Basic operation and maintenance tips
By Joe Escobar
Vacuum pumps have been getting a lot of press lately. Unfortunately, much of it has been negative, especially in the area of dry vacuum pumps. In this article, we will take a look at how vacuum pumps work as well as some maintenance considerations we need to keep in mind when working with vacuum pump systems.
In the early days, aircraft did not have vacuum pumps installed on them. Instead, the vacuum that was needed to run the gyro instruments was provided by a venturi tube mounted on the exterior of the aircraft. Although this was a fairly simple design and did not rely on the engine for operation, it did have its drawbacks. John Herman, vice president of marketing and sales for Tempest, explains. "Firstly, a venturi system is a dynamic system, meaning the aircraft must be moving at a fairly high speed to develop enough vacuum to run the instruments. Secondly, due to the amount of vacuum required to ’spin up’ the gyros a fairly long venturi was required. This, combined with the fact the venturi system is mounted on the outside of the fuselage in the air stream, makes it susceptible to icing conditions." Eventually, engine-driven pumps were developed to overcome these problems.
Measuring for vane wear.
Wet vacuum pumps
The first engine-driven vacuum pumps developed were "wet" pumps. Wet vacuum pumps are basically hydraulic pumps that pump air instead of liquid. The pump vanes are lubricated by engine oil. Wet pumps have a reputation for long service life. Because the internal parts are lubricated with oil (hence the term "wet" pump), the internal parts are more resistant to wear than dry pumps. However, the oil which contributes to the longevity of the design also was the leading drawback to wet pumps. The oil was pumped internally through the pump for lubrication purposes, and the oil and air were expelled out the exhaust port of the pump. Air-oil separators were used on the pump’s exhaust port to try to capture the oil. Unfortunately, some oil would usually bypass the separator, resulting in oil residue on the engine and aircraft exterior.
Damage to dry pump rotor caused by dropping the pump. Dry pumps
Around the 1970s, dry vacuum pumps were introduced to aviation. These pumps were engine-driven by a spline and did not rely on oil to lubricate the vanes of the pump. The carbon vanes of dry pumps are smaller in thickness than those on wet pumps. The vanes wear as they rub against the internal wall of the stator (or center section of the pump). As the vanes wear, they create a light carbon dust. This dust provides "self-lubrication" for the vanes.
The length of the vanes decreases over time through normal wear. If these vanes wear too much, the vane can bind in the rotor causing them to break. Until recently, mechanics had no way of determining the extent of vane wear. Recent designs in dry pumps have added the ability to determine vane life. Now, both Rapco and Tempest incorporate some type of wear indicator into their pump, thus ensuring the pump’s vanes have an adequate service life remaining.
Interior of wet vacuum pump. Note substantially thicker vanes.
Back to wet pumps
Recent high profile lawsuits have resulted in a renewed interest in wet vacuum pumps. This could be because of the perceived "bulletproof" design of the pump. It may also be because recent improvements in wet pump design as well as design improvements in air/oil separators by companies such as Airwolf and M-20 have resulted in little to no oil residue going overboard onto the aircraft.
Be careful not to get too complacent regarding the indestructable reputation wet pumps have received. Just like dry pumps, they are subject to failure if not maintained properly. Regardless of design improvements, there are several things that mechanics need to keep in mind to ensure they are not contributing to a shortened vacuum pump life. This is true for both wet and dry pumps.
Keep the system clean
One of the biggest contributors to vacuum pump failure is contamination. This contamination can either be FOD or fluid contamination. Mechanics must be diligent in preventing contamination from damaging the pumps.
Foreign particles larger than 0.001 inch can cause damage to your pump. It is necessary to keep the entire pneumatic system clean. If any part of the system is disconnected for maintenance, be sure to provide proper protection by capping or covering all lines. Before re-connecting the system, be sure it is thoroughly clean by blowing dry, clean air through it. Of course, be sure to protect any instruments from damage from the air. Mike Lotzer, national sales manager for Rapco, Inc., also stresses regular filter changes. He recommends changing the inlet and regulator filters every annual or 200 hours. Also, Lotzer says if feasible, take the system lines off the aircraft and blow them out.
Another thing to keep in mind with the system lines is possible contamination caused by deteriorating flexible lines. This can especially be a problem in aging aircraft. Flexible lines should be checked carefully to ensure that the lines are not deteriorating, which could lead to pump contamination. Older lines should be replaced.
If an aircraft has a vacuum pump failure, the lines need to be thoroughly cleaned prior to installation of the new pump. Contaminants left in the lines can damage the new pump if not removed.Another item that can introduce FOD damage is Teflon tape. Be sure not to put Teflon tape on vacuum pump fittings. Lotzer explains. "Teflon tape is a definite no-no. The fittings are cut with pipe thread. They are self-sealing. If Teflon tape is applied, the threads cut the tape, and it can end up in the pump." He explains that even a small piece of tape can damage the pump.
Solvents or hydrocarbons can damage vacuum pumps. If oil or solvent gets into the pumping chamber it will gum up the dry carbon that normally provides the lubrication for the vanes. This will cause the rotor to seize-up in the pump. The end result can be a sudden failure of the pump. This type of contamination is typically introduced in two ways — through a leaking garlock seal or through contamination from engine cleaning. Ironically, the drain mechanisms that some pumps incorporate to prevent the oil from leaking garlock seals from contaminating the pump can allow solvents to enter the pump if someone is careless when cleaning an engine. Therefore, the pump should be covered during engine washer to prevent accidental contamination. Also examine for oil around the mating area during maintenance to identify leaking garlock seals and replace them immediately.
An emphasis on cleanliness and regular attention and care can help ensure proper operation of vacuum systems, wet or dry. Regardless of the type of pump used, improper maintenance can quickly destroy a perfectly good vacuum pump. Proper inspection and maintenance practices can help ensure the vacuum systems you work on will provide years of trouble-free service for your customers.
Airwolf Filter Corp.
(800) 326-1534 Tempest
(800) 822-3200 Rapco, Inc.
(800) 527-2726 M-20 Oil Separators