Katier Posted January 20, 2012 Share Posted January 20, 2012 My J72 (3mm/ft - 3SMR kit) has been improving but I'm still having problems, mainly associated with it running ok in one direction but not the other. The chassis (scratch built) is driven by the rear pair of drivers and the notchiness/stalling I'd noticed seems to be caused by the lead wheels 'lagging' behind the rears slightly.. so sometimes they cause the wheels to 'lock'. The reason I hadn't picked it up is when testing by hand I was either pushing it along the track ( so the wheels act as a connecting rod too removing the notchiness) or using the middle wheels, where the effect seems to basically disappear. I guess I made a mistake opening up the connecting rods slightly too much, but am thinking the easiest solution is to re-engineer the gearbox to drive the middle axle. thoughts? Link to post Share on other sites More sharing options...
gordon s Posted January 20, 2012 Share Posted January 20, 2012 Let me preface this by saying that I am not an expert on chassis issues, but it sounds like the wheel quartering may be slightly out on one pair, or even one driving wheel that needs to be rotated slightly on the axle to make the distance between the rod centres equidistant. No doubt others will be able to brief you on the exact process to bring them in line, but I would look very closely at each wheel and the position of the spokes and the crankpin itself within the hole in the coupling rod. It can be done by trial and error once you can see where the problem is and which wheel is out of alignment or you can purchase a wheel quartering jig to assist you. Link to post Share on other sites More sharing options...
Katier Posted January 20, 2012 Author Share Posted January 20, 2012 Shouldn't be a quartering issue as they are square ended/SQ axles. Link to post Share on other sites More sharing options...
gordon s Posted January 20, 2012 Share Posted January 20, 2012 Ah, I'll get my coat.... Link to post Share on other sites More sharing options...
Paul Cram Posted January 20, 2012 Share Posted January 20, 2012 Can you bush the rod where you think you have opened it out too much? Link to post Share on other sites More sharing options...
sixteen 12by 10s Posted January 20, 2012 Share Posted January 20, 2012 Are the axles square to the frames, and the centers of the axle bearings and rod centers the same? Gary Link to post Share on other sites More sharing options...
Katier Posted January 20, 2012 Author Share Posted January 20, 2012 Are the axles square to the frames, and the centers of the axle bearings and rod centers the same? Gary yep, yep and yep. Link to post Share on other sites More sharing options...
RMweb Gold The Stationmaster Posted January 20, 2012 RMweb Gold Share Posted January 20, 2012 Are the rods flexing at all? Link to post Share on other sites More sharing options...
Katier Posted January 20, 2012 Author Share Posted January 20, 2012 Are the rods flexing at all? Nope but there is 'play' in the bearing which makes me thing that's what is the problem. I'm debating scrapping the existing ones as I think I over-did the holes and hand 'crafting' some fresh ones Link to post Share on other sites More sharing options...
billbedford Posted January 21, 2012 Share Posted January 21, 2012 When you take the rods off check that the crank pins are square to the wheel faces Link to post Share on other sites More sharing options...
Poor Old Bruce Posted January 21, 2012 Share Posted January 21, 2012 Sorry to be pedantic but in order for future generations to be able to understand the correct technical term used for these particular bits of gubbins, they are actually 'coupling' rods. I hope you will take this as constructive rather than nit-picking. I only looked at the thread to find out why anybody would want to lag connecting rods (as you would a hot water tank perhaps) but that's me misreading the (possibly confusing) title. I'll get me coat - and tin 'at. Link to post Share on other sites More sharing options...
buffalo Posted January 21, 2012 Share Posted January 21, 2012 Don't worry about it Bruce, just think of it as a public service, saving others from the pedant of the day award. Mind you, when I saw the title I thought it was something to do with crankpins. After all, if one can be said to lead, then the other must lag. Talking of which, I'd second Bill's suggestion to check that the pins are square. It can make a much greater difference than you might expect. Nick Link to post Share on other sites More sharing options...
Poor Old Bruce Posted January 21, 2012 Share Posted January 21, 2012 Don't worry about it Bruce, just think of it as a public service, saving others from the pedant of the day award. It's a tough job but somebody's got to do it Link to post Share on other sites More sharing options...
Katier Posted January 21, 2012 Author Share Posted January 21, 2012 Pins seem square.. I've taken some advice (apparantly given on kit instructions in the past) from the 3mm Society and filled in the holes with solder then re-drilled. I'm 100% happy with the chassis as the wheels spin freely and everything measures up so it's definitely either a coupling rod ( @ Bruce) issue or the pins.. The pins look square and re-drilling the holes and using the oft used advice of 0-4-0 first I think I've cracked it - but won't count my chickens yet!! When one side is horizontal (which is the point they lock) the opposing crack pins should be vertical.. only they aren't... In short the 'quartering' issue mentioned earlier in the thread seems to be correct only it's more subtle and caused by the holes in the Coupling rods to be VERY slightly out. I'm hoping ovalising that hole slightly will cure it (although they are already running much better even with that apparant issue). Link to post Share on other sites More sharing options...
billbedford Posted January 21, 2012 Share Posted January 21, 2012 Don't do that. Take the wheels off the axle and remove any burrs on either the axle end or the wheel and then reassemble them. Link to post Share on other sites More sharing options...
Katier Posted January 21, 2012 Author Share Posted January 21, 2012 Replaced the wheels ( the old ones had been on and off that many times they weren't exactly in the best of health ) and she's no better. The new wheels have no burrs of any type on them and definitely sit properly on the axles ( the old ones were a tad wobbly). They still lock slightly when pushing the driven wheel round at a point where the one rod is fully low and the other at the mid point. My analysis is that the non-powered axle doesn't always quite pass the 'halfway' point so tries to 'reverse' but I'm not 100% sure what's causing it. SQ is 100% Chassis I'm sure is fine. coupling rods are the bit I'm not certain about ( they are a bit ropey ) - bit also not sure how to fix them if indeed they are the problem. Wheels I'm happy with. Link to post Share on other sites More sharing options...
John_Hughes Posted January 22, 2012 Share Posted January 22, 2012 In my own experience the only way to get the coupling rods right is by using jigs - and using them intelligently! I generally use the Hobby Holidays gadget to set the axles dead 'congruent' with the rods, but I also then check and recheck them with some old Percy axle jigs, which fit into the axleboxes and are turned down at the ends to fit the holes in the rods; any error here and the loco just won't run smoothly. And any fore-and-aft slop in the axleboxes is similarly a frequent cause of problems. Don Boreham's advice is apt here, though very hard to take - if it doesn't work perfectly, scrap it and start again as botching will never fix it. Wasn't it Dugald Drummond who, in his single-driver phase, compared a coupled locomotive to 'a wee lad running wi' his breeks doon'? Link to post Share on other sites More sharing options...
billbedford Posted January 22, 2012 Share Posted January 22, 2012 To check the quartering:- Have a short piece of track on a board Hold it at eye level, so that you are looking down the track. Run your loco back and forth along the track and watch the crank pins. If one of the crank pins on either side rises or falls before the others then the quartering is out, otherwise it's you rods. Link to post Share on other sites More sharing options...
Katier Posted January 22, 2012 Author Share Posted January 22, 2012 It would appear she's fixed.. had to 'ovalise' the center holes slightly and she's now running pretty well in both directions and down to VERY slow speeds. Had her cover 6 inches without any trouble at a scale 3mph, and in fact in reverse goes as slow as 1.6mph. Link to post Share on other sites More sharing options...
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