WO2016000733A1

WO2016000733A1 - Method for detecting damping performance of a torsional damper

Info

Publication number: WO2016000733A1
Application number: PCT/EP2014/001846
Authority: WO
Inventors: Sami AHO; Fredrik SJÖQVIST
Original assignee: Volvo Truck Corporation
Priority date: 2014-07-04
Filing date: 2014-07-04
Publication date: 2016-01-07

Abstract

The invention concerns a method for detecting damping performance of a torsional damper (212) in a clutch disc (200). The method comprising the steps of registering a present value of a rotational motion characteristic of a gearbox shaft (105) and detecting damping performance of the torsional damper (212) by comparing the present value of the rotational motion characteristic with at least one of: - a predetermined value, - a stored value of the rotational motion characteristic of the gearbox shaft (105) registered when the torsional damper (212) was new, or - a registered present value of a corresponding rotational motion characteristic of a propulsion unit output shaft (104). The invention also relates to an arrangement for detecting damping performance of a torsional damper in a clutch disc.

Description

Method for detecting damping performance of a torsional damper

TECHNICAL FIELD

This invention relates to a method for detecting damping performance of a torsional damper in a clutch disc. The invention also relates to an arrangement for detecting damping performance of a torsional damper in a clutch disc. The invention can be applied in powertrains of heavy-duty vehicles, such as trucks, buses and construction equipment, but also in other types of vehicles.

BACKGROUND ART

Ignition-induced irregularities in rotational speed of the crankshaft generate torsional vibrations in the powertrain of vehicles driven by an internal combustion engine. The vibrations in the powertrain in turn cause disturbing noises in the vehicle body and rattling in the transmission. To reduce these torsional vibrations, it is common to provide the clutch disc with coil springs that allows a limited degree of rotation between the crankshaft and the transmission input shaft. However, at the resonance frequency of the powertrain, the torsional vibrations are amplified rather than attenuated by the coil springs. Therefore, it is known to supplement the coil springs with a torsional damper in the hub of the clutch disc. The torsional damper effectively reduces the vibrations at the resonance frequency by means of friction between the driven plate and intermediate plates in the clutch disc. However, as the surfaces of these plates wear, the damping performance is reduced. This is usually not detected until the driver of the vehicle is disturbed by noise caused by the vibrations or during a regular service. At that stage, the vibrations might already have caused unnecessary wear or damage to the clutch disc and to the gearbox.

There is thus a need for a method removing the above mentioned disadvantage. SUMMARY OF THE INVENTION

An object of the present invention is to provide an inventive method for detecting damping performance where the previously mentioned problem is at least partly avoided. This object is achieved by the features of claim 1. The invention concerns a method for detecting damping performance of a torsional damper in a clutch disc. The method comprises the steps of registering a present value of a rotational motion characteristic of a gearbox shaft, and detecting damping performance of the torsional damper by comparing the present value of the rotational motion characteristic with at least one of:

- a predetermined value,

- a stored value of the rotational motion characteristic of the gearbox shaft () registered when the torsional damper was new, or

- a registered present value of a corresponding rotational motion characteristic of a propulsion unit output shaft.

The task of a torsional damper in a clutch disc is to prevent torsional vibrations in the gearbox shaft. Torsional vibrations are irregularities in rotational speed. Hence, registering a rotational motion characteristic, such as rotational speed variation or angular acceleration level, of the gearbox shaft is a means of measuring the amplitude of the torsional vibrations in the gearbox shaft. The larger amplitude of the torsional vibrations in the gearbox shaft, the poorer is the damping performance of the torsional damper. In order to judge the damping performance, the registered present value of the rotational motion characteristic may be compared with a predetermined value, for example a value known to represent a dividing line between sufficient and insufficient damping performance.

Alternatively, or in combination with the previous method, the registered present value of the rotational motion characteristic is compared with a stored value of the rotational motion characteristic of the gearbox shaft registered when the torsional damper was new. As used in this context, "new" means when the vehicle power train has just been run in, e.g. after driving a distance of for example in the range of 1000 km - 50 000 km, specifically in the range 5000 km - 20 000 km, and more specifically about 10 000 km. The damping performance of a new torsional damper is supposed to be very good. Consequently, by comparing the present value of the rotational motion characteristic with a value registered when the torsional damper was new, it is possible to estimate the present damping performance. The more the present value deviates from the value registered when the torsional damper was new, the larger the deterioration of the damping performance.

Still more alternatively, or in combination with one or both of the previous methods, the present value of the rotational motion characteristic of the gearbox shaft is compared to a registered present value of the corresponding rotational motion characteristic of the propulsion unit output shaft. The propulsion unit output shaft is for example the engine crankshaft. During proper damping of torsional vibration, the amplitude of the rotational motion characteristic of the gearbox shaft lies below or within a certain range from the corresponding amplitude of the rotational motion characteristic of the propulsion unit output shaft. A value outside of that range indicates that torsional vibrations are amplified which means that the damping performance is relatively poor.

An advantage of the inventive method is that it allows monitoring the damping performance of the torsional damper. Keeping track of the damping performance allows for detection and correction of poor damping performance before the clutch disc and gearbox have suffered from wear and damage due to torsional vibrations.

Further advantages are achieved by implementing one or several of the features of the dependent claims.

The registered rotational motion characteristic of the gearbox shaft may be rotational speed variation of the gearbox shaft. Rotational speed variation is closely related to torsional vibrations, as torsional vibrations cause rotational speed variations. Rotational speed variation is a measure of the amplitude of the torsional vibrations, which in turn is inversely related to the damping performance. The larger the amplitude of the vibrations, the lower is the damping performance. Consequently, rotational speed variation is a relevant parameter to measure if attempting to detect the damping performance.

The registered rotational motion characteristic of the gearbox shaft may alternatively be angular acceleration level of the gearbox shaft. Angular acceleration is the time derivative of rotational speed and is equal to the rate of change of rotational speed. With large amplitude torsional vibrations, the rotational speed changes quickly, and hence the angular acceleration is high. Therefore, the level of angular acceleration is indicative of the amplitude of the torsional vibrations which in turn is inversely related to damping performance. Consequently, angular acceleration level is a relevant parameter to measure if attempting to detect the damping performance of a torsional damper.

The method may comprise the additional step of determining that the torsional damper is worn when the detected damping performance is below a predetermined level. This enables determining when the torsional damper needs to be replaced, long before the vibrations cause noise audible by the driver of the vehicle. Excessive wear on the clutch disc and gearbox caused by poorly damped vibrations can thus be avoided.

The method may comprise the additional step of informing a driver, a service management, or a fleet management that the torsional damper needs to be replaced when the torsional damper is determined to be worn. The recipient of the information may then have the worn torsional damper replaced before the clutch disc and gearbox are exposed to excessive wear caused by poorly damped vibrations.

In one example of the invention, the present value of the rotational motion characteristic is registered when the following conditions are fulfilled:

- the clutch is fully engaged,

- the propulsion unit output shaft or gearbox shaft rotates at a speed within a predefined speed range.

If the clutch is not fully engaged, neither torque nor ignition-induced vibrations in the propulsion unit output shaft are fully transmitted to the clutch disc due to slip at the friction surface of the clutch disk, i.e. at the friction surface that is arranged to engage the flywheel for transmission of torque via the clutch. For the torsional damper to be fully active, the clutch has to be fully engaged. Therefore, in order to evaluate the performance of the torsional damper, it is advantageous to register the rotational motion characteristic when the clutch is fully engaged. The powertrain of a vehicle comprises a number of masses interconnected by shafts with torsional spring characteristics. Such a system tends to oscillate with greater amplitude at some frequencies than at others. That is, vibrations at some frequencies tend to be amplified while vibrations at other frequencies are attenuated. The frequency at which the response amplitude is a maximum is known as the power train's resonance frequency.

The torsional vibrations in a power train mainly emanates from the ignitions in the cylinders of the combustion engine. For example a six-cylinder engine ignites three times per revolution of the propulsion unit output shaft. Each ignition produces an acceleration of the crankshaft, resulting in that the rotational speed of the propulsion unit output shaft varies slightly over each revolution. This rotational speed variation cause torsional vibrations. As the engine ignites a certain number of times per revolution of the crankshaft, the frequency of the torsional vibrations is directly related to the rotational speed of the propulsion unit output shaft and hence also to the speed of the gearbox shaft if the clutch is engaged. As the amplitude of the vibrations in the system is frequency dependent, it is advantageous to always register the rotational motion characteristic when the propulsion unit output shaft or the gearbox shaft rotates within a predefined speed range. This makes it easier to properly evaluate the damping performance of the torsional damper, as well as comparing current damping performance with historic damping performace.

Preferably, the predefined speed range corresponds to generating vibrations at about the resonance frequency of the powertrain. As the vibrations are maximally amplified at the resonance frequency, the vibrations are most easily measured and detected at a speed range corresponding to the resonance frequency, and the damping performance is most easily verified at said speed range.

The present value of the rotational motion characteristic may be registered when at least one of the following additional conditions is fulfilled:

- a predefined gear is engaged, or

- a torque transferred by the clutch is within a predefined torque interval. The engaged gear and the torque transferred may affect the torsional vibrations in the powertrain. Well-defined conditions in the driveline during the registering of the rotational motion characteristic enables more reliable and accurate comparisons of the present rotational motion characteristic value with previously registered values or predefined values. A low torque transferred by the clutch inherently results in relatively low amplitude vibrations, whereas high torque transferred by the clutch inherently results in relatively high amplitude vibrations. The rotational motion characteristic is thus more easily measure and detected at relatively high torque transfer levels of the clutch. The step of registering a present value of a rotational motion characteristic of a gearbox shaft may comprise registering a plurality of values of said rotational motion characteristic of said gearbox shaft and calculating an average value of the plurality of recently registered values of said rotational motion characteristic. Using an average value calculated from a plurality of registered values reduces measurement uncertainties caused for example by white noise.

The method may further comprise the step of detecting and recording the damping performance at predefined mileage intervals. Mileage is the distance driven by the vehicle. For example, the damping performance may be detected and recorded each 10 000 km. This enables detecting the damping performance regularly and discovering that the torsional damper needs to be replaced in time, but still the control unit of the vehicle is not overwhelmed from performing damping performance calculations or storing damping performance data. It also allows for monitoring the development of the damping performance as a function of total mileage. The method may further comprise the step of predicting when the torsional damper will become worn based on the recorded damping performances. By extrapolating the curve of the damping performance as a function of total mileage, it is possible to conclude at what driving distance the damping performance likely will pass below the predetermined level that determines that the torsional damper is worn. This allows for predictive maintenance. For example, the torsional damper may be replaced at the last regular service scheduled before the torsional damper is predicted to become worn. As a result, the number of visits at the repair shop can be limited which saves time.

The method may comprise registering the rotational motion characteristics using a rotational speed sensor on the gearbox shaft and/or on the propulsion unit output shaft. Rotational speed sensors are often already provided on the gearbox shaft and on the propulsion unit shaft in modern vehicles. The registering of the rotational motion characteristic can thus be performed using at least partly already existing sensors. This is advantageous as additional sensors would add to the cost of the vehicle and require space as well as installation work.

In one example of the invention, the torsional damper is configured to provide frictional damping of ignition-induced irregularities in rotational speed in the propulsion unit output shaft by means of a resilient member pushing at least one intermediate friction plate against a driven plate of the clutch disc. Torsional vibrations in the driven plate relative to the clutch disc output hub cause friction between the intermediate friction plates and the driven plate. The generated friction results in that the vibrational energy at least partly is dissipated as heat. When the wearing surfaces of the intermediate friction plates and the driven plate get worn, the damping performance of the torsional damper is reduced. The at least one friction plate is generally made of metal, such as steel.

The clutch disc may further comprise a set of coil springs, each located in a window in the clutch disc and configured to enable a limited degree of relative rotation between the driven plate and clutch disc output hub. The coil springs functions as to even out ignition-induced irregularities in rotational speed in the propulsion unit output shaft in the transfer of torque to the gearbox shaft. For most frequencies of torsional vibrations, this works out well. However, at and close to the resonance frequency of the powertrain of the vehicle, the vibrations are amplified despite the presence of coil springs. Therefore, it is advantageous to complement the coil springs with a torsional damper providing frictional damping. The invention further relates to an arrangement for detecting damping performance of a torsional damper in a clutch disc. The arrangement comprises at least one rotational speed sensor provided on a gearbox shaft and an electronic control unit. A control algorithm of the electronic control unit (ECU) is arranged to detect damping performance of the torsional damper by registering at least one present value of a rotational motion characteristic of a gearbox shaft, and detecting damping performance of the torsional damper by comparing the present value of the rotational motion characteristic with at least one of:

- a predetermined value,

- a stored value of the rotational motion characteristic of the gear box shaft registered when the torsional damper was new, or

The arrangement is arranged to perform the inventive method described above, and benefits from the same advantages as the method.

In the arrangement, the torsional damper may comprise a resilient member pushing at least one intermediate friction plate against a driven plate of the clutch disc, such that frictional damping of ignition-induced irregularities in rotational speed in propulsion unit output shaft is provided. The clutch disc of the arrangement may further comprise a set of coil springs, each located in a window in the clutch disc and configured to enable a limited degree of rotation between the driven plate and clutch disc output hub.

The invention further relates to a vehicle comprising an arrangement as described above. The invention further relates to a computer program comprising program code means for performing the steps of the method described above when said program is run on a computer. The invention further relates to a computer readable medium carrying a computer program comprising program code means for performing the steps of the inventive method when said program product is run on a computer.

The invention further relates to a control unit (ECU) for controlling a utility vehicle , the control unit being configured to perform the steps of the method as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

In the detailed description of the invention given below reference is made to the following figures, in which:

Figure 1 shows a schematic overview of a truck,

Figure 2 shows a schematic overview of the powertrain of a vehicle,

Figure 3 shows a schematic overview of a clutch disc,

Figure 4 shows a schematic overview of a torsional damper in a clutch disc,

Figure 5a shows an exemplary graph of rotational speeds of the driven plate and the clutch disc output hub as a function of time,

Figure 5b shows an exemplary graph of angular acceleration of the driven plate and the clutch disc output hub as a function of time,

Figure 6 shows an exemplary graph of maximal angular acceleration of the driven plate and the clutch disc output hub as a function of angular frequency, Figure 7 shows an exemplary graph of angular acceleration levels of the gearbox shaft registered at different mileages,

Figure 8 shows a flow chart for carrying out the inventive method according to one aspect of the invention,

Figure 9 shows a flow chart for carrying out the inventive method according to a another aspect of the invention,

DETAILED DESCRIPTION

Various aspects of the invention will hereinafter be described in conjunction with the appended drawings to illustrate and not to limit the invention. Variations of the inventive aspects are not restricted to the specifically shown embodiment, but are applicable on other variations of the invention. Like designations denote like elements.

Figure 1 shows an example of a vehicle 1 in which the invention may be implemented. In this example, the vehicle 1 is a truck but the invention would be equally advantageously implemented in the powertrains of other utility vehicles and also in other kinds of vehicles having a clutch disc provided with a torsional damper, such as buses and automobiles. In this example, the vehicle comprises e.g. wheels 3, a load carrying arrangement 6 and a driver's cabin 8.

Figure 2 shows a schematic overview of a powertrain 100 of a vehicle 1. A clutch 101 is situated between the combustion engine 102 and the gearbox 103, connecting the propulsion unit output shaft 104 of the engine 102 to the gearbox shaft 105 of the transmission of the vehicle. When the clutch 101 is in an engaged position, it transfers torque between the shafts 104; 105, and when the clutch is in a disengaged position, the shafts 104; 105 are decoupled from each other and no torque is transferred between them. The clutch 101 comprises a clutch disc rotationally conntected to the gearbox shaft. The clutch disc is arranged to engage a friction surface of a flywheel, which is rotationally connected to the propulsion unit output shaft 104. The clutch disc comprises a torsional damper (not shown in figure 2) in order to prevent ignition- induced irregularities in the rotational speed of the propulsion unit output shaft 104 to be transferred as torsional vibrations to the gearbox shaft 105 when the clutch is engaged. A rotational speed sensor 107, 108 is provided on each of the propulsion unit output shaft 104 and the gearbox shaft 105. The sensors 107, 108 are coupled to an electronic control unit 109 which is capable of registering, processing and recording measured values from the sensors 107, 108. Figure 3 shows a schematic overview of a clutch disc 200. The clutch disc is a part of the clutch 101 shown in Figure 2. The clutch disc 200 comprises several different parts. Along the circumference of the clutch disc 200 is a driven plate 220 covered with a friction surface 221. The friction surface 221 is pressed against a flywheel 199 rotationally fixed to the propulsion unit output shaft when the clutch is engaged by means of a clutch spring, such as a diaphragm spring. The clutch disc 200 is generally mounted rotationally fixed but axially movable to the gearbox shaft 105 by means of splines 205 provided in the middle opening 204 in the clutch disc output hub 210. The splines 205 are adapted to interact with corresponding splines 206 in the gearbox shaft 105. The clutch disc 200 is provided with a set of coil springs 201, each of which is located in a window 202 defined in the clutch disc between the driven plate 220 and the output hub 210. The coil springs 201 are located in a single radial plane and are configured to enable a limited degree of relative rotation between the driven plate 220 and the output hub 210. The function of the coil springs 201 is to smoothen out the variations in rotational speed of the propulsion unit input shaft induced by the ignitions in the cylinders of the combustion engine, while torque is transmitted from the driven plate 220 to the output hub 210. The presence of the coil springs 201 between the driven plate 220 and the output hub 210 causes the rotational speed and angular acceleration levels of the output hub 210 to be displaced in time from the rotational speed and angular acceleration levels of the driven plate 220 as shown in Figures 5a-b.

Directly after a cylinder ignition the sudden increase in propulsion torque in the propulsion unit output shaft 104 causes a longitudinal compression of each coil spring because the relative rotation between the driven plate 220 and output hub 210. The compressed coil springs results in increased propulsion torque being transmitted by the torsional damper, thereby generating an increased propulsion torque also at the output hub 210. The speed variation of the output hub is primarily caused by propulsion shafts acting as springs when a large torque is applied, i.e. twisting of the propulsion shafts. The motion characterisitic of the garbox shaft will be slightly out of phase with the motion characteristic of the propulsion unit output shaft 104 due to the delay characterisitic of the coil springs 201.

Figure 4 shows a schematic illustration of a torsional damper 212 in a clutch disc output hub 210. The torsional damper 212 comprises intermediate friction plates 215 and a resilient member 230 which presses the intermediate friction plates 215 against the driven plate 220. The intermediate plates are also pressed against each other and against the output hub 210. Figure 4 also shows the gearbox shaft 105 to which the clutch disc output hub 210 is axially moveably but rotationally fixedly mounted via splines 205, 206. The driven plate 220 is made to rotate along with the propulsion unit output shaft 104 by pressing a friction surface 221 of the driven plate 220 against a flywheel 199 rotationally fixed to the propulsion unit output shaft 105. The ignition- induced irregularities in rotational speed, i.e. torsional vibrations, of the propulsion unit output shaft 105 are evened out by the coil springs 201 (shown in Figure 3) in order to pass on a smoother rotational speed to the clutch disc output hub 210 and hence to the gearbox shaft 105. However, around the resonance frequency of the powertrain, the coil springs 201 do not manage to attenuate the torsional vibrations. In fact, the torsional vibrations tend to be amplified rather than attenuated by the coil springs at frequencies close to the resonance frequency, resulting in large amplitude torsional vibrations in the output hub 210 if nothing is done to prevent it. The purpose of the torsional damper 212 is to dampen the large amplitude torsional vibrations in the output hub 210. The torsional damper firmly presses the driven plate 220 against the output hub 210 via a number of intermediate friction plates 215 by means of a resilient member 230. The resilient member 230 is for example a Belleville spring, but other designs are possible, such as compression springs, coil spings, etc. As the coil springs 201 causes the angular acceleration level of the output hub 210 to be out of phase with the angular acceleration level of the driven plate 220, the output hub 210 and the driven plate 220 accelerate relative to each other, and out of phase relative time. The relative acceleration, i.e. relative angular motion, leads to friction between the surfaces of the driven plate and the intermediate plates 215. As a result, the vibrational energy is at least partly dissipated as friction losses and the torsional vibrations become attenuated. However, friction causes wear. As the wearing surfaces of the intermediate friction plates are worn, the thicknesses of the intermediate plates 215 are reduced. This is compensated for by the resilient member 230 which expands a corresponding distance in order to still press the intermediate friction plates 215 firmly enough against the driven plate. However, after substantial wear, the intermediate plates 215 become so thin that the resilient member 230 cannot expand to compensate for their reduced thickness while still retaining enough pressing force. With diminished pressing force, the damping performance of the torsional damper 212 is reduced.

Figure 5a shows an exemplary graph of rotational speeds ω of the driven plate 220 and the clutch disc output hub 210 as a function of time t. The solid line 300 represents the rotational speed ω of the driven plate 220. The dashed line 310 represents the rotational speed ω of an output hub 210 provided with an adequately working torsional damper 212. The dotted line 320 represents the rotational speed ω of an output hub with a worn-out torsional damper. The rotational speed curves 300, 310, 320 oscillate due to the presence of torsional vibrations. The amplitude of these oscillations, i.e. the rotational speed variation, is a measure of the amplitude of the torsional vibrations. The rotational speed variation 305 of the driven plate is defined as the difference between the peaks and the troughs of the driven plate rotational speed curve 300. The rotational speed variation 315 of the propely damped output hub is defined as the difference between the peaks and the troughs of the damped rotational speed curve 310. The rotational speed variation 325 of the poorly damped output hub is defined as the difference between the peaks and the troughs of the un-damped rotational speed curve 320. In the example of Figure 5a, the rotational speed variation 325 of the poorly damped output hub is larger than for the rotational speed variation 305 of the driven plate. This means that the torsional vibrations have been amplified instead of attenuated by the coil springs which indicates that the frequency of the vibrations are close to the resonance frequency of the powertrain. However, the rotational speed variation 315 of the properly damped output hub is smaller than the rotational speed variation 305 of the driven plate, even though the frequency of the vibrations is close to the resonance frequency of the power train. Hence, the torsional vibrations in the driven plate are attenuated while torque is transferred to the damped output hub.

As can be seen in Figure 5a, the peaks of the rotational speed curves 310, 320 of the clutch disc output hub are displaced in time relative to the rotational speed curve 300 of the driven plate. This displacement is due to the compression-relaxation sequence of the coil springs 201 which interconnect the driven plate and the clutch disc output hub.

Figure 5b shows an exemplary graph of angular acceleration a of the driven plate 220 and the clutch disc output hub 210 as a function of time t. The solid line 350 represents the angular acceleration a of the driven plate 220. The dashed line 360 represents the angular acceleration a of the output hub 210 provided with a proper torsional damper 212 for damping of torsional vibrations. The dotted line 370 represents the angular acceleration a of an output hub 210 with a worn-out torsional damper. The angular acceleration curves 350, 360, 370 oscillate due to the presence of torsional vibrations. The maximum angular acceleration level 355, 365, 375 of these oscillations is a measure of the amplitude of the torsional vibrations in the driven plate, the properly damped output hub, and the poorly damped output hub respectively. In the example of Figure 5b, the angular acceleration level 365 of the properly damped output hub is smaller than the angular acceleration level 355 of the driven plate. Hence, the torsional vibrations in the driven plate are attenuated while torque is transferred to the output hub. However, for the poorly damped output hub, the angular acceleration level 375 is larger than the angular acceleration level 355 of the driven plate. This means that the torsional vibrations have been amplified instead of attenuated by the coil springs (which indicates that the frequency of the vibrations are close to the resonance frequency of the powertrain). The amplified torsional vibrations tend to results in accelerated aging of the gearbox, power train vibrations and mechanical noise.

As can be seen in Figure 5b, the peaks of the angular acceleration curves 360, 370 of the clutch disc output hub are displaced in time relative to the angular acceleration curve 350 of the driven plate. This displacement is due to the coil springs 201 which interconnect the driven plate and the clutch disc output hub. The coil springs allow some relative motion between the driven plate and the output hub while torque is transferred. Positive angular acceleration means increase of rotational speed and negative angular acceleration means decrease of rotational speed. When the clutch is fully engaged, the driven plate is rotationally fixed to the propulsion unit output shaft and the clutch disc output hub is rotationally fixed to the gearbox shaft. Hence, if the clutch is engaged, the curves 300, 310, 320, 350, 360, 370 shown in Figure 5a-b also represents the rotational speed and angular acceleration of the propulsion unit output shaft and the clutch disc output hub respectively.

Figure 6 shows an exemplary graph of angular acceleration level a of the driven plate and the clutch disc output hub respectively as a function of rotational speed ω. The solid line 400 represents the angular acceleration level a of the driven plate 220. The dashed line 410 represents the angular acceleration level a of an output hub 210 provided with properly functioning torsional damper. The dotted line 420 represents the angular acceleration level a of an output hub 210 with a worn-out torsional damper. As can be seen in Figure 6, the angular acceleration levels of the properly damped as well as the poorly damped output hub are lower than the angular acceleration level of the driven plates for most rotational speeds ω. This is thanks to the coil springs which allow some relative motion between the driven plate and the output hub. However, at or close to resonance frequency rotational speed u)_r, i.e. a rotational speed corresponding to generation of ignition-induced vibrations at the resonance frequency of the powertrain, the coil springs amplifies the vibrations instead of attenuating them. This is why the angular acceleration levels of the properly damped as well as the poorly damped output hub are higher than the angular acceleration level of the driven plates close to the resonance frequency rotational speed ω_η Here, the angular acceleration level 420 in the hub with a worn-out torsional damper is much higher than the angular acceleration level of the driven plate. Such high-amplitude vibrations may cause damage in the transmission as well as unpleasant noise. However, in the hub provided with a properly damping torsional damper, the angular acceleration level 410 at the resonance frequency rotational speed u)_r is substantially lower than for the poorly damped hub. In fact, the angular acceleration level 410 is only slightly higher than the angular acceleration level 400 of the driven wheel. Low acceleration levels means small-amplitude vibrations which do not cause substantial damage or wear to the transmission and/or clutch disc. Figure 7 shows an exemplary graph of angular acceleration levels a of the gearbox shaft registered at different mileages M. The mileage interval between each registration may be for example 10 000 km. The acceleration level of the gearbox shaft is equivalent to the acceleration of the clutch disc output hub if the clutch is engaged. The dotted horizontal line represents a wear limit 700. The wear limit 700 is predefined and represents an angular acceleration level above which torsional vibrations are deemed as potentially harmful or causing excessive wear to the transmission. The angular acceleration level is a measure of the amplitude of the torsional vibrations. As can be seen in the example of Figure 7, the angular acceleration level is initially well below the wear level 700. However, as the total mileage M of the vehicle increase, the registered angular acceleration levels starts to increase. At the eighth registration 701, the angular acceleration level is above the wear level 700 for the first time and the torsional damper is deemed worn. If the torsional damper is not replaced, the angular acceleration level will continue to increase as the mileage increases.

Figure 8 shows a flow chart for carrying out the inventive method according to one aspect of the invention. The first step 501 is to register a present value of a rotational motion characteristic of the gearbox shaft 105. The rotational motion characteristic may be rotational speed variation, angular acceleration level, or any characteristic from which the amplitude of torsional vibrations can be deduced. The second step 502 is to use the registered present value in order to determine the damping performance of the torsional damper of the clutch disc. This is done by comparing the registered present value with a predetermined value, a stored value of the rotational motion characteristic of the gearbox shaft registered when the torsional damper was new, or with a registered present value of a corresponding rotational motion characteristic of a propulsion unit output shaft of the vehicle. The method is carried out by an electronic control unit and continuously updated information concerning for example propulsion unit output angular speed, gearbox angular speed, transmitted torque, clutch engagement status, etc. is generally constantly available in a vehicle operating register, ot the like, which is supplied with information from various sensors. Figure 9 shows a flow chart for carrying out the inventive method according to a further aspect of the invention. Just as in the aspect of the invention described in conjunction with Figure 8, the first step 601 is to register a present value of a rotational motion characteristic of the gearbox shaft 105, and the second step 602 is to determine the damping performance of the torsional damper. In this aspect of the invention, a third step 603 is to determine if the damping performance detected in the second step 602 is below a predetermined level. If the answer is yes, the fourth step 604a is to determine that the torsional damper is worn. Here, "worn" means worn to such an extent that the torsional damper does not provide adequate damping of torsional vibrations, i.e. the remaining torsional vibrations may cause excessive wear or damage to the transmission or clutch disc. Alternatively, "worn" means almost worn to such an extent that the remaining torsional vibrations may cause excessive wear or damage to the transmission or clutch disc. After step 604a follows an optional fifth step 605a of informing a user that the torsional damper needs to be replaced. The user may be a driver, a service management, or a fleet management. The information to the user may include information about how urgent the replacement of the torsional damper is. For example, the torsional damper may perhaps need to be replaced more or less immediately, or before a certain additional distance has been driven. The additional distance may be e.g. 1 000 km. If the answer to the third step 603 ("Is damping performance below a predetermined level?") is no, the fourth step 604b is to determine that the torsional damper is not worn. In such case, no further action needs to be taken.

The term gearbox shaft used herein refers to a gearbox input shaft, or a gearbox internal shaft or output shaft that is rotationally connected to said gearbox input shaft. Reference signs mentioned in the claims should not be seen as limiting the extent of the matter protected by the claims - their function is only to make the claims easier to understand. As will be realised, the invention is capable of modification in various obvious respects, all without departing from the scope of the appended claims. Accordingly, the drawings and the description thereto are to be regarded as illustrative in nature, and not restrictive.

Claims

1. A method for detecting damping performance of a torsional damper (212) in a clutch disc (200), the method comprising the steps of:

registering a present value of a rotational motion characteristic of a gearbox shaft (105), and

detecting damping performance of the torsional damper (212) by comparing the present value of the rotational motion characteristic with at least one of:

- a predetermined value,

- a stored value of the rotational motion characteristic of the gearbox shaft (105) registered when the torsional damper (212) was new, or

- a registered present value of a corresponding rotational motion characteristic of a propulsion unit output shaft (104).

2. Method according to claim 1, wherein the registered rotational motion characteristic of the gearbox shaft (105) is rotational speed variation (315, 325) of the gearbox shaft (105).

3. Method according to claim 1, wherein the registered rotational motion characteristic of the gearbox shaft (105) is angular acceleration level (365, 375) of the gearbox shaft (105).

4. Method according to claim 1, wherein the method comprising the additional step of determining that the torsional damper (212) is worn when the detected damping performance is below a predetermined level.

5. Method according to claim 4, wherein the method comprising the additional step of informing a driver, a service management, or a fleet management that the torsional damper (212) needs to be replaced when the torsional damper (212) is determined to be worn.

6. Method according to any one of the preceding claims, wherein the present value of the rotational motion characteristic is registered when the following conditions are fulfilled:

- the clutch (101) is fully engaged,

- the propulsion unit output shaft (104) or gearbox shaft (105) rotates at a speed within a predefined speed range.

7. Method according to claim 6, wherein the present value of the rotational motion characteristic is registered when at least one of the following additional conditions are fulfilled:

- a predefined gear is engaged, or

- a torque transferred by the clutch (101) is within a predefined torque interval.

8. Method according to any one of the preceding claims, wherein the step of registering a present value of a rotational motion characteristic of a gearbox shaft (105) comprising

- registering a plurality of values of said rotational motion characteristic of said gearbox shaft; and

- calculating an average value of the plurality of recently registered values of said rotational motion characteristic.

9. Method according to any one of the preceding claims, further comprising the step of detecting and recording the damping performance at predefined mileage (M) intervals.

10. Method according to claim 9, further comprising the step of predicting when the torsional damper (212) will become worn based on the recorded damping performances.

11. Method according to any one of the preceding claims, comprising registering the rotational motion characteristics using a rotational speed sensor (107, 108) on the gearbox shaft (105) and/or on the propulsion unit output shaft (104).

12. Method according to any one of the preceding claims, wherein the torsional damper (212) is configured to provide frictional damping of ignition-induced irregularities in rotational speed in propulsion unit output shaft (104) by means of a resilient member (230) pushing at least one intermediate friction plate (215) against a driven plate (220) of the clutch disc (200).

13. Method according to claim 12, wherein the clutch disc (200) further comprises a set of coil springs (201), each located in a window (202) in the clutch disc and configured to enable a limited degree of rotation between the driven plate (220) and clutch disc output hub (210).

14. Arrangement for detecting damping performance of a torsional damper (212) in a clutch disc (200), the arrangement comprising at least one rotational speed sensor (108) provided on a gearbox shaft (105) and an electronic control unit (109), characterised in that a control algorithm of the electronic control unit (109) is arranged to detect damping performance of the torsional damper (212) by:

registering at least one present value of a rotational motion characteristic of a gearbox shaft (105), and

- a predetermined value,

15. Arrangement according to claim 14, characterised in that the torsional damper (212) comprises a resilient member (230) pushing at least one intermediate friction plate (215) against a driven plate (220) of the clutch disc (200), such that frictional damping of ignition-induced irregularities in rotational speed in the propulsion unit output shaft (104) is provided.

16. Arrangement according to claim 14 or claim 15, characterised in that the clutch disc (200) further comprises a set of coil springs (201), each located in a window (202) in the clutch disc (200) and configured to enable a limited degree of rotation between the driven plate (220) and clutch disc output hub (210).

17. A vehicle (1) comprising an arrangement according to any of previous claims 14- 16.

18. A computer program comprising program code means for performing the steps of any of claims 1-13 when said program is run on a computer.

19. A computer readable medium carrying a computer program comprising program code means for performing the steps of any of claims 1-13 when said program product is run on a computer.

20. A control unit (109) for controlling a utility vehicle (1), the control unit being configured to perform the steps of the method according to any of claims 1-13.