Tappet

A tappet is a projection that imparts a linear motion to some other component within a mechanism.

The term is first recorded as part of the valve gear of Thomas Newcomen's atmospheric beam engine, a precursor to the steam engine. The first Newcomen engines had manually worked valves, but within a few years, by 1715, this repetitive task had been automated. The beam of the engine had a vertical 'plug rod' hung from it, alongside the cylinder. Adjustable blocks or 'tappets' were attached to this rod and as the beam moved up and down, the tappets pressed against long levers or 'horns' attached to the engine's valves, working the cycle of steam and injection water valves to operate the engine. This operation by tappets on a plug rod continued into the early twentieth century with the Cornish engine.

The term tappet is widely used in relation to internal combustion engines, but imprecisely. It is most commonly encountered as a maintenance task for overhead valve engines, that of 'adjusting the tappets'. This operation adjusts the overall clearance in the valve actuation system: typically 20 thousandths of an inch (0.5 mm). The name arises because it is the clearance of the tappets that is being adjusted, even though the adjustment is not made to the tappets themselves.

Strictly speaking, the tappet, also termed a cam follower, or lifter, is that part that runs on the camshaft and is made to move vertically by the action of the rotating cam. In an overhead valve engine, this tappet is fitted low down in the engine block. From there it drives a long thin pushrod, up to the top of the engine, above the cylinder head. Here the rockers, arranged on a rocker shaft beneath the rocker cover, reverse the direction of the valve movement to press the valves downwards to open them.

Early tappets had rollers to reduce wear from the rotating camshaft, but it was found that the roller pivots wore even faster and also that the small radius of the rollers also tended to accelerate wear on the expensive camshaft. Tappets then developed plain flat ends, although these were slightly radiused as 'mushroom' tappets as a perfectly flat end led to 'slamming' against a steep camshaft face.

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Wikipedia