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Questions about BALANCING
Balancing – how do we measure?
– The balanced part is placed with a handle on the balancing spindle, which we set into motion.
– Force sensors measure centrifugal forces due to mass imbalance around the actual axis of rotation.
– Centrifugal forces are measured in two different planes around the support points of the balancing spindle. We determine the size of the signals and their phase shift relative to the spindle.
– Force signals are used to calculate the results of unbalance measurements in relation to the balancing planes. The calculated unbalances will change if the position of the balancing planes changes.
– Unbalance compensation (real correction weight value) is calculated based on the calculated unbalance values.
Balance tolerance is a value of allowable residual unbalance, which may appear on the rotor after unbalance correction process has been finished. This value, most often express in grammillimeters [gmm] (or related units), is chosen by rotor’s design engineer or with help of ISO 21940 balance quality grades G. It is assumed that rotor balanced within balance tolerance will operate properly in nominal conditions. Value of allowable residual unbalance is affected by: rotor weight, maximum service rotational speed and machine type in which this rotor operates.
Balancing and rotational speed
Rotational speed of balancing depends on the rotor type. For rigid rotors amount of unbalance does not change significantly for all rotational speeds, up to maximum service speed. Those rotors are balanced using one, most often lower than rated, rotational speed. On the other hand elastic rotors’ unbalance changes as the rotational speed changes because of emerging rotor deformations (deflections). Such rotors have to be balanced at several rotational speeds, most often during ramping up to near service speed.
What is compensation?
In case when not only the rotor rotates on the balancing machine, but also the technological parts of the balancing machine, such as spindle, a compensation procedure is necessary. Elements rotating together with the balanced rotor are causing that measurement result contains not only rotor unbalance, but also unbalance of other rotating parts. To eliminate their influence and the runout error, so called compensation procedure is made. The algorithm calculates the unbalance signals of the rotor in its several angular positions against fixed position of the spindle/chuck and makes the correction so that balancer indicates only the rotor unbalance, and the signals from other parts are deleted. This procedure is used in machines with a vertical axis of rotation, balancers with a cardan drive and in case of using technological shafts
Balancing of rotors on a stationary balancer
Balancing on a stationary balancer allows to balance individual elements separately which results in reducing initial unbalance of full assembled rotor. The possibility of selecting the rotational speed and drive type allows to achieve proper operating conditions. Specially designed measuring unit of balancing machine, which is calibrated to the specific type of workpiece, ensures greater accuracy in comparison to portable equipment balancing. Using stationary balancer makes it possible to use automatic correction station. It is realized by installation of drilling, milling, welding etc. units. Additionally it is possible to use automatic holders or conveyors. As a result, there is opportunity to balance large series of rotors in reduced time.
Electric drives are constantly developing and they are important element in many fields of industry. The increasing demands of consumers and development of technology results in higher requirements of their properties as well as balancing. Therefore, balancing machines manufacturers, including CIMAT Ltd., introduced wide range of balancers specialized in unbalance measurement and automatic correction of such components. Depending on the type of workpiece, these are balancers with horizontal and vertical axes of rotation. The type of drive used to accelerate the rotor is also specialized, from non-standard belt drives to dedicated electric drives for assembled electric motors. To cope with high requirements of allowable residual unbalance there are precise, highly sensitive measurement systems with properly prepared software and electronics.
What is static and dynamic unbalance?
In static unbalance principal central axis of inertia is moved parallelly to rotation axis (couple unbalance equal zero). It means that statically unbalanced rotor placed on two measuring supports will swing until its “heavy point” is directed, according to force of gravity, downwards. Such rotor can single-plane balanced. Dynamic unbalance is a general case of unbalance, where central axis of inertia is twisted against rotation axis (combination of static and couple unbalance). To reduce dynamic unbalance it is necessary to add or remove weight at minimum two correction planes.
How to quick-check balance machine repeatability?
- Balance the rotor to about half of balance tolerance.
- Measure unbalance 10 times one by one, after each measurement stop the rotor.
- Difference between the smallest and the biggest measured value on one plane should be within 20% of balance tolerance.