sunbeam.20 Posted June 6, 2012 Share Posted June 6, 2012 Having acquired a DJH 7F kit with a rigid chassis partially completed to P4, very poorly, and with no coupling rods, I have been buying parts to build my own chassis from scratch. The design for this entails flat brass bar cut to accommodate square hornblocks and coil spring compensation along the lines used to build American brass locos, a NWSL gearbox that will be mounted on the drive axle with carden shaft drive. The original coupling rods according to the instructions are for straight eight coupled drive rod per side. My questions/thinking is that if the axles are all compensated, this straight coupling rod would not allow wheel movement up and down, and that three rods creating a multiple two coupled system would be required. Is this correct? The next question is what is the location of the pivot points in the coupling rod? Is the crankpin the obvious choice, with a half lap type of joint at two crankpins, as this appears the easiest to me? If not, how is this point calculated? Another query I have is if a wheel moves up or down on one side only, given that the hornblock will stay in the same relationship with the axle, how does one overcome the fact that the slots in the hornblock are no longer parallel to the chassis side frame as it now trying to pivot about its opposite chassis side? Is this accomplished with slop in the hornblock groove, and I expect there is only a mm or two of travel, if that. The easy answer may be chassis that is already available, eg Comet, but I wish to try to build my own as I never do anything the easy way, and I also have the remains of a Tenshodo 2-8-0 that also has no chassis and there would certainly be no kit for this. Matthew DJH 7F Chassis.pdf Link to post Share on other sites More sharing options...
John_Hughes Posted June 7, 2012 Share Posted June 7, 2012 My questions/thinking is that if the axles are all compensated, this straight coupling rod would not allow wheel movement up and down, and that three rods creating a multiple two coupled system would be required. Is this correct? Officially Yes, though I've seen models where the rods have been kept rigid as the total vertical movement is actually very slight (or ought to be!) The next question is what is the location of the pivot points in the coupling rod? Is the crankpin the obvious choice, with a half lap type of joint at two crankpins, as this appears the easiest to me? If not, how is this point calculated? Do whatever the prototype does; proper knuckle joints aren't particularly difficult to do - though I avoided them for years because I thought they'd be tricky - but of course you'll need extra sets of rods. If possible drive one of the centre two axles to avoid too much lost motion. Another query I have is if a wheel moves up or down on one side only, given that the hornblock will stay in the same relationship with the axle, how does one overcome the fact that the slots in the hornblock are no longer parallel to the chassis side frame as it now trying to pivot about its opposite chassis side? Is this accomplished with slop in the hornblock groove, and I expect there is only a mm or two of travel, if that. That's right. My own preference for the frames is for thinnish n/s - no more than about 20 thou max and preferably less, which is nice and easy to work with and solders like a charm - and recently I've been using Gibson horn-blocks aligned with the Hobby Holidays jig to match the rod settings, though I know some people can't get on with those horn-blocks. John Link to post Share on other sites More sharing options...
sunbeam.20 Posted June 8, 2012 Author Share Posted June 8, 2012 Thanks for the reply John. It looks like a little experimentation may be called for. It is my hope to progress through the construction here, after I have finished a couple of ongoing projects. Matthew Link to post Share on other sites More sharing options...
John_Hughes Posted June 8, 2012 Share Posted June 8, 2012 CLAG has an extensive section on compensation on rod-driven loco's: http://www.clag.org.uk/beam-menu.html I spent a long time at the EMGS show last year looking that these systems, and they do look neat (and apparently effective) but fairly complex; I'm not sure I would feel willing to home-brew one, though I'd certainly give it a go if it came in a kit I was building. Link to post Share on other sites More sharing options...
sunbeam.20 Posted June 8, 2012 Author Share Posted June 8, 2012 I have had a browse through the clag site, and believe I shall try the CSB system, as this could be set up as a way to retain the hornblocks with an eye bolt in the hornblocks with the beam through each and through similar in the chassis. If there is information about coupling rods on the site I must have missed it. Matthew Link to post Share on other sites More sharing options...
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