Sanitation Industry Professionals Explore the Process Behind Pumping & PTO Switches

Understanding how power is transferred from your truck’s engine to the vacuum pump could save you the headache of an inconvenient and costly breakdown in the field.
Sanitation Industry Professionals Explore the Process Behind Pumping & PTO Switches

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QUESTION: I’ve been an owner/operator for almost three years and I’m not exactly sure how my vacuum system works. All I know is I pull out the PTO switch and my pump comes on, and in minutes, I’m ready to pump. Can you tell me what happens between the PTO switch and the vacuum pump? Someday my luck with this thing might run out, and I need to know what’s going on underneath me.

Denny Macklin
St. Louis, Mo.

ANSWER: This is a good time to review the whole process of pumping. We’ll start, like you say, turning on the power takeoff. This is done electronically with the truck in neutral. The PTO now does what it is supposed to do, take power from the truck transmission. It is connected to the truck transmission and it turns at the same speed as the truck transmission.

Just a side note, new trucks bought from a dealer may not have the PTO provision, which allows connection of the PTO. Without the PTO provision, there can be no way to operate the pump. In this case, an engine-driven setup would need to be used.

With the PTO running, another shaft extends to the right angle gearbox. The right angle gearbox (they also make them with no right angle) serves two purposes. First, it properly lines up the drive shaft from the gearbox outlet to the vacuum pump. Second, it converts the rpm from the PTO to match the rpm required for the vacuum pump to operate properly.

For example, let’s say in our diagram the transmission is turning out 550 rpm. That speed goes to the PTO and then to the gearbox. The manufacturer of the pump recommends that their pump run at 1,100 rpm. Basic math tells us we need to double the speed of the shaft going to pump. Consequently, the assembler or the manufacturer of the system will install a gearbox with a 2:1 ratio.

With the driveshaft now turning at 1,100 rpm, there are two options to bring torque to the pump. The truck builder can run the shaft directly into the pump and be done with it.

Or, as I strongly recommend, the builder can install a Woods coupler between the gearbox and the pump. This is a small, flanged unit with hard rubber discs housed in a steel case that will save the pump in case the driver pulls the PTO switch when the truck is running and the driver is in a hurry.

PREVENT DAMAGE

The Woods coupler is designed to break away and prevent worse damage to the vacuum system in cases of a sudden, jerky start to the PTO drive. It is much easier to replace a Woods coupler and perform some shaft repairs than buy a new pump. The Woods coupler is made in two parts. One half accepts the shaft from the gearbox, and the other half, running at the proper rpm, is connected to the pump. In cases of sudden, high rpm starts, the shaft from the gearbox to the Woods coupler breaks at the coupler. The vacuum pump itself now has its two inlets filled: The shaft from the Woods coupler, which is driving the pump, and the inlet coming from the secondary moisture trap that protects the pump from waste damage. The outlet connects to the oil catch muffler, which quiets the pump and retrieving excess oil given off by the pump.

Depending on the type of pump you are using, there may also be an oiler. Some pumps have an automatic oiler that feeds the rotor on the inside. Other pumps have a separate oil tank that feeds the rotor.

It is important for every driver to know and understand these basic parts and their purpose. Any one of these items can cause problems, but some can be avoided if you know how the system works and monitor it properly every day.

As you see in the graphic, your vacuum system utilizes a relatively small number of moving parts. Familiarize yourself and your drivers with each component. If you take the time to do that, a lot of repairs and routine maintenance can be performed at your shop and you will get the maximum life out of your pump.



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