A continuously variable gear transmission

TECHNICAL FIELD

The present invention relates to a continuously variable gear transmission of the kind set forth in the preamble of claim 1.

BACKGROUND ART

In continuously variable gear transmissions of this kind it is known to provide an axial force generator in the form of a spring to ensure the necessary contact pressure between the surfaces of the input and output discs and the traction balls. A continuously variable gear transmission of this kind is e.g. known from <patcit id="pcit0001" dnum="US2469653A"><text>US-A-2,469,653</text></patcit>, in which the tension of the spring for providing the necessary contact pressure can be adjusted by means of a nut.

DISCLOSURE OF THE INVENTION

It is the object of the present invention to provide a variable gear transmission of the kind referred to above, in which the torque transfer is improved under varying loads and during fast change of transmission ratio, and having an efficient, simple and compact design compared to the prior art. This object is achieved with a continuously variable gear transmission of said kind, which according to the present invention also comprises the features set forth in the characterising part of claim 1. With this arrangement, the forces from the pre-spanning ring, having a curvature with a radius larger than the radius of the traction balls and a pre-span, will provide the necessary normal forces in the contact points when the transmission is running with a constant ratio, in which situation the traction balls and the pre-spanning ring will have contact where the pre-spanning ring has its largest inner diameter. When the gear transmission ratio is increased by tilting the ball axes, the output shaft has to accelerate the driven unit and therefore a higher transfer torque is necessary. The inertia of the driven unit often has a size so that the acceleration torque is larger than the transferred torque when the gear is not in the transient phase.

The fast tilt of the traction ball axes will make the pre-spanning ring move axially in the same direction as the ball surface. When the pre-spanning ring moves axially, the normal force will change because of the changed inner diameter at the contact point. When the transmission ratio stops changing, the pre-spanning ring will move back to its normal position with contact point at the largest inner diameter with a speed related to the operating speed.

The exact curvature of the inner surface of the pre-spanning ring can be designed to match the requirements of the specific usage, e.g. a smaller radius will mean an ability to accelerate units with greater inertia.

The continuously variable gear transmission may further be provided with an axial force generator which will increase the normal forces on the traction balls when the torque requirements rise, and decrease the forces when the torque requirements fall.

By means of the combination of the axial force generator and the curvature of the pre-spanning ring, the transmission will automatically adjust to the prevailing load situation.

Preferably all forces for providing the required torque are internal and limited to the input shaft, from the axial force generator to the input disc, the traction balls, the pre-spanning ring, the output disc and through a thrust bearing back to the input shaft. Thus, these forces are not transferred through the housing.

The position of the traction balls is controlled by their three-point contact, and the angle of the traction ball axles can be controlled by a rotatable iris plate. The iris plate is a disc or plate with spiral grooves, and the iris plate is shaped to fit around the curvature of the traction balls, keeping a constant distance to the traction balls.

The axles of the traction balls pass through the radial grooves in the support plate and the spiral grooves in the iris plate and when the iris plate is rotated, the axles will tilt. In order to provide room for the tilting movement, the grooves in the iris plate are wider than the diameter of the axles and in order to prevent play, the axles are fitted with iris rollers having a diameter equal to the width of the grooves in the iris plate. Different systems for rotation of the iris plate can be envisaged, such as e.g. using a step motor for controlling the transmission ratio.

If the step motor or similar actuator is connected to the support plate with the radial grooves and the support plate has a minor rotational play, the actuator forces under rapid movement can be minimised. This will be explained in the following.

If the axle of each traction ball is lying in a plane through the input shaft respectively, the balls are performing a pure rolling and the ball axles are stable.

If the radial grooves in the housing and support plate are not perfectly aligned, the ball axle is tilted and the ball will behave like a turning wheel on a car. Because is it axially fixed it can only start tilting in the groove.

This reaction will continue until the ball axles hit the stops in the grooves and the reaction time and direction will be dependent upon the size of the misalignment and the turning direction of the support plate relative to the rotational direction of the input disc.

If the rotational direction of the input disc is clockwise and the support plate is provided with a small rotation clockwise, the front ends of the ball axles will move towards the rotational axis of the input shaft. If the support plate is rotated counter clockwise, the front ends of the ball axles will move away from the rotational axis of the input shaft. Thus by controlling the rotational position of the support plate it is possible to enforce and support the activation of the iris plate.

If the actuator housing is connected to the support plate and the actuator arm is connected to the iris plate, these will always move in opposite directions, and if the support plate is flexibly mounted with springs forcing it towards the ideal aligned position, the actuator forces can be kept to a minimum and at the same time providing a quick actuation.

As a simplistic alternative the supporting rotation of the support plate can be provided by means of that part of the force from the iris plate to the traction ball axles, which is not directed in the radial direction of the grooves in the support plate. This solution will only require that the turning of the iris plate for reducing the transmission ratio is chosen in accordance with the corresponding rotational direction of the input disc, and naturally that the support plate is flexibly mounted with a possibility of a small rotation.

In an alternative embodiment the continuously variable gear transmission further comprises a disengagement mechanism, which lets the driven unit freewheel relative to the driving unit, when no drive of the driven unit is needed. Preferably the disengagement mechanism is controlled by the iris plate in such a way that with the transmission in its lowest ratio further turning of the iris plate keeps the traction ball axles in the same position and ramps on the iris plate transfers a force through a clutch plate to the input disc, which consequently is disengaged from the traction balls, thus disengaging the connection between the driving unit and the driven unit.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described more fully below with reference to the drawing, in which <ul id="ul0001" list-style="none" compact="compact"><li>Fig. 1 shows a sectional view of a continuous variable transmission according to the invention seen in the direction A-A in Fig. 2,</li><li>Fig. 2 shows a side view of the continuously variable transmission showing the radial grooves in the support plate,</li><li>Fig. 3 shows the iris plate with spiral grooves,</li><li>Fig. 4A shows a sectional view of the axial force generator,</li><li>Fig. 4b shows an exploded view of the axial force generator,</li><li>Fig. 5A shows a sectional view of the pre-spanning ring and a traction ball, in the constant ratio state,</li><li>Fig. 5B shows a sectional view of the pre-spanning ring and a traction ball after a quick tilt of the traction balls axles</li><li>Fig. 6A shows the iris plate with modified grooves and ramps for the alternative embodiment with disengagement mechanism,</li><li>Fig. 6B shows the iris plate of fig. 6a in a perspective view more clearly showing the ramps,</li><li>Fig. 7 shows the clutch plate and control pins used in the alternative embodiment</li><li>Fig. 8 shows a sectional view of the continuously gear transmission in accordance with the alternative embodiment with the disengagement mechanism cut through a traction ball,</li><li>Fig. 9 shows a sectional view corresponding to fig. 8, but cut between the traction balls.</li></ul>

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Fig. 1 shows a section through a continuously variable transmission according to an embodiment of the invention. The transmission comprises an input shaft 1 whose rotation is to be converted into rotation of an output shaft 10, whose one end extends out of the gear, said output shaft 10 being axially aligned with the input stub shaft 1. As shown in figs. 4A and 4B, the input shaft 1 is connected by an axial force generator to an input disc 8. The axial force generator consists of a number of ramps 14 on the input shaft 1, a number of ramps 15 on the input disc 8 and in between a number of balls 16. This axial force generator provides an axial force varying with the torque of the input shaft 1. The slope of the ramps 14, 15 is calculated to create an axial force to result in the needed normal force on the traction balls 2 to give the required traction.

This axial force presses the traction balls 2 towards the pre-spanning ring 11. When the input shaft 1 rotates the traction balls 2, they will start spinning, and the contact point between traction balls 2 and pre-spanning ring 11 will move to the axial centre of the pre-spanning ring 11, where the inner diameter is largest, as shown in fig. 5A.

When the traction balls 2 are tilted rapidly, the pre-spanning ring will follow axially and the contact point will move to a point with a smaller inner diameter of the pre-spanning ring 11, as shown in fig. 5B, which results in a larger normal force at the contact points of the traction balls 2.

The positions of the traction balls are defined by the three contact points with the input disc 8, pre-spanning ring 11 and output disc 9, and the axles are supported by the grooves in the housing 5 and the support plate 6 shown in fig. 2.

The iris plate 7 shown in fig. 3 controls the angle of the traction ball axles 3. The axles 3 of the traction balls 2 supported by the radial grooves 4 in the housing 5 and the support plate 6 passes through the grooves 13 in the iris plate 7. When the iris plate 7 turns, the axles 3 will tilt. In order to make it possible for the axles 3 to pass through the grooves 13 at an angle, the grooves 13 in the iris plate 7 are wider than the diameter of the friction ball axles 3. To prevent play, the friction ball axles 3 are equipped with iris rollers 17, which have the same diameter as the grooves 13 in the iris plate.

The iris plate 18 shown in figs. 6A and 6B comprises grooves 19, the inner part of which maintain a constant radius, in order to provide the minimum transmission ratio in connection with disengagement. The disengagement is provided by means of the ramps 20 on the iris plate 18, said ramps 20 forcing the clutch plate 21 shown in fig. 7 towards the thrust bearing 22, when the iris plate 18 is turned further onwards after reaching the minimum transmission ratio. The thrust bearing 22 will thus push the input disc 8 away from its engagement with the traction balls 2 whereby the driven unit is disengaged from the driving unit. In this situation i.e. the disengaged position of the input disc 8, the torque on the input shaft 1 will be close to zero and thus if an axial force generator is present, the axial force will be at a minimum, further supporting the disengagement of the transmission. The rotational position of the clutch plate 21 may be controlled by means of pins 23 inserted in the housing, whereby the movement of the clutch plate 21 is limited to an axial movement.

The figures 8 and 9 show different cross-sectional views of the transmission with the disengagement mechanism implemented. In fig. 8 the cross-sectional view is provided through a traction ball 1 and in fig. 9 a cross-sectional view is provided between the traction balls 1.

Above the invention has been described in connection with a preferred embodiment, however, many deviations may be envisaged without departing from the scope of the following claims, such as having the pre-spanning ring positioned on the inside of the traction balls 2 and the input and output discs positioned with contact on the outside of the traction balls 2, or other possible mechanisms for tilting the traction balls 2, etc.