# How much power can a discworld axle generate? **URL:** \ **Category:** Factual Questions\ **Created:** [July 28, 2024, 7:57am UTC](https://boards.straightdope.com/t/how-much-power-can-a-discworld-axle-generate/1004885 "2024-07-28T07:57:06Z")\ **Posts on this page:** 18\ **Page:** 3
**Author:** ![mbh](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@mbh](https://boards.straightdope.com/u/mbh)\ **Post date:** [August 1, 2024, 8:42pm UTC](https://boards.straightdope.com/t/how-much-power-can-a-discworld-axle-generate/1004885/41 "2024-08-01T20:42:34Z")
What is the conversion rate between kilowatts and spherical-cow-power? ---
**Author:** ![Baal\_Houtham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/baal_houtham/32/13604_2.png) [@Baal\_Houtham](https://boards.straightdope.com/u/Baal_Houtham)\ **Post date:** [August 1, 2024, 8:49pm UTC](https://boards.straightdope.com/t/how-much-power-can-a-discworld-axle-generate/1004885/42 "2024-08-01T20:49:33Z")
> [@Chronos](#): > > Something like that is probably a practical idea to avoid accidental damage to the equipment, but it won’t change what the ultimate limit is. Not an engineering/physics guy, but if the device was rigged so both sides spun would that double the potential output? I’ve been assuming that the weak point was the 6” diamond receiver hole, so would splitting the load between two diamond receivers (one on each side) help matters? ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [August 1, 2024, 8:56pm UTC](https://boards.straightdope.com/t/how-much-power-can-a-discworld-axle-generate/1004885/43 "2024-08-01T20:56:17Z")
Unfortunately not. You need two diamond receivers either way, since the forces are the same. But splitting the load just means each side is rotating half as fast. Energy is force times distance, and half the speed means you’re covering distance at half the rate. So the power output is also halved (but there’s two sides, so it comes out equal overall). ---
**Author:** ![md-2000](https://avatars.discourse-cdn.com/v4/letter/m/9d8465/32.png) [@md-2000](https://boards.straightdope.com/u/md-2000)\ **Post date:** [August 1, 2024, 10:53pm UTC](https://boards.straightdope.com/t/how-much-power-can-a-discworld-axle-generate/1004885/44 "2024-08-01T22:53:13Z")
> [@mbh](#): > > What is the conversion rate between kilowatts and spherical-cow-power? Depends… are we discussing horse-power equivalent or methane output? ---
**Author:** ![kaylasdad99](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kaylasdad99/32/3398_2.png) [@kaylasdad99](https://boards.straightdope.com/u/kaylasdad99)\ **Post date:** [August 2, 2024, 12:38am UTC](https://boards.straightdope.com/t/how-much-power-can-a-discworld-axle-generate/1004885/45 "2024-08-02T00:38:38Z")
> [@Peter\_Morris](#): > > But you could attach a wheel to the axle, and it would still turn once every 6.9 seconds. > > Imagine a merry-go-round turning at that speed, one complete rotation every 6.9 seconds. Would you think it’s slow or very fast? For the purposes of getting a, say, ten-foot diameter drive wheel slipped over the 6-inch cube, I’d be inclined to call it slow. And the merry-go-round idea illustrates this nicely. If the axle is already turning when the carousel is installed, you only have to worry about matching the speed of the hub to that of the axle. I certainly wouldn’t try stepping onto a twenty-five foot diameter carousel rotating at 8.7rpm, but a 12-inch diameter axle rotating at that rate would seem more doable. ---
**Author:** ![vbob](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/vbob/32/4439_2.png) [@vbob](https://boards.straightdope.com/u/vbob)\ **Post date:** [August 2, 2024, 1:05am UTC](https://boards.straightdope.com/t/how-much-power-can-a-discworld-axle-generate/1004885/46 "2024-08-02T01:05:22Z")
> [@md-2000](#): > > Depends… are we discussing horse-power equivalent or methane output? Spherical cows are a result of NOT having an output for methane. ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [August 2, 2024, 1:13am UTC](https://boards.straightdope.com/t/how-much-power-can-a-discworld-axle-generate/1004885/47 "2024-08-02T01:13:31Z")
That’s not my spherical cow! ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 2, 2024, 12:04pm UTC](https://boards.straightdope.com/t/how-much-power-can-a-discworld-axle-generate/1004885/48 "2024-08-02T12:04:11Z")
To be clear, I meant that a clutch that would slip above some torque (presumably a little less than the ultimate torque of the interface yoke) would be a good idea to prevent accidents. Making both sides turn might have some practical purpose, but if so, I’m not sure what it would be. It’d also be fairly easy to make that clutch, since you could do that at a much larger radius, so you wouldn’t need extreme forces there. Oh, and… > [@Dr.Strangelove](#): > > I don’t recall if they can create synthetic diamond (via magic) in the Discworld universe. Not exactly synthetic, but they might be able to get the same effect. Troll teeth are diamonds, their teeth grow somehow, and they can be very large. You’d just need to get a troll’s tooth to grow around the Device. ---
**Author:** ![andrewm](https://avatars.discourse-cdn.com/v4/letter/a/c57346/32.png) [@andrewm](https://boards.straightdope.com/u/andrewm)\ **Post date:** [August 2, 2024, 1:57pm UTC](https://boards.straightdope.com/t/how-much-power-can-a-discworld-axle-generate/1004885/49 "2024-08-02T13:57:48Z")
> [@Baal\_Houtham](#): > > but if the device was rigged so both sides spun would that double the potential output? This is, for all practical purposes, currently the case. As the OP properly says “the two halves rotate with respect to each other”. Without some third reference point on the device to declare stationary, you cannot declare that only one side is the one actually moving - all you can say is that the two pieces are moving relative to each other. As already stated, the forces on each half (whether against a generator or stationary anchor) are necessarily equal. ---
**Author:** ![md-2000](https://avatars.discourse-cdn.com/v4/letter/m/9d8465/32.png) [@md-2000](https://boards.straightdope.com/u/md-2000)\ **Post date:** [August 2, 2024, 2:32pm UTC](https://boards.straightdope.com/t/how-much-power-can-a-discworld-axle-generate/1004885/50 "2024-08-02T14:32:17Z")
> [@Chronos](#): > > To be clear, I meant that a clutch that would slip above some torque (presumably a little less than the ultimate torque of the interface yoke) would be a good idea to prevent accidents. Making both sides turn might have some practical purpose, but if so, I’m not sure what it would be. Isn’t this essentially the purpoe of a key in an axle, to shear before catastrophic damage is done to more imprtant parts of the mechanism? ---
**Author:** ![Saffer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saffer/32/6440_2.png) [@Saffer](https://boards.straightdope.com/u/Saffer)\ **Post date:** [August 2, 2024, 2:36pm UTC](https://boards.straightdope.com/t/how-much-power-can-a-discworld-axle-generate/1004885/51 "2024-08-02T14:36:05Z")
I suspect that in this case, those more important parts correspond to the indestructible device so such a protection mechanism is not required. ---
**Author:** ![Measure\_for\_Measure](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/measure_for_measure/32/557_2.png) [@Measure\_for\_Measure](https://boards.straightdope.com/u/Measure_for_Measure)\ **Post date:** [August 3, 2024, 11:40pm UTC](https://boards.straightdope.com/t/how-much-power-can-a-discworld-axle-generate/1004885/52 "2024-08-03T23:40:21Z")
> [@Dr.Strangelove](#): > > The compressive strength of diamond is somewhere around 60 GPa, which I’ll derate to 40 GPa. … > > If you have to use steel instead, the power goes down by about two orders of magnitude: ~700 kilowatts total. What material would be used on Earth Prime? From wiki’s article on [Young’s modulus](https://en.wikipedia.org/wiki/Young%27s_modulus) (" the slope of the linear part of the [stress–strain curve](https://en.wikipedia.org/wiki/Stress%E2%80%93strain_curve) for a material under tension or compression." GPa is a gigi[pascal](https://en.wikipedia.org/wiki/Pascal_(unit)), if anyone is wondering.), diamond tops the list followed by [Graphene](https://en.wikipedia.org/wiki/Graphene), a Single-walled [carbon nanotube](https://en.wikipedia.org/wiki/Carbon_nanotube), and [Carbon nitride](https://en.wikipedia.org/wiki/Carbon_nitride) (CN2) if you can settle for less than 80% of the strength of diamond. Are any of these available to governments, industry, or mail order in a way that could be fashioned into a useful grip-grommet? Or do we have to settle for [Tungsten carbide](https://en.wikipedia.org/wiki/Tungsten_carbide), about 65% the strength of diamond at most? ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [August 3, 2024, 11:54pm UTC](https://boards.straightdope.com/t/how-much-power-can-a-discworld-axle-generate/1004885/53 "2024-08-03T23:54:13Z")
Gonna be tough to match covalent carbon bonds. Tungsten carbide is likely your best bet for conventional materials. If a government really wanted to throw a billion or ten into a project, though, I think solid diamond blocks are achievable. Synthetic diamond is already cheap as chips. Making a large block of it is harder, but doable with a refinement of current tech. > [@Chronos](#): > > Troll teeth are diamonds, their teeth grow somehow, and they can be very large. I’d forgotten about that. A large tooth is actually perfect for what I had in mind, depending on the orientation of the crystalline structure. Cleave off the tip to leave a flat spot. Arrange four teeth around the circumferential sides of the cube on the forward-moving half. Being vaguely conical, the tooth spreads that high load to a much larger area on the other side, where a conventional wheel structure is built (out of steel or the like). If you were evil, you could probably also capture a young troll and surgically modify the jaw to produce teeth in whatever arrangement you needed. Maybe even have it produce diamond cylinders of indefinite length if you can keep the teeth growing via some medical means. ---
**Author:** ![mixdenny](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mixdenny/32/2962_2.png) [@mixdenny](https://boards.straightdope.com/u/mixdenny)\ **Post date:** [August 4, 2024, 12:07am UTC](https://boards.straightdope.com/t/how-much-power-can-a-discworld-axle-generate/1004885/54 "2024-08-04T00:07:22Z")
I have never heard of this device but is it even established that you have to anchor one cube while the other spins? Or does it just spin unstoppably by itself. I mean it is fantasy so… ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [August 4, 2024, 12:11am UTC](https://boards.straightdope.com/t/how-much-power-can-a-discworld-axle-generate/1004885/55 "2024-08-04T00:11:43Z")
The information about it is limited, but it gets moved around easily and handled as if it were an ordinary mechanical device. It doesn’t seem like one side of it is locked in space, rotationally or otherwise. Maybe there’s a hidden button that enables the anchoring system, like the [immovable rod](https://www.dndbeyond.com/magic-items/4662-immovable-rod) in D&D. ---
**Author:** ![Snarky\_Kong](https://avatars.discourse-cdn.com/v4/letter/s/a183cd/32.png) [@Snarky\_Kong](https://boards.straightdope.com/u/Snarky_Kong)\ **Post date:** [August 4, 2024, 12:48am UTC](https://boards.straightdope.com/t/how-much-power-can-a-discworld-axle-generate/1004885/56 "2024-08-04T00:48:33Z")
Young’s modulus is a measure of stiffness, not strength. ---
**Author:** ![Measure\_for\_Measure](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/measure_for_measure/32/557_2.png) [@Measure\_for\_Measure](https://boards.straightdope.com/u/Measure_for_Measure)\ **Post date:** [August 4, 2024, 1:00am UTC](https://boards.straightdope.com/t/how-much-power-can-a-discworld-axle-generate/1004885/57 "2024-08-04T01:00:11Z")
Noted. I am neither a physicist nor a material engineer. I found [this webpage at Cornell](https://courses.cit.cornell.edu/mclaskey/cee3710/stimulation3/strengthStiffness2018.html) to be helpful: > The words ‘stiffness’ and ‘strength’ both imply a sense of resistance and are both determined by geometry and material properties. However, a strong object may not necessarily be stiff, and vice versa! … > Strength [N/m2] is the maximum stress that the material can resist before deformation or fracture. > Stiffness [N/m] is the rigidity or resistance to bending. So strength is resistance to damage, while stiffness is merely resistance to bending. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [August 4, 2024, 7:33pm UTC](https://boards.straightdope.com/t/how-much-power-can-a-discworld-axle-generate/1004885/58 "2024-08-04T19:33:42Z")
> [@andrewm](#): > > Without some third reference point on the device to declare stationary, you cannot declare that only one side is the one actually moving - all you can say is that the two pieces are moving relative to each other. Rotation is not relative in that way. Attach a disc to both rotating parts and place ball bearings on both discs - the rotating one will fling them off and no declaration of different reference frames will change that. [Previous page](https://boards.straightdope.com/t/how-much-power-can-a-discworld-axle-generate/1004885.md?page=2)