PUSH BELT WITH UNWIND ELEMENTS The invention relates to a push belt, in particular for a continuously variable transmission. The principle of such a push belt and continuously variable transmission is known from for instance European patent no. 688 407. In such a transmission the push belt is driven by a first friction disc, and the push belt in turn drives a second disc. A transmission ratio is obtained due to the difference in diameter between the two discs, and the transmission ratio can thus be varied by varying the position of the push belt.

The links of the push belt circulate between the contact surface with the first friction disc and the contact surface with the second friction disc. The push belt links hereby move relative to each other in this transition zone. This relative movement takes place by means of a sliding movement.

This sliding movement results in friction, which has an adverse effect on the efficiency of the transmission.

It is an object of the invention to at least alleviate or even obviate this drawback. This object is achieved according to the invention with a push belt comprising successively placed push belt links forming a belt, wherein an unwind element is placed in each case between two adjacent push belt links. This unwind element makes it possible for two adjacent push belt links to make an unwinding movement relative to each other. A sliding or shifting movement typical of known push belts is prevented by this unwinding movement. As a result the friction in a push belt according to the invention is considerably lower, since the friction resulting from the sliding movement is reduced

considerably and there remains only a rolling friction.

The unwind element makes it further possible that in the transition zone, in which a link moves from contact with the first friction disc to contact with the second friction disc, the misalignment between the two links is compensated and the forces are transferred better. It is hereby possible to transmit a greater torque with a push belt according to the invention than with conventional push belts.

The position of the unwind elements is not determined by contact surfaces with the friction discs. The position of the unwind elements can hereby be adjusted for a good force transmission between the adjacent push belt links.

In addition, the use of unwind elements has the advantage that inaccuracies in the push belt links can hereby be compensated. The use of the unwind elements has the further advantage that for these unwind elements a material can be chosen which is different from that of the push belt links. The material for the unwind element can thus be adapted to the unwind function thereof, while the material of the push belt links can be adapted to the push belt function.

In a preferred embodiment of the push belt according to the invention each unwind element comprises at least two first unwinding surfaces arranged on either side, and the push belt links adjacent to the unwind element each comprise a second unwinding surface co-acting with a first unwinding surface. At least one of the first or the second unwinding surfaces is herein curved. This curvature defines the unwinding movement two push belt links can make relative to each other.

In another preferred embodiment the radius of curvature of the first unwinding surfaces differs from the radius of curvature of the second unwinding surfaces. The advantage is hereby gained that the push belt links and

unwind elements tend to slide to a fixed position relative to each other. A fixed position in relation to each other is thus ensured.

The first unwinding surfaces can herein be concave and the second unwinding surfaces convex.

In yet another embodiment of the push belt according to the invention, the push belt links each comprise adjacently of the unwind elements at least one recess into which the adjacent unwind element at least partly protrudes.

This ensures that the push belt links and unwind elements can no longer shift relative to each other in radial direction.

In another embodiment the unwind elements each comprise at least one recess into which one of the adjacent push belt links at least partly protrudes. Such an embodiment has the same advantages as the above stated embodiment.

These and other features of the invention are further elucidated with reference to the annexed drawings.

Figure 1 shows a cross-sectional view of a continuously variable transmission in which a push belt according to the invention can be applied.

Figures 2a and 2b show in cross-sectional view a prior art push belt.

Figure 3 shows in cross-sectional view and with exploded parts a first embodiment of a push belt according to the invention.

Figure 4 shows the push belt of figure 3 in the assembled state.

Figure 5 shows a second embodiment of a push belt according to the invention.

Figure 1 shows a continuously variable transmission, the principle of which is known from for instance the European publication EP-B-0 688 407. This transmission 1 has an input shaft 2 which is arranged

displaceably. On this input shaft 2 is arranged a first friction disc 3 on which a push belt 4 is placed. In the shown position the push belt 4 makes contact at the position 5 with friction disc 3. The driving force of input shaft 2 is hereby transmitted by push belt 4. Push belt 4 further lies against two friction discs 6. The driving force of input shaft 2 is thus transmitted via push belt 4 to friction discs 6, which can then be coupled to an output shaft. A transmission ratio is thus obtained through the difference in diameter between the first friction disc 3 and the second friction discs 6.

Figures 2a and 2b show a detail of push belt 4.

This push belt is a prior art embodiment. In figure 2a can be seen a part of friction disc 3 and the push belt 4 arranged therein. Figure 2b shows a side view of push belt 4. This push belt 4 consists of push belt links 7 which are provided on both sides with recesses 9. Cylindrical coupling parts 8 fall into these recesses 9. Coupling parts 8 and recesses 9 ensure that the position of the adjacent links 7 relative to each other remains fixed. When there is a change of shape however, as happens in the case of a continuously variable transmission 1, links 7 will slide relative to each other, wherein coupling parts 8 slide in recesses 9 and thus cause friction.

Figure 3 shows the components of a push belt according to the invention. This push belt comprises push belt links 10 having an unwind element 11 placed therebetween. Figure 4 shows these components 10 and 11 assembled to form a push belt 12. The unwind element 11 has a substantially rectangular cross-section. On both sides there are provided first unwinding surfaces 13 which in this embodiment take a flat form. These first unwinding surfaces 13 can also take a concave form.

Push belt links 10 each comprise on either side a recess in which a second unwinding surface 14 is arranged.

This unwinding surface 14 is convex and has a radius R1. In an embodiment with concave unwinding surfaces 13 the radius of unwinding surfaces 13 must be greater than the radius Ri.

As can be seen in figure 4, a rolling or turning movement of a push belt link 10 relative to unwind element 11 is hereby made possible. The recess in links 10 moreover ensures that the position of links 10 and unwind elements 11 relative to each other in radial direction is fixed.

Figure 5 shows a second embodiment of a push belt 20 according to the invention. This push belt 20 once again comprises push belt links 21 with unwind elements 22 placed therebetween. Links 21 are provided with a curved unwinding surface 23. Unwind elements 22 are provided with a straight unwinding surface 24. These unwinding surfaces 24 are arranged in a recess of unwind element 22. This once again ensures the relative position of links 21 and unwind elements 22 in radial direction. It should be noted that figure 5 is a cross-sectional view. Further arranged on links 21 are friction surfaces (not shown) which can be brought into contact with friction discs 3 and 6. It is furthermore evident that unwind elements do not have to be arranged between all push belt links. However, where they are absent on specific parts, the friction due to sliding will return once again.