GB2257493A - Torque-limiting device and continuously-variable ratio transmission device - Google Patents

Abstract

A mechanical torque-limiting device comprises an annular member (6) carrying blade springs (8) which bear against an eccentric (3) around the input drive shaft (1). When the load on the annular member (6) increases above a predetermined value, slipping occurs, with the eccentric moving around the springs at a faster speed of rotation than the annular member. The device can be combined with an epicyclic gear set (30) to provide an automatic continuously-variable transmission which is suitable, inter alia, for a pedal-driven vehicle. The inner ends of the springs may be restrained by grooves (Fig 9a) and the springs may alternatively be helical (Fig 10a). <IMAGE>

Description

TOROUE-LIMITING DEVICE AND CONTINUOUSLY-VARIABLE RATIO TRANSMISSION DEVICE This invention relates to a torque-limiting device, and to a continuously-variable ratio transmission device incorporating such a torque-limiting device. The transmission device is suitable for use on a pedal cycle, for example.

Various devices are known for changing the ratio of the number of turns of the pedals to those of the driven wheel or wheels in a pedal cycle to suit the prevailing conditions. For example, the derailleur gear system causes the drive chain to jump between different size sprocket wheels at the rear wheel to change ratios. This can only be effected whilst pedalling, and is therefore more suitable to continuous cycling, as in a race, than to cycling requiring frequent stops, where the correct ratio for restarting has to be selected in advance. An alternative device is the hub gear, in which epicyclic gear sets are used. With this type of gear device, generally only three or four ratios are available, and these can be selected only if pedalling is stopped temporarily.This can be disadvantageous, for example in racing, where the delay in effecting the change of ratio may be important, or in ascending a hill, where the loss of momentum during the change in ratio may be too great. Additionally, hub gears require careful adjustment to ensure that the different ratios are correctly selected.

A problem with providing a pedal cycle with a continuously-variable ratio transmission device is that of providing a simple torque-limiting device which does not give rise to significant losses of power in use.

Accordingly, one aspect of the present invention provides a torque-limiting device comprising: a) a shaft carrying an eccentric generally cylindrical surface; b) a drive member connected to an annular member concentric with the shaft and surrounding the eccentric surface; c) a plurality of springs extending between the annular member and the eccentric surface, and arranged such that the drive member rotates with the shaft when the output torque of the device is at or below a predetermined value and the eccentric surface moves relative to the annular member when the output torque is greater than the predetermined value.

Preferably, the shaft carries two eccentric cylindrical surfaces relatively displaced by 180 degrees, and the annular member surrounds both eccentric surfaces and carries two sets of springs, one set extending to one eccentric surface and the other set extending to the other eccentric surface. In one preferred embodiment of the invention, each eccentric surface carries a freely-rotatable sleeve, and the sleeves are linked together via a cranked connection whereby rotation of the sleeves relative to the annular member is prevented. The cranked connection preferably comprises a plurality of coupling members located between respective annular flanges upstanding from the sleeves, each coupling member being rotatably connected to each flange. Each coupling member is preferably in the form of a circular bobbin, and four of these, equally spaced, may be provided between the flanges.

The springs are most suitably blade springs, preferably a flat strip of spring steel pivotally mounted in the annular member and being formed with a convex portion bearing against the eccentric surface. The eccentric surface preferably carries a freely-rotatable sleeve. The or each sleeve is preferably provided with at least one ridge to limit rotation of the sleeve relative to the springs.

Alternatively, the sleeve is provided with lateral grooves into which correspondingly-shaped ends of the springs are located.

In another embodiment of the invention helical compression springs extend between mountings on the annular member and mountings around a sleeve on the eccentric surface.

Although the torque-limiting device of the invention can operate as a continuously-variable ratio transmission device on its own, a preferred embodiment of the invention provides a torque-limiting device having an epicyclic gear-set operatively connected thereto whereby the range of ratios of the rotational speeds of the shaft and the drive member may be extended.

The epicyclic gear-set may comprise planet gears driven by the drive member, a sun gear driven by the shaft, and an annulus gear coupled to a driven member. Alternatively, the epicyclic gear-set may comprise planet gears driven by the shaft, an annulus gear driven by the drive member, and a sun gear driven by an input drive member.

The invention also provides a pedal-driven vehicle, provided with a torque-limiting device in accordance with the invention, forming part of the drive train transmitting driving force from the pedals to the driven wheel or wheels.

For example, the shaft may have a pedal crank at each end thereof, and the driven member is a toothed chain wheel linked to a driven wheel via an endless chain and a drive sprocket wheel. Other configurations are possible. For example, the torque-limiting device, with or without the epicyclic gear-set, may be incorporated in the driven wheel of a pedal driven vehicle.

Although the transmission device of the invention is particularly suitable for use with pedal-driven vehicles such as pedal cycles, it may also be used in motor cycles or as an alternative to existing automatic or continuously-variable transmission devices in motor vehicles. The device could also be designed for use as a highly-efficient retarder mechanism for heavy transport vehicles such as buses and coadhes.

The transmission device is entirely self-regulating in all operating conditions and would require only an elementary manual control system to select forward or reverse drive.

The transmission device of the invention would offer a quality of performance exceeding that of any system currently available for pedal-driven vehicles, and would extend the use of pedal transport in the more adverse operating conditions. The device could be appropriately be used in pedal tricycles, which might prove more attractive for local journeys by those who would hesitate to use two-wheel machines on busy roads.

One alternative application of the torque-limiting device of the invention could be in the drive line of heavy machinery, for example that used in agriculture, where the machinery is iiable to fouling and jamming. The device could be designed to provide a specific torque limit to allow slippage in the drive and avoid serious damage until the power is shut down. The device could also be employed as a fail-safe mechanism in machine-shop equipment and to allow slippage in the drive line whilst running up large generators to their operating speeds. Further, it would be possible to use the torque-limiting device as a continuously variable ratio mechanism without being combined with an epicyclic gear set, in certain conditions.The narrower ratio band achieved would be usable, for example in cycles where heavy duty use is not expected, or in racing cycles where high effort and high speeds are the intended criteria.

In each case, the specific torque rating would be designed for the application.

Reference is made to the drawings, in which: Figure 1 is a sectional side elevation of a torque-limiting device forming part of a transmission device in accordance with a preferred embodiment of the invention; Figure 2 is a section on line A-A in Figure 1, with the springs omitted for clarity; Figure 3 is a sectional end view of the transmission device, from which the torque-limiting device of Figures 1 and 2 has been omitted for clarity; Figure 4 is an end elevation of the transmission device, showing in exploded sectional form the components of the outer casing and the attachment of a pedal crank; Figure 5 is a side elevation of a bicycle in accordance with the invention, incorporating the transmission device as illustrated in Figures 3 and 4; ; Figure 6 is an enlarged part-sectional front view of the transmission device installed in the frame of the bicycle illustrated in Figure 5; Figure 7 is a sectional end view corresponding to that of Figure 3 of a transmission device in accordance with an alternative embodiment of the invention, installed in the rear wheel of a pedal-cycle; Figure 8 is a side elevation of the rear wheel of a bicycle, including the transmission unit illustrated in Figure 7; Figure 9a is a side elevation of a torque-limiting device according to another embodiment of the invention; Figure 9b is a section on line IX- IX of Figure 9a; Figure 10a is a side elevation of a heavy-duty torque-limiting device according to yet another embodiment of the invention; Figure 10b is an exploded part-section on line X-X of Figure 10a; and Figure 11 is a front sectional view of a transmission device incorporating the torque-limiting device shown in Figures 10a and 10b.

Referring first to Figures 1 and 2, the torque-limiting device comprises central member 1, which has a splined bore 2 for receiving a correspondingly externally-splined drive shaft. The central member 1 has integrally formed therewith a pair of eccentric cylindrical members 3 and 4 arranged on either side of an intermediate member 5 which is coaxial with the bore 1. The cylindrical members 3 and 4 are displaced around the axis of the bore 2 in the central member 1 by 180 degrees. The external surface of the intermediate member 5 provides a bearing surface on which is rotatably mounted an outer annular member 6 having around its circumference ten pairs of slots 7, each comprising a larger transverse tubular bore 7a opening inwardly of the annular member 6 through a narrow neck 7b.

Mounted in each slot 7 is a spring steel blade 8 having a rolled tubular section 8a at one end which is received in the bore 7a with the blade emerging through the narrow neck 7b to be directed generally radially inwardly.

Each spring blade 8 is shown in the installed condition with a bend 8b approximately mid-way along its length and a rolled convex bearing portion 8c at its free end. Each blade in its free condition would lie straight and centrally along the radial line from its pivot location to the centre of the annular member 6. This feature of the blade profile is necessary to provide a pre-determined pre-stress loading which will vary according to the amount of deflection of each blade relative to its position with the eccentric cylinders 3 and 4. Each of the bearing portions 8c engages the surface of one of two sleeves 9 freely rotatably mounted over the respective eccentric cylindrical members 3 and 4.

Each sleeve 9 is provided with a diametrically opposed pair of transverse ribs 10 whose function is to prevent rotation of the sleeves relative to the spring blades 8.

In use, drive is input via the central member 1 and is output via the outer annular member 6. When the output load or torque is below a predetermined value, which will depend upon the combined spring force, the whole assembly will rotate as an integral unit, so that the output speed is the same as the input speed. When the output load increases above the predetermined value, for example, in the case of a pedal-driven vehicle, upon encountering an incline or head wind, the cylindrical members 3 and 4 begin to move relative to the springs and the outer annular member 6, so that the tension in each spring successively increases and decreases as its flexion increases and decreases with passage of the cylindrical member 3 or 4.The degree of movement of the cylindrical members relative to the outer annular member 6 depends upon the output load or torque, and thus the device serves to limit input torque required.

The device could be used between the pedal crank and the chain wheel of a pedal cycle, as a continuously-variable ratio drive mechanism, with the crank passing through and engaging the splined bore 2 and the chain wheel attached to the casing. Table 1 below illustrates the performance of the mechanism in this arrangement.

TABLE 1 : TEST RESULTS FOR FRAME MOUNTED MECHANISM TORQUE RATING 18 1b-ft (24.4 Nm)

SUN GEAR S 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 rpm PLANET CARRIER C 50.0 45.0 40.0 35.0 30.0 25.0 20.0 18.0 16.0 OUTPUT TORQUE TI 18.00 20.00 22.50 25.71 30.00 36.00 45.00 50.00 56.25 1b-ft (24.4) (27.1) (30.5) (34.9) (40.7) (48.8) (61.0) (67.8) (75.1) (Nm) PEDAL EFFORT PE 28.8 28.8 28.8 28.8 28.8 28.8 28.8 28.8 28.8 1b (128) (128) (128) (128) (128) (128) (128) (128) (128) (N) OUTPUT RATIO RD 2.50 2.25 2.0 1.75 1.50 1.25 1.0 0.9 0.8 VELOCITY V 9.00 8.1 7.2 6.3 5.4 4.5 3.6 3.24 2.88 mph (MPH) WHEEL EFFORT 7.20 8.0 9.0 10.29 12.00 14.40 18.0 20.0 22.5 1b (32.0) (35.6) (40.0) (45.8) (53.4) (64.0) (80.0) (89.0) (100) (N) 1 2 3 4 5 6 7 8 9 The device could also be used at the rear wheel of a cycle, with the drive applied via the sprocket wheel to the central member 1 and the wheel mounted on the casing of the device.Table 2 below gives test results for a typical cycle constructed in this way.

TABLE 2 : TEST RESULTS FOR FRAME MOUNTED MECHANISM TORQUE RATING 7.5 1b-ft (10.2 Nm) (P=1, S=1 1/2, A=3 1/2)

SUN GEAR S 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 rpm INPUT F 50.0 40.0 30.0 25.0 20.0 17.0 17.0 16.0 rpm ANNULAR HOUSING 125.0 100.0 75.0 62.5 50.0 42.5 42.5 40.0 rpm A OUTPUT TORQUE TI 7.50 9.38 12.50 15.00 18.75 22.06 22.06 23.44 1b-ft (10.2) (12.7) (16.9) (20.3) (25.4) (28.2) (29.9) (31.8) (Nm) PEDAL EFFORT PE 30 30 30 30 30 30 30 30 1b (133) (133) (133) (133) (133) (133) (133) (133) (N) OUTPUT RATIO RD 2.50 2.0 1.50 1.25 1.0 0.9 0.85 0.8 VELOCITY V (MPH) 9.00 7.2 5.4 4.5 3.6 3.24 3.05 2.88 mph 1 2 3 4 5 6 7 8 One practical application of the device described with reference to Figures 1 and 2 is illustrated by Figures 3 and 4. These Figures show a continuously-variable ratio transmission device for use on a pedal-driven vehicle, while Figures 5 and 6 illustrate the transmission device installed in a pedal bicycle.Referring first to Figures 3 and 4, the transmission device 29 comprises an epicyclic gear set 30 integrally coupled to a casing 31, which combines with the outer annular member 6 of the torque limiting device shown in Figures 1 and 2. For clarity, the torque-limiting device itself is not shown in Figure 3.

The casing 31 comprises first and second end plates 32 and 33 joined by bolts 34 which also connect the casing to the annular member 6, each passing through the tubular section 8a of a spring 8 within its slot 7. The first plate 32 is formed with a boss 35 through which a splined crankshaft 36 extends with clearance for the fitting of a pedal crank as hereinafter described. A synthetic rubber O-ring 37 fits into a groove 38 to provide an oil seal for the pedal crank.

The shaft 36 has three splined sections, a central section 39 which engages in the splined bore 2 (Figure 1) of the torque-limiting device, and a section 41 or 42 at each end for engagement with the pedal crank. The sun gear 40, which drives the epicyclic gear set 30 hereinafter described, is integral with the shaft 36, although a splined connection between these components would be feasible.

The second end plate 33 of the casing 31 provides a central bearing surface 43 for the shaft 36, and bearing support for the sun gear 44 of the epicyclic gear set 30.

The plate 33 also provides an integrally structured carrier cage 45 for the three planet gears 46, which revolve on spindles 47. The annulus gear 48 of the epicyclic gear set 30 engages the planet gears 46 and has a machined boss 49 which runs on the crankshaft 36. The boss 49 provides a location for a ball-bearing assembly 50 (Figure 4), and is machined and threaded to provide the drive and the fixing for a chain wheel 51 (Figures 5 and 6). An internal groove 52 receives a synthetic rubber O-ring oil seal 53.

As may be seen from Figure 4, a pedal crank 54 is provided with a boss 55 which has an internally-splined socket engaging on the splined section 42 of the crankshaft 36. A fixing bolt 56 passes through the boss 55 and engages a threaded bore in the end of the crankshaft 36 to attach the crank. A similar crank (not shown) is fitted to the other end 41 of the crankshaft 36. An outer cover consists of two parts 57 and 58 rivetted together and mounted on the ball-bearing assembly 50 on the boss 49, and a corresponding ball-bearing assembly 59 on the boss 35. Each part 57 and 58 of the outer cover is also provided with an oil seal 60 and 61, with part 58 also having an oil filler plug 62 so that the casing can be kept partially filled with lubricating oil.

The bicycle illustrated in Figures 5 and 6 has a frame 63 adapted to mount the device 29 illustrated in Figures 3 and 4 and comprising four principal members 64, 65, 66 and 67 extending between a mounting 68 for front forks 69 carrying the front wheel 70 and a rear mounting 71 for the rear wheel 72. An upright frame support 73 is mounted between these mountings 68 and 71 to carry a saddle 74. The principal frame members 64-67 are divided into two pairs each having an upper tube 64 or 65 and a lower tube 66 or 67, the pairs being spaced apart laterally so as to accommodate the width of the transmission device 29, which may be slightly greater than that of a conventional bicycle crank arrangement. Figure 6 shows the frame mounting arrangement, with a bracket 75 welded to the two frames and secured to a tubular mounting 76 into which the device 29 is bolted.

In use, under normal cycling conditions, ie on the level and without a significant head wind, the output torque is such that the torque-limiting device rotates as a single unit, with the outer annular member 6 turning at the speed of rotation of the pedals. The cycle thus has a fixed transmission ratio determined by the relative sizes of the chain wheel 51 and the rear wheel sprocket wheel 77 (Figure 5). This is because the sun gear 44 (Figure 3) is driven by the crankshaft 36 and the planet gears 46 are driven by the outer annular member 6 at the same speed. Thus, the annulus gear 48 also rotates at the same speed, and the epicyclic gear set 30 is effectively a solid unit, up to the specific torque rating for the unit.

However, when the load on the rear wheel 72 increases, for example on encountering a hill, or a head wind, this is transmitted via the annulus gear 48 to the planet gears 46.

This causes the torque-limiting device to begin a limited slip motion as described with reference to Figures 1 and 2, with the input shaft rotating at a greater speed than the outer annular member 6. This results in the planet gears moving around the shaft 36 more slowly than the rotational speed of the shaft itself, and the speed of the annulus gear 48 is thus also reduced. Thus, if the speed of pedalling is kept constant, with a constant pedal force, as the load on the rear wheel 72 increases, its speed of rotation decreases. That is, the effective drive ratio varies automatically and continuously with the output load.

The following example illustrates the principle of the invention. The springs 8 are selected such that the torque-limiting device will just transmit 18.00'lb-ft (24.4 Nm) of torque without relative rotation of the elements or relative movement within the epicyclic gear-set 30. At this stage the pedal effort is 28.80 lbs (128 N), and the road surface effort is 7.20 lbs (32.0 N), and using a chain-wheel to chain-sprocket ratio of 2.50 to 1.00, or one turn of the pedals to 2.50 turns. of the driven wheel. With these conditions, and using 24 inch (600 mm) diameter wheels, the road speed is 9.00 mph (4.02 m/s), for 50.00 turns of the pedals per minute; this implies ideal operating conditions.

When the operating conditions cause the road surface effort to increase beyond 7.20 lbs (32.0 N), the crankshaft 36 of the variable ratio mechanism will start to rotate faster than the annular member 6 in which the spring blades 8 are fixed and to which the planet gears 46 are attached.

Consequently the annulus gear 48 and the chain-wheel 51 are no longer turning in the ratio of one to one with the sun gear 44 and spindle 36. The output drive ratio is now 2.50 multiplied by a fraction derived from the rotational speed of the chain-wheel divided by the rotational speed of the pedals.

The following tabulated results show the performance of the variable ratio mechanism according to the specification chosen for this example.

As the harmonic slip occurs between the crank element which is driven by the sun gear S, and the housing that drives the planet gear carrier C, these two items are given predetermined rotational speeds in rpm, and are used as the input values to the formulae to produce the following results. TABLE 3 :TEST RESULTS

SUN GEAR S 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 rpm PLANET CARRIER C 50.0 45.0 40.0 35.0 30.0 25.0 20.0 18.0 17.0 PLANET GEAR P 50.0 35.0 20.0 5.0 -10.0 -25.0 -40.0 -46.0 -49.0 rpm ANNULUS GEAR A 50.0 42.5 35.0 27.5 20.0 12.5 5.0 2.0 0.50 rpm OUTPUT TORQUE TI 18.00 20.00 22.50 25.71 30.00 36.00 45.00 50.00 52.94 1b-ft (24.4) (27.1) (30.5) (34.9) (40.7) (48.8) (61.0) (67.8) (71.8) (Nm) PEDAL EFFORT PE 28.8 28.8 28.8 28.8 28.8 28.8 28.8 28.8 28.8 1b (128) (128) (128) (128) (128) (128) (128) (128) (128) (N) OUTPUT RATIO RD 2.50 2.13 1.75 1.38 1.00 0.63 0.25 0.10 0.03 VELOCITY V 9.00 7.65 6.30 4.95 3.60 2.25 0.90 0.36 0.09 mph (MPH) WHEEL EFFORT 7.20 8.47 10.90 13.09 18.00 28.80 72.00 180.0 720.0 1b (32.0) (37.7) (48.5) (58.2) (80.0) (128) (320) (800) (3200) (N) 1 2 3 4 5 6 7 8 9 The table shows that when taken to the extreme, the device according to the invention will produce high torque values with no increase in pedal effort when the operating conditions are extremely adverse. The result-s in column 7 indicate the useful operating range of the unit where the pedal effort is 28.80 lbs, and the output drive ratio is 0.25 to 1.00, or 4.00 pedal turns to 1.00 turn of the driven wheel.

Referring now to Figures 7 and 8, the transmission device according to this alternative embodiment has its major components in common with the embodiment illustrated in Figures 2 and 3 (the device of Figure 2 being incorporated into the transmission device) r and the same or similar parts are numbered accordingly. The device comprises an axle 70 which is secured to the rear frame 71 of the cycle (shown in part only, for clarity) by means of nuts 72 and washers 73. End float setting is catered for by an adjustable collar 74. Drive input is via a sprocket wheel 75 which is screwed on to the sun gear 76 of the epicyclic gear set 30. The planet gears 46 are connected to the torque-limiting device via a tubular support shaft 77 which is externally splined at one end 78 to engage the internally-splined bore 2.The annulus gear 48 is integrally formed with the second end plate 33. The casing of the torque-limiting member mounts the inner circular flange 79 of the cycle wheel 80, from which spokes 81 extend to the outer rim 82 of the wheel which carries the tyre 83.

Drive is input via the sprocket wheel 75 and the sun gear 76 and is transferred via the planet gears 46 and the annulus gear 48 to the casing of the torque-limiting device, and hence to the wheel of the cycle. In this case, the load on the wheel is such that the torque-limiting device operates as a solid unit, and the drive is direct. As the output load is increased, the torque on the planet gears 46 increases, and with it the torque on the central member 1 of the torque-limiting device. Slipping begins to occur, with the cylindrical members 3 and 4 beginning to move at a greater speed around the axle 70 than the casing. As with the device described with reference to Figure 3, the drive ratio varies continuously according to the load on the wheel, without the need for the user to exert any control.

The wheel-mounted version illustrated in Figures 7 and 8 requires a lower torque rating than the frame-mounted version of Figures 3 to 6, as the torque is magnified at the pedals by the ratio of the chain wheel to the sprocket wheel. This version could be designed to be interchangeable with standard rear wheels. It could possibly be mounted in each rear wheel of a tricycle, in which case the devices would not only provide a continuously variable drive, but would also accommodate the differential speeds of the rear wheels when turning, providing drive to each rear wheel accordingly.

Table 4 gives test results for a typical arrangement as illustrated in Figures 7 and 8.

TABLE 4 : TEST RESULTS

SUN GEAR S 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 rpm PLANET CAGE C 50.0 40.0 30.0 25.0 20.0 18.0 17.0 16.0 PLANET GEAR P 125.0 62.75 0.50 -30.6 -61.8 -74.2 -80.4 -86.7 rpm ANNULUS GEAR A 125.0 89.25 53.5 35.63 17.75 10.6 7.03 3.45 rpm OUTPUT TORQUE TI 7.50 9.38 12.50 15.00 18.75 20.83 22.06 23.44 1b-ft (10.0) (12.5) (16.7) (20.0) (25.0) (27.8) (29.5) (31.3) (Nm) PEDAL EFFORT PE 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 1b (133) (133) (133) (133) (133) (133) (133) (133) (N) OUTPUT RATIO RD 2.50 1.79 1.07 0.71 0.36 0.21 0.14 0.07 VELOCITY V (MPH) 9.00 6.43 3.85 2.57 1.28 0.76 0.51 0.25 mph WHEEL EFFORT 7.50 10.50 17.52 26.32 52.83 88.44 133.5 271.7 1b (33.4) (46.7) (77.9) (117) (235) (393) (594) (1209) (N) 1 2 3 4 5 6 7 8 Referring to Figures 9a and 9b, the outer annular member 84 is similar to the annular member 6 shown in Figure 1, except that the slots 86a in the spring fulcrum bosses 86 are angled away from the radial centre lines so as to make maximum use of the available space to accommodate longer spring blades 85. The bearing sleeves 88 around the eccentric cylindrical members are provided with lateral grooves 90 of part-circular cross-section, which receive and restrain the inner ends of the spring blades 85, which are curved over to provide eyelets 87. The spring blades 85 are formed with a curved profile such as to accommodate variations in length during the harmonic progression of the bearing sleeves 88.

Unlike the sleeves 9 shown in Figures 1 and 2, the bearing sleeves 88 do not have inner flanges. This is to simplify the machining of the lateral grooves 90. In place of the flanges, spacers 91 are interposed to provide a retaining surface for the blade eyelets 87 and to absorb any axial forces produced by the blades. An intermediate drive member 89, corresponding to the intermediate member 5 shown in Figures 1 and 2, is provided.

The design of this embodiment allows greater versatility in specifying a range of torque capacity ratings for a given size of assembly and installation. The device is adaptable for heavier applications than that shown in Figures 1 and 2, which is applicable only to relatively low torque machines such as pedal driven machines. For heavier applications it would be feasible to arrange the double banks of spring mechanisms in ganged assemblies.

The sleeves 88, as with the sleeves 9-in Figures 1 and 2, maintain a constant radial position relative to the annular member 84, in that they do not rotate around the offset centre AR but perform an orbit journey around the annular member's centre FR. The sleeves are therefore maintained approximately in this relative position by the resilience of the spring blades, which approximate to the connecting rods in a reciprocating simple harmonic mechanism.

For higher power transmission purposes still, a greater versatility of torque capacity variation is achieved by using coil springs in place of the flat blade springs.

A mechanism of this type is shown in Figures 10a, 10b and 11. It should be noted that in this embodiment the action of the sleeves 88 in the embodiment of Figure 9 in holding their relative positions by the resilience of the flat spring blades no longer applies.

Referring first to Figures 10a and 10b, the mechanism consists of an outer annular member or casing 100 into which are mounted two banks of eight coil springs 109. These are fixed at their outer ends to fulcrum pivots 101 in the casing 100, and to anchor devises 104 on sleeves 105 at their inner ends. The springs are permanently secured by an inactive coil at each end in mounting sockets 110a and 110b at their inner and outer ends respectively. The sockets 110a and 110b are secured pivotally by pins 108 to the anchor devises 104, and by bolts 111 to the outer fulcrum pivots 101.

Figure 10b shows only one half of the installation, and is intended to illustrate the method by which the sleeves 105 are maintained in their relative radial positions, and also how the power from the central drive member 102, essentially the same as that in Figures 1 and 9, is transmitted via the sleeves 105 to the casing 100.

Because the longitudinal stiffness of the flat springs is not available, it is necessary to restrain rotation of the sleeves 105 around the eccentric members by the addition of orbit discs 103, which transmit a variable rate of drive to the casing 100 by means of four bobbins 107 which operate in bores equally spaced about the casing centre FR in the central rib 112 of the casing 100.

Each bobbin 107 has two diametrically-opposed holes each of which accepts one of four machined pins 106 solidly mounted in a respective one of the orbit discs 103. The pitch radius of the bushed holes in the bobbins 107 is the same as the offset radius of the journals in the central drive member 102 (Figure 10a) which may be identical to that shown under reference numeral 5 in Figure 1. The rotation of the bobbins will therefore reflect exactly the orbit circle of the sleeves 105 about the axis centre FR. The anchor centres of the devises 104 will also maintain the inner ends of the coil springs in exactly the same orbit circle.

Referring now to Figure 11, which shows a front sectional view of the mechanism shown in Figure 10, the two banks of coil springs 109 are connected at their inner ends to the bearing sleeves 105 by the pins 108 and at their outer ends to the fulcrum pivots 101 by bolts 111. These bolts also serve to secure the assembly of the casing 100, the left-hand closing plate 113, and the right-hand closing plate 114 together.

The two banks of eight springs operate on either side of the central rib 112 of the casing 100, and by virtue of the 1800 opposing journals of the central drive member 102, the springs on one side are always at opposite positions of travel to their equivalent springs on the other side. For clarity of illustration, the springs are drawn as though they were aligned radially from the main axis centre FR.

However, Figure 10a shows the springs in their correct alignment, with a pronounced cant angle attitude. This arrangement of the springs is essential to the efficient performance of the mechanism, and the choice of cant angle will be fundamental to the torque rating of the device.

The functioning of the mechanism is identical to that described with reference to Figures 1 to 3. The central drive member 102 and the annular driven member (casing) 100 will rotate in a 1:1 ratio while the load resistance does not require a torque effort above that of the rated capacity of the mechanism. When the load resistance requires a torque output above the rated capacity, the central drive member will rotate faster than the annular driven member 100. This will cause the planet gear carrier, which is connected to the annular driven member, to lag behind the speed of the driven input sun gear, and further cause the annulus gear, which carries the chain wheel, for example, to lag behind the speed of the planet gears.This will effectively reduce the output ratio from 1:1 to a naturally regulating ratio according to the power supplied and the resistance to motion.

The torque output of the device will equal the rated capacity of the unit multiplied by the quotient of the sun gear RPM and the annulus gear RPM, which quotient will only be greater than a value 1.0 coincidentally with output ratios of less than 1:1.

This mechanism embodies the perfect application of variable speed power transmission, having accurate and immediate response to the conditions of applied power and resistance to motion. In this respect, a mechanism in accordance with the invention will truly fulfil the "law of work" in that, what is lost in force is gained in speed, and vice versa, there being no loss of energy during the infinitely variable changes in speed, apart from the unavoidable losses due to friction between the moving surfaces of the mechanism. By a naturally equalising process, the device will overcome a greater resistance by a corresponding reduction of speed by means of an infinitely variable power transmission.

In the mechanism according to the invention, any "slippage" within the mechanism to vary the speed occurs as a result of a harmonic progression. There is no loss of energy as with a slipping clutch mechanism, where there is a direct loss of energy which goes into the production of heat in the mating surfaces of the device.

As hereinbefore described, in replacing the flat blade springs with coil springs, it was necessary to develop a means of transmitting the torque from central drive member to the annular driven member. The flat blade springs, whilst subject to angular deflection by the action of the bearing members, are also subject to linear stress and these forces produce a resultant circular force which transmits the drive from the central member to the annular driven member. A fundamental requirement is that the bearing sleeves should not revolve relative to the annular member, but to follow an orbit circle around the main axis centre.Rotation of the bearing sleeves is constrained by the addition of ridges on the bearing sleeves in the embodiment shown in Figure 1, and more effectively by locating the formed ends of the spring blades in shaped lateral grooves on the bearing sleeves, as illustrated in Figure 9.

In using coil springs, these must be installed so that both their inner and outer ends are free to pivot.

Consequently, they will only accept compression forces from the bearing sleeves to which their inner ends arse mounted.

The reaction to the compression loads in the coil springs is provided by the bearing sleeves, and these are restrained in relation to the annular member by their connection to the four bobbins which rotate freely within the bores in the central rib during the harmonic progression process.

The bobbins and bearing sleeves, and hence the inner anchor pivots of the coil springs, will follow a circle equal to that of the offset journals of the central drive member as the bearing sleeves orbit the main axis centre.

This mechanism could be used to advantage in radial engines and also in multi-cylinder pressure pumps.

The alignment of the spring axes is fundamental to the effective work output of the mechanism in maintaining the torque specification. For this reason, the linking of the two sleeves via the cranked bobbin connection, as shown in Figures 10 and 11, may also be employed in mechanisms with blade springs, particularly when a high degree of accuracy in maintaining the specific cant angle of the springs is demanded by the conditions of operation. This may be important in all but the lightest loading conditions.

Claims (4)

1. A torque-limiting device, comprising: a) a shaft carrying an eccentric generally cylindrical surface; b) a drive member connected to an annular member concentric with the shaft and surrounding the eccentric surface; c) a plurality of springs extending between the annular member and the eccentric surface, and arranged such that the drive member rotates with the shaft when the output torque of the device is at or below a predetermined value and the eccentric surface moves relative to the annular member when the output torque is greater than the predetermined value.

2. A torque-limiting device according to Claim 1, wherein the shaft carries two eccentric cylindrical surfaces relatively displaced by 180 degrees, and the annular member surrounds both eccentric surfaces and carries two sets of springs, one set extending to one eccentric surface and the other set extending to the other eccentric surface.

3. A torque-limiting device according to Claim 1 or 2, wherein the or each eccentric surface carries a freely-rotatable sleeve.

4. A torque-limiting device according to Claim 3, wherein the or each sleeve is provided with at least one ridge to limit rotation of the sleeve relative to the springs.

4. A torque-limiting device according to Claim 3, wherein the or each sleeve is provided with at least one ridge to limit rotation of the sleeve relative to the springs.

5. A torque-limiting device according to Claim 2, wherein each eccentric surface carries a freely-rotatable sleeve, and the sleeves are linked together via a cranked connection whereby rotation of the sleeves relative to the annular member is prevented.

6. A torque-limiting device according to Claim 5, wherein the cranked connection comprises a plurality of coupling members located between respective annular flanges upstanding from the sleeves, each coupling member being rotatably connected to each flange.

7. A torque-limiting device according to Claim 6, wherein the coupling members are each in the form of a circular bobbin.

8. A torque-limiting device according to Claim 6 or 7, wherein four coupling members are provided equally spaced around the flanges.

9. A torque-limiting device according to any preceding claim, wherein the springs are blade springs.

10. A torque-limiting device according to any of Claims 1 to 3 or 5 to 8, wherein the springs are helical springs extending between mountings on the annular member and mountings around a sleeve on the or each eccentric member.

11. A torque-limiting device according to any preceding claim, having an epicyclic gear-set operatively connected thereto whereby the range of ratios of the rotational speeds of the shaft and the drive member may be extended.

12. A torque-limiting device according to Claim 11, wherein the epicyclic gear-set comprises planet gears driven by the drive member, a sun gear driven by the shaft, and an annulus gear coupled to a driven member.

13. A torque-limiting device according to Claim 11, wherein the epicyclic gear-set comprises planet gears driven by the shaft, an annulus gear driven by the drive member, and a sun gear driven by an input drive member.

14. A pedal-driven vehicle, provided with a torque-limiting device according to any preceding claim forming part of the drive train transmitting driving force from the pedals to the driven wheel or wheels.

15. A pedal-driven vehicle according to Claim 14, wherein the torque-limiting device is coupled to the pedals such that the shaft is provided with a pedal crank at each end thereof, and the drive member is linked to a toothed chain-wheel linked to a driven wheel via an endless chain and a drive sprocket wheel.

16. A pedal-driven vehicle according to Claim 14, wherein the torque-limiting device is mounted on the driven wheel, drive being output directly to the wheel, and the device is driven from the pedals via an endless chain.

17. A torque-limiting device, substantially as described with reference to, or as shown in, the drawings.

18. A continuously-variable ratio transmission device, substantially as described with reference to, or as shown in, the drawings.

19. A pedal-driven vehicle, substantially as described with reference to, or as shown in, Figures 5 and 6 or Figures 7 and 8 of the drawings.

20. A motor vehicle, provided with a torque-limiting device according to any of Claims 1 to 13, forming part of the drive train transmitting driving force from the motor to the driven wheel or wheels of the vehicle.

Amencknents to the clans have been tied as folows

1. A torque-limiting device, comprising: a) a shaft carrying an eccentric generally cylindrical surface; b) a drive member connected to an annular member concentric with the shaft and surrounding the eccentric surface; c) a plurality of springs equally spaced around the annular member and extending between the annular member and the eccentric surface, and arranged such that the drive member rotates with the shaft when the output torque of the device is at or below a predetermined value and there is continuous relative rotation between the drive member and the shaft when the output torque is greater than the predetermined value.

2. A torque-limiting device according to Claim 1, wherein the shaft carries two eccentric cylindrical surfaces relatively displaced by 180 degrees, and the annular member surrounds both eccentric surfaces and carries two sets of springs, one set extending to one eccentric surface and the other set extending to the other eccentric surface.

3. A torque-limiting device according to Claim 1 or 2, wherein the or each eccentric surface carries a freely-rotatable sleeve.