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1. Backlash
The speed change principle of gear can be regarded as the relative motion of two friction wheels with different pitch diameters. Assuming that there is no relative sliding at the contact point, that is, the tangential velocity of the same circumferential surface is the same, but the two wheels have different pitch diameters, then the speed is different and the speed ratio is a constant value that is only related to the pitch diameters.
There is no relative sliding friction wheel only in ideal, in fact, in order to ensure that no relative sliding, people use the gear and evenly arranged on the circumference of a circle with a certain number of tooth profile to make sure that no relative sliding, at the same time in order to avoid the rotation of meshing tooth interference to each other, and set up a certain amount of tolerance, this is the origin of back clearance.
Figure B - 19
A good gear transmission system requires a certain "clearance" between the transmission parts. Clearance prevents interference, wear, overheating, ensures certain lubrication, compensates for manufacturing tolerances, etc. Gear meshing clearance means that the gap between the teeth of a gear should be slightly larger than the width of the teeth. Similarly, we will also find a certain gap on the rolling bearing, that is, the small gap between the inner ring of the bearing, the rolling body (ball, roller) and the outer ring. Usually, there will be gaps in the keys and keyways of the shaft or hole. The clearance of the main parts of the gearbox (mainly from the toothside clearance) causes the output shaft to rotate at a small Angle during load reversal even though the input shaft is locked (not rotating). The "no-load Angle" of the shaft is called the rotating back of the gearbox
Gap. This is shown in Figure B-20. Figure B - 20
In theory, no torque is needed to generate backgap, but in practice, a certain amount of torque is needed to overcome the friction of the parts. When the clearance is eliminated, the part deforms elastically as the torque increases. From the point of view of the output shaft, it seems that the Angle of rotation depends on the torque and its size is the stiffness of the gearbox. The torque characteristic curve in practical application is shown in Figure B-21. The higher the slope of the curve, the worse the rigidity of the reducer.
In applications where there is no load reversal or where the post-reversal position is not critical, gearbox backlash is not a significant issue. Figure B - 21
In the precise positioning applications (such as robots, some automatic control equipment, etc.) where the load frequently reverses, the backgap directly affects the positioning accuracy. As a result, servo gearboxes designed for these applications are designed to have very low, tightly controlled clearances and high stiffness.
2. Definition and measurement of backgap
How to define and measure backgap
It is an unwritten industry standard to define backgaps in terms of output. The effect of the backgap measured at the output end on the input end basically depends on the deceleration ratio.
Output back clearance = i × The input end is idle
Attention! The above formula is theoretical. There are deviations in actual measurements, especially in multi-stage gearboxes, because the effect of each stage clearance depends on the position of the clearance in the whole gear chain, and the clearance is not exactly the same in each engagement link.
Backclearance of servo gearbox is usually divided into units of Angle, 1 Angle minute =1 degree /60. Unfortunately, the backgap is often referred to as an "arc minute", which is a mathematical or physical fable, because the definition of an Angle in radians does not have the concept of minutes.
For example, if the gap between gears with a diameter of 500 mm is 0.4 mm, the backclearance would be:
[0.4mm/ ( 2 * 3.14 *500mm)] x 360The degree of x 60points = 2.75 Corner points
All the values indicated in the backclearance of the gearbox sample are actually "angular minutes". The backclearance of the low backclearance precision reducer should be less than 5 Angle, and the backclearance value of the standard backclearance precision reducer ranges from 5 Angle to 30 Angle.
3.Method of measurement of backgap：
Although it may seem trivial, proper measurement of the backlash of a gearbox requires proper testing tools and instruments. Fixtures holding the gearbox and input shaft should be as stiff as possible. The rotation Angle of the output shaft can be measured directly by a high-precision encoder or by an indirect method. The indirect method refers to the installation of a relatively long rigid moment arm on the output shaft, the dial gauge is used to measure the displacement of a certain distance and calculate the corresponding rotation Angle.
Since a certain amount of torque is required to overcome all gaps in the system, the most accurate method is to measure a complete load reciprocation cycle of the gearbox (from zero to the clockwise rated load torque value, then unload and reverse to the counterclockwise rated torque value), as shown in Figure B-21 on the right. Through this method, the complete hysteresis curve of the gearbox can be obtained, which can not only determine the actual recoil, but also determine the torsional stiffness of the gearbox and the loss motion under any load. (We'll explain what that means later.)
A simple approximation is also possible: make sure the output shaft is preloaded clockwise and unloaded, and apply 2% of the rated load torque in the opposite direction. A dial gauge is used to measure the displacement of the rigid arm mounted on the output shaft at a certain distance from the center of rotation, and the corresponding rotation Angle is calculated.
3.stiffness
Rigidity (the reciprocal of the elastic value) describes the distortion of the output shaft with load due to the elastic deformation of the gear component. Rigidity and elasticity are determined by the ratio of the measured torsion deviation of the gearbox to the load. As shown in Figures B-20 and B-21. The unit of stiffness is the torque required to produce a unit torsion deviation. The units commonly used are:Nm/rad。
4.lost motion，Hysteresis loss
As shown in Fig. B-20, at the output end, after gradually applying the torque to the rated torque, the torque is gradually released to 0. At this time, the transmission Angle does not return to 0 as well, but there is a small lag value, which is called the lag loss.
Gearboxes such as harmonic, sejin, and RV have no backgap but have such errors due to elastic deformation.
The concept of loss motion is not used in planetary reducers, but as you can see in Figure B-21, it is only in these relatively low precision gearboxes that the backgap is a major factor, so manufacturers usually do not mention this error, but attribute it to torsional rigidity.
Fig.B-22
5.Angular Transmission Accuracy
Theoretically speaking, when the input Angle θ passes through the deceleration ratio I, the output Angle should be θ× I, but in practice, any precision reducer can not meet this requirement, there will always be errors, the size of the error is the Angle transmission accuracy of the reducer. Not all manufacturers will give this parameter in their samples.
6.Efficiency
It refers to the ability of the reducer to transfer motor power, and the main influence factor is the friction force in the process of the internal gear transmission of the reducer.
7.The service life of the
The efficiency of reducer is generally due to the reduction ratio, input revolution, load torque, temperature, lubrication conditions and different. In general, the efficiency provided in the sample refers to the efficiency at 3000rpm input speed and 25℃ temperature. It should be noted that this life needs to be corrected for low temperature use.
Fig.B-23
The service life marked by most manufacturers in the sample refers to the life of the bearing L10, that is to say, when the gear box is used in accordance with the specifications, the first failure should be the bearing. Therefore, the service life will be mentioned in the provision of torque index. These indicators are interrelated. Service life is the foundation parameters, a lot of data in the sample was calculated based on the service life and after the test to determine, for example, when you increase the input speed from the rated speed to maximum allowable speed, if the output torque unchanged, the results must be reduced life, also, if you improve the input speed of 30%, but decreases the output torque, May be able to maintain the original service life.
In some applications with very short lifetime, the application beyond the target is allowed, but it is best to depend on the experimental results.
If the radial or axial force at the output end is too large, the life will be shortened. In addition, attention should be paid to the calculation of life when used in the case of frequent swing at a fixed Angle.
There is a factor affecting life is easy to ignore, when the output shaft speed for a long time in a very low operation (below 0.02r/min) interval, the lack of lubrication of the bearing, can lead to the aging of the bearing and the drive side of the load rise. Special attention should be paid to this in applications such as single-crystal silicon pull-up furnaces.
When the reducer is used perpendicular to the horizontal plane, especially when the output shaft is used in the upper part, some brands of reducer in the input end without oil seal, need special attention, because this situation for a long time, will lead to the reducer gear surface due to the lack of enough lubricating oil and lead to gear surface damage.
8.Noise
Noise is measured at a distance of 1 m from a gearbox operating at an input speed of 3000rpm in a sound-proof environment.
Turbine worm reducers are generally less noisy, and today it is generally accepted that helical gears will be less noisy than straight gears. In fact, there is an error, the servo reducer precision due to the limit, backlash is relatively small, the gear meshing is tight, under the condition of same back gap, if the gear manufacture precision is low, the noise is relatively large, this, whether it's straight tooth, or helical, all the above problems, the difference is that under the precondition of the same thickness of the gear, The oblique teeth will have more of the kneading part than the straight teeth. More meshing parts can make up for the lack of precision. In terms of the ordinary general reducer can through so-called (such as car car noise is higher, usually for a period of time, gear noise can significantly lower) to reduce the noise, but this is the result bring back gap increases, it is not allowed in the servo with reducer, so people through enhancing the surface hardness of the gear to improve the wear resistance. But even so, the servo reducer still has the problem of accuracy decline after a period of time with load operation. Eisele reducer adopts German gear, which has achieved a very high level of comprehensive performance in accuracy, tooth surface hardness, bending resistance, impact resistance and other aspects. In addition, by increasing the thickness of the gear, increase the meshing part. Therefore, even if the use of straight gear, the same can achieve the effect of ordinary reducer helical gear.
9.Maximum input shaft diameter
This metric is sometimes not noticed, but in a sense, it limits the maximum torque that can be put in, and that torque times the deceleration ratio and the efficiency is the maximum torque that can be put out, and you might find that in some manufacturer's sample, the rated torque is much less than the actual maximum torque that can be put out. This may be due to some kind of "competitive skill".
10.Radial and axial forces are allowed
This may seem clear, but since most manufacturers do not have an index for overturning torque, neglecting the moment arm can be very serious if you just look at the force, especially if the moment arm of the axial force is not equal to zero in many applications (the force does not act on the axis).
The radial force is also important, and it is important to note the equivalent moment arm indicated in the sample, because this force cannot be applied at the root of the shaft. When applied to synchronous belts and pinion and rack, the radial force acting on the gearbox shaft is not carefully measured, resulting in the shaft being twisted off at the root, and the customer blames the manufacturer for the material problem. We should know that according to the principle of material mechanics, when a shaft is subjected to alternating axial force and torque at the same time, the stress concentration at the shaft root is far more than that of the torque alone, especially under the action of changing alternating stress, the situation will become very serious.
There are a few other things that are easy to overlook in the application:
A） When the motor with brake is running at high speed, it starts to brake, and the inertia torque of the external load will all be borne by the gearbox. Especially if the load center of mass and the gearbox axis do not coincide, the problem will be more serious.
B） On-board systems, such as radar, antenna, gun carriage, etc., can put a lot of extra force on the gearbox because of vibration and centrifugal force when the carrier is driving on rough roads and turning quickly.
C） Even during installation, especially for gearboxes with flanged output, excessive torque during tightening of the setting screws can cause damage.
Therefore, the shaft output planetary reducer is usually not suitable for direct pinion and rack drive, this kind of mechanism, the best use of flange output planetary reducer.
