Field of the Invention
The present invention relates to drive mechanisms for transmitting
a drive from a driving rotating member to a driven rotating member and particularly,
but not exclusively, to mechanisms used for operating valves, sluices, penstocks
and the like for controlling the flow of fluids. The term "fluids" In this context
covers liquids, gases and vapours, and multi-state fluids such as liquids containing
solids in suspension.
Background to the Invention
In a typical example of an actuator drive designed to be
used to operate a motorised valve, the driving member is an electric motor and the
reduction drive to the driven shaft is in the form of a worm and worm wheel, the
latter being keyed or otherwise fixed to the actuator output shaft or "column" which
operates the moving element of the valve.
In valve actuation technology it is common practice to
arrange the drive system such that only forces and torsion generated by the driving
rotating member or members of the actuator can be transmitted through the gearing
to the driven rotating member; this is to eliminate the problem that can arise when
no power is being provided to the driving member and the unbalanced forces generated
by the pressurised fluid within the valve are sufficient, when acting on the valve
moving member, to "back drive" the actuator gear train resulting in the valve moving
member being displaced from its seating or from its last position in the fluid path
determined by the last operation of the valve actuator.
One common method used to eliminate- the back driving possibility
in an actuator gear train is to use a substantial reduction ratio on the worm/worm
wheel mesh, the resulting low value of the worm lead angle combined with the normal
friction coefficient at the meshing surfaces causing the drive to lock when torque
is applied to the worm wheel side of the drive.
In a motorised valve this locked in" torque remaining after
a powered cycle operation can be considerable, particularly in valves where the
moving member is forced on to a seat in order to eliminate fluid leakage between
the upstream and downstream valve ports.
In order to be able to operate the valve in the event of
a power failure or when installing or servicing the pipe line equipment, a two position
clutch and a hand operated wheel are provided. These members are normally provided
on the actuator output shaft which carries the worm wheel. The clutch is operated
by a manual lever. The lever is spring biased to a parked position and the normal
state of the drive mechanism is to be clutched into the power driving mode. Operating
the lever over its full arc of travel uncouples the clutch moving member from the
power driving position and moves it into the manual driving position. Simultaneously,
the lever operates a latching mechanism which ensures that the clutch remains in
the manual driving mode when the lever is released and returns to its parked position.
The latching mechanism, is so designed that it will hold the actuator in the hand
drive state until the power drive is energised again. Rotation of the de-clutched
member of the drive train then automatically releases the latch mechanism and allows
the clutch moving member to return, under spring load, to its power driving position.
This automatic reversion to the power driving mode is a
common feature of Valve Operation Technology and has come about to avoid the need
to send operating staff out, sometimes long distances, to un-manned sites to attend
to valve actuators which have been inadvertently left in manual drive mode after
a service visit etc.
The high torque generated in the actuator output shaft
and remaining locked into the shaft by the non-back-driving worm/worm wheel mesh
generates high forces on the clutch engaging members when the said clutch members
are mounted on the output shaft and so become part of the force/torsion path between
the seated valve and the locked meshing teeth on the worm wheel. These relatively
high torques and forces can cause difficulties to arise when trying to disengage
the clutch from the power drive position.
Further, on large bore valves, where considerable forces
need to be used to both seat and un-seat the valve, the output shaft mounted hand
wheel as well as the lever used to uncouple the clutch from the power mode and move
into hand drive mode need to be of substantial, all metal construction, adding considerably
to the size and weight of the actuator.
A partial solution to this problem is revealed in
U.S. Patent 4,370,902
in which an additional spur gear reduction train is inserted between the
worm wheel shaft and the actuator output shaft or column. This reduces the torsion
on the worm wheel shaft approximately in proportion to the respective teeth numbers
on the additional meshing spur gears but still leaves the torque from a -seated
valve spindle reduced by the gear drive ratio locked into the power drive side of
the actuator dutch.
The present invention sets out to eliminate the locked
in torque from the power driving side of the clutch assembly and to further reduce
the operating torque required at the hand wheel so that, in addition to requiring
less manual effort to operate the lever and hand wheel, the components that make
up the clutch, lever and hand wheel assemblies can be reduced in size and, if required,
can be manufactured from mouldings made of plastic or reinforced plastic materials.
A typical existing method, used in Valve Actuation Technology
to hold the actuator drive in hand mode after operating the lever, but automatically
to change to power drive as soon as the drive motor starts to rotate, is achieved
by providing a propping mechanism between one face of the worm wheel and the clutch
moving member. This mechanism holds the clutch moving member in its hand drive position
and compresses the spring means which urges the clutch moving member into its power
driving position. The essential feature of the prop mechanism is a pivoted latch
member which forms the end of the prop adjacent to the worm wheel. The latch member
is provided with a light centering spring which holds the member at right angles
to the worm wheel surface when the hand operation mode has been selected.
As soon as the worm wheel starts to rotate under power
drive the friction between the wheel moving surface and the foot of the latch member
overcomes the light centering spring means and causes the latch member to rotate
approximately 90 degrees to a trailing, parallel position on the worm wheel surface.
The resulting movement of the latch member pivot, in an axial direction relative
to the worm wheel shaft is sufficient to allow the clutch moving member to engage
Whilst this existing automatic re-engaging to power drive
arrangement is satisfactory, it does rely on the use of small wire centring springs
which may fail in service. Also the need to rely on surface friction between the
latch member and the worm wheel surface in order to operate the mechanism results
in the latch not being "positively locked" into the hand driving mode.
Another prior system that is back-driveable and over which
the present invention is characterised is disclosed in
. Other prior systems are disclosed in
DE-35 21 398
It is a further object of this invention to provide a latching
mechanism for the automatic clutch re-engagement operation which does not rely on
friction forces to release the latch and further, to provide a mechanism in which
the latching member is not in contact with the rotating members of the drive when
the clutch is in the power drive mode.
Summary of the Invention
According to a first aspect of the present invention, there
is provided a motorised valve actuator, the actuator having an alternative manual
drive, the manual drive comprising a hand wheel and a clutch mechanism and lever
to change from motor power drive to hand drive mode, the actuator further having
an intermediate shaft between the motor and output shaft, the hand wheel being operatively
linked to the intermediate shaft to drive the intermediate shaft, the intermediate
shaft being positioned between the motor and the gear mesh whereby when the lever
is operated to bring the actuator into hand drive mode the clutch mechanism operates
substantially freely, the output shaft torque generated by the previous motor powered
run and locked into the output shaft by the non-back driving gear mesh not being
locked into the clutch mechanism characterised in that the drive from the motor
to the output shaft of the actuator is via a gear mesh which is incapable of being
back driven, and the clutch mechanism comprises a moving clutch member and a co-operating
clutch operator having co-operating conical surfaces.
Suitably the non-back-driveable gear mesh comprises a worm
and worm wheel assembly and preferably the clutch mechanism is associated with/mounted
on the intermediate shaft.
Preferably the actuator further has a gear reduction drive
associated with the intermediate shaft to yet further increase the ease with which
the output shaft of the actuator may be manually driven.
Since the actuator of the present invention has no locked
in torque at the clutch to be overcome to disengage the clutch from the motor to
instead engage the hand drive, and since the hand drive input torque required is
reduced by the provision of the additional reduction drive between the intermediate
shaft and the worm, at least one of and preferably each of the hand wheel, lever
and certain clutch components may be formed of moulded plastics materials.
As a result of its unique drive configuration, the actuator
of the present invention may thus be constructed more cost effectively as well as
being more compact, lighter and much easier to use than the existing actuators.
Further improvements to the actuator include forming the
moving clutch member and co-operating clutch operator with cooperating conical surfaces
whereby the clutch may be moved from motor drive engagement to hand drive engagement
yet more easily. The sliding conical surfaces on the moving clutch member and the
clutch operator are preferably configured with the clutch operator's conical surface
wrapping at least partially round the moving clutch member's conical surface so
that there is substantial area contact at the start of a de-clutching operation,
the area rapidly diminishing towards a line contact as the clutch moving member
rises to the hand drive position. Suitably the clutch operator's conical surface
wraps up to 180 degrees round the moving clutch member's conical surface.
Preferably the actuator has a rotating cam member that
is driven by the lever and which cooperates with the clutch operator in order to
vary the mechanical advantage between the lever movement and the moving clutch member
via the sliding conical surfaces. This cam member suitably provides a higher mechanical
advantage at the start of a declutching operation in order to overcome the higher
static friction coefficients and provides a lower mechanical advantage ratio or
ratios for the remaining travel of the lever when operating against the relatively
lower dynamic friction coefficients, the lower mechanical advantage ratio or ratios
producing a smaller total angular travel of the lever.
Particularly preferably the lever's rotation axis is positioned
parallel and adjacent to the hand wheel's rotation axis and suitably the lever has
a curved shape and is configured to swing underneath the hand wheel circular periphery
and partly surround the hand wheel boss, both these features being used to reduce
the space required to accommodate the actuator when installed on the valve.
Preferably the actuator further comprises a latch member
to latch the clutch in engagement for hand drive and suitably the latch member is
provided with teeth, which teeth engage with teeth of a gear wheel of the drive
train of the actuator to latch the actuator in its hand drive state, thereby providing
a positive engagement when in hand drive mode and a positively driven dis-engagement
operation when returning to power drive mode.
Preferably the gear wheel is driven by the motor and associated
with/ mounted on the intermediate shaft, the gear wheel, by operation of the clutch,
being selectively engageable with the intermediate shaft to drive the intermediate
Advantageously the latch member is pivoted and has adjacent
thereto a flat spring means with which the latch member co-operates, the flat spring
means serving to bias the latch member to pivot toward its hand drive engaged latch
Preferably the latch member is provided with a faceted
boss and which boss co-operates with the flat spring means so that in a "trailing"
disengaged position the latch member is positively held out of contact with the
member with which it otherwise engages in its hand drive engaging latch position.
Suitably the latch member co-operates with the flat spring
in order to provide three parked positions for the latch member, these being the
two "trailing" positions depending on the direction of rotation of the gear wheel
29 and the central engaged position when the mechanism is latched in hand drive
The faceted boss is preferably substantially rectangular/
Suitably the latch member is pivotal and latch mechanism
of the actuator further comprises at least one static post means to deflect the
latch member to pivot positively from a disengaged "trailing" position substantially
into the latched position and preferably there are a pair of spaced apart such static
post means between which the latch member is moved as the lever is operated in order
to move the latch member from either of two trailing positions.
Whereas the clutch has a moving clutch member, the intermediate
shaft suitably has pinion gear teeth to co-operatively engage with a co-operating
gear on a shaft for driving the worm/ worm wheel, and the intermediate shaft suitably
further has splines that cooperate with the moving clutch member whereby the moving
clutch member is slidable longitudinally of the intermediate shaft but rotates therewith,
the pinion gear teeth are particularly preferably extended longitudinally of the
intermediate shaft whereby the splines are formed of these longitudinal extensions
of the pinion gear teeth.
Preferably the longitudinal extensions of the pinion gear
teeth on the intermediate shaft are cut back to a reduced diameter. The reduced
diameter of the extensions provides a location on the intermediate shaft about which
the gear wheel driven by the drive motor may freely rotate and also provides an
abutment step serving as an axial location for the motor driven gear wheel, or for
an interposing thrust washer.
In a further aspect of the present invention there is provided
a motorised valve actuator of the type having an alternative manual drive, the manual
drive comprising a hand wheel and a clutch and lever to change from motor power
drive to hand drive mode, wherein the actuator comprises a latch member to latch
the clutch in engagement for hand drive or motor drive, the latch member being provided
with teeth which engage with teeth of a gear wheel of the drive train of the actuator
in order to provide a positive engagement when in hand drive mode and a positively
driven dis-engagement operation when returning to power drive mode.
Brief Description of the Drawings
Some background technology and preferred embodiments of
the invention will now be described, by way of example, with reference to the accompanying
drawings and-diagrams in which:
Description of the Preferred Embodiments
- Figure 1 illustrates the essential drive line features of a motorised valve
actuator using the existing known technology.
- Figure 2 shows the drive line features of a valve actuator referred to in
U.S. Patent 4,370,902
. This drawing also shows, in simplified form, the automatic re-engaging
clutch mechanism employing the existing known art
- Figure 3 is a sectional view of a motorised valve actuator, the subject of this
invention, showing the principal drive line components including the clutch and
clutch operating mechanisms. For clarity some of the rotating components have been
shown in a single plane.
- Figure 4 is a partially exploded isometric sketch showing some of the components
comprising the intermediate shaft with the adjacent clutch operating mechanism.
- Figure 5 shows the actuator components that comprise the latching and automatic
release feature of the clutch, the said components relative positions corresponding
to the actuator being in the power drive mode.
- Figure 6 illustrates the same components as in Figure 5 but with their relative
positions corresponding to the actuator being in the hand drive mode.
- Figure 7 is a view looking on to the face of the hand wheel with the adjacent
clutch operating lever shown in both its parked and its operating positions.
- Figure 8 shows an embodiment involving the intermediate shaft in which the integral
driving pinion gear on the shaft is extended and further machined to form the splines
which locate, radially, the moving clutch member.
Beginning with the prior art and referring firstly to Figure
1, showing a simplified view of the drive line of a currently designed valve actuator,
the electric motor 1 drives the worm wheel 2 by means of the worm 3. The worm wheel
2 is free to rotate on the actuator output shaft 4. A moving clutch member 5 is
free to move axially on the output shaft 4 but is located radially on the shaft
by the key or splines 6.
A hand wheel 7 is also mounted on the output shaft 4 being
free to rotate independently of the shaft. The clutch moving member 5 can be moved
into engagement with either the worm wheel 2 by means of the dogs 8 and slots 9
or into engagement with the hand wheel 7 by means of the dogs 10 and slots 11.
The lead angle on the worm 3, meshing with the worm wheel
teeth, is normally sufficiently small to prevent back driving of the mechanism when
torque is applied to the shaft 4. In this non-driving mode with the clutch moving
member 5 still engaged with the dogs 8 on the worm wheel 2 the torque locked into
the output shaft 4 is transmitted to the locking mesh of the worm wheel and worm
via the dogs and slots 8 and 9. This results in a significant axial force being
required to disengage the clutch from the power drive mode.
Following clutch disengagement from the power drive and
movement into hand drive mode, the hand wheel 7 is then able to operate the valve
via the actuator output shaft 4; the hand wheel torque required being equivalent
to that supplied by the power drive. For this reason, on medium and large size motorised
valve installations, the hand wheel has to be provided with extended spokes 12 to
enable the operator to generate the significant manual torque required to operate
the valve manually.
Figure 2 shows a known partial solution to the problem
of reducing the hand wheel torque by placing the hand wheel drive with the associated
clutch mechanism and the worm wheel on an intermediate shaft 13 carrying pinion
gear 14 which meshes with a gear wheel 15 mounted on the actuator output shaft 16.
Figure 2 also shows one form of the known mechanism used for automatically re-engaging
the power drive when the motor 1 is energised.
In this mechanism the axially movable shaft 17 is raised
by the lever 18 to push the clutch moving member 19 out of engagement from the worm
wheel 2 and into engagement with the hand wheel 20 and compressing the spring 21.
On the lower end of the shaft 17 is pivoted a latch member
22, shown in its trailing attitude with the clutch still engaging the worm wheel.
This latch member is urged into a vertical position from either a left or right
hand trailing position (depending on the rotation direction of the worm wheel) by
means of a self centering wire torsion spring (not illustrated) surrounding the
With the electric motor 1 stationary and the lever 18 operated
to bring the actuator into manual drive, the shaft 17 is raised allowing the latch
22 to rotate into its central position in axial alignment with the shaft 17. In
this state the latch member 22 is acting as a "prop" holding the clutch moving member
19 into its hand drive mode via the shaft 17.
On restarting the motor 1 the friction force between the
distal end of the latch member 22 and the rotating surface of the worm wheel 2 is
sufficient to overcome the centering torque generated by the self centering spring
at the pivot 23 causing the latch member to be rotated into one of its two "trailing"
positions (depending on the direction of rotation of the worm wheel) and so allowing
the shaft 17 and the clutch moving member 19 to descend into the power drive position
under the action of the spring 21.
The various improvements to the actuator drive line assembly
are illustrated in Figures 3 and 4, Figure 3 being a semi-diagrammatic sectional
view through the principal shaft centres of the drive and Figure 4 showing an isometric,
vertically exploded view of some of the components making up the clutch mechanism
and which need further clarification. In the sectioned design illustrated, the intermediate
shaft carries the moving clutch member 25 and is provided with an integral pinion
26. The motor shaft 27 has an integral pinion 28 which drives the gear wheel 29
also located -on the intermediate shaft 24 but free to rotate independently of the
The moving clutch member 25 is provided with splines 30
which cooperate with splines 31 allowing the moving clutch member to slide freely
on the shaft 24 but to be fixed rotationally relative to the shaft. In the sectioned
view, as drawn in Figure 3, the moving clutch member 25 is in its descended power
driving position with a protruding peg 32 cooperating with a circular hole or arc
shaped blind slot 33 formed in the gear wheel 29. In this mode the motor 1 is able
to drive the intermediate shaft 24 via the meshing pinion 28 and gear wheel 29 the
torque being transmitted to the shaft 24 via the peg 32 cooperating with the hole
or blind slot 33 and the splines 30 on the moving clutch member 25 meshing with
the extended length splines 31 on the shaft 24.
The intermediate shaft 24 is permanently, drivably coupled
to the actuator output shaft 4 via the pinion 26 meshing with the gear wheel 34
which, in turn, is keyed to the shaft 35 on which is machined the worm 3. The worm
meshes with the worm wheel 2, the latter being keyed or otherwise permanently attached
to the actuator output shaft 4.
One significant advantage of the actuator drive disclosed
in Figure 3 follows from the removal of the clutch assembly from the worm or wormwheel
shaft to an intermediate shaft driving the said worm shaft 35 suitably through an
intermediate gear reduction train (pinion 26 and gear wheel 34). Unlike the actuator
drives illustrated in Figures 1 and 2, the locked in torque from the non-back-driving
worm and worm wheel mesh is no longer transmitted via the clutch dogs and slots.
Further, since the reduction drive from the motor to the intermediate shaft is via
involute gear teeth (on pinion 28 and gear wheel 29) which can be back driven to
rotate the motor 1 armature, none of the driving torque transmitted when in power
drive mode can be left locked into the clutch dogs and slots when the power to the
motor is switched off.
The disengagement of the clutch from power drive to hand
drive mode can therefore be done without having to overcome any significant locked
in forces in the gear train resulting from the previous power run cycle. As a consequence,
the clutch assembly and the associated operating lever 36 can be made of light construction
and, in particular, can make use of plastics mouldings in place of the relatively
heavy metal components in the designs depicted in Figures 1 and 2. The additional
gear reduction drive via the intermediate shaft further enables the hand wheel assembly
to be of lighter construction than that required in the designs shown in Figure
1, enabling replacement of the extended spokes 12 and the large hand wheel 7 with
the modest size, moulded wheel 37 and turning handle 38.
In order to keep the actuator housing to the smallest dimensions
possible and to be comparable with the reduced sizes achieved by the hand Wheel
and lever, a new compact design of clutch and lever assembly is revealed in Figures
3 and 4. A significant step in achieving this compact design is to arrange for the
lever 36 rotation axis to be parallel and adjacent to the hand Wheel 37 axis, the
axes centre distances being such that, if required, the lever axis can be positioned
within the peripheral diameter of the hand wheel as shown in Figure 7. The curved
shape of the lever is a further feature of the compact design, the lever profile
in its parked position curving round the hand wheel boss so that it is partly hidden
beneath the hand wheel.
The means by which rotation of the lever 36 raises the
moving clutch member 25 from its spring loaded power drive position is as follows.
The moving clutch member 25 is provided with a conical
outer surface 39 which cooperates with an extended inner conical surface 40 of the
clutch operator 41. The cone half angle is approximately 45 degrees as shown in
Figure 3: the surrounding clutch operator conical inner surface 40 is cut away as
shown in Figure 4, the "wrapping" angle surrounding the moving clutch member 25
being equal to, or less than 180 degrees in order to allow the clutch operator 41
to travel in a horizontal direction to the right as viewed in Figure 3.
The clutch operator is constrained by the surrounding housing
to move only in a single horizontal plane, the travel to the right being converted
into an equivalent vertical lift of the moving clutch member 25 by the sliding action
of the extended conical surface 40 on the surface 39, the vertical movement disengaging
the power drive and engaging the set of radial face teeth 42 on the moving clutch
member 25 with a cooperating set of teeth 43 on the hand wheel shaft assembly.
The movement of the lever 36 (clockwise as viewed in Figure
7) from its parked position to its operating position 44 causes the clutch operator
41 to travel in its constrained horizontal direction by the action of the profiled
slot 45 in the rotating cam member 46 on the integral post 47 moulded on to the
clutch operator 41. The cam member 46 is keyed, or otherwise attached, to the shaft
48 of the lever 36. The profited slot 45 is so shaped that, from its parked position,
a significant portion of the movement of lever 36 is used to provide a small initial
deflection of the post 47 in order to increase the mechanical advantage of the lever/cam
profile and so overcome the higher static friction force between the operating conical
surfaces 39 and 40 at the start of a dedutching operation.
Once movement starts, the friction coefficient between
the lubricated conical surfaces will decrease to a value, typically about half the
static value coefficient enabling a lower mechanical advantage to be used for the
remainder of the lever 36 travel.
It should be understood that the male cone surface 39 and
its cooperating half circular surface 40 are only in full surface contact at the
start of a power declutching operation. This allows the maximum available contacting
surface area to be used with consequent minimum surface contact pressure when overcoming
the force generated by the higher static friction. As soon as the clutch operator
41 starts to move and the moving clutch member rises, the surface area in contact
rapidly starts to diminish because horizontal (as drawn in Figure 3) sections taken
through the contacting cones will show, in theory, circles of different diameters
in line contact. In practice the surface deflection produced by the operating forces
will provide a narrow width contact area in place of the theoretical line contact
of the two moving conical surfaces.
The lever 36 is spring biased so that, on release, it returns
to the parked position parity shielded by the overhanging hand wheel 37. Referring
to Figure 4, this is achieved, typically, by means of a coil tension spring 49,
one end being anchored to the actuator housing and the other end being attached
to the cam member 46 at a position such that the spring tension force produces the
return travel torque on the cam member 46 and thence to the lever 36 via the lever
The principal components of the new design latching system
are also illustrated in Figures 3 and 4 with diagrams, Figures 5 and 6, showing
the relative positions of the latching components in their unlatched (power drive)
and latched (hand drive) states respectively.
The post 47 is provided with a hole in its base into which
is inserted a small shaft 50 acting as a pivot for the new design latch member 51:
this last component operates in the same manner as the known component 22 in Figure
2 but provides significant improvements.
In the non-latched power drive mode (Figure 5) the latch
member 51 is held clear of the gear wheel 29 by the action of the flat spring 52
which holds the latch member in one of three angular positions 90 degrees apart
by cooperating with the sides of a square boss 53 surrounding the pivot centre.
This arrangement provides three definite parking positions at 90 degree intervals,
the central position being the "latched" state.
The flat spring ends are located by slots, or other means,
in the clutch operator 41 and therefore stay in the same position relative to the
pivot and shaft 50 during a de-clutching operation. During this operation the latch
member 51 is forced to rotate through nearly, 90 degrees by being withdrawn between
the two posts 54 located in the actuator housing. The final angular travel to the
latched position is achieved by the flat spring 52 being first deflected by the
corner of the rotating square boss 53, shown chain dotted in Figure 5, and then
returning to it's initial flat position forcing the latch member the final few degrees
into the position shown in Figure 6. In order to achieve this final latched position,
the horizontal travel has to be a little longer than the distance between latched
and unlatched positions. This is to allow the teeth 55 on the latch member to pass
over the teeth on the gear wheel 29 before engagement
The return to power drive mode is initiated by the rotation
of the gear wheel 29 driven by the motor 1. The latch member 51 now rotates; being
in mesh with the engaged teeth of the gear wheel, and so allowing the clutch operator
41 to return towards the power drive mode position. The final few degrees of rotation
cause the latch member 51 to "spring" into the Figure 5 position with all the latching
teeth 55 held clear of the gear wheel teeth.
Important advantages of this design of latching mechanism
with the associated automatic release feature over the known mechanisms used in
valve actuator technology are listed below:
- 1) The latch member 51 is positively located against the gear wheel 29 in the
latched (hand drive) condition and is positively driven to its non-latched position
when the gear wheel starts to rotate. There is no reliance, therefore, on rubbing,
friction generating components in order to secure a change from hand drive to power
- 2) In power drive and the motor operating, the latch member 51 is held clear
of the rotating gear wheel 29. This is a significant improvement on the known mechanism
where the trailing latch fever is in continuous sprung contact with the worm wheel
adding slightly to power drive friction; but, more importantly, producing wear debris
which may eventually contaminate the lubricating oil over a long running period.
- 3) The rotation of the latch member 51 from the power drive mode to (engaged)
hand drive mode is achieved primarily by the arrangement wherein the latch member
passes between the two posts 54 and is deflected. This is a more positive operation
than relying on the known method of using a self centering wire spring to achieve
the change to a latching position. Use of spring 52 to complete the last few degrees
of the 90 degree rotation of the latch member completes the positive latching operation
while safeguarding against risk of "locking" of the latch 51 in hand drive by the
posts 54. -
An alternative embodiment of the invention aimed at reducing
the costs of manufacture of the intermediate shaft 24 is illustrated in Figure 8.
Referring firstly back to Figure 3, the shaft 24 shown
in Figure 3 carries an integral meshing pinion 26 having teeth of standard involute
form and an upper, as drawn, male spline section 31 which cooperates with the splined
bore of the moving clutch member 25. Apart from the additional tooling costs in
having different sets of teeth (pinion and splines) the enlarged central diameter
on which the shaft 24 can rotate relative to the gear wheel 29 (in hand drive) prevents
the use of a through broaching operation when forming the teeth.
Referring to Figure 8, the intermediate shaft 56 carries
only one set of gear teeth which are extended to form the driving pinion at the
lower end of the- shaft as drawn. An abutment step 58 is formed on the shaft by
machining down the teeth, typically to a diameter 59 approximately equivalent to
the involute gear circular pitch diameter; that is machining off the gear teeth
The bore of the moving clutch member 25 can now be machined
out to form a cooperating splined hole to slide on the cut down teeth on the shaft
56 as shown in the upper local transverse section. The gear wheel 29 can rotate
over the cut down teeth as shown in the middle local section in Figure 8. A thrust
washer 60 may be interposed between the abutment step 58 and the moving clutch member
25. The shaft 56 only rotates at relatively slow speed within the gear wheel 29
and without transmitting any torque to the wheel when the actuator is in hand drive
so there will be no significant wear due to the interrupted circular contacts between
the gear wheel and the shaft.