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BACKGROUND 
   The invention relates to a drive system for chain sprockets of chain drives, preferably for driving chain scraper conveyors or chain-drawn ploughs for underground mining, with a drive assembly formed of an asynchronous motor and a gear mechanism, the gear mechanism being designed as an overload and load equalisation gear unit having a controllable multiple-disk clutch for overload equalisation with which multiple-disk clutch the force flow between the asynchronous motor and the chain sprocket can be disconnected. 
   A drive system of this type is known from DE 40 24 830 A1 or U.S. Pat. No. 5,551,902 for example. In the known chain drive, unregulated three phase a.c. asynchronous motors are used. With the downstream arranged overload and load compensation gearing, during operation of the drive system load compensation between the main drive and the auxiliary drive of the coal plough or chain scraper conveyor is brought about in order to optimise the operation of the coal plough or chain scraper conveyor and to prevent unfavourable loading conditions for the chain. In the case of the drive system in DE 40 24 830 A1 an auxiliary motor is assigned to the overload and load sharing gearing which can be switched on and off when the asynchronous motor is at a standstill in order to tension the chain. At the same time, in order to tension the chain in the drive connection between the asynchronous motor and the overload and load compensation gear mechanism, a blocking device which blocks the asynchronous motor from turning is provided. 
   In place of unregulated three phase a.c. asynchronous motors (rotary current asynchronous motors), pole reversing asynchronous motors (DE 37 41 762 A1) and computer-controlled three phase a.c. asynchronous motors (U.S. Pat. No. 6,008,605) have been proposed, in which for each motor the individual torque-revolution characteristic curve is stored in an assigned computer. By means of a revolution counter, when the motors are in operation the current revolutions are permanently determined in a potential-separated manner in order to regulate the motor by way of comparing the current revolutions with the individual revolutions according to the characteristic curve. Due to the proportionality of revolutions and torque, with appropriate rotary current asynchronous electric motors the given torque can be adjusted. 
   In underground mining, efforts are being made to use three phase a.c. motors with frequency converters, known as frequency converter motors, as electric drives. With frequency converter motors constant adjustment of the revolutions is possible. The rough underground ambient conditions, with dust, moisture and corrosion, as well as the statically determined coupling of the frequency converter motor with the overload gearing cause problems for the use of frequency converter motors which have hitherto prevented the broad possibilities of using frequency converter motors. However, one use of frequency converter motors is known in which a rotary elastic and breakthrough-proof claw coupling is arranged between the overload coupling and the frequency converter motor. The intermediate claw coupling considerably increases the space required for the drives in the underground working faces so that correspondingly assembled drive system can only be used in an underground working face if there is sufficient space available. Furthermore, when using frequency converters on chain drives there are still considerable difficulties in achieving the breakaway effect required for starting a loaded chain scraper conveyor or releasing a plough jammed in the working face. 
   SUMMARY 
   The aim of the invention is to create a drive system for chain sprockets of chain drives in which revolution and torque control of the asynchronous motors is possible and in which the required breakaway effect can be exerted on the chain sprocket and thereby the chain. 
   In accordance with the invention this task is achieved in that the asynchronous motor comprises a frequency converter motor and in the drive assembly a two-gear toothed wheel gear mechanism with a start gear and normal gear is placed between the motor shaft and the gear mechanism. By way of the intermediate two-gear toothed wheel gear mechanism all the advantages of a frequency converter can be exploited in underground drive system and at the same time, through changing from the normal gear to the starting gear position, it is possible to increase the torque for the breakaway effect in an overproportional manner. A further advantage of the toothed wheel gear mechanism with the two gear positions is that chain tensioning systems, as otherwise provided by auxiliary motors or other devices in the state of the art, can be dispensed with. 
   In a preferred embodiment the toothed wheel gearing is designed as a forward gear mechanism so that it is arranged between the frequency converter motor and the overload gear mechanism. In a particularly preferred embodiment the toothed wheel gear mechanism has a returning transmission gear mechanism with a gear transmission of 1:3 to 1:4 as the starting gear, so that the drive torque delivered in the starting gear is approximately two to four times the nominal torque. 
   The toothed wheel or forward gear mechanism preferably also has a drive side central wheel borne in a rotating manner on the drive shaft and a output side central wheel borne in a rotating manner on the drive shaft, between which a control gear is connected in a torsion-stable manner to the drive shaft, whereby the control wheel can be coupled by means of a gearing system either to the drive-side or output-side central wheel. In order to simplify controlling the gear mechanism, both central wheels and the control wheel have an aligned and adjacent toothed section with identical toothing. The gear system can then comprise, in particular, a control hub which can be slid over the toothed sections of two adjacent toothed wheels of the forward gear mechanism in order to lock the adjacent wheels together so that they rotate together. In a preferred embodiment the control hub is moved by means of a gear fork, which can be displaced parallel to the gear shaft axis by means of a gear rod. In particular, the toothed wheel gear mechanism can be designed so that operation of the control system and therefore changing the gear position can also take place under load. 
   The gear shaft may be borne on the gear mechanism casing on the output side and may be supported on the motor drive side by the drive side toothed wheel, which is borne on the gear mechanism casing in a rotating manner. Particularly preferred is a transmission gearing for the start gear position including a secondary shaft borne in a rotating manner on the gear casing, which has a first gear toothing engaging in the drive side central wheel and second gear toothing engaging in the output central wheel. At least the first gear toothing can be part of a single wheel connected in a torsion-stable manner to the secondary shaft in order to facilitate assembly of the toothed wheel gear mechanism and the secondary shaft. The toothed wheel or forward gear mechanism is expediently designed as a spur gear. It is particularly advantageous if the drive-side central wheel is designed as a bushing and/or is provided on the inside with a hub connection for the motor shaft. By way of this embodiment a radial displacement of the motor shaft of the frequency converter motor can be easily intercepted. The frequency converter motor and the toothed wheel gear mechanism can also be arranged in a common casing, whereby the central wheel on the motor drive side is arranged on the motor shaft or is part of the motor shaft, so that an additional bearing can be saved if necessary. 
   It is desirable to minimise the maximum space required for underground chain drives and to improve a statically determined connection of the frequency converter motor to the toothed wheel gear mechanism. Therefore, in the case of a frequency converter motor which is provided with a stator with stator windings, a rotor and a frequency converter switch, and a motor shaft, the motor shaft is borne in a rotating manner on the motor flange side and rear of the motor casing, and which motor shaft is preferably designed as a hollow shaft, the axial boring of which is penetrated by a torsion rod which is coupled in a moving manner only at the rear end of the hollow shaft. 
   In a preferred embodiment, in the frequency converter motor according to the invention, the torque rod passes trough the axial boring and/or the motor case in a contact-free manner on the motor flange side, i.e. there is no bearing provided for the torsion rod on the motor flange side and no support on the hollow shaft. By way of this measure, and without an intermediate torsionally elastic coupling, statically determined coupling can be achieved between the motor and the downstream drive system, even when due to manufacturing inaccuracies or assembly imprecision there is no exact alignment of the motor shaft with the input shaft of the downstream drive system. For the drive coupling between the motor and the downstream drive system it is particularly advantageous to be able to push the torsion rod into the hollow shaft from its rear end and through the hollow shaft. 
   Expediently the motor flange side end of the torsion rod is provided with a pinion gear, toothing or a shaft connection, the external diameter of which is smaller than the minimum internal diameter of the axial boring in order to allow the torsion rod to be passed through from the rear end of the hollow shaft, which is always accessible even when the motor is assembled. The rear end of the torsion rod can expediently be provided with a spur gear, a hub connection or hub connection toothing, the external diameter of which is greater than the motor flange side of the torsion rod and the internal diameter of the axial boring. 
   As additional safety for the motor used in underground mining, it is recommended to create a breakage point on the rear end of the torsion rod and to arrange fastening means for assembly/dismantling aids for the torsion rod between the breakage point and the motor flange side of the hollow shaft, so that even after a breakage of the torsion rod at the breakage point dismantling of all the parts of the torsion rod can be carried out without the motor having to be loosened on the motor flange side of the downstream drive system. The fastening means can, in particular, include an axial threaded boring in the face side of the torsion rod. 
   In frequency converter motors it is particularly preferable if the frequency converter control is integrated into the frequency converter motor, more particularly arranged in a control box integrated into the motor casing. In a preferred embodiment the torsion rod projects from the motor casing with its motor flange side end or the motor flange side pinion gear. The motor flange side end of the torsion rod can be coupled with the input shaft of the toothed wheel or forward gear mechanism in a statically determined manner. By coupling the motor to the two-gear toothed gear mechanism, the torque required for the breakaway effect can be easily attained without significantly increasing the required assembly space for the chain drive by switching the toothed wheel or forward gear mechanism into the starting gear position. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     Further advantages and embodiments of the invention are set out in the following description of an example of embodiment set out schematically in the drawings. In the drawings: 
       FIG. 1  shows a simplified, schematic view of an underground extraction device with two chain drives with frequency converter motors 
       FIG. 2  shows a cross-section of a frequency converter motor coupled to a forward gear mechanism, and 
       FIG. 3  shows in a diagram the motor torque attainable with the frequency converter motor and the forward gear mechanism. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows two chain drives, designated  50 , for driving an endless chain  2  running around both sprockets  1  of the chain drives  50 . In the case of a face or drift conveyor designed as a chain scraper conveyer, the chain  2  is a scraper chain band and in the case of a plough system, the chain is a plough chain which moves the coal plough, which is not shown, along the working face. The chain sprockets  1  turn the chain  2  around and are each driven with drive units with which they connected via the chain wheel shaft  3  in a torsion-stable manner. The drive units of both chain drives  50  comprise an electric rotary current asynchronous motor, designed as a frequency converter motor  10  with an integrated control box  11  for controlling the frequency converter and connected to an overload protection and load equalisation gear mechanism  4  with an intermediately arranged forward gear mechanism  30 . The overload protection and load equalisation gear mechanism  4  is, more particularly, designed as a planetary gear mechanism with two planet positions, whereby a hydraulically operated disk coupling is assigned to the hollow wheel of one of the planet gears in order to achieve load-free starting of all motors  10 , to be able to effect load equalisation between the two drive units  50 , and, in the case of blockages of the chain  2  to release the drive connection between the motors  10  and the chain sprockets  1 . The assembly and corresponding functioning of the overload and load equalisation gear mechanism is known, for example, from DE 43 16 798 A1. 
     FIG. 2  shows a longitudinal section through the frequency converter motor  10  and the forward gear mechanism  30 . The frequency converter motor  10  has a motor casing  12  with an integrated control box ( 11 ,  FIG. 1 ) for controlling the frequency converter. Inside the motor casing  12  is a stator  13  with stator windings  14 , whereby arranged at a distance of an air gap inside the stator  13  is the rotor  15  of the frequency converter motor  10  with which motor shaft designed as a hollow shaft  16  is connected in a torsion-stable manner. The fundamental assembly of a rotary current asynchronous motor designed as a frequency converter motor is known to a person skilled in the art, so that a more detailed description of the electrical method of operation of the frequency converter motor  10  is not given here. The hollow shaft  16  is borne both at the rear end  17  and the motor flange end  18  via bearings  19 ,  20  in a rotating manner on the rear bearing plate  21  and the motor flange plate  22  respectively and is provided with an axial boring  23 , in which a torsion rod  24  is arranged, which passes completely through the axial boring  23  and projects at the motor flange side end  18  of the hollow shaft  16  with a drive pinion  25  from the axial boring  23  and the motor casing  12 . On the rear end of the torsion rod  24  there is a further pinion  26  which is provided with appropriate toothing and engages in a torsion-stable manner in counter-toothing  27  on the internal circumference of the rear end  17  of the hollow shaft. Between the counter toothing  27  and the pinion  26  of the torsion rod  24  there can be transitional play in order to facilitate the assembly of the torsion rod  24  through an opening which can be closed with a closing lid  28  in the rear bearing plate  21  of the motor casing  23 . The external diameter of the pinion  25  is preferably slightly smaller and the outer diameter of the pinion  26  is preferably slightly larger than the internal diameter D i  of the axial boring  23 . In the area of the pinion  26  a nominal breakage point  29  is formed on the torsion rod  24  by way of a shearing groove, whereby a threaded boring  8  arranged on axis A of the torsion rod  24  extends beyond the nominal breakage point  29  in the direction of the gear side pinion  25  of the torsion rod  24  so that even in the event of breakage of the torsion rod  24  in the area of the nominal breakage point  29  a dismantling tool (not illustrated) can be screwed into the threaded boring  8  and the torsion rod  24  pulled out of the axial boring  23 . As the torsion rod  24  is only coupled to the hollow shaft  16  at its rear end  17  and supported relative to the hollow shaft  16 , alignment errors between the casing  12  of the frequency converter motor  10  and, respectively, axis A of the hollow shaft  16  and the casing  31  of the forward gear mechanism  30  can be compensated for. There is therefore no necessity to arrange a coupling, such as a torsion elastic claw coupling, between the frequency converter motor  10  and the forward gear mechanism  30 . 
   In the shown example of embodiment the forward gear mechanism  30  is designed as a two-gear toothed wheel gear mechanism whereby switching between a starting gear position and a normal gear position is carried out by way of a control ring or a control hub  32 . In the lateral view in  FIG. 2  the control hub  32  is shown in the lower half in the starting gear position and in the upper half in the normal gear position as will be explained. 
   The two-gear toothed wheel gear mechanism  33  of the forward gear mechanism  30  designed as a returning transmission gear mechanism has a gear shaft  34  which on the output side is borne in the output side bearing plate  36  by means of bearing  35 . The gear shaft  34  extends on the frequency converter motor  10  to close to the pinion  25  of the torsion rod  24  with a gap remaining between the pinion  25  and the gear shaft  34 . On the motor side end of the gear shaft  34  a drive-side central wheel  37  of the toothed wheel gear mechanism  33  is borne, which in this case is designed as a bushing, and the section of the central wheel  37  provided with gear toothing  39  is borne in a rotating manner on the free end of the gear shaft  34  by way of bearing  40 . The central wheel  37  tapers vis-à-vis the section with the spur gear toothing  39  to a connection section  41 , which has toothing  42  on its inner circumference and is, or can be, connected as a hub in a torsion-stable manner to the pinion  25  of the torsion rod  24 . On its outer circumference the connection section  41  has a cylindrical band  43  which is borne by way of bearing  44  on the motor-side bearing plate  45 , which is an integral part of the gear casing  31 . Axially displaced vis-à-vis the gear shaft  34  there is a secondary shaft  46  borne in a rotating manner on both bearing plates, whereby the secondary shaft  46  has a cam  47  with gear toothing  48  as well as a shaft section on which, for example, a single wheel  49  with gear toothing  51  is borne in a torsion-stable manner by means of a feather key connection. As further components the toothed wheel gear mechanism  33  has a control wheel  52  with spur gear toothing  53  connected in a torsion-stable manner to the gear shaft, as well as an output side central wheel  54  with spur gear toothing  55 , which is supported by means of bearing  56  in a freely rotating manner on the gear shaft  34 . 
   In the normal gear position in which the control hub  32 , which is movable over the control shaft  57  and the control fork  58  firmly connected thereto, connects a section of the gear toothing  53  of the control wheel  52  with a section of the gear toothing  39  of the drive side central wheel  57 , the control wheel  52  and therefore the gear shaft  34  rotates at the same speed as the torsion rod  24  connected to the central wheel  37 . This position of the control hub  32  therefore corresponds to the normal gear position of the gear box  33  with a gear transmission of 1:1. 
   The toothed wheel transmission gearing brought about by the engaging spur wheel and gear toothing  39 ,  51 ,  48  and  55  of the toothed wheels and pinions  37 ,  49 ,  47  and  54  respectively has a slow transmission ratio of 1:4 in the starting gear position in the shown embodiment. The starting gear position is only active if the control hub  32 , as shown in the lower half of  FIG. 2 , is in the left position in contact with the flank  59  of the output side central wheel  54 . In this position the control hub  32  simultaneously covers a section of the gear toothing  53  of the control wheel  52  and a toothed section  60  on the output side central wheel  54  which is formed on a collar  61  of the central wheel  54  projecting in the direction of the electric motor  10 . The position of the control hub  32  and the coupling of the toothed sections  60  and  53  causes the speed of the central wheel  54  to be transmitted to the control wheel  52  and therefore to the gear shaft  34 . In contrast to this, as has already been set out above, in the normal gear position the control hub  32  engages with the gear toothing  39  of the drive side central wheel  37  and the toothing  53  of the control wheel  52  in such a way that the gear shaft rotates at the same speed as the torsion rod  24  of the frequency converter motor  10 . 
   The starting gear position brought about by the two-gear toothed wheel gear mechanism is only initiated if a breakaway effect is to be achieved with the frequency converter motor  10  and the chain drive  50  in order to start the loaded chain scraper conveyor or to release the plough. As shown schematically in the diagram in  FIG. 3 , in the starting gear position the motor torque M d  brought about on the output side by the combination of frequency converter motor  10  and toothed wheel gear mechanism  33  and/or forward gear mechanism  20  increases to the breakaway torque Md A  which in this case is around four times the nominal motor torque Md N . The starting gear position can only initiated at low revolutions or low chain speeds V k . 
   This invention is not limited to the illustrated example of embodiment. The use of a frequency converter motor with a hollow shaft and torsion rod forms the preferred embodiment of the invention. The toothed wheel transmission gear mechanism and the frequency converter motor can also be arranged in a common casing, whereby the drive-side central wheel then coincides with the motor shaft so that one of the two bearings  20 ,  44  can be dispensed with. In this embodiment it is obviously also unnecessary to form the motor shaft as a hollow shaft with a torsion rod.

Summary:
A drive system for chain sprockets  1  of chain drives, more particularly for driving chain scraper conveyors or chain-drawn ploughs for underground mining, with a drive assembly formed of an asynchronous motor and a gear mechanism  4 , the gear mechanism  4  being designed as overload gearing and having a controllable multiple-disk clutch for overload equalisation with which the force flow between the asynchronous motor and the chain sprocket can be disconnected. The asynchronous motor comprises a frequency converter motor  10  and in the drive assembly between the motor shaft and the gear mechanism  4  a two-gear toothed wheel gear mechanism is arranged as a forward gear mechanism, with a starting gear position and a normal gear position, with which in the starting gear position the breakaway effect required for starting the loaded chain scraper conveyor or for releasing the plough can be attained.