Ryland3210 Posted January 31, 2008 Share Posted January 31, 2008 It may be the case in very highly tuned race motors but think about the the practicality of machining an *accurate* ellipse into a rod? yes, rods will stretch and deform, generally though unless they are taken beyond their design parameters the amount of deformation will of been taken into account in the design and manufacture of the rod. Remember, we are talking about 'Production' vehicles here. Not some incredibly expensive, 'Hand Tooled' perfectionists device. It has to be easily repairable by people with access to *ordinary* tooling. But I do imagine that you are correct in the fact that the clearances used in your average rod WILL take into account deformation of the eyes. Hopefully to a point where interference with the bearing's ability to wedge won't be effected. Pete FWIW, elliptical hydraulic cylinders have been in production at least for 5 years that I know of. Why on earth, you might ask, would anyone want to deal with the challenges in involved? In the particular application, there was limited space in one direction, but a need to increase the cross-sectional area of the cylinder. Cheers, John Link to comment Share on other sites More sharing options...
pete roper Posted January 31, 2008 Author Share Posted January 31, 2008 OK, moving right along..... Why are bearings and their journals usually made of dis-similar materials? Most of the 'Mechanics' here will know the answer but lets leave it up to the others who have just discovered the stuff above to work it out. NB. It's not hard. There is a very good reason, just think about the engine holistically and then consider it's expected life span. Don't be afraid of saying something silly. You'll probably be right Pete Link to comment Share on other sites More sharing options...
mike wilson Posted January 31, 2008 Share Posted January 31, 2008 It may be the case in very highly tuned race motors but think about the the practicality of machining an *accurate* ellipse into a rod? Not too difficult when you move away from one-off, "back room" work. After all, pistons are not circular in plan nor do they have straight sides. Only by a few thou' but they are critical dimensions for maximum durability. It is the modern capability for mass accurate machining, in many ways, that has brought us to the reliability point we are at now. My car is up to 160,000 miles, doesn't burn oil, has never had the head off and still does 50mpg. A couple of generations ago, that would have been very much the exception. Link to comment Share on other sites More sharing options...
raz Posted February 3, 2008 Share Posted February 3, 2008 ... but seeing as nobody seemed to want to find out more about bearings I'm quite happy to leave it if it is deemed uninteresting or irrelevant. Pete, what makes you think we're not interested? Are you waiting for us to take a shot on that question? If I got it right, the bearing is very soft compared to the journal and that means hostile particles can (to an extent) embed in the cheap bearing instead of ruining the crank. One particular thing I don't get is how the little end bearing gets its oil. Please continue! Link to comment Share on other sites More sharing options...
pete roper Posted February 3, 2008 Author Share Posted February 3, 2008 Pete, what makes you think we're not interested? Are you waiting for us to take a shot on that question? If I got it right, the bearing is very soft compared to the journal and that means hostile particles can (to an extent) embed in the cheap bearing instead of ruining the crank. One particular thing I don't get is how the little end bearing gets its oil. Please continue! Yup. Said it was simple. The shell bearings, (Or the mains!) Are sacrificial. They could run the rod direct on the crank, but when something went tits you'd have to buy a new crank and rods, (Greg will probably weigh in here with a carpet-chewing rant about how Guzzi still replace crank and rods after a big end failure simply because the parts system says this is the way to do it ). As for the little ends? Amazingly these survive fed by splash! As has recently been mentioned in another thread though the whole inside of the case, including the bores and little ends, are in a constant fog of oil droplets. The movement in the gudgeon pin is also not rotational so the forces acting upon it are different but it's one of the reasons you run VERY close tollerances on little ends. pete Link to comment Share on other sites More sharing options...
dlaing Posted February 3, 2008 Share Posted February 3, 2008 Please continue! Yes, Please! People might not reply, but we are reading, learning and enjoying! Here is a related post by Pete about the lube system http://www.v11lemans.com/forums/index.php?...amp;#entry95471 Link to comment Share on other sites More sharing options...
pete roper Posted February 4, 2008 Author Share Posted February 4, 2008 Good-Oh. I like this 'teaching' thing, its fun. So what do people want to go for? Should i do a bit on piston rings? They're great fun! Everyone thinks they're simple Rings are the work of the devil and probably cause more fuss and bother than just about anything else! Pete Link to comment Share on other sites More sharing options...
raz Posted February 4, 2008 Share Posted February 4, 2008 OK, then one thing I have had in mind is the piston rings not being keyed into position. Are they rotating slightly all the time? If so I guess there is reason to have them do that. The 50cc two-strokes I've seen has had a key so the rings were fixed. I would have thought they could brake in to better seal if they were not rotating. So, is this a 2-stroke vs 4-stroke thing or are there keys in some big 4-strokes too? Maybe this should go to a fresh thread so people can find it. Thanks! Link to comment Share on other sites More sharing options...
mike wilson Posted February 4, 2008 Share Posted February 4, 2008 Good-Oh. I like this 'teaching' thing, its fun. So what do people want to go for? Should i do a bit on piston rings? They're great fun! Everyone thinks they're simple Rings are the work of the devil and probably cause more fuss and bother than just about anything else! Pete Something on clearances wrt lubrication might work well at this point. Link to comment Share on other sites More sharing options...
mike wilson Posted February 4, 2008 Share Posted February 4, 2008 OK, then one thing I have had in mind is the piston rings not being keyed into position. Are they rotating slightly all the time? If so I guess there is reason to have them do that. The 50cc two-strokes I've seen has had a key so the rings were fixed. I would have thought they could brake in to better seal if they were not rotating. So, is this a 2-stroke vs 4-stroke thing or are there keys in some big 4-strokes too? Maybe this should go to a fresh thread so people can find it. Thanks! Non-keyed rings have the _ability_ to float but they don't necessarily do so. It's my _impression_ that a lot depends on how the individual tolerances match up on a piston/barrel combination, as to whether this happens. Certainly, ones I have seen that have had significant damage (not necessarily siezure but noticeable scratches in the barrel) seem to "key" themselves into one orientation. Keyed rings (on two strokes) don't rotate but they are keyed only to prevent the end of a ring finding itself floating past a port, springing out of its groove and then being trapped by the edge of the port, bringing everything to a rapid stop or causing severe discomfort to the piston and barrel. Link to comment Share on other sites More sharing options...
Guest ratchethack Posted February 4, 2008 Share Posted February 4, 2008 Should i do a bit on piston rings? They're great fun! Everyone thinks they're simple Rings are the work of the devil and probably cause more fuss and bother than just about anything else! Yes, please. Link to comment Share on other sites More sharing options...
raz Posted February 4, 2008 Share Posted February 4, 2008 Keyed rings (on two strokes) don't rotate but they are keyed only to prevent the end of a ring finding itself floating past a port, springing out of its groove and then being trapped by the edge of the port, bringing everything to a rapid stop or causing severe discomfort to the piston and barrel. Man, that was really obvious. Note to self: use brain. Link to comment Share on other sites More sharing options...
mike wilson Posted February 5, 2008 Share Posted February 5, 2008 Man, that was really obvious. Note to self: use brain. It's the obvious thiings that trip us up really badly, usually. 8-) WRT your point about breaking in - yes, if all rings were keyed they would probably break in quicker. But they would also wear out quicker, too. As they rotate, wear is spread around the ring. This is a _good thing_ . Link to comment Share on other sites More sharing options...
raz Posted February 5, 2008 Share Posted February 5, 2008 WRT your point about breaking in - yes, if all rings were keyed they would probably break in quicker. But they would also wear out quicker, too. As they rotate, wear is spread around the ring. This is a _good thing_ . I was not thinking quicker, but better. Like the rings are not 100 % symmetrical nor is the cylinder so a keyed ring could break in to an exact match giving a better seal than a floating ring could ever reach. But I guess there are more things consider. Not to mention I may be completely wrong here Link to comment Share on other sites More sharing options...
mike wilson Posted February 7, 2008 Share Posted February 7, 2008 I was not thinking quicker, but better. Like the rings are not 100 % symmetrical nor is the cylinder so a keyed ring could break in to an exact match giving a better seal than a floating ring could ever reach. But I guess there are more things consider. Not to mention I may be completely wrong here If you were right, I expect we would see many more machines with pegged rings. Link to comment Share on other sites More sharing options...
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