Scroll vacuum pumps, or dry pumps, are mainly used in vacuum systems requiring a clean condition, free of oil vapor contamination from normal oil-sealed pumps like rotary vane pumps. Scroll vacuum pumps have one or two stages of orbiting plates, compressing gases to the center of the plates and sucking the molecules out. The video from Pfeiffer Vacuum demonstrates the concept nicely.
The first pump I will dismantle come from Varian (Now Agilent). The model is TriScroll TS600. This model is one of the best available on the second hand market, mainly applied in helium leak detectors.
With two stages, the pump can achieve an ultimate pressure of 9.3E-3mbar.
When I got the pump, it has been used for about 30,000h, and it could not pump down the pressure. I dissembled and found that its tip seals were all broken and the inner part was extremely dirty. The second stage is displayed first, then the first stage according to the sequence of dismantling.
You need to remember the circle’s position here, otherwise it is extremely time-consuming to resemble the piece as it is not concentric with the outer circle.
This is the first plate. You can see the long using time causes severe scratches on the channels and the tip seals are abraded and need to be replaced, as they can no longer hold the vacuum by forcing the gas to be compressed. The three movable things on the sides are sync cranks to support the movement of the first plate. Don’t remove them
Second plate of the first stage , opposite to the piece above. The aluminum case has been removed and the bearing assembly is shown. There are three sync cranks at the sides. Don’t remove them.
First plate of the first take (the plate behind the one above).The above piece can only be removed after taking off the bearing assembly, where the black nut is secured by four screws, and crocodile plier should be used to hold the nut so it won’t move while taking off the screws. The black ring in the center is positioned by a special glue, so please be careful not to take it off if you don’t want to spend extra money on the glue. You can see that here some tip seals have already come off.
Next is the last plate, the one in the first stage. There is a crankshaft. Here you need to be careful not to apply any force on it as you will ruin all the bearings if you do so.
This is the back side of the plates, where the bearing appears. The originally white grease has become black and needed to be replaced. After removing the fan and the balancing weight, you should see the first picture, and the second picture is after removing the cap in the first picture.
After dissembling, the problem has been clear—the tip seal should be replaced and new grease should be added to the breading module.
Firstly the plates are cleaned using pure isopropanol. It is critical to critical to clean up all the dirt stick to the wall of the channels, but giving that the contamination is formed from the grease, it is impossible to clean it using a tissue with isopropanol. After some trials, I found out that it is the best to use forceps with a sharp plastic end to get rid of the dust. Using stainless steel forceps will result in scratches; I don’t know whether this is acceptable so I insist on plastic forceps.
Later, the new tip seals are placed.
Then, the old grease on the bearing is carefully removed by a towel without any solution. Cleanup grease that is visible to you is enough. Don’t apply any force to the bearing or try to take bearings out since it is very difficult to put it back and the old bearings are easily broken. The grease is then put directly on the face of the bearings. The grease for bearings are GPL224. A thin layer of the same grease is also applied on each o-ring for better vacuum sealing performance.
The only important part of the installation is that one need to use a depth meter to install the black nut on to the crankshaft. Firstly, the distance from the face of the nut to the crankshaft end is measured before the tip seals are installed. Then, after installing the tip seals, the depth is remeasured and the final depth should be the first reading + 0.009 inch. This is to ensure a correct resistance between the plates and the tip seals. Too tight hurts the bearings and causes overheating, too light cannot ensure a good vacuum.
After fixing everything, I assemble the whole system (make sure to notice that the two circles when installing the final plates are not concentric). The pump reaches < 5Pa easily.
The second pump, XDS10 from Edwards, has the same problem of not able to pump to vacuum. After dismantling, it is easily determined that this is also caused by the broken tip seals. In fact, replacing tip seals solves 9/10 of the cases. This pump has only one stage, so the ultimate pressure is 6.5Pa.
The tip seals at the center has been abraded severely, where its thickness is only half of that at the sides.
Again, the orbiting plates are clean with isopropanol and forceps, and new tip seals are installed.
After resembling, the pump can now go down to 4Pa, similar to a new one.
The last candidate is the mini scroll vacuum pump from ScrollTec. The labeled ultimate vacuum is 10Pa, while it only weight 2Kg. This is also a single-stage pump.
For this tiny pump, it does not apply any tip seal, so it is maintenance-free. It applies a special plastic material for its orbiting plate, and this plastic is used as tip seals. However, after using for years, the ultimate vacuum can greatly reduced due to the wearing plastic seals. One of my SVF20 can only attain a pressure of 200Pa. To solve the problem, the screws at the outer case can be tighten using a torque wench. The torque can increase from 4N, 0.5N at a time. The torque should no longer increase after <20Pa is obtained, since further tightening might cause damages to the aluminum case or the plastic case—the aluminum case is not that reliable; one time I break it when I apply 8N torque to a screw for the gas outlet.
Although this small pump is pretty good in its ultimate vacuum, it should be aware that the noise is much higher than even the big pumps due to the lack of tip seals. This can partially be solved by customizing an anti-noise case for it.