Lock up?

darth sidious

darth sidious

New Member
From the thread "Overdrive for an automatic?":-

chrisyork said:
Yep, lock up means that in third and fourth you are effictively driving a manual gearbox. There is no rise and fall in engine revs as you press the throttle - the engine is directly coupled to the wheels. The advantage of this is that you eliminate torque convertor losses, so you get much better economy, aside from the already better economy from the overdrive top ratio.

Chris
Click to expand...

Now I've got a question! As I've possibly said a few (thousand!) times on this forum before, we had an old 1972 2000Auto until we changed it to a manual.

How can you get lock-up (i.e. direct drive from the engine to the gearbox input shaft) on a torque converter which, unless I'm mistaken, works solely by fluid coupling? :?
 
To put a bit of detail on that.

First thing to understand is that the body, or stator, of the torque convertor is bolted to the engine crankshaft and takes the place of the flywheel on a manual car. Inside the torque convertor is a rotor or turbine. This is connected to the gearbox in a normal automatic like the Borg Warner.

The gerarbox itself is just that. In a manual transmission there is a mainshaft and a layshft with different sets of helical gears connecting the two. Gears are changed by different sets of gears being released or locked to the two shafts. In an automatic gearbox the gears are epicyclic. A gear is engaged by locking the outer ring of the gear with a brake or band. Daimler used to produce manual gearboxes like this - they were known as pre-selector gearboxes. This type of gearbox is particularely suitable for an automatic because the gears can be merged into each other by slipping the brake bands on two adjacent gear sets until only one is engaged.

Back to the torque convertor. When drive is engaged, the convertor is filled with oil, and the churn between the stator and rotor transmits drive to the gearbox. But as with any churning motion in a fluid, heat is produced too and therefore the TQ wastes some of the energy - it slips.

In a torque convertor fitted with lock up there are two shafts entering the convertor instead of one. The normal one engaging with the rotor is still there, but there is a second saft, concentric with the first, that engages with the body or stator of the TQ. in normal drive the shaft engaging with the rotor transmits drive to the gearbox. When lock up is engaged the concentric outer shaft that is coupled to the stator or casing of the torque convertor is locked to the inner shaft by a clutch so that direct drive is established from the crankshaft, through the casing of the torqe convertor to the input shaft of the gearbox.

Simples!

Chris
 
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