ZF12M gearbox

[vic008]

Be OK to run full time in reverse ?
Also, why do makers have different ratios forrd/ reverse? Would have thought easier to have identical size gears

[skipperpete]

Good question vic008, even ZF doesn’t categorically state that running constantly in astern will damage your ZF12, they skate around the issue by showing which lever position is ahead and also suggest that if cooling is provided..... maybe the smaller Hurth/ ZF’s can be run constantly in reverse ( volvo imply this). Here’s the power flow through the typical ZF mechanical transmission that explains why the ratios are different.
I’ve never installed a ZF Hurth to run using the astern lever position as ahead and have rebuilt or replaced a number of these gearboxes that have.

[FPNC]

Don't run any ZF M gear in left hand propeller rotation for FWD. The little intermediate gear is woefully inadequate for the task. The intermediate gear bearings will fail and you will destroy the box with circulated bearing metal.

[vic008]

Thanks, ZF Australia answered and said not designed to run in reverse, so that is that. Again, thank you