12 February 2011

Alternator pulley done

One of the important steps was to make the outer circumference of the bearing as concentric (centered) as possible with the inner bore for the bearing. If the two are off, the bearing will run eccentric and hammer on the belt. To ensure concentricity (neat word eh?), I indicated my 4-jaw chuck on the bored hole to within .001", the most accurate I've ever been on my lathe.

Once indicated, I turned the outer circumference just enough to true it up. Then I mounted it on my rotary table -- that's a cool little tool that lets you mount things to it and then as you turn a handle, it turns a fairly precise number of degrees. So if you want six holes in a circle, you drill a hole, spin your rotary table 60° and drill another one, and so forth. This (theoretically) spaces the holes evenly, which is important for balance if your part will be spinning 15,000RPM.

My rotary table is 3" so normally the largest item I can work with is maybe 2", to allow room for clamping. However, since I had already made a hole in the middle of my pulley, I was able to do some creative clamping to just barely fit a 3" pulley on a 3" table. Here I'm drilling pilot holes -- smaller bits drill straighter, and then when I enlarge the hole with the larger bit it will "follow" the existing hole and you end up with a straighter and more accurate hole.

They say a poor workman blames his tools, well, my cheap Chinese drill press really wanders and worse, is way too fast for drilling 1/2" holes in metal. In spite of my care, some of my holes ended up in significantly the wrong place but oh well, we'll see how badly out of balance it is once the car is running. Once the Jetta is done there are some things I can do to the drill press to improve accuracy (just like you have to do on a Chinese lathe).

Then it was time to beat on it:

This was the most critical part of the whole pulley. Bearings are "press fit" which means that the bearing is smashed into a hole that is slightly (.0005" to .001") TOO SMALL for the bearing. This means the aluminum is actually deformed -- stretched -- and the springiness in the metal itself will squeeze on the bearing tightly so it doesn't fall out. In my case, the hole I bored on my lathe was still smaller than the bearing but not by as much as I wanted, so I used some red Loc-tite (essentially, a special form of super glue for metal) to ensure good grip.

...and here it is, installed on the car. Looks nice, even if I do say so myself.

So, the reason I did this was to push tension on the outer edge of the belt. This increases the "wrap" on the alternator pulley to nearly 180° as seen above. The stock setup I replaced pulls tension on the inner edge of the belt, thus reducing the "wrap" on the alternator pulley, which is then more likely to slip and squeal and eat belts.

I hope this works.

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