The Magic of Vacuum

Simple empty engineering explains how pumps can hold vacuum until you need it to empty restroom holding tanks efficiently

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I’ve been cleaning portable restrooms for three years but I really don’t understand how the vacuum pump works. How can it pull the air out of the tank without letting more air in? I see the shaft and how it seems to be centered in the pump and since that is the case, I don’t understand why it isn’t allowing as much air into the tank as it is pulling out.

Hank Sanders

Gillette, Wyo.



The answer to your questions is relatively simple and explains the difference between a blower and a vacuum pump. Let’s take a look at the two types of machines.

First, we’ll look at the blower. The blower has a housing in which the shaft and the vanes are centered in the housing. The vanes are equally spaced and their purpose is to move air, a lot of air. The vanes are in a fixed position on the shaft. As the blower receives power, the shaft turns at high speed and the rotation pulls the air out of a tank and evacuates it into the atmosphere.

The air is being removed from the tank faster than it can be replaced. The blower, in that sense, acts like a vacuum cleaner. The air is moving so fast that a temporary vacuum is created. When the blower is turned off, the vacuum that has been created dissipates. This is one of the major differences between a vacuum pump and a blower.


When you create vacuum using a vacuum pump, the vacuum remains in the tank until a valve is opened and the outside air rushes in, or when the valve is hooked to a hose, the material in the hose is pulled into the tank.

The vacuum pump is structured differently than a blower pump. The first major difference is that the shaft is not centered inside the housing. The vanes are placed in a rotor with slots for the vanes to slide back and forth. The vanes are not fixed to the shaft. As the rotor turns, the vanes slide out to the lubricated housing. As the vanes near the narrowing of the housing they slide back into the rotor, making sure the air is evacuated from the tank and not returned to the tank.

The air is then pulled from the vacuum tank and allowed to exit through the exhaust port (which is usually the path to the oil catch muffler) and eventually to the outside air. The vanes, sliding up against the inner housing, do not allow the air to return to the tank. They act as seals to prevent the air from returning.

Consequently, the vacuum remains in the tank when the pump is turned off. There is no relief for the vacuum until the valve is opened. By utilizing the off-centered shaft and rotor, the pump locks the vacuum in place and becomes an economical way, as you all know, for pumping septic tanks and portable restrooms.

It’s critical to pay close attention to the oil being used in the vacuum pump. While a blower requires oil for the bearings due to the high-speed rotation of the shaft, the vacuum pump is somewhat different. It needs oil for the bearings, but it also needs oil throughout the interior of the pump housing so that the vanes are able to slide over the surface at approximately 1,100 rpm.

Always use the type of oil specified by the manufacturer. Each pump manufacturer constructs vanes from different materials. Those materials react differently to different types of oil.

As they slide back and forth in the housing, the vanes are likely to wear down due to friction. As they wear down, the efficiency of the pump decreases. This is a common maintenance issue and it’s fairly easy to diagnose. Pump manufacturers sell rebuild kits to ensure you can continue to use their pumps for years to come. A rebuild kit consists of vanes, seals and bearings and is intended for a specific pump.

When opening the end plate of a vacuum pump to install a rebuild kit, you’ll see exactly what I have described.


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