GB2234034A

GB2234034A - Drive mechanism for converting rotary motion into reciprocating linear motion

Info

Publication number: GB2234034A
Application number: GB8915856A
Authority: GB
Inventors: Daniel Bone
Original assignee: Black and Decker Inc
Current assignee: Black and Decker Inc
Priority date: 1989-07-11
Filing date: 1989-07-11
Publication date: 1991-01-23
Also published as: GB8915856D0

Abstract

A drive mechanism for a powered hand tool eg an electric saw, for converting rotational motion into reciprocating linear motion comprises a reciprocatable drive shaft (55) coupled to a rotatable drive means by a transfer mechanism incorporating a right angle drive. The transfer mechanism incorporates a gear (35) which is rotatably mounted on a bearing block (40), the reciprocatable shaft (55) being slidably mounted in front and rear bearing assemblies (80, 75), the rear bearing assembly (75) being incorporated into the bearing block (40). As shown the rear bearing assembly is formed by each prong (85) of a bifurcated end of the shaft (55) being slidably located in an annular groove 90 in the bearing block (40). Various cam and follower arrangements for converting rotary to reciprocating linear motion are disclosed (see Figs 5-10). <IMAGE>

Description

DRIVE MECHANIZE The present invention relates to a drive mechanism for converting rotational motion into reciprocating linear motion, and in particular to a drive mechanism suitable for use in a reciprocating hand tool. In such tools, it is desirable that the drive mechanism be capable of high speed but also be compact and have low noise and vibrational characteristics. The invention is particularly suitable for driving a small electric saw.

According to a first aspect of the present invention, we provide a drive mechanism for a powered hand tool or the like comprising a reciprocatable drive shaft coupled to a rotary drive means by a transfer mechanism to convert rotary motion of the drive means to linear motion of the drive shaft, and wherein the rotational axis of the drive means is parallel to the longitudinal axis of the drive shaft and the transfer mechanism incorporates a right angle drive mechanism.

According to a second aspect of the present invention, we provide a drive mechanism for a powered hand tool or the like comprising a reciprocatable drive shaft coupled to a rotary drive means by a transfer mechanism to convert rotary motion of the drive means to linear motion of the drive shaft wherein the drive means drives an output member in the form of a disc rotatable about a central pivot, the disc being provided with a cam track on one face thereof which is eccentrically located relative to the central pivot, there being a cam follower drive pin on one end of the drive shaft which engages in said cam track so that rotation of said disc is translated into reciprocating movement of the shaft.

According to a third aspect of the present invention, we provide a drive mechanism for a powered hand tool or the like comprising a reciprocatable drive shaft coupled to a rotary drive means by a transfer mechanism to convert rotary motion of the drive means to linear motion of the drive shaft wherein the drive means drives a gear which is rotatably mounted on a bearing block and the reciprocatable drive shaft is slidably mounted in a bearing means carried by the bearing block.

Preferably, the rotational axis of the drive means and the longitudinal axis of the drive shaft are substantially in line.

Preferably, the drive means is a pinion located on the output shaft of an electric motor and meshing with a gear, preferably through a bevelled arrangement providing a right angle drive.

In a preferred embodiment of the invention, the gear, either directly or through at least one intermediate gear, drives an output member in the form of a disc rotatable about a central pivot, the disc being provided with a cam track on one face thereof which is eccentrically located relative to the central pivot, there being a cam follower drive pin on one end of the drive shaft which engages in said cam track so that rotation of said disc is translated into reciprocating movement of the shaft.

Preferably, the disc is provided with balancing means to offset the effect of the eccentric cam track. The balancing means may be an offset mass integral with the disc, or one or more holes in the disc.

In one construction, the pinion meshes with a bevelled gear mounted for rotation about a pin fixed in a housing at right angles to the rotational axis of the pinion, and about which pin the disc is also rotatable, the disc being spaced from the bevelled gear. Alternatively, the disc may be fixed to the gear directly, for example, by press fitting, or a one piece cam/bevel gear may be provided.

In an alternative construction of the preferred embodiment, the pinion meshes with a bevelled gear mounted for rotation about a pin fixed in a housing at right angles to the rotational axis of the pinion, there being an intermediate gear wheel mounted for rotation about the same pin, and which meshes with teeth at the circumference of the disc.

In each construction of the preferred embodiment, the reciprocatable shaft is preferably mounted in plain bearings carried by a housing for the mechanism.

In an alternative embodiment of the invention, the gear, also preferably a bevelled gear, is rotatably mounted on a bearing block and, the reciprocatable drive shaft is slidably mounted in a bearing means carried by the bearing block. Preferably, the drive shaft is coupled to the gear by crank means and a connecting rod and the reciprocatable shaft is mounted in front and rear bearing assemblies, the rear assembly providing said bearing means, and the bearing assemblies confining the reciprocatable shaft to movement in a direction along its length.

In this alternative embodiment, the rear end of the reciprocatable shaft is preferably bifurcated. The rear bearing assembly may comprise each prong of the bifurcated end of the reciprocatable shaft slidably located in a groove in the bearing block.

In this alternative embodiment, the front bearing assembly may comprise a front bearing block, a cylindrical bearing located in a transverse bore in the block, there also being a through hole in the block and the bearing, through which holes the reciprocatable shaft passes.

According to a fourth aspect of the present invention, we provide a drive unit comprising a drive mechanism according to any one of the first three aspects of the present invention coupled to an electric motor. Preferably, the electric motor is battery powered and the mechanism, motor and batteries are located in a single housing, in line. Alternatively, the motor may be driven by a rechargeable battery pack.

According to a fifth aspect of the present invention we provide an electric saw comprising a drive unit according to the fourth aspect of the present invention accommodated in a cylindrical casing and wherein a saw blade extends from and is secured to the forward end of the reciprocatable shaft, and projects from a front end of the casing.

Two embodiments of drive mechanism according to the present invention are now described by way of example with reference to the accompanying drawings, in which: FIGURE 1 is a longitudinal sectional side elevation of an electric saw incorporating, one embodiment of the mechanism; FIGURE 2 is a perspective view of the drive mechanism only of Figure 1; FIGURE 3 is a plan view of a bifurcated reciprocatable shaft incorporated in the mechanism of Figures 1 and 2; FIGURE 4 is a front elevation and side elevation of the front bearing assembly incorporated in the drive mechanism of Figures 1 and 2; FIGURE 5 is a detail of a drive mechanism, of one alternative construction of a preferred embodiment of the invention; FIGURE 6 is a section on the line VI-VI of Figure 5;; FIGURES 7 and 8 are views similar to those in Figures 5 and 6 of a second alternative construction of the preferred embodiment of the invention; FIGURE 9 is a partial detail of a drive mechanism, of a third alternative construction of the preferred embodiment of the invention; and FIGURE 10 is a plan view of the crown wheel gear and cam disc of Figure 9.

In the drawings, like parts in the various figures are identified with the same reference numerals.

Referring to Figure 1 of the drawings, the saw 10 has a substantially cylindrical outer casing 5 in the rear of which is a battery compartment 15 and an electric motor 20, and at the front of which there is a blade guard or nose 12.

The motor is mounted with its axis of rotation substantially along the central longitudinal axis of the outer casing 5.

A switch 25 is provided to turn the motor on and off.

Optionally, the speed of the motor may be variable. A reciprocatable saw blade 105 projects from the front end of the casing 5 through a slot 109 in the nose 12. The blade 105 is made to reciprocate by the rotating output shaft of the motor 20 by means of a right angle gear drive and a type of crank and connecting rod mechanism, which will now be described in detail.

A bevelled pinion 30 is mounted on the drive shaft of the motor 20 and the pinion 30 meshes with a bevelled crown wheel drive gear 35 which is rotatably mounted on a bearing block 40 for rotation about an axis normal to that of the motor shaft. The bearing block 40 is mounted within the outer casing 5 by means of fixing screws 45. Hence, a right angled drive train from the motor causes rotation of the crown wheel gear 35.

A crank pin or pivot pin 50 projects from a top face of the crown wheel gear 35 and pivotally supports one end of a connecting rod 60, the other end of the connecting rod 60 being pivotally connected to a reciprocatable drive shaft 55 for the saw blade 105 by a pivot pin 65, which projects upwardly from the shaft 55, the shaft 55 and rod 60 being spaced apart by a sleeve 62 located on the pin 65. The pin 50 on the crown wheel gear 35 is spaced from the centre of rotation of the crown wheel gear, and thus acts as a crank means to convert the rotational motion of the crown wheel gear 35 into a reciprocating linear motion of the reciprocatable drive shaft 55 in a largely conventional manner through the connecting rod 60. The crank means and connecting rod allow the drive mechanism to run quietly at high speed.

The reciprocatable shaft 55 is bifurcated at its rear end (see Figure 3) and supported within the casing by means of front and rear slide bearing assemblies 80 and 75. The rear slide bearing assembly 75 is formed by each prong 85 of the bifurcated rear end of the reciprocatable shaft being located with a close sliding fit in an annular groove 90 in the bearing block 40. Location of the prongs 85 in the groove 90 confines the reciprocatable shaft 55 to move generally parallel to the longitudinal axis of the saw. It will be appreciated that the bearing block 40 could be of a different shape, and instead of having a cylindrical groove therein, two opposed grooves could be provided, one for each arm of the bifurcated rear end of the shaft 55.

The orientation of the output shaft of the motor 20 substantially parallel to the reciprocatable shaft 55, coupling of the motor 20 to the crown wheel gear 35 by means of a right angle drive train, and incorporation of the rear bearing assembly in the bearing block 40 on which the crown wheel gear 35 is mounted result in a highly compact drive unit.

The forward slide bearing assembly 80 consists of a cylindrical self-aligning slide bearing 82 provided in a transverse bore 94 in a block 95, the block being secured to the walls of the outer casing 5 by screws engaging in tapped holes 96. The reciprocatable shaft passes through a slot 100 in the block 95 and a further through slot in the cylindrical bearing 82 which confines the front end of the reciprocatable shaft to move parallel to the longitudinal axis of the saw. The front bearing assembly is maintained in its assembled condition by the walls of the outer casing 5. Slots 57 are provided in the shaft 55 to keep it as light as possible.

The saw blade 105 extends from the forward end of the reciprocatable shaft 55, and projects through the slot 109 in the nose 12. A rear end of the blade is located in a groove 102 and housed within a housing 107, the housing and blade being secured to the reciprocatable shaft 55 by screws 110 which engage in tapped holes 112. The nose 12 acts as a guard for the blade 105 and stops a user trapping his finger behind the housing 107, and also assists the user in using the saw, as it also provides a reaction member against which the workpiece can be placed during use.

The length of stroke of the saw blade is determined by the diameter of the circular path traced by the rotation of the crank pin 50. Typically, the length of stroke may be IOmm, Referring now to the construction of the preferred embodiment of the invention shown in Figures 5 and 6, the motor 20 is supported by a casing 101, and the bevelled drive pinion 30 on the output shaft of the motor meshes with the bevelled crown wheel gear 35, which is supported for rotation within the casing 101 by means of a fixed pin 103 located in the casing wall.

In the construction of Figures 5 and 6, the pin 103 not only rotatably supports the crown wheel gear 35 for rotation about the longitudinal axis of the pin, but also an inverted cam disc 125 which is axially spaced from the crown wheel gear 35. The disc 125 has a circular cam track 127 in its lower face, which is eccentrically located relative to the rotational axis of the disc (defined by the longitudinal axis of the pin 103), and a drive bush 111 on the end of a drive pin 129 engages within the track 127. The pin 129 projects from one end of a reciprocatable shaft 113 supported for reciprocating sliding movement in the casing 101 within spaced plain bearings 115, 117, the shaft 113 being equivalent to the shaft 55 in the embodiment of Figures 1 and 2, but somewhat lighter, due to the absence of the bifurcated end. This means the shaft has less inertia when it is reciprocating.

The cam disc 125, due to the eccentrically located cam track 127 would be out of balance when rotated at high speed if it was otherwise of uniform construction. To overcome this potential problem, the disc is provided with an integral balancing portion 119 of increased thickness, and, optionally, a recess 121 within the area defined by the cam track.

To cause the shaft 113 to reciprocate at high speed, the motor is operated, and by virtue of the pinion 30 engaging the crown wheel gear 35, the disc 125 is caused to rotate at high speed. This in turn causes the shaft to reciprocate due to the engagement of the bush 111 on the pin 129 in the eccentric cam track 127. The length of stroke of the shaft is determined by the amount of offset of the cam track 127 relative to the rotational axis of the disc 125.

This mechanism has advantages over that of the embodiment of Figures 1 and 2, because of the absence of the crank shaft 60 and crank pin 50. This means that when the mechanism is operating at high speed there are no sideways out of balance forces; the only out of balance forces are located along the longitudinal axis of the shaft 113, but when this is used to support a saw blade (its primary, but by. no means only use), such forces are quite acceptable.

The second alternative construction of the preferred embodiment shown in Figures 7 and 8 differs from that of Figures 5 and 6 in that an intermediate gear is provided between the pinion 30 and the cam disc 108. Accordingly, the bevelled pinion 30 meshes with bevelled crown wheel gear 35 which is supported for rotation in a modified casing 132 by the fixed pin 103, but instead of the cam disc 108 being supported on the pin 103 for rotation with the crown wheel gear 35, an intermediate gear 104 is supported on the pin 103 for rotation with the gear 35. The intermediate gear 104 is in meshing engagement with gear teeth 106 on the periphery of a modified cam disc 108 which is mounted for rotation in the casing 102 about the longitudinal axis of å fixed pin 120.As in the previous embodiment, the cam disc 108 has an eccentric, circular cam track 102 in its lower face, with which the drive bush 111 on the drive pin 129 carried by reciprocatable shaft 113 engages. As in the previous construction, the cam disc 108 is provided with integral balancing means, this time, the whole disc is relatively thick to provide a support for the peripheral gear teeth 106, and to balance the disc so that there are no lateral out of balance forces when it is rotated at high speed, it is formed with two recesses 122, 114.

Alternatively, the recess 122 may comprise one or more holes in the cam disc 108 and recess 114 may be omitted.

Referring now to the third alternative construction of the preferred embodiment illustrated in Figures 9 and 10, this construction has, in common with the construction of Figures 5 and 6, a pin 103 which rotatably supports both the crown wheel gear 35 and a cam disc 140. In contrast to the construction of Figures 5 and 6, however, the lower face of the cam disc 140 press fits directly in the upper face of the crown wheel gear via pegs provided in the lower face of the cam disc fitting into holes provided in the upper face of the crown wheel gear, the eccentric, circular cam track 127 being provided in the upper face of the cam disc. The cam disc 140 and crown wheel gear 35 are constructed of a hardened sintering and unhardened sintering respectively.

Alternatively, the cam disc and crown wheel gear comprise a one-piece hardened sintering. As in the constructions of Figures 5 to 8 the cam disc 140 is provided with integral balancing means, this time in the form holes 142 oriented in a crescent shape.

It will be appreciated that in each of the constructions of the preferred embodiments of Figures 5-10, the shape of the cam track can be altered to achieve different displacement, velocity and acceleration for a given speed of rotation of the motor.

Furthermore, by introducing the intermediate gear 104 as shown in Figures 7 and 8, it is possible to vary the output torque.

The outer casing may be constructed entirely of die cast zinc or may,' alternatively be constructed in two semicylindrical halves, the lower half of die cast zinc and the upper half of pressed steel.

It is of course also possible to keep down costs by providing one piece sintering for the composite parts.

It will of course be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention. For example, the reciprocatable shaft 55 (or 113) could have a file or other tool mounted thereon.

Also, the electric motor could be mains or battery powered, and/or the batteries could be rechargeable, or in the form of a replaceable power pack. Alternatively, the motor could be driven by compressed air or by some other means.

Furthermore, the various integral balancing means described could be interchanged or varied. For example, the face cam shown in Figures 9 and 10 could be used in the embodiments of Figures 5 and 6 and 7 and 8 instead of those described.

Claims

CLAIMS:

1. A drive mechanism for a powered hand tool or the like comprising a reciprocatable drive shaft coupled to a rotary drive means by a transfer mechanism to convert rotary motion of the drive means to linear motion of the drive shaft wherein the drive means drives a gear which is rotatably mounted on a bearing block and the reciprocatable drive shaft is slidably mounted in a bearing means carried by the bearing block.

2. A drive mechanism according to claim 1 wherein the rotational axis of the drive means is parallel to the longitudinal axis of the drive shaft.

3. A drive mechanism according to claim 2 wherein. the rotational axis of the drive means and the longitudinal axis of the drive shaft are substantially in line.

4. A drive mechanism according to any one of claims 1-3 wherein the transfer mechanism incorporates a right angle drive mechanism.

5. A drive mechanism according to claim 4 wherein the right angle drive mechanism comprises a bevelled pinion, adapted to be located on the output shaft of a motor, meshing with the gear.

6. A drive mechanism according to any one of claims 1-5 wherein the drive shaft is coupled to the gear by crank means and a connecting rod and the reciprocatable shaft is mounted in front and rear bearing assemblies, the rear assembly providing said bearing means, and the bearing assemblies confining the reciprocatable shaft to movement in a direction along its length.

7. A drive mechanism according to any one of claims 1-6 wherein the rear end of the reciprocatable shaft is bifurcated.

8. A drive mechanism according to any one of claims 1-7 wherein the front bearing assembly comprises a front bearing block, a cylindrical bearing located in a transverse bore in the block, there also being a through hole in the block and the bearing, through which holes the reciprocatable shaft passes.

9. A drive mechanism according to claim 1 and substantially as hereinbefore described with reference to Figures 1-4.

10. A drive unit comprising a drive mechanism according to any one of claims 1-9 coupled to an electric motor.

11. A drive unit according to claim 10 wherein the electric motor is battery powered.

12. An electric saw wherein a drive unit according to claim 10 or 11 is accommodated in a cylindrical casing and wherein a saw blade extends from and is secured to the forward end of the reciprocatable shaft, and projects from a front end of the casing.