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Spur Gear Diff

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john kennedy 1	09/12/2011 09:19:54
214 forum posts 24 photos	Hello everyone I am getting bits together to build a 1" Minnie.I have had Mr Masons book for years and have just decided to have a go. I came across this chap who has built one and incorporated a diff which he shows on his website Minnie It shows a small picture of it and he tries to explain in words how it works.Call me thick if you like but I don't understand a word of it . I've done a few searches but cant find a simple explanation for it. Has anyone come across this mechanism before and is it a worthwhile addition. Thanks .. John Edited By Katy Purvis on 12/12/2011 11:18:47
ady	09/12/2011 09:54:59
612 forum posts 50 photos	I see what you mean...sounds intruiging though got to go out now...will try to suss later if no-one else does http://plsntcov.8m.com/differential.htm From wiki This is another type of differential that was used in some early automobiles, more recently the Oldsmobile Toronado, as well as other non-automotive applications. It consists of spur gears only. A spur-gear differential has two equal-sized spur gears, one for each half-shaft, with a space between them. Instead of the Bevel gear, also known as a miter gear, assembly (the "spider") at the centre of the differential, there is a rotating carrier on the same axis as the two shafts. Torque from a rime mover">prime mover or transmission, such as the drive shaft of a car, rotates this carrier. Mounted in this carrier are one or more pairs of identical pinions, generally longer than their diameters, and typically smaller than the spur gears on the individual half-shafts. Each pinion pair rotates freely on pins supported by the carrier. Furthermore, the pinions pairs are displaced axially, such that they mesh only for the part of their length between the two spur gears, and rotate in opposite directions. The remaining length of a given pinion meshes with the nearer spur gear on its axle. Therefore, each pinion couples that spur gear to the other pinion, and in turn, the other spur gear, so that when the drive shaft rotates the carrier, its relationship to the gears for the individual wheel axles is the same as that in a bevel-gear differential wikipedia Edited By ady on 09/12/2011 10:01:01 Edited By David Clark 1 on 12/12/2011 11:01:18
John Stevenson	09/12/2011 12:18:12
5068 forum posts 3 photos	Austin 7 had a spur gear diff. John S.
Phil P	09/12/2011 13:02:45
851 forum posts 206 photos	You just beat me to it John. I have one on my Austin Seven and it is the simplest of things once you twig how it works. I googled and found this:- Photos Phil Edited By David Clark 1 on 12/12/2011 11:04:14
john kennedy 1	09/12/2011 13:28:49
214 forum posts 24 photos	Posted by Phil P on 09/12/2011 13:02:45: You just beat me to it John. I have one on my Austin Seven and it is the simplest of things once you twig how it works. I googled and found this:- Photos Phil it is the simplest of things once you twig how it works. Thats the bit I'm struggling with Edited By Katy Purvis on 12/12/2011 11:19:52
John Stevenson	09/12/2011 16:03:25
5068 forum posts 3 photos	Posted by john kennedy 1 on 09/12/2011 13:28:49: Posted by Phil P on 09/12/2011 13:02:45: Phil it is the simplest of things once you twig how it works. Thats the bit I'm struggling with What the simples bit or the twig bit ? John S.
john kennedy 1	09/12/2011 16:21:23
214 forum posts 24 photos	Thanks for your replies gents.I'm just going to have to study this and work out how he did it. Suppose I could build a bigger one with a proper diff, but it scares me looking at Reeves' price list.
mgj	09/12/2011 21:20:44
1017 forum posts 14 photos	It is a very simple mechansim. The second half of your question was - is it worth it? Probably not, unless you want to make it. It might even be a bad idea. Could a 1" Minnie pull you - probably on a very smooth surface, but its going to need every bit of traction available - and if you want traction, locked (no) diffs are good things to have, because the slightest bump and one wheel will spin. People will say a diff will help steering. Well yes, particularly on vehicles whose l/w ratio is relatively high. You can get round that problem without a diff by removing a driving pin, but then you lose traction. If you want to build a bigger one with a proper diff that comes in the form of cast gears and bevels, and which will most certainly pull you and others (at the same time), and is pretty compact and doesn't have lunatic prices for castings, look at littlesamson.co.uk (Little Samson models). The 3" Little Samson is a simple economic choice with plenty of grunt and is still family car transportable.(its a tiny bit bigger than a 2" Minnie at a sane price)
Alex gibson	09/12/2011 21:39:29
35 forum posts	Hi all, this site is beginning to do my head in. I can't read the above posts because of the ads. I know this is an old problem and the 'suits' will just tell people to stop posting long links but will they ever fix it? Hacked off alex Edited By David Clark 1 on 12/12/2011 11:02:02
The Merry Miller	09/12/2011 22:16:09
484 forum posts 97 photos	Alex, If it's just a momentary thing that pees you off while you are reading the posts, as posted before, just hit the escape button. That will freeze them. Len. P.
Robert Dodds	09/12/2011 22:42:27
324 forum posts 63 photos	Hi All, Can I make an attempt at explaining this fascinating little gear assembly the like of which I haven't seen before but as they say you never stop learning! The fundamental bit for a diff is that it has 3 input/output nodes. For the Austin diff that is, the crown wheel in and 2 half shafts out. If the input is locked stationary the two output shafts should rotate in opposite directions. The cut away drawings of the Austin diff need a bit of elaboration because the left half shaft does not show its gear end but it is like the right hand one with an integral gear. See the second drawing as this shows the width of the shaft gear more clearly and it is approx ½ the width of the small “planet” wheels that are mounted onto the various shafts around the cage. These planet wheels operate in pairs, meshing with each other but offset so that one engages with the RH half shaft gear and the other one of the pair engages with the LH half shaft gear. There will be a gap between the faces of the two shaft gears equal to the mesh width of the two planet wheels, otherwise you would get everything locked up. With this in mind, and keeping the crown wheel stationary, rotate the RH shaft clockwise. The planet it engages with rotates CCW and that in turn makes the second one of the planet pair rotate CW. Because this second planet is offset from the first planet gear it will engage with the LH shaft gear (out of sight in the drawing) but nevertheless making the LH shaft rotate CCW and so satisfies the requirement to reverse the rotation of the two half shafts. Turning to the Minnie we see a similar setup with the bull gear equivalent to the crown wheel and cage and yet again a couple of gears hidden from view to confound us. The 25T gears are equivalent to the half shaft integral gear above and the two 20T gears are the planets but in the Minnie arrangement there is only one through shaft and no way of creating the gap between two half shafts that creates the overlapping planet gears. On Minnie this is done by introducing the 14T gear that is pressed onto the shaft that also carries the hidden 20T planet (and that also engages with the 25T hidden gear that sits on the through shaft and provides drive to the left side) Ideally the 14T should be 20T but doing that would make it mesh with the RH 25T gear and lock everything up. The down side to the 14T compromise is that there is a 14:20 gear ratio between LH and RH sides. This does not upset the running speeds but it does affect the torque balance, one side having more drive than the other and would be seen, if ever pressed hard enough, by the one wheel always breaking away first under wheel slip conditions Bob D
Alex gibson	09/12/2011 22:59:16
35 forum posts	Hi Len, thanks for the reply however I'm not talking about the flashing ads (annoying as they are). I'm talking about the ads running down the right hand side of the page that superimpose themselves over previous posts if a long link has been posted (sorry Ady, no offence). There are lots of other posts regarding this problem on this site but nothing seems to be getting done about it. ps; sorry to the op for having a rant ( I've had snow on monday and tuesday, hailstones on wednesday, hurricane on thursday and in-laws on friday) even more hacked off alex Edited By Alex gibson on 09/12/2011 23:01:38
John Olsen	10/12/2011 08:49:15
1294 forum posts 108 photos 1 articles	It is a pity the long link is causing some folks problems because the pictures there are quite helpful. The main thing they lack is motion, since it is much easier to figure out how a diff works, whether bevel or spur. when you can see the bits going around, preferably while twiddling the shafts yourself. One way to look at it is that it is a form of analog adding device. If the road wheels are regarded as the input, then the rotations of the drive shaft are proportional to the sum of the rotations of the drive wheels. (The ratio of the crown wheel will decide what the proportion is.) Mr Babbage was planning to use this in his differential engine, among other things. So considering it more the normal way, when the drive shaft is turned, the two driven shafts will turn so that the sum of their rotations is proportional to the input rotations. If one wheel is fixed the other will rotate at twice the speed. This is the classic problem when one wheel is in the mud and the other on the tarmac. The Lego people make, or used to, a differential for some of the Lego Technics kits. This is a bevel type, but only has one bevel gear on the spider, which does help to see what is going on. (The input drive is a spur gear.) When one of my sons was about 11 or 12, he had been shown how a differential worked, so he built a spur gear differential out of the Lego parts that we had. It was a bit cumbersome but it really worked...so as a reward he got bought the Lego technics car, the big one with moving "pistons" and all. He is working as a programer for Google now. regards John
john kennedy 1	10/12/2011 09:24:45
214 forum posts 24 photos	Hi again, I can see there are some very clever people on this forum.Thanks to Bob and John for going to so much trouble to explain how it all works,but unfortunately most fell on stony ground.mgj has answered my question adequately and I now believe it may not be worth the effort due to the low power out put etc. Had a look at the Samsons and the look very nice and the prices not so scary. My main project on the go is a 5" Springbok which is running on air and I'm about to solder up the boiler.Before I start messing with all that very expensive copper I thought I would do the Minnie with a nice little boiler and hopefully have a nice little 'sideboard' model at the end of it. Thanks again, John...
Ian S C	10/12/2011 10:20:43
7468 forum posts 230 photos	Oh for meccano, Unfortunately my set did not have enough gears. Ian S C
john kennedy 1	10/12/2011 10:53:25
214 forum posts 24 photos	Talking about Meccano, found this on you tube.Nice to see it actually working... http://www.youtube.com/watch?v=imwSdJBFOBQ
Michael Gilligan	10/12/2011 11:06:27
23121 forum posts 1360 photos	John, Thanks for starting this thread ... the discussion has been very useful. There is one simpler alternative that you might consider: It's not really a Diff, but does share some of the useful attributes. ... Put a freewheel on each side. I believe this idea was used in some of the earliest Motor Cars, but my first acquaintance was when friends at 'Loughborough University of Technology' built a racing pedal-car [sometime around 1970]. This was a recumbent, with a central chain, and bicycle freewheel units each side. As I said, it is not really a Diff ... but it works! MichaelG.
Robert Dodds	10/12/2011 11:24:20
324 forum posts 63 photos	Michael G, You must be a good soldier (Who never looks back) Two free wheels won't reverse unless you have a locking pin to put in one of them. I think there were instances of one free wheel in the days when a man with a red flag went in front. Regards Bob D
wotsit	10/12/2011 12:26:24
188 forum posts 1 photos	For Alex Gordon (sorry its a bit OT for the rest of you) I know what you mean, Alex. If its any help, I am running Firefox under Ubuntu 11.10 (no Windoze - absolutely marvellous!), and I had the same problem, so I installed Adblock (find it under Firefox Add-ons - it is also available if you are running Windoze rubbish, and also for other browsers). This allows you to block selected bits and bits (not only advertisements), so I blocked all the idiot flashing advertisements on this site. The publishers have shot themselves in the foot as far as I am concerned if they will not address the problems with this forum - now I get no adverts unless I choose to read them. The only problem now is that the log-in boxes extend halfway across the page, so they sometimes block part of the first post in the thread, but I can live with that - just scroll it up and down. Edited By wotsit on 10/12/2011 12:26:51 Edited By wotsit on 10/12/2011 12:32:55
Michael Gilligan	10/12/2011 15:52:35
23121 forum posts 1360 photos	Bob D. Fair comment ... but would that really cause much difficulty on a small-scale model? The pedal racecar was oblivious to the problem: If you need reverse, you have probably already lost the race Far more important to have drive to the appropriate wheel. MichaelG. Not Soldier of any persuasion, just an old motorcyclist

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