An interesting connection between a crossbow and an airgun.

[ccdjg]

The other day I came across a fascinating video about an “assassin’s crossbow”.
https://www.youtube.com/watch?v=eM9t3Zk4KCs

What particularly interested me was the cocking method, which used the principal of a helical thread to gain mechanical advantage needed for drawing the powerful prod.






It struck me that this principal was also proposed a century or more later for cocking airguns.

First there was the twist handle air cane proposed by Lanes Bros. ca. 1890, which may or may not have gone into production, but for which no surviving examples are yet known. This was discussed in a previous thread (https://www.airgunbbs.com/showthread...ghlight=pistol)





The thread pitch (turns per inch, or tpi) is much larger than that used in the crossbow, and so the mechanical advantage would be less, but on the plus side it only took four twists of the handle to fully compress the spring.


Then there came the twist-grip pistol patented by Clarke, Johnstone and Fearne in 1923:





Now the thread pitch was even greater, and spring compression was achieved by only ¾ of a turn. As a result of the loss in mechanical advantage this meant that the pistol had to use a relatively weak spring and a short piston stroke and so was quite low powered.





I very much like the convenience of this cocking system in a pistol, and to get round the low power problem I had toyed with the idea of lengthening the piston stroke, and using a ratchet system, so that the pistol could be cocked with more rotations. Then I realised the obvious from the crossbow and walking cane examples: you do not need a ratchet to hold the spring compression when you release your hand pressure if the thread pitch is high enough.

This set me to wondering what the minimum pitch would need to be, for this to happen? I suppose there are equations for working this out, but this is beyond me. I would probably have to resort to trial and error using some models, unless anyone has any better suggestions?

It makes you wonder if there is anything that is truly new under the sun.

[Garvin]

It seems to me (as very much a lay-person in these things) that what the Clarke, Johnstone and Fearne pistol lacked that both the cane and the crossbow appear to have, is a handle at right angles to the thread. Surely this is what gives the mechanical advantage to achieve real power?

Actually, having watched the video, I see that the crossbow doesn't have a handle on it... Doh!

[draitzer]

The question of maximum thread lead for "stick-in-place" [ratchet-free] operation is interesting, but one that's been addressed. From the good old Interweb, we see the principle that a load will slip down an inclined plane if the tangent of its angle is greater than the friction coefficient between load and plane. This value varies by materials, lube, etc. but a number of .15 is reasonable for lubricated steel/steel friction, for an angle of around 8.5 degrees which becomes the helix angle for our thread. Then, using a thread diameter of 1.0 inch would give around one-half inch advance per revolution. Not totally ridiculous, only 5 revolutions for a 2.5 inch piston stroke.

This is clearly very approximate but at least may serve as a starting point - one could make up a 1"-2 tpi combo and try compressing a spring with it.

Don R.

[ccdjg]

It seems to me (as very much a lay-person in these things) that what the Clarke, Johnstone and Fearne pistol lacked that both the cane and the crossbow appear to have, is a handle at right angles to the thread. Surely this is what gives the mechanical advantage to achieve real power?

It is just a question of perception Danny. With the cane you would probably hold the cane tube fixed in one hand and rotate the cane handle in the other hand. With the pistol, the grip can in principle be fixed in one hand and the barrel/barrel housing section of the gun rotated in the other, in which case the leverage component of the mechanical advantage of the cane and pistol would be more or less identical. In reality, I find that when cocking the pistol it seems least effort, and most natural, to rotate the barrel about one third of the cycle and the grip about two thirds. I suspect however you do it, the same amount of energy would be expended by your muscles.

The question of maximum thread lead for "stick-in-place" [ratchet-free] operation is interesting, but one that's been addressed. From the good old Interweb, we see the principle that a load will slip down an inclined plane if the tangent of its angle is greater than the friction coefficient between load and plane. This value varies by materials, lube, etc. but a number of .15 is reasonable for lubricated steel/steel friction, for an angle of around 8.5 degrees which becomes the helix angle for our thread. Then, using a thread diameter of 1.0 inch would give around one-half inch advance per revolution. Not totally ridiculous, only 5 revolutions for a 2.5 inch piston stroke.

This is clearly very approximate but at least may serve as a starting point - one could make up a 1"-2 tpi combo and try compressing a spring with it.

Don R.

Thanks Don. I knew that someone with a better grasp of these things than me would come up with an answer. Its been 65 years since I was last taught any mechanics. Your suggestion sound like an excellent starting point, and it is now on my project list.

Cheers
John

[Garvin]

It is just a question of perception Danny. With the cane you would probably hold the cane tube fixed in one hand and rotate the cane handle in the other hand. With the pistol, the grip can in principle be fixed in one hand and the barrel/barrel housing section of the gun rotated in the other, in which case the leverage component of the mechanical advantage of the cane and pistol would be more or less identical. In reality, I find that when cocking the pistol it seems least effort, and most natural, to rotate the barrel about one third of the cycle and the grip about two thirds. I suspect however you do it, the same amount of energy would be expended by your muscles. Cheers John

Thanks John. My next (probably daft) question is if the pistol had a folding handle (a bit like the WR Highest Possible), could you then put a stronger spring in it?

[ccdjg]

Thanks John. My next (probably daft) question is if the pistol had a folding handle (a bit like the WR Highest Possible), could you then put a stronger spring in it?

An interesting and quite do-able idea (I am assuming you mean that the handle would be at right angles to the grip, and not in line with it as in the Highest Possible?). This would then not only give an increase in mechanical advantage but greatly improve the ability to grip the cylinder. Well worth trying.

[Garvin]

An interesting and quite do-able idea (I am assuming you mean that the handle would be at right angles to the grip, and not in line with it as in the Highest Possible?). This would then not only give an increase in mechanical advantage but greatly improve the ability to grip the cylinder. Well worth trying.

Yes, at a right angle to the grip.