While many small farmers still love to hand-milk their cows, commercial dairying usually employs the milking machine.

The modern milking machine looks complex, but the principle on which it operates is actually quite simple. The machine pulls a vacuum on the teats of the cow, causing the milk to flow.

Here’s how it works:

1. The cow’s teats are attached to the teat cups. Each teat cup contains a rubber or silicone liner inside a plastic or stainless-steel shell. The liners are the only parts of the machine that touch the cow. They form a seal between the teat and the short milk tube, used to transport the milk. All of the liners are worked by a pulsator valve, which in turn is connected to a vacuum pump. The area between the liner and the shell is the pulsation chamber.
2. The pulsator pulls a vacuum on the pulsation chamber, causing the liner to open up.
3. A constant vacuum is maintained on the short milk tube. As the liner opens due to the equalization of the vacuum pressure between the short milk tube and the pulsation chamber, the teat is exposed to the vacuum of the short milk tube, causing milk to flow.
4. The pulsator then releases the vacuum and exposes the liner to air again. Because now the air pressure in the pulsation chamber is greater than that in the short milk tube, the liner collapses and tightens on the teat in a massaging motion. This maintains proper blood circulation in the teat.
5. The pulsator operates at a rate of about 60 cycles per minute.
6. The short milk tubes attached to the teat cups meet at the part of the machine known as the claw. Milk from all four teats mixes at this point.
7. The vacuum in the long milk tube pulls the milk in a column through the line.
8. As the milk flows through the long milk tube, it enters a receiving jar. Any trapped air pockets in the milk column are released at this point. Milk from other cows attached to other milking units is mixed in.
9. As the receiving jar fills up, a pump kicks on and pushes the milk into the bulk tank, where it is refrigerated.
10. When the cow’s udder empties, the milking machine automatically shuts off. Various types of meters are used to detect the decrease in milk flow.
11. The teat cups automatically detach from the cow.
12. The cow’s teats are dipped in iodine to reduce the risk of infection caused by contaminated milk flowing back into the teats when the pulsator lets air into the teat cup.
13. The machine is cleaned to prepare it for another use.

Smaller operators might use a variation on this system in which the milk flows into a clean can or bucket instead of a receiving jar. In this system, the pulsator generally sits on top of the bucket. When the milking process is completed, the apparatus is removed from the bucket, leaving a container of farm-fresh milk.

Helpful Resource

How the Milking System Works
Includes plenty of photos and a useful diagram.