Selection of Attachment Buttons for a Textile Shopping Trolley

Selection of Attachment Buttons for a Textile Shopping Trolley
Edward Calleja Stafrace
Dept. of Mechanical Engineering
University of Malta.

After performing a decision matrix to select the means of attachment of the trolley to the frame, it was concluded that buttons will be used. However, various types of buttons exist in the textile industry, and unfortunately no numerical data regarding strengths were available.

Thus, it was decided that tests would be performed and information gathered to better decide what type of button will be used. Note that this is a very important exercise, since the trolley and the attached basket are expected to withstand 100kg of weight (including safety factor) without failure. If the trolley were to fail (failure includes the buttons fracturing or simply detaching) the user would be put in danger since the wheelchair is closely attached to the trolley.
Different buttons
Thus a simple test was performed. The first step was to find a supplier to professionally prepare samples. Yorkie Clothing Ind. Ltd was more than eager to help, as we promised to share our results with them. The samples prepared mimicked loops, which are similar to what will be used in the design. The sample buttons were pressure buttons, shirt buttons and jeans buttons. In addition, we prepared samples with 2 buttons per loop, in order to identify the relationship with doubling the number of buttons to strength.
After applied load
The set up consisted of a rod clamped to a table, the sample acting as a sleeve to the rod with buttons facing upwards, and weights attached to the underside of the loop sample. We started with the single pressure button sample, and started loading weights in the order of 200g. The experiment was performed twice per button each time. The pressure button failed non-destructively ( it detached) at 4.9kg first and second time 5.2kg. Thus it was concluded that it could safely hold up to 4.5kg of force per button loop. Next, the loop sample with 2 pressure buttons was tested, and the experimental load it failed at was 8kg of force and hence is safe to use with 7.5kg of weight.[ Note that the load is not double but approximately x1.5 times.]

Next the shirt-button sample was tested, and it managed to sustain an astonishing 29kg before suddenly failing. However, it was actually the material underneath which failed, as a pinhole was left behind and the thread of the button still perfectly intact. Hence it was concluded to be safe up to 25kg of load. Note that the only disadvantage with using a shirt button is that it suffers from fatigue and could fail after use for a number of years, it is also not very durable and could fracture under impact.

The final button type tested was the jeans button. The sample fractured at the large load of 33kg. Hence it was concluded that it could safely withstand up to 30kg of weighted loads. The button remained in perfect condition after testing, it did not fracture, but exerted a large stress at the point of contact and forced the material to rupture. The final sample tested was the double jeans button, and the test could not be completed as it withstood 40kg of weights and the lab could not provide us with more loads. However, it is thus safe to say that the best choice in terms of strength is the loop consisting of two jeans buttons.
Load applied on button
The selected button, however, will not only be selected by evaluating maximum strength but also on other factors. Firstly is the cost.
Button Type
Raw Cost/€
WIP Time/s
Labour Rate (china)* /€
Labour Rate (Malta)*/ €
Total China/€
Total Malta/€
*The typical wage rate in china is 12c/min and in Malta is 25c/min

As can be seen from the above table, the costs considered are very low, for both possible situations if manufactured in China or in Malta. Hence cost will be ignored for the component selection exercise. Therefore, the best suited button type will be the jeans button as it is of outstanding load capability and is of average labour time and cost. The jeans offers the advantage of being highly durable, impact and degradation resistant and does not suffer fatigue failure. The only disadvantage over pressure buttons is that it requires a little more time to close through the button hole, however this too is insignificant.


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