GB2309272A - Infinitely variable ratio gearbox comprises two differential gearings in series - Google Patents

Description

IMPROVED GEARBOX The gearboxes used in motor vehicles (and other machines) essentially provide the vehicle with a number of different ratios between the power unit (the engine) and the road resistance. This is necessary both to overcome the initial road resistance and any undulations in terrain so that the engine can develop sufficient revolutions per minute (rpm) to supply an adequate torque to overcome the resistance.

A constant, force, if it could be applied throughout an infinitely variable range of revolutions per minute, would provide acceleration up to the point where wind resistance would counteract the power output and stabilise the velocity at a peak figure.

However, an engine does not have an infinite range of rpm and therefore the gearbox is necessary to keep the rpm within a reasonable range where the engine can supply adequate torque output to overcome the resistance. As the speed rises, a higher ratio becomes necessary to preserve the rpm within the range or, if the load increases, a lower ratio may need to be brought into play to achieve this.

An engine, in fact, has an optimum speed (rpm) where it is producing maximum torque and any reduction or increase on this will reduce the power output.

Normally speaking, four gear ratios are found to be adequate for most cars although some high performance cars may have many more.

The necessity to change gears (ratios) has a number of implications: 1. The engine is rarely at peak output during acceleration and therefore the acceleration of the vehicle is not optimised.

2. Similar constraints apply to fuel economy since optimum power is related to fuel usage and the energy output produced from this.

3. Changing of gears involves a power cessation during which acceleration is actually negative, unless the clutch is slipped (which is a destructive activity), so the motion is not smooth.

From this it is readily seen that the ideal fuel economy and/or acceleration characteristics are inhibited by the decision between many ratios, which would potentially give better results, and the avoidance of too many gear changes, which would destroy this advantage in practice.

Automatic gear boxes apparently overcome some of these problems by optimising the gear change pattern and employing some form of torque conversion which, effectively, slips the clutch during changes.

However, in practice, this often results in an uneven (even nauseating), motion and it is a fact that a good driver with a manual change will always outperform an automatic.

Therefore, there is an unsatisfied need for an infinitely variable gear ratio that will adjust between engine speed and road speed, thus supplying exactly the right ratio at any given instant of acceleration, deceleration or steady velocity and keeping the engine at the optimum (efficient) speed.

The requirement has been known for some time to automotive engineers the world over and some solutions have been designed, varying from the fluid transmission of the 1960's U.S. large motor cars, often called "gas-guzzlers" to the DAF rubber band solution. None of these were particularly satisfactory since inherent inefficiency or unreliability of the methodology spoilt the potential advantages.

A constant mesh, infinitely variable ratio gearbox would confer benefits in efficiency of fuel consumption, better performance and less wear and tear and is therefore very desirable, provided the efficiency can be maintained through the transmission and the costs of manufacture are not inordinate.

The present invention aims to provide an efficient constant mesh infinitely variable ratio gearbox which is based upon proven technology and can be relatively inexpensively manufactured.

According to the present invention, there is provided a constant mesh infinitely variable ratio gearbox which comprises a pair of differentials connected in series to drive a shaft which constitutes the output shaft of the gearbox.

The invention will be better understood from the following particular and non-limiting description, given with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic representation of a well-known vehicle differential; and Figure 2 is a diagrammatic plan view of one embodiment of the present invention.

Referring firstly to Figure 1 which illustrates a classical differential, a drive shaft 10 has a bevel gear 12 which is in engagement with a ring gear 14 which is also referred to as a crown wheel. An apertured casing 16 is fixed to the ring gear 14 and carries a pair of diametrically opposite differential pinions 18a and 18k, carried by short shafts 20a, 20k which are journalled in the casing 16. Also carried by the casing 16 is a pair of differential side gears 22a,22k, which are in toothed engagement with the pinions 18a, 18k and which are each attached to a respective output shaft 24,24k.

Figure 2 illustrates the basic principle of the present invention. An engine 30 is connected via a drive shaft 32 to a drive pinion 34 which is in toothed engagement with a crown wheel 42 of a first differential 40. The differential 40 may take the form illustrated in Figure 1 and has two output shafts 44 and 45.

Shaft 44 is connected to a first toothed gear 46, and output shaft 45 is connected to a second toothed gear 47. A second differential 60 is arranged in series downstream (as regards power transmission) of differential 40. The differential 60 has a crown wheel 62 and two output shafts 64,65 respectively connected to a first gear 66 and a second gear 67. The former is in engagement with the gear 46 and the gear 67 is in engagement with the gear 47. The shaft 65 is also connected to an inhibitor arrangement 70, in the presently-preferred embodiment of the invention, which comprises a fixed drum 72 filled with a viscous fluid.

A paddle wheel 74 is located within the drum for rotation therein and is fixed to an end of the shaft 65.

It will be realised that in the absence of the inhibitor 70, the power is channelled through the low ratio gear chain 47,67 because it has the lower resistance to overcome. If this gear chain were locked, then all power would be channelled through the high ratio gear chain 46,66. Consequently, one would have two ratios from the same box, without the need to change gear. In the example shown, the lower ratio would be 5:2 and the higher ratio 2:5.

In accordance with the preferred embodiment of the invention, the inhibitor 70 is included. The inhibitor would exert little resistance at low speeds of the shaft but the resistance increases quickly as the speed of the shaft 65 rises, feeding resistance back through the entire chain and causing some of the power to be channelled off through the high chain (46,66).

This would result in progressively more of the power being fed through the high gear chain as the speed increased, exactly the conditions desired in practical application of the constant mesh infinitely variable ratio gearbox disclosed herein.

Each time the inhibitor shaft speed rises it self-limits its rotational speed and channels more energy to the high ratio chain. The rotational speed at which this inhibition is most effective is chosen to substantially coincide with the optimum revolutions per minute of the engine.

In the example shown, the gear ratio range would vary from 5:2 to 2:5 which (factorised down), gives a range of 1 to 6.25. A different ratio between the drive gears (46,47;66,67) would give a different range. However, the inhibitor characteristics would be adjusted to accommodate this. In practice, the desired ratio would be decided upon, and the inhibitor designed to provide the appropriate load of resistance, e.g. by changing the spacing between the valves of the paddle wheel and the internal surface of the cylindrical wall of the inhibitor 70.

The ratio range obtainable by different size gears in the high and low ratio drives is given by: (high gear)2 (low gear)2 An important advantage of a gearbox according to the invention is that its gears are in constant mesh, resulting in less wear, and there are relatively few moving parts so production cost is unlikely to be more than for existing conventional gearboxes and could be less. A simple centrifugal clutch would be included to supply the engage/disengage mechanism as in conventional automatic gearboxes.

It will be appreciated that modifications may be made to the embodiment disclosed, without departing from the invention, for example, an inhibitor of a different design could be included, such as a braking device.

Claims (5)

1. A constant mesh infinitely variable ratio gearbox which comprises a pair of differentials connected in series to drive a shaft which constitutes the output shaft of the gearbox.

2. A constant mesh infinitely variable ratio gearbox, in which power can be transmitted through the gearbox via one or both of two gear trains having different ratios, and in which an inhibitor is included to provide increasing resistance in the lower ratio gear train.

3. A gearbox according to claim 2 in which the inhibitor comprises a paddlewheel or like rotatable member operating within a closed environment which contains a viscous fluid.

4. A gearbox substantially as herein described with reference to and as illustrated in the accompanying drawings.

5. Any novel combination or sub-combination of features disclosed and/or illustrated herein.