Poor quality gears

[Maurice Cox 1Participant @mauricecox1]

[Maurice Cox 1Participant @mauricecox1]

I am helping a friend to complete a "Centaur" gas engine. I have a problem with the timing gears supplied by one of our advertisers. The finish on the flanks of the teeth on the larger of the two helical gears is very poor. Each flank is a series of flats and peaks. This is so all the way round the gear. There seems to be a bit of a pattern to it, perhaps caused by a blunt hob? I have rejected one pair, but the replacement is little better. I was thinking of getting a straight cut spur gear of the same D.P. and running the two together, with some lapping compound, to improve the surface. I would of course have to move the "lap" endwise to cover the full length of the teeth. Should this work? Is there any other way that I can improve the teeth?

Regards Maurice

[MuzzerParticipant @muzzer]

Have you got any photographs?

[Maurice Cox 1Participant @mauricecox1]

I did try, but failed miserably. The finish is like hammered pewter but smaller dents, and with some of the "dents" elongated. In bright light, the "good" flanks have a similar finish but not so deep. It is very disappointing! I will try again with the pictures.

Regards Maurice

[JasonBModerator @jasonb]

Maurice can you set one of the gears onto a horizontal shaft held in say the mill vice so it is free to turn and hold the other on a mandrel in the spindle and then run the two together with some fine lapping powder & a drop of oil or a lapping compound like "Timesaver"

My thinking is it would be better to lap teh two mating gears rather than try lapping them separately.

 

J

Edited By JasonB on 11/07/2015 19:18:16

[AjohnwParticipant @ajohnw51620]

If you do lap them make sure that the same teeth don't keep coming into contact all of the time as they rotate.

John

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[Maurice Cox 1Participant @mauricecox1]

Thanks Jason and John. I did think of lapping the two together, but I was afraid of spoiling the small gear as the finish on the large one is so poor. I suppose that if they are only ever going to run together then an" unusual" profile will not matter too much. I do have a couple of acceptable pictures now. I have them in my album. Just got to find how to put the into the thread and then I will do so. Done it in the past, but it seems to have changed. Or is it me? Probably!

Regards Maurice

[JasonBModerator @jasonb]

Just click the little black "camera" icon at the top of the reply box and it will take you to your album(s) where you can select the image.

Does look like it has a "hammered finish"

 

J

Edited By JasonB on 11/07/2015 19:52:43

[Maurice Cox 1Participant @mauricecox1]

Thanks Jason, here are the pictures. The second teeth down show the problem best. These gears came from a gear specialist via our advertiser, and I am very surprised that they consider this to be an adequate finish!

[DMBParticipant @dmb]

Strewth They do look poor quality. As you have already exchanged them, I would have thought that the supplier would have made an effort to send better quality replacements. You will have to make a decision whether to return this junk for a complete refund or to bodge what you have got. To send a replacement of equally poor quality suggests that all their supplies of these gears are all poor, If you go down this road they may quibble, in which case you should shout "Sale of Goods Act" (Goods must be of merchantable quality). On the other hand, if you "put up" with it, they effectively get away with it and all subsequent customers suffer as well. You could get prices for real quality replacements from reputable gear manufacturers and demand the cost from your original supplier. This is what I would do.

Good luck with whatever you decide, but let us all know what transpires.

John

[DMBParticipant @dmb]

Maurice,

I`ve just had a closer look and there seem to be little holes like they havent been machined at all but cast from some metal that has fizzed and bubbled. What is it? Ali/zinc /"pot-metal"/air-drying toffee? or

Txxxx Cxxx, like a famous jeweller once said about his company`s goods!

John

[Anonymous]

Posted by DMB on 11/07/2015 21:21:59:

Maurice,

I`ve just had a closer look and there seem to be little holes like they havent been machined at all but cast from some metal that has fizzed and bubbled.

Quite so, just what I thought on first viewing; it looks like a cast 'finish', not a machined one. It is possible to get faceting on the tooth flanks when hobbing a gear, but it looks nothing like the picture.

Andrew

[Maurice Cox 1Participant @mauricecox1]

To be fair, I must point out that the photos always make machined surfaces look poor due to light reflecting off any minute imperfection. The apparent pitting or "holes" are not visible to the naked eye. It is the faceting that is the source of my complaint. It does make you wonder if all this firm's output is made down to this standard!

Maurice

[Maurice Cox 1Participant @mauricecox1]

Further to my last post; I could go down to ask for my money back. In fact our advertiser did offer this. But then I end up with no gears at all! I have been unable to find another source of gears that will deal with small quantities, that is, one of each. I think I shall have to make the best of what I have; hence my enquiry.

Regards Maurice

[DMBParticipant @dmb]

Maurice,
Try S.H. Muffett Ltd in Tunbridge Wells.
Easy found on Google. Many years ago they used to advertise in ME, gears for the model engineer.A long time ago I started building a GHT bending rolls for which they supplied the 4 gears, I think they were ?7.50 – yet another nearly finished project! Anyway, I recall that delivery was prompt and looked very nicely made.Give them a try!
John.

Edited By DMB on 11/07/2015 23:17:42

Edited By DMB on 11/07/2015 23:18:53

[Nigel McBurney 1Participant @nigelmcburney1]

That gear looks awful,either a blunt gear cutter or the result of using a poorly supported mandrel,as described above this is the larger gear of the pair so it fits on the side shaft which must be small in diameter.The way to make such gears is to turn the gear blank at one end of a solid shaft with a longer collar,and a centre hole.set up in a universal dividing head,shaft in chuck and dividing head tailstock brought up to support the shaft via the centre hole,The teeth are then cut,and the shaft is then put back in the lathe and the gear parted off, the long collar on the gear is then held in a collet and the gears is then faced,drilled (or bored)and reamed and it can then be reversed in the lathe and the collar reduced to correct length.With such small gears there is not much room or clearance when getting the cutter set at an angle up to the gear blank but also missing the tailstock.I have made a lot of stationary engine gears straight cut and spiral ,the small ones are the difficult and expensive ones and unless the engine is very old and valuable the cost of the gears is too high for a new model.For general info the Spiral magneto gear on Petter M stationary engines is another gear with a small tapered hole to suit the magneto spindle,I worked out how they made them,the gear has a machined recess on one face and short machined spigot on the other face,the machining is very accurate so that one gear can mate up with one or more gears via the recess/spigot. so a stack of gears can be held accurately together, there are also three equi spacedcast holes in the gear,these are clearance holes take studs or bolts so they can be held firmly together on a tooling fixture which also has a mating spigot ,the fixture is held in the spindle of the gear cutter or a dividing head,with the other end of the stack supported by a centre,I just wonder how many were made at a time ,as Petters made a lot of the M type engine. Regarding the poor purchased gears its catch twenty two,either return them and then have no gears or just accept them and live with it, I know the gears look rough but I can assure you that there are a lot of gears on vintage stationary which look worse than yours having survived possibly 80 odd years of use , disuse ,abuse,and rust, If desperate I would run them,do not lap them in ,grinding grit is awful stuff and can cause further difficulties. Another point is that some engines mainly pre 1900 had the spiral side shaft gears cast and just dressed up with a file and they run very well,the only snag with such gears is that the pattern was made to DP sizes and subsequently the cast gear shrunk on cooling ,a pair made like this ran OK making a modern replacement for one or a pair causes a few headaches, though they can be overcome with some adjustment of the cutter depth etc etc all good fun.

[Roderick JenkinsParticipant @roderickjenkins93242]

Maurice,

I can understand that you are dissatisfied with the finish on your gears- having paid, I should imagine, a pretty high price for them. However, I think you will find that the gear is perfectly functional. The drive for the cam shaft doesn't need any great accuracy: the power transmitted is low, any noise from the gears will be drowned by the exhaust and the only important thing is the 2:1 timing. I made the gears for my Wyvern, which is very similar to the Centaur, by milling (rather than hobbing) and this involves all sorts of approximations, yet the gears work fine and the engine runs.

Out of interest, what size are the gears? ETW just specified anything that will fit to give the 2:1.

Cheers,

Rod

[Maurice Cox 1Participant @mauricecox1]

Hi Rod,

Your reassurance about the functionality of these gears is reassuring. I will lap them together as advised previously, and use them like that; see how it goes.

The gears have 9 and 18 teeth. The diameter of the larger gear is 1 5/8" by 1/2" face. The small one is 5/16" (aprox) by 1/2"face. This means that small one is only just large enough to bore out to fit the crankshaft, and the bed will have to be "excavated" to clear the large gear. I may turn the crank shaft to a smaller diameter on one side, as far as the gear, fit the gear, then sleeve the shaft back up to 1/2" diameter. I'll decide what to do when I actually do the job. All good fun!

regards Maurice.

[Roderick JenkinsParticipant @roderickjenkins93242]

It looks like you have a pair of 45 degree gears, so the 18 tooth is twice the diameter of the 9 tooth. Forty 45 degree gears tend to be stock gears from the usual suppliers suggested by various people above. ETW mentions using gears that are the same OD and for this you would need gears of approx 60 and 30 degrees. A 60 degree 32DP 18 tooth gear has an OD of 0.712 inches and the 9 tooth 30 degree gear is .625 OD. (helical gears of any angle will mesh with any other at right angles provided that the total angle adds up to 90 degrees – the magic angle for exactly the same OD is 26.6 degrees and 63.4 degrees). 60 and 30 degree gears would fit in the space better so that is why I decided to make my own for the Wyvern

Not that it did any good, I still had to relieve the cam shaft bracket to make the cam drive gear fit ! Oh well, I'm still an admirer of Mr Westbury, all part of the fun.

Rod

Edited By Roderick Jenkins on 12/07/2015 12:48:37

[AjohnwParticipant @ajohnw51620]

If you lap them as they are both based on the same prime number the same teeth will keep coming into contact so "wear" wont be even. You might get round that by lapping for a bit and then advancing the mesh by 1 tooth etc. The other problem is due to tooth count the big gear will win. Maybe you should ask the suppliers for the returned one back and lap two of the same size using the mesh advance idea to keep things even. Then lap the actual parts at the correct spacing. I suspect from the quality they may be a bit of a loose fit to each other. Sort of very noisy things that get fitted into power tools.

I've had some strange interests – just how did they make very accurate big telescope drives using spur gears. Lap with a very careful choice of tooth counts against gears that wont be in the final arrangement and then assemble. They may have done some more lapping then if counts were suitable. As the teeth should have a rolling contact parts that don't roll should wear off.

Maybe if the imperfections are shallow they could be cleaned up using more conventional methods, needle files, emery cloth pegwood or whatever.

As I had no idea what a Centaur was a search turned up Westbury's design. No details of the gears but mention of simple lathe attachments described in ME. It would be interesting to know how these worked as I doubt if they just geared up the lead screw for a screw cutting type arrangement. There are few ideas on youtube. This is one but he went cnc later

He doesn't give details but from elsewhere on the web I suspect he wraps a triangle round the part of the mandrel that can't be seen. There is mention of 1/8in aluminium being used in a forum. He does mention a template in this one

I've only done this sort of thing on a horizontal miller via a dividing head geared from the table so ways of doing anything like this at home are always of interest.

John

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[Roderick JenkinsParticipant @roderickjenkins93242]

John,

Chuck Fellows has written up his long lead fixture here (it's the pdf).

I managed to fiddle around with the change gears to get a maximum of 5 inches lead on my Myford – that's enough to make helical gears suitable for model engines. I've never managed to find a detailed description of the calculations necessary in 60 odd years of ME. It's all in Machinery's Handbook but needs quite a bit of teasing out, they always seem to start the explanation from somewhere I'm not.

Rod

[BazyleParticipant @bazyle]

I'm having trouble thinking of any machining method that would leave such marks. There don't seem to be traces of marks from a regular hob or a cnc endmill process going awry. It looks more like they were cleaned up by shot blasting with small bearings.

[AjohnwParticipant @ajohnw51620]

Posted by Roderick Jenkins on 12/07/2015 14:19:51:

John,

Chuck Fellows has written up his long lead fixture here (it's the pdf).

I managed to fiddle around with the change gears to get a maximum of 5 inches lead on my Myford – that's enough to make helical gears suitable for model engines. I've never managed to find a detailed description of the calculations necessary in 60 odd years of ME. It's all in Machinery's Handbook but needs quite a bit of teasing out, they always seem to start the explanation from somewhere I'm not.

Rod

I have to await moderator approval before I can access that.

Not sure I would want to punish my leadscrew like that but I probably have the gears to push the pitch up a lot. I suspect I wonder about doing nasty things to a cheap compound slide and use the same gear method milling machines use. The training exercise was a perfectly formed 3/4 in dia twist drill apart from grinding the end – sadly in mild steel but i also machines some replacement vice screws the same way which could have been done on a lathe.

From time to time I regret loosing my training notes and handouts but I do have some info on milling gears produced by the machine manufacturers. I'll see what I can dig out. Your correct though, Info on many things often isn't explained very clearly.

John

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[Roderick JenkinsParticipant @roderickjenkins93242]

Not really any particular strain on the leadscrew, in this configuration the leadscrew is driven from the handwheel – the only strain is on my wrist

Rod

[clogsParticipant @clogs]

Hi Maurice, just get ur money back…..they are junk,……won't belong before they will be unusable….

.I was in a similar position as u…..found several English firms making gears around ur size …I wanted case hardened, a matched pair 4 my job ranged from £45 – £340…..

I settled for just under £100 they were perfect……cant find the receipt….when I do I'll post it….

clogs

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