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Captain Obvious

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Everything posted by Captain Obvious

  1. I considered something like the lathe, but the problem is that even though you can chuck something up and adjust it to be perfectly centered (within the limits of your measuring equipment), that centering is still only at one point along it's length. The jaws are never good enough that you can assume the shaft sticking out of the chuck is perfectly normal to the chuck jaws. The optical method you described crossed my mind as well, but I don't know if the gap would be large enough to see. I've got a granite block designed for such things, but don't know if I would be able to see a quarter thousandth gap even with a strong light. And a quarter thousandth off from center at the journal could translate to more than that at the edge of the outer bearing race. Back when they made the stub shaft, they turned it "between centers" (That's the two little holes in the ends.) To produce the shaft, they mounted the raw stock between two points that were located in those holes. But the problem now is that those holes are all rusty and worn and I doubt you can trust the accuracy anymore to the level required to check the journals for runout. But if you could, that's probably the right way to do it. Fixture the whole thing up on the granite block... Mount the stub between centers and indicate each journal. I'm not a machinist and I'm sure someone working in a metrology lab would be able to do this in a snap.
  2. Woof. I feel for ya. Do the math. Get the grades. Collect the paycheck. In theory, it'll be worth it in the end.
  3. If the control loop is working properly, then theta should be very small. Problem is to make the control loop work properly. Actually for the second section, if you are allowed to use the feedback pot on the pendulum shaft, "theta" becomes a "concept". You just need volts: "The pendulum angle is measured with a single-turn potentiometer, with ±10V output corresponding to ±160˚ of rotation" That means you've got +10 V when the pendulum is almost horizontal in one direction and -10V when it's almost horizontal in the other. You've got +/-16 degrees per volt or (the inverse of that) 125 mV per degree around vertical. You're striving to keep the pendulum pot output at 0.00000 volts. And every 125 millivolts you are away from that is one degree off vertical and the sign of the voltage will tell you which way you are off.
  4. Cool. It's money, but those bearings aren't the kind of thing you want to do any more times than necessary. Glad to help and good luck with the rest of it. Keep us posted as to how it all turns out. I'm having a hard time picturing a good way to measure things to look for problems (like a bend) with a stub axle. You need good reference points and I'm not sure what you would use and how you would fixture it. The only precision surfaces on the axle are the bearing journals, but how would you detect a minute bend in the shaft between the two? What you're really looking for is that the two bearing journal circles are not on the same axis center anymore. How would you fixture to detect that?
  5. Wow... That's no fun. At all. Due in two days, huh? I'm assuming this is a final project for a control theory class? I can't give you any hard answers, but might be able to help a little bit in some areas. First thing to do would be to define the project... It appears to me that there are two major approaches that should be applied to solve the problem: a) First approach (the "classical control method") in which the single output of the system should be the cart position (NOT the pendulum angular position). So the only thing you have is the force required to move the cart. If the pendulum is not vertical, the force required to move the cart in the direction of the (off center) pendulum will be greater than if the pendulum were perfectly vertical (in equilibrium). And the force required to move the cart AWAY from the (off center) pendulum will be LESS than equilibrium. So it sounds to me that the "classical control method" would be to rock the position of the cart back and forth (sinusoidal) and measure the force required to move it. That force should be able to be derived by the current necessary to rotate the motor. That's where the PD lead-compensator stuff comes in... You should be able to keep the sine wave constant and as small as possible. And there is some doubt about the success of this approach as laid out in the original problem. You are supposed to program it all up this way and see if you can make it work. And if not, explain why not. For example, the control loop may not be stable and you may find that the sine wave required is increasing in amplitude until you run out of track length. I suspect this approach is not very robust and resistant to outside applied interference (the bump nudge described in the problem). b) The second approach allows the student to utilize the position feedback on the pendulum and it becomes a more direct control loop. Move the cart in the direction the feedback pot tells you to and strive to keep the output of the feedback pot at 0V. That should be much faster and more accurate feedback than using the cart force and should be able to do a much better job of compensating for externally applied interference. (Proof is left to the student.) If I understood the project correctly, then I would break it down into pieces: Write some equations for the force required to move the cart independent of the pendulum. Write some equations for the force on the cart due to the position of the pendulum (this is the part that Blue started working on above). Then combine the two. Write some equations for the relationship between the rotation of the shaft and the linear position on the cart. Write some equations for the electrical energy required to rotate the shaft. Etc... I'm so glad I graduated!!
  6. Well that's pretty definitive about the stub shaft. Hopefully the replacements from ebay will solve the problem! That would be a really easy solution. A lot easier than pulling everything back apart and starting completely over. Unless you have a neat way of pulling the outboard bearing off the stub shaft without putting pressure on the balls, you might consider using yet another new outboard bearing on the new replacement ebay stub. Depends on how you feel about reusing the one you already pressed on. And all this talk about wheel bearings and the resurrection of this thread reminds me... We never heard back from @Jason240z about the final outcome with his car. I believe his car is back on the road (so his issues have been resolved), but I don't think the loop was ever closed about what the problem was. Hopefully he can fill us in?
  7. Yes, the vacuum source is ported. There should be no vacuum at idle, and then slightly above idle (think light cruise), the vacuum peaks. From there, the vacuum will drop off above that point down to zero at WOT. So it never "retards"... It just provides no vacuum at idle, so the timing just reverts back to the base setting And about the temp sensor... Unless you are in Antarctica, I don't think there should ever be a situation where your engine would be cold enough that it would need the sensor to be completely disconnected to run right. Even with constant cool water from a hose, that temp sensor will be getting way warm enough to be providing meaningful input to the ECU. If you need to disconnect it to get the engine to run, then I think there's a problem somewhere in the system.
  8. Yeah, that's not cool. If that shaft is straight, there's no way you should be seeing that. Maybe out at the edge of a big rotor or something, but no way you should be seeing that at the axle without the mechanical amplification of big disk. Are you seeing that on both axles? One of them turned out OK, right?
  9. I understand the ASCII art perfectly. And if the seating surfaces are not square to eachother, there will be big problems. There will unquestionably be dimensional changes caused by the welding operations and my expectation would be that T3 would have started with raw materials and designed the housings such that after welding, there would be extra material that would have to be machined off to square up the surfaces and make them precise. Above, you mentioned looking for a thousandth or two, but when it comes to bearing placement, you're actually looking for the tenths of thousandths. A thousandth or two is usually way way way too much. In fact, a thousandth or two would make the difference between 200T required to press a bearing in vs. one that drops in free with just it's own weight. If those bearing bores are out of round and/or not coplanar to within a fraction of a thousandth, then I think you ought to put in a call to T3.
  10. So when the temp sensor input is actually making it to the ECU, how does it run? How do the plugs look? I'm a little concerned about the pic you posted of the insides of the AFM. Since you have a good report with the PO's mechanic, can you (did you) ask him if he adjusted the AFM to try to lean things out?
  11. Excellent!! So maybe all of the fuel pressure stuff was a red herring caused by questionable test equipment and procedures? In any event it reinforces the belief that it just looks like most other old 280Z's that are running either really rich or really lean because of a whole bunch of interrelated issues like dirty connectors, improperly placed connectors, and vacuum leaks. So you found the first, highest priority of those, but there are probably others of lower contribution and impact. The remaining issues are probably what the previous owner's mechanic was chasing to get it to pass smog.
  12. Haha!! No, they do not! So... Now it's been three days since this post was started, and the OP hasn't ever signed back into the forum since. Brand new user, all hot to trot about purchasing a car (or two!!) and hasn't been back to discuss all the input from the fine members here. Maybe he's totally consumed with a huge Easter party prep and just too busy to come back, but I just get a bad vibe from the whole thing. @Mike, I'm thinking maybe give it another day and then delete the whole thread (and user) if nothing changes? Been a while, but wasn't there a rash of spam bot users that signed up a while ago? Year ago maybe? And all of their user names were the same format... Name followed by a couple numbers IIRC?
  13. Glad to help. Before you rip everything completely apart and start over from scratch, I like your idea about moving the stub axle from the other side (that spins ok) and seeing what happens when you install that stub axle in the problematic housing. I agree that could be a good clue if the problem follows the housing, or the stub axle. In any event, I hope you get to the bottom of it.
  14. Been another half day and still nothing from the OP. Wonder how many additional hits have his links seen since he posted this?
  15. Couple of thoughts. First off, yes... if the distance piece is too short, it will completely bind the bearings when tightened. So getting accurate measurements of all the parts in question would be the best thing to do. As Zed Head mentioned, I modified a pair of (accurate, but) disposable calipers to fit down into the bearing recess and measure the distances between the bearing seating surfaces to make sure everything was within spec. And on one of the cars I did, I found that one of the housings was narrower than the other and one of the distance pieces was slightly shorter than the other. So even though all of the parts were "within spec", I mixed and matched parts and used the shorter distance piece in the narrower hub just because I knew. Pics of the modified calipers are in one of those other threads. Next thought is that one thing I have seen cause problems are raised burrs on the hub machined surfaces caused by pressing a bearing in while not completely square and raising a bump. After that happened, the bump caused some misalignment in the bearings and resulted in rotational binding like you described. So, question would be... Did you have any troubles pressing the bearings in or did they go in smooth, even, and easy from the start? Or did they go in cocked to one side and eventually snap into alignment. Lastly, while a surface grinder *CAN* be a very accurate device for removing material, if someone had messed with the distance piece in the past and screwed up the squareness of the end cuts, slapping it down onto the mag chuck of the grinder and removing a little material won't fix the problem. It will just propagate the error to the new length.
  16. Have you got some pics of what you're working with? And I'm not sure what you mean when you say "they are using the "B" series housing as a baseline." Are you doing the work yourself, or is someone doing it for you?
  17. Dig around on the web for input on this. Engravers have been making indexing fixtures for this sort of thing forever. With a little creativity, I'm sure you can come up with a way to index either the stamp or the target less than the width of the punch shank.
  18. Actually I don't like all the links in the OP's post. Links to a whole bunch of auction sites and pics nabbed from copart. Just joined. One and only one post. And hasn't even responded to all the input from other members about his original post. I'd like the OP to participate in the discussion he started and convince us that he isn't a spammer or a bot.
  19. Haha!! I had to look up the Coffman reference, but it reminded me that it's fiddler crab season!
  20. If the battery is tip-top charged, it should be able to run the EFI and ignition for quite some time even without an alternator.
  21. The what? Did you say computer???
  22. What he said. You might not have to replace ANY injectors. Have you verified that the issue is inside the injector itself and not something else?
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