The stretcher is a tool for individually pretensioning each spring cord in the torsion spring bundle. This article describes how the stretcher is used and how to avoid common pitfalls when arming the torsion spring.
Before you start, you need to make an eye-splice to one end of the spring cord. The other end of the cord should be tapered, so that it can be pushed through the limited amount of space inside the washers when the washer is almost full of cord. You will obviously need a stretcher, too.
Mounting the field-frame to the stretcher Edit
The stretcher has four holes that match the locations of the pi-brackets in the field-frames. To mount the field-frame simply push rectangular wooden beams through the holes in the stretcher and the pi-brackets:
If you can, push a wooden beam through each pi-bracket. If you can't, three will suffice. If some beams are loose, beat wedges between them and the pi-brackets.
Making the first pass Edit
Take one of the washers and pass the loop in the cord through the washer bar:
If necessary, place small U-shaped metal spaces against the inside edges of the washers bars. These spacers are primarily used to emulate a washer whose core widens towards the bottom. They can additionally be used to make minor adjustments to the effective diameter of the washer, should a washer be significantly larger than the others:
Push the cord through the field-frame and the other washer, placing the washer with the loop inside the top field-frame ring:
Turn the stretcher upside down, so that the washer with loop is at the bottom:
Pull the cord through the hole in the winch axle at the top:
Rotate the winch to tighten the spring cord.
Make sure you rotate the winch so that the cord presses tightly against the washer bar. If you're doing the first pass, this means that the upper washer will either rise up from the field-frame ring or rotate to the side:
To fix this issue push a pin through the washer and a hole in the field-frame ring:
Now you need to set the spring cord to correct tightness. This is done by measuring the pitch (frequency) of the cord when it's plucked or tapped with a small steel rod while the winch axle is locked. To ensure you get consistent results push the spring cord against the other cords or the washer bar with a wooden dowel.
Smartphones tend to have tuner applications which you can use to measure the pitch of the cord; this particular one is an Android Tuner application from the F-droid repository running on a Jolla smartphone:
The three pictures above have been taken several seconds apart. As you can see, the frequency slowly drops until it finally settles down. In my experience you need to tighten the cord to about 20hz more than what you're aiming for to reach the correct final tightness. After some practice you will be able to roughly gauge the correct frequency even without the tuner application. I try to keep the frequency of all cords within 10hz range, which seems good enough for all practical purpose.
Once the cord is of correct tightness place a small steel plate on top of the washer rim, pushing it as close to the cords as possible. Next lock the cord in place with locking pliers. Make sure that you push the cord inside the jaws instead of leaving it right at their edge:
The next step is very important. Tighten a bar cramp to the stretcher frame so that its bar prevents the handle of the locking pliers from rising up when tension on the winch is released:
Without the bar cramp it is next to impossible to prevent the cord from slipping sideways, ruining cord alignment. This is the especially when pretension level is high.
Now slowly loosen the cord from winch and push it through the washer and the hole in the winch axle on the opposite side:
Tighten the cord with the winch. When the tension is about correct, slowly loosen the clip at the other side: the spring cord should then snap into its designated place. Adjust the pitch just like before.
Optionally insert a (fake) arm inside the two cords:
Inserting an arm was suggested by Nick Watts and probably makes sense as it ensures that all cords are equally tight when the arm is in the bundle.
Stretching the remaining cords Edit
Now you can just repeat the process until the entire washer is full. There are several things to keep in mind, though:
- Ensure that cords are placed tightly against each other
- Lay out the cords in order from one side to the other
- Once you've reached the edge of the washer lay the next cord on top of the previous two
In the end there will be at least 3-4 layers of cords, depending on cord thickness and washer diameter. Besides this the process is the same as for the first cord:
If you're using fairly thick cord like the 4mm nylon three-ply here you also want to ensure that the washer diameter is exactly the same for each washer. If it's not, use the U-shaped steel pieces (see above) to adjust effective washer width. If you do not do this, then a cord on the upper layer could squeeze itself between the cords in the lower layers, ruining the otherwise perfect cord alignment. Recovering from this is challenging, so you can either live with it or start from scratch.
When you're about to lay out the last layer of cords you will need to remove the fake arm from the bundle:
Push the tapered end of the cord through and pull cord through behind it, possibly with pliers. Once you cannot get any more spring cord through the washers, lash the last cords together.
Common issues Edit
A number of things can happen when stretching the cord. Here are the most common ones and the ways to recover from them.
Cord slips from the clip Edit
This is pretty bad, as the tension of several previous rounds of cord will be lost. If you're using small-diameter cord then the damage probably remains fairly localized.
To recover, quickly winch the cord that slipped really tight. If you now test the tightness of a few previous rounds of cords, you will notice that they're a bit sloppy. Find the first cord which seems tight enough and lock it into its previous cord using locking pliers. Now you can simply unwind all the loose cord and start the stretching process again from an earlier point. Remove the locking pliers when the you've provided enough tension using the winch.
Cord slips sideways when the clip is removed Edit
When you're winching the cord with the clip on the other side, ensure that the clip and the cord stay in the exact right position. If they've moved sideways, loosen the winch a bit and use wedges to force the cord to its place. Still, sometimes the cord just slips into the wrong place. Recovery is essentially the same as above: clip the last two cords together with locking pliers, unwind the cord up to that point and continue from there.
Cord slips between cords in the previous layer Edit
This can happen easily if your washer diameters do not match exactly. It's probably not a big deal if you're using thin cord, as you can simply fit in as much cord as you need for each layer. With large diameter cord this becomes an issue because the difference between having, say, 14 and 12 cords in the layer becomes rather significant in the end. For my cheiroballistra and 4mm nylon cord the difference would amount to
(7 + 6 + 5 + 1) * 2 = 38 cords (6 + 5 + 4 + 1) * 2 = 32 cords
This is about 19% difference, which is definitely not insignificant.
Recovery from this kind of slippage is the same as above. To prevent this kind of issues use the U-shaped spacers (see above) to equalize the width of the washers. Also remember to push each round of cord right next to the previous one.
Cord squeezes itself through the clip to some degree Edit
This means that the clip is not providing sufficient pressure to keep the spring cord under tension. You will need to rethink your clipping arrangement or to use smaller diameter spring cord.
The difficulty of keeping even a fairly thin cord (4mm nylon) in place when the tension goes above 250hz seems to indicate that in the original weapons fairly thin cord was used to assist clipping. The description of the stretching and clipping process in the ancient manuscripts also seems to point towards this direction, the clip apparently being a very simple device that grabbed the cord with wedging action.
Locking pliers move when tension on the winch is released Edit
With 4mm nylon cord and 350hz tension it becomes very difficult to keep the locking pliers in place with wedges as suggested in earlier versions of this article. Quite often the jaws of the pliers "dove" inside the washer and in doing so also slipped sideways. This made it impossible to align the cord correctly. The only way to recover from this was to rewind the cord through the winch and to apply tension to lift the jaws up again. This process took time and caused unnecessary damage to the cords due to repeated clipping. Fortunately the process described above is much more reliable and you should not really encounter this problem.