A tappet is a projection that imparts a linear motion to some other component within a mechanism.
The first recorded use of the term tappet is as part of the valve gear in the 1715 Newcomen engine, an early form of steam engine. Early versions of the Newcomen engines from 1712 had manually operated valves, but by 1715 this repetitive task had been automated through the use of tappets. 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.
Internal combustion engines
In an internal combustion engine, a tappet (also called a 'valve lifter' or 'cam follower') is the component which converts the rotation of the camshaft into vertical motion which opens and closes the intake or exhaust valve. In an overhead valve engine, the tappets are located down in the engine block and operate long, thin pushrods which transfer the motion (via the rocker arms) to the valves located at the top of the engine. In an overhead camshaft engine, the tappets are located at the top of the cylinder head and often act directly on the valves. The types of valve lifters (i.e. tappets) used by automotive engines are solid lifters, hydraulic lifters and roller lifters.
An alternative to the tappet is the 'finger follower', which is a pivoting beam that is used to convert the camshaft rotation into opening and closing of a valve. Finger followers are used in some high-performance dual overhead camshaft engines (instead of bucket tappets), most commonly in motorcycles and sports cars.
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 of the tappet always running on the same point of the camshaft. However, in some relatively small engines with many cylinders (such as the Daimler '250' V8 engine), the tappets were small and non-rotating.
Most 'flat' tappets (i.e. without rollers) usually contain a slight radius which creates a subtle mushroom-shaped surface, since a perfectly flat surface leads to 'slamming' against a steep camshaft face.
Adjusting the tappets
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A common, yet imprecise, use of the term 'tappet' is the engine maintenance task referred to as "adjusting the tappets" in an overhead valve (OHV) engine, which was a widely used engine configuration between the 1940s and the 1990s. The task involves adjusting the clearance of tappet from the camshaft, however the adjustment is not actualy made to the tappets themselves.
On most OHV engines, the adjustment was made by turning a screw set in the end of the rocker that pressed on the end of the pushrod. With the engine rotated to give the widest gap between the camshaft and a particular tappet, the rocker screw was adjusted until this gap was at the correct spacing, as measured with the use of a feeler gauge. If the gap was too wide, this could result in an audible 'tappet rattle' from the rocker cover. If the gap was too narrow, this could result in engine damage such as bent pushrods or burnt valves. The adjusting screw was locked by a locknut. Failure of the locknut to hold the adjustment in place could cause catastrophic engine failure, which has led to fatal aircraft crashes.
On some OHV engines in the 1960s, such as the Ford Taunus V4 engine and Opel CIH engine, the tappet adjustment was done by setting the height of the rocker pivot point (rather than the typical method of a rocker-end adjustment screw). On the 1965-1970 versions of the Opel CIH engine with solid tappets, the tappet adjustment was conducted with the engine running.
Hydraulic tappets are tappets that contain a small hydraulic piston, pressurised by the engine's lubricating oil supply. Although the piston does not move appreciably or frequently, it acts as a hydraulic spring that automatically adjusts the tappet clearance according to the oil pressure. This makes the valve actuation self-adjusting and there is no need to adjust the rockers. Hydraulic tappets depend on a supply of clean oil at the appropriate pressure. When starting a cold engine, with low oil pressure, hydraulic tappets are often noisy for a few seconds, until they position themselves correctly.
Early automotive engines[when?] used a roller at the contact point with the camshaft,(p44) however as engine speeds increased, 'flat tappets' with plain ends became far more common than tappets with rollers. Some modern high-performance automotive engines use roller tappets, in order to use a camshaft with a steeper profile.
The earlier sidevalve engine had a similar arrangement, but simpler. As the valves were mounted at the sides of the cylinder and faced upwards, the camshaft could be placed directly beneath them and there was no need for a rocker. With lower cylinder blocks, the tappets could drive the valves directly without needing even a push rod.
Sidevalve engines also required their tappets adjusting, and in this case it was the tappets themselves that were adjusted directly. Small access plates were provided on the sides of the cylinder block, giving access to the gap between the valves and tappets. Some tappets had a threaded adjuster, but simpler engines could be adjusted by grinding down the ends of the valve stem directly. As the adjustment gap need only be expanded over the engine's working life (re-grinding valves into their valve seats during de-coking makes them sit lower, thus closing up the tappet gap), adjustment by removing metal was acceptable. Care was obviously needed, to not remove too much. Eventually the valves would be replaced entirely, a relatively common operation for engines of this era.
Overhead cam engines
Overhead cam engines were first developed as high performance aircraft and racing engines, with the camshafts mounted directly over the valves and driving them through a simple 'bucket tappet'. To give the best valve position for gasflow through a crossflow cylinder head, valves were usually mounted in two rows and with two separate camshafts. These bucket tappets were usually adjusted by a small shim, either above or below the tappet. Shims were made in a range of standard thicknesses and a mechanic would swap them to change the tappet gap. The engine would first be assembled with a default shim of known thickness, then the gap measured. A set of different shims would then be installed, each one chosen according to the change needed from the measured gap to the ideal gap. The gaps would then be measured again, in case of mistakes and also in case the cam position had shifted slightly. As the camshaft had to be removed to change the shims, this was an extremely time consuming operation, especially as the precise height of the camshaft above the cylinder head could change, depending on how carefully it was re-installed.
A much-improved system placed the shims above the tappet. This allowed each shim to be changed without removing either the tappet or camshaft, usually by pressing the tappet and valve down against the valve spring with a lever tool and removing the shim with tweezers. A difficulty with this system is that the rubbing surface of the tappet becomes the surface of the shim, which is a difficult problem of mass-production metallurgy. The first mass production engine to use this system was the Fiat twin-cam engine of the early 1960s, followed by engines from Volvo and the water-cooled Volkswagens.
The development of more efficient OHC mass-production car engines in the 1960s, combining overhead crossflow valves with the low cost of a single camshaft encouraged engines with overhead rockers, directly beneath a single overhead camshaft.[note 1] These rockers combined the function of sliding tappet, rocker and adjustment device. Adjustment was usually by a threaded pivot stud beneath the rocker. Linear sliding tappets became a problem for wear and demanded careful lubrication. Some engines, such as the Ford Pinto, developed a poor reputation for camshaft wear and encouraged aftermarket improvements to their lubrication systems.
The term 'tappet' is also used, obscurely, as a component of valve systems for other machinery, particularly as part of a bash valve in pneumatic cylinders. Where a reciprocating action is produced, such as for a pneumatic drill or jackhammer, the valve may be actuated by inertia or by the movement of the working piston. As the piston hammers back and forth, it impacts a small tappet, which in turn moves the air valve and so reverses the flow of air to the piston.
In weaving looms, a tappet is a mechanism which helps form the shed or opening in the warp threads (long direction) of the material through which the weft threads (side to side or short direction) are passed. The tappets form the basic patterns in the material such as plain weave, twill, denim, or satin weaves. Harris tweed is still woven on looms in which tappets are still used.
- The Newcomen Memorial Engine. Dartmouth, England: Newcomen Society.
- Woodall, Frank D. (1975). Steam Engines and Waterwheels. Moorland. pp. 31–34. ISBN 0903485354.
- Setright & Anatomy of the Motor Car, p. 33
- Hillier, V.A.W. (1981). Fundamentals of Motor Vehicle Technology (4th ed.). Stanley Thornes. p. 44. ISBN 0-09-143161-1.
- "Valve system operation" (PDF). www.thecarguys.net. Retrieved 11 February 2020.
- Setright, L. J. K. (1976). "Valve gear". In Ian Ward (ed.). Anatomy of the Motor Car. Orbis. pp. 29–36. ISBN 0-85613-230-6.
- "Hydraulic vs. Solid Lifters". www.summitracing.com. Retrieved 11 February 2020.
- "What is the difference between a flat tappet and a roller camshaft?". www.summitracing.com. Retrieved 11 February 2020.
- "About Those Finger Followers On Sportbike Engines". www.cycleworld.com. Retrieved 11 February 2020.
- Alexander, Robert Charles (1999). The Inventor of Stereo: The Life and Works of Alan Dower Blumlein. Focal Press. ISBN 0-240-51628-1.
- "Tuning the 1.9 Litre Opel (Part I)" (PDF). www.opelclub.com. Retrieved 21 February 2020.
- Setright & Anatomy of the Motor Car, p. 33
- Hillier, Victor Albert Walter (1991). Fundamentals of Motor Vehicle Technology. Nelson Thornes. ISBN 978-0-7487-0531-3. Retrieved 11 February 2020.
- Setright & Anatomy of the Motor Car, p. 34
- Setright & Anatomy of the Motor Car, p. 34
- Kennedy, Rankin. The Book of Modern Engines and Power Generators. VI (1912 ed.). London: Caxton. p. 162-166.