SETUP AND CONTROL OF BALANCING MACHINE OPERATION

V. T. Tikhonov

Problems of balancing rapidly rotating parts of mining machines—for example, the working wheels and the fan rotor of the “Prokhodka-500,” SVM-5, and the rotors and fans of electric drives—are becoming exceptionally important in the manufacture of machines, in connection with the increase in the rotational speeds of individual parts of mining equipment.

At present, at mining electrical-machine plants, domestic balancing machines of the DB-4 and DB-3 types have become widely used, as have balancing machines manufactured by the workshops of the Department of Applied Mechanics of the Moscow Institute of Railway Transport Engineers. Neither in the instructions for the machines nor in the modern technical literature has the question of quantitatively evaluating the operation of balancing machines been considered in detail. The setup and verification of the operating assessment of balancing machines requires standards for the bodies being balanced. A control rotor may serve as the standard for the body being balanced; in its weight, dimensions, and shape it must correspond to the rotors of the machines being balanced. Checking the balance of the control rotor under production conditions is difficult because of changes in the scale of operation of the balancing machine. For setting up and monitoring the operation of a balancing machine, we consider it possible to apply the well-known method of dynamic balancing without a balancing machine, by means of making a circular traversal of the body being balanced with a weighed load in two balancing planes. This method makes it possible to perform a quantitative evaluation of the operation of the balancing machine and to determine:

1) the magnitude of the unbalance in the balanced body in scale divisions;

2) the magnitude of the unbalance in scale divisions of the unbalance indicator, caused by the control weight;

3) the division value of the scale of the imbalance indicator of the balancing machine, g·cm;

4) the residual unbalance in the balanced body, g·cm;

5) the number of divisions on the scale of the imbalance indicator on the balancing machine corresponding to 1 g·cm.

For carrying out inspection and adjustment of balancing machines by the circular traverse method, control balanced bodies are manufactured having the same shape and weight as the parts to be balanced. The journals of the control balanced bodies are fitted with bearings. In the balancing planes of the control bodies, holes for control weights are drilled symmetrically with respect to the axis of rotation. The number of holes is chosen depending on the dimensions of the body being balanced; for bodies of medium size it is convenient to adopt eight holes, equally spaced around the circumference and threaded for the control weights; the minimum number of holes is 4. The dimensions of the holes and the weight of the weights are selected on the condition that the control weight create an imbalance of one, two, or more gram-centimeters. The end faces, the outer cylindrical surface, and the journals of the control body are ground so that runout on these surfaces is absent. The control bodies are subjected to careful dynamic balancing.

The operation of the balancing machine is checked by an additional control imbalance created by a control weight, by screwing the latter into a threaded hole on the end face of the body being balanced. The control weight is a screw weighed on an analytical balance. The weight is adjusted by changing the length of the screw. The circular traverse with the control weight is carried out clockwise, for which the control weight is screwed into the plane of the left-hand and then the right-hand end face, successively in the holes. Since the holes are at equal distances from the axis of rotation, they create identical control imbalances in addition to the unbalance of the rotor.

Having determined from the scale of the balancing machine, in each of the positions of the control weight, the magnitude of the imbalance, one can calculate the magnitude and location of the residual unbalance. The readings of the needles of the imbalance indicator on the balancing machine will be different at opposite points of the body being balanced. At one point the residual unbalance will increase the deflection of the instrument needle, and at the opposite point it will decrease it, owing to the “heavy” spot at the diametrically opposite point.

Having selected the points with maximum and minimum imbalance readings (Fig. 1), one may assert that the “heavy” spot is located close to the axial plane passing through these points. The imbalances created by the control weight \(P\) at all the indicated points...

as in a rotor, the check unbalances \(m=Pr\) each time cause (within the limits of linearity of the instrument readings) identical deflections of the instrument pointer by an equal number of divisions \(R\), while the residual unbalance of the rotor causes a deflection by \(\delta\) divisions of the instrument scale.

If the check weight \(P\) is screwed into the “light” place on the body being balanced, then, owing to the residual unbalance, the deflection of the pointers of the unbalance indicator on the balancing machine decreases; i.e., the value of the reading \(n\), in scale divisions, is equal to

\[ n = \kappa - \delta . \tag{1} \]

When the check weight \(P\) is screwed into the “heavy” place, on the contrary, owing to the residual unbalance of the body being balanced, the deflection of the pointer of the unbalance indicator increases, i.e., the value of the scale reading

Fig. 1. Circular diagram of the distribution of unbalance in the check body when traversed by a weighted load.

\[ m = \kappa + \delta . \tag{2} \]

From the data of the circular traverse and from the magnitude of the check unbalance, we determine the axial plane in which the unbalance is located and the magnitude of this unbalance \(\delta\). Solving equations (1) and (2) with respect to \(\delta\), we find the number of divisions of the indicator scale that characterizes the residual unbalance in the balancing plane,

\[ \delta = \frac{m-n}{2}. \]

Solving equations (1) and (2) with respect to \(\kappa\), we find the deflection of the pointer of the unbalance indicator caused by the check unbalance,

\[ k = \frac{m+n}{2}. \]

From the magnitude of the check unbalance \(M = Pr\), we determine the value of one division of the scale of the balancing-machine indicator (in \(g\cdot cm\)); 1 division \(= \dfrac{M}{k}\), whence the residual unbalance \(\Delta\) (in \(g\cdot cm\)) will be

\[ \Delta = \frac{M\delta}{k}. \]

To determine the number of divisions of the scale of the balancing-machine indicator \(w\) corresponding to \(1\ g\cdot cm\), we divide the number of divisions of the indicator scale by the magnitude of the check unbalance \(M\) in \(g\cdot cm\),

\[ w = \frac{k}{M}. \]

Coincidence of the indication of the magnitude of the residual unbalance on the scale of the unbalance indicator, in scale divisions, with its value obtained by circular traverse testifies to the accuracy of the measurements made, to correct adjustment, and makes it possible to draw a conclusion about the proper operation of the balancing machine.

After the balancing machine has been checked or adjusted, it is necessary to plot the sensitivity curve. For this purpose, on a carefully balanced test body, separately in the right and then in the left balancing planes, weighed pieces of plasticine weighing several grams are placed at one and the same point. The weight of the pieces is then increased. In most cases the curve (Fig. 2) has a linear relationship. Having plotted the sensitivity curve for the right and left balancing planes, it is possible to establish the sensitivity threshold of the balancing machine and to set the scale divisions of the unbalance indicator that ensure the residual unbalance within the tolerance limits.

The adjustment of the balancing machine must be recorded in the adjustment table.

Table for setting the handles of the electrical measuring panel for balancing armatures of a single series

Dynamic balancing of bodies operating on ball bearings is recommended to be carried out on the bearings intended for installation in the machines. If technological dismantling of the bearings is necessary, a mark must be made onઆરી

the inner race of the bearing and the shaft, and during assembly attention should be paid to the alignment of these marks. All operations of the technological machining process—turning, milling of keyways, grinding, and even painting—must be carried out before the dynamic balancing operation.

A setup table for the balancing machine and sensitivity charts for the right and left balancing sides must be compiled for each type of balanced part produced by the plant.

Fig. 2. Setup chart for the balancing machine.

(Graph labels: vertical axis—“Reading of unbalance, divisions”; horizontal axis—“Unbalance, g·cm.”)

In the DB-4 and DB-3 balancing machines, the “heavy” spot on the reference body, obtained by the circular-traverse method, is used for setting up and checking the operation of the neon lamp. Increasing the sensitivity of balancing machines, especially when balancing rotors weighing up to 3 kg, can be achieved by using a lightweight design of inserts made of light alloy or wood (Fig. 3).

(Drawing labels: \(A\), \(B\), \(R=35\), \(R=10\), 45°, 50, 7; “Section along \(AB\)”; 6; 14; 18; “Material—light alloy.”)

Fig. 3. Lightweight support for DB-3 and DB-4 balancing machines.

Dynamic balancing of parts consisting of the rotor and impellers of a “Prokhodka 500” type fan, and others, should always be performed together—in the assembled state.

References

1. A. P. Pilitsin. A Device for the Dynamic Balancing of Rotors. Mashgiz, 1953.

2. A. A. Dimerman. Balancing of Turbine Rotors. Mashgiz, 1946.

3. Testing of balancing machines. Express Information, State Scientific and Technical Committee of the USSR Academy of Sciences, Machine-Tool Building, Issue 38, No. 149—152, October, 1958.

4. B. V. Shitikov. Dynamic Balancing of Rotors. Transzheldorizdat, 1951.