6sigma

I changed my brake pads and fluid the other day. Had the car on jack stands with all four wheels off. At one point I started the car with clutch in, trans in neutral. But when I released the clutch with trans still in neutral, the rear axles began to turn, as if in gear, but at a slower rate. I don't fully understand how the transmission works, becuase I thought neutral meant mechanically disengaged - no gears connected to transfer power. What am I missing? Thx

dhayner

The oil in the transmission is whirled around by the input shaft, which then transfers momentum to the output shaft. There is no mechanical connection.

moparacker

^ thats not it

It has to do with friction. The input and output shafts share a bearing between the two. The little bit of rolling reistance on the bearing from the input shaft spinning at idle speed has a slight affect on the output shaft.

Thats the simplest way I can explain it, but I'm not very good at doing stuff like that.

steven975

the actual forward and reverse gears are on the driveshaft, or output shaft. They are spun by a layshaft, which is geared to the crankshaft, or input shaft. They (F and R gears) are not connected to it, though, they have ball bearings so that the gears can spin freely on the drive shaft.

when in neutral, the collar (which is another gear, with bearings on the outside and is connected to the shaft on the inside, that slides along the shaft when you switch gears and thus locks the output shaft's motion to the selected gear) isn't connected to any of those forward or reverse gears. The layshaft spins when the motor is on, and thus all forward and reverse gears are spinning freely on the driveshaft (via bearings).

This spinning on the driveshaft isn't frictionless, though, and some of the energy from the forward gears spins the driveshaft. both the ball bearings AND the oil viscosity contribute to this, by the way. the gears' spinning creates a circular current in the fluid which also helps spin the driveshaft.

You would only notice this with the rear wheels up, as the viscous and frictional force on the drive shaft isn't all that much; it's not enough to make the car move.

alexf20c

Old man. Still calls it a layshaft.


But you guys are thinking too hard. His clutch isn't fully disengaging. There is NOT enough friction in the transmission to spin those rear wheels.

steven975

i'm 28, not that old. and I'll call it a lay shaft if i please.

BTW, what is the other name so I don't sound so old fashioned?

moparacker

His clutch is engaged, he said that. And there is enough friction to do that, go give it a try.....



And Getrag still uses the term "layshaft", but I've usually refer to it as the countershaft.

alexf20c

I was just messin' with ya, man! The terminology "lay shaft" is just as common and as accepted as "counter shaft."



Nice description, but in layman's terms (hopefully) and for a synchro-equipped tranny...

There are 4 shafts in a traditional manual transmission: Input shaft, counter shaft, main shaft, and reverse idler shaft. The input shaft is driven by the engine crankshaft, and mounted inline to the main shaft, which drives the driveshaft. The counter shaft is mounted alongside, parallel to the mainshaft. The reverse idler shaft houses the reverse idler.

The input shaft is inline to the main shaft, and the two in fact are "connected" - the input shaft slides into the main shaft and rides on bearings - but they spin freely of one another. The only way power gets from the engine to the drive wheels is by the countershaft, which is driven directly by the input shaft and is always spinning at an rpm directly proportional to the input shaft. On the countershaft all gears (drive gears) are fixed to the shaft. On the main shaft the gears (driven gears) are free-spinning as a solitary gear or as a cluster gear; they ride on bearings. Those gears (drive and driven) for each "speed" are always in contact, but because the driven gear is free-spinning, it must be locked to the main shaft to deliver power.

To lock the driven gear onto the main shaft, the main shaft uses a synchronizer. A synchro has a collar which slides over the synchronizer hub (the hub is integral to the shaft and this is what provides the "lock"). The collar is moved in either direction via the shift fork. The way the synchro works is very simple. The collar will push the blocker ring against the gear. Between them is a tapered cone with a friction material which will help synchronize the speed of the hub (or main shaft) to the free-spinning driven gear, by slowing or speeding up the input shaft. (This is possible because the input shaft, though it's connected to the crank, is actually free-spinning when you put the clutch in.) Once the speed is synchronized, the collar will slide onto and grab the clutching teeth on the driven gear, thus locking that gear to the synchonizer hub, or the main shaft. And once you release the clutch, power will be delivered from the crankshaft, through the flywheel and clutch, to the input shaft, then the countershaft, the main shaft, and finally to the drive shaft, rear differential, half shafts, and drive wheels.

Each forward gear has the synchronizer, which helps smoothen shifting and negates the need for double clutching. Reverse, however, has straight-cut gears (that's why they whine in reverse; the forward gears are helical and thus much quieter in operation). Reverse is engaged by locking the driven gear (usually integral to one of the synchronizer collars) to the reverse idler gear. The reverse driven gear is always locked to the main shaft so the reverse idler shaft provides the free rotation, just as the forward driven gears had free rotation via bearings.

TrueDrezzer

Cool info, I didn't know about the reverse gear being straight cut. It all makes sense now, and explains why quickly shifting into reverse sometimes makes a "snack" noise in most cars.

///Robin