Abstract:
A roller bearing comprising an inner and outer raceway and a row of rollers arranged between and contacting the raceways, the axes of the rollers being substantially parallel to the axes of the raceways when the latter are coinciding, the rollers and the raceways having curved longitudinal section profiles in which the curve radii are substantially greater than the greatest distance between the central axis of the bearing and the surfaces of the raceways and wherein the rollers are axially movable between the raceways without being obstructed by axial limitations at the raceways or by limited axial cage play for permitting relative inclination and axial displaceability being limited only by the rollers being squeezed radially between the raceways.

Description:
This is a continuation-in-part of application bearing Ser. No. 780,428 filed Sept. 26, 1985 entitled RADIAL ROLLING BEARING, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to rolling bearings and particus the left-hand loudspeaker 3. The L signal is fed through an L-channel circuit to drive the center loudspeaker 4. The L-channel circuit includes an attenuating circuit and a main amplifier 16c connected in series with each other. The attenuation circuit attenuates the L signal at a small attenuation factor. The attenuating circuit may be taken in the form of a voltage divider connected between the L-channel output of the pre-amplifier 12 and electrical ground. The voltage divider has resistors 15a and 15b. The junction of the resistors 15a and 15b are connected to the main amplifier 16c which drives the center loudspeaker 4. 
     Each of the balance controllers 13 and 14 can be operated, independently of the other, to vary the volume of sound produced from the corresponding loudspeaker. Assuming now that the passenger M1 shifts his vehicle seat forward or rearward, he moves to a position with respect to which the right-hand and center loudspeakers 2 and 4 are located at different distances, degrading the listening quality of stereophonic sound. To achieve high stereophonic sound listening quality, the passenger M1 may operate the balance controller 13 in a manner to re-establish proper tone balance. This operation has no effect on the tone balance for the other passenger M1. 
     FIG. 2 shows a modified form of the stereophonic signal generator circuit which employs a single main amplifiers 16d in order to reduce the number of main amplifiers in the acoustic driver circuit and reduce the cost thereof. The main amplifier 16d receives L and R signals from the pre-amplifier 12. The main amplifier 16d has an R-channel output coupled to the right-hand loudspeaker 2 through a balance controller 17 and also to the left-hand loudspeaker 3 through a balance controller 18. The L channel output of the main amplifier 16d is coupled through an attenuating circuit to the center loudspeaker 4. 
     FIG. 3 shows another modified form of the stereophonic signal generator circuit which is substantially similar to that of FIG. 1 except for the arrangement of the balance control circuit. In this modification, the balance control circuit includes first and second balance controllers 20 and 25 for the passengers M1 and M2 seated on the right- and left-hand vehicle seats, respectively. 
     The first balance controller 20 includes drivingly associated variable resistors 21, 22 and 23. The variable resistor 21 is connected between the R-channel output of the pre-amplifier 12 and electrical ground. The variable resistor 22 is connected between the L-channel output of the pre-amplifier 12 and electrical ground. The variable resistor 23 is connected between the R-channel output of the pre-amplifier 12 and electrical ground. similarly, the second balance controller 25 includes drivingly associated variable resistors 26, 27 and 28. The variable resistor 26 is connected between the movable contact of the variable resistor 21 and electrical ground. The movable contact of the variable resistor 26 is connected through the main amplifier 16a to the right-hand loudspeaker 2. The variable resistor 27 is connected between the movable contact of the variable resistor 22 and electrical ground. The movable contact of the variable resistor 27 is connected through the main amplifier 16c to the center loudspeaker 4. The variable resistor 28 is connected between the movable contact of the variable resistor 23 and electrical ground. The movable contact of the variable resistor 28 is connected through the main amplifier 16b to the left-hand loudspeaker 3. 
     The variable resistors 21, 22 and 23 of the first balance controller 20 are drivingly associated in such a fashion that when the first balance controller 20 is operated in a direction increasing the volume of sound produced from the right-hand loudspeaker 3, the volumes of sound produced from the left-hand and center loudspeaker 3 and 4 decrease at the same rate, and vice versa when the first balance controller 20 is operated in the opposite direction. Similarly, the variable resistors 26, 27 and 28 of the second balance controller 25 are drivingly associated in such a fashion that when the second balance controller 25 is operated in a direction increasing the volume of sound produced from the left-hand loudspeaker 3, the volumes of sound produced from the right-hand and center loudspeakers 2 and 4 decrease at the same rate, and vice versa when the second balance controller 25 is operated in the opposite direction. 
     Assuming now that the passenger M1 shifts his vehicle seat forward or rearward, he moves to a position with respect to which the right-hand and center loudspeakers 2 and 4 are located at different distances, degrading the listening quality of stereophonic sound. In order to achieve high stereophonic sound listening quality, the passenger M1 may operate the first balance controller 20 in a manner to re-establish proper tone balance. This operation has no effect on the tone balance for the other passenger M2 although it causes a change of the volumes of sound produced from the left-hand and center loudspeakers 3 and 4 to the same level. A similar operation is made when the passenger M2 operates the second balance controller 25. 
     It is to be noted that the R signal may be coupled to drive the center loudspeaker 4 in which case the L signal is coupled to drive the right- and left-hand loudspeakers 2 and 3. In addition, the stereo signal reproducing system of this embodiment may be used for listeners seating on the rear seat. This embodiment permits a listener to create a stereophonic sound image in front of the listener independently of the sound image localization for the other listener by adjusting the volume of sound produced from the corresponding loudspeaker after he shifts his seat. 
     Referring to FIGS. 4 and 5, there is illustrated a second embodiment of the present invention which employs three loudspeakers 5, 6 and 7. In addition to the front loudspeakers 2, 3 and 4, for passengers M3 and M4 seating on the right- and left-hand rear seats. The front loudspeakers 2, 3 and 4 are located on the instrument panel 1 in a manner substantially similar to the first embodiment of FIG. 1. The right-hand loudspeaker 2 has a primary acoustical axis directed to the passenger M1 and the left-hand loudspeaker 3 has a primary acoustical axis directed to the passenger M2. The right-hand and center loudspeakers 2 and 4 are positioned at the same distance (l 1 ) relative to a passenger M1 seated on the right-hand front seat. Similarly, the left-hand and center loudspeakers 3 and 4 are positioned at the same distance (l 1 ) relative to a passenger M2 seated on the left-hand front seat. The right-hand loudspeaker 5 is located relative to the right of the right-hand rear seat. The left-hand loudspeaker 6 is located relative to the left of the left-hand rear seat. The right-hand rear loudspeaker 5 has a primary acoustical axis directed to the passenger M3 and the left-hand rear loudspeaker 7 has a primary acoustical axis directed to the passenger M4. The loudspeakers 5 and 6 may be mounted on the vehicle rear doors, respectively. The center loudspeaker 7 is located on the rear end portion of the center console in such a fashion that the right-hand and center loudspeakers 5 and 7 are positioned at the same distance (l 2 ) relative to a passenger M3 seating on the right-hand rear seat and the left-hand and center loudspeakers 6 and 7 are positioned at the same distance (l 2 ) relative to a passenger M4 seated on the left-hand rear seat. The loudspeakers 2 to 7 are connected to receive stereophonic signals from a stereophonic signal generator circuit for acoustical reproductions of stereophonic sound corresponding to the stereophonic signals. 
     The stereophonic signal generator circuit includes a control amplifier 32 connected to a program source 11 such as a cassette tape player, a stereo tuner, a compact disc player, or the like. The control amplifier 32 generated R and L signals at its R- and L-channel outputs, respectively. The R signal is fed through an R-channel circuit to the right- and left-hand loudspeakers 2, 3, 5 and 6. The L signal is fed through an L-channel circuit to the center loudspeakers 4 and 7. The R-channel circuit includes a first attenuating circuit 33 which is connected directly to power amplifiers 39a and 39b and also through a second attenuating circuit 34 to the power amplifiers 39c and 39d. The power amplifier 39a drives the left-hand front loudspeaker 3, the power amplifier 39b drives the right-hand front loudspeaker 2, the power amplifier 6 drives the left-hand rear loudspeaker 6, and the power amplifier 39d drives the right-hand rear loudspeaker 5. The L-channel circuit connects the L signal directly to a power amplifier 19e and also through a third attenuating circuit 35 to a power amplifier 39f. The power amplifier 39e drives the center front loudspeaker 4, and the power amplifier 39f drives the center rear loudspeaker 7. 
     With respect to the passenger M1 seater on the right-hand front seat, the front loudspeakers 2 and 4 are positioned at the same distance (l 1 ) and the loudspeaker 3 is positioned at a distance (l 3 ) greater than the distance (l 1 ). Consequently, the R sound recreated from the right-hand front loudspeaker 2 reaches the passenger M1 before the R sound recreated from the left-hand front loudspeaker 3. The difference (Δt) between the time at which the sound produced from the left-hand front loudspeaker 2 reaches the passenger M1 and the time at which the sound produced from the left-hand front loudspeaker 3 reaches the passenger M1 is normally several milliseconds (1 to 2 msec) and is given as: 
     
         Δt=(l.sub.3 -l.sub.1)/Vo 
    
     where Vo is the sound velocity. As a result, the passenger M1 will listen the R sound as produced from the right-hand front loudspeaker 2. This is well known in the art as &#34;harness effect&#34;. 
     The R sound generated from the left-hand front loudspeaker 3 is super imposed on the R sound produced from the right-hand front loudspeaker 2 to increase the R sound acoustical pressure at the passenger M1. The volume of sound produced from the center front loudspeaker 4 is greater than the volume of sound produced from any of the other loudspeakers since no attenuating circuit is placed in the line connecting its associated power amplifier 39e to the control amplifier 32. The acoustic pressure of the L sound produced from the center front loudspeaker 4 and the increased acoustic pressure of the R sound are well balanced at the passenger M1 so as to create a stereophonic sound image at a suitable location in front of the passenger M1. As shown in FIGS. 4 and 5, the right-hand rear loudspeaker 5 is located near the passenger M1 at a distance (l 4 ) which is smaller than the distance (l 1 ). It is, therefore, required to avoid the influence of the sound produced from the right-hand rear loudspeaker 5 on the location of the sound image created by the front loudspeakers 2 and 4. This can be achieved by the provision of the second attenuating circuit 34 which restricts the volume of sound produced from the right-hand rear loudspeaker 5 to a small level. Similar considerations are applicable for the passenger M2 seated on the left-hand front seat. 
     With respect to the passenger M3 seated on the right-hand rear seat, the rear loudspeakers 5 and 7 are positioned at the same distance (l 2 ). In addition. the front loudspeakers 2 and 4 are positioned in front of the respective rear loudspeakers 5 and 7. However, the passenger M3 will listen the R sound as produced from the right-hand rear loudspeaker 5 and the L sound as produced from the center rear loudspeaker 7 due to a &#34;harness effect&#34; since the front loudspeakers 2 and 4 are positioned at a greater distance from the passenger M3 than the respective rear loudspeakers 5 and 7. Consequently, a stereophonic sound image is located at a proper position in front of the passenger M3. The R sound generated from the right-hand front loudspeaker 2 is superimposed on the R sound produced from the right-hand rear loudspeaker 5, whereas the L sound produced from the center front loudspeaker 4 is superimposed on the L sound produced from the center rear loudspeaker 7. As a result, the acoustic pressure for the passenger M3 is substantially the same as the acoustic pressure for the passenger M1 in spite of the fact that the volumes of sound produced from the rear loudspeakers 5 and 6 are smaller than the volumes of sound produced from the front loudspeakers 2 and 4 because of the provision of the second and third attenuating circuits 34 and 35, respectively. Similar considerations are applicable for the passenger M4 seated on the left-hand rear seat. 
     Referring to FIG. 6, there is illustrated a modified form of the stereophonic signal generator circuit of the second embodiment. In this modification, the second and third attenuating circuits 34 and 35 are removed and replaced by first and second delay circuits 37 and 38, respectively. The first and second delay circuits 37 and 38 provide the same time delay with respect to their input signals. The power amplifiers 39c and 39d receive an R signal delayed a predetermined time with respect to the R signal applied to the power amplifiers 39a and 39b. Similarly, the power amplifier 39f receives an L signal delayed the predetermined time with respect to the L signal applied to the power amplifier 39e. As a result, the rear loudspeakers 5, 6 and 7 receive drive signals delayed the predetermined time with respect to the drive signals applied to the front loudspeakers 2, 3 and 4, respectively. 
     For the passengers M1 and M2, the stereophonic sound image can be located at a proper location in front of each of the passengers in the same fashion as discussed in connection with FIGS. 4 and 5. The location of the stereophonic sound image is free from the influence of the sound produced from the rear loudspeakers due to a &#34;harness effect&#34; since the first and second delay circuits 37 and 38 delay the arrival of sound from the rear loudspeakers to the passengers M1 and M2 with respect to the arrival of sound from the front loudspeakers to the passengers M1 and M2. 
     With respect to the passenger M3 seater on the right-hand rear seat, the rear loudspeakers 5 and 7 are positioned at the same distance (l 2 ). Since the delay circuits 37 and 38 provide the same delay with respect to the R and L signals to the power amplifiers 39d and 39f, the R sound produced from the right-hand rear loudspeaker 5 and the L sound produced from the center rear loudspeaker 7 will arrive the passenger M3 at the same time. As a result, a stereophonic sound image is positioned at a proper location in front of the passenger M3. Similar considerations are applicable for the passenger M4 seated on the left-hand rear seat. Since the volumes of sound produced from the rear loudspeakers 5, 6 and 7 are substantially the same as the volumes of sound produced from the front loudspeakers 2, 3 and 4, respectively, the passengers M1 to M4 can listen reproduced stereophonic sound under the same condition. 
     Although the attenuating circuits are used in the circuit of FIG. 5 and the delay circuits 37 and 38 are used in the circuit of FIG. 6 for minimizing the influence of sound produced from the rear loudspeakers on the localization of a stereophonic sound image for each of the passengers M1 and M2 seated on the vehicle front seats, it is to be noted that the attenuation circuits 34 and 35 and the delay circuits 37 and 38 may be used in a single circuit in order to insure avoidance of the influence. 
     Referring to FIG. 7, there is illustrated another modification of the second embodiment which can eliminate the need for the attenuating circuits 34 and 35 as used in the circuit of FIG. 5 or for the delay circuits 37 and 38 as used in the circuit of FIG. 6. In this modification, sound shield members 40, 41 and 42 are provided for the respective rear loudspeakers 5, 6 and 7 to minimize sound radiation from the rear loudspeakers to the passengers M1 and M2 seated on the vehicle front seats. This can minimize the influence of sound produced from the rear loudspeakers 5, 6 and 7 on the localization of a stereophonic sound image for each of the passengers M1 and M2 seated on the vehicle front seats. In addition, the volume of sound produced from the rear loudspeakers 5, 6 and 7 may be equal to or greater than the volume of sound produced from the front loudspeakers 2, 3 and 4, respectively. It is, therefore, possible to control the volume of sound produced from the rear loudspeakers independently of the volume of sound produced from the front loudspeakers. 
     Referring to FIG. 8, there is illustrated a third embodiment of the present invention. The right-hand loudspeaker 2 is located relative to the right of the passenger M1 seated on the right-hand front seat. The right-hand loudspeaker 2 is positioned a distance (l 5 ) away from the passenger M1. The left-hand loudspeaker 3 is located relative to the left of the passenger M2 seated on the left-hand front seat. The left-hand loudspeaker 3 is positioned at a distance (l 6 ) away from the passenger M2. The center loudspeaker 4 is located between the right- and left-hand loudspeakers 2 and 3 at a distance (l 6 ) from the passenger M1 and at a distance (l 5 ) from the passenger M2. These distances are determined according to the fashion in which parts are arranged on the instrument panel 1. The loudspeakers 2, 3 and 4 are connected to receive stereophonic signal from a stereophonic signal generator circuit for acoustical reproductions of stereophonic sound corresponding to the stereophonic signals. 
     The stereophonic signal generator circuit includes a pre-amplifier 12 which receives stereophonic signals from a program source 11. The pre-amplifier 12 generates R and L signals at its R- and L-channel outputs. The R signal is fed to a time delay circuit 51 and also to another time delay circuit 52. These time delay circuits 51 and 52 provides a predetermined time delay, for example, ranging from 0.3 msec to 0.6 msec to their input signals. The delayed signals are fed through a volume control circuit 53 to the main amplifiers 16a and 16b which drive the right- and left-hand loudspeakers 2 and 3, respectively. The L signal is fed from the pre-amplifier 12 through the volume control circuit 53 to the main amplifier 16c which drives the center loudspeaker 4. 
     With respect to the passenger M1 seated on the right-hand front seat, the right-hand loudspeaker 2 is positioned at a distance (l 5 ) and the center loudspeaker 4 is positioned at a distance (l 6 ). It is now assumed that the distance (l 5 ) is smaller than the distance (l 6 ) and the difference of the distance (l 5 ) from the distance (l 6 ) is in the range of about 10 cm to about 20 cm. 
     If the R and L sounds are produced at the same time from the right-hand and center loudspeakers 2 and 4, the L signal will arrive at the passenger M1 with a time lag relative to the arrival of the R signal at the passenger M1 because of the difference of the distance (l 5 ) from the distance (l 6 ). If the distance (l 6  -l 5 ) is in the range of about 10 cm to about 20 cm, the time lag will be in the range of about 0.3 msec to about 0.6 msec, causing such a &#34;harness effect&#34; that a sound image is located in a direction of travel of the sound which comes to a lstener earlier. It is known in the art that such a &#34;harness effect&#34; will occur when the time lag is in the range of 0.4 msec to 40 msec. A listener will have a sense of oppression due to the preceeding sound if the time lag is less than 1 msec and particularly in the range of 0.3 msec to 0.6 msec. 
     In this embodiment, the time delay circuit 51 is arranged to provide a time delay ranging from 0.3 msec to 0.6 msec to the R-channel signal which drives the right-hand loudspeaker 2. Consequently, the L sound produced from the center loudspeaker 4 can arrive at the passenger M2 at the same time when the R sound produced from the right-hand loudspeaker 2 arrives at the passenger M1. This permits creation of a stereophonic sound image in front of the passenger M1. The passenger M2 can have a stereophonic sound image in front of him in a similar manner. 
     Referring to FIG. 9, there is illustrated a modified form of the second embodiment, wherein the right- and left-hanINVENTION 
     Accordingly, it is an object of the present invention to provide a protection circuit for a shuttle drive transistor of a dot line printer which can reliably protect the drive transistor in the motor circuit from destruction as well as improve the starting performance. 
     It is yet another object of the present invention to provide a protection circuit for a dot line printer which can reliably shut down the printer when a sudden load is applied to the print head to prevent damage to the printer itself. 
     In keeping with the principles of the present invention, the objects are accomplished with a protective circuit for a shuttle drive transistor of the dot line printer and for the protection of the printer itself including a comparator for comparing the drive electric current for the head drive motor with a referencwe value and for stopping the motor when the drive electric current exceeds the reference value and a temperature detecting element being arranged in the vicinity of the drive transistor for the motor in order to detect the temperature and to supply a temperature signal to the comparator in accordance with the current temperature of the transistor and controlling the regulated value of the electric current in the comparator in accordance with the temperature fluctuation, whereby the drive electric current for the motor is controlled to be under the regulated value to protect the drive transistor, and the initial driving performance can be improved by an increase of the regulated value during the initial stage of motor driving. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view showing a preferred embodiment of a shuttle type dot line printer which adopts the protection circuit in accordance with the teachings of the present invention; and 
     FIG. 2 is a circuit diagram showing a preferred embodiment of the protection circuit in accordance with the teachings of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring more particularly to the drawings, FIG. 1 shows a preferred embodiment of a shuttle type dot line printer adopting the teachings of the present invention. A platen 16 is pivotally mounted to side plates 12 and 14 which are fixed to a base frame 10, and printing paper 18 is installed along the surface of the platen 16. The printing paper 18 is held with paper guides 20 and 22 engaging with the edge sprocket perforations along the side margins of the paper, and rotation of an advance shaft 24 feeds the paper toward the direction of arrow A. 
     A shuttle 26 is prepared on the base frame 10 so that the shuttle 26 can be driven in horizontal reciprocating motion to the directions B and C perpendicular to the paper feeding direction A, and a plurality of styluses are arranged along a line in the shuttle 26 in order to impact toward the printing paper 18. The impact of the requested stylus toward the printing paper 18 linked with the reciprocating motion toward the B and C directions of the shuttle 26 enables simultaneous uniform imprinting along both directions (line directions) through an ink ribbon (not illustrated) in detail on the paper 18. Each of the stylus compositions and solenoid devices is not shown in detail in the Figures, but every stylus is composed in the same manner with one in a normal dot printer 
     In order to drive the shuttle 26 in reciprocating motion in the directions B and C, on the base frame 10 is fixed a drive motor 28 consisting of a DC motor, etc. Its motor shaft has a flywheel 30 mounted thereon, and a crank shaft mechanism is prepared in front of the flywheel 30, although it is not illustrated in the Figures. Th crank shaft engages with a connecting rod mechanism 32. Since the other end of the connecting rod 32 engages with the shuttle 26 by the shaft 34, it is understood that the shuttle 26 is driven in reciprocating motion in the directions B and C through the crank mechanism including the connecting rod 32 in accordance with rotation of the drive motor 28. 
     The shuttle 26 mentioned above includes in its inside a plurality of styluses and stylus actuators to drive the stylus, the drive section of which is unitized as a hammer bank and is heavy. In the driving operation by the drive motor 28 there may be a problem which causes unnecessary oscillation (or vibration) or the like to the complete device because of its large inertia. In order to absorb this inertia the device is equipped with a counterweight 36 driven in reciprocating motion in the opposite direction to the shuttle 26. The counterweight 36 is coupled with the above mentioned crank mechanism by the second connecting rod 38. Accordingly, the shuttle 26 and the counterweight 36 move in opposite directions from each other, and the employment of the counter forces from each other can cancel the counter force arising from the force of inertia and acceleration to delete the generation of oscillation applied to the complete device. The shuttle 26 and the counterweight 36 mentioned above are pivotally held by supports 40 and 42 which are fixed to the base frame 10 so as to move along the directions B and C. 
     On the tail end of the motor 28 mounted thereon is a slit disc 44 and the position in the reciprocating motion of the shuttle 26 can be electrically detected by the slit disc 44 in cooperation with a photo-interrupter 46. 
     In FIG. 2 shown therein is a portion of a drive circuit for the motor 28. The drive electric current is supplied through the collector and emitter of a drive transistor 50 to the energizing coil of the motor 28, and the motor 28 is controlled by the drive input signal of pulses supplied to its base input 52. It is also possible that the input signal to the base input 52 can be any direct current controlling signal. 
     As mentioned above, the rotation of the motor 28 is controlled by the drive transistor 50. In this kind of motor driving circuit a comparativey large electric current flows through the drive transistor 50, and there may be a problem that a sudden increase of the electric current flowing through the transistor 50 caused by an extraordinary load increase destroys the transistor 50 itself and also causes damage to the printer itself. In order to protect the transistor 50 from such destruction and to prevent damage to the printer itself, in the present invention the drive electric current for the motor 28 is compared with a reference value and regulated to be under a predetermined electric current. In other words, the drive current of the motor 28 is detected as the voltage between both ends of a resistor 54 and it is supplied to the inverted input terminal of a comparator 58 by way of a resistor 56. On the other hand, the tapped value between resistors 61 and 62 is supplied to the noninverted input terminal of the comparator 58 as a reference value. Accordingly, when the motor driving electric current exceeds the regulated electric current determined by the reference value of the resistors 61 and 62, the comparator 58 outputs a stopping signal for stopping the motor 28 to a terminal 64 to switch off the above mentioned drive transistor 50, and the cut-off of the electric current reliably prevents the drive transistor 50 from destruction, especially from heat destruction, and prevents damage to the printer itself by the high speed operation of the protection circuit. 
     As mentioned hereinabove, according to the present invention, the drive electric current for the motor 28 can be always regulated to be under the predetermined electric current value, and the circuit can be well protected. In the shuttle type dot line printer, however, the motor 28 requires a large drive torque during its initial drive, and there arises a problem in that it is difficult to determine the reference value to be supplied to the above mentioned comparator 58. In other words, since the shuttle includes in its inside the hammer bank having a plurality of styluses which is comparatively heavy, the motor 28 requires a large drive torque during its initial driving stage. This necessary torque lowers rapidly as the shuttle 26 is continuously driven in reciprocating motion after the initial stage is over. Accordingly, when the reference value of the regulated electric current is determined on the basis of the necessary torque during the initial driving stage, there may be the problem of heat destruction of the drive transistor 50 due to the heating action of the transistor 50 during the continuous operation after the initial driving stage and the sensitivity to sudden load increases on the print head is reduced. 
     On the other hand, when the reference value is determined in accordance with the light load torque in normal operation, a sudden large electric current flow frequently stops the motor 28 during the large load torque required in its initial driving stage. In such initial driving stage of the motor 28 the drive transistor 50 is in the cool state and strong enough to resist heat destruction, but, on the other hand, in the continuous operation in which the transistor 50 is put in a comparatively high temperature state it is troublesome to stop the motor 28 frequently. 
     In the present invention, since in order to meet either case mentioned above the temperature in the vicinity of the drive transistor 50 is detected to be added to the reference value and the regulated electric current for the drive motor 28 is changed during the initial driving state or during the continuous operation, the protecting action can be extremely well performed in accordance with the real action. 
     In other words, in FIG. 2, a temperature detecting element, a thermistor 66 in the embodiment, is arranged in the vicinity of the drive transistor 50 in order to electrically detect the temperature in the vicinity of the transistor 50. The thermistor 66 is connected to a power source determining the reference value on one end an grounded by way of a resistor 68 on the other end. The voltage between both ends of the resistor 68 is supplied to the noninverted input of the above mentioned comparator 58 by way of a resistor 70 together with the motor drive electric current mentioned hereinabove. 
     Consequently, in this embodiment, both the motor drive electric current and the above mentioned temperature detecting signal are compared with the reference value. In the initial driving stage of the motor, in other words, during the cool state of the drive transistor, the temperature detecting signal is comparatively small so that the electric current flow does not exceed the reference value even if the motor drive electric current becomes larger and its regulated electric current can be substantially increased. Accordingly, in the initial driving stage of the motor when the initial electric current flows larger than the electric current in the continuous operation, there arise no such case which frequently happened in the prior art devices that the motor stops many times. 
     On the other hand, when the motor is driven in the continuous operation, as the temperature rises up in the transistor 50, the thermistor 66 increases the temperature detecting signal supplied to the comparator 58, and the regulated value of the drive electric current substantially decreases to the drive motor 28. Furthermore, the comparator 58 immediately outputs the signal to stop the motor 28 in such an extraordinary case that the drive electric current increases due to an abnormal increases of the load torque or the other external factors. 
     As described heretofore, according to the present invention, a change in the regulated value of the drive electric current between the cool state and hot state of the drive transistor performs the protecting action for the drive transistor and the printer itself extremely adequately for the operation of the dot printer and can achieve an extremely preferable protecting action without having complicated composition, since a single comparator can handle the regular electric current regulation and the temperature corrections to the value of the regulated electric current. 
     The shuttle type dot line printer is described in this embodiment, but this invention applies to the other type of dot printer in the same manner.