Why aren't there screw-based cocking mechanisms for crossbows?

I want to say that there are some screw-based ones, but I’ve been looking for an illustration and haven’t found one yet. I think I have seen such an illustration or even an actual one in a museum, and I think it’s what this fellow describes here as a ‘screw and handle’ crossbow . I’ll keep looking.

I (also) think it can be done, but would my intuition says it may be more expensive / difficult to build than other designs, and I think it would likely trade off potential pull with speed, but potential pull may generally have been limited by other factors to a maximum that was as attainable by other mechanisms, so my guess is that for some combination of those factors, it wasn’t often done. I’d expect if it were more efficient, we’d see more of them, and they’re rare enough I’m finding none so far.

Thanks for your response @Zaku. Am I wrong to think that given the correct gearing and threads that are sufficiently close together along the screw that travels the rail, a simple motor like the one in a battery-powered pencil sharpener would be able to draw back a 100–200 lbs. crossbow?

Crossbows are rather fical. There are several parts of a crossbow’s anatomy that are very susceptible to a problem.

The ones I used to work with were at most $800. Maybe more expensive ones are better. The ones I sold were cool, but we’re easy to break.

The cocking assist cords and crank you mentioned were the only mechanisms I was aware of.

The idea of a thread,or screw assist seems feasible.

It almost doesn’t matter how loud it is. Deer or boar hear the crossbow, and even a bow sometimes. It’s a one shot thing usually. With most crossbows you cock it, and then hunt with the safety on. It’s unlikely you’ll get a second shot on the same animal.

I found a book I have that has some drawings of a supposedly historical design. I think I’ve seen at least one actual other historical type. Again it calls it a “screw and handle” crossbow, and involves a handle and screw built into/onto the handle. It doesn’t suggest any particular drawbacks.

The ones I’ve seen are medieval military designs, so they don’t use motors and are more like 700+ lb draw weight. The threading in the illustrations is relatively tight. I don’t know enough about the practical engineering of battery motors and screws to know what to expect for performance or required devices.

You might try emailing the guy in this video I just found, Donald Duckman, who has made this battery-powered crossbow, though it doesn’t seem to be screw-based.

The author reportedly wrote on a now-offline web site:
“The specification I devised called for a full auto-firing rate of 1 bolt per second; with 6″ of draw length and a minimum of 30lbs draw weight. The magazine was to house 10 bolts 9″ long. The bow was to be capable of discharging the full magazine with imperceptible slowing down and have an effective range of 25 yards.

The draw length is limited by the throw of the crank that pulls the bow carriage plus the relationship this has with the near stall torque that can be pulled out of the motor / gearbox assy. The need to maximise these relationships must be balanced against the voltage / number of batteries / weight.

The above was achieved using 8 AA rechargeable NiCd batteries giving 9v.”

When I picture this it seems like the force on the threads would be extremely high, and they would have to be deep, making a large shaft necessary. That is just a guess from the movie in my head of such a device.

You could also check out this Patent US6799566 – Automatic cocking device in a crossbow , which is an electrical crossbow design using a ball screw, and discusses the design and has diagrams, etc. Of course, it’s a patent document… :-P

Screw based designs aren’t always practical. The threads would need to be covered. Or the smallest debris or hair could become stuck in it.

When I worked at the animal ER we used syringe pumps that used a screw system. You always had to be careful with them. They would catch on anything.

Considering the draw weight on most hunting crossbows, it would be very dangerous if it wasn’t done just right….

@MrGrimm888
*fickle

On top of what @MrGrimm888 offered, I would imagine a screw based cocking mechanism would work but take time to implement that in a fire fight or pursuit of game you have little room to twist the screw cocking mechanism. Time is of the essence in these situations.

The coefficient of friction and the direction of the normal force are the nemeses of lead screws. Screws can have a huge mechanical advantage but the effect of friction reduces their usefulness to items that are usually intended to stay in place like: nuts and bolts, screws, or car bottle jacks.
It is a rare screw mechanism that can “overhaul”; pull on the screw shaft and have the outside mechanism turn.
Pulleys and lever arms are incredibly efficient. Bearing designs and lubrication further mitigate the effects of friction. But screw mechanisms put the load right on axis with the friction normal force. It is hard to fight the laws of physics.

I use a pulley cord to cock my Barnett Raptor FX. It reduces the required 150 pound pull to 75 pounds which is quite doable.

The patent linked above has a ball screw, with ball bearings riding in the threads, so the friction is low.

I didn’t know such a thing existed until recently when I found this Hackaday post

I like the Hackaday post above. Unfortunately a mechanism like that is much more complicated, weighs more, is orientation sensitive, and costs much more than a simple pulley.
In general, when out in the woods you want to minimize the weight you have to carry. The pulley mechanism is about as light and simple as you can get.

@LuckyGuy Thanks for the in-depth analysis. That was exactly the kind of answer I was hoping to hear! I realize that a cocking rope is light/simple, but they are unarguably awkward-as-hell. Coords get tangled, you can only really use them if you’re on level ground, and it’s strange to require a secondary object in order to use the primary object.

You make excellent points about a screw-based system though. The way I pictured it working you’d have a sled that traveled back the length of the rail riding the screw. Once the rope is fully cocked, the sled would hit the end of the screw and disengage from it. A band of some sort would retract it back down the length of the rail in a second track that’s parallel to the screw where it would return to the initial position. I suspect, as you have pointed out, that the issue would be the friction and the weight involved. Also, keeping the threads clean would be a challenge for sure as someone pointed out. It would have to be enclosed.

If those challenges could be overcome (which they probably couldn’t) wouldn’t it be really nice to be able to push a button and draw your crossbow? If it were possible, affordable, didn’t add much weight, and reliable, wouldn’t that be 1000x better than using a rope cocking device?

@gorillapaws Yes it sure could be nice to just push a button. That would open up the posrt to the elderly and infirm (and lazy ;-))
Let’s look at it another way. My crossbow delivers the bolt with a rated kinetic energy of 97 ft-lb. Let’s call it 100 ft pound for easier math. 1 ft pound = 1.36 Watt seconds. So under ideal circumstances we need to cock the device with 100×1.36 = 136 Watt seconds. We can safely assume the screw mechanism would be in the 40% efficiency range. so it would take 1.36/.4 = 340 Watt seconds per shot. Assume you need at least 15 shots before changing a battery so that means we need a battery of 5100 watt secs. A good small Li ion battery is 3 volts at 1.5 amp hours = 3×1.5×3600 = 16,200 Watt seconds So it is theoretically possible with today’s technology.
Now we just need to make it cheap, light, and durable.

^FYI… In my area crossbow hunting is usually limited to those with physical reasons for not being able to draw a conventional bow…

Too slow. You want to recock the bow as quickly as possible, whether hunting or fighting. And one that was a screw type would also probably be too heavy to be practical.

Let’s look at another part of the problem. We know the battery technology will work. Now we need a motor. First let’s decide how much time we can accept for cocking. Ideally we’d like ti to be fast but that means a huge motor. What is the longest we can tolerate. Let’s call it 10 seconds. That means we need an electric motor capable of at least 34 Watts. Better make it 50 Watts to be sure. Himax sells a motor in that size for $43. Next you need to work on the mechanism. worm gear, shaft, latch mechanism, release, etc.
I’m sticking with a pulley.

This was a good thought experiment. Thanks.

@LuckyGuy TenPoint sells an ACUdraw system for something like $200, and that’s a manual crank built into the stock—it looks clumsy/slow/awkward. High-end crossbows can be north of $2,000 when decked out with gear. a $43 motor is actually pretty reasonable, especially given the size/weight. I’d bet people would be willing to pay $300 for this feature. This really is just a thought experiment thought.

Wouldn’t the addition of a motor pretty much defeat the purpose of using a crossbow? Not much historical authenticity. What about adding a range finder? What about adding a computer chip to automatically turn on the motor and then shoot?

@LostInParadise Historical authenticity pretty much went out the window when they started using exotic metals, laminated fiberglass and carbon fibers. The genius who figured out how to use cams and compound mechanisms really brought bows into the 21st century. The devices now store so much energy the arrows/bolts move at speeds well above 350 ft/sec. At those speeds the arrow will pass right through a full sizes deer sometimes without them even noticing until they walk away and start getting a little light headed and wobbly and lie down. Only 20 years ago 200–250 fps was considered fast.
Recently Benjamin/Crosman came out with a new class of weapon they call an Airbow. It will push an arrow to 450 fps!!! (500 fps is the legal definition of a firearm in Canada!) It takes a very special and complex arrow to withstand the peak forces. There are some serious discussions going on now trying to decide if this should be considered a bow, or rifle or something else and what hunting season is appropriate for its use?
It uses 3000 psi compressed air to propel the arrow. That is not an misprint. 3000 psi air!!! That is 200 atmospheres!!! Scary high pressure!

I’m not a hunter but I’d sure like one just to study!!!

@gorillapaws. I just watched the ACUdraw video you posted above. Wow. That sure sounded complicated, and with so many ways to really injure yourself. Yikes!

350 fps is almost 240 mph. I would also like to see one of these up close and I have no interest in hunting.

The fastest one I ever shot was around 270 fps. The bolt just appeared in the target. It was far too fast for my eye to track.No kick, very accurate. Scary to think how deadly it would be if you put broad heads on the bolts.

The same crossbow broke when firing once though. Sent the broken string flying over my shoulder. It could have severely injured me. Turns out the company sent bolts with that crossbow that were too light. The lighter bolt couldn’t stand the force of the string so the bolt shattered, and the string popped too.

I’ve had several other incidents with them. They were awesome when I first started using them, but now days I stick with my Mossberg 500.

I highly recommend everyone shoot a high powered of each once at least.

The sporting goods store I used to work at had a range in the archery department. So most people can ask to test fire one for free…

I have a Barnet Raptor FX. rated at 330 feet per second. It comes with a scope. Every shot is the same and as @MrGrimm888 stated, you pull the trigger, and thwack!, the bolt is in the target. No recoil, no sound.

I want to try an Airbow! I’d actually pay money to shoot with one. I imagine it would be quite expensive. The arrows have to be in the $20–40 range each and at those power levels you don’t want to reuse arrows too frequently.

^Yeah. That’s a cool crossbow. I sold Barnets where I worked. They’re the ones who accidentally sent the wrong bolts. Honest mistake. Good product for the price.

If I had more disposable income, I might get one, one day…

@MrGrimm888 Did you see the new Ghost 410? Yep! 410 fps! Holy Rocketship Batman!
At those energy levels bolt design is super critical. They operate on the edge of disaster.

How can there be no recoil? Is the arrow lighter in weight than the lead from a bullet? Maybe the acceleration time is comparatively longer for the arrow, even though we are talking fractions of a second in both cases.

@LuckyGuy . Yes. They are probably right on the edge of a injury, or death whilst using the contraption. But I’ll watch…

I assume they use carbon fiber bolts for it. As you’re probably aware, they are strong, but an unnoticed crack, or the wrong weight ,or just wear and tear could result in disaster.

Cool stuff though. It’s getting pretty unfair for the animals we hunt…

@LostInParadise . Recoil from a gun comes from the explosion of gunpowder, which propels the projectile (bullet.)

The larger the caliber of projectile, the more powder there is used to push it down the barrel (with some exceptions, like a 5.56 mm round.)

So it’s not the weight of the projectile that causes the recoil, but rather the propellant.

The recoil is used in many different semiautomatic weapons to push the bolt back to eject the recently spent shell, and then as it comes forward pushing a new round into the chamber from the magazine.

Crossbows do not have an explosive propellant. So there is little, to no recoil. All the gathered force is pushed onto the back of the loaded bolt. The bolt absorbs the force, that’s why too light a bolt is bad.

I’m sure @LuckyGuy could explain it more scientifically than me…

@LostInParadise Yep. No recoil. The thing is pure genius. Do you see the bent arms of the bow? They are being flexed along a line perpendicular to the arrows flight. When the arrow is released they spring outward in opposite directions and balance each other perfectly. That takes up the loin’s share of the recoil. Theoretically there is still some but it is infinitesimally small. I can’t feel it at all.

The new compound bows do the same thing. They have arms that bend down and up. When the arrow is released they move in opposite directions and cancel out the recoil.
Watch this video of Olympic archers. They barely hold the bow. Watch carefully how it flops down after the shot. There is a wrist strap that prevents them from dropping it since they “hold” it so loosely – some with their hands open.
My compound bow is so scary and powerful I was sure it would sproing away and kill me if I tried that. Nope. I did it a few times and it was perfect.
Incredible engineering!

@LuckyGuy Here’s your block and tackle style sled for cocking the crossbow built into the bow. More elegant than ball screws, though it would only work with a the short draw length of this particular crossbow. Cool design though. I saw this and remembered this discussion.

@gorillapaws Thanks! I enjoy seeing old posts reawaken. @MrGrimm888 I figured you’d enjoy this too!
Just yesterday I got an advertisement from Pyramyd Air introducing the Ravin R500; crossbow – 500 ft/sec. 220 foot pounds of energy (fpe)
They even have an electric draw version of it. Ravin R500E

I had a bad experience with a crossbow. Just be careful with them…

@MrGrimm888 Oh man… I’ll bet there’s an interesting story behind those 8 words!

Oh yeah. I was selling a bow to some customers. After a few shots I suddenly didn’t see the bolt, and noticed that one of the strings had flown over my shoulder. Turns out that the bolt didn’t weigh enough. The manufacturer had provided the wrong bolt. We didn’t know, about the bolt, after until weighing it.