(and why physics/mechanics knowledge is still very relevant in the digital age!)
After the first two days of the lunar new year, where lots and lots of relatives from my parents-in-law side came to visit, I decided that it was time to do something AGAIN about the sliding door to the toilet … (somehow, when many different visitors use it, the “flaws” of my first repair attempt, described below, showed up quite a bit).
Now, those of you who have sliding doors or windows at home might know that when the sliding mechanism does not work well, it means that either 1) too much dirt and yuck have accumulated onto the tracks and/or 2) the roller wheel needs replacement. So, after trying to see if it was 1) (by cleaning the track and laying a coat of candle wax on it), I figured it was the roller wheel that was causing the problem. Upon removing the door, I saw that one of the wheels had cracked.
The nice lady at the hardware shop sold me two of the wheels for $1.50 each, and suggested that I should buy two wheels so that I could place THREE (3) wheels on the door to spread the load (i.e. weight of the door) on three wheels instead of two. Yup, sounds like a good idea, I thought (assumption here is that it was the weight of the door that caused the wheel to crack), and duly went about removing the old wheel and housing and fitting in the two new wheels bought (to join the other older wheel that was still OK). One new wheel was fixed to the end of the door and the other was secured in the middle.
This job can cost a little bit if you get the hardware shop to do it, especially if the one that needs replacing has been riveted into position (which mine was) instead of having a screw to secure it (see below):
(to remove the cracked wheel that was riveted down – use a drill to remove the rivet head by drilling it away; the rest of the rivet should be accessible and can be pulled out using round-nose pliers).
By the weekend before Chinese New Year, the sliding door (the one half of it that had been fixed) had three wheels attached, was sliding better than before the wheel replacement, BUT still not very smoothly!!
Applying some mechanics principles to trouble shoot, (and noting the “stress test” of the first two days of relatives using the door), I decided to remove the third wheel (the one in the middle), leaving behind the two wheels at both ends of the door.
(the interested reader may wish to see if he/she can figure out why this solution works; note that all three wheels are running smoothly. Answer in italics at the end).
VOILA!! it really works much better now … 🙂
Lessons learnt: TWO WHEELS GOOD, THREE WHEELS BAD!! … and
1) the advice given by the hardware store people may not actually work
2) you never know when Physics/Mechanics principles learnt decades ago can be very relevant to daily life 🙂
Why two wheels work better than three (most of the time):
imagine that the three wheels are not perfectly aligned on the guiding track, which is very likely the case. Then, the door is actually supported by only two of the wheels. If the two wheels are the ones in the middle and at one end, then there is actually a turning effect when the door is pushed, which makes the door NOT slide smoothly. This turning effect will not be present when there are just two wheels, secured at the two ends of the door.