Tappet

A tappet, also known as a valve lifter or cam follower, is a vital component in a valve train. Its primary function is to convert rotational motion into linear motion. In an internal combustion engine, this means turning the spinning motion of a camshaft into the up-and-down movement needed to actuate valves. These valves control the intake of air and the exhaust of gases. Without this precise movement, an engine could not function.

The mechanism works through direct or indirect contact. In many engines, the rotating camshaft features lobes, which are raised sections on the shaft. As the camshaft spins, these lobes press against the tappet. This contact forces the tappet to move vertically. This linear motion is then transferred to the intake or exhaust valves. In some designs, the tappet pushes a long rod called a pushrod to reach the valves. In other designs, the tappet might act on a rocker arm or even the valve directly.

Engineers use different types of tappets depending on the machine's needs. Solid tappets are simple components that require manual adjustment of the clearance, or the gap between parts. Hydraulic tappets, or hydraulic valve lifters, use a small internal piston filled with pressurized engine oil. This oil acts as a hydraulic spring to automatically adjust the clearance. This self-adjusting feature removes the need for manual maintenance. Another type is the roller tappet, which features a wheel at the contact point. These rollers reduce friction and drag, which increases engine efficiency.

To prevent damage, tappets are often designed to rotate in place. This circular movement helps minimize wear on the camshaft. If the camshaft always hit the exact same spot on a stationary tappet, it could create grooves. Many plain tappets also have a slightly convex, or rounded, profile. This shape helps soften the impact when the camshaft lobe makes contact. In some small engines with many cylinders, such as the Daimler "250" V8, the tappets were small and did not rotate.

The history of the tappet begins with early steam power. The first recorded use of the term was in 1715 for the Newcomen engine. This was an early form of steam engine that used manually operated valves. By 1715, tappets were used to automate this repetitive task. In these engines, a vertical "plug rod" hung from a beam. Adjustable tappet blocks were attached to this rod. As the beam moved, the tappets pressed against long levers called horns to work the valves. This method was used in Cornish engines into the early twentieth century.

Engine layouts have evolved significantly over time. In older sidevalve engines, valves faced upward and the camshaft sat directly beneath them.

In these systems, mechanics often adjusted the clearance by grinding down the ends of the valve stems. Modern high-performance engines often use a Double Overhead Camshaft (DOHC) design. These engines were originally developed for racing and aircraft. In a DOHC setup, the camshaft sits directly over the valves. They often use "bucket tappets" to move the valves. Adjusting these requires using small metal pieces called shims of specific thicknesses.

Tappets are also essential in other mechanical systems. In pneumatic cylinders, such as a rock drill or jackhammer, a tappet can be part of a bash valve. As a piston moves, it impacts a small tappet that moves an air valve to reverse airflow. Tappets also play a role in weaving looms. They act as a mechanism to form the "shed," which is the opening in warp threads. This allows weft threads to pass through and create patterns like denim, twill, or satin. Even modern Harris tweed is still woven on looms that use these mechanisms.