BACKGROUND AND SUMMARY

The present invention relates to a method for controlling a gear shift in a transmission arrangement. The invention also relates to a control unit for controlling the gear shift and a vehicle comprising such a transmission arrangement and control unit. The invention is applicable on vehicles, in particularly working machines such as e.g. wheel loaders, articulated haulers, dump trucks, etc. Although the invention will mainly be described in relation to an articulated hauler, it may also be applicable for other type of vehicles such as e.g. trucks.

In connection with transportation of heavy loads at construction sites or the like, a working machine is often used. The working machines may be utilized for transportations in connection with road or tunnel building, sand pits, mines, forestry and similar environments. Thus, the working machine is frequently operated with large and heavy loads in rough terrain and on slippery ground where no regular roads are present.

In order to fulfil the desired demands from the fields where the working machine is frequently operated, high quality of the vehicle gearbox is necessary. The gearbox is arranged for adjusting the speed and tractive effort of the vehicle in dependency of the specific driving scenario. The gearbox comprises a transmission arrangement and depending on the specific type of gearbox, the transmission arrangement may comprise e.g. ordinary gear sets with cylindrical gear wheels in meshed connection with each other or planetary gear sets comprising a respective sun gear, ring gear and a planet carrier, or a transmission arrangement having a combination of ordinary gear sets and one or more planetary gear sets.

SE 527 108 describes a vehicle transmission arrangement, particularly suitable for working machines. The transmission arrangement in SE 527 108 comprises an input shaft and an output shaft. Five planetary gear sets are arranged between the input shaft and the output shaft for providing the desired gear ratios. Also, the transmission arrangement comprises three clutches and five brakes for achieving the desired gear shifts. Hereby, nine forward gears and four reverse gears are provided for the transmission arrangement in SE 527 108.

Although the transmission arrangement in SE 527 108 provides for a sufficient number of gears, the transmission arrangement is still in need of further improvements in terms of costs and functionality.

It is desirable to provide a method for controlling a gear shift in a transmission arrangement that improves the gear shift when shifting from a forward gear to a reverse gear in comparison to the prior art.

According to a first aspect of the present invention, there is provided a method for controlling a gear shift in a transmission arrangement, the transmission arrangement comprising a first, a second, a third, and a fourth planetary gear set comprising a sun gear, a planet carrier and a ring gear, respectively, wherein the transmission arrangement further comprises a transmission housing, an input shaft and an output shaft, wherein the planet carrier of the first planetary gear set is operatively connected to the output shaft of said transmission arrangement; the ring gear of the first planetary gear set and the planet carrier of the second planetary gear set are operatively connected to each other; the sun gear of the first planetary gear set and the sun gear of the second planetary gear set are operatively connected to each other; the ring gear of the second planetary gear set and the planet carrier of the third planetary gear set are operatively connected to each other; two of the sun gear, the planet carrier and the ring gear of the fourth planetary gear set are each operatively connected to a respective one of the input shaft, the sun gear, the planet carrier and the ring gear of the third planetary gear set; a first locking mechanism is arranged for selectively locking the ring gear of the third planetary gear set to the transmission housing; a second locking mechanism is arranged for selectively locking the planet carrier of the second planetary gear set to the transmission housing; and a first connecting mechanism is arranged for selectively connecting the planet carrier of the second planetary gear set to the ring gear of the third planetary gear set; the method, when shifting from a forward gear to a reverse gear, comprising the steps of: positioning the first locking mechanism and the first connecting mechanism at least in a partially engaged state for reducing the rotational speed of the planet carrier of the second planetary gear set; and positioning the second locking mechanism in an engaged state when the rotational speed of the planet carrier of the second planetary gear set is below a predetermined threshold limit.

The wording “operatively connected to” should in the following and throughout the entire description be interpreted such that the components thereof are fixedly connected to each other, i.e. the rotational speed of the components which are operatively connected to each other is the same. Hence, no connecting mechanism or the like is arranged between the components that are operatively connected to each other and they can therefore not be disengaged from one another during operation. Accordingly, the planet carrier of the first planetary gear set is always connected to the output shaft of the transmission arrangement.

Furthermore, it should be readily understood that the wording “two of the sun gear, the planet carrier and the ring gear of the fourth planetary gear set are each operatively connected to a respective one of the input shaft, the sun gear, the planet carrier and the ring gear of the third planetary gear set” implies that one of the members of the fourth planetary gear set, i.e. the sun gear, the planet carrier or the ring gear, is operatively connected to one of the members of the third planetary gear set or to the input shaft of the transmission arrangement. At the same time, another one of the members of the fourth planetary gear set is operatively connected to another one of the members of the third planetary gear set or to the input shaft of the transmission arrangement.

The wording “selectively connectable to”, “selectively connecting” and “selectively locking” should in the following and throughout the entire description be interpreted as an element being connectable at desirable points in time to another element. Hereby, gear shifts of the transmission arrangement can be executed by either connecting or disconnecting elements to/from each other. Components may be selectively connectable to each other by means of connecting mechanisms and controlled by e.g. a control unit or the like. When a connecting mechanism is positioned in an engaged state the components are connected to each other.

Still further, it should be readily understood that the step of reducing the rotational speed of the planet carrier of the second planetary gear set can be executed by either locking the first locking mechanism to the transmission housing and slipping, i.e. at least partially engaging, the first connecting mechanism until the rotational speed of the planet carrier of the second planetary gear set is below a predetermined threshold limit, or by positioning the first connecting mechanism in an engaged state and slipping, i.e. at least partially engaging, the first locking mechanism until the rotational speed of the planet carrier of the second planetary gear set is below the predetermined threshold limit. It should also be readily understood that the above described step of reducing the rotational speed of the planet carrier of the second planetary gear set can also be executed by slipping, i.e. partially engaging, both of the first locking mechanism and the first connecting mechanism until the rotational speed of the planet carrier of the second planetary gear set is below the predetermined threshold limit. Hereby, the first connecting mechanism and the first locking mechanism are at least positioned in a partially engaged state.

The predetermined threshold limit should be understood as a rotational speed below which it is suitable to engage the second locking mechanism.

An advantage of the present invention is that the above method enables for the second locking mechanism to be designed as a so-called dog clutch. Thus, according to an example embodiment, the second locking mechanism may be a dog clutch. A dog clutch is advantageous to use since it does not use friction to connect the planet carrier of the second planetary gear set to the transmission housing. Rather, a dog clutch uses interference between connecting parts. Hence, the second locking mechanism is more durable in comparison to e.g. a friction brake. Also, lower drag losses occur when the locking mechanism is disengaged.

According to an example embodiment of the present invention, the method may comprise the step of positioning the first locking mechanism in a disengaged state when the second locking mechanism has been positioned in the engaged state.

Hereby, when the engagement process of the second locking mechanism is ended, the first locking mechanism can be positioned in the disengaged state, thus completing the gear change process.

According to an example embodiment of the present invention, the transmission arrangement may comprise a third locking mechanism for selectively locking the sun gear of the first planetary gear set and the sun gear of the second planetary gear set to the transmission housing; and a second connecting mechanism for selectively connecting the sun gear of the third planetary gear set to the planet carrier of the second planetary gear set and to the ring gear of the first planetary gear set. Accordingly, also components of the first planetary gear set may be selectively connectable to the transmission housing.

According to an example embodiment of the present invention, the method may comprise the step of positioning the third locking mechanism in a disengaged state when reducing the speed of the planet carrier of the second planetary gear set.

Hereby, the sun gear of the first planetary gear set will not be fixated to the transmission housing and the output shaft will not be in mechanical connection with the planet carrier of the second planetary gear set which will enable the output shaft to continuously rotate when the second locking mechanism is positioned in the engaged state.

According to an example embodiment of the present invention, the method may comprise the step of positioning the second connecting mechanism in a disengaged state when reducing the speed of the planet carrier of the second planetary gear set.

Hereby, the input shaft can continuously rotate when the second locking mechanism is positioned in the engaged state since no mechanical connection will be present between the input shaft and the planet carrier of the second planetary gear set.

According to an example embodiment of the present invention, the sun gear of the fourth planetary gear set may be operatively connected to the ring gear of the third planetary gear set; the ring gear of the fourth planetary gear set may be operatively connected to the sun gear of the third planetary gear set; wherein the transmission arrangement may further comprise a third connecting mechanism for selectively connecting the input shaft to the ring gear of the fourth planetary gear set and to the sun gear of the third planetary gear set; and a fourth connecting mechanism for selectively connecting the input shaft to the planet carrier of the fourth planetary gear set.