the answer is five past one, but only on dry days
Gear Braking?
Moderators: slparry, Gromit, Paul
Re: Gear Braking?
The "parting apart" you mention is a mechanical fact relating to bevel gears, yes. But it is directly proportional to the amount of torque transmitted. The blipping of the throttle won't affect it, in fact during blipping the load will lessen on the bevel gears, I would think.BMbler wrote:Had few sherry's and got in a slow, deep thinking mode... to wet to be riding
What's your opinion on this bike?
Bleeping to change gears down is surely a good thing, but then considering shaft loads as for "parting apart" from mechanical point of view?
If you're particularly brutal in changing rapidly down the gears, basically too soon, the back drive torque will obviously be quite high. I'm not sure that scenario would necessarily be the highest load the bevel gears would ever see though.
What made you ponder that? Obviously a gallon of sherry would help.
I'm no expert, but I just can't see the maximum potential for back drive torque to be as high as the maximum potential for forward driving torque. Nowhere near in my eyes. How the helix angle and direction of rotation come into play in applying forces to the teeth, shaft and bearings I'm not confident, when it comes to spiral bevel gears.BMbler wrote:Not that many bikes are shaft driven and considering some work done on my FD assembly, some articles(as one above) made me think about FD teeth wear and the way it loads, lubes itself... I blame those, not the cherry or the amount of, that got consumed in the afternoon tea time.
Back on that steam train of a thought- surely gradient progression of pinion assembly indicate tolerance of load one way (accelerating), whilst upon gear braking it gets to work in opposite direction- with higher impact angle , hence direction of load would be "parting away from each other", loading up bigger torque force on pinnion, with less lubricating, hence heat generating and quite importantly- no lesser (maybe even higher load) on transmission due to simple fact, gearing down by engine could be to the full potential of compression of engine (negative HP), than accelerating (ignited petrol HP)... ( I get the feeling of not being to wise now and standing up out of my trenches
Saying that, on one of them damp days I had nearly flattened myself out on cold tires once or twice clearing out away from the traffic, by having rear to spin by throttle...
With back drive versus forward drive torque, in one direction the pinion does the driving and in the other direction the crown wheel does the driving. But there are still a combination of radial and axial loads acting in both cases. These loads are directly proportional to the torque being transmitted although the direction of loads applied will vary with the direction of rotation with respect to the helix angle and helix direction. I guess.
BMbler, I've re read your last post and maybe some of the things you are saying is the same as me. I'm not sure, as there may be something lost in the translation! Sorry.
A few things you have said which seem a bit different to my own understanding:
Successful gear lubrication. Lubricant able to withstand the pressure applied is determined to a large extent by the operating viscosity. Lubricant able to maintain contact with tooth face is determined to a large extent by the m/sec circumferential speed at the tooth contact diameter.
Gears always tend to "part away" from each other. They have to in order to transmit torque. Just how much they want to part away and in what direction will determine how that load is placed on the shaft and bearings supporting the gears.
Maybe that's wrong? If I've misunderstood you, I apologize.
A few things you have said which seem a bit different to my own understanding:
Successful gear lubrication. Lubricant able to withstand the pressure applied is determined to a large extent by the operating viscosity. Lubricant able to maintain contact with tooth face is determined to a large extent by the m/sec circumferential speed at the tooth contact diameter.
Gears always tend to "part away" from each other. They have to in order to transmit torque. Just how much they want to part away and in what direction will determine how that load is placed on the shaft and bearings supporting the gears.
Maybe that's wrong? If I've misunderstood you, I apologize.
Ha ha. I think you may be punishing that bmw of yours. No pipe and slippers for you I'm guessing.BMbler wrote:Corvus,
We are "on a same boat", your English reading skills are perfect and I have warned at the beginning- its is one them "cherry induced deliberations" on my part.
All and every-one knows as how oils are graded, classified and appreciated(I believe)- by the :
Friction reduction upon stress
by temperatures, before lubrication become non affective
So, seeing the article(above), I thought what happens on negative load- still unsure mate! as patch of impact gets "sharper" angle, as to what is normal torque load is upon acceleration...
Also, on a road, it happens, once in a while, I get to slip rear when down-shifting (mind you- combining brake and engine) to have a tiny slip (skid) on rear at about 4's or maybe 3'rd or even 2'nd once in a while... meaning (not necessarily) that such forces are way bigger than those of acceleration!? I mean I do get wheelies on 2'nd, maybe clutched at 3'rd at best. so the force on FD could be higher on gear barking?
In spite of all the punishment you are giving that bike of yours, I can't see how the loads on back drive can be higher than on forwards full power. I'm confused by the sharper angle you talked about? Which angle?
Go steady with that sherry!
Re reading your attached link. To my knowledge the klingenberg profile is strictly speaking a spiral bevel and not helical.
When setting contact patch between teeth on a spiral bevel, my understanding is that preference be given to achieving best contact in the direction of highest load. In our case that is driving the bike forwards. Setting a "perfect" contact patch in one direction, then checking in the other direction may show an imperfect contact. Engineering is not perfect to my knowledge. Compromises abound.
I fully agree with the overall appraisal of the article in that fitting a deep groove ball bearing is asking for trouble.
When setting contact patch between teeth on a spiral bevel, my understanding is that preference be given to achieving best contact in the direction of highest load. In our case that is driving the bike forwards. Setting a "perfect" contact patch in one direction, then checking in the other direction may show an imperfect contact. Engineering is not perfect to my knowledge. Compromises abound.
I fully agree with the overall appraisal of the article in that fitting a deep groove ball bearing is asking for trouble.
BMbler said: "Also, on a road, it happens, once in a while, I get to slip rear when down-shifting (mind you- combining brake and engine) to have a tiny slip (skid) on rear at about 4's or maybe 3'rd or even 2'nd once in a while... meaning (not necessarily) that such forces are way bigger than those of acceleration!? I mean I do get wheelies on 2'nd, maybe clutched at 3'rd at best. so the force on FD could be higher on gear barking?"
Yes, I hear what you're saying. But are you forgetting the effect of weight transfer? That will mean that the rear tyre will break traction with a great deal less torque than under acceleration. The opposite applies under acceleration. The weight transfer is rear wards so to make the rear tyre break traction will require significantly more torque.
No?
Also, to do the experiment, you can't apply the rear brake.
Yes, I hear what you're saying. But are you forgetting the effect of weight transfer? That will mean that the rear tyre will break traction with a great deal less torque than under acceleration. The opposite applies under acceleration. The weight transfer is rear wards so to make the rear tyre break traction will require significantly more torque.
No?
Also, to do the experiment, you can't apply the rear brake.
http://www.pkservis.com/data/web/upload ... ola/21.pdf
The above has a good set of diagrams showing reactions for spiral bevel gear pairs. Ok, it's an article on Gleason cut gears but I think the reactions are generally the same. I chose it because the diagrams are pretty clear and helpful.
I think our set up is the first diagram. Top line, left hand side?
If so, that explains more about the bearing arrangement in the boxer FD in question. I've also proven myself wrong. That I don't fully understand how spiral bevel gears work (although I did kind of admit that).
Yes, gears always separate, but the spiral bevel is a peculiar thing and difficult to get your head around. If I've understood the diagrams I've attached then in our case the axial component for the crown wheel is a tendency to draw it deeper into mesh. Hence the position of the taper bearing in the FD I guess.
What BMbler seems to be asking is whether these reactions reverse when shifting down. If they did then the pinion shaft has no means at all of supporting itself axially. Except the gear teeth themselves, which I now see is what BMbler is suggesting, I think? Perhaps BMbler can clarify I have understood that right. It's a very fair question.
Personally I don't think the reactions do reverse for two reasons. Firstly, the directions of rotation are still the same. Secondly, bmw have deemed it perfectly ok to not make any allowance for axial thrust in that direction, with regards to the pinion shaft. It's explains to me why they have a ball bearing on that side of the crown wheel too. Although I would have thought that loads from the dynamics of the bike would warrant something with more capacity for axial load?
Anyone throw any more knowledgable light on the subject?
The above has a good set of diagrams showing reactions for spiral bevel gear pairs. Ok, it's an article on Gleason cut gears but I think the reactions are generally the same. I chose it because the diagrams are pretty clear and helpful.
I think our set up is the first diagram. Top line, left hand side?
If so, that explains more about the bearing arrangement in the boxer FD in question. I've also proven myself wrong. That I don't fully understand how spiral bevel gears work (although I did kind of admit that).
Yes, gears always separate, but the spiral bevel is a peculiar thing and difficult to get your head around. If I've understood the diagrams I've attached then in our case the axial component for the crown wheel is a tendency to draw it deeper into mesh. Hence the position of the taper bearing in the FD I guess.
What BMbler seems to be asking is whether these reactions reverse when shifting down. If they did then the pinion shaft has no means at all of supporting itself axially. Except the gear teeth themselves, which I now see is what BMbler is suggesting, I think? Perhaps BMbler can clarify I have understood that right. It's a very fair question.
Personally I don't think the reactions do reverse for two reasons. Firstly, the directions of rotation are still the same. Secondly, bmw have deemed it perfectly ok to not make any allowance for axial thrust in that direction, with regards to the pinion shaft. It's explains to me why they have a ball bearing on that side of the crown wheel too. Although I would have thought that loads from the dynamics of the bike would warrant something with more capacity for axial load?
Anyone throw any more knowledgable light on the subject?
I'm not at all confident of being right. I can't imagine anyone is going to come along and explain it with any degree of authority, so I guess that's end of it!BMbler wrote:A bit like to trying to explain Mona Lisa... underwater
I have to admit, that started by asking in clear terms... for myself ...and reading your posts again, it seems finally come to the point that we are in correct understanding as what we are discussing.
Unfortunately! I am not a bit sure if my concern is truly valid! the load on a pinion in reverse- is it much greater in reverse, opposing "normal" load forward?
Asking again, as I just can not get my head round myself anymore!
Interesting thought, while it lasted.
Cheers.
Having followed this thread with a little interest and a great deal of bemusement (you lost me after a short while), what I will say that in over 40 years of dealing with and rebuilding gearboxes, (industrial not automotive) using the gear types you talk about, I have personally never seen a gearbox fail due to tooth contact wear alone.
Some which have tried to chew bearings or cages. Its almost always bearings that fail first. The biggest cause of this is a failure of the shaft seal allowing the ingress of crud which over time dries the bearing and causes its demise.
Most of these gearboxes have a high torque output and have to brake heavy loads from speed to rest under the effect of gravity possibly giving the negative torque you describe.
These gearboxes, and I suspect our FD gears are engineered to cope with greater loads than anything we can throw at them in normal abuse!
Nothing to worry about, have another beer!!
Some which have tried to chew bearings or cages. Its almost always bearings that fail first. The biggest cause of this is a failure of the shaft seal allowing the ingress of crud which over time dries the bearing and causes its demise.
Most of these gearboxes have a high torque output and have to brake heavy loads from speed to rest under the effect of gravity possibly giving the negative torque you describe.
These gearboxes, and I suspect our FD gears are engineered to cope with greater loads than anything we can throw at them in normal abuse!
Nothing to worry about, have another beer!!
Mick
2001 R1100s Frost Blue
Its not going the fastest,
Its stopping the quickest
2001 R1100s Frost Blue
Its not going the fastest,
Its stopping the quickest
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Steve1200S
- Member
- Posts: 736
- Joined: Wed Apr 20, 2011 7:48 am
- Location: Sheffield
I understand they've had a few years practice with shaft drives ..... although they seemed to have forgotten it with the bevel boxes on the new millenium bikesSteve1200S wrote:I know the answer!
I'm sure BMW thought of it, and if engine braking does create more 'reverse' torque on the gears than accelerating, it will have been designed for it. Done.
--
Steve Parry
Current fleet: '14 F800GS, '87 R80RS, '03 R1100S BoxerCup, '15 R1200RT LE Dynamic, '90 K1, '05 K1200S
Steve Parry
Current fleet: '14 F800GS, '87 R80RS, '03 R1100S BoxerCup, '15 R1200RT LE Dynamic, '90 K1, '05 K1200S
Cheers! I'll take you up on that suggestion very shortly. I'm not worried.Merecat wrote:Having followed this thread with a little interest and a great deal of bemusement (you lost me after a short while), what I will say that in over 40 years of dealing with and rebuilding gearboxes, (industrial not automotive) using the gear types you talk about, I have personally never seen a gearbox fail due to tooth contact wear alone.
Some which have tried to chew bearings or cages. Its almost always bearings that fail first. The biggest cause of this is a failure of the shaft seal allowing the ingress of crud which over time dries the bearing and causes its demise.
Most of these gearboxes have a high torque output and have to brake heavy loads from speed to rest under the effect of gravity possibly giving the negative torque you describe.
These gearboxes, and I suspect our FD gears are engineered to cope with greater loads than anything we can throw at them in normal abuse!
Nothing to worry about, have another beer!!
Industrial gearboxes are a different prospect in one very important way. In my eyes at least. There are usually a range of different types and many sizes within those types. A particular gearbox, from the catalogue, will find itself in many and usually varied applications. It is selected from the catalogue using various service factors which, if applied correctly, will home in on the appropriate size for the job. A motorcycle gearbox is designed for one job only and there is emphasis put on compactness and weight. How a particular size of gear finds itself in a motorcycle is a very different process to how a particular size of gear finds itself in a general industrial application. There are exceptions. Some industrial gearboxes in more specialized applications are designed more specififically for that application, but that's the exception rather than the rule.
I don't know if you would agree with that?
Going back to the case put forward by BMbler, what seems to happen with the back drive torque scenario is that the opposite side of the gear tooth comes into contact, even though the direction of rotation stays the same. (That goes back to my earlier post about setting up tooth markings. It is unlikely to be perfect on both sides. One side will have to be a compromise).
Does that sound right?
Now whether the back drive torque reverses the direction of the reactions I'm not sure. But based on the fact that bmw know what they're doing you'd have to say that they stay the same. In the bmw design BMbler attached, the pinion appears to be catered for axial load in only one direction. Or certainly any heavy axial loads anyway. It seems to be the case that directions of reactions are linked into relative directions of rotation and the direction of the helix (or spiral), based on the diagrams I attached.
So, if any of that boxerbollox is right then, no, BMbler you don't have to worry! Have another sherry.
