VOL. 11 January ISSUE YEAR 2010
in Vol. 11 - January Issue - Year 2010
The Good, The Bad, And The Useful
Picture 1: Sound is desired, noise is undesired vibration
Picture 2: Vibration is often a problem to be solved in airplane engines
Picture 3: Imbalance motor with imbalance weights on its top and bottom to create vibration for a round tub vibrator
Picture 4: Round tub vibrator with imbalance motor
The two sides of vibration
The title of this column hints at one of the most important physical forces for Mass finishing installations. But it is not the only one, centrifugal force being another very important mechanical basis for mass finishing machines. Of course there are a couple of other modern and very specific techniques for polishing, deburring and further surface refinements, but these are often just applicable for finishing purposes and not for industrial mass finishing.
If we take a look at the question, why vibration is such a useful force for the industry and especially for mass finishing, a definition of the word vibration itself may give us support: Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.
Furthermore it can be divided between free and forced vibration. The first occurs when a mechanical system is set off with an initial input and then allowed to vibrate freely, for example a tuning fork. The fork is vibrating at one or more of its natural frequencies and damps down to zero. On the other hand the forced vibration occurs when an alternating force or motion is applied to a mechanical system, for example a shaking washing machine due to an imbalance or transportation vibration caused by a truck engine. In this case the frequency of the vibration is the frequency of the force or motion applied and the order of magnitude is dependent on the actual mechanical system. This last point is very important for vibratory finishing techniques. As many other things in the technical world also vibration can have a good and a bad side.
Sometimes vibration is desired, sometimes not. Desired is for example the motion of a tuning fork, the reed in a woodwind instrument or harmonica, the cone of a loudspeaker and last but not least the movement of an unbalanced motor of a tub vibrator. Here this force is necessary for the correct functioning of the various devices.
However, often vibration is not desired, when it wastes energy and creates unwanted sound – noise. Apart from tub vibrators, the vibrational motions of engines, electric motors or most mechanical devices in operation are typically unwanted. Often caused by imbalances in the rotating parts, uneven friction or the meshing of gear teeth etc., a careful design usually minimizes unwanted vibrations. This vibration is bad vibration. It can burden the environment and the personnel. Certain frequencies threaten human health and human well being and under special circumstances they can be deadly in the worst case. Vibrations reduce the lifetime of equipment and push the total cost of ownership of production capital, as they push up the service costs for vibrating installations. Vibration can even push up the operation risk of machines as they put heavy strain on material and construction. With that strain the risk for a material fracture rises enormously and it has to be reduced again by engineers. Also noise is a bad effect of vibration, which must be regularly handled by constructors. Sound absorbing lids and cabins are often necessary to protect the operator.
So handling the bad effects of vibration is often one of the main challenges when sketching up first drafts for large and strong mass finishing installations. Vibrating parts on machines can be one consideration on a risk analysis of a machine. Where things are vibrating, the machine operator has a higher risk of being injured, for example by a bruise to his fingers at a narrow spot of the machine. Here again constructors have to consider these risks in order to reduce them. Even European guidelines like the EC machine guideline 2006/42/EC describe vibration as a possible risk in its appendix one, chapter 1.5.9. The international norm for a risk analysis DIN EN ISO 14121 also identifies vibration as a possible risk, which has to be considered by engineers and constructors. In the end, all these bad vibrations can be handled by an experienced and skillful product layout, which aims at transferring bad vibrations to good.
Now we have seen many types of undesired vibration. In the mass finishing world, things are the opposite. Vibration is desired. Careful design is needed for the creation of the most effective vibrational force and at the same time for the suppression of the bad vibrational effects. Here vibration is needed to fulfill a certain task. The different types of motors and the construction of the machines have to be combined in a very sophisticated way to generate a forced vibration in a working tub, on a vibratory conveyor or on a vibratory screening machine. For that purpose different types of imbalanced motors and of magnetic drives are used. A classic round tub vibrator for example generally uses a motor with imbalance weights. The size and angle of these weights influence the generated oscillation. These factors and the position and suspension of the motor in addition to the size and shape of the machine, result in a certain kind of swing amplitude – vibration.
Within the tub, where the working mass – abrasive media and the workpieces – is treated, we can see that vibration as a lasting acceleration. So the mixture of media and workpieces moves continuously from the outer border to the inner rand of the tub, punching one another steadily on the way. It becomes clear that the combination of the above mentioned vibrational adjustment factors like imbalance weight size, angle, machine shape and motor position is a very complex interplay of forces. The complexity of construction is pushed by the fact that engineers have always to consider the bad effects of vibration to create an effective and solid machine with an impressive lifetime and low service costs.
Mass finishing uses forced vibration to treat industrial workpieces in very different and effective ways and it uses vibration to transport masses of workpieces over conveyors and storage hoppers to the treatment machines or to containers. Even where centrifugal force is used for the central treatment vibration is often still the force of choice to transport pieces.
At the final count it is often impressive to see how engineers find a way out of the vibration dilemma to build machines which mainly use the good sides of vibration and which are bringing good vibrations to many different industries around the world.
by Mathieu Geuting
Spaleck Oberflächentechnik GmbH & Co. KG