By Howard Tring
As stated last month, in Part 1, this article talks only about one and two stage “medium vacuum” oil sealed rotary vane vacuum pumps that can produce a catalog ultimate vacuum of about 1 x 10-2 Torr (0.01 Torr or 10 microns) for a one stage model and about 1 x 10-3 Torr (0.001 Torr or 1 micron) for a two stage model.
Smaller vacuum pumps such as those used in the heating, ventilating and air conditioning industry (HVAC) are not included as they are often only for intermittent use and do not have the design features built into the laboratory sized continuous running vacuum pumps used in industry and science.
|Fig. 1 Direct Drive vacuum pumps.|
Larger rotary vane vacuum pumps, ones that require ball or roller bearings to support the weight of the rotor are not included either. Although they have many similar features to the laboratory sized vacuum pumps, they also have a variety of options to suit different applications.
Last month we looked at the old design vee belt drive oil sealed rotary vane vacuum pumps. These pumps, although reliable and able to cope with contamination from industrial and scientific vacuum applications, were labor intensive to make and assemble as they needed a baseplate to mount the pump, motor and drive components on.
One point that I missed in this discussion was that in most laboratory applications, you see oil sealed vacuum pumps sitting on a tray of some sort. This is to catch oil leaking from the shaft seal and prevent it from making a mess on and perhaps damaging the floor. This tends to hold true for the newer direct drive oil sealed vacuum pumps as well, even though shaft seal technology has improved. Vacuum pumps are often under a bench, behind and instrument or in a fume hood cupboard and do not receive the regular service that should be carried out. It probably isn’t much different in industrial vacuum pump applications. The most important part of a machine or production line is the production itself, little thought is given to the vacuum pump – if it does its job. It only gets attention if it fails for some reason, often neglect.
I must be fair here; many companies have very good maintenance departments and have routine maintenance schedules for each piece of equipment that needs it. These days this schedule is often a computer program and daily/weekly task lists are generated for machines that need to be checked or worked on. However, in my experience of business cycles, when times get tough the maintenance schedule is often one of the first things to be reduced. The plant moves from a regular schedule to a “fix it when it breaks” schedule. This can also be true when high production requirements overrule the need for a routine shut down. Again the fix it when it breaks rule takes over.
This is a long introduction, let’s move on.
Growth of the Vacuum Pump market in North America
|Fig. 2 Cross-section of DD vacuum pump.|
In the previous article I said that European vacuum pump companies started to open offices in North America in the fifties. This is when the old Edwards High Vacuum Ltd first ventured here, starting in Hamilton, Ontario around 1953 and then opening another branch on Grand Island, NY several years later. Due to this I assume that Leybold Vacuum arrived in the USA at about the same time, in Export, PA. I searched for some history on Leybold and other European vacuum pump manufacturers arriving in the USA but couldn’t find much. There was a reference to Alcatel dating from 1952 and another for Leybold dated 1962. One source was about twelve copies of a monthly magazine called “Vac Tech” which was part of Research and Development magazine. They were published by F D Thompson Publishers, from Chicago, in 1962 and 1963.
Looking at the advertisements and articles in these magazines indicated that the vacuum coating industry was an important one. There were a number of US companies offering vacuum coaters as well as some who were strictly offering vacuum pumps. The old Edwards High Vacuum advertised in nearly every issue but did not show one vacuum pump, just coaters and freeze dryers. Their focus did change in later years to less vacuum systems and more vacuum pumps.
I was looking for any references in these 1960s magazines to direct drive vacuum pumps, and found very little. Welch, Kinney, Stokes, Central Scientific and Lapine Scientific were all showing VBD vacuum pumps. Lapine Scientific’s pump was a “Vacuum Master” made by Leybold. This leads me to think that Leybold was relatively new in the USA at that time.
There were three companies mentioning direct drive vacuum pumps:
- A Langdon water cooled single stage small 35 l/m was advertised by Hevi-Duty, a part of Basic Products Group from Wisconsin.
- CVC offered 9 sizes of two stage “no vibration” pump, and
- Standard Scientific Supply from NY was offering its “Vacu-Pump which was a vertical design.
I don’t think any of these companies are still in existence.
|Fig. 3 Direct drive pump rotor and blades.|
By the 1970’s the European manufacturers all had a presence in the USA, and Leybold opened a manufacturing facility in Export, PA. Companies such as Leybold, Alcatel and Edwards became more successful as they overcame the reluctance to buy vacuum pumps from an offshore company. I believe much of the reluctance was due to a perceived difficulty in obtaining spare parts quickly and also a lack of technical support across the country. Companies such as Balzers and Pfeiffer also established bases in the USA during this time. These companies supplied vacuum pumps for industrial and scientific applications and also had the gauges, valves and fittings to widen the range of products. Over the years they also opened branch offices and repair centers across the USA to serve local markets better.
American manufacturers were slow to react to the European invasion of these small direct drive vacuum pumps. By the time they had developed their own designs the European companies were all well established in North America. (Fig. 1) Companies such as Welch, Precision Scientific and Kinney did make direct drive pumps successfully, but by then they had lost a great deal of market share which I don’t think has ever been recovered.
Differences between VBD and Direct Drive Vacuum Pumps
Looking at the two types of pump the direct drive pump is more compact and weighs less. Most early models had carrying handles until ISO standards limited the weight of a pump with a lifting handle to about 25 kg (55 lbs.). Now, for example, the Edwards RV3 and RV5 have a carrying handle, while the RV8 and RV12 have a lifting eye instead.
The second main difference is that instead of running at a rotational speed of around 500 rpm like the VBD pump, the direct drive pump has a 1750 rpm motor.
Running faster does allow a higher pumping speed (l/m, m3/h or cfm) to be generated in a smaller package, but there are also drawbacks.
|Fig. 4 Shaft seal designs.|
One of the early problems in pumps that I am familiar with was that the UK engineers didn’t take into account the difference in motor speed between the UK and the USA.
Pumps that ran at 1450 rpm in the UK failed earlier when running at 1750 rpm in the USA. This was considered for generation 2 of the direct drive pumps!
The interior design of the pump also had to be modified. (Fig. 2)
A faster running speed also makes the surface speed between the rotor, stator and end faces faster. This meant a pressurized lubrication system was needed to ensure good lubrication to all parts of the pump.
To reduce the surface speed of the rotor it was redesigned to make it a smaller diameter but longer in length to retain the pumping volume. (Fig. 3)
Earlier designs of rotor for two stage pumps were either a separate rotor for each stage with a small drive coupling between them, or for smaller pumps a one piece rotor for both stages. The rotor depends on a good oil film at the bearing to make sure it doesn’t actually touch the bore in the stator. A lighter rotor “floats” on the oil film better.
Another physical change was in the blades (vanes) area. Slower running VBD pumps required steel blades and springs between the blades to ensure that the blades contacted the surface of the stator. With faster running direct drive pumps the centripetal forces are higher and the blades could be made of lighter weight composite materials. Later models made the blades longer to push each other and the springs were eliminated.
Shaft seals in early VBD pumps were simple designs, and frequently leaked oil, so these had to be upgraded when direct drive pumps were introduced. The faster speed generates more heat at the seal surface and better materials and innovative lip designs were used. One shaft seal that I am familiar with has a wavy line molded into it as the seal lip. This tends to wipe a wider area of the shaft surface and reduces the likelihood of a groove being worn in it. Another design of seal has a spiral molded in it at the sealing face which tends to “pump” the oil back into the oil box. This seal must be selected for direction of shaft rotation – i.e. it is unidirectional – otherwise it can “pump” the oil out of the oil box. (Fig. 4) Due to a lack of attention mainly, shaft seals still leak but generally last a number of years before failing.
One model of Edwards direct drive pumps added a thick felt material pad under the seal outside the pump to adsorb any oil leakage from the shaft seal. It could be checked to see if any oil was in it by removing one of the plastic side plates. Laboratory pump users thought that this was a good idea.
|Fig. 5 Hydraulic inlet valve.|
The running temperature of direct drive pumps is higher than the old VBD pumps and this is partly responsible for vacuum pump oil specifications being improved over the years. One main change is from SAE30 viscosity oil to thinner SAE20 viscosity for the direct drive pumps.
A longer term result of the higher running speed is that direct drive vacuum pumps need a minor service about every two years if they are in continuous use. A complete change of valves, seals, o-rings and gaskets should be carried out and the internal parts of the pump examined for wear.
Items such as exhaust valves, either metal or elastomer, tend to lose some flexibility and elastomer o-rings will slowly become harder and more brittle. If the exhaust valves do not seal the exhaust holes when the pump stops, oil can leak down into the stator void. This will cause starting problems the next day and may cause fuses to overload or the motor thermal protection to cut in.
Continuous pressure from a shaft seal can also wear a groove in a rotor shaft. That can sometimes be polished out, but may require a new shaft or shaft sleeve. Another way to extend the life of those parts is to insert a suitable spacer next to the new shaft seal on installation, which moves the seal lip onto a clean part of the shaft or shaft sleeve.
Blades do not generally wear out quickly, but may become scratched across the sealing face due to foreign material entering the pump. The vacuum blades and the oil pump blade should be checked at every minor service and replaced as necessary. In a worn pump the arcuate seal area may also have a few marks running across it. It is not recommended to machine this area as it will affect the clearance for the oil film. If there are rough burrs some careful light work with an oilstone may remove them.
If new blades are installed in a pump with a marked arcuate seal area, it may take a number of hours for the new blades to wear in. The vacuum reading may not be as good as expected until this has occurred.
The last improvement that the direct drive pump has over the VBD pumps is the ability to use the oil pressure to open and close a valve at the inlet of the pump. In VBD pumps the problem of oil “suck back” into the vacuum system was always present. Many systems would have a separate isolation valve mounted above the inlet that would open when the pump started and close once the pump was shut off. VBD pumps have a larger volume of oil above the exhaust valves because the oil drains into the pump for lubrication by gravity. If a pump shut off due to a power failure, for example, it was possible for oil in the pump to be pushed by atmospheric pressure through the pump mechanism and into the vacuum lines on the inlet side of the pump.
The pressurized oil circuit is now used to open and close a built in inlet valve that completely isolates the vacuum system from the pump interior – and the oil – if the pump stops causing the oil pressure to drop to zero. When the pump starts the oil pressure is used to hydraulically open the inlet valve and when the pump stops a spring will automatically close the valve. (Fig. 5) Then the interior of the pump is allowed to reach atmospheric pressure.
The oil pressure can be generated by a small blade in the shaft rotating in an eccentric bore in the pump body. This design requires a shaft seal on either side of it to seal the pressure. Another method is to use a gerotor; this is a small gear rotating inside a gear profile.
I hope this allows the reader to understand how the rotary vane, oil sealed mechanical vacuum pump market and technology have changed over the years. There are still many uses for this type of vacuum pump. As usual comments and or corrections are always welcome.
Howard Tring / Tel: (610) 792-3505(610) 792-3505 / E-mail: HowardT (at) VacuumAndLowPressure.com / Web: www.vacuumandlowpressure.com
Howard Tring is the owner of Vacuum and Low Pressure Consulting, a company that supplies vacuum pump accessories such as reconditioned inlet traps and exhaust filters and new replacement elements for exhaust filters. Howard also offers on-site vacuum technology and oil sealed vacuum pump repair training and consulting services, customized to the needs of the client. Howard is a member of ASM International and the Heat Treat Society, the AVS, the SME, the SVC and the American Society for Training and Development.
Copyright December 2014, Tring Enterprises LLC – Comments on this article are welcome. I do not profess to know everything about any specific vacuum related subject. However, I have worked in the vacuum pump industry a long time and have seen good, bad and ugly. Please contact me with any comment or question. All messages related to the content of the article will be answered.