jermfab
Cast Iron
- Joined
- Jul 25, 2013
- Location
- atlanta, ga
Sharing another piece of motion picture special effects equipment. The trade name is a “ratchet”, but basically a high pressure air motivated accelerator. I’m not positive, but I believe the overall design was probably taken directly from the previous generation aircraft carrier catapult. The main difference is that we use compressed nitrogen instead of steam.
I wish I could say this was my design, it isn’t, I did have a hand in fabrication.
Basically it’s a rope compound:
Think block and tackle, working in reverse. Instead of pulling the two blocks together with a rope, we’re pulling the blocks apart and taking up the rope.
The traveler is attached to a hydraulic ram:
Finally there’s a high pressure accumulation and firing system. Nothing special on this one, literally just open the ball valve and things start happening... QUICKLY!
The ball valve on the right is both a safety and a dump. It stays open until you’re ready to fill and prevents accidentally activating the system if the firing valve is open. In the picture above I gave the system a bit of pressure to keep the ram closed so I could stretch my rope out.
Before my time guys used wire rope in machines like this, but the business graduated to synthetic before I got into it. Today I’m using 3/4” Technora 12-strand. The working load is significantly higher than 3/4” cable while weighing far less. 3/4” cable or wire rope breaks at 25.6 tons: 51,200 lbs. 3/4” Tech breaks at 32.5 tons: 65,000 lbs.
In addition to weighing less than comparable cable, synthetic rope doesn’t have nearly as much energy in the event it does break. Tech-line, when it breaks, almost just “lays down”, unlike cable which tends to carry a lot of energy... enough to cut anyone unlucky enough to get caught by it, in half. You can also tailor the synthetic rope to the circumstances. Here the abrasion resistant characteristics of Tech line were why I chose it... in other situations I might choose something like Amsteel or Dyneema. Both Amsteel and Dyneema are lighter, Amsteel almost floats, but neither have the abrasion resistance that tech-line has. Running either Amsteel or Dyneema around a sheave typically welds the outer fibers together and the rope is unusable after only a couple pulls.
Today’s work is gonna happen very fast. I’m pulling a Lincoln Town Car into a phone pole. The ratchet compound is a 6:1. The cylinder strokes 8’ so I’m pulling about 50’ total. The producers don’t want me to break the pole, but more important is that the accident appears to be a fatal one. I’m thinking 1800 psi, with both accumulators filled, ought to do the trick. I relieved the crap out of the car. There’s nothing at all between the bumper cover and the crank pulley. Ideally the machine stops pulling a two or three feet before impact, so I’m not actually ever pulling against the pole.
Math period, isn’t my strongest suit and math on dynamic stuff like this is even more difficult. I’m guessing I’ll go from zero to 35-40 mph right before impact... if all goes according to plan.
I’ll update later tonight with aftermath photos.
Be safe
Jeremy
I wish I could say this was my design, it isn’t, I did have a hand in fabrication.
Basically it’s a rope compound:
Think block and tackle, working in reverse. Instead of pulling the two blocks together with a rope, we’re pulling the blocks apart and taking up the rope.
The traveler is attached to a hydraulic ram:
Finally there’s a high pressure accumulation and firing system. Nothing special on this one, literally just open the ball valve and things start happening... QUICKLY!
The ball valve on the right is both a safety and a dump. It stays open until you’re ready to fill and prevents accidentally activating the system if the firing valve is open. In the picture above I gave the system a bit of pressure to keep the ram closed so I could stretch my rope out.
Before my time guys used wire rope in machines like this, but the business graduated to synthetic before I got into it. Today I’m using 3/4” Technora 12-strand. The working load is significantly higher than 3/4” cable while weighing far less. 3/4” cable or wire rope breaks at 25.6 tons: 51,200 lbs. 3/4” Tech breaks at 32.5 tons: 65,000 lbs.
In addition to weighing less than comparable cable, synthetic rope doesn’t have nearly as much energy in the event it does break. Tech-line, when it breaks, almost just “lays down”, unlike cable which tends to carry a lot of energy... enough to cut anyone unlucky enough to get caught by it, in half. You can also tailor the synthetic rope to the circumstances. Here the abrasion resistant characteristics of Tech line were why I chose it... in other situations I might choose something like Amsteel or Dyneema. Both Amsteel and Dyneema are lighter, Amsteel almost floats, but neither have the abrasion resistance that tech-line has. Running either Amsteel or Dyneema around a sheave typically welds the outer fibers together and the rope is unusable after only a couple pulls.
Today’s work is gonna happen very fast. I’m pulling a Lincoln Town Car into a phone pole. The ratchet compound is a 6:1. The cylinder strokes 8’ so I’m pulling about 50’ total. The producers don’t want me to break the pole, but more important is that the accident appears to be a fatal one. I’m thinking 1800 psi, with both accumulators filled, ought to do the trick. I relieved the crap out of the car. There’s nothing at all between the bumper cover and the crank pulley. Ideally the machine stops pulling a two or three feet before impact, so I’m not actually ever pulling against the pole.
Math period, isn’t my strongest suit and math on dynamic stuff like this is even more difficult. I’m guessing I’ll go from zero to 35-40 mph right before impact... if all goes according to plan.
I’ll update later tonight with aftermath photos.
Be safe
Jeremy