Aug 132011

From time to time I receive requests from sideshow performers for me to build them a pump for their own gavage acts. I am not in the business of manufacturing such devices. You will have to find a machinist to fabricate this for you, or else have the tools and skills to do it yourself. I can only describe what I did to build my own. I did not have a lathe, only a drill press. You will almost certainly have better results if you use a lathe instead of a drill press. The dimensions I am giving work for a particular sized barrel; I just happened to pick up an acrylic cylinder as scrap of this size. If you start with a different sized cylinder none of the following dimensions will work, and you will have to modify everything.

I built two pumps back in the early 1990s when I invented this stunt. One was stolen in Copenhagen, and the other remains with me. Both were essentially the same size. I started off with a clear acrylic cylinder 10” in length, 4.485” OD, and 3.975” ID. Clearly this is a 4” ID nominal tube. From there I used a drill press to machine an end cap, again out of clear acrylic, to seal one end. This was the most labor-intensive part of the fabrication, as I had to machine it to a few thousandths of an inch over the ID of the barrel. If you plan to do this yourself, you will need a reliable dial or digital caliper. The end cap on my unit was .675” thick, again made of clear acrylic. When your end cap almost fits into the barrel, use a heat gun to soften the end of the barrel. I chose not to use adhesives to secure the end cap into the barrel, but perhaps one could; I honestly don’t know what would work best. Both of my pumps held liquid under pressure for years with no leakage or malfunctions. You will notice the presence of “crazing” on the end of my barrel; this is a common phenomenon with all many plastics under stress. I suppose one could further secure the end cap with a metal band, or drill screws or pins into the plastic, but I didn’t need to.

From there a brass plumbing fitting was installed in the end cap. This was threaded with pipe thread, so you will need a pipe tap that corresponds to the threads on the fitting you are are using. The fitting needs to dimensionally match the tubing you will be using. Since I used tubing whose ID was 3/16” you will need to choose a metal fitting that will allow that size of tubing to slide over the hose barb. The tubing is elastic, and will expand a bit to fit over a hose barb. I sealed the junction of the threaded hose barb and the acrylic end cap with plumber’s epoxy on the outside of the unit. Black vinyl tape was wrapped around the junction of the hose barb and tubing to provide strain relief. If I were doing this today I would probably use silicone tape instead.

The plunger of the pump is a series of acrylic disks which hold the O-rings in place on a stainless steel rod. The opposite end of the rod is a plastic T-handle. There are two sizes of disks, which are sized to allow just a portion of the O-ring to be exposed to the inner surface of the pump. Again, the dimensions I’m providing are for a barrel whose nominal ID is 4”. The inner disk is 3.550” across and .229” thick. The thickness is nominally ¼”. The larger disk is 3.917” across and the same thickness. I started out using 4 O-rings, but later on found I could get by with just 3. For a 3 O-ring stack you will need 4 large disks and 3 small disks. I chose to tap the centers of these disks to match the threads on the stainless steel rod, which in this case was 3/8” coarse threads.

Believe it or not I was initially unable to find O-rings that fit this application! I fabricated my own using over-sized O-rings and Loctite Prism cyanoacrylate (superglue) #11. The ends must be diagonally tapered at the butt joint. I’m quite confident that anyone building a gavage pump in the 21st century should be able to find 4” OD O-rings commercially. Obviously the thickness of the O-rings needs to match the thickness of the plunger spacer disks. The ones I used were .240” thick.

The stainless steel rod I used was 14” by 3/8”. Both ends were tapped for 3/8” coarse threads. The plunger disk end was tapped 3” and the T-handle end was tapped 2 & ¼” or just a tad longer than the T-handle is thick. I ground a couple of flat spots on the rod to allow the rod to be chucked into a vice while I tapped the rod. This will also allow the use of a crescent wrench to hold the rod while assembling and disassembling the plunger handle. While I used galvanized washers, I wouldn’t recommend it for anyone building one today. I would stick with all stainless steel washers and nuts. On the plunger disk end I used an ordinary stainless steel nut in the middle, with nylon locking nuts at the end. If I was building this today, I would use nylon locking nuts at all four points; two on the plunger end and two on the T-handle end. Use a washer under all four nuts. Besides a crescent wrench, you will need a socket wrench to access and rotate the nut on the outside end of the T-handle.

The T-handle I used was also plastic, in this case a section of polyethylene whose dimensions are 6” by 1& ¾”. The center hole was counter sunk to allow the nuts and washers to fit inside elegantly and to prevent the center rod from biting into the flesh of the gavage assistant. In my case the countersink was a 1” wide bore.

The tubing I used was Tygon R-1000, 5/16” OD and 3/16” ID. The part number I used was AAU00012. As of this writing, this part number is still current. The tubing length I used was 7’. The end was tapered, and a series of cuts were made into the sides near the end to facilitate fluid flow. These cuts were made with a diagonal wire cutter.

I used PAM brand non-stick spray to lubricate the plunger when performing my act.

 Posted by on 08/13/2011 Art, Personal History

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