Planetary Roller Type Traction Drive Unit for Printing Machine

Transcription

1 TECHNICAL REPORT Planetary Roller Type Traction Drive Unit for Printing Machine A. KAWANO This paper describes the issues including the rotation unevenness, transmission torque and service life which should be studied when applying traction drive speed reducers to printing machines. Specifically, comparisons between conventional gear type speed reducer were made to prove the advantageous characteristics such as efficiency, vibration and backlash. The results show the possibility for traction drive unit to be expanded in their practical applications in various fields in the future. Key Words: traction drive, rotation unevenness, printing machine, speed reducer, planetary roller 1. Introduction Recently, in accordance with remarkable progress in digital image processing technology, the printing machines have been improved to higher quality image. To support the quality of the printing image, extremely high rotational accuracy is required at power drive sections, which feed printing paper. In general, the rotational speed needed at the power drive section is from several to dozens of revolutions per minute in many cases and the combination of a driving motor and a reduction unit is adopted in most cases. For these applications, new-type speed reduction systems are increasingly required as there is a limit to the use of conventional speed reduction systems such as gears and belts. Koyo has focused on the high rotational accuracy of traction drive units (see Fig. 1) developed for electric bicycles and power steering systems, and has applied the units in the printing machine field. This paper describes Koyo's concept of traction drive units for printing machine applications. 2. Basic Principle As shown in Fig. 2, the power is transmitted from the drive side rollers to the driven side ones through the oil film existing between two pressure-loaded contact rollers in the traction drive unit. There exists a minute relative speed difference between the rollers, and the tangential force (traction force) T is generated so as to shear the oil film. The traction force T can be expressed as in the formula a. T = lp l : Traction coefficient P : Pressure-loaded force generated by rollers lis defined as traction coefficient and is similar to friction coefficient. The lubricant with high traction coefficient is termed the traction oil in general. The traction oil can generate high traction coefficient through glass transition under high pressure. a P High viscosity lubricant Traction force (rolling transmission force) P Fig. 1 Planetary roller type traction drive unit Fig. 2 Principle of traction force Koyo Engineering Journal English Edition No.16E (24) 9

2 3. Application to Printing Machines 3. 1 Features Figure 3 shows the structure of a printing machine where letters or images are printed on a sensitized film with halftone notes by applying laser beam. Fluctuation of the feeding speed of the film, however, causes uneven printing. Polygonal motor u11 Fixed axle Planetary rollers Ball bearing Sun axle Stepping motor Polarizing lens Laser generator 133 Fig. 4 Structure of planetary type traction drive unit (film feeder) Printed matter Feeding drum Traction drive Fig. 3 Structure of printing machine Practically, the driving motor is operated at high-speed range where the fluctuation of speed is comparatively small, and by reducing its rotational speed, the fluctuation at lowspeed range is restricted. Generally the traction drive units have the following features. q High rotational accuracy High rotational accuracy is easily secured due to rolling transmission. w High efficiency High efficiency is easily obtained due to low power loss. e Low noise and low vibration There is no vibration or noise from intermesh generally observed in gears. r Low backlash Backlash can be easily reduced due to rolling transmission Basic Structure Koyo has adopted planetary roller type traction drive units where the input from the sun axle is taken out as the revolution output of planetary rollers to reduce the speed. The reasons for adopting this structure of a traction drive reduction unit are as follows. q Simple structure and high reliability w Easy to provide pressure-loaded force, which is absorbed in the internal structure The traction drive unit is a unitized product integrating the driving motor in order to be handled easily at end-users. Any type of driving motor can be used, but the stepping-motor or servomotor is generally used. Figure 4 shows a structure example of the traction drive unit Angular Velocity Fluctuation Ratio (Rotational Fluctuation) The angular velocity fluctuation ratio is the measure of rotational fluctuation and is defined by the value of angular velocity fluctuation of the rotating axle divided by the rotation angular velocity. This is one of the most important quality characteristics to influence directly on the image quality of printing machines and is expected to be as small as possible. With the traction drive unit shown in Fig. 4, angular velocity fluctuation data as shown in Fig. can be obtained. By further performing the frequency analysis of these data, the spectral data as shown in Fig. 6 is obtained. The cause of rotational fluctuation can be predicted to some extent from the peak-generated frequency. On the other hand it is to be noted that the measured result of rotational fluctuation is not always the same depending on the measuring device even when the same product is measured. This is because the degree of influence of vibration elements such as resonance will fluctuate depending on the rigidity of the measuring device itself or the moment of inertia of the rotor. Careful attention must be paid to this point. Angular velocity fluctuation ratio, %rms Measuring time, sec. Fig. Waveform of angular velocity fluctuation (measurement example) 6 Koyo Engineering Journal English Edition No.16E (24)

3 Angular velocity fluctuation ratio, % p - p 3. 4 Transmission Torque The transmission torque T q of traction drive can be obtained by formula s. T q = lpzd/2 Z : Number of planetary rollers d : Outer diameter of sun axle As the traction drive unit is constructed to transmit the power by rolling movement, minute slip occurs during operation and the slip ratio is closely correlated with the transmission torque. A measurement example of the relation between the slip ratio and the transmission torque is shown in Fig. 7. Slip ratio, % Frequency, Hz Fig. 6 Result of frequency analysis on angular velocity fluctuation ratio Fig. 7 Relation between transmission torque and slip ratio in traction drive unit In Fig. 7, the inclination of the dotted line has a remarkable changing point at the transmission torque value of 1 to 1. N m. This varies depending on the size of the traction drive unit or the pressure-loaded force. If the transmission torque is applied exceeding the range of the inclination variation, the slip ratio increases and the life of the traction drive unit is seriously affected. Unlike gear reducers, the speed reduction ratio of traction drive units slightly fluctuates due to slipping, etc, thus has a characteristic of accumulating the deviation of position (zero return accuracy for one rotation). When this deviation of position is considered to cause any problem in actual operation, it is necessary to control it by inputting the output signal of the traction drive unit to the driving motor. s 3. Speed Reduction Ratio Speed reduction ratio of the traction drive unit is expressed in the formula d. N = (D / d) + 1 N : Speed reduction ratio D : Inner diameter of fixed ring The number of planetary rollers needed for the traction drive unit is more than 3 rollers considering its structure. Formula f is used to calculate the maximum speed reduction ratio from the number of planetary rollers. N max = 2 / {1 sin (p / Z)} N max : maximum speed reduction ratio Table 1 Maximum speed reducing ratio Number of planetary rollers Max. speed reduction ratio From the above, it is noted that the maximum speed reduction ratio per one-step reduction is approximately 14.9 as shown in Table 1. Speed reduction ratio of more than 14.9, when needed, can be obtained with duplex traction drive units in series. Most of the traction drive units in commercial production by Koyo are double speed reduction type whose speed reduction ratio is between and 1. There are dimensional tolerances in each raceway diameter of sun axles, planetary rollers and ring rollers in the traction drive unit. The acceptable error against the specified speed reduction ratio is less than or equal to 1% Efficiency The efficiency g of the traction drive unit can be computed by the formula g. g = 1 (T R ) / (T 1 R 1 ) T : Output torque R : Output rotational speed T 1 : Input torque R 1 : Input rotational speed d f g Koyo Engineering Journal English Edition No.16E (24) 61

4 Figure 8 shows a measurement example of the efficiency of the one-step reduction drive. Efficiency of more than 9 % can be secured on average in spite of the affects from various use conditions such as transmission torque. Efficiency, % Fig. 8 Efficiency of torque transmission 3. 7 Vibration Smaller vibration compared with other speed reduction structures is one of the features of the traction drive unit because of rolling transmission as described before. Figure 9 shows the comparison result of vibration values with other planetary gear reducers of the same size. Vibration value, m/s Vibration value comparison with planetary gear Rotational speed of sun roller, min 1 Fig. 9 Comparison of vibration value : traction drive : planetary gear 3. 8 Life The following items are the concepts of life for the traction drive unit. The shortest life in the following cases is the actual life. q Flaking life 1) Flaking life caused by rolling fatigue of raceways. 2) Flaking can be prevented by adjusting the contact pressure on raceways. 3) Flaking, however, cannot be prevented at edge load section or when slips occur. w Raceway wear 1) The condition when normal load reduces due to raceway wear and when torque cannot be transmitted. 2) Raceway wear occurs when the oil film on raceway runs out. "Main causes of starvation of oil film" Depletion and insufficient supply of lubricants Excessive contact pressure on raceway Excessive roughness of raceway finish e Starvation of grease (raceway wear) 1) Applicable only with grease lubrication 2) With grease lubrication, the unit generally reaches the life when this occurs 3) Loaded torque or the slip at raceways scrapes away the oil film causing the oil film to run out Grease lubrication without re-greasing the system is mainly adopted for traction drive units for printing machines in commercial production to attain maintenance free as its functional requirements. If properly used, traction drive units reach their life when the grease runs out as described in e above. Figure 1 shows an example of accelerated life test results of traction drive units for printing machines. Slip ratio, % Fig. 1 Fluctuation of transmission torque by accelerated life test Before life test h 1 h 1 17 h In this example, the initial slip curve is maintained until the grease starts to run out (~ 1 h) as can be seen in Fig. 1. With the running out of the grease, the raceway wear starts to occur and reaches the life in a short time. Also, as for rotational fluctuation, the initial level is almost maintained until the raceway wear starts Backlash In traction drive units, backlash due to rolling transmission doesn't theoretically exist. However, the bearing clearance in planetary rollers causes. In the planetary type traction drive unit with the structure as shown in Fig. 4, backlash of 3.9 arc s (angle second) is secured by pre-loading and eliminating the clearance. In this case, backlash is not observed, but the most are hysteresis elements. When a worm gear of the similar size was measured, the backlash was more than or equal to 6 arc s. The value of 3.9 arc s can be said to be very small. 62 Koyo Engineering Journal English Edition No.16E (24)

5 Compactness of the whole unit can be achieved by adopting roller or slide bearings in place of rolling bearings for applications where the backlash is not so important. Output axle torsional angle, 1 3 rad Driving Motor There is no limitation of the types of driving motors to be used for traction drive units. However, servomotors are often used at low rotational speed when quietness and high rotational speed are needed, while stepping motors are used in many cases when high driving force is required. The amount of positional deviation* is reduced in proportion to the speed reduction ratio of traction drive units. However, since the rotational fluctuation is directly output, it is necessary to use driving motors with rotational accuracy as high as possible. *In terms of positional deviation: the deviation between the ideal and actual value of the motor axle rotation angle against positioning command to the motor. 4. Conclusion Backlash 3.9 arc s Output axle torsional rigidity rad/ N m..2.2 Output axle torsional torque, N m Fig. 11 Traction drive unit backlash For printing machines where higher printed image quality will supposedly be pursued, requirements for speed reduction structure with smaller rotational fluctuation will increase further in the future. In adopting the traction drive unit for these applications, it is necessary to understand the characteristics that slips occur. Traction drive units are expected to be adopted for wider applications as a new type of speed reduction device because of quietness, smaller backlash and responsiveness in addition to the power transmission function which cannot be achieved with limited technology of gear reduction systems. A. KAWANO * * Unitized Product Engineering Department, Bearing Business Operations Headquarters Koyo Engineering Journal English Edition No.16E (24) 63