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Ram-Proof City Gate

For the good old days before gunpowder was discovered.
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A truly classic problem is how to protect your city's main gate (set in a city wall, of course) from a battering ram. Classical answers include such things as putting the gate at the end of a tunnel, and there are "murder holes" in the walls of the tunnel through which defenders can shoot arrows at the attackers trying to use the battering ram. Filling the tunnel with corpses tends to reduce the ability to use the ram.

But here is something that might be a new Idea. Consider the capital letter "D" --and now consider tilting it over so that its flat side is upward, and the curve is downward. Let us make that D from a quite-large stone block, and put it in the ground such that the upward/flat part of the stone is at ground level. The stone block is as wide as the city's main gate-way, and people walk across the top of the block to enter/leave the city.

There is a winch and gearing system to affect the large stone "D"- shaped block. When danger threatens the block is rotated 90 degrees, so that its flat side is vertical, and basically flush with the city's wall. Half the stone block is still below ground level, and half is above ground level. There is now a great big PIT in front of the block, occupying the exact place where a battering ram should be located, in order to attack the gate. The big stone "D"-shaped block IS the gate, and the thickest part of the stone block is near ground-surface level --right at the place where the attackers would be pounding, IF they could get a battering ram in the right position in front of the gate!

Vernon, Apr 05 2015

Antikythera Mechanism http://en.wikipedia...tikythera_mechanism
As mentioned in an annotation. [Vernon, Apr 07 2015]

Romans and ball bearings http://historybecau...ake-nemi-ships.html
As mentioned in an annotation. [Vernon, Apr 07 2015]

On moving large/massive blocks http://www.rense.com/general39/coral.htm
Not necessarily so tough as [MechE] indicates, but exact details are missing. Ah, well. [Vernon, Apr 07 2015]

[link]






       This is excellent, and I shall incorporate it into the very next fortified city I build.
MaxwellBuchanan, Apr 05 2015
  

       By the time the mechanism existed to move around a slab of stone of that size, repeatably, and without counterbalancing, gunpowder was in widespread use.   

       Even ignoring that, you don't have to break the stone, you just have to break the mechanism holding it in place.
MechE, Apr 05 2015
  

       wouldn't it be easy to fill the hole, or am I missing something? Keeping in mind that the Romans built a mountain sized ramp to get into Masada, I'm not sure this is a sufficient deterrent.
theircompetitor, Apr 05 2015
  

       If you fill in the hole, you're still facing the flat side of a quarter-circle of stone.   

       If the hole were filled in, you could then turn the stone a further 90 degrees. This would flatten anyone standing on the newly-filled-in hole, and leave the attackers having to attack the curvey-side of a half-circle of stone.
MaxwellBuchanan, Apr 05 2015
  

       [+] hmm....   

       I think you'd want more than a semicircle, so the axle wouldn't be exposed to the attackers. Maybe a ( • ) which is balanced more than a D
FlyingToaster, Apr 05 2015
  

       It seems to me that the attackers need only employ a sufficiently long ram to reach over the pit. I think a more workable solution would be to stretch a series of steel chains across the walkway at variable heights, and some of them vertically going floor to ceiling, which can easily be bypassed individually by crouching and high- stepping and sidestepping, but carrying a shield overhead to protect from murder holes would be nigh impossible without snagging the leading edge of the shield on the chains, and charging through with a battering ram would be all but impossible because all the people holding onto the ram would be forced to go high or go low at different times, breaking their stride while trying to achieve ramming speed. They also wouldn't be able to attack the mooring points of the chains themselves very easily since their hands are full of ram.
21 Quest, Apr 06 2015
  

       [MechE], if you google for ["ton block" megalith] (the brackets represent the search box), you will find that ancient cultures moved around quite a few large stones, of similar size to the one proposed here.   

       Regarding ram-forces breaking the mechanism, I did happen to think that an axle-based pivoting block would be more vulnerable than a cog-track system. If multiple curved "racks" were carved into the underside of the block, the half-circular block becomes the equivalent of half-a-gear.   

       You can now put a significant number of smaller gears (drive gears, all of them) in the quarter-circular subsurface area that the block, in both the "up" and "down" positions, always contacts. The equivalent of a "pawl" (multiple pawls, actually) could be used to ensure the block stays rotated to the "up" position. The pawls could be located near the bottom of the pit, such that impact-forces against the block at ground level wouldn't be affecting them much. It may also be possible to support the block in other ways, to ensure impact-forces cannot affect the drive-gear axles.   

       The other subsurface quarter-circle could simply have slide-tracks. They would of course be exposed to the enemy, and while easily destroyed, they also are easily replaced, after the enemy goes away.   

       [theircompetitor], the Romans used that ramp to get the top part of a siege tower at about the same level as the top of the wall of Masada. They didn't bother with the fortress gate. For this city, the Idea is to make a battering ram essentially useless.   

       Note that if the hole was filled in, the block can no longer be rotated, so it doesn't matter if the drive-mechanism gets broken (unless the attackers' plan includes digging out the hole again --but that is why I described some things about protecting the drive-mechanism).   

       Until/unless the drive mechanism is broken, the attackers still need to break the THICKEST part of this gate! Siege towers will probably have to be used instead --which means the gate qualified as "ram- proof", after all.
Vernon, Apr 06 2015
  

       MechE this is one idea Vernon is allowed to pursue without being harassed with meticulous pedantic scientific facts.
pashute, Apr 06 2015
  

       That's why I was careful in my phrasing. The issue is not moving the stone, the issue is making a mechanism to move the stone. Simply hauling the stone up and down a ramp, or even with a block and tackle is simple.   

       Gearing it to rotate around an axle, and have the gearing function repeatably and reliably is another story. Ditto producing the motive power other than through brute force, and the mechanism to lock it in place. You don't see that sort of equipment at the sophistication this would require until the industrial revolution.
MechE, Apr 06 2015
  

       ^ That's me with the axle; Vernon doesn't mention one at all. Either way, as written, a fully semicircular chunk of rock is physics-ally a poor implementation: in order to get it into a defensive position you have to actually lift tons of stone (in terms of center-of-gravity); destroy the chock and it slips right back.
FlyingToaster, Apr 06 2015
  

       I've modified the main text slightly, to more clearly describe the result of rotating the block 90 degrees.   

       [MechE], regarding mechanical sophistication, might I remind you of the Antikythera Mechanism (linked)? The Romans had/used ball bearings (link), although I'm not implying here that they are appropriate for this Idea. It is just that some things considered "modern" aren't necessarily all that modern.   

       [FlyingToaster], I mention chock (as "pawl") in an annotation above --and plural, not singular. With enough chocks in enough places under the rotated block, they could protect the axles of the small gears also mentioned in that annotation, and be too numerous for any significant impact-force of a battering ram, against the block, to break them --the impact- force would be spread too thinly.
Vernon, Apr 07 2015
  

       There's a bit of a difference between the 6" cast bronze gears in the Antikythera mechanism and ones capable of handling 20 tons.   

       I'm not saying the mechanism is impossible, although it is difficult, and not in keeping with the period of fortifications you are talking about. Yes, the Romans were better at this than the middle ages, but heavy gearing of this sort simply wasn't happening. Wooden gearing, the most common in both Roman times and the middle ages isn't going to be strong enough. Stone gearing isn't significantly stronger, stone is tough under impact, but teeth would tend to crack under load.   

       Bronze and iron castings on the scale you're talking about still fail as often as not, even with modern metallurgy.   

       And consolidating sponge iron on that scale isn't going to happen, so iron doesn't happen until you get a puddling furnace during the industrial revolution.   

       Not to mention the issues of all of this mechanism being buried under a 20 ton rock you only move once every several years, which is going to lead to some issues with rot (wood), rust (iron) or spalling (stone). Bronze won't corrode so bad, but as I said, not strong enough in any size that can be cast.   

       And, as an aside, lead ball bearings aren't going to handle 20 tons of rock.   

       The best possible mechanism for moving stone on this scale, during this time period is a windlass, but that doesn't work well for causing rotation unless it can be pulling on the far edge. Which in this case means the windlass is either pulling rope under and has all the rot issues mentioned unless it's human accessible (and therefore a weak point) or over in which case either the belly of the d- shape is out, or the windlass is.   

       Your best bet for the locking mechanism would be heavy wooden beams through openings in the stone, and that's only going to be as strong as the beams, less the weight of the stone already on them.   

       A drawbridge has the same basic concept (block the opening with something that covers the ditch otherwise), and an iron-bound drawbridge out of heavy wood will be just as resistant to a ram. This is especially true since the inner gate was rarely in a straight line with the outer gate, making the maneuvering go a large ram rather difficult.   

       This is also why most actual recorded uses of rams, contrary to Hollywood, were aimed at the stone walls rather than the gates.
MechE, Apr 07 2015
  

       Why not simply shape the gate in a delta fashion that deters Rams with a glancing blow?
RayfordSteele, Apr 08 2015
  

       [MechE], in another annotation I thought I made it clear that a lot of gears could be used, which would dilute the weight of the block that any one gear experiences. Of the materials you mentioned, bronze is the one most likely to be appropriate. If you have a 20- ton block and 40 gears, then that is only 1/2 ton per gear. Such weight-dilution could also make the other thing I described (carving "racks" of gear-teeth into the underside of the block) more durable.   

       With respect to using a ram against a wall, you are talking about a castle wall, not a city wall, which was usually a whole lot thicker than a castle wall. And this Idea is about a CITY gate.
Vernon, Apr 08 2015
  

       Like I said, make the gate into a V-shape so that you can reinforce it and simply can't hit it straight on. Use the power of triangles, man.
RayfordSteele, Apr 08 2015
  

       Um, city walls were usually thinner than castle walls, when there was a difference. And yes, city walls had all of the same tricks to keep a ram away from the gate as the castles did. If the city walls were thicker, it was due to a stone/rubble/stone construction, which isn't a lot stronger.   

       And it doesn't matter how many gears you have. If they're hand finished, most of the load is going to be on one, because they won't mesh uniformly.   

       Regardless, if you agree that the minimum gear size is more than a couple of inches thick, then the technology didn't exist to cast bronze in that thickness. It still really doesn't today. Modern large bronze gears are cast very carefully with a thin web and the gear is machined into a rim that is as thick as possible. The best versions are cast in a centrifugal mold, such that the rim is under very high pressure during casting to force metal into the rim, and voids and gas bubbles out to the center.
MechE, Apr 08 2015
  

       I disagree with your load comment. If you were talking about high-strength steel alloys then the loading wouldn't distribute well. But in the days of malleable pig-iron, not so much a problem.   

       I see for this maybe a geartrain made of steel-clad large wooden or even stone gears.
RayfordSteele, Apr 08 2015
  

       Starting with a horizontal cylinder of rock, if you slice the bottom and top off, you end up with a ( ). The bottom portion could be built back into a cylinder in a framework of metal, much lighter than the original stone, incorporating runners. The mess can then be slid to rotate the cylinder.
FlyingToaster, Apr 08 2015
  

       Or just orient the cylinder vertically, like some kind of normally-open revolving door.
RayfordSteele, Apr 08 2015
  
      
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