Field of the invention

The present invention relates to a chain drive and a tensioning device for a chain drive. In particular, but not exclusively, the present invention relates to a chain drive and a tensioning device for use with chain-driven vehicles, such as all-terrain vehicles. The invention also relates to a belt drive system.

Background

A typical chain-driven mechanism or“chain drive” includes a driver sprocket wheel and a driven sprocket wheel interconnected by a linked chain. The chain couples the rotational motion of one sprocket wheel to the other, allowing torque to be transferred between them. In order for the mechanism to function effectively, the chain cannot be too loose (to prevent the chain from slipping from the teeth of the sprockets) or too taut (to avoid excessive forces on the sprocket wheels). Excessive slack in the chain may also result in a number of undesirable effects such as: delaying the transfer of torque between the sprocket wheels (so called“play in take-up”);“chain slap” caused by the chain contacting parts of the mechanism or neighbouring structures; or a lack of precise registration of the chain links with the teeth of the sprocket wheels. Maintaining the chain at an appropriate tension is therefore an important consideration in chain- driven mechanisms.

Optimal chain tension is also essential to the performance of chain-driven vehicles. In particular, chain tension is a key parameter for chain-driven“all-terrain” vehicles, which are required to operate under a wide range of conditions, including large gradients and very rugged surfaces. Chain runs on this class of vehicle are typically long and space constrained and the chains are prone to stretching under normal usage conditions. There are a number of key challenges for this type of vehicle, including:

• Chain forces are very large and can occur in forward and reverse directions;

• Manual adjustment is difficult to do correctly and needs to happen frequently ;

• Automated adjustment mechanisms are difficult to design and typically deal with only a small amount of stretch between manual interventions. In many chain-driven mechanisms, the driver sprocket wheel and driven sprocket wheel remain at a fixed separation from one another and the length of the chain between the sprocket wheels is adjusted to establish the desired chain tension. However, as the chain wears it elongates, causing the tension to diminish. The effects of chain wear are exacerbated where the separation between sprocket wheels is large compared to the chain pitch size. In these cases, excessive chain slack can accrue rapidly, rendering the mechanism ineffective. A particular problem occurs when the direction of rotation of the drive sprocket is reversed as this causes the slack portion of the chain to move from one side of the mechanism to the other, potentially jamming the mechanism or causing the chain links to come off the teeth of the sprocket wheel(s).

A common approach to tensioning is to deflect the un-tensioned side of the chain run in order to take up slack. A simple spring-loaded mechanism attached to the vehicle structure works well where rotation is always in the same direction, such as in a bicycle or a motorcycle. However, if the mechanism must also be able to rotate under load in the opposite direction, as is the case with all-terrain vehicles, the fixed, spring-loaded mechanism will not resist large drive loads, particularly as the forces involved may be considerable depending on chain and tensioner geometry. One approach to this problem has therefore been to develop“floating” chain tensioning mechanisms which draw the reaches of the chain towards one another to maintain tension, but are otherwise free to move along the chain. One such “floating” chain tensioner is described in US61 17034.

“Floating” chain tensioners suffer from the disadvantage that that the location of the tensioning device is largely uncontrolled on the chain run, resulting in large, often violent and unpredictable excursions from its starting location. This movement creates vibrations and the tensioner may even impact other parts of the mechanism, which can be particularly hazardous if the tensioner unseats and becomes entangled in the chains and sprocket wheels. The unconstrained behaviour of such tensioning devices, with high susceptibility to catastrophic failure and limited range of effective tensioning, severely limits their utility. Floating tensioners are also generally ineffective in chain drives in which the drive and driven sprocket wheels are of different sizes because the tensioner tends to be deflected towards the smaller sprocket wheel. Known chain tensioners are also typically ineffective for long chain runs because they are unable to accommodate the large amounts of slack that may develop.

Summary

According to a first aspect of the invention there is provided a chain drive comprising first and second sprocket wheels configured to rotate about respective fixed axes. A chain is passed around the sprocket wheels in a closed loop for transferring torque between them. The chain drive comprises a chain tensioner for biasing together reaches of the chain, the chain tensioner being movable along the chain. A stator sprocket wheel is engaged between the reaches adjacent the chain tensioner to hold the reaches apart between the chain tensioner and one of the sprocket wheels and thereby limit movement of the chain tensioner along the chain.

The term“movable” is used to mean that the chain tensioner is not constrained by anything other than its engagement with the chain or the stator sprocket wheel. Locating the stator sprocket adjacent the chain tensioner, i.e. between the chain tensioner and one of the first and second sprocket wheels, means that the chain tensioner and stator sprocket act on different sections of the chain, with the chain tensioner bringing together two sections of the chain and the sprocket wheel holding apart two other sections of the chain. This arrangement causes the stator sprocket to act in opposition to the chain tensioner, thereby reducing the amount of chain slack that needs to be taken up by the chain tensioner. Surprisingly, the arrangement of the stator sprocket and the chain tensioner remains stable when the chain drive is operated.

The stator sprocket wheel and chain tensioner may be supported by only the chain.

The length of the chain tensioner may be less than the diameter of either of the first and second sprocket wheels.

The chain tensioner may comprise first and second chain guides joined by an elastic member, the chain guides being located outside the chain to bias together the reaches of the chain under the tension provided by the elastic member.

The elastic member may comprise a spring or an elasticated cord. Each of the chain guides may comprise at least one sprocket wheel engaged with a respective one of said reaches.

The chain tensioner may be tethered to the stator sprocket to further limit the movement of the chain tensioner.

The chain tensioner may be tethered to the stator sprocket by a mechanical linkage between one or more of the chain guides and a spindle attached to the centre of the stator sprocket wheel.

The chain drive may comprise a second chain tensioner for biasing together the reaches of the chain on a side of the stator sprocket wheel opposite the chain tensioner, the second chain tensioner being movable along the chain, the stator sprocket wheel limiting the movement of the second chain tensioner along the chain by holding apart the reaches of the chain. Both chain tensioners may be tethered to the stator sprocket wheel.

According to a second aspect of the invention there is provided a system comprising two or more chain drives according to the first aspect of the invention. The chain tensioner is shared by each of the chain drives.

According to a third aspect of the invention there is provided a vehicle comprising the chain drive according to the first aspect of the invention.

The direction of the chain drive may be reversible.

According to a fourth aspect of the invention there is provided a chain tensioner for a chain drive comprising first and second sprocket wheels configured to rotate about respective fixed axes. A chain is passed around the sprocket wheels in a closed loop for transferring torque between them. The chain tensioning device comprises: first and second chain guides joined by an elastic member for biasing together the reaches of the chain; and a stator sprocket wheel tethered to at least one of the chain guides for engaging between said reaches adjacent the chain tensioner to hold them apart between the chain tensioner and one of the sprocket wheels and thereby limit movement of the chain tensioner along the chain.

According to a fifth aspect of the invention there is provided a drive system comprising first and second wheels configured to rotate about respective fixed axes. A flexible chain or belt is passed around the wheels in a closed loop for transferring torque between them. A tensioner is provided for biasing together the reaches of the chain or belt, the tensioner being movable along the chain or belt. A stator wheel is engaged between said reaches adjacent the tensioner to hold them apart between the chain tensioner and one of the first and second wheels and thereby limit movement of the tensioner along the chain or belt.

Brief description of the drawings

Figures 1 is a schematic perspective view of a chain drive;

Figure 2 shows a schematic perspective view of another chain drive;

Figures 3 and 4 show the chain drive of Figure 2 during operation in respective forward and reverse directions; and

Figures 5 to 8 are schematic perspective views of further chain drives.

Detailed description

This invention addresses and overcomes the principal shortcomings of floating tensioners by introducing one or more, generally lightweight, stator sprocket wheels capable of modifying the chain geometry to limit longitudinal excursions of the floating tensioner and constrain vibration and slap effects. The inclusion of a stator sprocket wheel also allows the tensioner to operate over a shorter and pre-defined length of chain, which means that larger amounts of chain slack can be accommodated by the tensioner before it becomes ineffective. The invention also allows a floating tensioner solution to be used with chain runs that are not parallel, and to manage the part of the chain run over which tensioning is effected in order to achieve optimal placement and stability of the tensioning device(s). Optimal placement may take into account obstructions which might otherwise impact or displace the tensioning device(s), such as chain run separation, access constraints or many other practical factors. The invention also provides a mechanism whereby one or more tensioners, which may be considered floating by virtue of not being connected to any external structure, are nevertheless tethered to a precise and fully constrained location on the chain run whilst being entirely supported by the chain itself.

Figure 1 shows a chain drive 100 comprising two sprocket wheels 101 , 102 mounted on fixed axles (not shown) and a chain run 103 for transferring torque between them. The chain run 103 comprises a series of links which engage with the teeth of the sprocket wheels 101 , 102 such that the chain run 103 is pulled around the sprocket wheels 101 , 102 when they are rotated. The chain drive 100 also comprises a chain tensioning device 104 which draws or biases together the upper and lower reaches of the chain 103A, 103B, i.e. the sections of the chain on opposite sides of the sprocket wheels 101 , 102, in order to provide tension in the chain 103.

The chain tensioning device 104 comprises a pair of chain guides or“pucks” 105 which are provided on the outside of the chain run 103 and drawn together by an elastic member 106 in order to provide tension to the chain 103. The pucks 105 each have a smooth, convex surface facing the chain 103 in order to reduce frictional load and sidewalls to prevent the pucks 105 from slipping off the chain 103. The elasticity of the elastic member is typically chosen so that the chain tensioning device is able to maintain chain tension for all conditions, e.g., during start-up, normal operation, light loading, heavy loading, direction change, slow down, or stop conditions, but can also be optimised for particular applications. In this example, the elastic member is an elasticated cord, but other types of elastic member can be used, e.g. an elastic member comprising one or more springs.

The chain drive 100 further comprises a stator sprocket wheel 107 located adjacent to the chain tensioning device 104 with its teeth engaged with the upper and lower reaches of the chain 103 in order to force them apart against the tension provided by the chain tensioning device 104. The stator sprocket wheel 107 is positioned between the chain tensioning device 104 and one of the sprocket wheels 101 to hold the reaches of the chain 103 apart between the chain tensioning device 104 and the sprocket wheel 101. The chain tensioning device 104 and the stator sprocket wheel 107 are“floating” or movable in the sense that they are supported only by the chain 103, i.e. they are not mechanically coupled to any external frame or support and can therefore be positioned anywhere along the chain run 103. This flexibility is particularly useful when the chain drive 100 is incorporated in vehicles with demanding spatial constraints.

The stator sprocket wheel 107 and tensioning device 104 act synergistically because the stability of the stator sprocket wheel 107 is dependent upon the tension provided by the tensioning device 104, whilst the stability of the tensioning device 104 is itself dependent on the modification of the chain geometry caused by the stator sprocket wheel 107.

In operation, an engine or motor (not shown) drives one of the sprocket wheels 101 in rotation so that the chain run 103 is pulled around it in a loop, thereby also driving the other sprocket wheel 102 and the stator sprocket wheel 107 in rotation. Although “floating”, the stator wheel 102 remains firmly in place, constrained by its engagement with the chain 103, which in turn constrains the movement of the“floating” tensioning device 104 along the chain 103. The tensioning device 104 is therefore confined to a specific sector of the chain run, which increases the efficiency of take-up of chain slack and prevents random excursions along the chain 103. The stability resulting from the combination of the stator sprocket wheel 107 with the chain tensioning device 104 allows the chain drive to be operated in either direction and under lightly or heavily loaded conditions.

The chain drive 100 allows tensioning of the chain 103 within a tightly constrained space envelope, e.g. within a width defined by the diameter of sprocket wheels 101 , 102 and the chain 103. When space constraints are not a concern, the pucks 105 may be replaced by alternative forms of chain guide such as idler sprocket wheels or rollers, which may extend outside of this space envelope. In this example, the sprocket wheels 101 , 102 and the stator sprocket wheel 107 are all the same size, but they can be different sizes. In general, the stator sprocket wheel 107 must have a diameter which is large enough to partially counteract the tensioning device 104 by holding apart the upper and lower reaches 103A, 103B of the chain run 103.

Figure 2 illustrates a further chain drive 200, which is similar to chain drive 100, except that the pucks 105 are attached to scissor arms 201 , which are in turn attached to a spindle 202 for support. The spindle 202 is free to rotate about the rotational axis of the stator sprocket wheel 107. The stator sprocket wheel 107 limits the movement of the pucks to pre-defined arcs and the tensioning mechanism is therefore entirely constrained, i.e. the pucks are not free to migrate to any extent along the chain run. This substantially reduces the level of vibration associated with the system and prevents a common failure mode characteristic of floating tensioning devices, in which the device is damaged or dislodged through impact with an external object in the vicinity of the chain run 103.

Figures 3 and 4 illustrate the disposition of the chain drive 200 when it is operated under load in forward and reverse directions. In Figure 3, the chain tensioning device 204 is displaced upwards as because chain slack is initially located in the lower reach of the chain 103. Reversing the direction of chain drive 200, as in Figure 4, causes the chain tensioning device 204 to be displaced downwards as the chain slack is transferred to the upper reach of the chain 103. In general, changing the direction of the chain drive 200 will cause the chain tensioner 204 to move rapidly up and down resulting in impacts with structure surrounding the chain drive. Flowever, this vibration is constrained by the presence of the stator sprocket wheel 207 and particularly, by the scissor arms 201 tethering the pucks 105 to it. These features mean that the chain tensioner can be made of light-weight materials (which would otherwise undergo large amplitude vibrational motion when the direction of the chain drive is reversed) and that the chain run can be operated in a space with limited external clearance.

Multiple stator sprocket wheels 107 and tensioners 104, 204 can be used in numerous combinations and permutations, limited primarily by space available.

Figure 5 shows a further chain drive 500, which is variation of the chain drive 100 described above and which utilises two floating tensioning devices 104 and a single stator sprocket 107 to stop the tensioning devices from contacting one another. The ability to deploy two constrained floating tensioners 104 on a single run of chain allows a greater amount of chain slack to be taken up before the chain tensioning devices 104 become ineffective, e.g. when the chains touch or the elastic members no longer provide enough tension.

In general, the constraining effect of the stator sprocket wheel 107 allows the chain run to be separated into multiple segments, each of which can be separately tensioned using independently constrained tensioners, with a cumulative tensioning effect. For long chain runs, multiple stator sprocket wheels may be used to increase the number of separately tensioned segments, i.e. any number of stator sprocket wheels can be used. Figure 6 is similar to Figure 5, except that one of the tensioning devices is tethered to the stator sprocket wheel 107.

Figures 7 and 8 illustrate partially and fully constrained tensioning for a system with different sized drive and driven sprocket wheels 701 , 702 and with multiple chains 703A, 703B. The use of a suitably sized stator sprocket 702 allows floating pucks 705 to be constrained between the stator sprocket wheel 707 and the larger sprocket wheel 702. The pucks 705 extend across both chains 703A, 703B in order to provide tension to them both simultaneously. An additional tensioner 804 (not shown) can also be provided between the stator sprocket wheel 707 and smaller sprocket wheel 701 .

In Figure 8, the tensioner 804 is tethered to the stator sprocket wheel 707 by a pair of scissor arms 202.

Whilst the invention is presented in the context of a chain drive system, to which it is particularly well suited, it is also applicable to other systems employing other types of flexible drive elements, such as tensile belts and wires over various forms of pulley, to transfer drive from one rotating axle to another.