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Analysis and concept development
The work in this report started of by first studying the machine so that a profound understanding of its hardware and software could first be inquired. Then, proceeded by analyzing why the noise, vibration problem occurred and then, ultimately, creating a new construction design that can solve the problem along with making it correspond to its specifications.
The machine was studied by learning about its software, electrical wiring and electrical components, that is, the actuator and the load cell. When the understanding of these was inquired, it all was documented by taking photos of the machine (appx. 2), illustrating the machine in the CAD software SolidWorks (appx. 3), creating a wiring diagrams for its electrical wiring (appx.4), and saving its original labview programming code (appx. 5). Along all this, the field of tribology and friction test machines was studied as well, because of its high relevance for the machine.
The noise, vibration problem was analyzed by performing random tests. The results from the random tests were then interpreted. The purpose of this was to find the source for this problem and by that also knowing how to solve it. The original labview G code were not programmed with a friction force indicator, which showed how much friction force the load cell was censoring. This was important to have when analyzing the problem because it enabled one to sense the whole picture of the problem. A friction force indicator was therefore added to the G code and was also verified to function correctly (appx. 6).
As soon as the cause of the noises and vibrations was found, the development of a new construction design was carried out. The secondary problem of the machine was its incongruity with its specifications. So, while the machine anyway was being reconstructed for the purpose of solving the noise, vibration problem, the specifications were also taken into consideration by integrating them to the process. As mentioned in the background subsection of this report, it had been decided that some specifications would not be fulfilled; and that some were adjusted due to time limitation and to make the work less extensive.
Analyzing the noises and vibrations
Random tests
The purpose of this step was for the author to look at the noise, vibration problem for himself. The experience gathered this way would then create a general picture of the noise, vibration problem. When the general picture of the problem had been created, the problem was interpreted and analyzed doing an analogy with similar problems.
To execute several random tests with the machine became the first starting point in the aim of looking at the machine’s noise, vibration problem and understanding why it was happening. Two flat specimen and three pin specimens was in total used for all the executions. The two flat specimens were made out of gray iron, one of the pin specimens were made out steel, one out of gray iron and one had a bearing ball on its tip. The pin with the bearing ball was used more than once (Picture 4).
When the test was executed, they all had different test settings i.e. what speed the flat specimen oscillated in, how long the distance of the oscillations were, the duration of the oscillation and also what materials that they reciprocated against each other with. The same 47,7N load was however used for each of the tests.
In the beginning of very test, it was assured that both the pin specimen and the flat specimen were tightly fastened to their respective places. When they were tightly fastened, the foldable arm was folded down, and the load was placed on the arm, making the specimens being forced against each other. The arm had a horizontal position at this point (picture 1).
When the specimens were fastened and forced against each other, the actuator was turned on; and, by that, initiating the oscillating friction movement between the two specimens. Because of the combination of both the specimens being pushed against each other and at the same time oscillating, they started to wear out each other.
What all the tests more or less had in common were that they; yes indeed ended up creating noises and vibrations. It was heard that the noises came from the specimens wearing interaction and it was felt that it was the foldable arm that was vibrating the most.
Another observation was that the steel and the gray iron pin specimens had gained tilted wears during their tests. To gain tilted wears was implicitly known in this work as an undesired test result. It was undesired because that would make calculating the wear volume more difficult, than compared with a non-tilted wear.
In every test, the load cell had been mounted next to the pin specimen, recording the friction forces in both the oscillating directions. The load cell showed to record larger forces in the direction towards the arm base than in the direction away form the arm base (It was in the direction to the arm base that the noise, vibration problem occurred).
The following bulletin point summarizes what had been observed at the random tests:
It was the arm that vibrated when vibrations occurred
It was from the friction interaction of the specimens that made the noises
The friction forces recorded by the load cell was not the same in the two oscillating directions
The pin specimens were gaining tilted wears during the tests
Interpreting
Here, the observations from the random tests are interpreted.
The general picture that had been obtained during the random test was that the ability of the foldable arm to gain a folding angle was somehow the source to the vibrations and noises. During the tests, the pin specimen seemed to be “digging” into the flat specimen while moving towards the arm base, while in the other direction, the pin specimen was sliding on the flat specimen more gently, like a nail would do on a gramophone.
It was speculated that an analogy between the noises and vibrations of the machine and the squeals of a drum break could be made; because, like the shoe in some drum breaks might oppose the rotation of the drum when in a heel-and-toe contact [7], so did the pin specimen appear to do against the moving flat specimen. Figure 15 demonstrates this analogy where the force F for the drum break is the analogy to what the load L is for the machine.
A literature study was performed in the field of drum breaks when this analogy was made. After the research, it appeared that this kind of problem is within the field of friction-induced vibrations.
Two friction induced vibration phenomenon, believed to be the explanation to the noises and vibrations of the machine, was extracted from the literature study. The first one was stick-slip and the other one was sprag-slip. These were then compared with the machine.
Starting with stick-slip. Stick-slip is defined to happen to slow moving friction systems when the static coefficient of friction is higher than the kinetic coefficient of friction. The machine did only manifest the noises and vibrations in one direction. If stick-slip would be what was happening to the machine, the noises and vibrations should have appeared in both the directions. Stick-slip was therefore deemed to not be the explanation to the noises and vibrations of the machine.
Proceeding to sprag-slip. Sprag-slip is defined to happen when the objects are angled against each other as they move and as they approach
Where µ is the coefficient of friction and θ is the angle. Figure 16 shows where the angle θ of the machine is. This angle θ was measured to be about 3,3° in the CAD illustrated version of the machine. Then, by applying the 3,3° in equation 2, the resulting coefficient of friction µ becomes equal to about 17,4. The specimens’ coefficient of friction µ was not near that. Hence the noises and vibrations of the machine could not be addressed as being a sprag-slip case.
However, the sprag-slip definition had shed light on the angle θ of the machine and that it should be considered. By taking the moment about O, like the explanation to sprag-slip had done, and observing the two resulting equations
one could draw the direct conclusion that Ff ≠ µL and FN ≠L in the machine, which is the contrary to what has been believed up until this point, i.e., Ff =µL and FN=L. What this in other word implied was that, when the actuator moved towards the arm base, arm, causing the arm to vibrate (Figure 17). This was deemed to be the explanation for the noises and vibrations of the machine. A possible solution for this would be to reconstruct the pivot point to a position where the angle θ is equal to 0°.
Simply making θ = 0° by reconstructing the pivot point was a possible solution to the vibration, noise problem; but, in that case, the recently discovered problem of the pin specimen gaining a tilted wear would still be occurring. A more desirable solution would instead be to substitute the pivot point O with a point Q that only have a horizontal degree of freedom (DOF). The angle θ would in that case be irrelevant. (Figure 18)
The force diagram in figure 18 was settled to be the theoretical solution for the noise, vibration problem. This theoretical solution would be integrated into the development of a new construction concept.
Development of a new concept
This subsection presents the development of a new construction concept that solves the noise, vibration problem along with fulfilling the specifications for the machine. A step-by-step approach was done in the development process (Figure 19).
Function breakdown
In this first step of developing a new construction concept of the friction test machine, a functional breakdown was developed. As given in the objective, the aim was to solve the noise, vibration problem while making the machine correspond with the specifications. To do so, the machines specification and the theoretical solution were converted into a function breakdown table. This table acts as the foundation for both the coming brainstorm session and the morphological matrix.
Brainstorm
A brainstorm session was done in order to find at least one idea for every partial function in the function breakdown table. All the ideas were sketched down with pen and paper. These ideas were then used in the morphological matrix.
Morphological matrix
All the ideas generated from the brainstorm session were added into a morphological matrix, arranging the ideas in columns and the partial function, which the ideas were supposed to solve, in rows (Figure 20). In order to create new concepts, partial functions were mixed and matched. Certain combinations were chosen. This was done three times, which resulted in three concepts.
Go/No-go matrix
A Go/No-go matrix was used to sort out the most optimal concept out of the three concepts created in the morphological matrix step (Figure 21).
According to the results of the Go/No -go matrix, concept 2 was the concept that should be further developed. Concept 1 did not perform well under the “Reasonable expensive” part of the matrix because the pneumatic table partial function of the concept was considered to redundant. The money and the effort that the pneumatic table requires would probably not yield much better results in comparison with the other concepts that instead had a rolling mechanism. The rolling mechanism was also considered to be a more simple idea. Concept 3 received a bad rating under the “Safe and ergonomic” part because of its free hanging weight partial function. The free hanging weights was considered to not be ergonomic and especially not when compared to the other concepts idea within that partial function i.e. having a lever mechanism.
Crap-up
To assure that the selected concept 2 would work physically, a crap-up of the concept was made.
Cardboard, paper and tape were used to create the crap-up. The purpose of this crap-up was to evaluate if the concept would work physically by achieving a sense of its partial functions in a physical form (Picture 4).
An evaluation of the concept crap-up was done as soon as the crap-up had been created. All of the partial functions seemed to be able to function properly, except for the sliding bar unit. The sliding bar unit of the concept crap-up did not seem to be able to function properly because it looked like to easily would start tilt when it would be oscillated by the actuator (Picture 5). Examples of consequence this could lead to is lever forces being lost because of friction forces from the sides it tilts against, or the specimen’s surfaces receiving irregular wears.
The consequences that could arise from the tilting of the sliding double rod unit were not acceptable. Having load force lost to friction in this way would make it hard to fulfill the partial function of creating load forces up to 120N. Therefore, it was decided that the sliding double rod unit in the concept crap-up had to be swapped for something else.
A runner unit was selected to be the new unit that the sliding bar unit would be substituted with. The runner unit consisted of a runner that, in contrast to the bars of the sliding bar unit, was a standard manufactured part. A much more linear motion with minimum influence from friction was possible with this unit because of its fine-engineered bearing ball sliding mechanism. Hence, load force lost due to friction and irregular wears on the specimens was not expected to happen with this new selected unit The concept crap-up with its new runner unit was again created, but this time out of SikaBlocks. The purpose for recreating it in SikaBlocks was to make the concepts’s function even more defined. Having the concept more defined in the form SikaBlocks made it easier to do an even better evaluation of the concepts conceptulaized functions (Picture 7).
The concepts functions was again evaluated. Except for the height of the fulcrum unit (originally known as the arm base unit), all of the conepts partial funtions seemed to function as they were intended to. The height of the fulcrum unit became too low when the runner unit was selected to swap place with the sliding double rod unit. This issue was decided to be taken cared of in the next step where the concept would be illustated in the CAD software SolidWorks
CAD
The concept in its final crap up form was measured and recreated in SolidWorks (Figure 22). In the proces of doing this recreation, the parts geometry became simplified and minor various design gaps that had not been defined in the concept became defined. This can be observed by looking at the parts from the SikaBlock crap-up of the concet in appendix 7 and the eqvivilent parts in the CAD version of the concept in appedix 8.
Aluminum is what most of the designed components was assigned to be made out of. The two components that had their material assigned more accurately was the balancing weight and load weight. This is because their density mattered in order for the lever of the concept to function properly.
Contents
1 Introduction
1.1 BACKGROUND
1.2 PROBLEM PRESENTATION
1.3 DELIMITATIONS
1.4 OBJECTIVES
1.5 OUTLINE
2 Theoretical framework
2.1 TRIBOLOGY
2.2 FRICTION-INDUCED VIBRATIONS
2.3 LEVERS
2.4 THE FRICTION TEST MACHINE
2.5 PRODUCT DEVELOPMENT
3 Analysis and concept development
3.1 ANALYZING THE NOISES AND VIBRATIONS
3.2 DEVELOPMENT OF A NEW CONCEPT
4 Concept evaluation
4.1 CONCEPT VS FUNCTION BREAKDOWN
5 Discussion and conclusions
5.1 DISCUSSION OF ANALYSIS
5.2 DISCUSSION OF THE CONCEPT DEVELOPMENT
5.3 DISCUSSION OF THE NEW CONCEPT
5.4 DISCUSSION OF THE OBJECTIVES
5.5 CONCLUSIONS
6 References
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REDESIGN OF A TRIBOLOGICAL TEST MACHINE
