Many car enthusiasts are familiar with the high pitch whine a transmission with straight cut gears makes, but not many people know why race cars use straight cut gears.
Most people think that straight cut (actually called spur) gears are stronger than helical gears and that’s why their used. While generally race transmissions with straight gears are very strong, its not because of the straight teeth on the gears. What may shock you is that straight cut gears are actually weaker than a same sized helical gear. The reason for this is quite simple: helical gears have more teeth meshing and thus have a greater load bearing surface area. Another myth is that straight cut gears are more efficient than a helical gears; but the difference in efficiency is negligible with both spur and helical gears having about 98-99% efficiency.
So why are straight cut gears used in race transmissions? Well, there is only one real reason, and that is because straight gears don’t produce any axial loads (thrust loads). Its very easy to build strong gears that are either straight cut or helical, so that’s not the problem. The problem is when you start putting a lot of torque through helical gears, the axial loads become so high that you need a very strong transmission case and shafts (axles) to contain the loads; this can add significant weight to the transmission. Straight cut gears simplifies the transmission case design and also the types of bearings used. This allows for a very light weight transmission to be designed – and we know that weight is everything in a race car. The one negative of straight cut gears, and its a big one, is that they produce a lot of noise.
The above image shows the axial loads (red) produced by helical gears. The transmission case and bearings must be designed to handle this load as well as the normal force (blue) of the gears pushing away from each other.
In many instances, the axial loads produced from the helical gears can be limiting factor of a transmissions torque capacity. That is, the case fails before the teeth on the gears do. By installing an aftermarket straight cut gear set into a case that came from the factory with helical gears, it is possible to drastically increase the load capacity of the transmission. Depending on the application, straight cut gears may not be necessary because it would increase the transmission capacity well beyond the power the engine outputs. Simply reducing the helix angle of helical gears can increase transmission capacity and keep the noise at a more acceptable level. This was the case with the famous Muncie M22 4 speed that was prolific in many GM muscle cars. Even though that transmission used helical gears, the helix was at a shallow 20 degrees compared to 45 for the M21 (the low performance version of the same trans). The M22’s gears did make some noise which earned it the nickname “Rockcrusher”.
Interestingly, there is a gear design called a double helical (or herringbone gears) which cancels out the thrust loads; but, I’m not aware of any car transmissions that use this design.
These gears are from an industrial application. The larger gear is over 12′ tall (3.65m)
I’m not sure if these types of gears are suitable for a car transmission since all of the double helical gears I have seen are very wide. There’s probably not enough of a benefit to justify the extra cost and manufacturing difficulty in using this type of gear.
If you like this post, be sure to check out my post on Dog Engaged transmissions. “Dog Boxes” or, incorrectly, “Crash boxes” as they are also called, are high performance race transmissions that typically use straight cut gears. These transmissions provide clutchless shifting and are the fastest transmissions outside of twin clutch paddle shifts.
—
Muncie M21/22 information source:
http://442restoration.homestead.com/files/muncie_4_speeds.htm
Gear types and efficiencies:
http://www.roymech.co.uk/Useful_Tables/Drive/Gear_Efficiency.html
An example of double helical gears can be found here in the 1934 Jaguar SS owners manual:
http://www.tocmp.com/manuals/Jaguar/1934SS/pages/34_ss_manual_10_l_jpg.htm
Another question I see concerning straight cut gears is that dog engaged transmissions (aka crash boxes) only come with straight cut gears. This is not the case at all. The type of engagement, either synchromesh or dog, has nothing to do with the style of gears. Synchros and dogs are found on the face of the gear and do not limit the gear style.
If you didn’t know this, you should read my post on Dog Engaged transmissions that I have linked in the post. Here it is again! http://automotivethinker.com/?p=880
Hi, the image of the big cogwheels are from Andre Citroen , founder of Citroen Automobiles,
http://en.wikipedia.org/wiki/Andr%C3%A9_Citro%C3%ABn
Problem solved – no axial forces, take a look at http://www.belix.se
Hi-performace gearbox without no drawbacks
what is the furmula is helical gearcuttting and sitting thedrive gearand lead gear.pllease
Crash boxes and dog boxes are entirely different things, a crash box is a gearbox where the gears are not in mesh until you slide it in to gear, this also means that by default you cannot have a helical or herringbone crash box, these where common in big trucks 70 years ago..
Looking into this, I confirmed this to be true. I will edit the post to clear this up. I don’t know how the confusion in terms started, probably because most authors are don’t really know what the transmissions are called; but also because of the horrible sound when a shift on a dog engaged trans is poorly executed.
Also Citroen’s logo is a herringbone gear..
Herringbone gears would not work in an automotive transmission as the gears have to slide against each other
I think you should do some research into how modern manual transmissions work.
No, he is correct. How would such a gear engage another gear? The slide into eachother.
Modern transmissions don’t slide gears into each other like you are thinking. The gears in modern transmissions are of a constant mesh design. I suggest looking up a youtube video to learn how it works. https://www.youtube.com/watch?v=wCu9W9xNwtI – this video covers both the old ‘sliding mesh’ transmission which you are thinking of (also known as crash box) and the modern constant mesh which is used in every manual today.
Gears are engaged and disengaged from the shaft, not from eachother.
Yup, exactly. Dontcha just love the “know-it-alls” out here on the internet? …especially when it comes to “car stuff” for some reason….
Also herringbone are not quite the same as double helical…. although they posses similar characteristics and look similar, the double helical is 2 helical gears machined onto a common shaft or hub with a clearance groove down the middle, the herringbone has no such clearance groove down the middle and is much stronger, they where used in Citroens …