Vacuum pumps — most of us have been exposed to these aircraft components. For aircraft gyros, there have been two main choices for vacuum pumps — dry and wet. Both types of pumps use the same technology of rotary vanes to provide suction (or pressure for pressure-type systems). But there is a new kid on the vacuum pump block: a dry piston pump manufactured by Sigma Tek. This is a PMA direct replacement pump for many piston engine applications. The pump is a totally different design from current vacuum pumps. We will take a look at how it works and address some maintenance issues mechanics need to be aware of.
How it Works
When you first see a Sigma Tek vacuum pump, you will notice right away that it doesn’t resemble the rotary vane pumps it is designed to replace. Not only does it look different, it operates totally differently than rotary pumps.
The basic design of the piston pump is simple. There are a total of 16 umbrella valves, eight on either side of the piston. The piston is operated by a crank mechanism that is driven by the engine. The design of the crank is such that it drives the piston straight up and down in the cylinder, thus avoiding side loading on the cylinder wall. As the piston pushes toward one side of the pump, four of the eight valves shut with the pressure produced, while the other four open allowing air to go through and out the pressure side of the pump. On the other side of the piston, the opposite is happening — four of the valves close while the other four open up allowing air to enter from the vacuum side of the pump into the pump chamber. Once the piston reaches full travel, it reverses direction. Now, the air that was drawn into the pump is expelled out the pump pressure side while the other side is drawn into a vacuum. So, in each half of the full cycle, air is being drawn in and expelled, creating a constant vacuum and pressure during operation.
The piston pump design doesn’t lend itself to catastrophic failure. The design of the valves is such that several valves can fail and still not significantly affect the pump’s performance. However, failure of an excessive number of valves will result in a degradation of pressure. Monitoring pressure output is a good way to monitor pump life. For example, if you are normally at 5.2 inches pressure at cruise you may see that go down to 4 inches if some valves aren’t working properly. This is a good indication that the system and/or the pump needs to be looked at.
Although the design of the piston air pump makes it less susceptible to failure by foreign object ingestion, it is still necessary to clean out the vacuum system prior to installation. Ensure all lines are free of debris. This is especially critical if you have a rotary pump failure since there will be carbon dust and debris in the lines. This is also a good time to check the condition of all flexible lines and replace them as necessary.
The Sigma Tek pump installs on the same mounting pad as the original rotary vane pump. One thing that mechanics should keep in mind is that the design of the pump is such that it is bi-directional. That means that the vacuum port and the pressure port will remain the same regardless of whether it is driven clockwise or counter-clockwise. There is no need to know engine rotation or worry about stocking different pumps for different rotating drives.
The pump can be mounted in any orientation. The mechanic can mount it in the orientation that will allow for the best clearance and easiest plumbing. Sigma Tek shared that for the most part, the pump will either be installed vertically where the piston head is vertical up, or horizontally where the piston head is to the left-hand side toward the accessory case when you are looking toward the front of the aircraft. It boils down to installing it in a way that allows for the easiest installation. Each shipped pump comes with a flange gasket.
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