Quarryscapes Posted April 11, 2014 Share Posted April 11, 2014 This is a problem affecting both my Comet 0-6-0 chassis (Dean & Collett Goods). When built rigid the middle axle is not in contact with the rails. Just trying to think of a good solution to this problem - I could drop the bearings out and file the hole into a slot but I'm worried I'd end up putting the axle out of square. I could make that axle sprung using comet hornblocks - but would the spring rate be too high given as it'd be the only sprung axle thus creating the opposite effect of the chassis rocking about the centre? You can just see the gap under the centre drivers in this shot: How have you overcome this issue? Link to post Share on other sites More sharing options...
RMweb Gold Worsdell forever Posted April 11, 2014 RMweb Gold Share Posted April 11, 2014 A lot of chassis are designed like that, it's to stop the loco rocking on uneven track, after all with a rigid chassis there will only ever be three wheels in contact with the rails at any one time. Link to post Share on other sites More sharing options...
RMweb Gold RedgateModels Posted April 11, 2014 RMweb Gold Share Posted April 11, 2014 Agreed, I'm sure Geoff will confirm this, but that's how Comet design their chassis and why Link to post Share on other sites More sharing options...
buffalo Posted April 11, 2014 Share Posted April 11, 2014 As Paul says, it's probably intentional. The downside is that you're relying on flange contact on the central wheels for electrical pickup. Springing the centre axle would be an improvement and you should be fine with the usual springs that go with the hornblocks. In the unlikely event that the springs are too strong, just add a bit more weight in the boiler. Nick Link to post Share on other sites More sharing options...
Quarryscapes Posted April 11, 2014 Author Share Posted April 11, 2014 The thing that puts me off sprung hornblocks is Comet's instruction that the hornways will need filing. Is this a given or am I likely to find the hornblocks work as is? I had thought about going fully sprung on the Dean chassis but I really do not want to be filing hornguides. Link to post Share on other sites More sharing options...
buffalo Posted April 11, 2014 Share Posted April 11, 2014 I don't know the Dean and Collett chassis, but the ones I've dealt with have the cutout piece attached at top and bottom only. These are certainly not critical as far as filing goes. All that might be necessary on the sides is a gentle dressing with a fine file just to remove any cusp on the etch. They'll only need a few strokes, not enough filing to get seriously out of shape. Nick Link to post Share on other sites More sharing options...
Horsetan Posted April 11, 2014 Share Posted April 11, 2014 The thing that puts me off sprung hornblocks is Comet's instruction that the hornways will need filing. Is this a given or am I likely to find the hornblocks work as is? I had thought about going fully sprung on the Dean chassis but I really do not want to be filing hornguides. I think that if you use the hornblock bearings and springs supplied by Comet (actually the same as those that you'll find in Brassmasters loco kits, and also compatible with a good many SEF kits - especially those designed by the late Alistair Rolfe), little or no filing will be needed....and because the bearings are asymmetrical, you can flip them around if you need more or less sideplay. Link to post Share on other sites More sharing options...
Quarryscapes Posted April 11, 2014 Author Share Posted April 11, 2014 Thanks chaps, I'll bite the bullet and order some hornblocks then see how it goes! Link to post Share on other sites More sharing options...
Miss Prism Posted April 11, 2014 Share Posted April 11, 2014 When built rigid the middle axle is not in contact with the rails. Just trying to think of a good solution to this problem I think you should ask yourself how much of a 'problem' this really is. If it stays on the track, I wonder whether trying to put in a sprung replacement hornblock would be worth the hassle. Unless carefully set (height), the strong Brassmasters spring could be inappropriate for that weight of loco. Link to post Share on other sites More sharing options...
buffalo Posted April 11, 2014 Share Posted April 11, 2014 Good point, Miss P. Although I said earlier that I didn't know this chassis, I've since found a fully sprung version of the Collett Goods in a drawer. I'd replaced it with a High Level chassis with CSB when I changed from EM to P4 several years ago, but still haven't finished working on the body... Now, I haven't set up my usual spring test rig, just some quick and dirty measurements by hand. It takes around 45g to compress a single spring over the full 1mm of travel. However, about half of this is preload on the spring, suggesting that it is too long. With only the middle axle sprung, I'm guessing that at normal ride height it would need around 25-30g on each wheel just to keep the outer wheels on the track. So, the body weight would need to be around 200g to prevent the spring from lifting the outer wheels over a slight bump, and over 300g to allow full travel on the springs. It might be better, and little more effort, to spring all axles, but it will still be hard springing and need plenty of weight in the boiler. Another possibility is to cut one or two turns off the spring so it is only just compressing at the loer stop. This increases the spring rate but reduces the preload requirement. Nick Link to post Share on other sites More sharing options...
Theakerr Posted April 11, 2014 Share Posted April 11, 2014 I just made the centre hole a bit oval with a needle file. Seems to improve smooth running as well because it introduces a little slop, just like the RTR models. Link to post Share on other sites More sharing options...
Quarryscapes Posted April 11, 2014 Author Share Posted April 11, 2014 I suppose had I known about it before reaming the holes out to fit the bearings I could have filed it oval then filed the sides of the bearing down to act as a rough and ready hornblock... but of course it's not something you'd know about until the wheels are in or someone has already told you about it. I could always leave out the spring over the comet hornblock and let gravity do it's thing, or perhaps fit 'Simpson' springs as per 2mm scale practice, It is live framed after all. Link to post Share on other sites More sharing options...
Theakerr Posted April 12, 2014 Share Posted April 12, 2014 When I said hole I should have clarified - I meant the bushing Link to post Share on other sites More sharing options...
RMweb Premium Mallard60022 Posted April 13, 2014 RMweb Premium Share Posted April 13, 2014 Is this in P4? If not then.........sorry but I think you will find it is not a problem unless it always 'offends your eye' and then I sympathise. being a bit of a 'bodger' I almost always add a tad (technical measurement) of vertical 'slop' in my chassis rear and centre bearings, but I don't model steam in P4/EM P Link to post Share on other sites More sharing options...
artisan100 Posted April 14, 2014 Share Posted April 14, 2014 Hello, Someone mentioned this thread to me at Scalefour North over the weekend. As several people have mentioned the design philosophy behind the Comet rigid chassis has been tried and tested for about twenty years now and in all that time the centre axle hole has intentionally been displaced 0.25mm upwards. The result, as mentioned above as well, is that the chassis doesn't rock on the centre wheels when travelling over a raised area of track, such as a misaligned track join or point frog. We would always advocate having pickup from all drivers, rather than just the outer drivers, and then the loco has the best chance of proceeding smoothly through any track imperfections, which has always been the intention. (For a tender loco, adding pickups to the tender and taking the juice through to the loco makes a surprisingly large improvement. Well worth doing.) Incidentally, although this feature is standard throughout the Comet range of over 50 chassis, it's only in the past three of four years that anyone has queried it, and our assumption is that the standards of chassis building have risen over the years and modellers are now noticing what they haven't noticed before. Of course, the position of the centre axle hole is immaterial if the chassis is built using sprung hornblocks. And there our assumption is that in order to achieve maximum haulage capacity the loco body will be weighted to the maximum that the motor can comfortably handle, and then the loco rides on the 'stops' and the springs only come into place if an axle travels over a dip in the track. Other design philosophies are available, and some are much more complicated than others. The Comet one is simple and has a proven 'track' record. I'm always happy to answer any queries on Comet products, and can be reached by phone, email, post or pm here on RMWeb. Due to the sheer quantity of postings here though I may miss a thread concerning Comet products but a question direct to me will always get a response. Hope this helps. Geoff Brewin Comet Models Link to post Share on other sites More sharing options...
coachmann Posted April 14, 2014 Share Posted April 14, 2014 Having the centre axle bearing slightly higher than the outer ones had been good practice that was handed down for generations and so modellers knew why it was so. Like so many areas of life, the link was broken when loco building fell by the wayside. Two variations spring to mind on RTR chassis.......The Hornby LMS 2-6-4T chassis has axles in line with no springing and it is a pain on Peco Insulfrog diamonds......No solution other than to remove centre wheels and turn a bit off them. The Bachmann LMS Jinty 0-6-0T has a sprung middle axle that is kept in line by the keeper plate so is next to useless. Slip shims in either side of the middle axle and these allow it to drop below the outer ones on Insulfrog diamonds so that they collect current.....job solved. Link to post Share on other sites More sharing options...
Miss Prism Posted April 14, 2014 Share Posted April 14, 2014 Having the centre axle bearing slightly higher than the outer ones had been good practice that was handed down for generations and so modellers knew why it was so. Yes, but where the good practice became bad practice was where one-piece rods didn't also have a slightly raised centre axle hole! Link to post Share on other sites More sharing options...
Horsetan Posted April 14, 2014 Share Posted April 14, 2014 ....in order to achieve maximum haulage capacity the loco body will be weighted to the maximum that the motor can comfortably handle, and then the loco rides on the 'stops' and the springs only come into place if an axle travels over a dip in the track..... My understanding was that you should aim to weight the body so that the springs only compress to allow the bearings to sit in mid-position, so that there is leeway to travel up or down. Link to post Share on other sites More sharing options...
artisan100 Posted April 14, 2014 Share Posted April 14, 2014 My understanding was that you should aim to weight the body so that the springs only compress to allow the bearings to sit in mid-position, so that there is leeway to travel up or down. As I said Ivan, other design philosophies are available. The difficulty with partially compressed springs is that the loco body weight has to be equal front and rear, otherwise the loco adopts a nose up or nose down posture. That may be difficult to achieve. To avoid that there are systems where the spring rates can be adjusted, but then the system was getting far more complicated than Comet wanted to offer. We know from the number of packs of hornblocks we sell, and the number of EM and P4 spacers, that the majority of purchasers intend to build a rigid 00 chassis, and so the intention has always been to make life easy for the majority of our customers whilst at the same time offering options and enhancements for others who desire something more sophisticated. Regards, Geoff Brewin Comet Models Link to post Share on other sites More sharing options...
Miss Prism Posted April 14, 2014 Share Posted April 14, 2014 The Comet/Brassmasters spring, designed by Alistair Rolfe, needs in excess of 100g, probably nearer 150g (Nick's values above are interesting, but it would be nice to have some official empirical figures) to depress it fully over its full 2.25mm travel. Thus all locos will partially compress the springs, which was what was intended in the design. Link to post Share on other sites More sharing options...
buffalo Posted April 14, 2014 Share Posted April 14, 2014 ...(Nick's values above are interesting, but it would be nice to have some official empirical figures)... I've been out to the garage and set up my Heath Robinson spring tester (milling machine with steel rod in chuck, using the vertical feed to apply pressure to hornblock and spring mounted in a Comet Collett Goods frame resting on small electronic scales). For a sample of one spring: Free length 3.95mm Compressed length 1.45mm Spring rate 50g/mm (measured at 0.5, 1.0 and 1.5mm compression) With hornblock on the stops, depression 1.25mm and load 63g. The retainer slots in the frame are about 2.5mm deep, so why only 1.25mm compression? The top corners of the guide slot in the frames are slightly rounded so the hornblock binds before it reaches the top of the slot. Nick Link to post Share on other sites More sharing options...
Miss Prism Posted April 14, 2014 Share Posted April 14, 2014 The retainer slots in the frame are about 2.5mm deep, so why only 1.25mm compression? The frame 'tongue' length has to be sufficient to hold the spring's position, and besides which, a 1.25mm compression of that springrate is more than enough for 4mm scale axles weights. It would be possible to get a 1.5mm compression by filing square the top corners of the slightly rounded hornblock aperture, but what axle weight would be in the 150g region? I doubt the OP's Dean Goods could be more than 60g. (Hence my concern in #9.) No one type of coil spring will suit all loco applications, and although the Comet/Brassmasters one is probably more suited to heavier locos, I think the choice of springrate was a reasonable compromise. If it was significantly softer, there would be height-setting problems and loco balance would be more critical. Link to post Share on other sites More sharing options...
talyllyn1 Posted April 14, 2014 Share Posted April 14, 2014 I'm about to start a 6-wheeled chassis that I was intending to fit simple compensation onto the front two axles. However, there is insufficient clearance between the flange edges and the footplate for any significant upward movement, so I'd resigned myself to building it "rigid". Having read Iain Rice's book on chassis construction I had decided to set the centre axle 5-10 thou. higher - essentially as the Comet solution described here. I then thought about fitting Comet hornblocks to the centre axle only, in conjunction with jointed coupling rods - the thinking being that the springs would allow downward movement, but restrict the upward movement. There are no half-etched hornblock cut outs, and although I've fitted High Level blocks before after cutting out the slots, I can't imagine how slots for the Comet type (with the spring slots) can be done. Has anybody done this, and if so - how? Cheers Dave Link to post Share on other sites More sharing options...
KalKat Posted April 14, 2014 Share Posted April 14, 2014 I'm building a Comet chassis and fitting it with High Level Hornblock assemblies. All that's needed is to reduce the spring locator by about 1mm or so Emma Link to post Share on other sites More sharing options...
Quarryscapes Posted April 14, 2014 Author Share Posted April 14, 2014 Hello, Someone mentioned this thread to me at Scalefour North over the weekend. As several people have mentioned the design philosophy behind the Comet rigid chassis has been tried and tested for about twenty years now and in all that time the centre axle hole has intentionally been displaced 0.25mm upwards. The result, as mentioned above as well, is that the chassis doesn't rock on the centre wheels when travelling over a raised area of track, such as a misaligned track join or point frog. We would always advocate having pickup from all drivers, rather than just the outer drivers, and then the loco has the best chance of proceeding smoothly through any track imperfections, which has always been the intention. (For a tender loco, adding pickups to the tender and taking the juice through to the loco makes a surprisingly large improvement. Well worth doing.) Incidentally, although this feature is standard throughout the Comet range of over 50 chassis, it's only in the past three of four years that anyone has queried it, and our assumption is that the standards of chassis building have risen over the years and modellers are now noticing what they haven't noticed before. Of course, the position of the centre axle hole is immaterial if the chassis is built using sprung hornblocks. And there our assumption is that in order to achieve maximum haulage capacity the loco body will be weighted to the maximum that the motor can comfortably handle, and then the loco rides on the 'stops' and the springs only come into place if an axle travels over a dip in the track. Other design philosophies are available, and some are much more complicated than others. The Comet one is simple and has a proven 'track' record. I'm always happy to answer any queries on Comet products, and can be reached by phone, email, post or pm here on RMWeb. Due to the sheer quantity of postings here though I may miss a thread concerning Comet products but a question direct to me will always get a response. Hope this helps. Geoff Brewin Comet Models Thankyou Geoff, I'm not questioning your design policy, it just surprised me as I've never heard mention of it before and a lot of people build and like Comet chassis. I'd also much rather have a loco with all it's wheels on the ground, like the real thing, so will be popping in an order to you for some hornblocks later this week. I will however pull you on one thing - the EM spacers I have here are not 13.25mm, but closer to 13mm. The jigs are spot on but when assembled the spacers just fall through the frames with plenty of sideways movement available. Looks to me like t Link to post Share on other sites More sharing options...
Recommended Posts
Archived
This topic is now archived and is closed to further replies.