An electrically-controlled force-exerting actuator comprising a force-applying power spring arranged to exert a force on an output member, a force-applying control spring arranged with respect to the power spring such that the force exerted by the control spring is detracted from the force exerted by the power spring to determine the residual force exerted on the output member by the power spring, and an electric motor operable to vary the detractive force exerted by the control spring.

This invention relates to electrically-controlled force-exerting actuators. 
Such actuators are known for use, for example, in railway braking equipment 
where the actuators generate the braking forces required. 
Previously-proposed actuators have used an electric motor which itself 
generates directly the required braking forces or, alternatively, the 
braking forces have been generated by a power spring the force exerted by 
which is directly controlled by an electric motor of the actuator. In both 
these previously-proposed constructions, problems arise from the 
considerable hysteresis inherent in such direct systems. Attempts have 
been made to mitigate this problem by designing the actuator and its 
control system to operate on different parts of the hysteresis loop 
dependent upon the direction of operation of the actuator, that is to say, 
dependent upon whether the required braking force is being increased or 
decreased. However, such a solution presumes a particular hysteresis loop 
which may or may not be valid for any particular actuator and its 
operating context. 
The present invention minimises the hysteresis problem by using a control 
spring controlled by the electric motor and acting in opposition to the 
power spring. 
Accordingly the present invention provides an electrically-controlled 
force-exerting actuator comprising a force-applying power spring arranged 
to exert a force on an output member, a force-applying control spring 
arranged with respect to the power spring such that the force exerted by 
the control spring is detracted from the force exerted by the power spring 
to determine the residual force exerted on the output member by the power 
spring, and an electric motor operable to vary the detractive force 
exerted by the control spring. 
The power spring may be operative between the output member and a fixed 
part of a housing of the actuator, the control spring then being operative 
between the output member and a part the position of which is determined 
by operation of the electric motor. In such an arrangement, said part may 
be threadedly engaged with a threaded member, the operation of the 
electric motor effecting relative rotation of the part and the threaded 
member. Alternatively, said part may be a cam rotation of which by the 
electric motor determines the detractive force exerted by the control 
spring. 
There may be provided an adjustable stop variation of the position of which 
variably determines the minimum force exertable by the control spring. In 
this case, the adjustable stop may be threadedly engaged with a threaded 
part when such is provided, there then being provided a second electric 
motor operation of which effects relative rotation of the stop and the 
threaded part thereby adjustably to position the adjustable stop. The 
adjustable stop may be carried on the output member and may be 
collapsible. The stop may be engageable by a part mounted for rotation 
with but for axial movement relative to the threaded member when such a 
threaded member is provided. In this case, the threaded member may be 
normally restrained from rotation by a normally-energised 
electrically-controlled locking means effective to lock the threaded 
member until the locking means is de-energised. When the part the position 
of which is determined by operation of the electric motor is a cam as 
above described, the part engageable with the stop may be mounted for 
rotation with the cam but for axial movement relative to the cam. 
There may further be provided a final output member in addition to the 
output member and to which any residual force exerted on the output member 
can be transmitted, there then also being provided means for extending the 
final output member relative to the output member before any residual 
output force exerted on the output member is transmitted to the final 
output member. In this case, the transmission path for the residual output 
force between the output member and the final output member may include 
two relatively-rotatable and threadedly-engaged members of which one is 
clutchable to the output member and the other is engageable with the final 
output member. There may be provided a spring which urges the output 
member and the final output member to their relatively extended positions, 
the spring being operative on said other of the two relatively-rotatable 
and threadedly-engaged members. Sensing means may then be provided for 
sensing when the output member and the final output member are in their 
relatively extended positions and which, only when so sensed, allows 
operation of the electric motor which is operable to vary the detractive 
force. A further electric motor may be provided by operation of which the 
two relatively-rotatable threadedly-engaged members can be relatively 
rotated relatively to move the output member and the final output member 
against the effect of the spring urging these two output members to their 
relatively extended positions, to their relatively un-extended position. 
The sensing means may then allow operation of the further electric motor 
only after completion of operation of the electric motor which is operable 
to vary the detractive force fully to cease exertion of the residual force 
on the output member. 
There may be provided a final output member in addition to the output 
member and to which any residual output force exerted on the output member 
can be transmitted, means then being provided for extending the final 
output member relative to the output member before any residual output 
force exerted on the output member is transmitted to the final output 
member. The transmission path for the residual output force between the 
output member and the final output member may include two 
relatively-rotatable and threadedly-engaged members of which one is 
clutchable to the output member and the other is engageable with the final 
output member. There may also be provided a spring which urges the output 
member and the final output member to their relatively extended positions, 
the spring being operative on said other of the two relatively-rotatable 
and threadedly-engaged members. Sensing means may be provided for sensing 
when the output member and the final output member are in their relatively 
extended positions and which, only when so sensed, allows operation of the 
electric motor. There may be a further electric motor by operation of 
which the two relatively-rotatable threadedly-engaged members can be 
relatively rotated to move the output member and the final output member 
against the effect of the spring urging these two output members to their 
relatively extended positions, to their relatively un-extended position. 
The sensing means may also allow operation of this further electric motor 
only after completion of operation of the first-mentioned electric motor 
fully to cease exertion of the residual force on the output member.

The following embodiments of the invention will all be described in the 
context of railway brake actuators. It will, however, be understood that 
the concepts incorporated in the following described embodiment can 
equally well be used in brake actuators for other forms of vehicles. 
Indeed, they are applicable also to brake actuators for other forms of 
rotating machinery or, generally, to actuators for generating a force for 
other than braking usage. 
Referring to FIG. 1, the force-exerting actuator has a power spring 1 which 
extends between an intermediate wall 2 of a housing 3 of the actuator and 
a flange 4 carried on the end of an output member 5. Extending axially 
from the flange 4 is a cylindrical extension 6 which lies co-axially with 
the power spring 1 and, at its end remote from flange 4, has a radially 
inwardly projecting second flange 7. 
Trapped between the second flange 7 and a radially outwardly projecting 
flange 8 on a nut 9, is a control spring 10. The nut 9 is threadedly 
engaged with a ball-screw 11 which is in the form of a sleeve which passes 
through the wall 2 of the housing 3. Secured to the end of the ball-screw 
sleeve 11 on the opposite side of the wall 2 from the nut 10, is a gear 12 
meshed with a pinion 13 arranged to be driven by an electric stepping 
motor 14. 
As so far described, the actuator operates as follows: 
The actuator is shown in FIG. 1 in its "release" position. In this 
position, the control spring 10 is sufficiently compressed that it exerts 
on the flange 7 a sufficient force totally to balance the force exerted by 
the power spring 1. Thus, the power spring 1 is prevented from exerting 
any force on the output member 5. From this position, the stepping motor 
14 can be operated to rotate pinion 13 and, therefore, through gear 12, 
the ball-screw sleeve 11 in such a direction as to wind the nut 9 towards 
the left (as viewed in FIG. 1). Such movement of the nut 9 will allow the 
control spring 10 to expand and thus reduce the force which it exerts on 
the power spring 1. By such reduction, the detraction which the control 
spring 10 makes from the force exerted by the power spring 1 is reduced 
and the power spring 1 thus is freed to exert on the output member 5 a 
force which is the difference between the total force of which the power 
spring 1 is capable of exerting and the reduced force which the control 
spring 10 exerts in opposition to the power spring 1. It will thus be seen 
that, by controlling the operation of the stepping motor 14, the nut 9 can 
be positioned to control the force exerted by the control spring 10 and, 
therefore, the residual force allowed to be exerted by the power spring 1 
on the output member 5. 
In the context of railway braking equipment, the electric motor 14 would be 
controlled to determine the degree of braking required to be effected and 
this determined degree would be effected by the output member 5 being 
arranged to be operative on the braking members of the brake equipment. 
One characteristic sometimes required, in railway braking equipment, is 
that the maximum braking force allowed to be exerted shall be dependent on 
the loading of the railway vehicle. The facility to provide for this 
requirement is provided in the actuator of FIG. 1 by the following parts: 
Passing through the ball-screw sleeve 11 is a shaft 20 which, at its 
left-hand end as seen in FIG. 1, has a cup-shaped flange 21, and, adjacent 
its right-hand end, has a threaded portion 22. Engaged with the portion 22 
is a second nut 23 integral with a gear wheel 24 meshed with a pinion 25, 
and arranged to be driven by a second electric stepping motor 26. The 
motor 26 is arranged in a suitable electric circuit to be operated to an 
extent dependent upon the load of the vehicle on which the actuator is 
used. Variable operation of the motor 26 in dependence upon the load will, 
through pinion 25, gear wheel 24 and second nut 23 variably axially 
position the flange 21. The flange 21 being located in the axial path of 
the nut 9, the flange 21 will act as an adjustable stop variably to limit 
the maximum movement of the nut 9 and, therefore, the minimum value which 
the control spring 10 is allowed to detract from the force exerted by the 
power spring 1. Thus, the maximum residual force allowed to be exerted by 
the power spring 1 on the output member 5 (and, therefore, the maximum 
braking force which can be exerted) is controlled in dependence upon the 
loading of the vehicle. 
At least in certain railway braking contexts, the above described simple 
actuator suffers a major disadvantage. Because there is no provision for 
taking up the slack before the springs become operative to exert an output 
force on the output member, the springs would have to be such as to allow 
of their extension to take up such slack. The next described actuator has 
such a provision and allows for the slack to be taken up before the 
springs are allowed to be operative to exert the output force. 
Referring to FIG. 2, the right-hand half of the actuator is, in all 
essentials, substantially identical to the actuator of FIG. 1 and like 
reference numerals are used for like parts. These parts of the actuator 
operate in exactly the same manner as the like parts of the actuator of 
FIG. 1 and, therefore, here require no further description. The only point 
of difference between the two actutors in these parts is that the shaft 20 
in the FIG. 2 embodiment is tubular for a reason which will become more 
apparent hereinafter. 
The left-hand end face of the output member 5 is formed with a clutch face 
30 engageable with a complementary clutch face 31 on a nut 32. The 
periphery of the nut 32 is formed as a gearwheel 33 meshed with a pinion 
34 arranged to be driven by a third electric motor 35. Carried by the nut 
32 in the region of the gearwheel 33 is a bearing 36 by which the nut 32 
can rotationally engage a radially inwardly projecting wall 37 of the 
housing 3. 
The nut 32 is urged to the right by a spring 38 effective between the nut 
32 and a thrust bearing 39 carried by the housing 3. 
The nut 32 is threadedly-engaged with a reversible thread with a tubular 
member 40 which co-axially houses a spring 41 effective between the 
left-hand end face 42 of the cup-shaped flange 21 on the shaft 20 and a 
radially inwardly-projecting flange 43 of the tubular member 40. The 
tubular member 40 carries a pin 44 which projects into an axially 
extending slot 45 in a tubular final output member 46. 
The left-hand end of the final output member 46 is closed by an end face 47 
which carries a further thrust race 48 resiliently engaged with a flange 
49 which is trapped between the thrust race 48 and a spring 50 extending 
between the flange 49 and the flange 43 of the member 40. The flange 49 is 
formed on the end of a shaft 51 which passes through the actuator and, 
particularly, through the tubular shaft 20. Towards its left-hand end, the 
shaft 51 has a threaded portion 52 which is threadedly engaged with the 
internal periphery of the flange 43 of the tubular member 40. Adjacent its 
right-hand end the shaft 51 has a squared portion 53 slidably engaged by a 
complementary-shaped squared tube 54. At its right-hand extremity, the 
tube 54 carries an operating arm 55 arranged to control operation of a 
mechanical clutch 56 of the stepping electric motor 14. Positioned so as 
to be engaged by the operating arm 55 are a pair of electrical contacts 
57. 
The actuator of FIG. 2 operates in the following manner: 
The actuator, which is a railway brake actuator, is shown in FIG. 2 in the 
"brake-released" condition. In this condition, the clutch 56 is "made" to 
prevent rotation of the stepping motor 14 and the control spring 10 is 
thus held compressed equally to hold compressed the power spring 1. There 
is, therefore, no output force exerted by these combined springs on the 
output member 5 as is explained above in relation to the FIG. 1 
embodiment. Also, the integral clutch in the electric motor 35 is 
energised thereby holding the nut 32, the tubular member 40, the final 
output member 46 and the shaft 51 all in their positions as shown in FIG. 
2. 
To apply the brakes, the integral clutch in the electric motor 35 is 
de-energised and thereby released. Such release of this clutch allows the 
spring 41 to extend taking with it the tubular member 40 (spinning the nut 
32 on its bearing 36 through the threaded engagement of the member 40 with 
the nut 32), the final output member 46 (through the spring 50, flange 49 
and thrust bearing 48) and the shaft 51 (by its flange 49 being trapped 
between the spring 50 and the thrust bearing 48). Such movement of all of 
these parts will continue until the brakes are engaged. When such 
engagement occurs, the final output member 46 will be unable to travel any 
further. As the member 46 cannot now move any further, the spring 41, 
being the stronger, will compress the spring 50. To effect this, the shaft 
51 will be rotated on the thrust bearing 49 by virtue of the threaded 
engagement of the flange 43 with the threaded portion 52 of the shaft 51. 
In such compression of the spring 50, the tubular member 40 can move 
axially of the final output member 46 by virtue of the pin-and-slot 44/45 
connection between these two members. 
Rotation thus caused of the shaft 51 causes the squared tube 54 similarly 
to be rotated. This rotation of the squared tube 54 causes, firstly, the 
clutch 56 to be freed thus to free the motor 14 for operation and, 
secondly, by making the contacts 57, energises the motor 14. In the manner 
above described with reference to FIG. 1, the motor 14 can now be operated 
to reduce the force exerted by the control spring 10, thus reducing the 
detraction which this spring 10 makes from the force exerted by the spring 
1. Thus, the differential force is exerted on the output member 5 which is 
first moved to engage the clutch 30/31 and, thereafter, the force exerted 
on the output member 5 is transmitted through the nut 32, the tubular 
member 40, its flange 43, the threaded portion 52 of the shaft 51, the 
flange 49 of the shaft 51, and the thrust bearing 49, to the end face 47 
of the final output member 46. Thus, a braking force determined by the 
degree of operation of the stepping motor 14 is impressed on the final 
output member 46 and the previously-engaged brakes. 
Subsequently to release the brakes, the stepping motor 14 is first operated 
to re-compress the control spring 10. Such re-compression will, as above 
described with reference to FIG. 1, remove the braking force exerted on 
the output member 5 and will disengage the clutch 30/31. In this process, 
the nut 32 will be restored to its position in which its thrust bearing 36 
re-engages the wall 37 of the housing 3 carrying back with it the tubular 
member 40 and the final output member 46 as the previous stretch in the 
brake rigging recovers under relaxation of the previously-applied braking 
forces. When all the braking force has finally been relieved by the above 
actions, the spring 41 is freed to re-expand. This re-expansion of spring 
41 rotates the shaft 51 in the direction opposite to which it had been 
rotated during the brake application causing, through the squared tube 54 
and the operating arm 55, the contacts 57 to be broken and the clutch 56 
to be re-made. Hence, further operation of the motor 14 is prevented and 
the parts of the actuator controlled by the electric motor 14 are locked 
in their "brakes released" condition. The breaking of the contacts 57 also 
causes a pre-determined degree of operation of the electric motor 35. Such 
operation rotates the nut 32 and, thereby, moves the tubular member 40 a 
predetermined axial distance to the right. Through the pin-and-slot 
connection 44/45, such movement of the member 40 carries with it the final 
output member 46 to give a pre-determined brake clearance. The axial 
movement of the members 40 and 46 in this setting of the brake clearance, 
re-compresses the spring 41. 
The parts of the actuator have now all been returned to a "brakes released" 
condition with a pre-determined amount of brake clearance. Hence, it will 
be seen that the right-hand parts of the actuator are also, effectively, a 
slack adjuster for, irrespective of whatever may have been the degree of 
wear of the brakes in successive brake applications, the brake clearance 
is always adjusted during a brake-release operation to a pre-determined 
value. 
Should there be an electric-power failure, a brake application is 
automatically effected. De-energisation of the integral clutch of the 
motor 35 will allow the spring 41 to apply the brakes. Thereafter, the 
clutch 56 will be "broken" upon collapse of the spring 50 and the clutch 
30/31 "made" (both in the manner above described) so that the spring 
arrangement 1/10 is clutched to the final output member 46 to allow the 
exertion of braking forces. 
It will be noted that the motor 26, pinion 25, gearwheel 24, tubular shaft 
20 and flange 21 provide the facility for load limitation of the maximum 
braking forces allowed in exactly the same way as is described with 
reference to FIG. 1. 
Referring now to FIG. 3, there is here illustrated an actuator suitable for 
operation of railway disc-brake equipment. Some of the parts are the 
equivalency of the parts shown in the embodiment of FIG. 1 and for such 
parts, the same reference numerals are used in the two embodiments. 
The actuator of FIG. 3 includes the power spring 1 and the control spring 
10. The power spring 1 is operative between the intermediate wall 2 of the 
housing (not shown in totality in FIG. 3) and the flange 4 of the output 
member 5. The control spring 10 is operative between the second flange 7 
on the output member 5 and a nut 9 threadedly engaged with a reversible 
screw thread on a ball-screw shaft 11. 
Carried by the flange 4 of the output member 5 is a brake pad 140 
engageable with a brake disc 141. 
At its end opposite from the nut 9, the ball-screw shaft 11 carries a 
slotted disc 142 the periphery of which provides the gearwheel 12 meshed 
with the pinion 13 arranged to be driven by the electric motor 14. 
As thus far described, the actuator of FIG. 3 operates in the same manner 
as that of FIG. 1 in that, in the "brake released" condition shown in FIG. 
3, the control spring 10 is held fully compressed and overcomes the force 
exerted by the power spring 1 thus holding the brake pad 140 out of 
engagement with the brake disc 141. Operation of the electric motor 14 
will "let out" the control spring 10 and, thereby, reduce the force which 
it exerts in opposition to the power spring 1. By variably operating the 
electric motor 14 to vary the detractive force exerted by the control 
spring 10, the residual force exerted by the power spring 1 on the output 
member 5 can be varied to produce the required braking force exerted by 
the brake pad 141 on the brake disc 141. 
To lock the electric motor 14 in any desired position of its operation, is 
a pawl 145 operable under the control of a solenoid 146 to engage between 
the teeth of the pinion 13. The pawl 145 is loaded by a spring 147 
normally to be out of engagement with the pinion 13, energisation of the 
solenoid 146 driving the pawl 45 into between the teeth of the pinion 13 
against that spring loading. 
Located adjacent the slotted disc 142 is a slotted Hall-effect switch 150 
which measures the degree of rotation of the disc 142. 
Carried by the output member 5 is a first microswitch 148 which lies in the 
path of the nut 9. A second microswitch 149 carried by the intermediate 
wall 2 lies in the path of the output member 5 in its direction of travel 
to release the brakes. 
This FIG. 3 embodiment operates in the following manner: 
The actuator is shown in FIG. 3 in its "brakes released" condition and in 
this condition the solenoid 146 will have been energised to engage the 
pawl 145 with the pinion 13 and thus hold the motor 14 and slotted disc 
142 in a position in which the control spring 10 is held compressed 
sufficiently fully to overcome the power spring 1. Thus the output member 
5 will have been retracted and the brake pad 140 held out of engagement 
from the brake disc 141. 
From this "brakes released" condition, an electrical control signal is 
first generated to indicate the required degree of braking. The generation 
of this control signal will first de-energise the solenoid 146 so that the 
spring 147 will withdraw the pawl 145 from engagement with the pinion 13. 
The release of pinion 13 then frees, through the gearwheel 12, the slotted 
disc 142 and the shaft 11 for rotation. Freeing the shaft 11 for such 
rotation, allows the control spring 10 to expand driving the nut 9 to the 
left. At the same time, expansion of the control spring 10 reduces the 
force which it exerts in opposition to the power spring 1 thus allowing 
the spring 1 to expand driving the output member 5 to the left to bring 
the brake pad 140 into engagement with the brake disc 141. 
When the brake pad 140 is engaged with the brake disc 141, the output 
member 5 is prevented from any further leftward movement and the power 
spring 1 from any further extension. However, the control spring 10 is not 
so inhibited and it will continue to expand. Shortly after the brake pad 
140 engages the brake disc 141, the continuing expansion of the control 
spring 10 will carry the nut 9 to engage the microswitch 148. Operation of 
the microswitch 148 generates a signal to cause the slotted Hall-effect 
switch 150 to start measuring the rotation of the disc 142. Clearly, the 
rotation of the disc 142 is a measure of the axial movement of the nut 9 
which is, itself, a measure of the expansion of the control spring 10. In 
so far as the expansion of the control spring 10 is indicative of the 
reduction of the force which it exerts in opposition to the power spring 1 
and, therefore, an indication of the residual force exerted on the output 
member 5 by the power spring 1 and, consequently, the braking force being 
exerted by the brake pad 140 on the brake disc 141, the rotation of the 
disc 142 is an indirect measurement of the braking force being exerted. 
The output signal from the slotted Hall-effect switch 150 is therefore 
compared with the original electrical signal generated to indicate the 
required degree of braking. When this output signal indicates a degree of 
braking just short of that indicated as required by the original 
electrical circuit, the solenoid 146 is energized to prevent any further 
extension of the control spring 1. The control spring 1 will thus be 
brought to a halt at a point which will result in the required degree of 
braking. Should the solenoid 146 have been energised too early or too late 
so that the nut 14 "undershoots" or "overshoots", this will be seen by the 
switch 150 and the solenoid 146 and the motor 14 operated accordingly. 
When the switch 150 "reads" the rotation of the disc 142 as indicative of 
the braking force being that required, the solenoid 146 is energised to 
engage the pawl 145 with the pinion 13 and thus "lock-in" that required 
degree of braking. 
Should a variation in the required degree of braking now be indicated by 
variation of the original electrical signal, the pawl 145 will again be 
released and the electric motor 14 energised, if appropriate, to vary the 
degree of braking to bring it into accord with the variation to the 
electrical signal. 
To release the brakes, the electrical signal is suitably revised. Such 
revision will, again, first cause the pawl 145 to be released from 
engagement with the pinion 13 and, thereafter, the motor 14 energised. 
Energisation of the motor 14 will now, through the pinion 13 and the 
gearwheel 12, rotate the shaft 11 to "wind-back" the nut 9 and re-compress 
the control spring 10. Initial re-compression of the spring 10 will 
increase the force detracted by it from that exerted by the power spring 
1, thus reducing the braking force being exerted between the brake pad 140 
and the brake disc 141. When this braking force has been substantially 
wholly reduced, continued compression of the control spring 10 will pull 
the brake pad 140 away from the brake disc 141. Just prior to the brake 
pad 140 leaving the brake disc 141, the nut 9 will also leave the 
microswitch 148. Thereafter, the motor 14 is continued to be operated for 
a predetermined amount (again, measured by the slotted Hall-effect switch 
150) to provide the required clearance between the brake pad 140 and the 
brake disc 141. 
It will be observed, of course, that any electrical power failure will 
result in a full brake application as loss of power will result in the 
pawl 145 being retracted from the pinion 13 by the spring 147 with the 
motor 14 remaining de-energised. Hence, the control spring 11 can fully 
expand to allow the totality of the force exertable by the power spring 1 
to be applied as a braking force to the output member 5. 
It will have been noted that, so far in the description of the operation, 
no mention has been made of microswitch 149. Spring-applied brake 
actuators conventionally have a manual release facility. It will be seen 
that the microswitch 149 is positioned beyond the normal "brake release" 
condition of the actuator. After a manual release, the control spring 10 
will need to be compressed beyond its normal fully compressed condition so 
that the power spring 1 is, equally, more than fully compressed. The 
function of the microswitch 149 is to detect when the two springs 1 and 10 
have been sufficiently over-compressed as to permit re-setting of the 
manual release. 
Referring to FIG. 4, there is here illustrated another form of actuator 
incorporated into railway disc brake equipment. Again, like reference 
numerals are used for like parts in the preceding embodiments. 
The actuator again includes the power spring 1 and the control spring 10. 
The power spring 1 is operative between (in this case) the end wall 2 and 
the flange 4 of the output member 5. The control spring 10 is operative 
between (in this case) the flange 4 and the nut 9 which, in this 
embodiment, is extended into a disc-like form being slidable within the 
housing 3 but being held from rotation relative thereto by a key 61 which 
rides in a slot 62 in the interior wall of the housing 3. The nut 9 is 
threadedly engaged on the ball-screw shaft 11. 
At its end remote from the nut 9, the shaft 11 has a socket 63 of square 
cross-section which receives the squared-end 64 of the output shaft 65 of 
the electric motor 14. 
As so far described, the actuator of this embodiment operates exactly as 
those of the preceding embodiments in that: 
in the "brake released" condition of the actuator as shown in FIG. 4, the 
control spring 10 is fully compressed so as to overcome the force exerted 
by the power spring 1 and thus ensuring that there is no residual output 
force applied to the output member 5, and 
to apply the brakes, the motor 14 is freed to rotate the de-energisation of 
the latch 68. This permits the control spring 10 to expand thus reducing 
the detractive force exerted by it and, consequently, allowing an 
increasing residual force to be exerted by the power spring 1 on the 
output member 5. 
In this embodiment, the shaft 11 has integral with it a disc 66 which has a 
toothed-periphery 67 engageable by a solenoid-operated latch 68. This 
latch 68, in its de-energised state, frees the shaft 11 for rotation and, 
in its energised state, locks the shaft 11 against rotation. The latch 68 
is, of course, de-energised when the motor 14 is operated to rotate the 
shaft 11. 
It will be seen that the disc 66 is supported for rotation on thrust 
bearing 69. 
At its end remote from the disc 66, the shaft 11 is provided with a splined 
extension 70 which engages a similarly-splined axial bore 71 in a member 
72. The member 72 provides a re-entrant flange 73 between which and a 
flange 74 at the end of a tubular extension 75 of the nut 9 is a further 
thrust bearing 76. Thus, on the one hand, the member 72 can rotate with 
the shaft 11 and, on the other hand, it can move axially with the nut 9. 
Positioned in the path of axial movement of the flange 73 of the member 72, 
is a collaspsible or axially adjustable stop 77 the end face 78 of which 
facing the flange 73 constitutes a clutch face. 
The above-described construction provides for limiting the maximum output 
force which can be exerted by the actuator, particularly, although not 
only, in an "emergency application" of the brakes. 
From the "brakes released" condition of the actuator as shown in FIG. 4 in 
which the motor 14 will be de-energised and the latch 68 energised to lock 
the shaft 11, "emergency application" is effected merely by de-energising 
the latch 68. Without the motor 14 energised, de-energisation of the latch 
68 frees the shaft 11 to be rotated. As it was the locking of shaft 11 
which previously held the control spring 10 fully compressed, releasing 
shaft 11 allows the control spring 10 to extend. Such extension of the 
control spring 10, as in a "service" brake application, allows the 
residual output force to be applied to the output member 5. 
Extension of the control spring 10 will also drive the nut 9 to the left, 
the shaft 11 (now being freed to rotate by release of the latch 68) being 
thereby forced to rotate. Rotation of the shaft 11, similarly rotates 
member 72 through the splined connection of the shaft 11 with the member 
72. However, movement of the nut 9 to the left will move the member 72 
axially to the left in addition to its rotation imparted by the shaft 11, 
by the interconnection of the nut 9 with the member 72 through the tubular 
extension 75, the flange 74, the thrust bearing 76 and the flange 73 on 
the member 72. The flange 73 will thus be carried into engagement with the 
collapsible stop 77 the clutch face 78 of which, being engaged by the 
flange 73, will prevent further rotation of the member 72 and thus the 
shaft 11. Such prevention of any further rotation of the shaft 11, 
prevents further axial movement of the nut 9 and, therefore, any further 
extension of the control spring 10. Thus, by the positioning of the stop 
77, the minimum force can be set which the control spring 10 is allowed to 
detract from the power spring 1. Hence, the maximum residual output force 
allowed to be exerted on the output member 5 is determined by the 
positioning of the stop 77. 
It will be noticed that the driving of the shaft 11 by the nut 9 in this 
way, will exert an axial loading to the left on the shaft 11. Hence, the 
inclusion of the thrust race 69. 
It will also be noticed that apart from effecting an "emergency 
application" by positive de-energisation of the solenoid latch 68, such an 
application will automatically be effected should there by an electrical 
power failure. 
As shown in FIG. 4, the above described actuator is suitable for operation 
of a railway vehicle's disc brakes. 
The output member 5 is arranged to operate a final output member 80. 
Between these two members may be inserted some form of manual-release 
means 81 by which the final output member 80 can be released independently 
of the actuator. 
The final output member 80 is pivotally connected at 82 to a lever 83 
itself pivotally connected at 84 to a tension bar 85. The tension bar 85 
is, in turn, pivotally connected at 86 to one of a pair of calliper levers 
87/88. The levers 87/88 carry the brake pads 89. 
Clearly, any movement of and force exerted by the output member 5 is 
transmitted to the brake pads 89. Micro-switches 48 and 49 are again, 
provided, they having the same functions as the similarly-referenced 
micro-switches in the embodiment of FIG. 3. 
The above described embodiment has a particular advantage over the 
previously-described embodiments. If a load-limited brake application is 
being made, whether as an "emergency application" or otherwise, this will 
be a particularly heavy brake application with likely consequential 
greater wear of the brake pad or block. Whilst a slack adjuster may be 
incorporated, this will not deal with the brake wear as it occurs during a 
particular brake application but will merely adjust for such wear which 
occurs during one application before the next is made. In the previous 
embodiments it will be seen that the element which sets the maximum limit 
of a brake application (the cup-shaped flange 21 in FIGS. 1 and 2) 
constitutes a fixed "land" once it has been positioned. With these 
embodiments, because of this arrangement, should there be any appreciable 
wear of the brake members during a load-limited brake application, the 
value of the brake application will rapidly fall-off with such wear. This 
will be so for the following reason: as the brake wears, the output member 
5 will further extend under the influence of the power spring 1 to 
accommodate such wear. Not only will, therefore, the residual force 
applied through the output member 5 drop with the consequential extension 
of the power spring 1 but, much more significantly, because the flange 8 
of the nut 9 not be "grounded" on the flange 21, the power spring 1 in 
extending will have to compress the control spring 10. Now, the control 
spring 10 is far higher rated than the power spring 1 (typically, in the 
ratio of 20:1) so in compressing the control spring 10, its detractive 
force will substantially be re-increased with consequential substantial 
reduction of the residual force applied to the output member 5. 
In contrast to this situation, it will be seen that, in the FIG. 4 
embodiment, the collapsible stop 77 is carried by the flange 4 of the 
output member 5 and, therefore, the stop 77 moves with the output member 
5. With this arrangement, assuming a load-limited brake application and 
wear of the brake during the application, as the brake wears, the power 
spring 1 will, again, extend to accommodate that wear. However, with this 
arrangement of FIG. 4, such extension of the power spring 1 will 
dis-engage the clutch face 78 from the member 72. Such dis-engagement will 
free the member 72 (and, therefore, the ball-screw shaft 11) for rotation. 
Such rotation will occur as the nut 9 is now freed for axial movement 
under the influence of the control spring 10. Thus, whilst the initial 
extension of the power spring 1 in accommodating the wear had tended to 
compress the control spring 10, such compression will not, in fact, occur 
because the control spring 10 is free to compensate for such tendency by 
further axially displacing the nut 9. Hence, the two springs 1 and 10 will 
remain balanced to generate a residual force exerted on the output member 
5 which is determined by the load setting of the collapsible stop 77 
irrespective of the position of the output member 5 as it moves with 
increasing wear of the brake. 
Referring now to FIG. 5, wherein, again, like reference numerals are used 
for like parts in the previously-described embodiments, the actuator has 
power spring 1 (in the form of a pair of co-axial springs 1A and 1B) and 
the control spring 10. In this embodiment, the control spring is in the 
form of a series of springs circumferentially arranged around the tubular 
output member 5. 
The power spring 1 extends between the intermediate wall 2 of the housing 3 
of the actuator and a flange 4 carried by the output member 5. The control 
spring 10 extends between a second flange 7 on the output member 5 and a 
pressure plate 100. On the reverse side of the pressure plate 100 from the 
control springs 10 is a thrust race 101 co-axial with the output member 5. 
The thrust race 101 lies between the pressure plate 100 and a gear wheel 
102 formed on its face opposite to that against which bears the thrust 
race 101, with a cam face 103. Under the effect of the control springs 10, 
the cam face 103 resiliently bears against a roller 104. The gearwheel 102 
is arranged to be driven through spur gears (generally indicated by 
numeral 105) by the electric motor 14. 
As so far described, it will be seen that the residual output force 
generated on the output member 5 is determined in the same manner as in 
the previously-described embodiments. The spur gears 105, the cam face 103 
reacting on the roller 104, the gearwheel 102 and the pressure plate 100 
collectively operate to permit the electric motor 14 variably to compress 
the control spring 10 in a fully comparable way to that, in the previous 
embodiments. Again as with the previous embodiments, the control spring 10 
effective on the output member 5 exerts a force which is detractive from 
the force exerted by the power spring 1 on the output member 5. Hence, the 
motor 14 is operable to determine the residual force exerted on the output 
member 5. 
The shaft 65 of the electric motor 14 is coupled to a shaft 106 which 
carries near its right-hand extremity a disc 66 the periphery of which is 
toothed at 67 and is engageable with a solenoid-operated latch 68. This 
arrangement operates in exactly the same way as the similarly-referenced 
parts in the embodiment of FIG. 4 to allow for the locking and un-locking 
of the control springs 10 exerting any particular force on the output 
member 5. 
Means similar to those in the embodiment of FIG. 4 are provided for 
determining the maximum residual force to be exerted on the output member 
5 in an "emergency application". These means include the collapsible stop 
77 together with its clutch face 78. In this case, the clutch face 78 is 
engageable with the left-hand face of the disc 66. It will be seen in FIG. 
5 that the shaft 106 passes through the pressure plate 100 and, whilst 
rotatable therein, is secured for axial movement therewith by the plate 
100 being sandwiched between, on one side, a circlip 107 and, on the other 
side, a bearing 108 the outer race of which is pinned at 109 to the shaft 
106. Thus, when the shaft 106 has moved a sufficient distance to reduce 
the detractive force allowed to be exerted by the control springs 10 to a 
desired minimum (and, therefore, the residual force exerted on the output 
member 5 to have reached a desired maximum) the disc 66 being rotated by 
the electric motor 14 operating to reduce the detractive force being 
exerted by the control spring 10, will be carried by the plate 100 into 
engagement with the clutch face 68 and will be braked thereby to prevent 
further rotation of the electric motor 14. 
It will be seen that, because again the collapsible stop 77 is carried on 
the output member 5 as in the embodiment of FIG. 4, the arrangement of the 
embodiment of FIG. 5 offers the same advantage as the comparable 
arrangement of FIG. 4 in ensuring that wear of the brake does not result 
in an unacceptable reduction of the brake force in a load-limited brake 
application. 
In this embodiment of FIG. 5, the collapsible stop 77 also provides for 
variable-load adjustment of the maximum, residual force permitted to be 
exerted on the output member 5. 
As has been observed above, in railway braking actuators it is frequently 
desirable to ensure that the maximum permitted braking forces are 
consistent with the load of the railway vehicle. 
To achieve such control, the shaft 106 is rotationally supported in a 
threaded tube 110. The threaded tube 110 is engaged by a nut 111 carrying 
a pin 112 axially slidable in a slot 113 in the adjustable stop 77. Means 
(not shown) are provided for rotating the stop 77 together with the nut 
111 to a degree dependent upon the load of the vehicle. Such rotation will 
cause axial movement of the nut 111 towards or away from, as the case may 
be, the disc 66. This movement of the nut 111 will carry with it the 
collapsible stop 77 under the influence of the spring 114 extending 
between the nut 111 and the end face of the stop 77. Thus, the spacing 
between the end clutch face 78 on the stop 77 and the disc 66 can be 
varied in accordance with the load of the vehicle. This will result in the 
maximum permitted rotation of the motor 14 and, therefore, the maximum 
residual force exertable on the output member 5 to be adjusted according 
to the load of the vehicle. 
The micro-switches 148 and 149 of the embodiment of FIG. 4 are also 
provided in this embodiment. 
There is provided, in this embodiment, a slack adjuster mechanism generally 
indicated by the reference numeral 115. This slack adjuster is of a 
well-known type and in so far as it forms no part of the present invention 
any detailed description of it is unnecessary in this Specification. This 
slack adjuster operates merely to ensure the maintenance of the desired 
clearance between the brake pads 89 and the brake disc 116. This it does 
by adjusting the extension of a final output member 46 by which the pads 
89 are operated, relative to the output member 5 in the well-known manner. 
Extending from the slack adjuster 115 through the output member 5 is a 
tube 117 and a shaft 118 terminating at their right-hand end in manual 
adjustment means generally indicated by the reference numeral 119. Again, 
the construction and function of these parts are well-known and the only 
observation here needing to be made as to these parts is that they are 
provided to permit of manual release of the brake. 
The operation of the embodiment of FIG. 5 is, in all essentials, similar to 
the operation of the embodiment of FIG. 4 so that no further description 
of its operation is here required.