Research on rotor balance problem of feed mill

The rotor is the core component of the feed mill. The balance of the rotor largely determines whether the feed mill can operate normally. Unbalanced rotor will cause the bearing position to bear dynamic load, bearing heat, abnormal noise, body shake, unstable movement of the whole machine, may cause unsafe hidden dangers. In order to slow the wear of rotors and bearings, improve their working efficiency and service life, and improve the working environment, rotor balance is an important project that must be considered when designing and manufacturing feed mills. Studying the balance principle and method of the feed mill rotor, the allowable imbalance moment and the reasonable design of the rotor are of great significance for balancing the rotor.

1 Principle and method of rotor balance

1.1 Rotor static balance

The rotor of the jaw crusher is a disc rotor (see Fig. 1), and the rotor mass can be approximately considered to be distributed in the same plane perpendicular to its axis of revolution. If the center of mass of the rotor is not on the axis of rotation, its eccentric mass will produce inertial forces as it rotates. This imbalance is also manifested when the rotor is static, so it is called static imbalance. GB 9239-1988 "Determination of the allowable imbalance of the balance quality of rigid rotors" stipulates in the standard that "if the bearing spacing of the disk rotor is sufficiently large and the axial runout of the disk portion is relatively small when rotating, the even imbalance can be ignored. A calibration plane can be used to correct the imbalance, ie single-sided (static) balance. Therefore, the rotor of the jaw crusher should be statically checked before installation, and the whole machine can be assembled for the rotor whose unbalance amount is within the allowable unbalance range. Since the static imbalance of the rotor is caused by the non-coincidence of the center of mass of the rotor and its axis of rotation, when performing static balance check, the centroid and rotation can be made by adding or removing a part of the mass in the same balance surface on the rotor. The axes are coincident so that the inertial forces of the rotor are balanced.

The common equipment for the static balance check of the claw crusher rotor is the static balance frame (see Figure 2). Place the shaft to balance the rotor on the edge of the static balance frame. If the rotor has eccentric mass, the center of mass a must be at the bottom, and the rotor will rotate freely. When it is stationary, the center of mass a will be directly below the axis. At this point, a balance mass is added directly above the axis, and then the test is repeated. If the rotor is still not stationary, the balance mass is increased or decreased until the rotor remains stationary at any position. When the rotor is stationary, it indicates that the center of mass a of the rotor has coincided with the axis of rotation, that is, the rotor has reached static equilibrium.

1.2 Rotor dynamic balance

In mechanical design, it is generally considered that when the ratio of the axial width b of the rotor to its diameter D, that is, b/D ≥ 0.2, the mass of the rotor cannot be regarded as being in the same plane. At this time, the eccentric mass is also distributed in several different planes of rotation. The rotor of a hammer mill generally has more than two hammer jaws, the axial width of which is much larger than the axial width of the rotor of the jaw mill, and there may be eccentric mass on each hammer jaw. Even if the center of mass of the rotor is on the axis of rotation, the centrifugal inertia forces generated by the eccentric masses are not in the same plane of rotation, and the resulting inertial couples are still unbalanced. Moreover, the action orientation of the couple is changed with the rotation of the rotor, so the rotor will generate a dynamic load, causing the machine to shake. This imbalance is not manifested when it is still, and it is only manifested when it is running, so it is called imbalance.

As shown in Fig. 3, it is assumed that the rotor of the hammer mill has eccentric masses m1, m2, m3, m4 in the hammer plates 1, 2, 3, 4, respectively, and their radius of gyrations are r1, r2, r3, r4, respectively, and the rotor The angular velocity ω operates. According to the structure of the rotor, it can be known from theoretical mechanics that a force can be decomposed into two component forces parallel to it. Select two hammer plates 1 and 4 at both ends of the rotor as the balance base surface, and decompose all the eccentric masses m1, m2, m3, and m4 onto the two balance base surfaces of 1, 4, in order to balance the rotor, set at 1, 4 A balance mass ma having a radius of ra and a balance mass mb having a radius rb are respectively added to the two balance base surfaces.

For a balanced base, the equilibrium conditions are:

A balanced rotor, no matter how many eccentric masses it has and how many hammer splints, can be balanced by adding or subtracting an appropriate balance mass from each of the two selected equilibrium surfaces. The dynamic balance check of the hammer mill rotor is generally carried out on a dynamic balancing machine, and the dynamic balance machine can directly measure the size and orientation of the balance mass that needs to be increased in the two balance bases.

2 Derivation of the rotor allowable imbalance moment formula

After the balanced rotor, there will inevitably be some residual imbalances, and if the imbalance is still acceptable within the allowable imbalance. JB/T 6270—1992 “Technical Conditions of Claw Crusher”, JB/T 9822.1—1999 “Technical Conditions of Hammer-type Feed Crusher”, LS/T 3604—1992 “Hammer Mill” Standard and Feed Crusher The production license implementation rules stipulate that the feed mill rotor balance should meet the requirements of G16. According to the standard of GB 9239-1988 "Determination of the balance of the quality of the rigid rotor balance":

Where: G - balance quality grade, 16 mm / s;

G——gravity acceleration, 9.8 N/kg;

Eper - the allowable imbalance (g·mm/kg);

Uper - the allowable imbalance (g·mm);

m - rotor mass (kg);

ω - rotor angular velocity (radian / s);

N——rotor speed (r/min);

Mper - allows the imbalance moment (N·m).

Substituting (4), (5), and (6) into (7) gives:

3 balance of special shape rotor

In the feed machinery industry, a small number of crusher manufacturers with weak scientific research capabilities have made the rotor of the hammer mill into a non-disc shape, as shown in Figure 4. Such rotors are commonly used in small-sized hammer mills, and there is currently no standard for balancing methods and allowable imbalance moments for such rotors. Therefore, it is difficult for quality inspectors to judge whether the rotor is balanced or unbalanced, and it is certain that such a rotor has defects in design. When the hammer is rotated at a high speed, its strong centrifugal force easily causes the hammer on the corner of the rotor. Stress concentration at the installation site can easily cause the corner portion of the rotor to rupture, posing a potential safety hazard.

4 Discussions and recommendations

4.1 Even after the balanced rotor, due to the uneven wear of the working parts such as the claws and hammers caused by the feed mill during long-term load work, it is highly likely that the rotor will be unbalanced. Therefore, the wear parts such as the claws and the hammers should be replaced in time. According to the JB/T 6270-1992, JB/T 9822.1-1999, LS/T 3604-1992 standards, the claw type pulverizer with the rotor diameter D ≥ 310 mm, The jaw crusher with the difference in mass of the same group should not exceed 1 g and the rotor diameter D<310 mm. The difference between the masses of the same group should not exceed 0.5 g; the mass of the hammers of the hammer mill is poor. Should not exceed 5 g.

4.2 By designing the speed and rotor mass of the pulverizer, the allowable unbalance moment of the rotor can be calculated, and then the allowable unbalance amount on the rotor radius can be calculated according to mper=Mper/(g·r real), and the measured unbalance amount and the allowable unevenness The comparison is more intuitive for testing rotor balance.

4.3 As a feed mill manufacturer, the rotor structure should be reasonably designed to reduce the safety hazards caused by the vulnerable parts of the rotor.