Looking after an R380

**Bush65** · 21st December 2009, 12:34 PM

Fifth gear can see much higher tooth loads.

For a given vehicle load the torque at the gearbox main shaft will be the same.

Now this torque at the main shaft is the same for all gears, but because fifth gear (on the main shaft) has the smallest pitch circle diameter, the tooth load will be higher (think of lever principle using the pitch circle radius).

But fifth gear on the gearbox main shaft besides having highest tooth load also has a smallest number of teeth than its mating gear.

The tooth profile of gears with small tooth number count is such that the thickness of the tooth near it's root is smaller compared to a gear with more teeth (assuming same module teeth) - this is the nature of involute tooth profiles. The tooth load creates high stresses at the root of the tooth, so the gear with a smaller number of teeth will not be as strong (everything else being equal).

Now the other issue is with wear, which is a primarily a function of Hertzian contact stresses. It turns out that the tooth profile for a gear with smaller number of teeth has a smaller radius at the contact points on it's flank - this is the nature of involute tooth profiles. This is the main reason why the Hertzian contact stresses are higher. The other main factor in Hertzian stress is surface hardness, but this is a material property nothing to do with tooth profile.

In all gear design, where different diameter gears are in mesh, the designer has to evaluate the strength and wear of both the smaller and larger gears (4 calculation for each gear pair).

Usually the pinion (small gear) is made from higher strength material than the wheel (not waste $$ on material not needed for wheel strength/wear).

The other things that designers can do is apply positive addendum modification to the pinion - this results in increased thickness at the root of the teeth. But there is a limit to how much addendum modification can be used because the tip of the teeth can get too thin which can be a problem for case hardening depth (case hardening materials produce the strongest gears).

The other main factor that the designer can control is the tooth width. Width is increased to increase tooth strength/wear capacity. But there is limit in how much the width can be increased, over which the returns diminish because the loads are distributed uniformly over the tooth width (mainly due to deflection).

IMHO, the gearbox designers have taken the view that drivers will/should use lower gears for pulling heavy loads and have saved $$ in the design of fifth gear.

Edited 3rd sentence to fix error in logic (brain fade when fist written) and small change in 4th sentence for clarification.

**vnx205** · 21st December 2009, 01:00 PM

That's just what I wuz gunna say.

Seriously, Bushy, I liked that explanation.

However, that was mostly to do with tooth loading and the likelihood of teeth failing. Aren't more gearbox problems bearing or shaft wear?

EDIT
Is that third sentence right. Wouldn't the bigger gear have lower tooth loading?

**Bush65** · 21st December 2009, 03:18 PM

Originally Posted by vnx205

...

However, that was mostly to do with tooth loading and the likelihood of teeth failing. Aren't more gearbox problems bearing or shaft wear?

EDIT
Is that third sentence right. Wouldn't the bigger gear have lower tooth loading?

I agree with all of those points. Thanks for picking up error - I will edit the post when I have more time.

I'm not sure though that there aren't problems with with 5th though.

Bearing wear is always a problem with gearboxes - leads to tooth alignment errors which leads to ....

My brain wasn't in gear when I wrote that - I am busy with, and thinking of other matters today.

**vnx205** · 21st December 2009, 05:04 PM

I feel a bit pleased with myself for spotting that mistake.

I'm not pleased just because there was a mistake there, but it means I must have understood a fair bit about what you were talking about to realise there was a typo.

Come to think of it, you probably should be pleased too. It means your explanation was good enough to make sense to a layman.

**Blknight.aus** · 21st December 2009, 05:48 PM

no thats correct...

if your talking about reducing tooth stress you use a smaller gear to drive a larger one or you make the gears wider (thats the easy way without going into exotic tooth shape modification and metalurgy canges)

heres a simple experiment.

grab a ruler and hold it at one end. now hang a can of soft drink from the other end.

in this example we'll use the can to represent the driving force because its trying to turn your hand.

as you slide the bit of string its hanging from closer to your hand the can has less influence on your hand meaning you can exert less effort to hold the can up.

its a very simple second order lever, a pivot with a load and then some work applied to it. (the center of rotation is the pivot, your thumb is the load and the work is done by the can)

now lets make it a 3rd order lever and call your thumb the work and the can the load and you notice as the can moves further out on the ruler the harder it is for you to keep it level.

now lets get a little more complicated.

grab 2 rulers, one in each hand and hold them at 90 degrees to each other so you can slide the contact point along both rulers.

lets call the ruler in your left hand the layshaft of the gearbox and the one in the right the output shaft.

lets put our imagainary box in gear. place the number 30 on the ruler in your right hand over the number 10 on the ruler in your left hand (which should be right net to your thumb. now turn your left hand. A lot of easy left hand motion produces a little movement on the right hand.

experiment with sliding the contact point along the 2 rulers eventually you'll get to a lot of effort over a short distance from your left hand produces a lot of motion without much force in your right hand.

thats basic gear principles.... now to proove tooth loadings.

go grab some of your kids playdough roll up a ball and place it between the 2 rulers and repeat the above experiment at all the various ruler positions re-rolling the same piece of dough each time.

which application produces the thinnest blob of playdough? Have you noticed that the surface of the ball cracks a little around the edges of the disc that is produced

now picture your blob of playdough as a tangle of polymer chains that make up your oil.

**Bush65** · 21st December 2009, 06:34 PM

Jumping in before Ron!

As the Queen is our only ruler, I have difficulty imagining her with a can of soft drink hanging off one end.

I'm sure you meant to type rule, not ruler.

**Captain_Rightfoot** · 21st December 2009, 07:02 PM

Originally Posted by Bush65

Jumping in before Ron!

As the Queen is our only ruler, I have difficulty imagining her with a can of soft drink hanging off one end.

I'm sure you meant to type rule, not ruler.

Yes, She is not amused.

**Blknight.aus** · 21st December 2009, 07:19 PM

As an Atheatic Republican SHE can go and get........

**County3.9** · 22nd December 2009, 09:54 PM

Originally Posted by Bush65

Jumping in before Ron!

As the Queen is our only ruler, I have difficulty imagining her with a can of soft drink hanging off one end.

I'm sure you meant to type rule, not ruler.

Long live queen Pauline!

**justinc** · 22nd December 2009, 10:37 PM

John,

Just an observation here,

I have noticed with ALL the R380 5th gear failures (Actual Broken teeth etc) I have encountered that the failure of the gear itself was IMHO brought about by the initial failures of one or both of the support bearings. This causes excessive deflection of the shafts under the side loads experienced by helical gear sets, and causes tooth failure by grossly exceeding the design stresses of the gear teeth. IMHO the 5th gears themselves in the R380 are adequate, it is the ecessive loads placed on the wrong parts of the teeth that cause them to actually break.

Good quality Synthetic oils, changed at appropriate intervals and keeping these gearboxes away from big 4 cylinder Jap diesels with turbos on them

will ensure their long lifespan

JC

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