LINEAR MOTION BRAKE DEVICES

Abstract: A lock device capable of supporting a load is given whereby a first threaded member (3) cooperating with a high lead angle second threaded member (12) is provided to control linear movement parallel to the axis of the threaded members. The high lead angle is sufficient to be backdriving or non self-locking. A first brake (14) opposing relative rotation of the threaded components is provided and a second brake (19, 22) having a surface concentric with the axis of the threaded members and contacting at least one threaded member to lock said members to preclude linear movement is also provided. The second threaded member (12) cooperates with the first threaded member (3) to carry the load when the system is locked. An actuator (20) is provided to selectively maintain the second brake in a substantially inoperative mode.

Description: LINEAR MOTION BRAKE DEVICES The present invention relates to a mechanical locking brake mechanism which can be used to safely control linear mechanical movement. In particular this invention relates to any extensible load-bearing rod, motivated by any suitable mechanical, electrical, pneumatic or hydraulic means. Mechanical locking mechanisms can be advantageously employed in fluid or hydraulic operations as well as other applications where linear movement is involved. Such structures include, but are not limited to single-acting cylinders such as, for example, hydraulic jacks and push rams; double-acting cylinders such as, for example, push-pull cylinders of the type used to support a crane telescopic arm; and multi-stage cylinders. In such systems, the cylinder may be driven relative to the piston when the latter is substantially static/or otherwise. The present invention provides a mechanical locking mechanism where threaded members are employed which are capable of supporting a load when locked to preclude mechanical movement. This invention additionally provides for a cylinder and piston which operates to prevent relative movement between the piston rod and the cylinder at any point along the extensible length of the rod. There are many applications for such a brake and locking mechanism either to achieve voluntary locking engagement of the rod and cylinder at any desired extension, or to provide an effective fail-safe mechanism. This can be employed, for example, when there is a failure of pressure under load, or to prevent dangerously rapid acceleration of the rod. It is important to provide s uch f ail-saf e mechanisms i n load bearing devices and, in particular, hydraulically operating lifting tackle, jacks, cranes, supports and struts. According to the present invention, a mechanical locking brake system is provided for preventing relative axial movement between a static member and an extensible load-bearing member in an a cylinder and piston arrangement or the like, which mechanism comprises a static member including at least one first bearing surface, a load-bearing member having a threaded portion on at least part of its extensible length, a locking member adapted to threadingly mesh with the thread portion of the load-bearing member for rotation thereabout being suspendably mounted on support means during axial movement of the load-bearing member relative to the static member, said locking member being provided with at least one second bearing surface, and support upsetting means actuatable by remote control means or automatically either upon acceleration of the load-bearing means beyond a pre-determined safety limited or upon moving faster than a pre-determined safety limit or upon moving faster than a pre-determined safe speed to bring respective opposing first and second bearing surfaces into locking interengagement thus restraining rotational movement of the locking member to retard or prevent relative axial movement between the static and load-bearing members. Thus in accordance with this invention a lock device capable of supporting a load is given whereby a first threaded member cooperating with a high lead angle second threaded member is provided to control linear movement parallel to the axis of the threaded members. The high lead angle is sufficient to be backdriving or non self-locking. A first brake opposing relative rotation of the threaded components is provided and a second brake having a surface concentric with the axis of said threaded members and contacting at least one threaded member to lock said members to prevent rotation and preclude linear movement is also provided. The said second threaded member cooperates with the first threaded member to carry the load when the system is locked. An actuator is provided to selectively maintain the second brake in a substantially inoperative mode. The two brake surf aces or first and second bearing surfaces are preferable substantially conical surfaces, but may be substantially spherical or planar surfaces, adapted for frictional locking engagement. Ratchet teeth and other friction surfaces may also be used. The threaded members or intermeshing threads may be square helical threads, and are preferably multiple start threads. Alternatively, the threads may be provided by an acme thread, ball thread, or other similar conventional designs. Advantageously, the bearing surfaces, locking members, support means, and support upsetting means or actuator of the mechanical locking mechanism are housed substantially within the static member, for example, within the cap portion of a static hydraulic cylinder. The support means may suitable be provided by a lever arrangement providing support for the locking member, whereby means are provided such as a spring to balance the lever in a support position. Suitable means are provided to upset the balance of the lever to remove support for the locking member to trigger engagement of the locking mechanism. It will be readily appreciated that the upsetting means may be remotely actuated by any suitable mechanical, electrical, electromagnetic or hydraulic means. The locking member support means may include a thrust bearing, preferable a ball or roller bearing suspendably mounted between the static member and the locking member wherein the locking member is provided by a lock type brake nut. According to the invention, there is provided a mechanism for regulating translational displacement of a body, comprising: a translational member, a static member, and a ro tation al memb er whi ch is ro ta ta b l e r el ative to both said translational member and said static member, the translational member being associative with said body for translational displacement relative to said static member, and the rotational member (a) being drivingly associated with one of said translational and static members for rotation of said rotational member during said relative translational displacement of said translational and static members, (b) having a plurality of cooperating dispositions relative to one of said translational and static members during said rotation of said rotational member, and (c) being rotation-retardingly cooperative with said one of said translational and static members, relative to which the rotational member has a plurality os cooperating disposition, in at least one of said dis p ositi on, to su bstan tially control or prev ent said relative translational displacement of said translational and static members. Said rotational member may have displacably friction members for retarding engagement against one of said translational and static members of the mechanism. In a preferred arrangement, said retarding engagement of the frictional members against said one of the translational and static members is dependent on the speed of rotation of said rotational member. Alternatively said rotational member may be associated with a movable member of a clutch-type device. Said clutch-type device may suitably be a fluid clutch. In a favored aspect, the invention provides a mechanism for regulating translational displacement of a body, comprising: a translational member, a static member, and a rotational memb er whic h is rotatable relati ve to both said translational member and said static member, the translational member being associative with said body for translational displacement relative to said static member, and the rotational member (a) being drivingly associated with one of said translational and static members for rotation of said rotational member during said relative translational displacement of said translational and static members, (b) being associated with the other of said translational and static members for limited translational displacement relative to said other member between a plurality of dispositions relative thereto, and (c) being rotation-retardingly cooperative with said other member in at least one o f said disposition, to substantially control or p re v e n t s ai d r e l a ti v e t r a ns la t i o nal d is p l a c e m en t o f sa i d translational and static members. In a preferred embodiment, said rotational member is drivingly associated with said one of said translational and static members by means of a non-self-locking screw thread. Said translational and static members are then most suitably relatively displacable in a direction which is substantially parallel to the axis of rotation of said rotational member. In said preferred embodiment, said rotational member most suitably has at least one friction surface for engagement against a cooperating friction surface of said other of said translational and static members. Said f riction surfaces of said rotational member and said other of said translational and static members are preferably frusto-conical, for locking said translational and static members against relative translational displacement in the direction of tapering of said surfaces, in an engaged condition of said surfaces. In said preferred embodiment, means may be provided for moving said f riction s ur faces o f said ro tational mem ber and s aid oth er of said translational and static members from a said engaged condition of said surfaces into a spaced apart disposition, so that relative translational displacement of said translational and static members may take place in said direction of tapering of said frusto-conical friction surfaces. The mechanism may also incorporate control means for preventing said movement of said friction surfaces of said rotational member and said other of said translational and static members into said spaced apart disposition in the presence of an axially-acting load in excess of a predetermined value on one or both of said translational and static members. In said preferred embodiment, one of said translational and static members may be the cylinder of a hydraulic actuator or a component fixedly associated with said cylinder, and the other of said translational and static members may be the piston of said actuator or a component fixedly associated with said piston. Said control means may include logic means for preventing pressurization of a chamber of a said hydraulic actuator for relative translational displacement of said static and translational members in a particular direction when said friction surfaces are in an engaged condition to lock said static and translational members against displacement in said direction. It is an object of this invention to have a lock that will apply irrespective of the orientation of the device with respect to gravity or inertial forces. It is a further object ot this invention to provide a lock that is double acting but may be opened in one direction only. It is a further object of this invention to provide a lock that can be used without the presence of keyways, splines or other anti-rotation structure which allows parts to be locked against movement under load while maintaining automatic length adjusting features associated with existing backdriving devices. A further object of this invention is a lock which will apply automatically if a predetermined acceleration is exceeded or if a predetermined velocity is exceeded. It is yet another object of this invention to provide a lock device that af ter extension locks immediately to prevent retraction and after retraction locks immediately to prevent extension in the presence of considerable axial thread clearances associated with high lead angle threaded members. And finally, it is an object of this invention to provide a device which is simple and inexpensive to manufacture while maintaining high load bearing capabilities and a high degree of safety in operation. Further objects and advantages of this invention will be seen from a full understanding of the instant specification. Preferred embodiments of a mechanical locking mechanism for a hydraulic cylinder and piston arrangement or the like will now be described in greater detail with reference to the accompanying drawings in which: Figure 1 is a longitudinal cross-section of a first embodiment of the invention comprising a single-acting push ram and lock type brake mechanism; Figure 2 is a plan view of the embodiment of Figure 1 on section line A-A; Figure 3 is a second plan view of the embodiment of Figure 1 on section line B-B; Figure 4 is a longitudinal axial cross-sectional view of another embodiment of the invention, showing a double-acting push-pull ram with a single-acting, internally releasable, lock type brake mechanism operating within the region of the ram containing the hydraulic oil; Figure 5 is a section showing a modif ication of the detailed structure shown in Figure 4; Figure 6 is yet another section showing a modification of the detailed structure shown in Figure 4; Figure 7 is a longitudinal axial cross-sectional view of an embodiment of the invention, shown a threaded rod lock type brake mechanism; Figure 8 is a partial section in the axially downward direction of mechanisms of the type shown in Figure 7 - showing a particular construction of a release mechanism in longitudinal cross-section; Figure 9 is a longitudinal axial cross-sectional view of an alternative arrangement of release mechanism; Figure 10 and Figure 11 are related with Figure 11 being a longitudinal axial cross-sectional view of the "control" portions of another embodiment of this invention in which a double-acting push-pull ram is provided with a double-acting lock type brake mechanism, the lock being releasable in either the extend or the retract sense, but not in both senses simultaneously; and Figure 10 being a downward continuation of Figure 11 on a different scale to allow for an improved understanding and showing the lock type brake region of the mechanism of Figure 10; Figure 12 shows centrifugal , weight arrangements for providing a speed-sensing automatic braking feature in a device according to the invention; Figure 13 is a cross-sectional view illustrating rolling contact by means of spring-loaded rollers; Figure 14 is a longitudinal axial cross-section of the lower portion of a fully retracted double acting fluid actuator fitted with a double acting brake with a first threaded member having two engageable portions, a composite nut or two connected nuts, and a rotating threaded bar; Figure 15 is a longitudinal axial cross-section of the piston portion of a duble acting fluid actuator fitted wth a double acting brake with two rotating nuts and a floating control piston; Figure 16 is a longitudinal axial cross-section the piston portion of a double acting fluid actuator fitted with a double acting brake wit tw rotating nuts and floating control piston; and Figure 17 is a longitudinal axial cross-section of fluid seal rings and rod brake arrangement. In Figures 1 through 3, a first embodiment of a mechanical locking brake mechanism in accordance with the present invention is shown. A hydraulic cylinder 1 houses a hydraulic piston 2 having a piston rod or ram 3 threaded along its length. A double start stub square thread is shown. A hollow end cap 4 screws onto cylinder 1. A rod bush 5 in turn screws into this end cap. A bushed "T" piece mounting 6 is provided at the free end of piston rod 3 and a trunnion mounting arrangement 7 is provided for the cylinder 1 in a manner known per se. An oil port 8 is also provided at the base of the cylinder and 9 are wear rings and seal 10 and 11 is provided for- piston 2. A conical locking type brake nut 12 is internally-threaded to mesh with the threads on the piston rod so as to be freely rotatable thereabout during extension of the rod. A thin plastic expansible disk 13 is seated on top of the lock nut 12 and is likewise threaded and split into three equal segments as shown in Figure 2. A first brake to oppose relative motion of the threaded components in the form of a garter or extension spring 14 is stretched around the circumference of the plastic disc structure to press it tightly against the threads of rod 3. Pegs 15 engage slots in the disc to cause lock nut 12 and disc 13 to rotate together. Ball bearing race 17 is held in position on the outside of the skirted portion of the lock nut 12 by an interference fit retainer ring 18. Lock nut 12 is suspendably supported by pins 22 projecting inwardly with an interference fit from an annular control ring 19, there being two pins above and two pins below the ball bearing race 17, such that in the raised position illustrated, lock nut 12 is supported on two diametrically opposed points of support. Ring 19 is free to tilt about lever 20 which itself can hinge about pin 21. This lever 20 is operable from the cylinder exterior. A thin circular plastic sheet 23 having a hole through its - center separates disc 13 and nut 12 and helps to prevent ingress of dirt into the mechanism. Operation of the locking mechanism embodied by the hydraulic cylinder and piston arrangement illustrated in Figures 1, 2, and 3 will now be explained. Drive force is transmitted to the piston 2 by increasing hydraulic pressure within the sealed cylinder space below the piston via -fluid port 8, in a manner known per se. so that in use the piston 2 will move relative to cylinder 1. Piston 2 will push the piston rod 3 in the sense that will move it towards the extended position, through rod bush 5. Because the conical lock nut 12 is threaded onto piston rod 3 and because of the breaking force generated by the first brake spring 14, the first threaded member, nut 12, may be "carried up" with the piston. However, the two pins 22 above the ball bearing race 17 limit the "upward" movement of the nut 12. These pins are in turn prevented from moving "up" by the fact that they are attached to control ring 19 which, although it can hinge via lever 20 about pin 21, is stopped by pin 16. Thus, nut 12 can rotate freely, but is prevented from moving axially "upwards" beyond a certain limit. The thread on rod 3 and nut 12 is not self-locking because of- the sufficiently large lead thread angle on the second threaded member which is greater than the angle of friction, whereby the axial force imposed on the nut 12 will tend to rotate it. As it rotates it will drive the disc 13 with it via pegs 15. Sheet 23 provides a dirt exclusion function, while the disc 13 scrapes and cleans the thread of rod 3. However, first brake garter spring 14 imparts a constant slight retarding brake force on the rotation of disc 13 as well as compensating for wear as the disc gradually wears down where it contacts threaded rod 3. This slight brake force is passed on to nut 12 via driving pegs 15. This has the desired effect of opposing the inertia of the nut 12 and stopping its rotation almost immediately the rod 3 stops. More importantly, garter spring 14 prevents the nut from back driving due to its own weight. If the cylinder is oriented upside-down for the form of Example 1, the cylinder would not work in the absence of the garter spring. Thus the rod 3 may extend freely, causing the nut 12 and disc 13 to rotate as it does so. If however the rod 3 starts to retract, it will carry the nut 12 "down" with itself. The nut will not be prevented from moving "down" by the ball bearing race 17 as this will simply push against pins 22 below it, causing ring 19 to hinge away slightly about hinge pin 21. Thus the conical bearing surface of the nut 12 will frictionally engage and lock against an opposing bearing surface on the hollow end cap 4 of the cylinder. Also, the rotational moment resulting from the rod 3 pushing "down" on the interengagement of the threads of 3 and nut 12 will be insufficient to overcome the moment of friction of the conical bearing surfaces, that oppose rotation, and cause the nut to lock. At this stage the rod 3 will have been prevented from moving "down" or retracting any further and the cylinder and piston will now be locked. It should be noted that the amount by which the rod 3 is able to move "down" or retract, before bringing about locking engagement of the mechanism is determined by the gap separating the opposing conical bearing surfaces, which in practice may be quite small and typically from less than 1 to 5 millimeters. The gap may easily be varied according to particular application requirements by for example, changing the vertical position of stop 16, and can be as little as a fraction of a millimeter. Thus, effectively the rod is lockable in all positions of extension. The conical bearing surfaces advantageously provide a self-centering effect, as well as increasing the frictional force and thereby the moment of the friction force which prevents rotation. It should also be pointed out that because control ring 19 can tilt freely about hinge axis 21, and because the vertical separation between pins 22, which are disposed parallel to hinge axis 21, is such that it leaves some clearance for the bearing 17, this arrangement acts as a universal mounting support. That is, the force imparted by the pins 22 on nut 12 is always axial with respect to the threaded rod 3 and so tends not to cant the nut over. In order to release the locking mechanism and to retract the rod 3 it is necessary to disengage the frictionally locked conical bearing surfaces. This may be done by applying a "downward" force to the end of the control lever 20, outside end cap 4. This has the effect of "raising" pins 22 which in turn lift or support ball bearing race 17 again. This will have the required effect and retraction of the rod 3 will simply rotate the nut 12 and disc 13 as during extension, but of course, the rotation will now take place in the o pp osi te sense, that is, clockwise as oppos ed to counter-clockwise or anticlockwise. The "downward" force referred to earlier that needs to be applied to the outside end of control lever 20 should preferably be applied via an extension spring (not shown). - The reasons for this will be explained later. One workable solution would be to have one end of the extension spring hooked onto the outside end of control lever 20, with the other end attached to one of any number of remotely controlable "pulling 11 devices. Examples are: flexible steel cable such as a Bowden cable (e.g. as used in vehicle clutch or hand-brake cables), an electromagnet, a pneumatic or hydraulic slave cylinder. As these pulling devices do not act directly on the control lever (i.e. they act via an extension spring) their stroke is not as critical. In practice one would arrange that when the pull was released, the extension spring would be unextended. When the pull was applied it would extend the spring more than the total movement available at the end of control lever 20. Another, most important reason for using an extension spring is that this allows the lock nut to act as an automatic brake under certain circumstances. Consider the case when the outside end of the control lever 21 has been pulled "down" by means of a force applied to the extension spring and that the conical bearing surfaces are not contacting wherein the rod 3 may retract (i.e. the "off" position of the locking mechanism). If the nut is not to retract or "drop" with the rod 3 it must rotate at the appropriate rate. The forces causing this rotation are, the threads on the rod pushing "downwards" and around and the ball bearing race 17 pushing "upwards". The forces opposing this rotation are due to the moment of the nut 12 and the friction between the threads of the nut 12 and the rod 3, and between the disc 13 and the rod 3. Clearly these opposing forces will increase as the rod 3 accelerates or moves too fast. The maximum magnitude of the force with which the ball bearing race 17 can push upwards is determined by the characteristics of the spring used. Thus by appropriate choice of spring and the tensioning of the spring, if the rod 3 were to accelerate too rapidly and/or move too quickly beyond a safe limit the aforementioned forces opposing the rotation of the nut will overcome rotational forces opposing the rotation of the nut will overcome rotational forces and the nut will move "down" with the rod, thereby extending the spring further. As the nut moves "down" the conical bearing surfaces will make contact such that frictional forces will retard the rotational speed of the nut and so act as an automatically applied brake. Normally, this will bring the rod 3 to a rapid halt. The severity of this braking action may be varied in the design and it depends on factors such as the lead angle and form of the intermeshing threads on the rod 3 and nut 12, the angle of the conical surfaces, the relative diameters of the rod and the conical surfaces and the various applicable coefficients of friction. The importance of the automatic brake feature cannot be overstressed as it means that the brake is ready to work at all times, even when the lock has been opened to allow the rod to retract. It should be noted that once the lock has engaged fully, or partially when the conical surfaces are in contact, and a substantial part of the external load is acting "down", (e.g. at T-piece mounting 6) the rod 3 is held against retraction by the nut and disengagement of the lock will not be immediately possible. In order to disengage the lock the cylinder will first have to be hydraulically re-pressurised until the rod 3 extends by at least a small amount. This will create a gap between the conical bearing surfaces and release the lock. At this stage retraction is achieved by first appropriately tensioning the extension spring attached to control lever 21 with the "pulling" device used, and then letting oil out of the cylinder via oil port 8, in a manner known per se.. To further reduce the ingress of dirt or water and also protect the mechanism from climatic influences, several methods known per se (not shown in Figures 1, 2 or 3) may be utilized. For instance, a light plastics sleeve, made from PVC for example, may be provided with a small hole to allow rod 3 through at one end adjacent to , T-piece 6, and loosely fitting at the other end over cap 4, wherein the open end would slide down over the cylinder 1 towards the hinge 7 upon retraction of the rod 3. Another solution would be to utilize a rubber bellows enclosing rod 3 in a known manner. It should be evident that when the nut acts as a lock or a brake, it imposes the same but opposite torque on the rod, thus tending to rotate the rod. In this embodiment the end mountings 6 and 7 are called upon to prevent rotation. The inherent strength of the machine components which bear these mountings, and the welds fixing the mountings to the cylinder, must be taken into account in designing safe components. The magnitude of this torque will depend on the axial load, the thread lead angle used, the thread form and of course the coefficient of friction. In another embodiment described below an alternative solution using a key and keyway arrangement is utilized to overcome this problem. It will be appreciated that this and other solutions (i.e. rolling contact and subsidiary rods) may be utilized in a modified version of the first embodiment, if desired. Service access to the hydraulic components may conveniently be made without having to disassemble the mechanical lock/brake mechanism and vice versa. For instance, to re-hone the cylinder 1 or to replace the wear rings 9 or oil seal 10, unscrewing cap 4 (with the whole lock/brake mechanism intact) will allow this to be done in much the same way as with a conventional push cylinder. On the other hand, the lock/brake mechanism contained in cap 4 can be dismounted for servicing without disturbing the cylinder wear rings and oil seal and without having to expel the piston 2 from same. This is made possible if rod 3 is an interference fit into piston 2 as illustrated in Figure 1. A small longitudinal keyway (not shown) would be cut along that part of the rod 3 which is pressed into piston 2 to allow air which would otherwise be trapped during fitting to escape. Thus, oil would first be pumped into port 8 to bring piston 2 to the end of its normal travel abutting against the "stop" section of cap 4. Next rod - 3 is pulled out of piston 2 by applying a pull with pullers or other appropriate devices, for example, to T-piece mounting 6. Finally cap 4 with the lock type brake- mechanism undisturbed may be unscrewed from cylinder 1. In the embodiments discussed so far, the threaded rod rotates while the nut moves axially along the rod without rotating. - In an alternative construction, the threaded rod may be fixed at one end or both ends, and the n ut rota tes while moving axially along the rod. In a single acting configuration similar to that shown in Figure 1 may be utilized. In a double-acting version, an arrangement similar to that shown in Figure 4 may be utilized. Referring now to Figure 4 of the present application, reference 2 denotes a double-acting piston and 29 part of a hollow rod, for a double-acting piston and cylinder arrangement. The cylinder is known per se and is not shown. Reference 12 indicates an internally threaded lock type brake nut that mates with a threaded bar, not shown. This threaded bar is permanently fixed at one axial end to the "bottom" of the cylinder. Reference 25 denotes the mating conical lock/brake surface on the inside of piston 2, which engages against the conical outer periphery of nut 12 in the locked condition of the mechanism. Reference 9 refers to a standard piston bearing ring. Reference 10 relates to a standard double-acting piston seal. Reference 26 denotes a release piston. Reference 33 indicates a spiral compression spring urging piston 26 outwardly against a piston travel stop pin 46. Reference 19 represents a release ring for the lock/brake mechanism, while reference 48 relates to a release ring arm. Reference 16 denotes a release ring arm "bottom" stop. A pair of slightly tapered release rollers 49 are radially opposed on ring 19 and rotate about axles 50 projecting radially inwards from the inside of release ring 19. The roiling contact surfaces of rollers 49 are slightly tapered to avoid skidding that would otherwise occur. A groove in the outside walls of nut 12 receives the rollers 49 and is correspondingly tapered to mate. The release ring 19 pivots about a pin 21 via a lever arm 20 extending radially outwardly of the ring 19 diametrically opposite release ring arm 48, to allow ring 19 to pivot about pin 21. A permanent "brake" against rotation of nut 12 is provided by one or more steel balls 31 urged against the threaded bar by spiral compression springs 30. The workings of the embodiment shown in Figure 4 will now be explained. To extend the piston 2 and rod 29 the extend chamber "below" the piston is pressurized. If the oil in the retract chamber is metered out, this allows controlled extension to take place, either under no load or with a "positive" or "negative" load, provided the magnitude of the loads is not excessive. The lock/brake nut 12 is not applied in the extend direction, as it is drawn along the threaded bar, in the "up" direction, by wheels 49, and thus release ring 19 is fully pivoted "down" about pin 21 in an counterclockwise sense in the view of Figure 4, with the arm 48 engaged against release arm "bottom" stop 16. This happens regardless of the position of piston 26. For the lock to carry out its intended function of preventing unwanted retraction taking place, piston 26 must be in the "up" position, i.e. against stop 46. Nut 12 is also fitted with some form of constant braking mechanism such as spiral compression springs 30 and steel balls 31. Two or more of these braking mechanisms are radially disposes and symmetrically spaced around the nut, although only one is shown in Figure 4. This is necessary as the threads are non-self-locking and so the nut might otherwise unscrew under its own weight. They also help to bring the rotating nut to a more rapid halt once the force causing the rotation has ceased. Hence, nut 12 will not unscrew under its own weight, and if the piston 2 drops or retracts due to leakage of the seal etc., the tapered outer skirt of the lock nut 12 contacts its mating surface 25. The lock nut 12 thus prevents further retraction of piston 2 from taking place. It should be noted that as the threads are non-self-locking, the load will try to rotate the threaded rod in one direction and at - the same time try to rotate the nut 12 in the opposite direction, since action and reaction are equal and opposite. The nut 12 will in turn try to rotate piston 2. However, in most applications, this cannot happen as the cylinder and rod mountings, e.g. hinge or lug mount, will prevent any such relative rotation. If the cylinder is only mounted at one end, as in the case of support or outrigger legs, then relative rotation between the cylinder and the hollow piston rod must be prevented by some other means. One solution is to have two or more symmetrically disposed keyways cut in the threaded bar and running longitudinally through and over the threads. Appropriate keys attached to the inside of the piston fit these keyways and prevent relative rotation, as required. Another solution is to utilize a special nut instead of nut 12, that provides rolling contact at low loads, but sliding contact at higher loads. The rolling contact is non-self -locking, but the sliding contact is self -locking. To allow retraction to take place, the retract chamber must be pressurized to a higher pressure than the extend chamber. . When this happens, the release piston 26, which is exposed to- retract chamber pressure, moves "down", and, via spring 33, exerts a downward force on release ring arm 48. This in turn causes release ring 19 to pivot "dawn", i.e. counterclockwise about pin 21, together with rollers 49 which both rotate freely and at the same time maintain separation of nut 12 from brake surface region 25. If, during retraction, the pressure in the retract chamber is reduced, e.g. due to over-rapid retraction, "downward" pressure on arm 48 will be insufficient to prevent nut 12 and surface 25 from contacting, and when they do, the retraction will be brought to a stop mechanically. This embodiment provides a double-acting cylinder with a mechanical lock/brake mechanism that works in the retract sense only. The lock may be opened only if the pressure in the retract chamber, i.e. the camber "above" the piston, is greater than the pressure in the extend chamber, i.e. the chamber "below" the piston. In m any prac tical applicatio ns, this condition is satisfied. In particular, if retraction takes place against a load, then the extend chamber may be connected to tank and the retraction pressure will be greater. Also, if retraction takes place under no load, then by metering, i.e. restricting, the return flow out of the extend chamber back to the tank, the retract pressure will again be greater. This is due to the fact that the retract area is less than the extend area. One example of an appropriate application of this arrange ment is f or support or outrigger legs for truck-mounted cranes and excavators. As a safety feature, retraction will not be possible if the load acting in the retract sense is too great. In situations where retraction needs to take place under a maximum load, such loading would cause the extend chamber to be subjected to maximum system pressure, so that the retract chamber could only be pressurized to a maximum pressure that would equal, but not exceed, this pressure. Hence, the release shown in the upper part of Figure 4 would not work for such applications. One way that this constraint may be overcome is to have a larger effective release piston area exposed to retract chamber pressure than is exposed to extend chamber pressure, i.e. a ratio greater than 1:1. This may be achieved by having an arrangement in which a small intermediate air-filled or evacuated chamber 51 contains a plunger 52 and is bounded or delimited by diaphragms 53 and 54, as shown in the detail view of Figure 5. This arrangement may replace piston 26 of Figure 4. The diaphragms may be steel or textile/braid-reinforced rubber. References 55 and 56 are oil-pressure-tight clamps for the diaphragms, and although they are shown welded on, in practice they may be bolted on, so as to securely pinch the edges of the diaphragms, or they may be crimped to these edges. .- An alternative to these diaphragms is the use of metal bellows. Another arrangement to met the requirement noted above involves the use of a "differential" piston arrangement, as shown in Figure 6. Reference 57 indicates a differential piston, with its larger area face facing the pressure in the retract chamber, and its smaller area facing the pressure in the extend chamber. Reference 58 denotes a piston seal and reference 59 a rod seal. Cavity 60 is vented to atmosphere. This is necessary, because the seals cannot be wholly relied upon not to leak, and therefore, without such a vent, entrapped oil in the cavity might prevent the piston from functioning properly as a differential piston. This venting to atmosphere may be achieved, for example, by running a miniature bore hydraulic pipe along the inside wall of the hollow piston rod 29. This may be done, for example, either by allowing additional clearance between the threaded bar and the inside wall of piston rod 29 or by cutting a keyway along the length of the threaded bar to provide the necessary clearance. This keyway may at the same time serve the further purpose of being one of two or more keyways symmetrically disposed along the length of the threaded bar to engage with piston-mounted keys and prevent relative rotation of the piston and the threaded bar. An alternative range of embodiments not shown in detail in any of the Figures will now be explained with reference to Figure 4. The same underlying principles apply to a double-acting lock, but for simplicity are explained for a single-acting case. In this further configuration, the release piston 26 is completely dispensed with, and a spiral compression spring is used, similar to the spring 33 which acts against arm 48, to allow retraction to proceed, but only under certain circumstances. In particular, this spiral compression spring is so chosen that it will be powerful enough to allow retraction to commence an continue, provided that the acceleration of the piston does not exceed a critical value. To show that this arrangement is practicable, an analysis of events during extension may first of all be undertaken. The threads are non-self-locking, which means that the nut would rotate under its own weight along the vertical threaded bar, if it were not for the permanent "brakes" 30 and 31. As the piston begins to extend, arm 48 abuts against stop 16 and the wheels 49 try to "pull" the nut 12 "up" along the threaded bar, while at the same time allowing it to rotate quite freely, as wheels 49 are mounted for free low-friction rotation. This upward force on the threaded nut 12 acts upon the non-self-locking threads and tends to cause nut 12 to „ rotate. Because of the laws of friction, the magnitude of this rotation-inducing force is substantially proportional to the magnitude of the axial force on the threads, which in turn is equal to the axial force that the pair of wheels 49 exert on nut 12 minus the weight of the nut. Thus rotation-inducing force increases as the wheels 49 exert increasing upward force on nut 12. This rotation-inducing force must be sufficient to overcome the static friction of the threads, (this having two components, a constant radial one and a variable axial one which varies in proportion to the axial load), the static friction of the "brakes" 31, (which is a constant force . independent of the axial load) and the static f riction of the wheels 49, which varies in proportion to the axial force they are imposing. For the purposes of the present analysis, the rotation force constant is designated as k, so that an axial force F has the effect of establishing a rotational force kF, and the constants for the variable friction forces are taken to be k j for the axial friction on the threads and k 2 for the friction of the wheels 49. Also the sum of the constant friction forces opposing rotation is denoted by M, i.e. the sum of the forces exerted by "brakes" 31 and the radial component of the friction of the threads themselves. Then if nut 12 is assumed to have a self-weight w, the rotation-inducing force = (F-w) k, and the friction forces opposing rotation = (F-w) k j + Fk2 + M. If it is assumed that the rotation-inducing force equals the frictional force plus the turning force N that is used to accelerate nut 12 and wheels 49, i.e. increase its angular velocity from zero, then (1) (F-w) k = (F-w) k χ + Fk 2 + M + N From equation (1), (F-w) k = (F-w) k^ + Fk 2 + M + N. If the threads have a sufficiently large lead angle and the wheels 49 are sufficiently free turning, then k>k j + k 2 . w(k - k χ ) + M + N i.e. F ss k - k t - k 2 1 w(k - k j ) + M i.e. F = N + k - k j - k 2 (k - k j - k 2 ) 1 w(k - k j ) + M The terms _ and k - k j - k 2 (k - k j - k 2 ) are constants and the first of these constant terms is positive (i.e. >0). Hence the equation is of the form F = RN + S, i.e. a linear equation in N and F, and the force F increases linearly with N. The analysis for retraction is very similar. In this case, if the force that the wheels 49 exert downwards is called G, then, the rotation force = (G + w) k. The maximum value of G is determined by the choice of spring 33. The frictional forces opposing rotation = (G + w) k j + Gk 2 + M. If rotation is to commence (G + w) k = (G + w) k j + Gk 2 + M + N. i.e. G(k - k α - k 2 ) = -w(k - k j ) + M + N 1 M - w(k - k j ) i.e. G = _ N + k - k χ - k 2 (k - k j - k 2 ) The terms k - k χ - k 2 (k - k χ - k 2 ) . are again constant. This equation is therefore again of the form, G = AN + B where A and B are constants. Hence rotation commences if the spring is sufficiently powerful to provide at least sufficient force on arm 48 to result in a force G being exerted by the wheels 49, in accordance with the criteria established by the latter equation. It follows that with the appropriate choice of spring, there will be a threshold value of N, which, if exceeded, will result in the spring not being able to provide a sufficiently large force to satisfy said equation, and thus the nut's angular velocity wilL not accelerate sufficiently rapidly for it to move "down" along the threaded bar ahead of the piston 25, and the lock will apply. It is a known fact that static friction is always greater than dynamic friction, so that, if the spring is powerful enough to start the nut rotating and commence its angular acceleration, then, assuming that the acceleration remains constant, or only decreases, to bring nut speed up to a metered retract velocity, then retraction will be able to continue unhindered. Hence, there is provided a brake control mechanism that will apply the brake immediately, if the rod accelerates too fast. As an example, the brake/lock may purposely be made to apply by having a retract valve in which the flow rate may be varied, hence, by opening this valve rapidly and fully, the high resulting acceleration may trigger off the brake/lock almost immediately, before hardly any retraction had actually taken place and before much speed had been gained. Furthermore, the brake/lock may be designed to give a gentle rate of retardation by various means, e.g. by choosing conical brake surface material with a lower coefficient of friction, altering the cone angle, or increasing the lead angle of the thread. Thus in order to retract, the piston is slightly extended, and then a more normal rate of retraction is carried out by opening the retrace valve gradually. This embodiment results in a relatively simple arrangement, which may nevertheless be suitable for certain applications, e.g. support legs or outriggers. Also, as mentioned earlier, this configuration is easily extended to a double acting brake, e.g. by using springs to keep nut 48 centralized. In another embodiment to be described in detail in regard to Figures 7 and 8 the invention provides a locking and brake mechanism for use, for example, with an elevator or two-post garage lift. An elongate threaded rod is rotatably mounted in fixed bearing means at each axial end to be axially displaceable over a small distance. Conical mating surfaces, respectively at the upper end of the rod and in the vicinity of the upper rod bearing, engage in frictional manner when the rod moves , in a direction away from one extreme axial disposition and are spaced apart by axial displacement of the rod in a direction towards said extreme axial disposition. A nut is mounted on the threaded rod and is associated with the elevator or lift carriage so that during raising or lowering of the lif t carriage or elevator, the rod is constrained to rotate. Such rotation takes place freely when said frictional surfaces are spaced apart, and such spacing is maintained during a raising movement by the dynamic interrelationship of the nut and the rotating shaft in conjunction with the end float of the rod or shaft, which urges the rod towards said extreme axial disposition. When this relative rotation ceases, the shaft may move away from said extreme end position to such an extent that the friction surfaces engage and the mechanism is locked. Thus the mechanism guards against collapse or excessive run-back of a raised elevator or lift. The lock may be released by endwise axial displacement of the shaft or rod in the direction of said extreme axial position, so again providing clearance between the frusto-conical friction surfaces, to which end, a release device is also associated with the mechanism. Figure 7 shows the "top" section of a safety rod lock/brake mechanism. A rod 3 is threaded and has a truncated conical "head" 38 as shown. The thread type used may be of virtually any form but a stub acme or stub trapezoidal form is perhaps the most preferred. Some of their suitable properties include reduced susceptibility to binding arising from the ingress of dirt and relative ease of manufacture by a thread rolling process. Reference 25 indicates a heavy "fixed" plate that has been bored out to mate with the truncated conical head 38 of rod 3, so as to substantially constrain rod 3 against downward axial displacement relative to plate 25. Reference 61 denotes a top hat shaped housing for a bearing 62, the bearing being received within the cylindrical portion of housing 61. At the closed axial end of the housing 61, a substantially semi-spherical seat is provided to accommodate a hardened steel ball 66. Housing 61 is fastened to plate 25 by bolts 63. Reference 67 indicated an axle that screws into the free axial end face of rod portion 38, the axle 67 being locked to the rod end portion by nut 68. The free axial end of axle 67 is provided with a further substantially semi-spherical recess for accommodating a second hardened steel ball 66 to engage against ball 66. This ball arrangement provides a simple, low friction, "top" bearing for rod 3. Axle 67 allows the gap between the matting truncated conical surfaces of plate 25 and rod portion 38 to be adjusted, if necessary, the gap in question being the spacing between these frusto-conical surfaces when rod 3 is lifted to place balls 66 in contact. Reference 12 indicated an internally threaded nut for mating engagement with the threaded rod 3. The threads are non-self-locking, i.e. they have a sufficiently great lead angle to enable the nut to drivingly rotate the rod. Guide bearings 5 are fitted at each end of nut 12. As an alternative, not shown, needle roller bearings may be used. Two horizontally opposed radial bores accommodate hardened steel balls 31 urged against the thread of rod 3 by helical compression springs 30, whose compressive force may be adjusted and balanced by grub screws 65 radially displaceable within threaded radial holes in nut 12. Figure 8 is the axially downward continuation of Figure 7, showing the "bottom" section of the mechanism and a particular construction of release mechanism. Reference 76 indicates a bearing that is part of a fixed plate 75. Reference 72 denotes a plunger/housing for a bush 71 and a hardened steel ball 74 forming part of a bearing arrangement similar to that at the top end of rod 3. The axially upper end region of housing 72 is externally threaded to receive a mating knurled ring 73. Reference 69 indicates an axle that screws into rod 3 and reference 70 denotes a nut for locking it in screwed engagement within the axially downward end of rod 3. The "bottom" or lower end face of this axle has a second hardened steel ball 74 accommodated within it. Reference 84 denotes a large helical compression spring whose upward thrust on ring 73 and, in turn, via housing 72, bearings 74 and axle 69, on rod 3, is adjustable by screwing ring 73 along housing 72. Reference 49 denotes a roller, mounted on an axle 50. Axle 50 is in turn mounted on one arm of "L" lever 20, which pivots about a pin 21 fixedly associated with plate 75. Reference 77 represents a further pin joint between the other arm of lever 20 and a control rod 78. When rod 78 is moved from right to left, plunger or housing 72 is raised. A combination control cylinder/rod guide 79 is fixed with respect to plates 25 and 75. Reference 26 denotes a control piston, reference 80 the piston's return spring, and reference 47 the piston seal. The cylinder 79 has a threaded port 44. Reference 82 denotes a lever associated with rod 78 by pivot pin 81. Reference 83 denotes a stop/fulcrum for lever 82, which is fixed with respect to cylinder 79, and thus also with respect to plates 25 and 75. Reference 33 indicates a helical extension spring, one end of which is fastened to lever 82 and the other end to a bowden cable 86. Reference 85 represents a bracket for the end of the bowden cable 86, the bracket being fixed with respect to fulcrum/stop 83, cylinder 79, and plates 25 and 75. A description of the workings of the mechanism of Figures 7 and 8 will now be given. The axle 67 is screwed into rod portion 38 to set or adjust the extent of its protrusion from the top face of portion 38. This adjustment controls the maximum gap that can exist between the mating truncated conical surfaces of plate 25 and rod portion 38 when the rod 3 is raised as far as possible, i.e. when the two hardened steel balls 66 contact or the conical surfaces of plate 25 and rod portion 38 come into contact, is only a few millimeters, this axle 67 is locked at the selected gap setting by nut 68. The axle 69 is screwed into the bottom axial end of rod 3 in such a way that it is not fully screwed home, i.e. subsequent adjustment in either the "in" or the "out" sense may take place if necessary. The setting of axle 69 is locked by nut 70. Knurled ring 73 is adjusted - until the spring 84 approximately balances the weight of threaded rod 3 together with its associated smaller components -- ball 66, axle 67, nut 68, axle 69, nut 70, bearing 71, balls 74, housing 72 and ring 73. In a practical application the nut 12 is fastened, for example, to an elevator or the carriage of a two-post garage lift. This fastening may be done by means of fasteners through the bores 64. As the nut 12 gets carried "up" by ascent of the elevator or hoist carriage, it initially causes rod 3 to "rise" until the hardened steel balls 66 come into contact. These prevent any further "rise" of rod 3 from taking place and provide a low friction thrust bearing, thus allowing rod 3 to rotate at an appropriate rate as determined by the rate of axial displacement of nut 12 and the thread pitch. As soon as the nut 12 ceases its upward motion, its rotational effect on rod 3 also ceases. However, rod 3 will tend to continue to rotate, due to the inertia of the rotating components. This has the effect of reducing the gap between the truncated conical surfaces of plate 25 and rod portion 38. This gap may close up entirely. In certain applications, such complete closing of the gap may be undesirable, e.g. it may be preferable for the lock to become operative only if the nut 12 drops more than some pre-determined amount from the position at which it stopped. In order to bring the threaded rod 3 to a rapid stop, before it reduces the above mentioned gap by "too much", the hardened steel balls 31 urged inwardly by - the springs 30 act as a form of brake. The grub screws 65 allow the braking force to be balanced between the "right and left" balls 31 and also to be varied overall, e.g. - a miniature torque wrench may be used to set the positions of grub screws 65. When the nut 12 descends on unintentional or inadvertent lowering of the elevator or hoist carriage, it causes rod 3 to descend with it until the truncated conical surface of rod portion 38 contacts the mating surface of plate 25. When this happens, rod portion 38 is prevented by said contact from either continuing to descend or commending to rotate. This in turn stops the descent of the nut 12, and the lock is now "on". The particular release mechanism-shown in Figure 8 is one applicable to a two-post lift application. The lock, may be released to allow the nut 12 to descend, only if both the control cylinder 79 is pressurized, via port . 44, and also bowden cable 86 is activated to extend spiral spring,, 33. These two requirements must be met, if control rod 78 is to be moved from right to left to release the lock. When rod 78 moves from right to left, it causes lever 20 to pivot about pin 21 in a clockwise sense, resulting in wheel 49 urging plunger 72 in an upward direction. This force, combined with that of spring 84, is normally sufficient to overcome the combined downward force consisting of the weight of rod 3 and its associated components and the downward force resulting from nut 12 descending along the rod. Hence the truncated conical surface of rod portion 38 is kept out of contact with its mating surface on plate 25, and the rod 3 is able to rotate at the appropriate rate to allow the nut 12 to continue its descent. If, however, at any time during descent, the nut accelerates "too rapidly", this will put additional downward force on wheel 49, that will be sufficient to cause spring 33 to extend further. Plunger 72 will then also drop, and the truncated conical brake surfaces of rod portion 38 and plate 25 will come into contact automatically and bring the rotation of rod 3 to a halt. This will at the same time stop the descent of nut 12. This brake action will also take place immediately, if, during descent, either the pressure in cylinder 79 drops or the bowden cable releases the tension spring 33. In the two-post garage lift application, the control rod 78 may extend to a second similar threaded rod brake mechanism, which would be located off the page to the left of Figure 8. This would mean that both mechanisms would be either on or off simultaneously. This concept may be extended to three or more such mechanisms. Figure 9 shows cylinder 87, piston 26, piston seal 47, and a spiral compression spring 33 located within the piston 26. It provides a simple hydraulic release mechanism as an alternative to that shown in Figure 8, which may be bolted on directly underneath plunger 72, so that spring 33 may act directly on the bottom of plunger 72. There are no mechanical features, such as lever 82, spring 33 and cable 86 of the arrangement of Figure 8. This rotating threaded rod lock/brake mechanism may be adapted to work in a double-acting sense by introducing a second pair of truncated conical surfaces similar to those of rod portion 38 and plate 25 at the other end, i.e. the "bottom", of the rod 3. This - "bottom" pair prevents upward motion when locked. The release mechanism then also requires to be suitably modified. A first arrangement provides two modes: 1) "lock on" in both the "ascent" and "descent" directions, and 2) "lock off" in both the "ascent" and "descent" directions. An alternative arrangement provides three modes: 1) "lock on" in both the "ascent" and "descent" directions, • 2) "lock on" in the "descent" direction but "off" in the "ascent" direction, i.e. allowing "ascent" only to take place, and 3) "lock on" in the "ascent" direction but "off" in the "descent" direction, i.e. allowing "descent" only to take place. In the embodiments discussed so far, the release mechanisms are all static in the sense that they do not travel "up" and "down" with the nut 12. It is possible to arrange for the release to be controlled from nut 12, while dual control is also possible. As a simple example of a travelling release, a modification of the embodiment shown in Figures 7 and 8 may be considered. The release mechanism shown at the bottom of Figure 8 is dispensed with, i.e. wheel 49, axle 50, pivot pin 21, lever 20, rod 78 etc. The spring 84 is replaced by a somewhat stronger spring. The grub screws 65 of Figure 7 are replaced by a mechanism that may be "biased" to compress the springs 30. However, this compression may be removed or reduced by mechanical, hydraulic, pneumatic, or electromechanical - "release". • The result is that in a "biased" mode, the nut may "ascend" but not "descend". In a "release" mode, "ascent" or "descent" may take place, but during descent, the brake is still applied automatically, if acceleration becomes too rapid, by virtue of the angular momentum of the various rotating components. In double-acting construction, a similar arrangement provides for the two modes discussed earlier. In a three modes arrangement, the bidirectional, i.e. clockwise and counterclockwise, braking provided by the steel balls 31 is replaced by some form of unidirectional brake, such as for example, a cam brake. Figures 10 and 11 are related as noted earlier with Figure 11 showing a hydraulic lock release mechanism for a double-acting embodiment, located in an external end cap 96 of a hollow piston rod 29. Reference 1 denotes a cylinder. A threaded bar 3 is rigidly fixed to the far or blind end of cylinder 1, not shown. A hollow end cap 97 accommodates a rod seal 98 and rod bearing ring/wiper combination 99 and has a static "O" ring oil seal 100. A static "O" ring oil seal 101 is provided between hollow piston rod 29 and end cap 96. A cartridge type valve body 102 fitted with four circumferential "O" rings 103 screws into end cap 96. A long release rod/spool. - valve combination 78 has a "bottom" end, Figure 10, which extends to and is pin-jointed to the usual lock/brake release ring, see Figure 10. There is a certain amount of axial play in this pin joint so that when member 78 is in a central position, as shown, the release ring may move up or down enough for the lock nut itself to be able to contact both its extend and retract brake surfaces. However, when member 78 is fully "down", it will not allow the retract brake/lock surfaces to contact, and when it is fully "up" it will not allow the extend brake/lock surfaces to contact. The top section of member 78 has an axial bore, as shown, to prevent oil that may have leaked past a seal 104 on member 78 from getting trapped in the chamber "above" member 78 and thus hindering its functioning as a spool valve. To this end member 78 is fitted with "O" rings 104, between which are located, as indicated in the drawing, the spool valve land. Oil bores or passages 105, 106, 107, 108, 109 and 110 are provided as shown. A floating piston 111 slidingly surrounds member 78 and has a piston seal 47 and a rod seal 111. A series of belleville springs 33 is provided "above" piston 26 and another series of belleville springs 33' is provided "below" piston 26. These are shown in their natural unspring state, and, in the absence of pressure in port 105 or port 106, centralize piston 26. Spiral compression springs 112 and 113 are provided between the piston 26 and respective circlips 114 and 115 on control rod 78. References 116 and 117 denote flow restrictors, and 118 and 119 non-return check valves. An oil bore 120 has an "O" ring 121. A steel oil pipe 88 is welded onto rod 29 in continuation of oil bore 120 and extends down along the wall of piston rod 29 to the piston of the unit, Figure 10. At the piston, it is again welded to line up with a bore that leads to the retract chamber. References 122 and 123 denote oil-tight tapered alien plugs, and 6 a "top" mounting bore. A hydraulic diagram in the top right hand corner of the drawing shows a center-open direction control valve that may be utilized for the unit. The working of this embodiment are quite straightforward. When extend port 105 is pressurized, it causes floating piston 26 to move up from its i n i t i al "central" positio n, compressi ng bell evill e spri ngs 33. It also compresses helical compression spring 112, to exert a "lift" force on control rod 78. Provided that the lock is not "on" in the extend sense, the spring 112 causes the control rod 78 to rise, which maintains the lock "off" during the following extension phase, while at the same time the spool valve connects oil passage 106 to passage 109 and passage 105 to passage 107, allowing a controlled or metered extension of the piston/cylinder unit to take place. If, on the other hand, the lock is "on" in the extend sense, (the spring 112 not being sufficiently strong to release it), the spool valve does not move from its central position, and the extend port is not pressurized, so that extension is not attempted in vain. An important advantage of this configuration is that the lock cannot be subjected to the dual load made up of the external load proper in combination with the hydraulically imposed load. Before extension can take place, the rod must first be slightly retracted. This may be automatically achieved by means of a small subsidiary oil bore 125 shown by a dotted line in Figure 11. A second similar bore 124 provides automatic extension, prior to retraction, if required. In a similar way, pressurizing the retract port will bring about automatic extension, if required, the opening of the lock in the retract sense, and the pressurizing of the retract chamber. Figure 10 is a downward continuation of Figure 11 (not drawn to the same scale) and shows an embodiment of a double acting lock/brake mechanism built into the piston with the first brake not shown. It is of the rotating lock nut variety. Release controls are not shown, as these would be located at the blanked-off far end of the hollow piston rod, substantially as shown in Figure 11 and described above. The threaded bar that is fixed to the "bottom" of the blind end of the cylinder is not shown, and the cylinder itself is also omitted for clarity. The various parts are now identified. Reference 29 denotes the downward continuation of hollow piston rod of Figure 11 and 2 the hollow piston. A piston lock/brake surface sleeve 94 screws into the hollow piston 2 and mates with lock/brake nut 94. As in previous embodiments, this nut is internally threaded to mate up with the threaded bar, not shown. Its outside lock/brake surface differs from previous embodiments in that it is a zigzag in cross-section as shown, with alternating peaks and valleys along its axial extent. This design feature allows the lock/brake nut to be much more compact in the radial sense, without adding to its axial dimension, i.e. making it longer. This is most important in applications where a heavy, large-diameter, threaded bar must be used, e.g. of the order of one-half of the full bore cross-sectional area of the cylinder, to avoid buckling in the case of a long stroke and/or heavy loading. References 89 indicate a pair of keys protruding radially from the inside of the hollow piston 2. These are shown radially opposed but in practice three or more may alternatively be used, preferably equally spaced circumferentially. They run in appropriate keyways cut axially in the outside of the threaded bar, not shown. These keys prevent any relative rotation from taking place between the piston/rod and the threaded bar. They may also act as a guide from the threaded bar. In applications where the hinge points of the piston rod and they cylinder are suitable for preventing such rotation, the keys 89 are not required. However, the keyways on the outside of the threaded bar, not shown, would still be needed, to accommodate the "retract chamber" steel tube 88, continuing downwards from Figure 11, and control rod 78, also continuing downwards from Figure 11. An oil bore 93 near the top of the piston 2 connects the retract chamber above the piston with its feed tube 88. A double acting piston seal 10 and a piston bearing ring 9 are provided on the exterior of the piston. Control rod 78 narrows at its "bottom" end, to pass through a guide bore in keyway 89. Stops 90 and 91 such as, for example, - 27 - circlips, are provided on rod 78, one above and one below a curved control arm 92. The narrow lower portion of control rod 78 is a loose fit in a bore in control arm 92, and also there is axial clearance, as shown in Figure 10, between stop 90 and control arm 92 and stop 91 and control arm 92. With the control rod 78 in its "central" position, as shown, these clearances are sufficiently large to allow control ring 19, which is attached to control arm 792 to pivot about a hinge pin 21 to an extent sufficient for the "extension" and "retraction" lock/brake surfaces of sleeve 94 and nut 12 to come into contact. Each of a pair of radially opposed rollers is mounted on an axle 50 protruding radially inwards from the inside of control ring 19. Alternative keys 95, shown in dotted outline, may be used either with keys 89, or instead of keys 89. The workings are straightforward and need not be explained again. However, it should be understood that when, for example, the "retract" lock/brake surfaces are contacting, that is when the lock/brake nut is somewhat "up" from its position as shown in Figure 10, and control rod 78 is in its "central" position as shown in Figure 10, the gap between stop 90 and control arm 92 will be virtually zero. This means that if, at this time, retract port 106 of Figure 11 is pressurized, the control rod/spool valve 78 is prevented from moving "down". However, if extend port 105 of Figure 11 is pressurized rather than retract port 106, then in these conditions, with a substantial gap, approximately twice that shown in Figure 10, between control arm 92 and stop 91, control rod 78 is not prevented from moving up, and the extend port is pressurized. Thus, when on the "retract" lock, the retract chamber is not pressurizable, but the extend chamber is. In a similar way, when on the "extend" lock, the extend chamber is not pressurizable, but the retract chamber is. This is as it should be, so that the lock/brake nut and threaded bar are never subjected to the dual loads of external load and hydraulic load. A variety of assembly techniques may be employed to provide the undulating contoured surfaces of nut 12 and sleeve 94, including inter alia, stacking a plurality of ring-form members into a locked-together array, or the use of segmented nut and sleeve portions. In the most general application case, a double-acting ram may be required to extend or retract with or against a load, or act in either sense, totally unloaded. A general double-acting mechanical lock/brake mechanism, when in the lock, i.e. "on", position, must prevent both retraction and extension from taking place irrespective of whatever pressures prevail in either extend chamber 105 or retract chamber 106, either due to the load only or due to any hydraulic leaks. In addition it is highly desirable for this lock/brake to be remotely controllable, in particular releasable, by a standard direction control valve and two oil lines running from this direction control valve to the cylinder, these being required in any case to effect the hydraulic extension, retraction and hold of the cylinder. In addition, a simple and compact configuration of such and embodiment is applicable to many practical situation. The embodiment of Figures 10 and 11 satisfies this requirement. Many of the embodiments discussed work through a spring, have an acceleration sensing "automatic brake on" feature, e.g. if the lock nut accelerates too fast, then the brake is applied automatically. This is an important safety feature. However, in certain circumstances a speed sensing automatic brake-on feature may be preferred. There are many ways in which centrifugal force may be used to apply the brakes automatically. For example, in the case of rotating nut embodiments, kidney-shaped centrifugal weights 126 as shown in Figure 12 may be pivotably mounted on hinge pins 127. Figure 12 only shows one of these in full outline and part of , a second one in dashed outline. In practice two or more such weights are symmetrically placed around the circumference of the nut. The region 129 is part of the brake/lock nut. A garter spring, not shown, keeps these centrifugal weights "closed", as shown. As the angular velocity of the brake/lock nut increases, the centrifugal force also increases. This force eventually overcomes the garter spring and the weights start to open outwardly about their - respective hinge pins 127. This causes the "tail" end 128 of the kidney shaped weights 126 to hinge inwards, so that their brake surfaces 128 contact the threaded rod through gaps provided as shown in nut 129. The faster the rate of spin, the larger this braking effect will be. Figure 12 shows a simple brake surface 128. In applications where the centrifugal brake is expected to function frequently and/or from high speeds etc., a more elaborate hinging brake pad arrangement may be used. The amount of braking provided is only dependent on the angular velocity and not on the direction of rotation, i.e. clockwise or counterclockwise. In certain applications, the brake force may be required to be different, depending on whether rotation is clockwise or counterclockwise. T w o t o t a l l y different ways of actually applying these centrifugal brakes are now described. In the first, the centrifugal brake triggers off a normal conical brake/lock, if its angular velocity, i.e. r.p.m. or speed, exceeds a certain predetermined level. Thus, the brake may be applied: (a) by the release mechanism, (b) automatically by inertia, due to over-rapid acceleration, (c) automatically by the centrifugal brake, due to excessive velocity, or (d) by any combination of two or more of (a), (b) and (c) above. In a second totally different type of application, the centrifugal brake is used in its own right as a sort of automatic speed or revolution governor. An alternative to the centrifugal brake mechanism to govern the angular velocity of the rotational parts and thereby govern the actual linear velocity of extension/retraction, is to use some form of fluid brake. A fluid brake is similar to a fluid clutch but one of the cooperating elements in the fluid is kept stationary, while the other is attached to the rotating part to be governed. In a similar way to the centrifugal brake, the resisting torque acting against rotation increases with angular velocity, and so a governor effect is produced. One difference is that a centrifugal brake may not apply at all at low speeds whereas a fluid brake will normally have some small effect, even at very low speeds. Fluid brakes may be designed in such a way that all other things being equal, they provide a different brake force while rotating in one direction to that exerted in the other direction of rotation. In certain applications, it may be desirable to provide braking in one direction of rotation only. This may be achieved in both the centrifugal and the fluid brake versions by interposing either a ratchet mechanism, a sprag clutch, a roller clutch, or a cam clutch mechanism. These last four mechanisms may also operate on their own, combined with a thrust bearing, to provide a direct brake/lock. however they must be fitted with appropriate release mechanisms. In the embodiments discussed, a non-locking thread on the rod and nut is employed. However, it is possible to use a thread that is locking provided that under low load the nut and rod threads do not actually contact, but are separated by some sort of rolling components. One solution would be to use the familiar ball screw thread on the rod. Two nuts would be utilized, namely a ball nut in which the roller bearings are recirculated in a matter known per se. and an ordinary nut that mates with the special thread of the rod. This ordinary nut would not be a tight fit on the rod in the sense that a certain amount of axial play exists, for example about one millimeter. The ordinary nut and ball nut would be joined in such a way for example, by means of springs, that as long as the load was small, the ball nut would keep the ordinary nut centralized with reference to its axial play so that it would in fact not be contacting its thread. However, as the load increases, the springs would not be able to maintain this centralization and the ordinary nut would contact. In effect, this provides a transition from a non-locking thread at low load to a locking thread at higher load. The advantage of this fact is that the ' rod will no longer want to rotate once locked and also during braking. An alternative is shown in Figure 13 where a modified - version of piston 2 of Figure 14 is shown. Radial boring 130 and tapered spring steel pin 131 act as a spring axle for roller wheel 132. Referring to Figure 14, a cutaway view of a double acting fluid actuator is shown having cylinder 1 and hollow piston 2 surrounding rotating rod 3 having attached at one end brake 38 and extendable to support load above piston 2. Piston 2 is attached to hollow piston rod 29 to which the load (not shown) is attached. Self-locking nut threaded member 28 is attached to piston 2 and is engaged with a non-self locking threaded ' member, backdriving nut 12, by antirotation peg 36 which holds backdriving nut 12 and self-locking nut 28 in position to prevent relative rotation. This nut is preferably made a material such as Nylon which has ~ ' a lower coefficient of friction then the material such as Bronze from which the self locking nut 28 may be made. Positioning spring 37 surround antirotation peg 36 to centralizing the male and female threads between rod 3 and self-locking nut 28. Break spring 30 and ball 31 function as a first breaking means to contact break ball 31 and provide contact frictional engagement between channel threads 35 and the ball 31 and to position and nest ball 31 in spherical indentation 32 at the park position when the system is retracted as shown. Rod 3 is a threaded member having both sloping threads 35 and channel threads 35 as shown. Self-locking nut 28 is positioned to contact the sloping threads only when backdriving nut 12 is overloaded to the extent that springs 37 can no longer maintain the separation as shown. Backdriving nut 12 engages channel threads 35 only and cooperates with rod 3 to provide backdriving due to the high lead angle of channel threads 35. Brake 38 comprises male upper brake surface 39 and lower brake surface 24 which in the non-braking mode are positioned away from their respective upper and lower opposing brake faces 40 and 25. Bearing 17 is mounted by means of its outside race within brake 38 and control piston rod 27 contacts the inner race. Springs 33 and 33' centralize bearing 17 with respect to control piston rod 27. Control piston 26 having control piston extension 43 which extends through cap 4 is positioned between two chambers which communicate with control piston extend port 44 and control piston retract port 45. Brake position indicator 42 is fixedly attached to rod 3 to show the position of the brake and to allow sensing of the brake position at all times. Fluid passage 41 allows fluid to escape the braking zone. Extend chamber port 8 connects with the extend chamber. As shown, the system is in the retract position but brake 38 is not engaged, however, the first brake, spring 30 and ball 31, is nested with spherical indentation 32 putting the system in a fixed park position while retracted. To extend the system of Figure 14, control piston extend port 44 is pressurized which causes control piston 26 to move in a downward direction and away from piston 2 thereby compressing the springs 33 via ball bearing 17 which causes brake surfaces 24 and 25 to contact. Fluid is now pumped into extend port 8 which causes piston 2 to move in an upward direction and as it does, it carries back driving nut 12 with it due to the centralizing springs 37. These springs collectively are stronger than spring 33' which is urging 24 and 25 together. Hence, springs 37 are able- to maintain their centralizing function between nut 28 and the slope face 34 of threaded bar 3 while the engagement of the threads of 12 and 3 causes the intensity of contact between 24 and 25 to diminish to the extent that 24 slips on 25 and 3 begins to rotate due to backdriving of 12 at the appropriate rate to allow the extension of piston 2 to continue unhindered. When the desired extension has been achieved, introduction of fluid through extend port 8 is ceased. Piston 3 stops extending and the control piston extend port 44 is then depressurized and piston 26 returns to a neutral position by the centralizing force of springs 33 and 33'. If piston 3 retracts for any reason, constant or fixed brake 30 and 31 riding in channel groove 35 provides sufficient friction to cause rod 3 to move downwardly causing brake faces 24 and 25 to firmly engage to prevent rotation and prevent retraction. If further retraction does occur, springs 37 will no longer be able to maintain centralization and self-locking nut 28 will engage sloping grooves 34 on rod 3 to lock the system and further loading will not cause further retraction through backdriving. In a similar way extension is prevented as brake surfaces 39 and 40 contact to stop rotation. If extension continues springs 37 will be overcome and self-locking nut 28 will engage with slope threads 34 to provide locking thus stopping further extension. The first brake 31 is essential to prevent backdriving due to gravity. To retract the system, control piston retract port 45 is first pressurized. This will compress springs 33'. At the same time, as the fluid in the main extend chamber is entrapped, the extension of control piston rod into this area will displace the fluid in it's main extend chamber and cause piston 2 to extend some small amount. This will ensure that the brake surfaces 24 and 25 are no longer locked together by compressive load and likewise the self locking threads 34 are not engaged. Hence, springs 33' which are still in a compressed state will maintain 24 and 25 apart as fluid is metered out of extend port 8 and pumped into the retract port (not shown). The reason why the particular thread form of Figure 14 is used is backdriving nut 12 will cause threaded bar 3 to backdrive, but self locking nut 28 will not. Furthermore, in practice, it is desirable that 12 will consistantly backdrive stongly and on the other hand, that . self locking nut 28 be as decisively self locking as possible. This is done by three independent means that collectively provide the necessary result. First, the contact profile of the self lock threads is sloped as seen by -thread profile 34 whereas the contact profile of channel thread 35 is horizontal. The horizontal thread is more efficient. Secondly, the pitch diameter of the slope" thread 34 is clearly greater than the pitch diameter of channel thread 35 and as both threads share a common lead, the lead angle of the channel thread will clearly be the greater. Finally, a low friction material is used such as Nylon which has a coefficient of friction of for example 0.75 for the backdrive nut 12 and the higher friction material such as bronze which has a coefficient of friction of for example 0.125 for the self locking nut 28. Additionally, one may increase the tendency for the slope threads to self lock by cutting the nut slope angles at a slightly differenct angle from the threaded bar slope angles. Hence, that if threaded bar threads 34 are at a slope of 45 to the horizontal and the sloping nut threads are 46 , then the effective pitch diameter will increase as required to promote self locking. Numerous mechanical devices are known that convert rotary input to linear output. Among the more "efficient" of these are those that provide rolling contact between the threaded components. The ball screw is the best known and most widely used such device. The rotor device has needles which are mounted radially that have inner most tips engaging the threads and are provided with n eedle cages an d a thrus t cage to take the axial and radial loads respectively. Another approach is to use ring and groove rollers. These rollers may be mounted on axles that are parallel with the axle of the screw with which they mate. Alternatively the axles of these rollers may be inclined at a suitable angle such that the lands are substantially parallel to the threads of the screw at their point of contact. By these means, it is possible to obtain either point contact which provides lower friction known - as pure rolling friction or line contact which allows greater loads to be borne. Figure 15 will illustrate one such embodiment that can be utilized in a modified version of the embodiment of Figure 14. Backdriving nut 12 of Figure 14 and pegs 36 and springs 37 are replaced. The constant brake ball 31 and urging spring 30 are relocated, for example in a radial boring in self lock nut 28 to engage the section of the modified threaded bar 133, that is, parallel with the axle of the threaded bar (see Figure 15). Thus, Figure 15 shows one cross section 133 of the modified thread form of threaded bar 3 of Figure 14 that would mate with the ring and grooved rollers one of which is shown in Figure 15. In a preferred embodiment the threaded bar would have three starts and these would be three such rollers 134 each mounted on axles 135 equally equally spaced circomferentially 120 degrees apart, that would be rigidly screwed into piston 2 and/or self locking nut 28 (140, Figure 15) in a similar fashion to 36, all shown on Figure 14. These axles may be either parallel or skewed with respect to the axle of the threaded bar as discussed earlier. The needle bearings 136 provide a friction free means for roller 134 while at the same time allowing it free axial movement. The springs 137 above and below thrust bearings 138 in combin ation with circlips 139 provide a precise centralizing means for roller 134. In a similar way to the functioning of the embodiment of Figure 14, this modified embodiment provides that rollers 134, backdrive nut 3 (of Figure 14) while the centralizing springs 137 are capable of maintaining the self locking threads 3 and 28 from contacting. As was the case in the Figure 14, for backdriving to occur, when required, it is essential that springs 33 (Figure 14) and 137 (Figure 15) are so chosen that 33 will yield before the springs 137 collectively. Again as before, if the rotation of 3 is prevented by brake 38 then any load greater than what the springs 137 can support, will cause self lock nut 28 to engage and oppose the back driving of 3 by rollers 134. For a particular axial load on 3 the point is reached where the backdriving torque of the rollers 134 is completely cancelled out by the self lock of nut 28. Any further loads will simply, cause the net result of the rollers and nut to be more and more firmly self locking. In an embodiment similar to the one just described, standardly threaded forms such as for example acme or stub acme may be used on the bar with the self locking nut also being standard. The only variation from standard being the provision for a more then customary amount of axial play or run out between these two components.- Figure 16 is an embodiment that comprises and internal floating piston type of release. It also features a back driving drive nut and a self locking load bearing nut. The relatively large area and long stroke of the floating piston with respect to the rod has the desired effect that it displaces a considerable amount of fluid and so is more likely to relieve the lock of its load, allowing it to open. Of course this also happens to a lesser extent in the embodiment shown in Figure 4. Also, because of the near full bore rod used, it is far easier to pressurize the retract chamber to a pressure that will exceed the extend chamber pressure, even when the extend chamber is highly pressurized due to a high compressive load.. It should be noted these two highly desirable features may carry over to a simpler embodiment to that shown in Figure 16 where the usual back driving nut only is utilized instead of the two nuts as in the Figure 16 embodiment. Figure 16 is a half section view with the dotted line representing the center line. Reference 1 is a section of the wall of a double acting cylinder with its blind end towards the bottom which is not shown. Extending from the center of this blind end, not shown, is a threaded bar similar to 3 of Figure 14. The top hollow cap, through which the hollow piston rod 29 extends, together with the usual rod seal, wear ring, scraper, and retract port are also not shown. The lower beginning of hollow piston rod 29 is shown. A floating piston 141 is fitted with two way piston seal 10 and rod seal 10'. Hollow piston 2 is threadedly fastened to hollow rod 29 having wear rings 9 and 9'. The piston 2 has two or more symmetrically spaced axial boarings 143 through which control rods 142 pass. The top portion of these rods 142 are fastened to the floating piston 141 while the bottom portion passes through a hole in the control ring 146. Near its bottom end, a centralizing circlip 145 is located. It, together with centralizing springs 33 and 33' have a centralizing effect on control ring 146 whose lower portion also forms the outer race of ball race 17, the inner race of which is formed by a lower outside skirt of back driving nut 12. This nut is similar to nut 12 of Figure 14 and is likewise preferably a low friction material, at least the threaded portion such as e.g. Nylon. Ball 31, the first brake, acts as a constant brake and is urged against the threaded bar by spring 30. Self locking threaded nut 144 is similarly threaded to nut 28 of Figure 14 and is preferably made of more normal higher friction, higher load boring material such as bronze. However the threads unlike those of nut 28 of Figure 14 do not need to have more than the usual amount of axial play when mating with the threaded bar. Both nuts are fitted with two or more axial borings symmetrically spaced and facing one another, 147 and 148. They are, furthermore, so alined that when floating piston 141 is central as shown and springs 33 and 33' have control ring 146 centralized as shown. Then, when borings 147 and 148 are exactly lined up and both nuts 12 and 144 are threaded onto the threaded bar, the nut 144 is axialy central in the sense that its top and bottom sloped brake surfaces are equidistant from the mating sloped ring 40 and 25 being threadedly attached to piston 2. Drive pins 149 float in bearing 147 and 148. While not shown, threaded components 29, 2, 40, 25. and 2 are locked by means to prevent inadvertent unscrewing. In operation, to extend the rod 29, the extend chamber is pressurized. Oil from this chamber will cause floating piston 141 to extend upwards to contact the stop above it on rod 29. Control rod 142 which is attached to piston 141 acts through springs 33 to urge control ring 146 upwards which likewise through bearings 17 urges backdriving nut 12 to rise against the threads of threaded rod (not shown). As nut 12 backdrives, it will drive non backdriving nut 144 via drive pins 149 likewise until the upper sloped frustro- conical brake face of self locking 144 contacts its mating brake surface 40. Simultaneously the fluid pressure in the extend chamber will cause the rod 29 and piston 2 together with floating piston 141 and other associated structure to extend together. When the desired extension is reached, introduction of fluid is ceased and it will be noted that there is no gap between to brake surface of 144 and its mating break surface and, also, its threads are engaged such that any axial thread lash is only upward. The rod 29 is then prevented from any fall back. To retract the rod, the retract chamber is pressurized to a higher pressure than the pressure in the extend chamber. This will cau&e floating piston 141 to retract towards piston 2 until it abuts the top of the surface of piston 2. This will displace fluid in the extend chamber causing rod 29 to extend with reference to cylinder 1. The extension of the rod 29 plus the movement of control rod 142 collectively causes self locking piston 144 to move downwardly away from contacting 40 until it contacts 25. Continued pumping of fluid into retract chamber while simultaneously metering out fluid from the extend chamber will result in the required retraction. In the embodiment of Figure 16, during and after extending against a load (compressive load), there is no fall back. After retracting with a compressive load, and stopping by first discontinuing the metering of the fluid out of the extend chamber and then . depressurizing the retract chamber, the load and entrapped fluid in the extend chamber cause the piston rod to retract while the floating piston will advance. After a predetermined fixed amount of same, the lock will apply, hence a fixed fall back is established. Retraction may be stopped under compressive load by depressurizing the retract chamber. As soon as this is done the float piston . shifts with respect to the rod to assume its normal position which occurs,. any time extend pressure exceeds- retract and the brake will come on to stop retraction. At this point the metering valve of extend chamber may— be closed. If retraction is subsequently continued, the retract chamber is first pressurized to cause a small extension of the piston rod to release the lock and then fluid is allowed to escape from the extend chamber out through the metering valve, while metering retract chamber pressure. When considering what happens under tension load, retraction against a tension load occurs by depressurizing the extend chamber and filling the retract chamber at a rate which will control the rate of retraction. As before, either during or after such retraction there will be no fall back onto the lock. However, it should be made clear that extension will not be able to take place with a tension load as it will not be possible to get the floating piston to assume its position which deactivates the lock to allow extension. In practical cases when the tension load is relatively small, one way of overcoming this problem is to put sufficient friction load onto the rod to oppose extension. This often occurs in practice due to the friction of the rod seal, rod wear ring, and rod scraper. If not, one solution is to employ two rod seals and pressurize them against one another. This is illustrated in Figure 17. Figure 17 illustrates the top portion of a hollow piston rod 1 extending through hollow end cap 150, 152 and 152' and rod seals. Fluid passage 151 allows the small cavity 153 between the rod seals 152 and 152' to be pressurized. This will cause the friction that these rod seals impose on the motion of the rod to be increased. Clearly other types of self energizing seals besides the "O" ring type may be advantageously used. As an alternative modification to allow an embodiment similar to that of Figure 16 to extend under a small tension load such as self load, floating piston 141 of Figure 16 is spring biased in a position at extreme top end of its travel with respect to the rod 29 and piston 1. An important preferred embodiment which would be appropriate for outrigger applications would be to modify the embodiment of Figure 16 as follows. The frusto conical brake surfaces of 25 and 144 are dispensed with as is spring 33'. Detent holes similar to that shown as 32 in Figure 14 are positioned along the fixed threaded bar in the path of the brake ball(s) 31 (one detent hole for each ball) so that the balls would simultaneously engage when the outrigger is fully retracted. This prevents the outrigger piston rod from inadvertently extending and thereby drag along the ground during movement of the vehicle to which the outriggers are fitted to. It should be noted that an embodiment identical to Figure 16 except that nut 144 is back driving, is also possible with the same features as Figure 16. The difference will be that when locked, the load will unsuccessfully attempt to back drive 29 and 1. In all embodiments such as shown in Figures 13 to 16, the turning moment generated first increases linearly with axial load on the thread, but once non backdrive portions contact, the turning movement begins to drop linearly with axial load to zero and linearly goes negative. It should be noted that the constant of linearity during backdriving will, in general, be different from to the constant of linearity once the self lock section of the threads contacts. This negative sign signifies that torque would actually have to be provided (as opposed to being generated) in order to get threads to rotate with regards to one another in the direction in which backdrive primarily occurs. The magnitude of this negative torque is a measure of the likelihood that due to unusual circumstances, backdriving will still occur. Naturally, the larger the magnitude of this negative torque figure, the more unlikely that backdriving will occur if the coefficient of friction drops due to e.g. special lubricants or vibration. Hence, in practical applications the friction of the various piston and rod seals, wear rings and scrapers are sufficient to provide the small anti-rotation friction initially necessary with the understanding that as additional load is added, the self locking threads carry this extra load and work to advantage as outlined above. An embodiment similar to Figure 14 is also possible with the modification that the two nuts 12 and 28 are replaced by a standard lock driving nut fitted with constant brake 30 urged by spring 31. In this case, of course, the rod 29 must be prevented from rotating with respect to the cylinder 1. Also, a further modification to such an embodiment would be to replace thrust bearing 17 by four more powerful ones that can transmit sufficient axial force to separate the brake surfaces 24 and 25 or 39 and 40 even when these are heavily loaded by the external load. The release piston 26 is also considerably beefed up and springs 33 and 33' are no longer utilized. It is clear that this embodiment has the advantage that the lock may be released under load, but caution must be exercised because of the disadvantage that this modification could have catastrophic consequences in certain applications. Instead, compound -threads as sho-wn in Figure 14 and used in embodiment 16 it is also possible to -have a multiple start tread with some starts being of a profile to mate with a back drive nut (sliding or rolling contact) while the other starts being of " a profile- to mate with, a self lock nut. In general the preferred embodiments would have M "Back Drive" Starts and MN "Self Lock" Starts where M is any integer 1, 2, etc. and N is also any integer 1, 2, etc. In a further embodiment not shown that is a modification of Figure 14 we make use of the fact that as indicated earlier the maximum torque generated by the two nuts is limited and in part eventually goes negative. Hence, we may replace the lower brake 38 and brake surfaces 25 and 40 by a suitable powerful thrust bearing arrangement, the friction of which is so chosen that in combination with 12 (and 31) backdriving occurs, but it combination with 28 self locks. Now all that is required is a relatively week independently operable on/off brake such as a disc or drum to either allow or prevent rotation. It is possible to have two identical threads forms of the same lead on two parallel threaded members, one which is self locking and the other backdriving due to the pitch diameter of the two threaded members. Hence, by means of suitable positive linkage, it is possible for the smaller pitch diameter threaded member to backdrive and thereby rotate the larger pitch diameter threaded member which is self locking at exactly the "correct" rate. Also it is possible to use a single backdriving thread to drive two or more non-backdriving threaded members. By varying the timing between the driving and driven thread pairs, the amount of "fall back" of the slef lock thread may be varied as required. Also, by gearing, it is possible to drive the self lock thread at other than the "correct" rate so that the amount of fall back will vary according to position. This is a desirable feature in applications there the load does not move in direct linear relationship to the extension of the cylinder. While this invention has been described with many embodiments, these embodiments are not limited thereof as one skilled in the art can make many modifications using the principles of the invention.

Claims: What is claimed is: 1. A brake device comprising a nut cooperating with a high lead angle threaded bar to control linear movement, said high lead angle being sufficient to be non self -locking; a brake surface concentric with the axis of said threaded bar; the said threaded bar cooperating with the nut in the brake operative mode to carry the load; and an actuator to- selectively maintain the brake in a substantially inoperative mode. 2. A lock device capable of supporting a load comprising a first threaded member cooperating with a high lead angle second threaded member to control linear movement parallel to the axis of the threaded members, said high lead angle being sufficient to be non self-locking; a first brake opposing relative, rotation of the threaded components; a second brake having a surf-ace concentric with the axis of said threaded members and contacting at least one threaded member to lock said members to preclude linear movement; said second threaded member cooperating with the first threaded member to carry the load; and an actuator to selectively maintain the second brake in an inoperative mode. 3. The lock device of Claim 2 wherein said nut comprises a threaded portion having a top and bottom surface normal to the axis of the nut. 4. Th e lock de vice of Claim 2 wherein said nut co mprises a circumferential surface having at least one member normal to the axis of the nut extending into the threads of the threaded member. 5. The lock device of Claim 2 wherein said actuator selectively maintains the brake in an inoperative mode allowing the relative rotation of the threaded components in only one direction. 6. The lock device of Claim 2 wherein said actuator selectively maintains the brake in an inoperative mode allowing the relative rotation of the threaded components in at least one direction. 7. The lock device of Claim 5 wherein one said threaded member cooperates with the second brake having means for sensing rapid acceleration of one of said threaded members with respect to the other said threaded members to engage said second brake. 8. The lock device of Claim 2 wherein said second brake is rendered operative by a sensing means capable of overcoming said actuator when maintaining said brake in an inoperative mode. 9. The lock device of Claim 8 wherein said sensing means is capable of sensing angular velocity of one of said threaded members. 10. The lock device of Claim 2 wherein said first threaded member has at least two threadable engagable portions engagable with said second threaded member whereby at least one engagable portion is selectively engagable to be self locking to oppose rotation. 11. The lock device of Claim 2 wherein said first brake directly opposes relative rotation of the threaded components. 12. The lock device of Claim 2 wherein said first brake is capable of having variable braking action dependent upon linear position. 13. The lock device of Claim 2 wherein said first threaded member has at least two threadable engageable portions engageable with said second threaded member and held in fixed angular alignment , with one another but not linear fixedly spaced, whereby at least one engageable portion having at least one second brake surface concentric with the axis of the threaded members and being selectively engageable to be self -locking to oppose rotation, said second brake surface capable of opposing rotation to preclude linear movement. 14. The lock device of Claim 2 wherein said first threaded member has at least two threadable engageable portions engageable with said second threaded member and held in fixed angular alignment with one another but not linear fixedly spaced, whereby at least one engageable portion cooperating with a high lead angle second threaded member and at least one engageable portion having at least one second brake surface concentric with the axis of the threaded members, said second brake surface capable of opposing rotation to preclude linear movement.

Inventors: KERTESZ STEPHEN J F, KERTESZ, STEPHEN, J., F.
Applicants: BRODMANN GEORGE L, KERTESZ STEPHEN J F, BRODMANN, GEORGE, L., KERTESZ, STEPHEN, J., F.
Application year: 1986
Main classification: F16F11
IPC codes: B66F3, F15B15