Tappet

A tappet is a projection that imparts a linear motion to some other component within a mechanism. 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 valve 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 'rocker arm', 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, since a perfectly flat end led to 'slamming' against a steep camshaft face. To reduce wear from the rotating camshaft, the tappets were usually circular and allowed, or even encouraged, to rotate. This avoided grooves developing from the same point always running on the same point of the camshaft. In a few engines, particularly small V8 engines with little space, the tappets were small and non-rotating. This long and widely used engine arrangement was adjusted by turning a screw set in the end of the rocker that pressed on the end of the pushrod. This adjustment was made by rotating the engine until the cam was at its lowest point, giving the widest gap, then adjusting the rocker until this gap was at the correct spacing, as measured with the use of a feeler gauge. Mis-adjusted tappets can give rise to an audible 'tappet rattle' from the rocker cover, if the valve clearances are excessive. Over-tight clearances may, even worse, lead to bent pushrods or burnt valves. The adjusting screw was locked by a locknut. If the screw became loose, the adjustment would also work loose. Such a simple matter as a loose tappet locknut even led to an engine fire, then the aircraft crash that led to the death of WWII radar expert Alan Blumlein. Some mass-market car engines from the 1960s replaced the rocker-end adjustment screw with a non-adjustable rocker pivoted on a central stud with a hemispherical nut. Tappet adjustment was made by moving the pivot nut up and down. Some of these designs also required the tappet clearance to be set with the engine running. As this rapidly wore out feeler gauges, long rolls of disposable feeler strip were needed instead.