having a circular blade housing at one end and a circular blade carried by the housing the handle serving to enclose a drive shaft for driving the circular blade, the drive shaft having: an output end providing for the transmission of rotation to the circular blade; and an input end providing for rotation of the shaft by way of a drive means for the knife the drive means providing for the trimming knife to be linked to a motor characterised by the provision of a clutch functioning between the input end and the output end of the drive shaft; and a clutch operating means located on, or on an extension of, the handle so that a user of the trimming knife can use the clutch operating means to provide for the blade to be driven or not driven independently of operation of a drive means as aforesaid.

According to a first preferred version of the first aspect of the present invention the clutch means comprises a male member located for driving by the input end of the drive shaft and a female member located to drive the output end of the shaft member and the clutch operating means serve to displace the male member axially relative to the drive shaft from a first, coupled, configuration where the male member engages with the female member and a second, uncoupled, configuration where the male member is disengaged from the female member. Typically the male member is formed integrally with means defining the input end of the drive shaft.

According to a second preferred version of the first aspect of the present invention or of any preceding preferred version thereof the clutch operating means includes a lever pivotably mounted in the handle, or an extension thereof, whereby a user can operate the clutch.

According to a third preferred version of the first aspect of the present invention or any preceding preferred version thereof the clutch operating means provides for safe operation of the trimming by acting automatically to provide for the blade to be not driven in the event of anomalous operation of the trimming knife by a user including releasing hold of the knife According to a second aspect of the present invention there is provided a trimming knife unit comprising: a trimming knife according to the first aspect or any preferred version thereof; a motor (typically electric or pneumatic) for driving the trimming knife while being remote from the trimming knife; and a drive means (typically a flexible shaft) linking the motor to the trimming knife whereby the trimming knife can be driven by the motor.

BRIEF DESCRIPTION OF THE DRAWINGS A conventional trimming knife and unit followed by an exemplary embodiment of the invention will now be described with reference to the accompanying drawings of which: Figure 1 is a perspective view of a conventional trimming knife unit; Figure 2 is an enlarged plan view of a trimming knife, a component of Figure 1; Figure 3 is a cross section of a vertical elevation of a trimming knife according to the present invention; and Figure 4 is a cross section on a horizontal section of the knife of Figure 3.

MODE FOR CARRYING OUT THE INVENTION Figure 1 A conventional trimming knife unit 10 is made up of a trimming knife 11, an electric motor unit 12 and a flexible drive shaft 13 linking the motor unit 12 to the knife 11. The motor unit 12 is shown suspended by forked mount 14 from a suspension point above a working area in which the knife 11 is to be used. The forked mount 14 includes a hanger 15 in which the knife 11 can be mounted when not in use.

Figure 2 A conventional trimming knife 11 is shown here in more detail. The knife 11 is made up of a handle 20 having a circular blade housing 21 at end 22 of the handle 20. Within the housing 21 there is rotatably mounted a circular blade 23. The handle 20 contains a drive shaft (not shown) whereby rotation fed by flexible drive shaft corresponding to flexible drive shaft 13 (Figure 1). The flexible drive shaft extends into the other end 24 of the handle 20 to one end 22 and the rotation of the flexible drive shaft is fed directly by the drive shaft to a bevel gear (not shown) powering the circular blade 23. The trimming knife 11 incorporates a number of safety and other features which are not considered further here. The trimming knife 11 is of modular construction to enable individual items to be readily replaced.

Figure 3 and 4 This shows a trimming knife 31 which differs from that of Figures 2 in embodying the present invention. The invention involves the provision of means for isolating a trimming blade of the knife from the flexible drive shaft. The knife 31 is readily used in conjunction with the motor unit 12 and flexible drive shaft 13 referred to in connection with Figure 1.

The trimming knife 31 comprises a handle 32 having a circular blade housing 33 at one end 34 and a circular blade 35 whose edge is shrouded by the housing 33. The handle 32 serves to enclose a drive shaft 36 for driving the circular blade 35 by way of a bevel gear set 37. The drive shaft 36 has: an output end 38 to provide a power input to bevel gear set 37; and an input end 39 which can be driven by way of flexible drive shaft 40 (only one end being shown). The flexible drive shaft 40 has extending through its outer cover 41 a power transmitting shaft which projects from the cover 41 to provide a square ended output spigot 42. This spigot 42 engages with a complementary socket 44 provided in input end 39 of the drive shaft 36. The opposite end of the flexible drive shaft 40 to that from which spigot 42 projects engages in a similar manner to an electric motor which serves to power the trimming knife 31 by way of the flexible drive shaft 40.

In this case the drive shaft 36 is in two parts which can be coupled or uncoupled by means of a clutch 44 operable by a clutch operating means, in this case lever 45 (Figure 3) located by pivot 46 on the handle 32. The lever 45 enables a user of the trimming knife 31 to provide for the blade 35 to be driven or not driven independently of the operational state of the flexible drive shaft 40.

In this case the clutch 44 comprises a male member 47 located for driving by the input end 42 of the drive shaft 36 and a female member 48 located to drive the output end 38 of the shaft member 36. The clutch lever 45 serves to displace the male member 47 axially relative to the drive shaft 36 from a first, coupled, configuration (as shown in Figure 4) where the male member 47 engages with the female member 48 and a second, uncoupled, configuration where the male member 47 is disengaged from the female member 48.

The male member 47 incorporates a flange 50 on which seats one end 51A of a compression spring 51 whose other end 51B seats on a flange 52 on drive shaft 36. The compression spring acts to resiliently load flange 50 so that flange 50 is biased so as to be driven away from flange 52. Figure 4 shows the closed configuration of the clutch 44 when the handle 45 is pressed towards the body of handle 32. If the handle 45 is not held down in this way to cause the clutch 44 to be closed (so providing for drive shaft 36 to be driven by flexible drive shaft 40 then the compression spring 51 acts to drive the flanges 50,52 apart and so prevent further rotation of the blade 35 by means for the flexible drive shaft 40.

The provision of a clutch 44 as described serves to provide for substantially enhanced safe operation of the trimming knife 31 by functioning automatically to cause the blade 35 to cease to be driven in the event of anomalous operation of the trimming knife 31.

Such anomalous operation could arise for example by the user dropping the knife 31 or releasing their hold on the handle 32 for some other reason.

INDUSTRIAL APPLICABILITY The present invention provides a trimming knife and unit with greatly enhanced ability for safe operation. To stop the cutting blade operating it was previously necessary to switch off the motor powering the flexible drive cable and this could require a fair amount of movement between the position where the trimming knife is in use and a operating control position for the associated motor.

The newly proposed knife can be readily used in existing trimming knife units and so enable an existing unit to be readily and relatively cheaply uprated to provide a safer working configuration just by changing the knife which forms just one part of the unit rather than requiring the installation of a new trimming knife unit.

The invention is applicable to any knife of the type described in a range of such knives which are used, for example, to cope with a wide range of de-boning requirements..