On Wed, 16 Aug 2017 08:24:01 -0700, wrote:

On Wed, 16 Aug 2017 04:43:06 -0700 (PDT), Christopher Tidy
wrote:

Am Mittwoch, 16. August 2017 13:32:32 UTC+2 schrieb David Billington:

I don't think the guy I know that used to work for David Brown would
agree. While he is no longer full time he does get call upon to work on
North Sea oil rig gearboxes from time to time and has worked on large
ship gearbox installations in the past from what he has said.

It might just be that big gearboxes cost dramatically more than smaller ones, and so other solutions become economic. Could be that simple. I'm just interested to know if there are technical reasons about gearbox scaling which affect the choice as well.

Chris
It might be just that. Since the volume of metal increases by the
square of the diameter of the gear they must reach a point where
moving that volume of metal takes too much energy compared to the
amount of energy transmitted. So the cost of the gearbox plus the cost
of transmitting the energy just gets too expensive.
Eric

This may or may not be an issue, but when gears get really large, they
can become more expensive than the simple size proportion would
indicate.

It has to do with heat treatment and the relative risk (and cost) of
failure. Big ones are often case-hardened, teeth only. As you go
smaller, the tendency is toward flame hardening of teeth, and then
through-hardening for the still smaller ones. We're talking about
heavy-duty industrial gears here.

I've watched this being done on 36-inch Curvics or spiral-bevels (I
forget which) at Gleason Works, 30 years ago, and the really big ones
go through a lot of steps. They're spotted with rouge and lapped after
hardening in the biggest sizes. Lapping each gear can take a whole
day.

--
Ed Huntress