22 replies to this topic

[03jkirk]

this isnt about the gear but looking at the picture of the gearbox and there looks like there is a crack on the webbing above the main central bearing, i would advise checking this closely be4 even considering buildin bak up in this casing.

[A.Millican]

this isnt about the gear but looking at the picture of the gearbox and there looks like there is a crack on the webbing above the main central bearing, i would advise checking this closely be4 even considering buildin bak up in this casing.

Yes the casing is also cracked...the lay shaft was also in poor condition. I will be getting a new casing and servicing the box with replacement bits where necessary
You have good eyes!
By the looks of things with the wear the lay shaft and lay gear, the cracked casing. I think this gearbox was on a high hp engine prior to me owning it.. The full gear set is virtually unmarked and is nice and clean so I was just going to service the box and g a new lay gear as mentioned before.

But yea.. All in all a bit of a ball ache!

[03jkirk]

ye might have been but also could have just been on a poorly maintained engine, suprise it has cracked there to be honest because looks like an A+ casings and the main differance on these was 1 less bolt hole in this webbing specificly to prevent a crack in this place.

[tiger99]

This is not what you want to hear, but helical gears, particularly the A+ type, are usually stronger than straight cut. They do of course suffer from higher power loss. You don't get something for nothing.

Even so, it is surprising that it broke. Maybe the heat treatment during manufacture was incorrect? Too hard and teeth tend to fracture, too soft and you get rapid wear. The ideal gear has a soft, fatigue resistant core in the teeth and a hard outer surface to resist wear. Hardened right through is very bad.

A very close-up picture of the fracture surfaces could probably tell us something interesting, i.e. did it fracture due to simple overload, or was there already fatigue cracking at the root of the tooth?

Occasionally at work I get to see gears on non-automotive equipment which have failed, and when you see all the things that can happen, it is quite amazing that they work at all. A simple fatigue calculation, based on how steel generally behaves, would indicate that the teeth would fall off after at most a few hundred miles, but as we know, a good gearbox may do quarter of a million miles. Broken teeth are actually one of the more uncommon failures, chipped, noisy teeth due to foreign matter, and galling due to pressure welding as a result of poor lubrication are much more common, as eventually is general wear.

[A.Millican]

Hello,

I have a lot of knowledge in properties of materials. It looks like a clean brittle fracture..the lay gear seems to be very hard and brittle rather than 'tough'


Cheers Andrew

[tiger99]

I thought that may be the case, but could not be sure from the photos. Probably hardened incorrectly. That may be a risk with things like special gears, which are manufactured on a small scale. It would be rare with a mass produced item where the process is well controlled.

[A.Millican]

Yes it seems like the larger is a lot more brittle that the gear set. Possibly incorrect tempering of the material...either way it's knackered hah. Never mind!

Think I'm going to gradually repair and build the gear box as a little side project.

Cheers Andrew

[A.Millican]

Lay gear not larger ....stupid auto correct sorry!