GB2427455A - Drive mechanism - Google Patents

Abstract

A drive mechanism (screw actuator) comprises a nut on the leadscrew whereby relative rotary motion between them causes relatively linear motion of the nut 44 along the leadscrew, wherein the nut is configured to provide, for a constant axial loading between the nut 44 and leadscrew, a first mechanical efficiency (eg. speed, force) in a first direction (eg. upwards) of said linear motion, and a second mechanical efficiency less than said first mechanical advantage in a second direction (eg. downward) of said linear motion. The nut may be a split-nut made of two parts 50,52 with one of the parts 50 having a coefficient of friction higher than the other. Cam surfaces may cooperate with ball bearings to serve as a dog clutch 54 to provide the different efficiencies. The drive mechanism may be used in a pulley system in theatres to raise or lower flying bars and scenery.

Description

2427455

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DRIVE MECHANISM

The invention relates to a drive mechanism comprising a complementary threaded leadscrew and nut on the leadscrew, the drive mechanism is arranged such that the mechanism has a high mechanical efficiency when movement of the nut/leadscrew is in a first direction and a low mechanical efficiency when movement of the nut/leadscrew is in a second, opposing direction.

The invention is of particular use in any system where lifting and lowering of a load is required. For example, in the theatre setting, where lifting and lowering of scenery is required, a flying bar is a horizontal member suspended above a stage from which items of scenery can themselves be suspended. Each flying bar extends across the stage and there will normally be a series of flying bars, one behind each other, across the depth of the stage from front to back. Each flying bar is typically supported by a number of cables so that it can be moved up and down. When it is desired to have the piece of scenery in place the flying bar is lowered by a winch system so as to take the piece of scenery into the correct position on the stage. When the piece of scenery is no longer required, the flying bar is winched upward into the structure known as the fly tower above the stage to bring the piece of scenery out of view of the audience.

Figure 1 shows the general concept as previously described. Figure 1 shows one half of a stage structure 10, although it will be appreciated that the structure is substantially symmetrical and the other half will correspond to that shown in Figure 1. The stage structure 10 includes

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stage 12 and a proscenium arch 14, i.e. the arch separating the stage 12 itself from the auditorium. Behind and above the proscenium arch 14 is the fly tower 16 from which other equipment is suspended.

5 In particular, at least one flying bar 18 is suspended from the fly tower 16 by a number of cables 20 which pass over pulleys 22 (shown in their simplest form) to a system for raising and lowering the flying bar, for example a winch, which is not shown.

In the raised position, shown in solid lines in Figure 1, the flying bar 18 and a scenery panel 24 attached thereto are raised up into the fly tower 16 above the proscenium arch 14 and out of sight of the audience. When the flying bar 18 is lowered, shown in dashed lines in Figure 1, the scenery panel 24 rests on the stage 12 and the flying bar 18 is located a short distance above the proscenium arch 14 so as to be out of sight of the audience.

Traditional systems for raising and lowering the flying bars include rope systems, counterweight systems and 20 manual/motorized winching systems, each of which will now be briefly discussed.

In a rope system the cables/ropes attached to the flying bar are run over at least one pulley and the other end of the rope is manually pulled and then secured in position on a so called pinrail by tying off the rope around a belaying pin. As will be appreciated this system is very manually intensive, for each flying bar there will be more than one rope and therefore more than one person will be needed to raise/lower each flying bar. It will also be difficult to raise/lower the ropes at the same rate to

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ensure smooth movement of the flying bar. Furthermore, the weight of the equipment attached to the flying bars, e.g. scenery or lights, and of the flying bars themselves, is significant.

5 In a counterweight system the free end of the rope attached to the flying bar is itself attached to an arbor. Weights are placed on the arbor to balance the weight of the flying bar and the equipment attached thereto to reduce the effort needed to raise/lower the flying bar. In a single 10 purchase system the amount of weight put on the arbor is the same as the weight of the flying bar and its attachments, but this means that to move the scenery from the raised position to the lowered position the arbor will need to move the entire height of the wall, because for 1 metre movement 15 of the flying bar the arbor will also need to move 1 metre. In a double purchase system the arbor will only need to move 1 metre for a 2 metre movement of the flying bar and attachments, therefore reducing the space needed for the system. However, the weight placed on the arbor will need 20 to be twice that of the flying bar and any attachments.

This means that the people loading the arbor must typically handle large weights. Additionally this system is known to be complicated to use and maintain.

In motorised winch systems the various ropes can be 25 controlled by a single operator and the loads that can be lifted safely are far greater than with manual systems. Winches can also be used in conjunction with counterweight systems (which of course would still necessitate the manual loading of the counterweight). Some problems with winch 3 0 systems are that they can be noisy, sometimes complex and can need more maintenance than rope/counterweight systems.

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Also a significant problem is that simple winches can easily get out of synch, thereby restricting the versatility of the system.

It will also be appreciated that when using a motor to 5 facilitate the raising and lowering of the flying bar the power transfer requirements for raising the flying bar,

where gravity is opposed, are significantly different to the power transfer requirements when lowering the flying bar, where the system is working with gravity. In fact, it is 10 often the case that when lowering, the motorised system turns into a generator and the control system needs the means to dissipate this energy, often via relatively complex electronic systems that dump this energy into large resistors in the form of heat.

15 Traditional screw systems used generally utilise one of

2 screw systems which are different in terms of their mechanical efficiency. Mechanical efficiency of a simple machine, such a screw and nut system, is equal to the Actual Mechanical Advantage (AMA) of the screw divided by the Ideal 20 Mechanical Advantage (IMA) of the screw, i.e.

mechanical efficiency = %

IMA

Mechanical efficiency is measured as a percentage and an ideal machine would have an efficiency of 100%. Therefore, in reality, efficiencies are always less than 100%.

25 In a screw system the IMA is calculated by dividing the circumference of the screw by its pitch, the pitch being the distance between the threads of the screw. The IMA is the theoretical result achieved when all the work put into the

system is converted to work out. However, due to frictional losses in any screw system the AMA will always be less than the IMA, resulting in less than 100% efficiency.

The efficiency measurements of screw drive systems are further complicated by the fact that there are actually two different efficiencies. The first calculated efficiency is in the drive direction (i.e. torque to thrust conversion) and the second calculated efficiency is in the back-drive direction (i.e. thrust to torque conversion). Indeed if the back-drive efficiency is less then approximately 50% then the screw is said to be self-locking meaning that torque would need to be applied to enable a load to be lowered. If back-drive efficiency is greater than 50% then an additional breaking system will need to be applied to the system to sustain a load in equilibrium.

An example cf a screw system which is optimal for use when raising a load is a ball screw system. In a ball screw system, such as that shown in Figure 2, the placement of ball bearing between the shaft and nut minimises the frictional losses since the rolling friction of the ball bearing is less than the sliding friction of other screw systems, such as a plain nut and shaft system. This means that the mechanical efficiency is high, i.e. greater than 80%.

Figure 2 shows an example of a ball screw system 3 0 comprising a threaded screw shaft 32 and a ball screw nut 34. A number of ball bearings 36 are self-contained within the ball screw nut 34 and roll between the threads 3 7 of the screw shaft 32 and complementary grooves 38 which are provided on the internal surface of the ball screw nut 34.

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As the ball screw nut 34 rotates the ball bearings 3 6 rotate both about their own axis and that of the screw shaft 32 and are eventually forced into a return pipe 3 9 though which they move and are released back into the system between the 5 threads and grooves.

Due to the high mechanical efficiency of a ball screw system all ball screw systems are by nature non self-locking. This means that if a ball screw is to be used to support, or move, a load against gravity, such as when 10 lowering a flying bar, a rotational brake will be needed to slow/stop the movement of the screw and therefore the load. Nevertheless, the high mechanical efficiency is useful in raising a load since more of the power put into the system will be converted into power out, because of the minimised 15 frictional losses.

In contrast to the ball screw system a more standard screw system, such as a plain nut and shaft system, has significantly lower mechanical efficiencies, in the range of 2,0 to 70% and generally around 30 to 40%. Often the 20 mechanical efficiency of such a system is less than 50%, meaning the system is self-locking. Such a system is advantageous when lowering a load or maintaining it in a static state. In particular, it is advantageous if the system exhibits self-locking characteristics, thereby 25 negating the requirement for secondary braking systems.

As will be appreciated there are significant advantages in using motorised systems since they are less labour intensive, but those systems currently available inherently come with the disadvantages outlined above. It is therefore

desired to provide an improved motorised system for raising/lower flying bars.

SUMMARY OF THE INVENTION

According to the present invention there is provided a drive mechanism converting between linear and rotary motions as claimed in claim 1.

In brief the invention seeks to combine the properties of a high efficiency screw when raising a load, with the properties of a low efficiency screw, when lowering the load. Preferably, it does this by fitting a high efficiency nut and a low efficiency nut to a common screw and linking them via a "dog-clutch" mechanism. This "dog-clutch" effectively completely removes axial load from the lead screw when raising the load - allowing the high efficiency nut to supply all the axial load when lifting, but transfers both axial load and a small torque component to the low efficiency nut when lowering. Thus, the invention effectively provides a screw which is highly efficient when lifting, with all the advantages of smaller motor power for a given load, but relatively inefficient when lowering, with all the advantages of not requiring secondary braking of control system power regeneration.

In other words preferably the nut of the invention is designed such that a high efficiency nut portion has a first friction coefficient relative to axial loading, and a low efficiency nut portion has a second friction coefficient, which is higher than the first friction coefficient. The high and low efficiency nut portions being connected by a load transfer coupling, the "dog-clutch", which is arranged to transfer axial loading from the high efficiency nut

portion to the low efficiency second nut portion during linear motion in the second direction.

Preferably, the "dog-clutch" comprises respective cooperative cam surfaces on the high and low efficiency nut portions, operative to apply an axial separation force between the high and low efficiency nut portions in response to relative rotary motion between the nut and leadscrew corresponding to linear motion in a second direction.

Advantageously, the "dog-clutch" includes ball bearings between the respective cam surfaces.

Preferably the high efficiency nut is a ballscrew nut and the low efficiency nut is a plain nut.

Preferably the low efficiency nut is made from a high friction material from the group phosphor bronze, PTFE coated steel, mild steel, stainless steel, aluminium or plastic.

Advantageously, the system further comprises a linear convertor attached to the leadscrew nut, the linear convertor being constrained to linear motion. More preferably, the linear convertor is attached to the high efficiency nut portion, so that when the shaft is rotated the nut is constrained to linear motion.

Preferably the mechanical efficiency of the high efficiency nut lies in the range 80% to 100% More preferably the high mechanical efficiency lies in the range of 85% to 95%.

Preferably the mechanical efficiency of the low efficiency nut is less than 80%. More preferably, the low

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mechanical efficiency is less than 50% and even more preferably the low mechanical efficiency lies in the range of 25% to 35%.

The drive mechanism of the present invention may be 5 used in a theatre winch system which also includes a motor; a linear convertor attached to the nut, the linear convertor being constrained to linear motion so that rotational motion of the leadscrew is converted to linear motion of the linear convertor; and a connector to connect the linear convertor 10 to a piece of theatre equipment to be raised and lowered.

Preferably the theatre winch system further includes a pulley system having at least one pulley over which said convertor passes.

Preferably the pulley system has a mechanical advantage 15 of 8:1.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in detail, by way of example only, with reference to the accompanying drawings in which:

20 Figure 1 illustrates one-half of a stage showing apparatus for supporting a scenery panel;

Figure 2 shows a ballscrew arrangement;

Figure 3 shows a schematic diagram of a rotary to linear conversion system according to the present invention;

25 Figure 3a shows section AA through the system of Figure

3;

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Figure 4 shows a more detailed view of a lead screw nut and clutch according to the present invention in a free position;

Figure 5 shows the lead screw nut of Figure 4 in a locked position;

Figure 5a shows an isometric view of the leadscrew nut shown in Figure 5.

Figure 6 shows a more detailed view of a clutch assembly of the lead screw nut of Figures 4 and 5;

Figure 7 shows a more detailed view of one-half of the lead screw nut of Figure 6;

Figure 8 shows a side view of a winch system according to the present invention;

Figure 9 shows a perspective view of the winch system of Figure 8;

Figure 10a shows a schematic orientation system view for use when describing the winch system of Figure 8;

Figure 10b shows a axial orientation system for use when describing the winch system of Figure 8;

Figure 11 shows cross-section DD through Figure 8;

Figure 12 shows cross-section AA through Figure 8;

Figure 13 shows cross-section BB through Figure 8; and

Figure 14 shows a master and slave winch configuration according to the present invention.

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DETAILED DESCRIPTION OF THE DRAWINGS

Figure 3 shows a schematic representation of a system embodying the present invention. To the rear of the system is a rotary motor 4 0 which comprises a threaded motor shaft 42, also known as a leadscrew, extending therefrom in a forward direction. A nut 44, hereinafter referred to as a lead screw nut, is threaded on to the motor shaft 42 and is free to move along the motor shaft 42. The lead screw nut 44 will be described in more detail subsequently.

Attached to the rear end of the lead screw nut 44 is a linear guidance plate 46 which is constrained to linear movement, for example by the use of guide rails 48. As the motor shaft 42 is rotated by the rotary motor 4 0 the lead screw nut 44 is driven and the rotational motion is converted to linear movement of the linear guidance plate 46. By reversing the rotational direction of the motor 40 and thereby the motor shaft 42 the linear direction of the lead screw nut 44 will also be reversed. In this way the lead screw nut 44 is able to move along the motor shaft 42 in both rearward and forward directions depending on the sense of the rotation of the motor shaft 42.

Looking at the lead screw nut 44 in more detail, as shown in Figures 3 to 7, it can be seen that the lead screw nut 44 comprises two parts, a front low mechanical efficiency nut, for example a plain nut 50, and a back relatively high efficiency nut, for example a ballscrew nut 52. For clarity reasons the description will refer to a plain nut and ballscrew nut, although it will be understood that the description will apply equally to other forms of low and high efficiency nuts. The plain nut 50 and

ballscrew nut 52 are connected via a clutch, shown generally as 54. The clutch 54 transfers the load between the plain and ballscrew nuts when the direction of the motor shaft 42 is reversed.

In a specific example of a lead screw nut according to the present invention the plain nut 50 comprises a locking nut 51 and a corresponding locking dog 53, from which extends a first clutch portion 55. The first clutch portion 55 is a hollow cylinder and, at the end remote from the locking dog 53, has a general cam configuration as shown in Figure 6. The configuration of the end of the first clutch portion 55 remote from the locking dog 53 takes the form of a series of ramps 56 which are connected by interconnecting portions 58. Preferably where the interconnecting portions 58 and the ramps 56 merge is radiused.

The number and angle of the ramps 56 and interconnecting portions 58 will vary depending of the size of the lead screw nut and also the loads which the system is designed to lift and lower. The variables will be calculated to allow the required transfer of torque and axial force as will be described subsequently.

Adjacent the first clutch portion 55 there is a second clutch portion 60, which again is a hollow cylinder with the wall end adjacent the first clutch portion 55 having ramps 62 and interconnecting portions 64 which are complimentary to those of the first clutch portions. The ramps 62 and interconnecting portions 64 of the second clutch portion 60 do not need to be designed co interengage with the ramps 62 and interconnecting portions 64 of the first clutch portion 55. However, if desired they may take an identical form to

the first clutch portion 55 but essentially rotated 180° about a line perpendicular to the longitudinal axis. The ramps 56 and 62 and- interconnecting portions 58 and 64 of the first and second clutch portions 55 and 60 are arranged so that when assembled the ramps 56 and 62 complement each other to form an almost complete cylinder linking the plain nut 50 and the ballscrew nut 52. Between each of the corresponding interconnecting portions 58 and 64 is a ball bearing. The ball bearings 66 are retained in the clutch system 54 by virtue of concentric outer 68 and inner (not shown) retaining rings.

The first and second clutch portions 55 and 60 are designed so that when the nut is operating in a rearward (also described subsequently as lifting) direction as shown in Figure 4 the lead screw nut 44 is in a so-called "free" configuration, as depicted in Figure 4. In this free configuration, the clutch 54 is arranged such that the ball bearings 66 are in contact with the interconnecting portions 58 and 64 of both the first and second clutch portions 55 and 60 and only the ramp 56 of the first clutch portion 55. Rotating the leadscrew nut 44 clockwise (as looking from the motor), i.e. so the leadscrew 44 moves in the rearward direction, can rotate the plain nut 50 slightly to the free position shown in Figure 4, where it can move away from the ballscrew nut 42, thereby enabling it to "float" with a minimal axial load, and therefore minimal friction, between the threads of the plain nut 50 and the threads of motor shaft 42. In other words, the operable system in the free configuration is the ballscrew nut 52, the plain nut 50 simply being passively dragged behind. This means that

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negligible friction is produced between the plain nut 50 and the motor shaft 42.

When the direction of the rotary motor 40, and thereby of the motor shaft 42, is reversed so the lead screw nut 44 5 is operating in a forward (also described subsequently as lowering) direction, as shown in Figure 4, the lead screw nut 44 is in a so-called "locked" configuration as shown in Figure 5. In the locked configuration the clutch 54 is arranged such that the ball bearings 66 contact both ramps 10 56 and 62 and only the first clutch interconnecting portion 58. In other words, rotating the leadscrew nut 44 anticlockwise (as looking from the motor), i.e. so the leadscrew 44 moves in the forward direction, can rotate the plain nut 50 slightly to the position shown in Figure 5, 15 where it takes axial loading from the ballscrew nut 52,

thereby causing an increase in friction between the threads of the plain nut 50 and the threads of the motor shaft 42. Preferably the friction produced results in the plain nut 50 having a mechanical efficiency which is low enough that in 20 the lowering direction the lead screw nut 44 is self-locking .

Although the invention has been described relating to a travelling lead screw nut and a rotating motor driven shaft it will be clearly understood by a person skilled in the art 25 that it is equally applicable to the following arrangements: an axially fixed, non-rotating leadscrew nut and a motor driven rotating and translating shaft; an axially fixed motor driven rotating leadscrew nut and a travelling, but non-rotating, shaft; and a motor driven rotating and 30 translating leadscrew nut and a non-translateng, non-

rotating shaft. Furthermore, it will also be understood

that the system can be manually driven and that the motor is not an essential feature of the invention.

When the lead screw nut 44 of the present invention is utilized in the theatre setting the configuration of the clutch 54 means that an optimal lead screw which provides a high mechanical efficiency, in excess of 80%, in a lifting direction and a low mechanical efficiency, below 70%, in a lowering direction is available.

Figures 8 and 9 show an example of a specific winch housing 70 embodying the present invention. For ease of reference, the orientation of the winch housing 70 shown in Figures 8 and 9 will be used throughout the description. To clarify the orientation of the winch housing 70, Figure 10a shows a schematic view of the winch housing 70 and Figure 10b the axis system defining the system. As can be seen from Figure 10a the top of the winch housing 70 will be referred to throughout the specification as being the "top" and the bottom of the winch housing 70 as being the "bottom". Looking at Figure 10a, the left of the winch housing 70 will be described as the "front" and directionally as the "forward direction" and the right of the winch housing 70 will be described as the "back" or directionally as the "rearward direction". The relevant positions of features within the winch housing 70 will be described as being forward or rearward of each other as defined within this system. However, it will be appreciated that in use the orientation may differ from that as defined in Figure 10. For example, if the winch housing 70 is to be used in a vertical manner then the "front" as defined in Figure 10a would, in fact, be the "bottom" of the system.

The winch housing 70 comprises of a top and bottom plate, 72a and 72b respectively, which are held in a parallel spaced relationship by a number of attachment plates, which will be described in more detail subsequently. The top and bottom plate 72a and 72b have opposing inner faces 74a and 74b, which are configured to be in sliding engagement with a pulley carrier assembly.

The pulley carrier assembly comprises of a top carrier plate 78a and a bottom carrier plate 78b which are maintained in a parallel spaced relationship by virtue of having outer surfaces being in sliding engagement with the inner surfaces 74a and 74b of the top and bottom plates 72a and 72b. The pulley carrier mechanism preferably further comprises a front plate 80 which is positioned between the top and bottom carrier plates 78a and 78b at the front of the pulley carrier mechanism. In the embodiment shown the front plate 80 extends across the front end of the pulley carrier assembly and is attached to the inner surfaces of the top and bottom carrier plates 78a and 78b by flanges 81a and 81b, although it will be understood that other methods of attachment, including being formed integrally with the top and bottom carrier plates 78a and 78b is also envisaged.

At the back end of the winch housing 70 is a motor housing 82 which houses a rotary motor (not shown).

Attached to the rotary motor is a motor shaft 84, which extends in a forward direction longitudinally along the longitudinal axis C of the winch housing 70. The longitudinal axis is defined by the plates 72a and 72b. The motor shaft 84 is threaded with a helical thread configuration 85 and the lead screw nut 44, which is provided with a complementary helical groove configuration,

is threaded onto the motor shaft 84. The lead screw nut 44 is then free to rotate along the motor shaft 84. The lead screw nut 44 is configured generally as discussed previously to provide a high mechanical efficiency screw system in the lifting direction and a lower mechanical efficiency screw system in the opposing direction.

At the back end of the lead screw nut 44 there is attached, or integrally formed therewith, the linear conversion plate 46 which extends across the interior space of the winch housing 70 between the top and bottom carrier plates 78a and 78b. The linear conversion plate 46 is attached to the top and bottom carrier plates 78a and 78b via backward facing flanges 87a and 87b to the inner, opposing, faces of the carrier plates 78a and 78b. Although the linear conversion plate 46 is shown as being a single plate extending across the entirety of the interior space, with a hole formed therein for the motor shaft 84 to pass through, it will be appreciated that the linear conversion plate 46 could comprise of two separate plates. In this situation the first plate would extend from the lead screw nut 44 in an upperward direction and would be attached to the top carrier plate 78a and the second plate would extend from the lead screw nut 44 in a downward direction and would be attached to the bottom carrier plate 78b. The lead screw nut 44/linear conversion plate 46 arrangement results in rotary motion of the lead screw nut 44 being translated to linear motion. Since the linear conversion plate 46 is fixedly attached to the pulley carrier mechanism 76, it therefore follows that as the lead screw nut 44 rotates, the linear conversion plate 46 will translate this to corresponding longitudinal motion of the top and bottom

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carrier plates 78a and 78b and thereby of the pulley carrier assembly 76.

In front of the lead screw nut 44, using the orientation system defined earlier, there is provided a 5 floating pulley arrangement 89. In the embodiment shown,

the pulley arrangement is attached to the carrier plates 78a and 78b by virtue of top and bottom intermediate plates, 90a and 90b respectively. The top and bottom intermediate plates 90a and 90b are fixedly mounted to the inner surfaces 10 of the top and bottom carrier plates 78a and 78b respectively.

The floating pulley arrangement 89 comprises a first pulley set 92, which is located directly in front of the lead screw nut 44. The first pulley set 92 comprises two 15 parallel pulleys, first top and bottom pulleys 93a and 93b respectfully, which are oriented parallel to the inner surfaces of the top and bottom carrier plates 78a and 78b and are located equidistant on either side of the longitudinal axis C. The first top and bottom pulleys 93a 20 and 93b are attached to the intermediate plates 90a and 90b by virtue of shafts 94a and 94b, which are preferably perpendicular to the intermediate plates 90a and 90b. In front of the first pulley set 92 is a second pulley set 96. The second pulley set 96 comprises two parallel pulleys 97a 25 and 97b, second top and bottom pulleys respectively, which are again oriented parallel to the top and bottom carrier plates 78a and 78b. Again, the second top and bottom pulleys 97a and 97b are located equidistant from and on either side of the longitudinal axis C. However, the first 3 0 top and bottom pulleys 93a and 93b and the second top and bottom pulleys 97a and 97b are arranged such that the lines

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of action, i.e. the line along which the cable will run over the pulleys, of the two pulley sets is not coincident and, furthermore, is not subject to interference by the corresponding top or bottom pulley of the other pulley set.

5 Similarly to the first pulley set 92, the second top and bottom pulleys 97a and 97b are attached to the top and bottom intermediate plates 90a and 90b by virtue of shafts, in this instance given the reference numerals 98a and 98b respectively.

10 It will be understood that as the top and bottom carrier plates 78a and 78b move there will be an equivalent movement of the first and second pulley sets 92 and 96 meaning that these pulley sets are known in the art as "floating" pulleys.

15 As discussed previously, the top and bottom plates 72a and 72b of the winch housing 7 0 are held in spaced relationship by a number of attachment plates. At the back end of the winch housing 70 there is attached an opposing pair of left and right motor housing plates, 100a and 100b 20 respectively, as shown most clearly in Figure 11. The left and right motor housing plates 100a and 100b extend between the top and bottom plates 72a and 72b and are attached thereto to maintain the top and bottom plates 72a and 72b in parallel spaced relationship. The left and right motor 25 housing plates 100a and 100b each further comprise a motor holding portions 102a and 102b respectively. The motor holding portions 102a and 102b depend inwardly to the interior space of the winch housing 70, and towards the longitudinal axis C. The motor holding portions 102a and 30 102b are designed such that when the left and right motor housing plates 100a and 100b are fixed to the top and bottom

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plates 72a and 72b the motor is held firmly in position by virtue of the motor holding portions 102a and 102b "clamping" the motor in position.

Directly in front of, and adjacent to, the left and right motor housing plates 100a and 100b are an opposing pair of left and right motor shaft mounting plates, 104a and 104b respectively, shown most clearly in Figure 11. The left and right motor shaft mounting plates 104a and 104b extends across either side of the opening formed between the top and bottom plates 72a and 72b. Attached to the motor shaft mounting plates 104a and 104b, and extending inwardly into the interior space of the winch housing 70, are two holding arms 106a and 96b respectively, again shown most clearly in Figure 5. The holding arms 106a and 106b are configured no hold a seal 108 (?) (is this a flexible mounting or a seal?) on the back end of the motor shaft 84 and to maintain the motor shaft 84 in linear alignment.

Turning now to the front end of the winch housing 70, and as shown in Figure 12, there are attached adjacent to the front end of the winch housing 70 a pair of opposing left and right front end plates, 110a and 110b respectively. One of the left or right front end plates 110a and 110b has a fixed pulley 112 attached thereto. In the presently shown embodiment the fixed pulley 112 is attached to the left front end plate 110a, although it would be understood that it will also be possible to attach the pulley to the right front end plate 110b if the remainder of the pulley arrangement was also reversed. The fixed pulley 112 is attached adjacent to the left front end plate 110a such that its line of motion lies parallel to the left front end plate

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110a and perpendicular to the lines of motion of the first and second pulley sets 92 and 96.

Rearward of the front end plates 110a and 110b are an upper pulley housing assembly 114 and a lower pulley housing 5 assembly 116 as shown most clearly in Figure 13. The upper and lower pulley housing assemblies 114 and 116 are adjacent the front end plates 110a and 110b, but are offset from one another.

In the embodiment shown, the lower pulley housing 10 assembly 116 is being shown as being located in a more forward direction than the upper pulley housing assembly 114, although it would be understood that this orientation may be reversed if so desired.

The upper pulley housing assembly 114 comprises a first downwardly dependent wall 118 which is attached at its upper end to the left hand side of the top plate 72a and extends downwardly to approximately the longitudinal axis C, which extends into the page in Figure 13, where it merges with a inwardly and upwardly extending first pulley attachment wall 120. The first; pulley attachment wall 120 which in turn merges with a first upwardly extending wall 122, which is attached at its upper end to the right hand side of the top plate 72a.. Preferably, as shown in the described embodiment the first downwardly extending wall 118 and the first upwardly extending wall 122 are parallel to each other and perpendicular to the top plate 72a.

Attached to the first pulley attachment wall 120, via a shaft, nut and bolt arrangement 12 6, is an upper angled pulley 124. The upper angled pulley 124 has an input line 30 of action and an output line of action parallel to the lines

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of action of the second top pulley 97a and the first top pulley 93a respectively. The input and output lines of action are then offset by virtue of the angled configuration of the pulley 124.

The lower pulley housing assembly 116 is substantially symmetrical to the upper pulley housing assembly 114, but is longitudinally offset as discussed previously.

Nevertheless, for clarity the lower pulley housing assembly 116 will also be described. The lower pulley housing assembly 116 comprises a second upwardly extending wall 128, which is attached at its lower end to the left hand side of the bottom plate 72b and at its upper end, which is substantially coincident with the longitudinal axis C,

merges with an inwardly and downwardly extending second pulley attachment wall 130. The second pulley attachment wall 13 0 in turn merges with a second downwardly depending wall 132. The second downwardly depending wall is attached at its lower end to the right hand side of the bottom plate 72b and is preferably substantially parallel to the second upwardly extending wall 128. Attached to the second pulley attachment wall 13 0, by virtue of a shaft, nut and bolt arrangement 13 6, is a lower angled pulley 134. The lower angled pulley 134 has an input line of action and an output line of action which are parallel to, and coincident with, the lines of action of the first lower pulley 93b and the second lower pulley 97b respectively. The input and output lines of motion are offset by virtue of the angled configuration of the pulley 134. The angles of the upper and lower angled pulleys 124 and 134, as seen in Figure 7 are opposite, and symmetrical.

The operation of the pulley system as previously described is as follows:

• as shown in Figures 8 and 9 a pulley cable 138 is fixed at one end to the second downwardly depending wall 132 of the lower pulley housing assembly;

• the pulley cable 138 then enters to the right of the first bottom pulley 93b and exits from the left of the first bottom pulley 93b having passed thereover;

• the pulley cable 138 enters the left of the lower angled pulley 134 and passes around the helix before exiting from the right hand side of the lower angled pulley 134;

• the pulley cable 13 8 enters to the right of the second bottom pulley 97b before exiting to the left and returning through the lower pulley housing 116;

• the pulley cable 138 then enters to the bottom of the fixed pulley 112 and exits out of the upper side of the fixed pulley 112;

• the pulley cable 138 passes through the upper pulley housing assembly 114 on the left hand side and enters to the left of the second top pulley 97a before exiting to the right of the second top pulley 97a;

• the pulley cable 138 then enters the right of the upper angled pulley 124 and passes around the helix before exiting from the left of the first angled pulley 124;

• the pulley cable 138 then enters the first top pulley 93a on the left hand side and exits from the right hand side of the first top pulley 93a;

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• the pulley cable 138 then exits through the right hand side of the lower pulley housing assembly 116 and out of the open front end of the winch housing 70. The free end of the pulley cable 138 is then attached to the piece of equipment which is to be moved, for example the flying bar, as required (not shown).

Although the example of the winch system embodying the present invention shows a system having an 8:1 ratio it will be understood that the invention also embodies systems having different pulley ratios, for example a basic 1:1 system or a 4:1 system.

The invention may also include the use of the winch system previously described as a "master" winch to drive a number of slaves, each of which may be used to raise and lower a portion of a piece of theatre equipment , for example a flying bar.

Although the invention has been described with reference to a theatre winch system it will be clearly understood by a person skilled in the art that it will also find applications in many other areas where a leadscrew and nut arrangement is used to move a load.

Figure 14 shows a schematic diagram of a master and slave system 14 0 according to the present invention. In the master and slave system 140 rotary motion of the rotary motor 40 is converted to linear motion of the linear guidance plate 46 as described previously by virtue of the lead screw nut 44. The linear guidance plate 46 is then connected, for example, using metal struts or wires 144, to a first slave 142a, which in turn may be connected to a second slave 142b, again for example using metal struts or

wires 144. The connection of the master and slaves means that as the linear guidance plate 46 moves the slaves 142a and 142b (and additional slaves as desired) will also move simultaneously. Within each of the slaves 142a, 142b, 142c is provided a pulley system, which may be a single pulley or a set of pulleys providing a mechanical advantage. The pulley systems comprise a pulley rope or cable 146 which is attached at one end to the respective slave 142 and at the other to the piece of theatre equipment 148.

The result of the linear movement of the respective slaves 142 will, by virtue of the pulley systems contained therein, be to raise or lower rope/cable which is attached to the piece of theatre equipment 148.

Since each of the slaves 142 moves with the linear guidance plate 46 there will be no difference between the distance each slave 142 and therefore the cable attached thereto will move.

I

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Claims (18)

CLAIMS;

1. A drive mechanism converting between linear and rotary-motions comprising:

a lead screw;

5 a nut on the leadscrew, whereby a relative rotary motion between the leadscrew and the nut corresponds to a relative linear motion of the nut axially along the leadscrew;

said nut being configured to provide, for a constant 10 axial loading between the nut and the leadscrew, a first mechanical efficiency between said rotary and linear motions in a first direction of said linear motion and a second said mechanical efficiency lower than said first mechanical efficiency in a second direction of said linear motion.

15

2. A drive mechanism as claimed in claim 1, wherein said nut has first and second axially disposed nut portions, said first nut portion having a first friction coefficient relative to axial loading, and the second nut portion having a second said friction coefficient which is higher than said 20 first friction coefficient, and said first and second nut portions having a load transfer coupling arranged to transfer axial loading from said first nut portion to said second nut portion during said linear motion in said second direction.

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3. A drive mechanism as claimed in claim 2, wherein the load transfer coupling comprises respective cooperative cam surfaces on the first and second nut portions, operative to apply an axial separation force between the first and second nut portions in response to relative rotary motion between

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the nut and leadscrew corresponding to linear motion in said second direction.

4. A drive mechanism as claimed in claim 3, wherein the load transfer coupling includes ball bearings between said

5 respective cam surfaces.

5. A drive mechanism as claimed of any of claims 2-4, wherein said first portion is a ballscrew nut and said second portion is a plain nut.

6. The drive mechanism of any of claims 2-5, wherein the 10 second portion comprises a nut made from a high friction material from the group phosphor bronze, PTFE coated steel, mild steel, stainless steel, aluminium or plastic.

7. The drive mechanism of claim 1, wherein said system further comprises a linear convertor attached to said

15 leadscrew nut, said linear convertor being constrained to linear motion.

8. The drive mechanism of any of claims 2-6, further comprising a linear convertor attached to said first nut portion such that when the shaft is rotated the nut is

2 0 constrained to linear motion.

9. The drive mechanism of any preceding claim wherein said first mechanical efficiency lies in the range 80% to 100%.

10. The drive mechanism of any preceding claim wherein said first mechanical efficiency lies in the range 85% to 95%.

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11. The drive mechanism of any preceding claim, wherein said second mechanical efficiency is less than 80%.

12. The drive mechanism of any preceding claim, wherein said second mechanical efficiency is less than 50%.

13. The drive mechanism of any preceding claim, wherein said second mechanical efficiency lies in the range of 25% to 35%.

14. A drive mechanism as claimed in any preceding claim for use in a theatre winch system further comprising:

a motor;

a linear convertor attached to said nut, said linear convertor being constrained to linear motion such that rotational motion of said leadscrew is converted to linear motion of the linear convertor; and a connector to connect said linear convertor to a piece of theatre equipment to be raised and lowered.

15. The drive mechanism of claim 14 further comprising a pulley system having at least one pulley over which said convertor passes.

16. The drive mechanism of claim 15 wherein the pulley system has a mechanical advantage of 8:1.

17. A drive mechanism configured to provide motion in a first direction and a second opposing second direction substantially as herein described with reference to Figures 3 to 13.

18. A drive mechanism for use in a theatre winch system substantially as herein described with reference to Figures 3 to 13.

223474, SJT, SMD