Semi Truck Gearboxes

[Boz Mon]

How do semis have 18 gears? is there a really big "H" pattern setup or what? i know there is a button that changes the rear end ratio but thats all i know. anyone have a site or anything to help me?

 

[Emohawk]

I think there's two. Think of a mountain bike; 3 gears on the front, 7 on the back. They used to both be manual, but I think the secondary one is usually an automatic now.
This is what my dad told me, anyway. I can't seem to find any semi truck enthusiast sites.

 

[Duke]

That's exactly the way it works. Usually there's 6 forward speeds in 3 ranges. They can run up through some of the gears, then shift the axle, and run back up through the gears.

Given the load they carry (60-90,000 lbs) and the narrow rev range of a big diesel they need really really wide ratio coverage.

 

[eliseracer]

And they have a whole pile of reverse gears... Most likely using only one of the ranges (?).

 

[slimer90210]

That's exactly the way it works. Usually there's 6 forward speeds in 3 ranges. They can run up through some of the gears, then shift the axle, and run back up through the gears.

Given the load they carry (60-90,000 lbs) and the narrow rev range of a big diesel they need really really wide ratio coverage.

I have seen a truck that had a 6 gear tranny and a 3 gear axle but 2 gear axles are far more common. And the more common arangement is a split tranny.

In an 18 gear split tranny there are 6 primary gears in one part of the tranny and 3 secondary gears in another part of the tranny. Essentially there are 2 trannys, a 6 gear and a 3 gear in one tranny case. The power comes from the clutch to the primary gears (1-6), and gets passed on from the primary gears to the secondary gears(usually low/mid/high range), and from the secondary gears to the driving axle. I have also seen split trannys where the power come off the clutch to the secondary gears to the primary gears to the driving axle.

And once we got a truck that had a 18 gear split tranny and a 2 gear driving axle giving a total of 36 forward gears. I believe that trucks driveline setup was clutch to secondary gears to primary gears to axle gears.

I hope that explains it clearly enough for your to understand.

 

[Duke]

I have seen a truck that had a 6 gear tranny and a 3 gear axle but 2 gear axles are far more common. And the more common arangement is a split tranny.

By 'shifting the axle' I meant shifting the final drive, but your explanation is much more thorough.
👍

 

[slimer90210]

By 'shifting the axle' I meant shifting the final drive, but your explanation is much more thorough.
👍

I just felt the distinction beween a split tranny and multi-gear axle needed to be made...

 

[Flerbizky]

Where I come from with MB trucks, it's usually a 4 speed ZF box with dual high low selection - which, of course, equals 16 forward....

 

[slimer90210]

Where I come from with MB trucks, it's usually a 4 speed ZF box with dual high low selection - which, of course, equals 16 forward....

I must admit I have never seen a truck setup like that.... interesting...

 

[Flerbizky]

I must admit I have never seen a truck setup like that.... interesting...

It IS kinda cool actually.. Imagine grabbing the shifter as you would a joystick, then you'd have a flip at the top (thumb) and another one around your middle / index fingers... Quite a hoot to drive as well** !....

**That is, if I remember how

 

[slimer90210]

It IS kinda cool actually.. Imagine grabbing the shifter as you would a joystick, then you'd have a flip at the top (thumb) and another one around your middle / index fingers... Quite a hoot to drive as well** !....

**That is, if I remember how

Much the same way the secondary gears are shifted... and yes do look fun to drive...

 

[slowman]

Having driven a semi (more on that later) I can tell you about the one I drove... it was a '96 Freightliner Century Class, and it had 12 gears. It was H pattern, but arranged a little differently >>>

1 5 9
|_|_|
| | |
3 7 11

This looks weird, but there was a button on the shift knob that when you got to the end of first, you switched this button and it would shift to second. When second was winding out, and you were ready to shift to third, you moved the shifter down to third, and while you were crossing from 1-3, you switched the button back. That's third. When third was winding out, then you switched the button again to catch fourth. Same up to twelfth gear.

Reverse was a seperate stick, and could only be pushed into reverse if the forward gears box was in neutral.


I knew a guy that would drive truck, and I went with sometimes and made some money helping him load/unload the truck. He gave me the opportunity to drive one time going through a very empty highway in Oklahoma. Went for about 20 miles or so... It's hard to see where you are on the road.

 

[standard235]

A friend of my dad's owns a trucking company. he said they almost have completely switched to automatic.

 

[PS]

I have nightmares where I'm on a racetrack, and I look down only to find a semi truck's transmission underneath me. I then start mashing the clutch and shifting like crazy because I have no bloody idea on how to use them.

Now, when they (they, you, them, whatever) say there is 6 primary gears, and then 3 secondary gears, does that mean you have two different shifters or just one shifter with 3 different thingies on it? I'm not even sure what I'm asking anymore...

 

[slimer90210]

Having driven a semi (more on that later) I can tell you about the one I drove... it was a '96 Freightliner Century Class, and it had 12 gears. It was H pattern, but arranged a little differently >>>

1 5 9
|_|_|
| | |
3 7 11

This looks weird, but there was a button on the shift knob that when you got to the end of first, you switched this button and it would shift to second. When second was winding out, and you were ready to shift to third, you moved the shifter down to third, and while you were crossing from 1-3, you switched the button back. That's third. When third was winding out, then you switched the button again to catch fourth. Same up to twelfth gear.

Reverse was a seperate stick, and could only be pushed into reverse if the forward gears box was in neutral.


I knew a guy that would drive truck, and I went with sometimes and made some money helping him load/unload the truck. He gave me the opportunity to drive one time going through a very empty highway in Oklahoma. Went for about 20 miles or so... It's hard to see where you are on the road.

Is that how they use the secondary gears?? Having never driven a semi before (I have only worked on them) I just assumed you went through the 6 primary gears in low range, sifted to the next range and went through them again... interesting...