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What are the mechanical incentives for motor vibration caused by bearings and gears

Update:04-06-2020
Summary:

In small motors, ball bearings are usually used. The ro […]

In small motors, ball bearings are usually used. The rolling elements (steel balls) are in mechanical contact with the inner ring and the outer ring. They are surrounded by a layer of lubricating oil, which can slightly cushion the contact surface and expand it slightly.

 

If the rolling elements and the ring raceway are sufficiently round and undamaged, the contact point caused by the compressive force and lubricant movement is elastically deformed in the circumferential direction, and only a wide-band (spectrum) vibration is generated. The lubricant can suppress the vibration.

 

If there is not enough lubricant or the viscosity of the lubricant is incorrect, the vibration will increase ("metallic hard noise" noise), the pressure at the contact point increases, the material fatigue increases and the raceway damage, especially the axial overload of the bearing, will Causes faster material fatigue.

 

Radial ball bearings have radial bearing clearances due to the way they are manufactured and operated. If the radial clearance caused by the elastic axial preload on the outer ring or the inner ring is compressed to zero, the bearing ball cannot transmit the diameter optimally To force.

 

In this case, the bearing fit and assembly quality play an important role. At a certain rotational speed, the ball actually runs synchronously, rather than on a circular or wavy path. Self-induced axial vibration occurs in the bearing bracket, which is called howling noise. In the bearing, we mainly encounter movements that generate elastic forces, which in turn cause vibrations and forces that generate vibratory movements. It means that there are displacement excitation and force excitation, so both of them must be considered in the theoretical analysis of individual cases.

 

Vibration caused by imbalance

 

In the motor manufacturing technology, it is impossible to completely align the rotation axis of the main shaft and the rotor. Although the deviation can be kept low, the typical high speed of small motors will generate a large amount of centrifugal force, that is, the modal order r=1 at the support point. The circumferential radial force, in terms of elasticity, will cause the circumferential radial deflection of the support point to increase or decrease. Depending on the deviation of the axis of inertia from the axis of rotation (parallel or angular), there can be radial forces. In space, they are in-phase or out-of-phase, or the same combination, and therefore the motion caused by these forces (from shaking to Rolling motion), the parallel mismatch between the shafts is called static unbalance, and the angle mismatch is called dynamic unbalance.

 

Gear drive vibration

 

When it comes to gears, there are two main causes of vibration:

 

The first is that the gear does not reach the precise level and causes tangential or radial vibration;

 

The second is that when the gear teeth roll against each other, the instantaneous gear ratio changes due to the change in the radius ratio.

 

For large gears with many teeth, power transmission usually involves multiple teeth. As long as there is enough clearance and elasticity, the change between the radii is uniform, just like a helical gear. Small motors have smaller gears, fewer teeth, and fewer meshing (overlapping) teeth, which are rarely engaged by helical gears.

 

Therefore, even small fluctuations in the radius ratio will make it quite obvious, which will stimulate the rotational vibration on the shaft and the radial vibration at the bearing. These vibrations are equal to the product of the rotation frequency and the number of teeth and their multiples. These vibrations occasionally It will be modulated by the rotation frequency or its multiple, for example when the plastic gear warps and becomes loose.

 

Vibration is a form of displacement excitation, which is mainly forced motion that generates pressure or force. The size of vibration depends on the elasticity of the gear and other elasticity, inertial torque, and the gap between the gear and the lubricant pad.

 

Plastic gears are advantageous in terms of noise due to their elasticity, low mass, and material damping, but as temperature and humidity increase, their dimensions tend to increase to undesirable levels.

 

Therefore, they must be designed with play, and they are not suitable for relatively high loads. For multi-stage gearboxes and planetary gears, the change in radius largely compensates for the interaction in the entire transmission system. During the life of the motor, wear often causes gaps and backlash between gear teeth.

 

When the gear transmission system is opened and the direction is reversed, the gap and backlash will generate impact noise in the transmission system. During operation, the friction between moving parts will generate random impulsive noise and continuous noise. Continuous noise usually contains many harmonics, and gaps and backlash may occur between many parts. Typical examples are flange mounting of motors, no-load gearboxes, insufficiently tensioned ball bearings or loose ball bearings on shafts. Inner circle.

 

The gap between the gearbox teeth will cause vibration (displacement excitation), the two sides of the gear teeth suddenly collide and rebound elastically, and the resulting repetition frequency and amplitude depend largely on the size of the clearance, and they It becomes very obvious under no-load conditions, during start-up or when the speed changes. As the load increases, the vibration disappears. These vibrations can be minimized by good lubrication and elastic teeth that provide material damping (plastic). In large motors with clearance or worn metal gear teeth, this vibration is very harmful.

 

in conclusion

 

In each motor, in addition to the desired force, torque, and motion, it is inevitable that undesirable forces, torque, and motion will also be generated. Undesirable fluctuations (oscillating torque) are superimposed on the desired motor torque, which results in oscillation of the rotary motion. Radial forces caused by unbalance and magnetic effects cause radial motion. The frictional forces that fluctuate with time appear on the bearing and sliding contact surfaces and cause undesirable motion. When the gearbox is installed in the equipment, the gears can cause undesirable rotational vibrations. All these movements constitute structural noise and they are transmitted to the vibration surface of the motor.

 

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