garethashenden Posted March 11, 2015 Share Posted March 11, 2015 Trying to plan a N scale (1:160) locomotive and I'd like to check my calculations. The gear ratio I've come up with seems quite high. The prototype has 63" wheels which will be 10mm on the model. With a top speed of 60mph I've calculated that the wheels will turn at 360 rpm. I plan on using a Nigel Lawton 10x12 motor (because I have one) which has a maximum speed of 35000 rpm. From this I think I need a 109:1 gear reduction. This seems to be about twice the "normal" gear reduction. Is there something I've missed? One of these in case anyone wants to know. Link to post Share on other sites More sharing options...
buffalo Posted March 11, 2015 Share Posted March 11, 2015 I suspect the 35,000RPM figure is the maximum with no load. With a significant load it would be much lower, but I'll leave the 2mm folk to suggest appropriate gear ratios. Nick Link to post Share on other sites More sharing options...
RMweb Gold queensquare Posted March 11, 2015 RMweb Gold Share Posted March 11, 2015 Anything over about 40:1 will be fine, just don't turn the controller up full. Jerry Link to post Share on other sites More sharing options...
Adrian Posted March 11, 2015 Share Posted March 11, 2015 I seem to recall that Tim Watson built a rather fine Midland Baldwin loco in 2mm scale and published somewhere - perhaps details on that may help. Unfortunately not got access to it at the moment but MRJ Issue 19 lists No.1 shop : Photograph and description of 2mm scale Baldwin 2-6-0 chassis and drive system. http://www.modelrailwayjournal.com/issue.php?s=19 Hope this helps. Link to post Share on other sites More sharing options...
garethashenden Posted March 11, 2015 Author Share Posted March 11, 2015 I seem to recall that Tim Watson built a rather fine Midland Baldwin loco in 2mm scale and published somewhere - perhaps details on that may help. Unfortunately not got access to it at the moment but MRJ Issue 19 lists No.1 shop : Photograph and description of 2mm scale Baldwin 2-6-0 chassis and drive system. http://www.modelrailwayjournal.com/issue.php?s=19 Hope this helps. Interesting. I'm familiar with his model of a GNR Baldwin, I didn't know he did the Midland one as well. I have a growing list of questions for him when I see him next anyway, so I'll add the gearing to that list. Link to post Share on other sites More sharing options...
garethashenden Posted March 11, 2015 Author Share Posted March 11, 2015 Anything over about 40:1 will be fine, just don't turn the controller up full. Jerry Well yes, that is one solution. But it doesn't really answer the question of whether or not the gearing calculations are correct. I suspect that Buffalo is on to something with the unloaded speed, but I don't know what effect that will have. Link to post Share on other sites More sharing options...
Adrian Posted March 11, 2015 Share Posted March 11, 2015 Interesting. I'm familiar with his model of a GNR Baldwin, I didn't know he did the Midland one as well. I have a growing list of questions for him when I see him next anyway, so I'll add the gearing to that list. Now that you mention that I might be getting confused - maybe his Midland Baldwin was in 7mm. Best getting it direct from him - rather than my vague guesses. Link to post Share on other sites More sharing options...
RMweb Gold queensquare Posted March 11, 2015 RMweb Gold Share Posted March 11, 2015 Well yes, that is one solution. But it doesn't really answer the question of whether or not the gearing calculations are correct. I suspect that Buffalo is on to something with the unloaded speed, but I don't know what effect that will have. It depends what question you want an answer to. Do you want a practical solution that has worked in countless locos or a whole bunch of theory. You asked if there was something you missed, I would suggest experience. Tim would almost certainly tell you the same. Jerry Link to post Share on other sites More sharing options...
Nimbus Posted March 11, 2015 Share Posted March 11, 2015 Well yes, that is one solution. But it doesn't really answer the question of whether or not the gearing calculations are correct. I suspect that Buffalo is on to something with the unloaded speed, but I don't know what effect that will have. It would mean you'd only get your target max speed with no load - not even the loco - applied. ISTR a DC motor delivers maximum power when loaded such that it is running at 50% of its no-load speed, at a given applied voltage. In which case, I'd halve your calculated ratio, and end up not far from what the practitioners have already recommended on this thread. The Nim. Link to post Share on other sites More sharing options...
RMweb Gold queensquare Posted March 11, 2015 RMweb Gold Share Posted March 11, 2015 Now that you mention that I might be getting confused - maybe his Midland Baldwin was in 7mm. Best getting it direct from him - rather than my vague guesses. Tim's Baldwin was originally to be a Midland one as it's start predates Copenhagen Fields. With the advent of CF it seemed sensible to make a GN one. He built a stunning 7mm A4 but not a Baldwin. Jery Link to post Share on other sites More sharing options...
garethashenden Posted March 11, 2015 Author Share Posted March 11, 2015 It depends what question you want an answer to. Do you want a practical solution that has worked in countless locos or a whole bunch of theory. You asked if there was something you missed, I would suggest experience. Tim would almost certainly tell you the same. Jerry I don't want a lot of theory, but I don't really want to just be given the answer either. I want to know where I went wrong. "The gears and the weight of the locomotive add a load to the motor and reduce its top speed" seems to be the answer. There must be something about they way I ask questions. I get the same sort of answers from a friend at university. Instead of showing me my error she just gives me the answer. It's fine in the short term but not that useful in the long term. Link to post Share on other sites More sharing options...
RMweb Gold queensquare Posted March 11, 2015 RMweb Gold Share Posted March 11, 2015 I don't want a lot of theory, but I don't really want to just be given the answer either. I want to know where I went wrong. "The gears and the weight of the locomotive add a load to the motor and reduce its top speed" seems to be the answer. There must be something about they way I ask questions. I get the same sort of answers from a friend at university. Instead of showing me my error she just gives me the answer. It's fine in the short term but not that useful in the long term. I shan't bother in future then. Jerry Link to post Share on other sites More sharing options...
Phil Copleston Posted March 11, 2015 Share Posted March 11, 2015 I don't want a lot of theory, but I don't really want to just be given the answer either. I want to know where I went wrong. "The gears and the weight of the locomotive add a load to the motor and reduce its top speed" seems to be the answer. There must be something about they way I ask questions. I get the same sort of answers from a friend at university. Instead of showing me my error she just gives me the answer. It's fine in the short term but not that useful in the long term. I think the problem is, you're asking the wrong sort of question. It doesn't matter how to calculate gearing from a theoretical model top speed from a given motor. What matters is practical experience from empirical evidence - and Jerry has already given you the answer: 40 to 1 or above has proven to be a suitable ratio to produce a realistic performance in countless working 2mm locos. A decent motor and a quality controller will also be an important factor. Therefore, your question perhaps ought to have been "Which gear ratio will enable me to realistically control my loco." Link to post Share on other sites More sharing options...
2mm Andy Posted March 11, 2015 Share Posted March 11, 2015 I don't want a lot of theory, but I don't really want to just be given the answer either. I want to know where I went wrong. "The gears and the weight of the locomotive add a load to the motor and reduce its top speed" seems to be the answer. There must be something about they way I ask questions. I get the same sort of answers from a friend at university. Instead of showing me my error she just gives me the answer. It's fine in the short term but not that useful in the long term. There is a really useful gear reduction/speed calculator spreadsheet on the High Level Kits website (in the gearbox planning section). By the way, Nigel's website shows the 10x12 motor as having a 'no load' speed of 25000rpm, not 35000 as you've mentioned above. That should bring the gear ratio you need down somewhat. Andy Link to post Share on other sites More sharing options...
garethashenden Posted March 12, 2015 Author Share Posted March 12, 2015 There is a really useful gear reduction/speed calculator spreadsheet on the High Level Kits website (in the gearbox planning section). By the way, Nigel's website shows the 10x12 motor as having a 'no load' speed of 25000rpm, not 35000 as you've mentioned above. That should bring the gear ratio you need down somewhat. Andy I got the 35000 figure from the yearbook... Anyway, I think the question I wanted answered was "What's wrong with my maths?" not "What gear ratio should I use?". I had previously been told that 40 or 50 to 1 would be a good ratio to aim for, but when I calculated that I would need something more than twice that I knew it was wrong but I didn't know where the error was. I really hope I haven't managed to offend any o you in the process, that wasn't my intention. Link to post Share on other sites More sharing options...
Caley Jim Posted March 12, 2015 Share Posted March 12, 2015 As Phil has said, calculating the theoretical gear ratio from the no load speed of the motor is an academic exercise, as the speed will rapidly fall off as the load increases. The more gears there are, the less will be the mechanical efficiency of the mechanism and so the higher the load on the motor and therefore the lower its speed. With modern motors and dcc controlling the speed of the loco is most easily done with the controller, provided you have a smooth and free running mechanism. Jim Link to post Share on other sites More sharing options...
2mm Andy Posted March 12, 2015 Share Posted March 12, 2015 I got the 35000 figure from the yearbook... Anyway, I think the question I wanted answered was "What's wrong with my maths?" not "What gear ratio should I use?". I had previously been told that 40 or 50 to 1 would be a good ratio to aim for, but when I calculated that I would need something more than twice that I knew it was wrong but I didn't know where the error was. I really hope I haven't managed to offend any o you in the process, that wasn't my intention. You've not offended me, and I think others have suggested that the mistake was to use the motor no-load speed in your calculations. As regards the discrepancy between the yearbook data and Nigel Lawton's website, I notice that some of the dimensions of the motor are also different from the data in the yearbook, which suggests that Nigel is perhaps selling a slightly different motor from the original (the data in the yearbook goes back about 6 or 7 years). Anyway, I will feed this back for correction in the next issue. Andy Link to post Share on other sites More sharing options...
RMweb Gold queensquare Posted March 12, 2015 RMweb Gold Share Posted March 12, 2015 I'm not offended just, as Phil says, feel you need to be a bit more specific in your question. Also to dismiss someone's advice with "I don't really want to just be given the answer" is easily open to misinterpretation! To be honest I'm not really interested in no load speeds of a motor but how it performs in a chassis. I'm a big fan of the Nigel Lawton motors generally although in my opinion the 8mm motor is better than the 10mm, lower revving and more torque. Nigel thinks it has a better magnet. Jerry Link to post Share on other sites More sharing options...
ZiderHead Posted March 12, 2015 Share Posted March 12, 2015 First find the max axle rpm from Speed (mph) and DriverDiameter (inches): = (Speed*63360) / (60*DriverDiameter*PI) = (60*63360) / (60*63*PI) = 320rpm Next grab the no load speed from the motor specs - 25,000rpm in this case. As you can see from the graph below, the rated rpm of a motor (i.e. the max it should run under load) is about 80% of the no load rpm. So your required gearing for 60mph is: = 25,000rpm * 0.8 / 320rpm = 20,000rpm / 320rpm = 62:1 The max power is about 50% no load rpm, so arrives at a scale speed of: = 60mph * 0.5 / 0.8 = 38mph This is rather convenient because you need most power for the heaviest load at its highest speed (i.e. freight) and the above calc gives you just that at typical line/train speed limits for freight Link to post Share on other sites More sharing options...
richbrummitt Posted March 12, 2015 Share Posted March 12, 2015 Calculating theoretically you will have losses in each reduction too. A single start worm is less than 50% efficient and unless they are made very accurately the efficiency of spur gear pairs, although reasonable, to begin with, tails off. Link to post Share on other sites More sharing options...
RMweb Gold 2mmMark Posted March 14, 2015 RMweb Gold Share Posted March 14, 2015 Just because a motor will rev to 35,000 rpm, it doesn't mean that you must design for than speed. It's likely you'll find the resulting mechanism is quite noisy. I built an 08 chassis with a reduction in the 85:1 range, looking for good slow speed running. It runs fine but gear & motor noise is quite audible when the wick is turned up. Mick Simpson has a J27 with a single 28:1 reduction on 8mm dia wheels. That runs very well. I'm building a Peckett with a 28:1 reduction which has 7mm wheels, I'm hopeful that will run OK too. My first 2mm loco was a class 24 with 17:1 reduction, that performed well. My gut feeling when I saw your loco project was that 50 or 60:1 would be ample reduction. The calculation above looks to justify that feeling. When designing a loco chassis, accomodating the gearing and motor can often be the final arbiter of the reduction you can use. You might want to consider using a jig/mockup of the gear & axles shafts to lay it all out as I describe here: http://www.rmweb.co.uk/community/index.php?/blog/1345/entry-14132-peckett-y-class-framed-part-1/ about ½ way down the page. Very nice project, by the way. The challenge with US steam locos is the lack of a footplate to provide a convenient body/chassis boundary. Mark Link to post Share on other sites More sharing options...
ZiderHead Posted March 14, 2015 Share Posted March 14, 2015 This is my gearbox spreadsheet in case anyone finds it useful, its very simple - just enter the no load motor speed, loco top speed and driver diameter in feet & inches (or just inches) and it gives you the required gearbox ratio. Gearbox calc.xls Link to post Share on other sites More sharing options...
ZiderHead Posted March 14, 2015 Share Posted March 14, 2015 One of these in case anyone wants to know. Great looking loco, and thats an awful lot of pipework and detail to attempt in 2mm! Cant wait to see it Link to post Share on other sites More sharing options...
garethashenden Posted March 14, 2015 Author Share Posted March 14, 2015 Great looking loco, and thats an awful lot of pipework and detail to attempt in 2mm! Cant wait to see it Yeah, it's going to be "fun". Assuming it goes well, I want to do this one as well. Link to post Share on other sites More sharing options...
garethashenden Posted March 14, 2015 Author Share Posted March 14, 2015 Just because a motor will rev to 35,000 rpm, it doesn't mean that you must design for than speed. It's likely you'll find the resulting mechanism is quite noisy. I built an 08 chassis with a reduction in the 85:1 range, looking for good slow speed running. It runs fine but gear & motor noise is quite audible when the wick is turned up. Mick Simpson has a J27 with a single 28:1 reduction on 8mm dia wheels. That runs very well. I'm building a Peckett with a 28:1 reduction which has 7mm wheels, I'm hopeful that will run OK too. My first 2mm loco was a class 24 with 17:1 reduction, that performed well. My gut feeling when I saw your loco project was that 50 or 60:1 would be ample reduction. The calculation above looks to justify that feeling. When designing a loco chassis, accomodating the gearing and motor can often be the final arbiter of the reduction you can use. You might want to consider using a jig/mockup of the gear & axles shafts to lay it all out as I describe here: http://www.rmweb.co.uk/community/index.php?/blog/1345/entry-14132-peckett-y-class-framed-part-1/ about ½ way down the page. Very nice project, by the way. The challenge with US steam locos is the lack of a footplate to provide a convenient body/chassis boundary. Mark Thanks Mark, a mockup is definitely planned. Any thoughts on how/where to make the joint? Link to post Share on other sites More sharing options...
Recommended Posts
Archived
This topic is now archived and is closed to further replies.