Fusor Vacuum Pump Saga
This post will detail my efforts at getting a vacuum pump setup that is sufficient for fusor work. One of my goals was to do this without using a 2 pump setup. (Yes, I DO know that the vast majority of folks building fusors, use a 2 pump solution for their vacuum system.) I wanted to see whether I could dispense with using either a diffusion pump or a turbo pump. Primarily because having a second pump doubles the probability of failure of the pump station – 2 things that can fail instead of one. Furthermore turbos require a somewhat complicated electronic controller – which means if you are using a turbo, you now have 3 things that can fail – mechanical backing pump, turbo pump itself, and the turbo controller. Furthermore, using two pumps requires more vacuum plumbing than one, as you must plumb the pumps together in addition to plumbing them to the fusor.
It turns out that you CAN get a pump setup that only uses one pump, which will get you down to fusor level vacuums ie: ~1 micron, but you either have to get lucky, or be willing to spend a pretty penny.
Fortunately, I got lucky.
My very first vacuum pump setup on the fusor was using an Edwards E2M8 dual stage direct drive pump, but with that I could not get down to less than about 5 microns, and typically it liked to run after getting hot, at more like 12-14 microns. This was good enough to let me make a plasma, but not good enough to properly evacuate the fusor for D2 runs.
I also had an old Welch 1405, but that would only pump down to about 35 microns. Much worse and not even close to what is needed for a fusor.
However, I also had bought from a University of Washington (UW) surplus auction at some point about 10 years ago what looked to be a brand new old stock, still in the box Welch 1402 pump head, that appeared to be at least 40 years old, but never used. It still had the factory tags on it. It turns out that both the Welch 1405 and Welch 1402 use a 1/2HP electric motor, and the diameter of the drive pulley on both pumps is 10 inches. So, I decided to replace the 1405 pump head that only pulled down to 35 microns with the 1402. This entailed properly locating and drilling 4 new holes in the pump base plate for the 1402 (the hole layout IS different between the 1405 and 1402), and then bolting the 1402 head down, and then reattaching and adjusting the tension of the drive belt. One thing I discovered about these old Welch pumps is that they have plates that go across the bottom of the pump base, underneath the drive motor that the motor mount bolts screw into, which function just like conflat plate nuts, so you can just loosen the motor mount bolts and retighten them to remove and adjust the drive belt tension without ever having to access any nuts underneath the pump base. Very nice touch, IMO. Especially since the complete pump assembly is NOT light. The 1402 with motor and base weighs in at 120+ pounds. Unfortunately, the pump bolts do not screw into similar plates, so you are stuck tilting the base up and getting 2 wrenches on bolt head and nut to attach the pump head to the base. But even if there had been plates on the 1405, they wouldn’t have worked for the 1402 anyway because the bolt spacing is different.
The new old stock 1402 pump head came filled with oil that looked brand new – I drained it to take a look, and it was a clear pale yellow clean oil. I had purchased some Inland 19 Ultra vacuum pump oil, which I decided to use instead. So I saved the original drained oil and loaded in a batch of brand new completely clear 19 Ultra. Then since the pump had been sitting for decades, I manually turned the 10” pully drive forward, and backed it up a bit if it got stuck, and then moved it forward again. There is an arrow on the pulley indicating the normal direction of rotation. It has been several months since I first brought this pump up, so I don’t remember how much time I spent manually turning the pump. But it is very important that you are sure that the pump can turn manually in its normal direction without getting hard stuck, before you switch on the motor to run it. If the vanes are sticking, they will loosen up the more the pump is turned. Everytime it sticks in the forward direction, just move it back and forth a bit until it unsticks and you can turn it more in the forward direction. The pump will loosen up the more you turn it manually until it stops getting stuck. You may need to spend and hour or more working the pump manually to make sure it won’t get stuck.
Once the pump would turn in the forward direction normally without getting stuck, I plugged in the pump motor, and it ran just fine. For a while. Then I heard the start of what sounded like a clanking noise. Unfortunately the clanking noise got worse as the pump heated up. I wasn’t sure exactly how a new old stock Welch mechanical pump was supposed to sound, but I was pretty sure that a loud metallic clank was not good, so I powered it off. After letting it sit a while, I fired it up again, and at first again the pump sounded fine, but eventually it again started to clank. I thought perhaps that oil wasn’t circulating properly in the pump, so I pulled the 00 rubber stopper plugging the pump hose barb intake, and added a bit more Ultra 19 oil down the pump throat as it was running. Didn’t seem to make much difference to the clanks. Of course doing this caused the pump to gurgle loudly and spew oil mist “smoke” out its exhaust while the stopper was out of the intake.
Basically, whenever the pump started to heat up, the clanking would start.
I thought maybe it was the new oil I was using. The new oil looked a bit less viscous than the old oil, perhaps the pump required a heavier weight oil. So, I swapped in the original oil.
No difference. Still clanked when it got hot. Plus I noticed that the ultimate vacuum that I could get with the old oil (5+ microns) was significantly worse than the vacuum I could pull with the Island Ultra 19 (~2 microns). So, I swapped back in the Inland Ultra 19. It wasn’t the oil viscosity or type that was causing the clanks.
After getting frustrated with the repeated clanking, I mused that maybe the clank wasn’t really that bad after all, and perhaps the pump just clanked normally. So, I decided to just let it run and clank. (Really BAD decision… DO NOT do that if you have a Welch vacuum pump that clanks. They are NOT supposed to clank.) Pump ran for a while, clanking, and the clanking got louder and more frequent as the pump got hotter, and the pump kept getting hotter, and hotter, and started to slow down… and the motor was getting really hot. And the pump kept slowing down and slowing down until it stopped. At which point I realized I was being a total moron, and that I might have frozen up my brand new old stock pump head, and that I was probably going to burn out the motor as well! So, I immediately switched off the pump, and let it cool down for a several hours. (I’m telling you about my stupidity, so that you don’t make the same mistake.)
After the pump cooled off, I then tried carefully moving the 10” diameter pump pulley back and forth by hand to make sure it wasn’t actually frozen, and fortunately it was not. Once it would again rotate fully in the normal drive direction by hand, I drained the oil, and put in a brand new batch of Inland 19 Ultra. (1402 pumps actually hold quite a lot of oil – a little more than ½ a gallon.) The drained oil was full of tiny particulates, and was no longer clear, but instead slightly grey – likely due to the presence of both metallic and vane particulates in it. It looked much like the very first oil change made on brand new lawn mower engines – assuming that first oil change is made after 10 hours or so of use and the oil is still clear enough to see through.
After this epic fail, I decided to try a new tack, which was to run the pump until it just first started to clank and then immediately switch it off and wait for it to cool down. Then then turn it on again until it clanked, switch it off, cool, rinse and repeat. My theory was that most likely the pump vanes which are typically phenolic (the original label states they were fiber vanes) had absorbed moisture during the several decades the pump had been sitting at atmosphere in the Pacific northwest humidity. So, when the pump heated up, the vanes would get too constricted to slide freely up and down in their slots in the rotors, and would cause the clanking. I reasoned that if I ran the pump enough up to the point where the vanes started to be constricted, but not let it actually damage itself by clanking, I could slowly wear the sides of the vanes down (and dry them out by keeping them at vacuum pressures for significant time), and eventually the pump might run continuously with no clanking.
This is exactly what happened. At the beginning the clanking would happen when the pump got to about 95 to 100 degrees Fahrenheit. As the runs continued, the temperature at which the clanking would start slowly rose. Eventually, after many cycles of running until clanks started, and then stopping to cool off, the pump could get fully up to its running temperature of 120-125 degrees Fahrenheit, with no clanking! (An inexpensive Chinese made handheld electronic thermometer with a laser pointer was very useful in keeping track of the temperature of both the pump head and the motor during this process.)
Once the pump finally appeared to be able to run continously with no clanking, I decided to just let it run for 24 hours straight, which it did without any clanking. After that, I measured the vacuum the pump was pulling at the end of the hose it was connected to, and measured 1.5 microns.
Success!
Morals of the story:
1) Welch makes really nice mechanical pumps. The 1402 pumps are especially nice.
2) If you have a brand new (or brand new old stock) Welch mechanical pump, it can pull down enough by itself to run a fusor – down to 1 micron or less.
3) Never let a Welch pump clank. That is not how they are supposed to sound. When running they make a quiet gurgle, with perhaps some random aperiodic clicking that is not very loud. While they are pumping down from atmosphere, the gurgling sound is much louder. The manual specifically warns against letting the pumps pump against too high a pressure continuously. Pumping at 1 torr or higher for extended periods of time can damage the pump. (It also spews oil mist out its exhaust vent, or into the exhaust filter.)
4) If your pump has been stored a long time and clanks when it starts to warm up, bring it into service by repeatedly running it until it first starts to clank. Then shut it off, let it cool, and run it again. Do this repeatedly, until it can run continuously without clanking. Once it runs continuously, you should probably consider replacing the oil used during the bring up process with new clean oil. You will also probably have more success eliminating the clanking, if you replace any decades old oil right away before starting the bring up process, since the old oil is probably full of moisture as well, and will therefore slow down the process of getting the pump vanes to slide properly when the pump gets hot. (Since oil already loaded with moisture will take a lot longer to pull moisture out of the vanes, than oil with no moisture.)
5) The oil you use in your mechanical pump really does matter. Especially if you are trying to do a single mechanical pump solution for a fusor. Modern mechanical pump oil that has been doubly refined – like Inland 19 Ultra – is going to perform better than oil that has been absorbing moisture while sitting at atmospheric pressure for decades, and likely was never as refined as the new oil is anyway.
The original tags that came attached to the Welch 1402 pump head. Pretty clearly 60's or early 70's era tags.
The pump after swapping on the 1402.
The initial pressure measured at the end of the vacuum hose using the MKS901p and display loaned to me by Bruce Meagher.
An early pressure pulled on the fusor by the 1402. Later the pressure would drop significantly as the chamber outgassed its adsorbed moisture.
Joe.
