Big gearbox design

Any gearbox experts here? Just puzzling over why really big gearboxes are c ommonly avoided. Locomotives mostly use electric transmission and the relia
bility of gearboxes in things like wind turbines isn't great.
Is there some reason to do with scaling the geometry, like if you double th e size of every dimension, the shaft can transmit more torque than the teet h? At a quick glance, it doesn't seem that simple. Or is there more slip be tween the teeth and more wear? I can't figure out a concrete reason and it' s bugging me :-).
Chris
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On Tue, 15 Aug 2017 23:47:37 -0700 (PDT), Christopher Tidy

These are pretty big: https://goo.gl/7dcoqC
Regards,
Boris Mohar
Got Knock? - see: Viatrack Printed Circuit Designs (among other things) http://www.viatrack.ca
void _-void-_ in the obvious place
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On 16/08/17 07:47, Christopher Tidy wrote:

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

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

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
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On Wed, 16 Aug 2017 08:24:01 -0700, snipped-for-privacy@whidbey.com wrote:

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.
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wrote:

https://www.geartechnology.com/issues/0113x/large_gear_construction.pdf
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On Wed, 16 Aug 2017 17:39:57 -0400, "Jim Wilkins"

Yeah, for cast and fabricated mill gears. Notice the size range: 125 in. diameter and up.
Those that Gleason was making started as steel forgings.
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wrote:

http://www.ronsongears.com.au/the-worlds-largest-gear-%E2%80%93-an-australian-innovation-with-an-echo-of-german-ingenuity.php
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Any gearbox experts here? Just puzzling over why really big gearboxes are commonly avoided. Locomotives mostly use electric transmission and the reliability of gearboxes in things like wind turbines isn't great.
Is there some reason to do with scaling the geometry, like if you double the size of every dimension, the shaft can transmit more torque than the teeth? At a quick glance, it doesn't seem that simple. Or is there more slip between the teeth and more wear? I can't figure out a concrete reason and it's bugging me :-).
Chris ===========================From what I've read it seems the reasons are mismatches between the engine and load torque/speed requirements and problems keeping the shafts aligned in large, somewhat flexible structures like ships. http://www.navweaps.com/index_tech/tech-038.htm
https://www.wartsila.com/docs/default-source/product-files/gears-propulsors/gears/wartsila-o-gears.pdf
Gears worked better when the speed was relatively constant or the load demand could be controlled by varying propellor blade pitch as on large WW2 aircraft engines. http://www.newcomen.com/wp-content/uploads/2012/12/Chapter-13-White.pdf
http://www.machinedesign.com/news/comparing-cycloidal-and-planetary-gearboxes
Usually lowest overall cost drives the choice. -jsw
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The reason locomotives are diesel-electric is that the electric motors can deliver torque at 0 RPM - no need to slip a clutch until the huge mass got rolling.
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Am Mittwoch, 16. August 2017 22:52:01 UTC+2 schrieb snipped-for-privacy@sbcglobal.net :

n deliver torque at 0 RPM - no need to slip a clutch until the huge mass go t rolling.
Interesting comment. I can see this being a key problem. Just thinking abou t it, how do diesel engines scale? I mean, the bell housing on a small car engine has a diameter of what? 30 cm? And on a locomotive engine maybe 100 cm? So you can have something like a 25 cm diameter clutch in a car and a 8 5 cm diameter clutch in a locomotive? Let's assume it's a single plate clut ch for now.
Taking this a bit further, if you have maximum engine torque and 0 rpm at t he wheels, how much power are you briefly sinking into the clutch? Maybe 50 kW in the car and 1500 kW in the locomotive? Which means you have 11 times the clutch area and 30 times the power. Following this logic, I can see ho w clutch scaling is going to max out at about the size of a large truck, an d on top of that, trains accelerate quite slowly. The heating might not be so brief.
Nice thought, Randall.
Chris
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Interesting comment. I can see this being a key problem. Just thinking about it, how do diesel engines scale? I mean, the bell housing on a small car engine has a diameter of what? 30 cm? And on a locomotive engine maybe 100 cm? So you can have something like a 25 cm diameter clutch in a car and a 85 cm diameter clutch in a locomotive? Let's assume it's a single plate clutch for now.
Taking this a bit further, if you have maximum engine torque and 0 rpm at the wheels, how much power are you briefly sinking into the clutch? Maybe 50 kW in the car and 1500 kW in the locomotive? Which means you have 11 times the clutch area and 30 times the power. Following this logic, I can see how clutch scaling is going to max out at about the size of a large truck, and on top of that, trains accelerate quite slowly. The heating might not be so brief.
Nice thought, Randall.
Chris
========== http://www.railway-technical.com/trains/rolling-stock-index-l/diesel-locomotives/
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On Thu, 17 Aug 2017 19:52:19 -0700 (PDT), Christopher Tidy

A Loco doesn't use a clutch. The generator is directly coupled to the diesel. The traction motor is directly geared to the wheels. No clutch, just feild controls on the generator and motor to control voltage and current, and switching gear for reversing and braking.
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Am Freitag, 18. August 2017 05:43:55 UTC+2 schrieb Clare:

I know. It was hypothetical. I was trying to work out why not.
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The history of locomotive engineering is interesting in that the demands of being such a heavy and powerful moving vehicle made the simplest-seeming things like following a curve or wheel balance very difficult. https://en.wikipedia.org/wiki/Hammer_blow
One consequence was the gradual weakening of cast-iron bridge components: https://en.wikipedia.org/wiki/Tay_Bridge_disaster "A joiner who had worked on the bridge from May to October 1879 also spoke of a lateral shaking, which was more alarming than the up-and-down motion, and greatest at the southern junction between the high girders and the low girders. He was unwilling to quantify the amplitude of motion, but when pressed he offered 2 to 3 inches (50 to 75 mm). When pressed further he would only say that it was distinct, large, and visible."
In addition to hammer blow the necessary 90 degree piston offset between the two sides to avoid being stuck on top-dead-center makes the loco wiggle (yaw) sideways. -jsw
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Not "clutch". There were "diesel-hydraulic" locos in Britain. "Hymek" and "Warship Class" (???). Were they British "licences" of German designs? Seems a very German example of precision engineering - not easily replicated elsewhere. Advantage is said to be that hydraulic-mechanical transmission weighed less than electric transmission, so could pack more punch if the loco had to be small and light due to track / axle-load, etc. Check the real facts if interested.
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wrote:

Used primarily on narrow guage??
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snipped-for-privacy@snyder.on.ca writes:

Mainline - Standard gauge - Western region of UK. Hilly routes. https://en.wikipedia.org/wiki/British_Rail_Class_35 https://en.wikipedia.org/wiki/British_Rail_Class_42
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Underpowered locos on hilly routes were dangerous: https://en.wikipedia.org/wiki/Armagh_rail_disaster
It's a good example of a chain of small problems compounding into a deadly disaster.
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