Which gear puts the most stress on a transmission?

[YTNUKLR]

I know 1st in a 901 is a weak link because it's not supported by bearings on both ends.

My question is a little different: which gear receives the most stress when you get on the gas?

It seems like maybe 1st or 2nd, because those gears allow the engine so spin up the fastest. But maybe not, because the tires break loose a bit usually and take up some of the "slack" that the gears would receive otherwise.


Tapping the Pelican brain trust..

Thanks,
Scott

[Walko]

I think the stress is not caused by how fast the drive train spins up by rather which gear transmits maxium torque. All receive the same HP and torque but 1st is used to get off the line and in slow stop start so one assumes that this will be the gear that receives the slop (ie on teh throttle and off) allowing the wear to be greater.

My 2 cents

Michael

[psalt]

The answer to your question is First gear, but Second usually wears out sooner because it is the most frequently used gear. A gearbox is a torque multiplication device. The gearbox torque output of First Gear is around 130% higher than Second and around 260% higher than Fourth in a typical four speed box. Electric OD's fitted to gearboxes have max torque ratings, on small 4 cylinders cars OD could be engaged in 2, 3 or 4th. On higher output 6's, OD engagement is limited to top gear by electric switches because of the higher torque output in lower gears.

[hcoles]

I believe in a conventional trans layout with a solid layshaft.. first gear puts the most force on the bearings because it has the smallest dia. and therefore the load angle of the teeth is more directed toward the bearings. There is usually a "small" bearing in the middle of the "upper" section. This allows the input shaft to spin independantly from the other end which holds gears for 1, 2, 3 (in a four speed). This bearing can take a beating when in 1st gear. E.g. this was a weak point in the 1971 BMW 2002... the dang bearing would start to go at every 90k miles or so.

[richde]

If you're talking about shock loads to the transmission, it would probably be 2nd or 3rd from harsh up shifts, but that would find the weakest link in the transmission, not necessarily the gears themselves.

[JFairman]

Originally Posted by hcoles

I believe in a conventional trans layout with a solid layshaft.. first gear puts the most force on the bearings because it has the smallest dia. and therefore the load angle of the teeth is more directed toward the bearings. There is usually a "small" bearing in the middle of the "upper" section. This allows the input shaft to spin independantly from the other end which holds gears for 1, 2, 3 (in a four speed). This bearing can take a beating when in 1st gear. E.g. this was a weak point in the 1971 BMW 2002... the dang bearing would start to go at every 90k miles or so.

The answer to the origonal question is first gear.
I rebuilt the transmission in my '72 2002 back in '82. There were a couple things to do to improve it...
You change the ball bearing on the layshaft next to first gear to a 2 piece roller bearing. That ball bearing always had a short life because it was next to first gear and the engines torque would wear out the bearing if the driver liked to drive the car quickly.
The replacement 2 piece roller bearing handles much greater side loads from the torque of first gear so it would last alot longer. The remaining ball bearing at the other end of the layshaft next to third gear saw much less torque and would last. Also, being a ball bearing it would keep the end float of the layshaft correct.
The other update for 2002 transmissions up to '74 was to use the fine splined output shaft.
BMW changed the splines on the output shaft for mounting the driveshaft flange from course to fine in 1974 because the course splines would wear out and become loose causing a nasty clunk everytime you got on or off the gas.

The needle bearings you mentioned that located the back of the input shaft into the front of the main or output shaft never gave me a problem in my 2002's
They were only doing their rolling thing while in first through third gear because when you shifted to fourth gear you were locking the input shft to the output shaft in a 2002 getrag transmission and the bearing wasn't turning anymore then, just locating the 2 shafts while the transmission was in "direct drive".

[burgermeister]

1st.

(input Torque in ft*lb ) * 2 / (Working Diameter of selected gear on input side, in feet) is the force carried by the gear teeth (and the same force, opposite direction, is carried by the bearings). So, the smaller the gear, the higher the load.

[hcoles]

yep.. that's what I remember from the book learning days...

[YTNUKLR]

Thanks. Appreciate the knowledge !

Burger, where is that equation from? Engineering Mechanics, Physics?

[burgermeister]

Probably got used in an intro to transmission design class I took.

Really, input torque is in feet * pounds. So, if you have a moment arm (1/2 * the gear/wheel/whatever working diameter, or 1* working radius), you can divide the torque (pounds * feet) by the radius (feet). The "feet" cancel out, and you're left with pounds.

This type of "analysis" (Dimensional Analysis) can be done on all conversion problems. Just multiply by the appropriate constants, keep track of the units, and when the units you're left with are the ones you want, it will usually be the right answer.