COMPLEX THINGS IN SIMPLE WORDS OR PHYSICS AT HOME

To deal with the terms and understand the principles of balancing, higher education is not required at all, although at first glance you can’t say this after reading any State standard.

Not many realize that the terms of balancing can be considered on the example of an ordinary washing machine.

For example: the drum of a washing machine can be safely considered as a balanced rotor, in fact it is a cantilever type rotor.

In the field of balancing, it is customary to call any product subject to balancing a rotor .

The cantilever rotor is one of the three existing types of rotors. Its feature is that its main mass is outside of one support. In the case of a washing machine, the bearings are the bearings, and the drum is the rotor.

When balancing on the machine, vibration supports with roller blocks or prisms will act as supports.

There are also two-cantilever rotors. They differ from cantilever ones in that their main mass is taken out of both supports and in shape they resemble an ordinary dumbbell.

The last type of rotor is intersupport межопорный, and as you guessed from its name, the bulk of such a rotor is between the supports. A classic example of an intersupport rotor is the rotor of an asynchronous electric motor, which is equipped with almost all household appliances, including a washing machine.

By the way, the rotor of an asynchronous electric motor is shaft-type rotor, that is its length is 5 times its diameter. And if balancing is necessary to eliminate the imbalance, we will use at least two correction planes.

If the situation is reversed and the length of the rotor is five times less than its diameter, then such a rotor is called a disk-type rotor . Acceptable results for its balancing can be achieved using only one correction plane. The washing machine also has a disk-type part, what do you think it is? That’s right, it’s a pulley.

Correction planes are certain zones chosen by the designer or independently. They are located on the rotor and they remove, add or move material in order to balance the rotor.

The pulley can be balanced in many ways, but as a rule, this happens on the machine, although there are exceptions.

For balancing, you can use a special vertical machine – it’s more convenient, or an universal balancing machine with a horizontal axis of rotation. However, in this case, balancing will be carried out using a balancing mandrel, because the pulley doesn’t have its own bearing surfaces.

The balancing mandrel is a perfectly balanced shaft on which the rotor to be balanced is mounted.

Balancing machines can be divided into three groups: pre-resonant, resonant and over-resonant.

Resonant machines are used quite rarely, as the imbalance is determined at the resonant frequency and it’s very impractical, so we skip its description.

The resonant frequency is the frequency of rotation of the rotor, at which there is a sharp increase in the amplitude of oscillations. In other words, this is a frequency with a high level of vibration and heavy loads on the supports (bearings), the rotor itself and the entire structure.

Pre-resonant machines have a greater rigidity of the supports, which allows balancing at low frequencies without reaching the resonant frequency, which is very convenient. Machines of this group are very popular.

Over-resonant machines are machines that balance the rotor at frequencies above the resonant frequency.

So, we dealt with the equipment and types of rotors, it’s time to move on to the balancing itself.

Imagine the situation that we put a tennis ball into the drum of a washing machine, initially balanced at the manufacturer’s factory (as the manufacturer of our favorite Canada goose down jacket wrote in the washing instructions). In this case, the rotor has moved from a balanced state to an unbalanced state.

The ball in this situation can be safely called the unbalanced mass, and the radius vector from the axis of rotation to its center of mass is the eccentricity of the mass. Those. the addition of the ball resulted in an initial imbalance.

The value of the initial imbalance in this case will be the product of the distance from the axis of rotation to the center of mass of the ball and the mass of the ball. The formula looks like this: D=R*M

To balance the rotor means to determine the position and magnitude of the initial imbalance and eliminate, or rather compensate for, it.

What options can be in our case:

The first is to remove the ball. This will be called correcting the imbalance by removing the masses.

But what about your favorite jacket?! By the way, it can cost more than a washing machine. Then the second thing remains – to put the same ball on the opposite side – this will already be an imbalance correction, but by adding masses, and the second ball will be called the corrective mass.

Or the third: if there are several balls – let’s say three, distribute them around the circumference so that their imbalances compensate each other. For three identical balls, the angle between them should be 120⁰. This is called the correction of the imbalance by the method of mass displacement.

If in the second case the balls differ in mass or volume, the rotor may not be completely balanced, and the remaining imbalance will be called residual imbalance. Then it is necessary to make a decision to eliminate the residual unbalance or complete the balancing, since the amount of vibration may be acceptable for the operation of this rotor. If the amount of vibration is still acceptable, the residual unbalance is called the acceptable unbalance.

To determine the acceptable imbalance, the rotors are divided into balancing accuracy classes, which can be determined from the State standard 1940-1-2007.

The main central axis of inertia is, in simple words, such an axis, during rotation around which the rotor will be balanced.

It is due to the fact that the axis of inertia does not coincide with the axis of rotation of the rotor, there is an imbalance, vibration and deflection of the rotor. The radius vector of the displacement of the axis of inertia relative to the axis of rotation of the rotor is called the eccentricity (center of mass) of the rotor.

The product of the rotor eccentricity and the mass of the rotor is the main vector of rotor imbalances. It is also equal to the vector sum of all rotor imbalances.

Simply put, this value can be called the unbalance of the rotor. The State standard 1940-1-2007 normalizes the specific imbalance. What is it and why is it needed? Let’s figure it out.

Specific unbalance is the ratio of rotor unbalance to rotor mass. We take the mass of our ball, multiplied by the distance from the axis of rotation to the center of mass of the ball (D=R*M – we have no other imbalances), and divide by the mass of the drum. If we weigh the ball in grams, measure the radius of the drum in millimeters, and take the mass of the drum in kilograms and remember that 1000 mm is a meter and 1000 g is a kilogram, then we get the value of the specific imbalance in microns:

gr*mm/kg= gr*m/1000/1000gr= m/1 000 000 = m* 10^-6 = µm gr*mm/kg = µm

This is the displacement of the axis of inertia relative to the axis of rotation of the rotor!

And why is it needed? Knowing this value, we can judge the imbalance of different rotors. In our case, about the imbalance of the drum of our friends’ washing machine, even if they live in Australia.

Unbalance can be of different types, and now we will analyze them. The axes are straight lines, and the mutual arrangement of straight lines in space can be of three types:

1. The axes are parallel

Rotor static imbalance

2. The axes intersect at the center of mass of the rotor

Torque unbalance of the rotor

3. The axes do not cross or intersect at the center of mass

Dynamic imbalance of the rotor

Dynamic imbalance contains static and momentary imbalance.

The definition of moment and dynamic imbalance occurs during rotation on a balancing machine, static imbalance can in some cases be eliminated on special stands.

We hope that we have clarified the difficult terms used in balancing, however, if you wish to study this issue in more detail, we recommend studying State Standard 19534-74.

Timing:

We wish you good luck in your work!