Jens Vesterdahl Posted March 19, 2017 Share Posted March 19, 2017 Hello Gents. (This post was moved from the 7mm forum, as I just realised it should be here) I'm currently trying to improve the performance of a North West Short Line gauge 1 drive, which has a Mashima motor with two steel (?) worms. The worm gears on the axles are plastic and while I can in fact make the drive purr like a kitten, the tolerances for doing so are absolutely ridiculous. Also, it seems like the worm gears are slightly off center, so I suspect that the contact point between the worm and gear is not optimal. I am thinking about replacing the worm gears on the axles with brass gears. Where do I get such gears? The specs are: - Module 0.5 - 25 teeth skewed (20 degrees ?) - 3/16" bore The width of the gear should be approx. 3-4 mm but that's not critical. Best regards Jens Link to post Share on other sites More sharing options...
meil Posted March 19, 2017 Share Posted March 19, 2017 Hello Gents. (This post was moved from the 7mm forum, as I just realised it should be here) I'm currently trying to improve the performance of a North West Short Line gauge 1 drive, which has a Mashima motor with two steel (?) worms. The worm gears on the axles are plastic and while I can in fact make the drive purr like a kitten, the tolerances for doing so are absolutely ridiculous. Also, it seems like the worm gears are slightly off center, so I suspect that the contact point between the worm and gear is not optimal. I am thinking about replacing the worm gears on the axles with brass gears. Where do I get such gears? The specs are: - Module 0.5 - 25 teeth skewed (20 degrees ?) - 3/16" bore The width of the gear should be approx. 3-4 mm but that's not critical. Best regards Jens IMG_0638.JPG The worm gears are on the motor shaft and the motor shaft doesn't look like it's 3/16 on your photo. Link to post Share on other sites More sharing options...
Jens Vesterdahl Posted March 19, 2017 Author Share Posted March 19, 2017 Okay, English is not my first language, so I got my terms from Wikipedia. According to that definition, the worm is on the motor axle and the worm gear or (worm wheel) is - in my case - on the wheel axles. Anyway, I'm looking for a replacement of the gears on the wheel axles - not the ones on the motor axle. Jens Link to post Share on other sites More sharing options...
bertiedog Posted March 19, 2017 Share Posted March 19, 2017 Being Gauge one these gears are quite large sizes and a commercial gear factor may be the best to approach. Tracing exact matches is difficult, but not impossible. As you suspect the worm on the shaft, it would be best to order a complete set of gears. Most makers are Metric, but will supply imperial to order. The Mashima motor will be a metric shaft. The NWSL gear on the axle will be imperial bore to fit axle, unless NWSL used metric, and it must be checked to confirm. It looks from the photos that the worms and gears are high quality, just how far out are they? The motor might need packing up to leave clearances, but make sure the chassis in standing on the flat before checking. This takes up the axle bearings to the operating position. If run off the track it will appear to have more clearances and play. Worm sets will work even with large back lash and loose gears, as long as not for super heavy uses, but make sure they are well greased. Stephen Link to post Share on other sites More sharing options...
Jens Vesterdahl Posted March 19, 2017 Author Share Posted March 19, 2017 Thanks for the reply. The wheel axles are indeed 3/16". The manufacturer says so, my calliper says so, and so do the 3/16" ball bearings which fit snugly on the axles. I've found a brass gear wheel on the web, but it's expensive and the bore is wrong. The worm looks like it's high quality, yes, but I'm not so sure about the plastic gear wheel. NWSL does make a point about using different materials, though, and I don't have much practical long-term experience with gears. The thing is that the drive design is - by admission from NWSL - very old, and they run horribly, even after a long break-in run. On examination I found that the motor is glued to the housing with some caulk-ish goo and even though the motor axle bearings are mounted in guides, my conclusion is that the tension between the worm and gear is much too high. So - I've designed and have had 3D printed an insert for the housing, where I make sure that the worm and gear are correctly aligned to each other, the wheel axles are positioned correctly, and everything else adopts to those fixed points. Also, I replace the delrin bearings with ball bearings. Unfortunately I goofed the outer diameter of the ball bearings, so I need to make a second try, but the prototype does give me a chance to play with the drive, and while it runs significantly better than the stock design, it does have a noise that correlates to a revolution of a wheel axle. Looking at the plastic gear wheel, it looks like it is slightly off-center, but I need to investigate some more. I can remove that noise and make the drive purr as it should, but initial measurement shows that the axle distance then doesn't match the theoretical value. I'd need 10 of those worm wheels, so I think for now I'll go along with what I have and try to make it work. I'll do some more testing and measuring and see if I can make the real world meet the theory. Jens Link to post Share on other sites More sharing options...
bertiedog Posted March 19, 2017 Share Posted March 19, 2017 By the way, you may class the gears as "Plastic", but they look and from the description, sound like "Tufnol" Gears. This material is exceedingly tough, a cloth reinforced pressure phenolic resin material used when quietness and precision is required. It is used in many top class units. I suspect the whole motor has to be shimmed upwards to provide about 2 to 4 thou clearance over the gear, but tested only when the unit is standing on a flat, forcing the axles up wards. It may only need shims under the brass bearings on the motor shaft to do this, and once correct the motor re-glued with silicon caulk ( or a hot glue gun) into place, or held by a strap across it. Link to post Share on other sites More sharing options...
bertiedog Posted March 19, 2017 Share Posted March 19, 2017 If the Tufnol or plastic gears are running out of true, they can be put right by any competent machinist with experience of gears. Most home mechanics have never seen it done,....... first the gear is assumed to have its teeth "true", but forget the bore, which is what you are trying to correct. Around the teeth in a pattern to suit the lathe chuck or collet are fitted steel wires that fit in the tooth gap, so as to leave the wire top outside the overall diameter. This allows the chuck to grip the wires only and apply accurate hold on the gear. Once you are sure it is running in the lathe true, the bore is bored out to a bigger size, and the outside is faced. A bush with the axle sized hole is made to fit the bore in the gear, with a small rim to run against the faced surface. It is fitted with force or a looser fit with Loctite retainer. The rim is not a requirement with force fit, but should be used with Loctite. The result will be a true gear as good as the machining of the teeth was, and will be very much better. However be warned, it cannot repair the bad machining of the teeth if they are out of true around the pitch circle. But this is fairly rare to find, it is the bore that is nearly always wrong. Link to post Share on other sites More sharing options...
Jens Vesterdahl Posted March 19, 2017 Author Share Posted March 19, 2017 By the way, you may class the gears as "Plastic", but they look and from the description, sound like "Tufnol" Gears. This material is exceedingly tough, a cloth reinforced pressure phenolic resin material used when quietness and precision is required. It is used in many top class units. Thank you for directing my attention to the fact that plastic is not simply plastic. Reading on the NWSL web site, they say that their gears are made from Acetal or Delrin if you like. This is used also for their bearings, and a little research on the web tells me that the material is or certainly can be suitable for gears. Their worms are always made from steel. Assuming then that the gear wheel is OK, I cleaned the gaps and had another inspection run, and it sounds and looks much better now. I guess there could have been some debris stuck in the tooth gaps. With that and with your information here from the forum, I trust my gear wheels for now, and I'm moving on to redesigning the insert part and checking the dimensions. What I do know is that the ball bearing seating is too small, and that could in fact be the cause of the entire problem. Thanks, gentlemen. I'll post more results when I have them. Incidently, this is what I'm working on: Best regards Jens Link to post Share on other sites More sharing options...
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