A tappet is used to apply linear force from one part of a machine, such as a piece in a car's engine, to another part inside that machine. This is usually because if the two parts were to come into direct contact with each other it could cause damage to one or both of the parts. The tappet provides the contact between them so only the amount and direction of force needed to move part of the machine is applied.
The most common example of a tappet is found inside an internal combustion engine, the type of engine found in most modern cars. For the engine to work, valves must be opened and closed so air can enter the engine and cause the combustion that moves the car forward. The valves must close during the combustion itself, and open again after the explosion to let the air out and allow new air to enter for the next explosion.
These valves are usually lifted by a part known as the camshaft. The camshaft turns and, as it does, small egg-shaped pieces on it called cams push up on a rod which in turn pushes on other parts that open the valves. As the cam turns over to the flat side of its egg shape, the rod is lowered and the valve closes. The cam continues to spin around and this process opens and closes the valves.
Located at the bottom of this rod is the tappet. It is a cylindrically shaped piece of metal that the cams come into contact with. The cam pushes on the tappet, which is a part of the rod and thus the rod is pushed upward. The tappet is there to keep the force of the turning cams from bending the rod. It ensures the force pushes the rod upward and not sideways.
Another type of tappet is the hydraulic tappet. It performs the same basic function but is designed so the engine will sound quieter during use and adjustments for valve clearance do not need to be performed periodically. The disadvantage to using a hydraulic tappet is that it can sometimes rattle loudly when the car is first started and in certain engines might malfunction and keep the valves from opening at all. These problems typically depend on the type of car the hydraulic tappets are used in.