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Structural Design of an Office Chair

Analysis of Failure Modes

Presentation and Methodology

It is becoming increasingly difficult to ignore the fact that any piece of equipment or furniture has its potential to develop a fault sooner or later. This aspect has to be considered by designers and manufacturers on a regular basis so that they will be able to provide their customers with quality products. A potential to break or work improperly always exists that is why it is vital to pay attention to the improvement of quality and failure prevention. Therefore, the analysis of two major failure modes for an office chair is quite essential for the research as long as its design may rely heavily on the related findings and suggested solutions. The analysis of the failure models is also an appropriate source for detecting the most vulnerable and sophisticated elements of an office chair so that the research is able to contextualize much of the implications regarding a structural design of an office chair.
To start the analysis with, it is necessary to focus on the failure mode for bending supporting tube of the office chair. This element is more important than arm supporters and the methodology of reaction on failure can be applied to the second mode, as well. It is becoming increasingly apparent that a supporting tube starts to bend after being under a big weight for a long time. Physical damages are not regarded as a failure so that the analysis is particularly oriented at deformation after a long time under weight’s stress. In such a way, the material of the tube does not obtain the initial form because of the pressure as it is depicted on the illustration below.

The process of the failure consists of deformation, certain time span, weight placing stress on the tube, and characteristics of the tube material. As a consequence, the supporting tube bends more and more after a longer period of time.
Such type of failure mode is usually verified by a so-called creep testing. The supporting tube is put with a certain weight on the top for a particular time frame. The following illustration depicts this process.

The failure mode can be calculated via potential of flexible material. The formula is the following: Ep = k * x2/2 where Ep is potential energy, k – hardness pf material, and x – bending. Provided that k is 200 and bending is 250, potential stress energy is the following. Ep = 200 * (250 *250)/2 = 12, 499, 988. Needless to say, the chosen figures were chosen for clarity of explanation so that in practice calculations will demonstrate different results. Still, the failure mode can be calculated by that simple formula. It is worth saying that it can be also applied to the second failure mode, as well.
On the basis of the measurements and findings, a proper correlation between time, deformation recovery, initial parameters of the tube, and the weight of the pressing mass. In consequence, a failure framework has to be developed. These recommendations can be used for a manufacturing process of office chairs as well as for a drastic change in their design. A suggested design and failure prevention strategy are presented in the next section so that the details will be issued independently. As it has been mentioned, a similar methodology is applied to the failure mode for fracture of arm supporters. Therefore, that is also worth discussion.
The second failure mode is for a fracture of arm supporters. By the same token, the failure develops because of mechanical damage or high pressure on a certain area of the arm supporter. It is worth mentioning that the pressure does not spread over the entire supporters when a sitter puts their elbows flat on the arm supporters. Deformation usually occurs in cases when a sitter places their elbows at a particular angle. It can be 90 grades angle and sharper. The angles above 90 grades are not actually possible and they do not pose any threat to the fracturing of the arm supporters. In the same vein, fracture occurs after a certain time frame. It may appear in several points of the arm supporters as long as a sitter does not place the elbows in the same place for every single time. Hence, the same parameters for the failure measurements can be utilized: time frame, pressure weight, flexibility of the material, and area of potential pressure should be considered.
With regard to the numerous points of pressure, it is to be said that the failure framework has to be flexible. Therefore, the prevention method as well as changes in the design has to comply with intensity of pressure on a particular segment of the arm supporter. It is a pivotal aspect as a sitter does not sit still for a long time so that centre of pressure will be moved from one angle to another. The related recommendations will be given an account in the following section. This is the key statements concerning two major failure modes for the office chair. As they have been discussed, it is appropriate to proceed on the following section in order to conduct the analysis of the failure factors.

Analysis of Failure Factors

Traditionally, a failure mode for supporting tube of an office chair is addressed by the following preventive measure. First of all, the chair is designed in a way, which presupposes regrouping of weight on the other elements. It is usually a back of the chair so that a sitter bends on it and a considerable weight puts a pressure on a flexible back bend. The following picture describes that principle.

As long as sitter bends on the back, a weight is placed at different angle. Hence, tube faces a considerably lower pressure while a back bend simply leans towards. Some office chairs have an additional pole for a back bend but they are generally weak because of flexibility loss. Furthermore, they make a sitter lean with more effort in order to sit in a comfortable way. However, such prevention of failure is not totally valid since other elements are exposed to failures accordingly.
The other strategy to reduce a failure factor for that type of mode is to place an additional lumber plate under the sit. The plate helps to spread the pressure of the weight over the sit so that the supporting tube is pressed with a minimal weight. It is quite reasonable solution, but one should pay attention to the fact that office chairs are commonly used by people, who sit at the desk while writing or typing. Needless to say, such activities do not presuppose an individual to lean on the back of the chair. Thus, a back bend does not accept any pressure at all. A different angle of sitting changes the angle of pressure accordingly. That is why the supporting tube develops a failure. The entire weight is moved on the supporting tube because of a sitter’s position.
A prevention strategy for the second failure mode is commonly based on the adjustable arm supporters. A sitter is enabled to adjust them in accordance with the height or principles of a personal comfort. That requires a sitter to make less effort for placement their elbows in a desired way so that less pressure is placed on the arm supporters. On the contrary, this strategy is not actually effective. The strategy is relatively acceptable from the perspective of the comfort, but it is essential to note that it does not comply with technical peculiarities of work in office. Office workers usually need some hard basis for their elbows as long as they are writing, typing, or holding phone. In other words, an office chair does not have to relax a sitter but help them to make their work as efficient as possible. Hence, the described method of failure factor reduction is almost unused in a contemporary manufacture as it is obviously redundant.
Some manufacturers consider a change of material for arm supporters to be an object of potential search for alternatives. It is certainly true to a particular extent, but it is pivotal to take into account the following fact. Provided that an alternative material is harder than a standard one, arm supporters will become evidently uncomfortable from the perspective of office performance as office workers will get tired faster and feel pain in their elbows. In case an alternative material is softer, it will require additional bars for its stiff placement. Therefore, these bars can be fractured in the same way. A sitter places their elbow in some particular point on the arm supporters so that the related areas are pressed. As a result, that area produces a fracture. It is becoming increasingly apparent that traditional approaches to prevention of failure factors of arm supporters’ fracture are drastically ineffective. That is why alternative strategy is needed.

Recommendations

With regard to the failure mode for bending of the supporting tube, it is to be said that the tube needs a buffering. As a consequence, a pressure that is placed on the tube and sit will sit an opposite force because of buffering effect. One may argue that buffering can be gradual and stiff not sufficiently. That is why it is recommended to utilize hydraulic buffering as it responds to the pressure with according opposite force. In other words, the tube will bend less because hydraulic base will take all pressure force for production of buffering effect. The seat can be incorporated with anatomic sit so that a sitter can feel naturally while sitting in the office chair. Needless to say, such approach implies minimal risk factors as well as provides a sitter with more healthy sitting conditions as the seat will be more stiff and stable. Hence, this design is worth a further discussion.
In such a way, hydraulic tube will not develop excessive bending under a high pressure of a sitter weight. In contrast, it is appropriate to make a comment that hydraulic tube will imply a certain cost spending. The results, however, are relevant to the presupposed costs so that the suggested approach is worth investing. It is beneficial for the manufacturers as they are able to offer quality office furniture. As for consumers, offices will make profit from purchasing more reliable chairs for a considerably longer period of time. The design may require some time for experimenting and testing, but the general approach is quite simple. A similar design and manufacture strategy are applicable to the second type of failure mode so that the paper moves on the discussion of that mode.
Fracture of arm supporters is recommended to be addressed with placing additional below moving plate. As pressure increases, additional bars bend. In such a way, fracture is not possible as long as all force is directed to the bending. Therefore, it is worth saying that the additional bar is supposed to be as wide as possible in order to bend flexibly and spread the pressure over its area. In consequence, the pressure is divided over the entire arm supporters and no damage can be made to the main supporting bars. It is also possible to suggest hydraulic additional bars, but that strategy will imply more costs. Again, this solution is justified from the perspective of costs as well as comfort. There can be some contradictions concerning the suggested failure prevention strategy, but they can be refuted by the following fact. That is why it is necessary to describe them in a detailed way.
One may argue that such design is unreasonably costly and implies extra weight for the office chair, but a plain technology of bicycle hydraulic buffering can be utilized. The main challenges concerning the outlined strategy are based on a proper choice of material and factual manufacture. Needless to say, the design may require certain adjustments, but the suggestion is still reasonable. It is advised to apply some models of Total Productive Maintenance in order to minimize costs and raw material spending and maximize the effectiveness of the manufactured element. The TPM model is expected to focus on such parameter as balance between material, weights, and overall costs. The detailed solutions are worth an independent research so that it is important to place the emphasis on their significance. Still, a choice of the material for reduction of failure factor can be underpinned by calculations of potential stress placed on the chair. Provided that a typical seat of office chair is something like 50 cm2 and placed stress is equal to 70kg, a standard pressure potential can be calculated as follows: 50/70 = 0. 71. One should pay attention that a formula of Pascal pressure is not applicable as long as a stress on a hard material has to be indicated. Therefore, the stress can be calculated via dividing the area of potential pressure on the weight placed on it. Again, this principle can be utilized for addressing the first failure mode, as well. This is the main points regarding analysis and addressing of two major modes of an office chair so that the key points need to be summarized.
All in all, it is to be said that this section has focused on the analysis of two major failure modes of an office chair. They are bending on the supporting tube and fraction of arm supporters. The section has described the main peculiarities of the modes and gave an account to its main failure factors. As a consequence, it has been revealed that the main problem is related to the excessive weight placing a pressure on the tube/supporter. The research has suggested new approaches to addressing the failure prevention. The tube is recommended to be incorporated with hydraulic buffer, and arm supporters need additional lower plate. Extra hydraulic bars are also advised, but their applicability has to be experimentally justified initially. The overall methodology of the solutions is relatively simple as it presupposes a direct physical reaction on the excessive pressure instead of regrouping it at the other elements of an office chair.
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