GB2083793A - Wrist mechanism - Google Patents

Abstract

A wrist mechanism comprises a tool holder (1) carrying non-rotatably thereon a bevel gear (5), different parts of which are in mating engagement with first and second bevel wheels (21,23) having different pitch circle diameters. Coaxially rotatable first and second drive pinions (35,41) are in mating engagement with the bevel wheels (21,23) respectively. A bevel wheel shaft (13) rotatably carries the bevel wheels (21,23) and is itself rotatably journalled in a cylindrical housing (3). The bevel wheel shaft (13) carries a tool holder support shaft (9) on which the tool holder (1) is rotatably mounted so that when the bevel wheels (21,23) are driven in opposite directions the tool holder (1) rotates on the tool holder support shaft (9) and when the bevel wheels (21,23) are driven in the same direction the tool holder (1), tool holder support shaft (9) and the bevel wheel shaft (13) are angularly displaced about the axis of the bevel wheel shaft (13). <IMAGE>

Description

SPECIFICATION Wrist mechanism The present invention relates to a wrist mechanism and in particular to a wrist mechanism having a tool orwork holder which is rotatable and angularly displaceable.

A known wrist mechanism has a tool holder which is rotatable and displaceable over an arcuate path.

The tool holder carries a bevel gear which is in mating engagement with two bevel wheels which have the same pitch circle diameter and which are driven by respective remote actuators by way of respective chains. The actuators are controlled independently such that when the two bevel wheels are driven in opposite directions the tool holder rotates and when the two bevel wheels are driven in the same directions the tool holder is dispaced angularly in a plane perpendicular to the axis of the bevel wheels.

In the known wrist mechanism the chain drive from the drive motors to the bevel wheels requires that the actuators are aligned perpendicular to the axis of rotation of the tool holder and this in conjunction with the need for chains makes the mechanism necessarily bulky. It is desirable for many installations to have the wrist mechanism as compact as possible.

It is an aim of the present invention to provide an improved wrist mechanism which has a more compact drive mechanism which can be accommodated within a relatively small housing.

According to the present invention there is provided a wrist mechanism comprising a tool or workpiece holder carrying non-rotatably a bevel gear, first and second bevel wheels having different pitch circle diameters and in mating engagement with the bevel gear, and first and second drive pinions which are in mating engagement with the first and second bevel gears respectively and which have a common axis of rotation, drive means being provided for the first and second bevel gears whereby the first and second bevel gears can be rotated in either the same or in the opposite direction so as to angularly displace or to rotate the tool orworkpiece holder.

In a preferred embodiment the wrist mechanism has a housing in which is rotatably journalled a bevel wheel shaft which carries rotatably the first and second bevel wheels. The bevel wheel shaft carries a tool holder support shaft on which the tool holder is rotatably mounted. Thus when the bevel wheels are driven in opposite directions the tool holder rotates on the tool holder support shaft and when the bevel wheels are driven in the same direction, the tool holder, tool holder support shaft and the bevel wheel shaft are angularly displaced about the axis of the bevel wheel shaft.

In the preferred embodiment the first drive pinion is mounted on a drive shaft and the second drive pinion is disposed co-axially of the first drive pinion and journalled on the drive shaft. The drive shaft carries driveably a first bevel geared crown wheel which is engaged by sun wheels which in turn engage with a second crown wheel carried by the second drive pinion. The sun wheels are carried rotatably on respective spigot pins which depend from an annular member co-axially disposed within the housing. First and second clutch or brake mechanisms are disposed in the housing whereby the annular member can be selectively coupled to the housing or to the second drive pinions such that the first and second drive pinions are rotated either in the same direction or in opposite directions.

Thus the preferred embodiment of the invention has the advantage that the drive for the tool or workpiece holder is provided by a single drive motor and this enables the wrist mechanism to be accommodated within a generally compact cylindrical housing.

In a preferred arrangement the annular member carries two pairs of braking surfaces having a V-shaped cross-section. The first braking surface is engaged by friction members carried by a pair of annular piston members which are axially displaceable with respect to one another and which are non-rotatable, but axially displaceable with respect to the housing of the wrist mechanism.

The second braking surface is likewise engaged by friction members carried by a pair of annular piston members but in this case the piston members are non-rotatable, but axially displaceable with respect to the second drive pinion.

The present invention will now be described further, by way of example only, with reference to the accompanying drawing which illustrates a simplified cross-sectional view of a wrist mechanism according to the invention.

Referring now to the drawing there is shown a wrist mechanism having a tool or workpiece holder 1 and a housing 3. The tool holder 1 carries a bevel gear 5 having teeth 7 and is rotatably mounted and axially located on a tool holder support shaft 9 by means of bearings 11. The support shaft 9 has depending therefrom a bevel wheel shaft 13 and is journalled in bearings 15 in the housing 3 and has its axis of rotation perpendicular to the axis of rotation of the tool holder 1. The bevel wheel shaft 13 also carries first and second bevel wheels 21, 23 which are rotatably journalled on the shaft by means of needle roller bearings 25, 27. The first and second bevel wheels carry respective gear teeth 29, 31 which have different pitch circle diameters.

A drive shaft 33 which is driven by means of a motor (not shown), which may be an electric or a pneumatic motor, or any other suitable actuator carries a first drive pinion 35 and a first crown wheel 37 both of which are in driven connection with the drive shaft 33. The first drive pinion carries gear teeth 39 which are a mating engagement with the gear teeth 29 of the first bevel wheel 21.

A second drive pinion 41 is co-axially disposed with respect to the drive shaft 33 and is journalled thereon by means of needle roller bearings 43 and is journalled with respect to the housing in a ball bearing 45. The second drive pinion 41 carries gear teeth 47 which mate with the teeth 31 of the second bevel wheel 23. By virtue of this arrangement the second bevel wheel and second drive pinion neces sarily have a larger diameter than the first bevel wheel and drive pinion with which they engage respectively, although it will be noted that the gear teeth of the first and second bevel wheels have the same angle, and gear teeth of the first and second drive pinion also have the same angle, i.e. they both have the same drive ratio. The second drive pinion 41 also has secured thereto a second crown wheel 49.

The first and second crown wheels carry respective gear teeth 51, 53, having the same pitch circle diameter, which co-operate with teeth 54 formed on four sun wheels 57, only two of which are shown in the drawing. The sun wheels 57 are rotatable on respective spigot pins 59 which depend from an annular member 61. The annular member is journalled in the housing 3 and carries two annular braking surfaces 63, 65 which have a V-shaped cross-section and which are associated with first and second brake mechanisms generally indicated as 67 and 69.

The first brake mechanism 67 comprises two annular piston members 71,73 which are keyed together to permit axial displacement between the two and they are keyed to the housing so as to be non-rotatable but axially displaceable with respect thereto. The annular piston members 71,73 carry friction members 75, which co-operate with the V-shaped braking surface 63, when the brake is actuated.

The second brake mechanism 69 also comprises two annular piston members 79, 81 which are axially displaceable with respect to one another and which are keyed to the second drive pinion 41 so as to be axially displaceable but non-rotatable with respect thereto. The annular piston members 79, 81 carry friction members 77 which co-operate with the V-shaped braking surface 65.

In operation when it is required to angularly displace the tool or workpiece holder 1 in a plane perpendicular to the shaft 13 the drive shaft 33 is rotated by means of the drive motor and the first brake mechanism 67 is operated. Fluid pressure is introduced by way of passages (not shown) into a chamber 83 whereby the friction member 75 is pressed into engagement with the braking surface 63 so that annular member 61 is coupled nonrotatably with the housing 3. The second brake mechanism 65 is released so that the second drive pinion and the annular piston members 79,81 coupled thereto are free to rotate with respect to the annular member 61. Thus the first crown wheel 37 drives the sun wheels 57 which in turn rotates the second crown wheel, and the second drive pinion to which it is coupled in the opposite direction to the first crown wheel.In turn the second bevel wheel 23 is driven by the second drive pinion 41. Simultaneously, the drive shaft 33 drives the first bevel wheel 21 by way of the first drive pinion 35. Thus the first and second bevel wheels are rotated in the same direction on the shaft 13 and the tool or workpiece holder is thus displaced angularly about the axis of the bevel wheel shaft 13.

When it is required to rotate the tool or workpiece holder 1 the first brake mechanism 63 is released and the second brake mechanism 65 is applied. Fluid pressure is introduced into the chamber 85 whereby the annular piston members 79, 81 apply the friction members 77 to the braking surface 65. The annular member 61 is thus coupled to the second drive pinion 41. Upon rotation of the drive shaft 33 the first crown wheel 37 drives the sun wheels 57, but because the spigot pins 59 carrying the sun wheels 57 are now effectively coupled to the second crown wheel 49 the sun wheels 57 cannot rotate on the spigot pins 59 and instead the annular member 61 is rotated by the crown wheel 37 and in turn the second drive pinion is rotated in unison with the annular member 61.Thus both the first and second drive pinions 35,41 are rotated in the same direction whilst the first and second bevel wheels 21, 23 are rotated in opposite direction (because they are on opposite sides of the axis of rotation) and the tool or workpiece holder is rotated.

In an alternative embodiment not illustrated the brake mechanisms may comprise clutch mechanisms in the form of say a dog clutch whereby the annular member 61 is positively coupled with either the housing 3 or with the second drive portion 41.

In a further alternative not illustrated the first and second drive pinions 35,41 may be provided with their own drive motor whereby they may be driven independently in the same or opposite directions without the need for the brake mechanism and differential gearing used in the preferred embodiment.

An advantage of the particular arrangement is that the fluid and/or electrical power to the tool or work holder can be conveniently run by way of a passage or passages in the tool holder support shaft and bevel wheel shaft to the housing 3 and only a simple seal is required between the end of the bevel wheel shaft and the housing. This arrangement avoids the need for fragile flexible hoses.

Claims (9)

1. A wrist mechanism comprising a tool or workpiece holder carrying non-rotatably a bevel gear, first and second bevel wheels having different pitch circle diameters and in mating engagement with the bevel gear, and first and second drive pinions which are in mating engagement with the first and second bevel wheels respectively and which have a common axis of rotation, drive means being provided for the first and second bevel wheels whereby the first and second bevel wheels can be rotated in either the same or in the opposite direction so as to angularly displace or to rotate the tool orworkpiece holder.

2. A wrist mechanism as claimed in claim 1, further comprising a housing in which is rotatably journalled a bevel wheel shaft which carries rotatably the first and second bevel wheels.

3. A wrist mechanism as claimed in claim 2 wherein the bevel wheel shaft carries a tool holder support shaft on which the tool or workpiece holder is rotatably mounted.

4. A wrist mechanism as claimed in claim 1,2 or 3, wherein the first drive pinion is mounted on a drive shaft and the second drive pinion is disposed coaxially of the first drive pinion and journalled on the drive shaft.

5. A wrist mechanism as claimed in claim 4, wherein the drive shaft carries driveably a first bevel geared crown wheel which is engaged by sun wheels which in turn engage with a second crown wheel carried by the second drive pinion.

6. Awrist mechanism as claimed in claim 5, wherein the sun wheels are carried rotatably on respective spigot pins which depend from an annular member coaxially disposed within the housing.

7. A wrist mechanism as claimed in claim 6, wherein first and second clutch or brake mechanisms are disposed in the housing whereby the annular member can be selectively coupled to the housing or to the second drive pinion such that the first and second drive pinions are rotated either in the same direction or in opposite directions.

8. A wrist mechanism as claimed in claim 6 or 7, wherein the annular member carries two pairs of braking surfaces defining a V-shaped cross-section, the first braking surface being engaged by friction members carried by a pair of annular piston members which are axially displaceable with respect to one another and which are non-rotatable, but axially displaceable with respect to the housing of the wrist mechanism and the second braking surface being likewise engaged by friction members carried by a pair of annular piston members but in this case the piston members being non-rotatable, but axially displaceable with respect to the second drive pinion.

9. A wrist mechanism substantially as hereinbefore described with reference to the accompanying drawing.