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Help needed building an onager
#31
Thanks! Yes, I can understand that once assembled and the sod ramp having been put in place, they would be very ungainly things to shift, even the smaller ones. I made the trip to the shipyard today and even the oak pieces of about two meters long with sufficient thickness to form the sides were quite hefty!

Indeed we are working on many assumptions. I had assumed that when a siege took place, the attacking side would want to get the breaching of the walls and gates over as quickly as possible. Staying around too long will heighten the risk of counter attack by a relief force and would deplete the local recources at speed.

Along our stretch of the Limes, munition would have had to be carried, not chipped on the spot, as there are no rocks of any size in The Netherlands (geology feeding my assumption!). Perhaps in other places it was easier/faster to chip new ones instead of digging them out and lugging them back to a cart? I wonder how long it would take to chip a round ball of rock 15 cm across. Anyone know?
Salvete et Valete



Nil volentibus arduum





Robert P. Wimmers
www.erfgoedenzo.nl/Diensten/Creatie Big Grin
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#32
Robert,
Using your comments about the bent beams as inspiration, I've started working on a revised drawing of a DeReffye type onager. Hopefully I will have more time to work on it tonight. It seems a reasonable assumption that they would have used either a log specially selected for it's natural bend, or possibly steam bent one to fit rather than carving the curvature into it. Doing so would violate the grain and invite splitting under pressure.
One other thing I plan to incorporate is a winch drum that starts out thicker at the outer ends and tapers gradually towards the middle. If the rope is anchored near the
ends it will start out winding quickly when the draw weight is less and the winch will gain mechanical advantage, and the rope's angle will improve, as the arm is pulled further back and the tension increases.
P. Clodius Secundus (Randi Richert), Legio III Cyrenaica
"Caesar\'s Conquerors"
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#33
I wonder if the grain really matters in the sidebeams. The force is directed inwards by the washers as the ropes tighten, I would think. Grain matters most in something that bends, like a bow. The sidebeems do not bend.
Your drum design sounds interesting. I am trying to come to grips with the mechanical advantages, as the rotation would be the same. The amount of rope wound would just decrease. So it would act as a kind of pulley, sacreficing "way" for "force"?
Salvete et Valete



Nil volentibus arduum





Robert P. Wimmers
www.erfgoedenzo.nl/Diensten/Creatie Big Grin
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#34
Grain matters very much when the spring is installed and tensioned. As one of my friends found out, the grain needs to be running from stanchion to stanchion (or in this instance crossbeam to crossbeam) or the hole carrier will shatter.

That is also why placement of the beams is so critical on torsion engines. They need to provide the maximum support while allowing the optimal ammount of arm rotation. On most hobbyist reconstructions the layout of the stanchions isn't as much of an issue because they will never approach destructive force levels.

Despite my failure to provide a drawing of the tapered-drum concept, you seem to have grasped the basic idea. The ends of the draw rope would connect to the thickest part of the drum with the latch/trigger forming the apex of an inverted V attached to the arm. As the rope coils around the drum the two legs of the V converge and pull back/down at a more direct angle. Both effects should help the winch gain power as the load increases. It's essentially a variable speed winch with only one moving part.
P. Clodius Secundus (Randi Richert), Legio III Cyrenaica
"Caesar\'s Conquerors"
Reply
#35
Ah, OK, see the point. Will be very carefull when selecting the beams, there were several to choise from.

Wouldn't making it thinner in the middle make the drum weaker, with the maximum amount of force being excerted on the thinnest portion of the drum?
Salvete et Valete



Nil volentibus arduum





Robert P. Wimmers
www.erfgoedenzo.nl/Diensten/Creatie Big Grin
Reply
#36
Quote:Wouldn't making it thinner in the middle make the drum weaker, with the maximum amount of force being excerted on the thinnest portion of the drum?
Yes, that is a valid concern with this design. Provided that the center portion of the drum was no thinner than the axle ends that fit into the pockets in the side beams, it would be essentially the same strength as the winch in Duncan's Osprey illustration noted earlier. One major difference is that I would start from one large log shaped into an hexagonal timber and then lathe turn the axle stubs and center section, leaving a short hexagonal section on each end where the holes for the winch poles could be inserted. This further simplifies the construction and set-up of the machine by eliminating the problem of how to attach the drum ends to the rope drum. This one-piece winch would simply be clamped into place between the side beams when the frame is assembled and held there with the clamping force of the stretched spring bundle. When it's time to disassemble the machine, all you have to do is remove the spring cord and knock the side beams apart with a mallet.
For your educational and safety needs you could easily sandwich a ratchet cog between the side beam and one of the hexagonal sections.
P. Clodius Secundus (Randi Richert), Legio III Cyrenaica
"Caesar\'s Conquerors"
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#37
Robert,
Here is an updated drawing of the onager that includes your suggestion of the bowed beams as well as the tapered drum. I've also shown the angled stanchions/crossbeams to illustrate how they provide for increased arm travel. I'm sure that fine tuning and changes to the geometry will yield even greater improvements, but this shows that one can easily get more than the 40-80 degree arc possible with the traditional Payne-Gallwey or Schramm designs. All this with fewer parts, less hardware, and less chance it will try to destroy itself.
More important, as far as historians are concerned, is the fact that it follows the only source we have without inventing or disregarding anything.


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P. Clodius Secundus (Randi Richert), Legio III Cyrenaica
"Caesar\'s Conquerors"
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#38
Wow, that does look great! And very believable, too. The main challenge will be making the drum. I am thinking of attaching the rachet to the outside, with part of the axle protruding through the frame. Making that square and cladding in iron sheet will allow the ratchet to be slippen on and removerd with ease, also limiting wear on the axle.

You position at rest is not straight up, which will allow some pre-tensioning with the washers, too. That was a point which was bothering me, as there is no way of putting in some initial torsion if the arm is to be upright in rest.
Salvete et Valete



Nil volentibus arduum





Robert P. Wimmers
www.erfgoedenzo.nl/Diensten/Creatie Big Grin
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#39
Wouldn't it be easier to make a single rope whinch ( I know 2 ropes are stronger) because this migght get a simetric and so get some torsion on the whinch??
btw i made a scale model for my daughters "sinterklaas surprise "to shoot "pepernoten"they came about
6 meters. but i works.
AgrimensorLVCIVS FLAVIVS SINISTER
aka Jos Cremers
member of CORBVLO
ESTE NIX PAX CRISTE NIX
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#40
Quote:Wouldn't it be easier to make a single rope whinch ( I know 2 ropes are stronger) because this migght get a simetric and so get some torsion on the whinch??
btw i made a scale model for my daughters "sinterklaas surprise "to shoot "pepernoten"they came about
6 meters. but i works.
Agrimensor,
True, a single rope winch might be easier to build, but it would pull the arm out of alignment as it wound around the drum. Technicalities like this are seldom a problem at the force levels we generate with hobby projects or even larger scale models, but they can greatly impact the dynamics of full sized weapons.
From an historical perspective, Ammianus said "...four young stalwarts on each side, by pulling rearwards the bars to which the withdrawl ropes are connected, draw the arm down almost horizontal". Since he is our only real source, two ropes makes more sense.
I hope your daughter enjoys her present. I helped my friend's daughter build an egg trebuchet for her school project. It was fun and educational.
P. Clodius Secundus (Randi Richert), Legio III Cyrenaica
"Caesar\'s Conquerors"
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