METHOD FOR OPERATING A tamping unit of a track-laying machine as well as a tamping device for track bed compaction and a track-laying machine

Description

field of technology

[0001] The invention relates to a method for operating a tamping unit of a track construction machine and furthermore to a tamping device for track bed compaction and a track construction machine. State of the art

[0002] Rail-guided track maintenance machines are used to maintain track beds. These machines are equipped with a tamping device featuring a movable tamping unit for track bed compaction. During operation, the tamping unit is repeatedly moved between a reset position, in which it is disengaged from the track bed, and an engagement position, in which it is engaged with the track bed. This process subjects the tamping unit to high static and dynamic loads. To maintain the functionality of these heavily stressed components, regular, time-consuming, and costly inspections and maintenance are performed.

[0003] Such tamping units and methods for track bed compaction are known, for example, from WO 2017 / 129215 A1, GB 2 451 310 A and WO 2014 / 102401 A1. Summary of the invention

[0004] The invention is based on the objective of creating a method for operating a tamping unit of a track construction machine that increases the performance and economy of the tamping unit.

[0005] This problem is solved by a method with the features of claim 1. According to the invention, it has been recognized that the ballast force acting between the tamping unit and the track bed, particularly along a displacement direction of the tamping unit, is essential for the stress on the tamping unit and that this force can be precisely determined based on the drive force and the acceleration. By determining the stress acting on the tamping unit based on the ballast force, the tamping unit can be operated efficiently and economically. During operation of the track construction machine, the tamping unit, in particular the at least two tamping picks, the drive unit, and the bearing unit, are subjected to high mechanical stress. According to the invention, it has been recognized that the ballast force acting between the tamping unit and the track bed is essential for the stress on the tamping unit.By evaluating stress based on ballast forces, track construction machines can be designed to be robust and operated efficiently and economically. For example, highly stressed components can be identified and designed and maintained according to these stresses. Furthermore, track bed processing can be carried out while ensuring a high ratio between processing speed and energy consumption, and taking into account the ballast forces that significantly influence wear, thus reducing expected downtime for maintenance. Determining and evaluating stress based on ballast forces can therefore increase the performance and cost-effectiveness of the track construction machine.

[0006] The tamping unit is moved relative to the track bed at least, and in particular exclusively, in a vertical direction. This movement preferably occurs between the return position and the engagement position. In the return position, the tamping unit is raised and out of engagement with the track bed. In particular, the tamping unit can be arranged vertically in the return position such that it is positioned completely above the sleepers and / or rails. Preferably, the tamping unit has at least two, and in particular at least four, tamping tines. In the engagement position, the tamping unit, and in particular the at least two tamping tines, penetrates the track bed. In a delivery position arranged between the return position and the engagement position, the tamping unit comes into contact with the track bed.The track bed compaction can take place during the relocation from the delivery position to the intervention position.

[0007] To determine the ballast force, the driving force acting on the tamping unit and required for its movement is determined. The driving force is defined as the force required to move the tamping unit between its return position and its engagement position, particularly in the vertical direction. The driving force can be measured, for example, using a force sensor. The driving force can be measured at the tamping unit and / or at the unit carrier and / or at a drive device acting between the tamping unit and the unit carrier.

[0008] An acceleration sensor can be used to determine the acceleration acting on the tamping unit. The acceleration can be measured at the tamping unit and / or at the drive unit.

[0009] The ballast force is determined based on the driving force and the acceleration. Ballast force is defined as the force acting between the track bed and the tamping unit, particularly the at least two tamping picks, and is oriented along the displacement between the return position and the engagement position, especially in the vertical direction. By considering both the driving force and the acceleration of the tamping unit, the ballast force can be determined reliably and accurately despite harsh operating conditions.

[0010] A method according to claim 2 ensures the increased performance and cost-effectiveness of the track construction machine. The position of the tamping unit, particularly in the vertical direction, can be detected with exceptional reliability and robustness. Position sensors used to move the tamping unit can be employed, thus eliminating the need for additional sensors. Acceleration detection is therefore particularly economical. The position can be detected at the drive unit. Alternatively, the position can be detected at a bearing assembly by which the tamping unit is mounted relative to the unit carrier. The position can be detected using a position sensor, in particular a displacement sensor or a rotary encoder, in the form of a potentiometer, a Hall sensor, or a cable length encoder.To record the change in position over time, the position's progression can be differentiated according to time using an evaluation unit, or the change in position can be determined over a discrete time step. From the change in position over time, i.e., the velocity, the acceleration is determined as the change in velocity over time. Preferably, the position, and thus the acceleration, is recorded relative to the assembly carrier. Taking the acceleration due to gravity into account, the absolute acceleration of the tamping unit can be determined.

[0011] A method according to claim 3 ensures the increased performance and efficiency of the track laying machine. By taking into account the mass-dependent inertial force, the ballast force can be determined with particular accuracy. The mass of the tamping unit can be weighed before installation in the track laying machine or while installed on the machine. Alternatively, the mass of the tamping unit in the return position can be determined by measuring the drive force. In the unaccelerated state, the weight force, and thus the mass of the tamping unit, can be determined based on the drive force.

[0012] A method according to claim 4 ensures the increased performance and cost-effectiveness of the track construction machine. The fluidically actuated drive unit is robust in operation and ensures the provision of the power necessary for processing the track bed. Determining the drive force by sensing at least one fluid pressure acting on the drive unit can be particularly robust. By using pressure sensors necessary for pressure control, the tamping unit can be manufactured particularly economically by avoiding redundancies. The drive unit preferably has at least one hydraulic cylinder and / or at least one pneumatic cylinder. A piston guided within the respective cylinder is connected to a piston rod and has a piston ring surface facing the piston rod and a piston surface opposite the piston ring surface.Preferably, the fluid pressure is detected by detecting a piston pressure acting on the piston surface and / or a piston ring pressure acting on the piston ring surface.

[0013] A method according to claim 5 ensures the increased performance and efficiency of the track construction machine. When the tamping unit is moved between the return position and the engagement position, the ballast force acting on the tamping unit varies considerably. By determining the stress based on the time course of the ballast force, changes in the ballast force can be taken into account. Preferably, the stress on the tamping unit is determined over at least one tamping cycle. A tamping cycle comprises moving the tamping unit from the return position to the engagement position and back from the engagement position to the return position. The stress on the tamping unit can also be determined over the entire operating time of the tamping unit.Preferably, the stress on the tamping unit, in particular on the at least two tamping picks, is determined at least over the duration of one tamping cycle, in particular over several tamping cycles, and especially over the entire operating period. In addition to the static stress, the temporal profile of the ballast force also provides information about the dynamic stress on the tamping unit. Knowing the dynamic stress allows maintenance cycles to be optimized and maintenance effort to be reduced.

[0014] A method according to claim 6 ensures the increased performance and efficiency of the track construction machine. Depending on the track bed being worked, the ballast force varies within and between different tamping cycles. It has been recognized that the ballast force amplitudes, i.e., the amplitudes of the changing ballast force, are of crucial importance for the stress on the tamping unit. To determine the ballast force amplitudes, the temporal profile of the ballast force between a first and a second measuring point can be recorded, wherein the ballast force is the same at both the first and second measuring points, and wherein the second measuring point is defined by the first time this ballast force is reached again. The ballast force amplitude is determined as the difference between the maximum ballast force value and the minimum ballast force value between the first and second measuring points.

[0015] A method according to claim 7 ensures the increased performance and efficiency of the track construction machine. To determine the load spectrum, the cumulative frequency of the ballast force amplitudes is determined. Preferably, the range of occurring ballast force amplitudes is first divided into ballast force amplitude segments. To determine the load spectrum, the frequency of the ballast force amplitude occurring within each ballast force amplitude segment can be counted. The load spectrum thus provides information about the magnitude and frequency of the alternating stress acting on the tamping unit. The load spectrum is therefore particularly suitable for evaluating the dynamic stress acting on the tamping unit.

[0016] A method according to claim 8 ensures the increased performance and efficiency of the track construction machine. When the at least two tamping picks penetrate the track bed, the ballast work is transferred between the tamping unit and the track bed. The ballast work correlates with the stress on the tamping unit. The stress on the tamping unit can be determined particularly efficiently using the ballast work. To determine the ballast work, the ballast force and the position can each be determined after specific time steps. Subsequently, the change in position over this time step can be multiplied by the ballast force, in particular the average ballast force over this time step. Alternatively, the ballast force can also be integrated with respect to the position.

[0017] A method according to claim 9 ensures the increased performance and cost-effectiveness of the track construction machine. To determine the wear condition, the stress acting on the tamping unit can be compared with a maximum permissible stress. Based on the wear condition, a prediction can be made as to how long the tamping unit can continue to operate before failure occurs, in particular before individual parts of the tamping unit fail. The wear condition can also be used to determine the necessity of maintenance work, especially the replacement of the tamping unit. Knowing the wear condition allows the track construction machine, and in particular the tamping unit, to be operated for longer periods, thus utilizing its actual service life and reducing downtime and maintenance costs.

[0018] A method according to claim 10 ensures the increased performance and efficiency of the track construction machine. Adjusting at least one process parameter for controlling the tamping unit based on the load allows for influencing the load on the tamping unit. Suitable process parameters include, for example, the frequency and / or amplitude of the vibration and / or displacement component transmitted to the at least one tamping pick, the positioning speed of the tamping unit between the return position and the engagement position, the acceleration of the tamping unit, and the fluid pressure acting on the drive unit. Advantageously, this allows the at least one process parameter to be adjusted depending on the track bed being worked and the load resulting from the condition of the respective track bed.Depending on the track bed, energy consumption and processing speed can thus be optimized, taking into account the stress acting on the tamping unit.

[0019] A method according to claim 11 ensures the increased performance and efficiency of the track construction machine. By changing at least one process parameter when a stress threshold is exceeded, both overstressing of the tamping unit and insufficient processing speed of the track bed can be counteracted. Preferably, the at least one process parameter is reduced when an upper threshold is exceeded such that the stress on the tamping unit decreases. When a lower threshold is undershot, the at least one process parameter can be changed such that the stress increases. Advantageously, a difference between the upper and lower thresholds ensures that the at least one process parameter is not subject to constant change.

[0020] A method according to claim 12 ensures the increased performance and efficiency of the track construction machine. By adjusting the at least one process parameter such that a stress limit is not exceeded, failure of the tamping unit, in particular of the at least two tamping picks, can be reliably prevented. The stress limit can be a static and / or dynamic, in particular experimentally determined, strength value of the tamping unit, especially of individual parts of the tamping unit. The at least one process parameter can be continuously changed based on the stress, or the change can be made in discrete steps. For example, the vibration frequency of the at least two tamping picks can be continuously varied between 30 Hz and 50 Hz. Alternatively, the vibration frequency is 35 Hz in a first mode and 45 Hz in a second mode.The tamping unit can be operated in the first and second modes, switching between them based on the load. The tamping unit can operate in more than two modes. Each operating mode differs from the others in at least one process parameter.

[0021] Different types of tamping units can be compared and evaluated based on the ballast force and / or the stress. The ballast force and / or the stress can also be used to optimize the tamping unit, in particular its kinematics, bearings, materials, and / or design.

[0022] The invention is further based on the objective of creating a tamping device for track bed compaction that has increased performance and economy.

[0023] This problem is solved by a tamping device with the features of claim 13. The advantages of the tamping device according to the invention correspond to the advantages of the method according to the invention. The tamping device can be further developed, in particular, with the features of at least one of claims 1 to 12. Preferably, the tamping unit is mounted on the unit carrier so as to be slidably vertically. The drive unit can include a hydraulic cylinder. For engaging the track bed, the tamping unit preferably comprises at least two, in particular at least four, tamping picks. The drive force sensor system can include at least one pressure sensor and / or at least one force sensor. The acceleration sensor system can include at least one velocity sensor and / or at least one position sensor and / or at least one acceleration sensor. The position sensor can be designed as a non-contact sensor.The position sensor can be arranged between the tamping unit and the unit carrier, particularly on the drive unit. Preferably, the at least one position sensor is designed as a potentiometer and / or as a Hall sensor and / or as a cable length sensor.

[0024] The invention is further based on the objective of creating a track construction machine with a tamping device that has increased performance and economy.

[0025] This problem is solved by a track construction machine with the features of claim 14. The advantages of the track construction machine according to the invention correspond to the advantages of the tamping device according to the invention. The track construction machine can be further developed, in particular, with the features of at least one of claims 1 to 13. Brief description of the drawings

[0026] Further features, advantages, and details of the invention will become apparent from the following description of an exemplary embodiment. The illustrations show: Fig. 1 a schematic representation of a rail-guided track construction machine with a tamping device for track bed compaction, Fig. 2 a schematic front view of the tamping device in Fig. 1 , wherein the tamping device has a tamping unit with four tamping picks and wherein the tamping picks are in engagement with a track bed, Fig. 3 a schematic side view of the tamping device in Fig. 1 , wherein a driving force, an inertial force and a ballast force act on the tamping unit, Fig. 4 Graphs of the driving force, the inertial force and the ballast force over time and for a single tamping cycle, Fig. 5 Graph of the ballast force over time for six tamping cycles, Fig. 6 Graph of recorded load amplitudes of the ballast force over a number of load cycles and Fig. 7 Graphs of a position of the tamping unit, the ballast force and a ballast work over time. Description of the embodiments

[0027] A track construction machine 1 comprises a machine frame 2, at least two axles 3 mounted on the machine frame 2, a machine drive 4, and a tamping device 5 for track bed compaction. The axles 3 are arranged at different angles to each other along a horizontal x-direction on the track construction machine 1. The x-direction, together with a vertical z-direction and a horizontal y-direction, forms a machine-fixed coordinate system. Rail-guided wheels 6 are rotatably mounted on the axles 3. The machine drive 2 is designed to rotate the wheels 6 of at least one of the axles 3.

[0028] The tamping device 5 has an aggregate carrier 7 and a tamping unit 8 mounted in the z-direction relative to it. The tamping unit 8 comprises four tamping picks 8a and a compaction drive 8b. The tamping picks 8a are each attached to a tamping pick carrier 8c and rotatably mounted about a carrier axis 8d via this carrier. The tamping pick carriers 8c can be rotated about the respective carrier axis 8d by means of the compaction drive 8b.

[0029] The tamping device 5 is attached to the machine frame 2 via the aggregate carrier 7. The tamping unit 8 is movable relative to the aggregate carrier 7. For this purpose, a linear bearing 10 is provided between the aggregate carrier 7 and the tamping unit 8. The linear bearing 10 has bearing rails 11 attached to the aggregate carrier 7 and bearing sleeves 12 connected to the tamping unit 8.

[0030] The tamping device 5 further comprises a drive unit 9. The drive unit 9 includes a hydraulic cylinder 13. The hydraulic cylinder 13 acts between the unit carrier 7 and the tamping unit 8. A hydraulic piston 14 with a piston rod 15 attached to it is linearly displaceable within the hydraulic cylinder 13. The hydraulic piston 14 has a piston ring surface A KR facing the piston rod 15 and a piston surface AK facing away from the piston rod 15. A piston pressure p K of a hydraulic fluid located in the hydraulic cylinder 13 acts on the piston surface AK. A piston ring pressure p KR of the hydraulic fluid acts on the piston ring surface A KR. The piston pressure p K acting on the piston surface AK and the piston ring pressure p KR acting on the piston ring surface A KR result in a total drive force FA transmitted via the piston rod 15 to the tamping unit 8.

[0031] The stuffing device 5 has a drive force sensor system for detecting a first measured quantity pK, pKR, FA corresponding to the drive force FA. The drive force sensor system comprises a piston pressure sensor 16 for detecting the piston pressure pK and a piston ring pressure sensor 17 for detecting the piston ring pressure pKR. From the piston pressure pK acting on the piston surface AK and from the piston ring pressure pKR acting on the piston ring surface AKR, the total drive force FA acting on the stuffing unit 8 via the piston rod 15 can be determined. The drive force FA is calculated as follows: F A = p KR ⋅ A KR − p K ⋅ A K

[0032] The tamping device 5 has an acceleration sensor for detecting a second measured quantity corresponding to the acceleration az of the tamping unit 8, namely the position z and / or the velocity vz. The acceleration sensor is designed in the form of a displacement sensor 18. The displacement sensor 18 is mounted on the unit carrier 7 and on the tamping unit 8. The displacement sensor 18 is designed to detect the position z and the velocity vz of the tamping unit 8 relative to the unit carrier 7 in the z-direction.

[0033] To determine the ballast force FS acting on the tamping unit 8, the tamping device 5 includes an evaluation unit 19. The evaluation unit 19 is in signal communication with the piston pressure sensor 16, the piston ring pressure sensor 17, and the displacement sensor 18. The evaluation unit 19 is also in signal communication with a pressure regulator 20. The pressure regulator 20 is designed to control the piston pressure pK and the piston ring pressure pKR to specific setpoint values. The respective setpoint values ​​for the piston pressure pK and the piston ring pressure pKR can be preset by the evaluation unit 19.

[0034] The operation of track construction machine 1 and the operation of tamping unit 8 are described below:

[0035] To construct and / or maintain a track bed 21, the track construction machine 1 is moved along a track 22 in the x-direction by means of the machine drive 4. A central axis 23 of the tamping device 5 is positioned centrally above a railway sleeper 24 arranged on the track bed 21 and supporting the tracks 22.

[0036] At the start of the track bed compaction process, the tamping unit 8 is in a reset position 25. The bearing sleeve 12 is located at one end of the linear bearing 10, and the piston rod 15 is largely immersed in the hydraulic piston 14. The tamping picks 8a attached to the tamping unit 8 are not engaged with the track bed 21. The piston area AK is subjected to the piston pressure p K, and the piston ring area A KR is subjected to the piston ring pressure p KR. The drive force FA acting on the tamping unit 8 from the hydraulic piston 14 is determined by the evaluation unit 19. For this purpose, the actuator 9 is actuated. This increases the piston pressure p K and decreases the piston ring pressure p KR. The drive force FA acting on the tamping unit 8 via the piston rod 15 is increased in the opposite direction to the z-axis.The driving force FA results in the acceleration az acting on the tamping unit 8, which is oriented opposite to the z-direction and leads to an increasing velocity vz of the tamping unit 8 in the direction of the track bed 21. The tamping unit 8 is displaced opposite to the z-direction. Opposing the driving force FA is the inertial force FT, which is of equal magnitude. The ballast force FS is zero before the tamping picks 8a make contact with the track bed 21.

[0037] In the delivery position 26, the tamping picks 8a engage with the track bed 21. Between the delivery position 26 and the engagement position 27, the partial ballast forces FS1, FS2, FS3, and FS4 additionally act on the tamping unit 8 in the z-direction via the four tamping picks 8a. The partial ballast forces FS1, FS2, FS3, and FS4 add up to the ballast force FS. Due to the displacement between the delivery position and the engagement position 27, the ballast force FS is not zero.

[0038] The curves of the driving force FA, the inertial force FT and the ballast force FS are shown in Fig. 4 The movement of the tamping unit 8 over time t for the duration of a tamping cycle is shown in detail. The movement of the tamping unit 8 between the return position 25 and the engagement position 27 takes place in the approach phase 28. The return phase 29 follows the approach phase 28 at a later time interval.

[0039] During the reset phase 29, the tamping unit 8 is moved from the engagement position 27 via the feed position 26 back to the reset position 25. For this purpose, the drive unit 9 is actuated such that the piston pressure pK is reduced and the piston ring pressure pKR is increased. The hydraulic cylinder 13 thus generates the drive force FA, which is now oriented in the z-direction. The tamping unit 8 is accelerated in the z-direction due to the drive force FA. The acceleration az is oriented in the z-direction and results in an increasing velocity vz in the z-direction and the displacement of the tamping unit 8 in the z-direction. Between the engagement position 27 and the feed position 26, the ballast force FS acts on the tamping unit 8.Between the delivery position 26 and the return position 25, only the driving force FA and the equally large but oppositely oriented inertial force FT act on the tamping unit 8, whereby the ballast force FS is zero.

[0040] During the tamping cycle, the tamping picks 8a are set into vibration by actuating the compression drive 8b. For this purpose, the compression drive 8b drives the tamping pick carrier 8c essentially in a horizontal direction, causing the tamping pick carrier 8c and the attached tamping picks 8a to rotate about their respective carrier axis 8d. The movement of the tamping picks 8a about their respective carrier axis 8d essentially comprises two motion components. A vibration component causes only a small rotational amplitude of the tamping picks 8a about their respective carrier axis 8d, with a vibration frequency fS between 35 Hz and 45 Hz. This vibration component acts on the tamping picks 8a throughout the entire tamping cycle. In addition to the vibration component, the tamping picks 8a are subjected to a displacement component. The displacement component has a higher rotational amplitude than the vibration component and a displacement frequency of approximately 0.5 Hz.In the engagement position 27, the tamping picks 8a are rotated about their respective support axis 8d such that the tamping picks 8a, which are spaced apart from each other in the x-direction, move towards each other. In the return position 25, the displacement component is oriented such that the tamping picks 8a move away from each other again. The application of the displacement component to the tamping picks 8a follows in the displacement phase 30. The compaction of the track bed 21 is achieved through the superimposed application of the vibration component and the displacement component to the tamping picks 8a.

[0041] The tamping cycle is complete as soon as the tamping unit 8 returns to its return position 25. To further compact the track bed 21, the track construction machine 1 is moved in the x-direction until the central axis 23 is positioned directly above the nearest sleeper 24 in the x-direction. The tamping cycle is then repeated at this sleeper. The profile of the ballast force FS over time t is shown for six consecutive tamping cycles in Fig. 5 depicted.

[0042] The evaluation unit 19 determines the stress on the tamping device 5 based on the temporal profile of the ballast force FS. The stress is determined using ballast force amplitudes SFs. The ballast force FS is a time-varying, oscillating load. The ballast force amplitude SFs is determined as the difference between a maximum ballast force FS and a minimum ballast force FS within a single oscillation. In addition to the ballast force amplitudes SFs, the cumulative frequency NFs of each ballast force amplitude SFs is determined. To determine the stress, a load collective is defined based on the cumulative frequency NFs.

[0043] In Fig. 6 The graph shows the ballast force amplitude SFs versus the cumulative frequency NFs. By comparing the ballast force amplitude SFs versus the cumulative frequency NFs with a maximum permissible cumulative frequency NFs of ballast force amplitude SFs, the wear condition of the tamping unit 8 is determined. The wear condition is determined both for individual parts of the tamping unit 8, such as the tamping picks 8a, the drive unit 9, and the linear bearings 10, and for the entire tamping unit 8.

[0044] Depending on the load, at least one process parameter pK, pKR, fS is set for the operation of the tamping unit 8 by means of the evaluation unit 19. For this purpose, the evaluation unit 19 is in signal communication with the compaction drive 8b for controlling the vibration frequency fS and with the pressure regulator 20 for controlling the piston pressure pK and the piston ring pressure pKR. If a load threshold SW is exceeded, the at least one process parameter pK, pKR, fS is changed. For this purpose, the ballast force Fs is compared with the threshold SW by means of the evaluation unit 19, whereby the at least one process parameter pK, pKR, fS is changed such that the ballast force FS is reduced when an upper threshold SW1 is exceeded, and such that the ballast force FS is increased when a lower threshold SW2 is not reached.The ballast force FS is reduced by increasing the oscillation frequency f S and by reducing the pressure difference between the piston pressure p K and the piston ring pressure p KR, and increased in the opposite way. The process parameters p K , p KR , f S are modified by the evaluation unit 19 such that an optimum is achieved between low stress on the tamping device 5 and a high processing speed of the track bed 21.

[0045] As an alternative to determining the load spectrum, the ballast work WS can be determined using the evaluation unit 19 to ascertain the stress. The ballast work WS is determined from the ballast force FS and a change in the position z of the tamping unit 8. The ballast work WS corresponds to the work introduced into the track bed 21 via the tamping picks 8a. The change in position z is recorded over a discrete time period. This change in position z is then multiplied by the ballast force FS. The ballast work WS is determined as the sum of the products of the ballast force FS and the changes in position z.

[0046] In Fig. 7The graphs of position z, ballast force FS, and ballast work WS for one tamping cycle are plotted against time t. The ballast work WS can also be understood as the area under the curve of the ballast force FS against position z. By determining the ballast force FS acting on the tamping unit 8 using the evaluation unit 19, conclusions can be drawn about the stress on the tamping unit 8. Determining the ballast force FS taking into account the drive force FA and additionally the acceleration az is significantly more accurate compared to considering only the drive force FA to determine the ballast force FS. The stress on the tamping unit 8 can thus be reliably determined, and the wear condition of the tamping unit 8 can be reliably ascertained.Adjusting at least one process parameter pK, pKR, fS depending on the load enables the efficient and economical operation of the track construction machine. This results, particularly through optimization, in high processing speed, low energy consumption, and reduced stress on the tamping unit 8.

Claims

1. A method of operating a tamping unit of a track maintenance machine, comprising the steps of: - providing on a track bed (21) a track maintenance machine (1) having a tamping unit (8), - displacing the tamping unit (8) relative to the track bed (21), - determining a driving force (FA) acting on the tamping unit (8) and required for the displacement, - determining an acceleration (az) acting on the tamping unit (8), - determining a ballast force (FS), acting between the tamping unit (8) and the track bed (21), by way of the driving force (FA) and the acceleration (az), wherein the evaluating of the ballast force (FS) takes place in such a way that a strain acting on the tamping unit (8) is determined by way of the ballast force (FS).

2. A method according to claim 1, characterized in that the acceleration (az) is determined by measuring a temporal change of a position (z) of the tamping unit (8).

3. A method according to claim 1 or 2, characterized in that, for determining the ballast force (FS), an inertial force (FT) acting on the tamping unit (8) is determined by means of the acceleration (az).

4. A method according to one of claims 1 to 3, characterized in that the displacement of the tamping unit (8) occurs by means of a fluidically operated drive device (9), wherein at least one fluid pressure (pK, pKR) acting on the drive device (9) is measured for determining the driving force (FA).

5. A method according to one claims 1 to 4, characterized in that the strain is determined by way of a temporal progression of the ballast force (FS).

6. A method according to one of claims 1 to 5, characterized in that the strain is determined by means of ballast force amplitudes (SFs ) of the ballast force (FS).

7. A method according to claim 6, characterized in that, for determining the strain, a load spectrum is determined by way of a cumulative frequency (NFs ) of the ballast force amplitudes (SFs ).

8. A method according to one of claims 1 to 7, characterized in that, for determining the strain, a ballast work (WS) is determined from the ballast force (FS) and a change of a position (z) of the tamping unit (8).

9. A method according to one of claims 1 to 8, characterized in that a wear condition of the tamping unit (8) is determined by way of the strain.

10. A method according to one of claims 1 to 9, characterized in that at least one process parameter (fS, vz, az, pK, pKR) for controlling the tamping unit (8) is set in dependence on the strain.

11. A method according to claim 10, characterized in that the at least one process parameter (fS, vz, az, pK, pKR) is changed when a threshold value of the strain is exceeded or fallen below.

12. A method according to claim 10 or 11, characterized in that the at least one process parameter (fS, vz, az, pK, pKR) is set in such a way that the strain does not exceed a strain limit value.

13. A tamping device for track bed consolidation, comprising: - a unit carrier (7), - a tamping unit (8) supported on the unit carrier (7), - a drive device (9) for providing a driving force (FA) and for displacing the tamping unit (8) relative to the unit carrier (7), - a driving force sensor system for detecting a first measuring value (pK, pKR, FA) corresponding to the driving force (FA), - an acceleration sensor system for detecting a second measuring value (z, vz, az) corresponding to an acceleration (az) of the tamping unit (8), and - an evaluation unit (19) for determining a ballast force (FS), acting on the tamping unit (8) by means of the first measuring value (pK, pKR, FA) and the second measuring value (z, vz, az), characterized in that, the evaluation unit (19) is designed for determining a strain, acting on the tamping unit (8) by way of the ballast force (FS).

14. A track maintenance machine comprising - a machine frame (2), - at least two axles (3) supported on the machine frame (2), including rail-guidable wheels (6) arranged thereon, - a machine drive (4) for rotary actuation of the wheels (6) of at least one of the axles (3), and - at least one tamping device (5) according to claim 13, fastened to the machine frame (2).