INFO

This is an automatic pyrotechnic star pump. I designed and built it for my Olin self study (OSS) requirement at Olin College in the fall of 2012. It is designed to rapidly and precisely pump cylindrical pyrotechnic stars, using a wet star mixture. Using a wet mix and making the parts out of non-sparking materials ensures that there is minimal risk of accidental ignition. While in use, the machine utilizes a number of cams and sliders to perform a 4 step process to pump the stars.

Cutaway View

First, while the star chamber (A) is sealed, the supply chamber (B) is pressurized. The pumping piston (C) retracts, allowing the chamber to fill with star mix. Second, the main slider (F) opens the star chamber (A), and the piston (C) is pushed down, precisely extruding a star (Below Left). Third, the main slider (F) slides right, causing the blade (D) to cut the star (Below Right).

Star ExtrudingStar Cutting

Fourth, the blade retracts and the blade guard (E) scrapes the star off the blade as the main slider (F) moves to back to the right to seal the star chamber (A) in preparation for the next star. This cycle repeats itself as the machine runs, and after they are cut the stars fall onto a conveyor belt (G) which carries them to a container which they fall into (not shown, but any container will work). At the upstream end of the conveyor is an optional prime dispenser (H), which lays out a layer of prime powder onto the conveyor that the stars land on and are coated in. When there is prime on the belt, the stars roll down the belt instead of sticking to it, covering all sides of it with prime. This both prevents the stars from sticking to each other and primes them for easier ignition.

To use the machine, a 125g batch of any type of star mix is made and loaded into the supply chamber (B), the supply piston (J) is loaded behind it, and the chamber lid is sealed. The machine requires compressed air, which is hooked up to the male quick-disconnect on the machine. The pressure of the air is set depending on the viscosity of the star mixture (typically 50-80 psi), up to a pressure of 100 psi. The machine's motor is powered by 5 volts of DC power, and draws up to 3 amps. The machine operates at ~60 rpm, creating about one star per second. At this rate it creates ~175 stars in about 3 minutes, which is the about how many stars 125 grams makes. After the batch is done, a piston removal stick is screwed into the supply piston and it is removed, and the machine is cleaned. The machine is easy to clean and made from materials that will not rust, even when in contact with oxidizers present in pyrotechnic mixtures. To clean it, the motor is removed and the rest of the machine can be run under hot water to clean it. When switching between incompatible chemicals the machine should be disassembled and thoroughly cleaned to prevent any dangerous mixtures from being formed. Furthermore, any star mixtures that are incompatible with Delrin, Teflon, stainless steel or neoprene cannot be used, as the mixture comes into intimate contact with these materials.

Star Pump

The mechanical design of this machine is broken into two halves, the top star pump, and the receiving conveyor belt assembly. The top utilizes 3 sets of cam-spring-follower mechanisms on two shafts to operate the pneumatic valve, the pumping piston, and the main slider. I chose to use cams because these 3 parts of the machine (except the pneumatic valve) require precisely timed, complicated linear motion. To ensure proper timing between the 2 shafts that the 3 cams are on, I used a timing belt with static tensioner. This was chosen over gears for its simplicity and to ensure accessibility to the components behind it. The supply chamber is designed so that the supply piston is less dense than the star mixture it is pushing, so that it floats on top. This prevents the need for tight tolerances on the welded tube, while accomplishing its purpose of preventing compressed air blow-by. The supply chamber size is arbitrary, it is a size that suits my needs well, but its length and diameter can be increased or decreased (to a limit) depending on the application, allowing much larger batches to be run with changes only to the supply air pressure. The pneumatic valve is 3-way, 2-position. This means that when pressure is not being applied to the supply chamber the valve vents the chamber to the atmosphere to relieve the internal pressure, giving the machine its characteristic chugging noise. The motor is mounted with a simple clamp and transmits power through a gear, so that to remove the motor only 2 screws need be loosened (not removed) and the motor will slide out.

The bottom conveyor assembly is connected to the above pumping assembly via 4 cap screws and a round O-ring belt, allowing for easy disassembly for cleaning. This part of the machine uses molded acetyl gears to reduce the speed from the input at a ratio of 1:16 so that the conveyor belt moves at a slow enough speed for the stars to dry on the outside (when using 25% Isopropyl acohol or other more volitile wetting agents, such as acetone) before falling off the belt into a container. The slow speed of the belt keeps its motion smooth so that stars are not bounced off the belt. The sides prevent any stars from falling off the belt while the machine is moving, although they are not strictly necessary. One of the belt pulleys is mounted in a slot to allow for tensioning of the conveyor belt. The bottom assembly is mounted on a wide 1 inch square 80/20 extrusion base, providing stability while freestanding. This also makes it easy to securely mount a container for the stars to fall in, to add any other improvements, or to securely fix the machine to a work bench.

Video

Machine Running

CAD

All CAD are SolidWorks files. See Links for SolidWorks viewing software. The profile file are needed only to modify the frame extrusion profile. 05-00 is the top level assembly. A .zip file of all the CAD is available to save you clicking. It does not include the profile files.

CAD.zip
05-00 eAssembly
80/20 1010 profile

Technical Drawings

This is a full set of technical drawings. Some parts have to scale .dxf's that correspond to them, allowing you to upload them to a CNC machine to cut these parts. A .zip file of the whole package is available to save you clicking.

Drawing Package.zip