Which transaxles can be dog boxed?

Seymour Snerd

Lifetime Supporter
Helical gears are superior to spur gears. Beside the quietness, the load carrying capacity is far superior with an helical assembly as the mating surface is obviously wider with an helical and also because the load distribution is progressive.
Stephane --

I'm not sure your argument holds together, but perhaps we just have some terminology issues. First of all is "width" of mating surface really the right parameter? Don't you mean "area"? Also, don't you need to consider the effect of frictional losses and heat generation from the "progressive" load distribution? Load carrying capacity, after all, needs to be seen in at least two different lights: instantaneous and long-term. Are you arguing that a helical gear set has both higher resistance to shock loads such as clutchless shifting as well as being able to tolerate higher power transmission over the long term? Always, under all conditions?

If you have references to some reputable technical articles that support your assertion we'd be all ears. The generalization is just a little suspicious given the predominance of straight-cut gears in transmissions designed for motorsports competition. Can you explain?
 
Alan,

Thanks for putting some interest in my posts. You are right, english being unfortunately my 3rd language and not my mother tongue, my lexical is then sometimes (often..) limited.

I can see from your question that my answer was not enough detailed and structurated but I can also see , that you are mixing some considerations wich have to be considered separately.

Gear Strength :

I maintain that an helical gear offers better load capacity than a spur gear. This considering obviously a set of 2 gears with exactly the same specifications in terms of material, width, and module (n° of teeth).

Keeping it simple, the reason is that the helical gear tooth is effectively larger since it is diagonally positioned. Also the contact ratio is higher (number of teeth involved at the mesh point) than a spur gear assembly.

You asked for "references to some reputable technical articles that support your assertion "...My library is full of mechanical books but most of them are unfortunately in french or italian. Anyway , I have some books in english wich I consider as "reliable" and I am happy to recommend you this one :

Manual Gearbox Design by Alec Stokes - SAE International ( Society of Automotive Enginers) - ISBN 0 7506 0417 4

You can buy it here :

Amazon.com: Manual Gearbox Design (9780750604178): Stokes: Books

My edition is 1992, I guess some went after but I doubt he denied what he wrote in its first publication.

Also, Alek Stokes has been involved in designing Formula 1 gearboxes for over 40 years, so we may think he can be classified as a "reputable source".

On page 37 of the 1992 edition, we can read :

"Helical Gears :

Single helical gears are an alternative to the spur gear, for transmitting power between parallel shafts, but the action of the helical gear with its teeth cut at an angle to its axis is different from that of the spur gear whose teeth are parallel to the axis. Standards of accuracy being equal, helical gears are superior to spur gears in the quietness of operation and load carrying capacity. As a result of the angular displacement of the helical gear teeth, the contact with the mating gear will run diagonally across the tooth face and not parallel as with a spur gear. Thus, the tooth engagement and load distribution is gradual and therefore quietness of running is an inherent feature of helical gearing and shock load is practically eliminated. This becomes a major advantage in cases where speeds are too high for the successful application of spur gears....."

Also "Google is your friend", and a quick search reported many links like this one :

What Is the Difference Between Spur & Helical Gears? | eHow.com

So with an helical design, we need less material for a predeterminned load capacity than with a spur gear. The diagonal design allows a thinner gear. Thus, the rotating weights are decreased and , beside the quietness benefit, this is why you find helical cutting in synchronized gearboxes as the less inertia the better to allow quick synchronization.

A negative point is the axial forces created because of the tooth angle wich then ask for a reinforced housing and bearings, so here weight and more friction (bearings).

Also, because of the angles there are friction losses and so the efficiency is affected.

Efficiency :

Spur gears offer a higher eficiency than helical. A common acceptance when doing raw calculations, for example to determine the reachable speed of a car, then to determine the gear ratio sequence is to take a 85% efficiency for an helical gearbox and 90% for a spur gearbox.

Obviously these numbers are not definitive and only the ones got from the dyno test are the truth but they help for the very first calculations.

For example, taking a gt40 of 1057 kg with the following specs :

Frontal area : 1.30989 square meter
Drag coefficient : 0.39
Air density : 1.22 kg/m3
Tire resistance coefficient : 0.015
Other resistances (brake pads, steering move ..etc) : 0.003

and to not make it too complex 0% percent Grade.

We can then calculate that for a speed of 280 km/h, the opposite drag would be of 2072 N.

With a 0.85 gearbox you will then need 254.2 hp to maintain this speed, and with a 0.90 : 240.1 HP. Considering , for example, that 254.2 hp is our maximum engine output, using a 0.90 gearbox you would be able to reach 285.7 km/h and only 280 with a 0.85 gearbox.

Also because spur gears do not offer axial thrust, housing can be lighter, and you don't care about gear inertia as the dog engagement is a direct clamp.

In a world where racing teams are chasing the best efficiency it is quite a normal move to go to spur gears, but you will notice that in racing gearboxes ,crownwheels are most of the time helicals ...

I hope to have clarified my previous answer.

Greetings

Stephane
 

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Seymour Snerd

Lifetime Supporter
Stephane --

It was very generous of you to take the time to write such an extended and well-thought-out discussion for me; I really appreciate that and particularly value the new knowledge I have. And by the way your English is perfect. I dare not think how good your French and Italian must be. And I will buy Stokes' book. Thank you for that as well.

As you said, many of us are under the mistaken impression that spur gears are "stronger". Hopefully your article above will help dispel that notion.

Alan.
 
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Troy

Can you tell more about your experience with the TXL 300 ?

TOM
Hi Tom, She's not running yet, though I was really impressed with the way Elite go about their business. I have mounted my rear suspension from the transmission, rather than the chassis. The TXL300 is a big gearbox, no question, and the TXL250 would be great for most GT40 applications, but I wanted the flexibility to mount suspension hardware from it. (The front half of it's case and it's mounting points are the same as the Hewland NLT)

On the subject of paddles, I will be installing a set, however I will be steering well clear of electrically operated solenoids. They are way too harsh on the dogs as well as the selector shaft. Pneumatics are a great way to go, and are much easier on the gearbox. Of course, running a small compressor on-board presents some packaging problems, however I'd rather this than a transmission full of broken parts!

There are numerous companies that provide either type of set up, but you really should be on the hunt for a package that includes a throttle 'blipper', one that can handle shifts up and down the gearbox, and of course one which incorporates the ignition cut to the engine.
 
These are two different topics. Dog engagement and straight cut gears. Dog engagement does not imply straight cut gears and vice versa.

Straight cut gears are more efficient, eliminating thrust forces produced by helical gears but helical gears have a longer tooth (and thus stronger root) than straight gears.

Dog engagements must be shifted abruptly for long life of the dog teeth, and you don't have to use the clutch if rev matched.

Syncro setups last much longer as there are no dog teeth to round out but time into gear is much greater than with a dog box.


The 01E can be configured for syncro, or dog engagement as well as helical or syncro or a combination thereof.
 
Griffin Gearboxes can be DOG boxes..
We offer the 940 RACE, 950 RACE, and the 960 RACE.

4, 5 and 6 speeds...

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Got you covered... :thumbsup:
 
I was always under the impression that 'Dog Box' referred to the location of first gear in the shift pattern and had nothing to do with straight, or helical cut gears. No?
 
DOG BOX is slang, for a lug tooth engagement.

also see
Gearbox

" One of Webster's definitions for "dog“ is, "Any of various devices for holding, gripping or fastening something, as one consisting of a spike or bar of metal with a ring, hook, claw or lug at the end.“

Hold your hands out with fingers spread and your finger tips pointed toward each other. Now move your hands together such that your fingers slip between the fingers of the other hand. Now try to rotate one hand within the other. That is the concept of "Dog ring engagement.“ The fingers or dogs of one hand are engaged with the fingers or dogs of the other hand - they are hooked together.

Dog ring engagement is quicker to shift , in most cases, because there are less teeth to engage. Our dog boxess have 8 teeth compared to our synchronized street box which has 36 teeth to engage.
 
I have a VGC in my car which is dog engagement with straight cut gears, the noise for me is not an issue but I imagine in a road car it would be. The best description i ever read on the benefits of dog engagement was written by the guys at Albins.
The faster you can manage the gear change the longer the dogs will last, simple as that , forget the clutch once you are off the line, and be as agressive as you can.

The gears are indeed quite large and they need a really serious blip on the throttle, but it has proved to be really reliable. I have named my box Mike, after Mike Tyson, you only get one shot at a gear change and if you get it wrong you will pay the penalty so give it heaps the first time.

Iain
 
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