The present invention relates to a transmission system for industrial vehicles, of the type comprising a torque converter and a geared mechanical transmission which transmits the motion from the converter output to the driving wheels of the vehicle.

There are generally known transmission systems of the type in which a torque converter is used in association with a mechanical transmission, particularly a gearbox, typically designed for vehicles for agricultural or industrial use.

In these applications, the load on the engine is highly variable, and the use of the torque converter is particularly advantageous because the converter output torque can vary according to the operating conditions.

However, the presence of a torque converter leads to a number of drawbacks, mainly arising from the dissipation of energy in the form of heat due to the hydraulic transmission of the motion.

More precisely, as the difference between the engine speed and the vehicle speed increases, the quantity of heat dissipated also increases, and the efficiency of the converter decreases as a result.

In order to overcome this drawback, transmission systems have been developed in which the torque converter can be locked up, in other words in which a device is provided for fixing the pump and turbine of the converter to each other, with direct transmission of the motion of the driving shaft.

This device can be provided inside the converter, with an internal locking clutch, such as that described in GB 2389155, or externally, by providing a further external driven shaft, connected to the converter output, which can be locked by means of a clutch on to the converter input, on the casing of the pump.

An example of an embodiment of the latter solution is given in EP 2 169 270, which describes a transmission for industrial vehicles in which the torque converter is connected to two output shafts, the first of which is connected to the turbine, and therefore to the output of the converter, while the second is connected to the casing of the converter, in other words to its input.

The second shaft, connected to the casing, is hollow, and the shaft connected to the turbine is supported rotatably inside this second shaft. Additionally, the second shaft has a gear wheel which transmits the movement to a driven intermediate shaft. This intermediate shaft is normally idle and can be connected via a pair of gears which can be engaged by means of a clutch keyed on to to the driven shaft.

When the gears are engaged, the motion of the casing is transmitted directly to the turbine, so that these elements rotate jointly, and the torque converter is thus locked up.

However, both of these solutions have drawbacks. In particular, the positioning of a locking clutch inside the torque converter is rather complicated and entails high production costs.

As regards the second type of solution, the need to use a further intermediate shaft results in rather large overall dimensions. Moreover, during the operation of the torque converter the idle rotating components cause a dissipation of energy which is preferably to be avoided . A further sol ution is described in US Patent 5,819,587 in which the output shaft of the converter is hol low, and the input shaft extends within it and also projects beyond an end of the output shaft d istal from the converter. In this position , a clutch is provided for join ing the input and output shafts of the converter together with respect to rotation, thus provid ing the locking action . A similar solution is described in US 5, 129,870.

However, even these sol utions are not free of d rawbacks, because, d uring operation with the converter locked u p, in other words with the pump and turbine joined together with respect to rotation, the known systems do not work in an optimal way, since the damping effect provided by the hydraul ic transmission is no longer present; instead, there is a rig id connection between the transmission and the eng ine, resulting in an increase in the vibration transmitted from the latter, with an adverse effect on d riving comfort and the l ife of the mechanical components.

This problem is mitigated to some extent in the transmission described in International Patent Application WO 2010/070873, in which a damper is provided between the d riving shaft and the pump of the converter.

However, this sol ution also suffers from some d rawbacks, as the whole of the power entering the transmission always passes through the damper, which therefore has to operate in highly variable cond itions; in particular, its rigidity operates in series with the variable rig id ity of the converter when the motion transmission takes place hyd raul ical ly, whereas it acts separately during operation with the pump and turbine locked, with transmission of the mechanical motion .

This makes it particularly difficult to select and correctly design a damper capable of operating effectively in all the conditions of motion of the system. Consequently the reduction of the vibration entering the transmission in different operating conditions is less effective, and the cost is greater than that of a solution which has to operate only during the mechanical transmission of the motion.

Therefore, the technical problem which is addressed by the present invention is that of providing a vehicle transmission which can overcome the drawbacks mentioned above with reference to the prior art.

This problem is resolved by the transmission system according to Claim 1. The present invention has some considerable advantages. The principal advantage is the fact that the transmission system according to the present invention enables the torque converter to be locked up by means of a system which is simple to produce and which also reduces energy dissipation to a minimum. Furthermore, the converter is locked up in a soft manner, thus improving driving comfort in these phases. Additionally, the system provides effective vibration damping overall, in all conditions of motion, using a solution which is simple and effective in terms of construction.

Further advantages, features and methods of use of the present invention are made clear by the following detailed description of some embodiments, provided as non-limiting examples. Reference will be made to the figures on the appended drawings, of which :

- Figure 1 is a schematic illustration in sectional side view of a transmission system according to the present invention; and

- Figure 2 is a schematic sectional side view of a clutch and a corresponding damper which are details of the transmission system of Fig ure 1.

With reference to Fig ure 1 in the first place, a transmission system accord ing to the present invention is ind icated as a whole by the reference numeral 100.

The transmission system 100 comprises a torq ue converter 1 of a known type connected to a driving shaft, not shown in the d rawing , at an input 11. The d riving shaft receives a rotary motion from an engine, for example an internal combustion eng ine . Add itional ly, the input 1 1 of the converter 1 is formed at a hub 14 of the converter, on which the casing 13 of the converter 1 is supported .

The casing 13 thus forms a toroidal chamber which houses a centrifugal pu mp P, which is also fixed to the in put 1 1 and to the d riving shaft so as to act on a working fl uid present in the casing 13.

The converter 1 further comprises a turbine T to which the working fluid transfers energy so as to cause the rotation of an output 12 of the converter which is fixed to the turbine .

In this way, therefore, a hydraul ic connection is formed between the driving shaft and the output of the torque converter 1. In any case, the modes of operation of the torq ue converter 1 are known to persons skil led in the art, and therefore wil l not be described in further detail .

The transmission system according to the present invention comprises two separate shafts lead ing from the torq ue converter 1 , namely a first shaft 2 connected to the output 12 of the converter 1 and fixed to the turbine, which is therefore also referred to as the output shaft 2, and a second shaft 3, which is connected to the input 11 of the converter, and which is therefore fixed to the casing 13 of the converter, this shaft also being referred to below as the input shaft 3.

More precisely, the first shaft 2 is tubular, in such a way that a cavity 20 is formed , and the second shaft 3 can extend within th is shaft and is coaxial with it. The two input and output shafts are also referred to below, respectively, as the inner and outer shafts. It should be noted that this config uration is the opposite of that used in EP 2 169 270, in which the shaft connected to the casing l ies outside the shaft connected to the turbine .

The second shaft 3 extends from the hu b 14 of the converter 1 , to which it is rigidly con nected , to an output end 31 , at which an output of the transmission system is provided for connection to an ancil lary pump, not shown in the drawing , or to other means for actuating the working components of the veh icle, in other words those not involved in the phases of motion . Thus the eng ine of the vehicle can supply power to the ancil lary eq uipment of the vehicle by bypassing the converter 1 and therefore avoiding the d issipation phenomena associated with this component.

The second shaft 3 also has a g reater longitud inal extension than the first shaft 2, in such a way that, when the second shaft is inserted into the first, a first end 30a and a second end 30b, axially opposed to the first, are defined, each of these ends being outside the first shaft 2 and , more precisely, each of these ends projecting axial ly from the corresponding end 20a, 20b of the first shaft 2.

The first end 30a is connected to the h ub 14 of the converter 1 , while a mechan ical damper 6, described in greater detail below, is keyed on to the second end 30b .

The damper 6 therefore transmits the motion of the second shaft 3 to a clutch 4, coaxial with the input shaft 3 and output shaft 2, which al lows the motion of the second shaft 3, and therefore the motion of the d riving shaft, to be transmitted , on command, to the output shaft 2.

When the cl utch is engaged , the two shafts are joined together, and conseq uently the input 11 and output 12 of the torq ue converter 1 rotate at the same speed , or, in other words, the converter is locked up .

On the other hand , when the cl utch is d isengaged, the output shaft 2 rotates in accordance with the characteristic curve of the torq ue converter

1 , while the input shaft 3 again rotates jointly with the d riving shaft, thus driving the loads connected to the output 31 .

The second shaft 2 further comprises at least one output gear keyed on to this shaft in a longitudinally intermed iate position between the cl utch 4 and the converter 1.

The output gear enables the motion of the shaft 2 to be transmitted to a gearbox 7, shown purely schematical ly in the d rawing , from which the motion is transmitted to the wheels via an output 51 of a transmission shaft 5.

In the present embod iment, there are two output gears 21a, 21 b, which can be engaged selectively by means of respective clutches 22a, 22b .

Thus part of the gearbox 7 can be constructed d irectly on the output shaft

2, thereby helping to red uce the overall d imensions of the whole transmission system 100. The damper 6 and the connecting clutch 4 between the first and second shafts 2 and 3 will now be described in detail with reference to Figure 2. In particular, the damper 6 comprises a base 61 of plastic material or other material which is sufficiently rigid to allow the transmission of the motion, connected rigidly to the end 30b of the second shaft 3, and a flange 62 fixed to the base 61 and rigidly connected to the clutch 4 at the position of a driving element 43 of the clutch . One or more resilient elements 63 capable of damping the vibrations can be interposed between the flange 62 and the base 61.

The clutch 4 comprises a driven element 42, keyed on to the shaft 2, which can receive the motion from the driving element 43 when the clutch is engaged .

The damper 6 therefore makes the process of engaging the clutch 4 softer, and consequently improves driving comfort during the locking of the torque converter. It also limits the vibration transmitted to the shaft 2 and to the other components of the transmission in sequence, thus reducing the stress to which the system is subjected .

Additionally, by comparison with other known solutions using dampers, in the present invention the damper has the advantage of operating only when the converter 1 is locked up, whereas, when the transmission of motion takes place through the hydraulic connection, the damping effect is provided by the converter itself, thus avoiding the problems described above.

Furthermore, when the motion takes place with the shafts 2 and 3 locked, it can limit the vibration issuing from the gear changing mechanism, during gear changing for example, thus providing optimal protection for the converter 1 and the gearbox.

Clearly, therefore, the transmission system described above can be used to overcome the problems identified with reference to the present invention, aided by the use of two coaxial output shafts, of which the inner shaft is connected to the driving shaft, these shafts being connectable to each other via a clutch which is also coaxial with the shafts.

This feature enables the power of the engine to be made available outside the transmission system, using a compact and reliable solution.

A further advantage is provided by the fact that the clutch 4 can also be slip-controlled; in other words, a certain amount of slip can be provided between the driving element 43 and the driven element 42, in order to obtain the partial transmission of the power of the inner shaft 3 to the outer shaft 2, and thus provide intermediate operating conditions between those in which the motion is transmitted by the converter 1 and those in which the converter is locked up.

Additionally, this configuration advantageously allows the use of a damper between the inner and outer shafts, this damper being able to operate only when the converter 1 is locked up.