Hypoid oil? …why?

[Blackal]

There's never been one on mine, never really thought about it a poor quality pic I took today.

Modified type - for o-ring seal.

Could be worth a good clean and condition of O-ring before replacing.

Al

[nab 301]

]

Modified type - for o-ring seal.

Could be worth a good clean and condition of O-ring before replacing.

Al

Really Al? , I must admit it's the first time I had a close look at it in 10 yrs . Presumably o ring is on gearbox case? The knees and hips weren't up to grovelling yesterday before I read this but I couldn't feel anything and there's no oil leaks despite refilling with synthetic.
I'll have a close look next time assuming it doesn't do an Exxon Valdez in the mean time..

[Blackal]

Sorry - a bit of an optical illusion on the first view.

Looks like a thin o-ring, but on zoom-in it looks like a twin anular ridge.

Sorry

[Corvus]

Service manual states that the gearbox and end gear requires hypoid oil.

The difference between a regular bevel gear and a hypoid gear is that in the hypoid gear, the center of the pinion is offset from the center of the crown wheel.
See pic: http://en.wikipedia.org/wiki/Hypoid
This requires special oil, due to the contact point twisting as it’s rolling along the cogs.
But the end gear is not a hypoid gear! And the gearbox doesn’t even have a bevel gear (obviously, since the crankshaft is longitudinally mounted) in there!

So why does BMW say that that the gears require hypoid oil?

Must add: it’s not that I’m losing any sleep over this matter, I’m just wondering if anybody has any info on this.

//T

I wondered the same. In fact I'm wondering now, over a beer and not going to be losing sleep either!

I believe a hypoid gear has a degree of sliding, similar to a worm gear, whereas a helical gear, spur gear, bevel gear and I think also a spiral bevel gear all have a rolling action on a constant pitch circle.

If I've got that right will there be more oil shearing in a hypoid, requiring a different oil? Still, that doesn't explain why bmw need it in their gearboxes and final drives!

The gearbox has a torsional ramp action shock absorber. Would this need a hypoid gear oil? Not sure what is in the final drive which needs it. The spline?

At the end of the day maybe they found that they can increase tooth loads (pressure) by using this type of oil, thereby able to use smaller gears?

Just speculation though.

With all the recent talk of EP greases, this took my mind back to the above question, posed by tapio.

Just another guess really, and kind of the same conclusion I reached above, but with a guess of an explanation.

Perhaps the EP additives allow a lower viscosity oil, for the given gear tooth loadings, than could be obtained without EP additives? If so, then, given this is a motorcycle gearbox and not an industrial one (where the solution would be to simply raise the oil viscosity) it might be a way of achieving smaller gears and reasonable operation of the gear selection process, without a really thick oil spoiling that aspect of the gearbox.

Dunno. Pure guesswork!

[Tapio]

Achieving smaller gears with EP additives?
No, I really don't think so. Of course, if you have straight cut, spur type gears, then making gears wider would lessen the risk of having metal-to-metal teeth contact.

But not when it comes to helical gears. In a helical gear you don't have a full line of contact (like in spur gears) but a point of contact that moves along the tooth face as the gears turn. So making the gears wider doesn't help att in reducing oil film pressure in the contact point.

In fact, I don't think metal-to-metal contact is a big issue in gear tooth geometry designing. A much bigger issue is NVH. So i've heard. Gears can whine a lot if this is not adressed. A good example is the whining you can hear in a car with a manual transmission going in reverse. reverse usually is a non syncronised spur gear.

I have done hundreds of failure analysis on spiral cut hypoid gears when I worked at Getrag.
They were shipped to us from Land Rover's proving ground in Solihull.
Metal-to-metal contact was never a problem. Maybe one gear in a hundred had some very small pitting in the contact faces. You needed a magnifying glass to see it.

[Corvus]

Achieving smaller gears with EP additives?
No, I really don't think so. Of course, if you have straight cut, spur type gears, then making gears wider would lessen the risk of having metal-to-metal teeth contact.

But not when it comes to helical gears. In a helical gear you don't have a full line of contact (like in spur gears) but a point of contact that moves along the tooth face as the gears turn. So making the gears wider doesn't help att in reducing oil film pressure in the contact point.

In fact, I don't think metal-to-metal contact is a big issue in gear tooth geometry designing. A much bigger issue is NVH. So i've heard. Gears can whine a lot if this is not adressed. A good example is the whining you can hear in a car with a manual transmission going in reverse. reverse usually is a non syncronised spur gear.

I have done hundreds of failure analysis on spiral cut hypoid gears when I worked at Getrag.
They were shipped to us from Land Rover's proving ground in Solihull.
Metal-to-metal contact was never a problem. Maybe one gear in a hundred had some very small pitting in the contact faces. You needed a magnifying glass to see it.

My understanding of tooth contact with helical cut gears is different to yours. I've always understood that they will carry higher loads for a given width because more area is in contact, not less.

http://science.howstuffworks.com/transp ... /gear3.htm

The load carrying ability of the oil is fundamentally important to gear tooth life, not ultimate shear strength of the tooth, but literally pitting, as you've said. Better surface finish, material choice, hardening and also oil viscosity are all deciding factors in determining tooth width, although mainly ultimate shear strength. Also gear pcd. A smaller PCD, for a given torque, will increase pressure on the tooth. Imo.

If a certain oil allows higher pressures to be carried, then the gears can potentially be smaller and narrower, as long as the tooth ultimate shear strength is not the deciding factor. In a motorcycle gearbox, as low a viscosity as possible must surely help gear selection?

[Corvus]

http://automotivethinker.com/transmissi ... s-helical/

http://www.thomasnet.com/about/helical- ... 00404.html

[nab 301]

http://automotivethinker.com/transmission/straight-cut-gears-vs-helical/

interesting link , I never realised that helical gears produced very high axial loads

[Corvus]

http://automotivethinker.com/transmission/straight-cut-gears-vs-helical/

interesting link , I never realised that helical gears produced very high axial loads

Depends on the helix angle. Use trigonometry to find the value as a proportion of the tangential load. The axial load load created by a small helix angle could sometimes be carried quite adequately by a deep groove ball bearing. So it needn't always be very high. Usually it's relatively small. Imo.

[Corvus]

http://automotivethinker.com/transmissi ... s-helical/

interesting link , I never realised that helical gears produced very high axial loads

Also, on the above link, the component forces shown are not, to my way of thinking, the highest component. He shows the axial and separating components, whereas, in most applications the highest component by far is almost acting tangentially, but is tilted by the tooth pressure angle, which lengthens the vector line. Fig 8.1 on this link illustrates it perfectly. Force Fn they are calling it. This same force is felt at the bearings carrying the shaft (but in the opposite direction), to my way of thinking.

http://www.khkgears.co.jp/en/gear_techn ... 55-461.pdf

Fig 8.3 shows what I meant in my previous post regarding the axial component. You can see that, for a smallish helix angle which a lot of gearboxes have, the vector line is not that long. I'm definitely no mathematician, but if you are half decent at trigonometry, you'll get your head around it no problem.

[Tapio]

My understanding of tooth contact with helical cut gears is different to yours. I've always understood that they will carry higher loads for a given width because more area is in contact, not less.

From your first link:

When two teeth on a helical gear system engage, the contact starts at one end of the tooth and gradually spreads as the gears rotate, until the two teeth are in full engagement.

Wikipedia:

With parallel helical gears, each pair of teeth first make contact at a single point at one side of the gear wheel; a moving curve of contact then grows gradually across the tooth face to a maximum then recedes until the teeth break contact at a single point on the opposite side

ok, so the contact point moves, but its never utilizing the whole width of the tooth face. So how can you decrease the risk of metal-to-metal contact by widening the gears?

[Corvus]

...........ok, so the contact point moves, but its never utilizing the whole width of the tooth face. ..............

Gear tooth teamwork?

If my own understanding is correct then the links we have put up don't adequately explain the situation.

If we follow the action of one tooth only, in slow motion, then what happens is as described in the links. But if we look at our chosen tooth just as it begins to engage, then freeze frame and look at the tooth (or teeth, depending on the helix angle) preceding ours, we should see full tooth contact. In other words, before any given tooth begins to leave full contact, the next one coming around starts to engage. Maybe this is partly the reason why there is more contact area than spur gears. That, plus the fact that, because the tooth flank is set at an angle to the shaft centres, the tooth itself is longer than it would be on a spur gear of the same width.

[Corvus]

Achieving smaller gears with EP additives?
No, I really don't think so. .........

.........In fact, I don't think metal-to-metal contact is a big issue in gear tooth geometry designing. .................

My take on things.

To a certain extent I see what you mean. The action of an involute tooth is generally a rolling action, more than sliding. But still, if you roll one metal object over the same area time and again, with substantial pressure pushing onto it, then you will eventually get localised fatiguing of the metal. Initially a kind of flaking, leading to pitting.

The pressure I mentioned is the key issue. If the lubricant is inadequate it won't "support" the teeth and the metal will have to share a proportion of the pressure. Usually, given a basic mineral oil, the only real answer, if we don't want to proportion our gears larger, is to increase the oil viscosity.

This was my point about the EP additives. Could the addition of them allow a lower viscosity than would be possible with a basic mineral oil?