Balancing
The term balancing refers to reducing or eliminating an imbalance .
Each rotating about a fixed axis rigid body has an imbalance , to vibrations ( oscillations ), noise and increased wear, at high speeds may even lead to destruction. If the manufacturing tolerance leads to an unbalance that is too great, the mass distribution must be balanced individually on this body. In the case of balancing only after the workpiece has been assembled, one speaks of operational balancing, in which the unavoidable assembly irregularities can also be taken into account. The compensation can be positive or negative:
- With positive balancing, balancing compounds are applied by welding, gluing, clamping or screwing on weights.
- With negative compensation, masses are removed by drilling, grinding or milling.
A mixed form is the adjustment by screwing in or screwing out a screw.
Instead of changing the body, the axis of rotation can also be corrected in such a way that the imbalance is minimized. This balancing technique is called balancing centering .
The tolerances for balancing and other specifications are standardized in the DIN ISO 21940 series, in particular parts 1, 2, 11 to 14 (formerly DIN ISO 1940-1).
Vehicle wheel balancing
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When balancing vehicle wheels, a machine (“wheel balancer”) sets the wheel (rim with tire) in rotation (rotation). The axis on which this happens is equipped with sensors. An electronic evaluation system calculates the imbalance from the measured values of the forces occurring on the axis . Then balance weights are attached to the rim to compensate for the imbalance.
A distinction is made between balancing weights
- the application, d. H. Vehicle type (truck, car, motorcycle)
- Type of fastening (impact weight, adhesive weight, plug-in weight, clamp weight)
- Material and
- How it works (static or dynamic).
Following the decision of the EU Commission on June 27, 2002 on end-of-life vehicles, the use of lead in balance weights was banned. Since then, various other materials have been used as substitutes, mainly zinc and steel , as well as tin , tungsten and special plastics, which have a high density .
There are also dynamic balancing weights. These have not yet caught on, particularly because of their restricted functionality. It is an adhesive weight, which consists of a thermoplastic tube filled with freely moving steel balls.
The causes of the imbalance are uneven wear and tear on tires or treads and uneven density distribution of the wheel center. As a result, vehicle wheels must be rebalanced after changing tires or other reworking operations.
The aim is not only to adjust the center of gravity of the wheel to be balanced so that it falls on the axis of rotation , but also that the main axis of inertia , which is perpendicular to the wheel of revolution, not only intersects with the axis of rotation, but lies on it. Therefore, counterweights often have to be attached to both sides of the rim.
Balancing machines use a rotary movement and a dynamic force measurement on the axis of rotation to determine the mass, the angle and the side of the rim to which the counterweights must be attached.
Rotating machines and machine parts
Rotors or rotors and armatures of electric motors are often balanced by removing the laminated core of the finished rotor in the form of bores, notches or over a large area. These are also usually balanced dynamically, i.e. that is, material may need to be removed from both ends of the rotor. According to the terms “static” and “dynamic”, the static imbalance is already evident when the vehicle is stationary, while the dynamic one only becomes apparent during operation. With static balancing, balancing in one reference plane is sufficient, in contrast to dynamic balancing, which takes place on 2 planes. These should ideally be as far apart as possible.
In order to be able to operate in any position, the moving coil must of moving coil instruments are balanced. For this purpose, they have counterweights that can be moved or bent relative to the pointer. For a similar reason, the balance wheel of a watch must be carefully balanced. Otherwise the clock error will depend on the orientation of the clock. Vibration and bearing wear do not play a role here.
The rotating masses of washing machines, spin dryers and centrifuges for test tubes cannot be balanced due to the principle (because the contents change and the distribution is usually uneven). Therefore, their axes of rotation are movably mounted in a resilient and damping suspension in order to reduce the forces on the bearings and the environment. Modern washing machines often first perform a spin cycle at a low speed and then try to redistribute the laundry by moving back and forth before the spin cycle begins at full speed. They have an acceleration sensor on the drum suspension to monitor the imbalance.
Residual imbalances lead to a so-called critical speed at which the oscillating overall system (spring-mass system, consisting of rotor mass and shaft or of total mass and suspension / foundation) can come into resonance . The critical speed is a danger, especially on high-speed machines (turbines, centrifuges, etc.); it is reduced by good balancing, by resilient, damping suspension or by driving through the critical speed particularly quickly during run-up.
Field balancing
Most of the industrial balancing takes place on special balancing benches or balancing machines on which the bare rotor is clamped. The rotor is therefore not balanced in the operating state, but "for itself". Most balancing banks are "hard" and therefore "force-measuring", which means that the test object rotor hardly vibrates and the imbalance can be deduced directly from the measured force.
An alternative method is the so-called "operational balancing", in which the rotor is balanced in the installed state in the machine in which it is running anyway. The force measurement, which is usually not possible in this way, is replaced by a vibration measurement. In contrast to the balancing bench, it is not possible to immediately deduce the imbalance from the measured oscillation in the original state. Instead, one takes the detour via trial unbalanced masses, with which a defined, known unbalance change is introduced into the system. This can be used to calibrate the vibration measurement so that a linear relationship between unbalance and vibration is determined. These so-called "influence coefficients" are a complex 2x2 matrix in two-level balancing.
The actual imbalance can be inferred from the vibration measurement via the relationship determined in this way. Counterweights can be set accordingly.
Field balancing has the advantage that the rotor is balanced without dismantling on site and at the nominal speed under operating conditions. The success or failure is immediately visible.
literature
- Adolf Lingener : Balancing. Theory and practice . Verlag Technik, Berlin and Munich 1992, ISBN 3-341-00927-2 .
- Hatto Schneider: Balancing technology . 7th edition. Springer, Berlin a. a. 2007, ISBN 978-3-540-49091-3 .
- Booklet series "Balancing Practice" from Schenck RoTec GmbH .
- Tewes, Olaf: Innovation in balancing weights. In: AutoWheelsReifen - rubber tires. No. 6, 2006, pp. 113-114.
Individual evidence
- ^ Balancing technology , textbook by Hatto Schneider.
- ↑ EU end-of-life vehicle regulation ( Memento of the original from March 6, 2016 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ German Institute for Standardization (Ed.): DIN ISO 21940-13 . August 2012.
