A few times as a forum or portion of, we have ended up in a good discussion about how and why. I don't claim to be anything other than an enthusiastic learner but I love to know how stuff works.

 

I would like to know more about gearboxes today. Let's get to the bottom of;

 

The shafts inside, is it 3 in all gearboxes?

Exactly how synchros work, double and tripple synchro gears?

Helical cut and straight cut gears.

Sequential and dog boxes.

Can you get synchro sequential? I think you can?

 

Let us pool our knowledge then I will write up a summary at the end.

 

Ready, steady, google! :biggrin:

Interesting topic I've wondered it myself. I'm interested to know what the stock box is good for and what it's weak point is?

Manual transmission is fairly simple, input shaft from clutch which is inline with main shaft and output shaft so power is delivered along a continuous line from crank to prop.

 

When a gear is selected only the shift fork and synchro is moved, all gears are fixed and constantly meshed.

 

When a gear is selected the shift fork for the chosen gear moves a synchronising ring towards the synchronising cone of the gear,this causes the shafts to match speed allowing a smooth transition.

 

The counter shaft or lay shaft is a shaft of fixed gears meshed with the main shaft to give the different ratios.

 

Reverse has a small idler gear to allow output shaft to rotate in the opposite direction.

 

In most transmissions 4th gear is a 1:1 ratio, one turn of input is equal to one turn output, basically the transmission has no effect as it's crank speed at the output shaft.

 

Helical gears are cut a specific angle to give smooth noise free operation. Straight cut are exactly that, reverse is usually straight cut hence the wine most transmissions make in reverse.

 

Most reverse gears don't have a synchro hence why reverse cannot be selected whilst moving forward without grinding. Some transmissions do have reverse synchro and can be selected whilst moving forward without grinding.

I was at work yesterday and had time to read into all things gearbox. What Chris said above,

 

"All gears are constantly meshed..." was a 'penny drops' moment for me.

 

I understood about ratios and big/little cogs, what I really wanted to know was what the mechanics are inside, what moves and how do the cogs interact.

 

Once I looked at a lot of pictures and saw the constantly meshed ratios, this is what made sense to me...

 

 

The gears (cogs) on the input shaft and lay shaft are hard fixed to the shaft.

 

However, those on the output shaft are not, they are on bearings and as such freewheel around the output shaft, driven by their counter gears.

 

 

The way to engage a gear and lock a route through the gearbox is with the gear I have just drawn on above. That one 'is' fixed to the shaft and has a tube/collar around it that is splined on the inside and can slide left to right (on that diagram).

 

It engages on the face of the gear locking them to the shaft.

 

 

Like that ^

 

 

Or like that ^

 

- - - Updated - - -

Edited February 23, 201510 yr by Gaz 300
Duplicate

Moderator I broke it ^ :oops:

Synchros are entirely in the area of that shuttle locking collar. Their job is to guide the splines/teeth together so they mesh/dovetail properly rather than clatter face to face. They ar like tapered guides. Not only do they filter the teeth into a male vs female alignment, but they do this while gripping up the 2 shafts gradually. Slowing where necessary and speed matching the shafts.

 

 

[ATTACH=CONFIG]85703[/ATTACH]

 

"Tapered brass cones" would be the GCSE 1Mark answer.

Edited February 23, 201510 yr by tomfromthenorth

Straight cut gears then.

 

This isn't really to do with the strength of the gears, arguably straight cut ones are weaker due to the lower overall mating area of the teeth.

 

They overcome the problem of axial thrust, a consequence of helical gears. The axial thrust means that the bearings that locate the shafts need to be beefy, especially when you put 5 6 7 hundered newton flaps of torque through them.

 

Your gearbox wants to go in 5 different directions when you go full boost in second! When the bearings fail the the shafts then go all wobbly, make a lot of noise then they really will go in 5 different directions!

 

 

 

So straight gears, does not mean sequential, or fast shifts, just that the bearings are offloaded so your torque ceiling is higher.

 

This axial thrust helical gear thing is used in limited slip diffs, it is also how a Sea King measures it's torque for the tq gauge, that is the answer we are told to give every year on our check, "helical gears sir!" :)

Edited February 23, 201510 yr by tomfromthenorth

Did you find anything mentioned about the shape of the gear teeth. I always understood straight cut gears to refer to the shape that the ends of the gear teeth are cut.

 

Any idea why they're so much louder?

Yeah you are right, it is the constantly meshed gears that are either straight or helical. The shape of the teeth can vary by price application and manufacturer from what I read.

 

 

Bit of a crude drawing, as is the one coming below :lol:

 

This one below is meant to show that teeth can be different shapes, I am no engineer and this is a doodle, but most are vaguely Toblerone shaped! But ppg sell ones that are pentagonal, the bottom doodle. This means they mesh smoothly for a slightly quieter sound with nore mating surface area and no axial thrust.

 

 

The noise as I understand it is the noise of thousands of little claps as the teeth mate up.

 

Imagine lifting a hardback book an inch and dropping it square onto a desk. It will slap, this is the Tom analagy for square cut.

 

Now imagine just lifting one end of the book to form a sort of ramp then dropping that. It will be a more cushioned slap, more akin to the more gradual meeting of diagonally or helically cut gear teeth.

Dog engagement, this is instead of synchros, the engagement is like lego bricks coming together, you slam it across, it has coarse male and female castelation on the gear faces that lock together hard. Black or white, it is either in or out.

 

But if you don't smack them together you will knock all the knobbles of your lego bricks and knacker your gear box!

Last one :) just read into sequential, that property in isolation is simply a beautiful system of gears and linkages to make the shift selection a fore/aft motion.

 

It uses a sprung ratchet rotating cylinder but is basically just a different set of levers and rods.

 

 

However I imagine these are almost always sold as straight cut and dog engagement as well.

My wife asked me this question the other day (shes learning to drive at the moment and wanted to know about engines, clutches and gearboxes). We found this video that explains it well

 

https://www.youtube.com/watch?v=IKywZ730JFs

Yeah that is one of the things I watched, it is really good, but I did read most of the internet yesterday :lol: