Your bathtub faucet system has several parts that work together to produce water of the right temperature for your bathing or showering pleasure. Each part has a definite function and purpose and without one the whole unit would simply not operate.
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The handle is the most obvious part of the faucet. This is the part you turn to control the water. Turn it left and you'll get hotter water. Turn it right and you can take a cold shower. Handles come in all shapes and styles from elaborate, decorative lever handles to simple round ball-shaped handles. This part is connected to the faucet cartridge, the valve system in the faucet.
The cartridge is the valve system in the bathtub faucet. It is made of two ceramic discs that fit together with the flat sides facing each other. When the handle rotates, it turns the discs so holes align and water flows into the faucet. The cartridge is often replaced if the faucet starts leaking because that means the discs are scratched or damaged. There are also O rings that work in conjunction with the cartridge to keep water from escaping through the body of the faucet.
Bathroom faucets have trim pieces that protect the body of the faucet and your wall. The trim sleeve is a cylindrical object that fits over the cartridge and is located under the handle. The shower wall trim is called the escutcheon; it is a round plate that fits over the entire body of the faucet. The trim sleeve is removed to access the cartridge. However, on many faucets the escutcheon or back plate does not have to be removed to reach the cartridge.
Bonnets and Retaining Nuts
The bonnet and retaining nut hold the cartridge in place. Each faucet uses either one or the other to secure the cartridge. The bonnet is typically a round part that screws onto the base of the faucet over the cartridge. The retaining nut is also round but shaped more like a ring. It also screws onto the body of the faucet and holds the cartridge in place. Some faucets do not use either. They use a cartridge clip to secure the faucet in place. This clip inserts into the body of the faucet, securing the cartridge in place.
Adapters and Spacers
Many faucets have adapters and spacers. An adapter connects certain handles to the body of the cartridge and is located between the cartridge and handle. It is also referred to as a stem extension, because it extends the stem to fit the handle. A spacer is also located between the cartridge and handle. It is ring-shaped and simply provides a buffer when the handle does not fit directly up against the cartridge. Not all faucets use these parts, but many do.