Sunday, December 21, 2008

Basketball hoop

It's one of those Sunday afternoons when I feel like writing another blog post. Hopefully this one actually happens. So here it goes:

My kid have one of these from as a birthday gift. My wife told me I need to put a more positive tone to my design analysis of products. So in the spirit of Joel Osteen's the-glass-is-half-full approach to live (sarcasm implied), let me spend some time learning about the good things from this design.

The basketball hoop is composed of 5 main parts: the base, the static post, the telescoping post, the backboard, and the hoop. The height is adjustable on the telescoping post with notches that can be locked into position. The backboard is inserted into the telescoping post and snaps into place. The hoop is fixed to the backboard using interference friction fit.

First of all, I like the simplicity of the design. It can be easily manufactured with blow molding. It is modular enough to ship in a box. However, some of the snap-fit is poorly designed/manufactured. Notice in the picture below that the hoop is tilted/bowed down a few degrees. At the edge of the hoop, the deflection is about 3-4 inches. It is significant enough to affect the function of the design, maybe not necessarily negatively though, since it actually makes it easier to score :)

This can simply be improved in the tolerance design of the backboard hole and post assembly. In addition, having the post be inserted deeper into the hole and meet a positive stop inside the backboard will help locate the assembly with better precision. I'm sure there are even better attachment strategy between the backboard and the post that will allow self-datuming on rigid surfaces, but I cannot think of anything practically useful right now. Please comment on this post about your ideas for this.

The locking mechanism to fix the height of the hoop is ingenious design. The locking mechanism is self-datuming and self-tightening because the weight of the hoop and telescoping post actually rotates the locking piece in such a way that it increases the engagement and preload of the lock. Cool idea!

Finally, the base is fill-able with water or sand in order to increase weight and stability of the whole structure. Because the hoop sticks out forward though, I would argue that making the base's center of gravity offset to the rear (not symmetrical) will increase the load capacity of the hoop under usage. This is typical in most basketball hoop structure where weights are attached at that very rear end to compensate players dunking and hanging loads in the front.

Sunday, November 16, 2008

Keyboard tray annoyances

Ladies and gentlemen, finally, my second post is here. This time, I would like to highlight yet another annoyance I have regarding everyday products, namely the keyboard tray from a typical computer desk sold in Staples, Officemax, and the like. The keyboard tray is typically mounted on a linear ball bearing slide. The housing of the linear slide is fixed to the main desk platform with a simple right angle bracket, shown in the first figure.

Now the keyboard tray works just fine if you push it in and pull it out in the middle of the drawer. But if you push/pull at the left or right-most location, it will bind. This happens with my last two computer desk. They all have the same design. This binding is due to a loading condition that is different from the specified function. When you apply the load at the center of the tray, the loading condition is a pure force or translation forward. But when the load is applied on the extreme left or right points on the tray, it creates a translation AND rotation, as illustrated below.

The culprit of the problem is in the brackets that hold the linear slide frame. These brackets are stiff in the direction of the pure forward translation, but not very stiff when subject to side load. The induced rotation causes an amplified side load on the brackets. In fact, the wider the tray, the greater is the multiplication factor of this side load due to rotation of the tray. Because the bracket deforms as the tray rotates, this creates binding in the ball bearings along the linear slide. This is not anticipated by the design. One easy solution is to put gussets on the bracket. Another solution is to have side walls that coincide at the location of the slides so that the slides rest on hefty side walls or main frames of the desk. The pictures below illustrate alternate design that solves this problem inherently in the design without having to reinforce the brackets.

It is very important for a design to be robust, which means that its performance does not decrease as the assumed factors deviate from normal parameters. This parameter includes specifications such as loading conditions. If a simple table is designed to support your desktop items which weigh at most 10 lbs, why do you think it is designed for 300 lbs capacity? That’s right; somebody is going to sit on the table. People use textbooks as a step ladder, and who knows what people do with duct tapes. The point is, we always design things with safety factors as a margin. But safety factor is often specified for one particular loading condition. Here, the performance drops immediately to the point of non-functional as the assumption (tray is pushed/pulled at the center location) is violated. This is an important design principle – robustness.

Saturday, September 13, 2008

Plastic bottle cap design

Here is my first real post in this blog. Let's pick a typical plastic bottle cap that most of us interact with everyday from soda or mineral water. What we have here is three different bottle caps. The left one I found on the parking lot, probably from aquafina, the middle one is from ice mountain bottled water I drink everyday in the office, and the right one is from the coke I bought for the sake of writing this blog :)

The first thing I would like to comment on is the sealing design from the left and middle ones. To seal the bottle, they used a short 'tongue', probably about 1/8" inch high that mates into the bottle mouthpiece from the inside. The coke bottle cap accomplish this by coating the mating surface of the cap with a softer polymer. It has the feel of the rubberized coating popular in cell phone housing these days. I think using the 'tongue' to seal the bottle is a clever idea. It eliminates the need for coating the cap with a softer material (which eliminates a whole set of machines in your assembly line). As a consequence, the material used in the left and middle caps in the picture is also softer. This is acceptable since they are probably cheaper as well.

In addition, the tongue seal is more robust with respect to the variation in the cap preload. The surface seal style in the coke bottle cap relies heavily on preloading the softer material against the mouthpiece in the vertical direction. The tongue seal delivers equal performance as well as the tongue is engaged to the mouthpiece in the radial direction regardless of the preload from the cap tightening torque. The sealing direction are indicated in the next picture. This might not be an issue from manufacturing perspective because the assembly station will ensure a certain tightening load. But as a consumer, this means that you need to make sure you tighthen the coke bottle all the way while for the mineral water bbttle a snug twist is adequate.

Is it possible that the sealing capabilities of the coated cap (from coke) has better sealing performance, and hence used in carbonated beverages? Possibly. But I would argue that the tongue seal is capable of high pressures too. This is because the hydrostatic pressure due to the soda will exert force on the tongue against the bottle mouthpiece from the inside, and thereby increasing the seal as opposed to weakening it.

Plastic bottles is typically made with Polyethylene Terephthalate (PET) while its caps are made from high density polyethylene (HDPE). You can notice this by the resin identification code.

The second thing I always wonder is the broken threads. I have few guesses on why they do not have continuous threads around the cap, but I'm not sure. I think it has to do with the molding process. I will leave this up to the readers to comment on.

Friday, September 5, 2008

A change in plans

I have my doubts on blogging. Since writing is not an exciting task for me, if this blog is going to fly, then it has to be something compelling for me to write on. Now, I decided that this blog should not only be all things mechanical, but it also should be something that revolves around design. I would not consider myself an anal person in general, but there are exceptions. One of them is when I see something that does not work as it should, especially in mechanical design of products I buy. At the same time, I am regularly fascinated by elegant designs that are simple, out of the box, and of course, work as it should. So this is what this blog will be about: Reviews of annoying/flawed and ingenious product designs, mostly from a mechanical engineering perspective. 

Thursday, August 14, 2008

How to retrofit your car with R134a

The proper way to retrofit your car with R134a is to first evacuate the old R12 refrigerant, because it uses a different oil and additives from R134a. If you mix them, it might cause problems with your system, the compressor in particular. If you a cheap like me, however, and own a $500 car, it’s not worth it to pay $200 for a shop to retrofit your car with R134a. You can buy a retrofit kit, which comes with the valve adapter (pic attached) and few cans of R134a. All you have to do is screw the adapter onto your low and high pressure line of the system and it is ready to use with any R134a refill kits out there (which by the way, is legally available for anyone to buy). Do this at your own risk though. My Nissan 240sx is retrofitted without proper flushing of the system and so far it has survived 1.5 years (knock on wood).

A/C Refrigerant: R12 (Freon) vs. R134a

Today, R12 (Freon) is no longer used because it damages ozone. Not only is R12 banned for sale in the US, but also to evacuate the refrigerant from your system, you have to be a licensed car repair shop. They want to make sure you have the proper equipment to contain the old refrigerant. Cars built in the USA after 1993 uses R134a as the A/C refrigerant. You can notice cars using R12 by noticing the valve on the low and high side of the refrigerant line. They uses valves similar to the ones you have on your wheels/tires.

Wednesday, August 13, 2008

All things mechanical?

This is my first blog post. I am a person fascinated with mechanical things of all sort, hence my hobby to take things apart, screw it up in the process, and try to fix them for no obvious reasons. I also love working on my car.

So what will I post here? I have no idea, but my wife told me it might be ... "all things mechanical"