[Half-Inch Stud]

Does anybody have data on the TH400 2.48, 1.48:1 Straight-Cut gear strength in the 1/4 mile use?

To me they appear more efficient, but the Straight tooth appears to have more load across less tooth are?
Might make a nice loud 1st & 2nd gear noise on the street.

Never busted a stock helical planetary, nor seen a 1/4 mile run bust the stock helical planetary. Seen a cooked planetary set from a core that had > 45 minute continuous 2nd-1st gear ride limp home.

[Scott65]

Thrust load should theoretically be better with the straight cuts. Maybe less likely to cook bearings?

[Half-Inch Stud]

Yes, near-zero Thrust loads on all the HD-Torringtons is the low-hanging fruit, for MPG.

Helical-Cut Sun Gear just looks way stronger for TQ-Transfer, and Wheel-Hop shock levels.

[mgarblik]

Not a scientific answer, but the factory went to the straight cut gears in 1 ton and heavier applications of that transmission I believe. Since they are more noisy, extra strength seems to be the only reason they would have gone to that design. Just my observation. How much stronger? People racing them hopefully will chime in.

[Shiny]

Seems you already know the helical gearset will be stronger in "bending" normal to the teeth if all else is equal. That will follow shared loads across multiple teeth in the helical. But if the dominant fail mode is bearing or case failure from thrust loads, then clearly the helical loses. Maybe the truck example supports that?

But your question is interesting. Assuming a straight-cut was chosen for "efficiency", would it still have enough strength margin to handle the abuse of a race car?

My guess is the materials for an aftermarket straight-cut might be stronger than OEM helical so apples to oranges may be part of the answer.

[Half-Inch Stud]

@ Shiney Yea, we heard alot about M22 straight-cut gears, yet i was fine with the M21 HD, As for TH400 also fine with the helical cut and case loading because the High Density Torrington are slippery under thrust loads. Far as i recall Strong Steel modulous and Yield strength is a tight grouping across the alloys.

Any racers insist on the straight-cut planetarie for the TH400 ?

I have seen photos long ago the non-stock ratios (numerically lower 1st gear) loosing teeth in the 1/4 mile. Pretty ure those were traight cut teeth.

[chiefbigb]

Heavy truck transmissions Eaton Fuller meritor ect. In the main case use straight cut gears but in the auxiliary/ rear box use angled teeth. I know some power train engineers in my field. Good question to ask them

[Shiny]

"Successful engineering is all about understanding how things break or fail."
Henry Petroski

Originally Posted by Half-Inch Stud

... helical cut and case loading because the High Density Torrington are slippery under thrust loads.

That's a good point. Maybe helical gears have more "sliding" motion along the tooth contacts? If so, I guess wear at the contact surface could be another failure mode influencing the design decision. As I recall, no gear is "perfect" WRT rolling vs sliding

Originally Posted by Half-Inch Stud

Far as i recall Strong Steel modulous and Yield strength is a tight grouping across the alloys....

Yes, all steel alloys share the same elastic modulus but there can be big differences in yield and tensile strength.. typically correlated with cost! To me the answer to your question can't be answered without knowing "how it fails". If it's overstress from impact loads (ie "dump the clutch"), then "toughness" properties of an alloy will be important.

If the biggest risk is fatigue failure, then tensile strength is important but so is design and processing (ie stress risers, residual stress, microstructure). I'll have to look up the differences in tooth profiles between helical and straight gears. Maybe it's hard to keep the tooth stress the same when the ratios are the same?

Based on your descriptions and comments from others, it sounds like thrust loading of the case and subsequent thrust bearing or case failure may be the practical concern when helical gearsets get overloaded.

Originally Posted by chiefbigb

Heavy truck transmissions Eaton Fuller meritor ect. In the main case use straight cut gears but in the auxiliary/ rear box use angled teeth. I know some power train engineers in my field. Good question to ask them

Cool! Will be interesting to hear what they say if you think of it.

[ErikW]

Originally Posted by Half-Inch Stud

Does anybody have data on the TH400 2.48, 1.48:1 Straight-Cut gear strength in the 1/4 mile use?

To me they appear more efficient, but the Straight tooth appears to have more load across less tooth are?
Might make a nice loud 1st & 2nd gear noise on the street.

Never busted a stock helical planetary, nor seen a 1/4 mile run bust the stock helical planetary. Seen a cooked planetary set from a core that had > 45 minute continuous 2nd-1st gear ride limp home.

If you have them, use them but not worth trying to find them. They are a bit noisy also.

[Half-Inch Stud]

Thus far, i plan to tell the fella the helical setup is a freebie, unless he wants the noise of straight-cut gears.

[Formulas]

when i took apart my 1968 GTO th400 one of the thrust washers was gone from one helical gear and there was substantial wear in the drum from helical side loading, tossed that sucker decades ago

[taff2]

We like to use the st cut factory gear sets in our TH400 builds if they are available- trouble is,they are in short supply now and pretty expensive.
We have one st. cut set in a customers 6000lb LWB Chevy race truck, with a 632 cu in BBC and Procharger it makes around 1600hp. It has run flawlessly for the past two race seasons apart from the stock output shaft snapped. We renewed that with an aftermarket 300M shaft, changed the low roller sprag and checked out the rest of the trans - the factory st. cut gearset was perfect.
Also have another st. cut set in a 4000lb twin turbo small block street/ strip van, has run flawlessly for the past three seasons, 9.40@ 142.
Look in the big Chevy powered motor homes and trucks for a set, usually if the trans has the large drum brake on the rear of the trans case -it has the st.cut gear set.

[Half-Inch Stud]

@taff2 thank you, as that is the only data for me to go by. Boosted-engine data is definitely a Higher TQ load than any NA engine TQ output.

Yea, i got the rear drum brake tail kit with it.

[JSchmitz]

Straight cut gears are stronger than helical gears. They have more tooth contact area for better load distribution. They consume more power due to friction. They are also louder because they kinda crash together vs. rolling together progressively.