building a drawbridge retraction mechanism?

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  • building a drawbridge retraction mechanism?
  • Premier Icon maccruiskeen
    Subscriber

    It would be less complicated for your kids to just pour boiling oil over any potential intruders ๐Ÿ™‚

    Premier Icon mucker
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    What about a mtb wheel with a freewheel hub to take it up and held there with a mechanical disc brake.

    5lab
    Member

    I’ve been putting my shed-making skills to use building a castle/fort thingy in my back garden, but it looks a little incomplete without a drawbridge. I could put a permanent one in easily (with decorative chains), but I figure one which can go up and down would be much more fun ๐Ÿ™‚

    It won’t be massive – maybe 4’x2′, and thus fairly light but (given it’ll be made of wood), it will be heavy enough to have a requirement to stop one toddler dropping it on another toddler’s head, so I figure I need some sort of winder thing to wind the rope/chain in/out from inside the castle. The thing is, I’ve no idea what this sort of thing is called but I thought the STW hive mind would have a better idea. I know large roller garage doors have this sort of thing in them, but no idea of naming etc (I figure I’ll have to buy the roller, rather than build it from scratch myself, but I’m open to ideas).

    Over to you ๐Ÿ™‚

    [url=https://flic.kr/p/Z6hsNb]IMG_20171015_1650559[/url] by Hugh Lunnon, on Flickr

    [url=https://flic.kr/p/Zu2MNi]IMG_20171015_1650022[/url] by Hugh Lunnon, on Flickr

    5lab
    Member

    trouble with a ratchet is that would allow a kid inside to drop the door on kid outside without any slowing.

    I like the mtb wheel idea – maybe with a bodged brake so its always on.. alternately it looks like an engine hoist may be geared so heavily it’s effectively braked when the chain’s not being pulled..

    Premier Icon nickjb
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    What about a couple of counterweights? Very easy, just pop the ropes over a pulley then into a bit of drain pipe. I’d make the weight slightly heavy so the door stays closed then pop a brick/log on it to hold it open

    5lab
    Member

    interesting option. I’d probably have it the other way (so it stayed open, just), but it’d solve having the problem of dropping it on someone’s head. I guess the only thing lacking then would (maybe) be a winch to raise and lower it (for the sake of coolness), unless I could make the drawbridge stiff enough to have the counterweight on one pulley and the winch on the other without twisting it..

    You could counter weight it, but fill the drainpipes with rainwater and overflow (just so they don’t dry out). Use something really thin/heavy as the weight with a metal sheet on the bottom that matches the diameter of the pipe, then drill holes until the correct damping is achieved? It’d resist opening too quickly even if they were pulling it down on themselves.

    In reality though, whatever you do there’ll be ways of killing each other, one kid underneath it while the other attempts a (Ye Olde) Dukes of Hazzard jump out of the partially opened bridge on his bike?

    Premier Icon maccruiskeen
    Subscriber

    Use something really thin/heavy as the weight

    Sash window weights

    neilwheel
    Member

    I suggest a counter weight too, chain over pulley with a quick link on to M10 lifting eye attached to a bit of studding. You can then wrap lead flashing around the stud to tweak the counter weight to give the motion that you want. Large washer and another lifting eye on the bottom of stud. You can then attach a hauling line with a friction brake on the bottom of the weight.

    Premier Icon MoreCashThanDash
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    Can I just say, that is bloody marvellous and this 48 year old kid is very jealous

    scruff9252
    Member

    You could use a jockey winch – these have two handles, one for up and one for down. Ensures that both the lifting and dropping are controlled and not able to free fall – we used to use on the keel on our old trailer sailing boat – drop the keel uncontrolled and it could continue out the bottom of the boat…!

    One like these;

    http://www.stantonhope.com/Product/Tools-26-Equipment-170850-Tirfor-Jockey-Winch-with-Rope

    That said – the suggestion above of lifting a counter weight though a water filled drainpipe is pretty clever!

    5lab
    Member

    cheers for the suggestions. The water damper is an interesting choice, made me wonder if I could use a big enough set of pulleys to cycle an old fork damper instead (it’d work out as a ratio of ~10:1 though, which is a bit excessive, but would be fun)

    I recon if it’s counterbalanced correctly then damping isn’t too big an issue (as even if it’s moving quickly, the effective force is low). so the current plan is :

    l side – chain goes over a pulley to a counter weight on a single pulley (resulting in a 2:1 ratio), hidden in a drainpipe.

    r side – chain goes over a pulley then wraps around an old 26″ wheel mounted in a knackered fork lower which is screwed directly to a beam. This gives just enough ‘pull’ on a single rotation (1750mm for a 559mm rim) to pull in all the chain needed for a 1200mm drawbridge (1700mm needed..)

    so I then just need to figure out a handle for the old front wheel (something can probably be mounted to the disk mount points, or bolted onto the rim/spokes) and then a way of securing it ‘shut’ (little fabric loop for the aformentioned handle).

    Then I need to over-engineer the bridge itself enough to keep it nice and rigid. Simple ๐Ÿ™‚

    PS: aformentioned child is still only 1, so this *might* be more of a project for me than him for the next couple of years..

    Premier Icon luket
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    Gas struts? They seemed readily available in a vast array of specs when i needed one for a piece of furniture. Sized correctly it could be designed such that they have to apply force to open it all the way and it would not return all the way up without being deliberately push closed.

    Alternatively as a pure damping mechanism how about clamping around the pivot, as was i believe often done using leather but I can’t recall for what application.

    Premier Icon the-muffin-man
    Subscriber

    I’d hold off doing anything – the “take it down or else” letter from your friendly local planning office is on it’s way! ๐Ÿ˜€

    Bruce
    Member

    If you are going to make a drawbridge you might as well dig a moat, it will give the drawbridge context.

    Premier Icon nickjb
    Subscriber

    Over 2.5m so would technically need planning. There was a treehouse near us that caused a bit of a ruckus but they were eventually allowed to keep it.

    Hopefully mine will be allowed. Nearly finished…

    Not totally mine, but I did make a little bit ๐Ÿ™‚

    IHN
    Member

    Have you asked David Davis?

    (as even if it’s moving quickly, the effective force is low

    Still potentially has momentum though?

    I’d use rope not chain, chain has weight so as it lowers or raises will tend to speed up, although it would conveniently mean it would stay open/shut until pushed/pulled.

    TBH though, I doubt you’d ever get it ‘safe’ unless it was just a dummy/not opening one.

    johndoh
    Member

    Stop pratting around with that and mow your bloody lawn.

    Premier Icon nedrapier
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    Not sure if I can help with heavy wood. I had different design requirements for the low-slung racing castle I made for the 2002 Fernie Cardboard Downhill Derby.

    Due to the lightweight construction and the heavily raked, aerodynamic front wall of Castle Fast, I didn’t need any retention mechanisms beyond the friction in the cardboard spool and string.

    neilwheel
    Member

    Nice fort. I’m also not sure if you can make it safe unless it is a closed system, where there is no free movement in either direction.

    As there are baby’s head involved, I don’t think using old bike bits is the right way to go if you choose to make this.

    Premier Icon slowoldman
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    If you are going to make a drawbridge you might as well dig a moat

    What, and ruin that lawn?

    Premier Icon maccruiskeen
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    aformentioned child is still only 1, so this *might* be more of a project for me than him for the next couple of years..

    I wonder if thereโ€™s a way you can harness the power of your wifeโ€™s eyes rolling to operate it ๐Ÿ™„ ๐Ÿ™‚

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