![Cam-disc-3_3D_animated](https://upload.wikimedia.org/wikipedia/commons/5/58/Cam-disc-3_3D_animated.gif)
A tappet is most commonly a component in an
internal combustion engine
An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combus ...
which converts the rotating motion of the camshaft into linear motion of the valves, either directly or indirectly.
An earlier use of the term was for part of the valve gear in
beam engine
A beam engine is a type of steam engine where a pivoted overhead beam is used to apply the force from a vertical piston to a vertical connecting rod. This configuration, with the engine directly driving a pump, was first used by Thomas Newco ...
s beginning in 1715. The term is also used for components in
pneumatic cylinder
Pneumatic cylinders (sometimes known as air cylinders) are mechanical devices which use the power of compressed gas to produce a force in a reciprocating linear motion.
Like hydraulic cylinders, something forces a piston to move in the desire ...
s and
weaving loom.
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Beam engines
![Leawood Pump (geograph 2682446)](https://upload.wikimedia.org/wikipedia/commons/1/10/Leawood_Pump_%28geograph_2682446%29.jpg)
The first recorded use of the term tappet is as part of the
valve gear
The valve gear of a steam engine is the mechanism that operates the inlet and exhaust valves to admit steam into the cylinder and allow exhaust steam to escape, respectively, at the correct points in the cycle. It can also serve as a reversing ...
in the 1715
Newcomen engine
The atmospheric engine was invented by Thomas Newcomen in 1712, and is often referred to as the Newcomen fire engine (see below) or simply as a Newcomen engine. The engine was operated by condensing steam drawn into the cylinder, thereby creati ...
, 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.
This operation by tappets on a plug rod continued into the early twentieth century with the
Cornish engine.
From the 19th century onwards, most steam engines used
slide valve
The slide valve is a rectilinear valve used to control the admission of steam into and emission of exhaust from the cylinder of a steam engine.
Use
In the 19th century, most steam locomotives used slide valves to control the flow of steam into ...
s or
piston valves
Piston valves are one form of valve used to control the flow of steam within a steam engine or locomotive. They control the admission of steam into the cylinders and its subsequent exhausting, enabling a locomotive to move under its own power ...
, which do not require the use of tappets.
Internal combustion engines
![Pushrod2](https://upload.wikimedia.org/wikipedia/commons/d/d8/Pushrod2.PNG)
In an
internal combustion engine
An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combus ...
, 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
A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fittings ...
. The types of valve lifters (i.e. tappets) commonly 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 are usually circular and allowed, or even encouraged, to rotate. This avoids 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
convex
Convex or convexity may refer to:
Science and technology
* Convex lens, in optics
Mathematics
* Convex set, containing the whole line segment that joins points
** Convex polygon, a polygon which encloses a convex set of points
** Convex polytope ...
radius which creates a subtle mushroom-shaped surface, since a perfectly flat surface leads to 'slamming' against a steep camshaft face.
Adjusting the tappets
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 has been a widely used engine configuration since the 1940s. The task involves adjusting the clearance of tappet from the camshaft, however the adjustment is not actually 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
A feeler gauge is a tool used to measure gap widths. Feeler gauges are mostly used in engineering to measure the clearance between two parts.
Description
They consist of a number of small lengths of steel of different thicknesses with measureme ...
. 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
The Ford Taunus V4 engine is a 60° V4 piston engine with one balance shaft, introduced by Ford Motor Company in Germany in 1962. The German V4 was built in the Cologne plant and powered the Ford Taunus and German versions of the Consul, Capri, an ...
and
Opel CIH engine
The Opel cam-in-head engine (CIH) is a family of automobile engines built by former General Motors subsidiary Opel from 1965 until 1998. Both four- and six-cylinder inline configurations were produced. The name derives from the location of the cam ...
, 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
![CVH Kopf2](https://upload.wikimedia.org/wikipedia/commons/7/7b/CVH_Kopf2.jpg)
A
hydraulic tappet
A hydraulic tappet, also known as a hydraulic valve lifter or hydraulic lash adjuster, is a device for maintaining zero valve clearance in an internal combustion engine. Conventional solid valve lifters require regular adjusting to maintain a sma ...
, also known as a "hydraulic lash adjuster", contains a small hydraulic piston that becomes filled with pressurised engine oil.
the piston acts as a hydraulic spring that automatically adjusts the tappet clearance according to the oil pressure. Although the movements of the piston are small and infrequent, they are sufficient to make the valve actuation self-adjusting so that there is no need to manually adjust the clearance of the tappets.
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.
Roller tappets
Early automotive engines used a roller at the contact point with the camshaft, however as engine speeds increased, 'flat tappets' with plain ends became far more common than tappets with rollers. However in recent times, roller tappets and rocker arms with roller tappet ends have made a resurgence due to the lower friction providing greater efficiency and reducing drag.
Valvetrain layouts
Sidevalve engines
![Sidevalve engine with forced oil lubrication to crank and oil mist to camshaft (Autocar Handbook, 13th ed, 1935)](https://upload.wikimedia.org/wikipedia/commons/4/48/Sidevalve_engine_with_forced_oil_lubrication_to_crank_and_oil_mist_to_camshaft_%28Autocar_Handbook%2C_13th_ed%2C_1935%29.jpg)
In a
sidevalve engine
A flathead engine, also known as a sidevalve engine''American Rodder'', 6/94, pp.45 & 93. or valve-in-block engine is an internal combustion engine with its poppet valves contained within the engine block, instead of in the cylinder head, as ...
— a common design for car engines until the 1950s— the valves are mounted at the sides of the cylinder and face upwards. This means that the camshaft could be placed directly beneath the valves, without the 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 regular adjustment of the tappet clearance, 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 tappet adjustment always consisted of expanding the clearance (re-grinding valves into their valve seats during de-coking makes them sit lower, thus reducing the tappet clearance), adjustment by shortening the valve stems was a viable method. Eventually the valves would be replaced entirely, a relatively common operation for engines of this era.
Pushrod engines
In a
pushrod engine
An overhead valve (OHV) engine, sometimes called a ''pushrod engine'', is a piston engine whose valves are located in the cylinder head above the combustion chamber. This contrasts with earlier flathead engines, where the valves were located bel ...
, the tappets are located down in the
engine block
In an internal combustion engine, the engine block is the structure which contains the cylinders and other components. In an early automotive engine, the engine block consisted of just the cylinder block, to which a separate crankcase was atta ...
and operate long, thin
pushrod
A valvetrain or valve train is a mechanical system that controls the operation of the intake and exhaust valves in an internal combustion engine. The intake valves control the flow of air/fuel mixture (or air alone for direct-injected engines) ...
s which transfer the motion (via the rocker arms) to the valves located at the top of the engine.
Single overhead camshaft engines
In a
single overhead camshaft
An overhead camshaft (OHC) engine is a piston engine where the camshaft is located in the cylinder head above the combustion chamber. This contrasts with earlier overhead valve engines (OHV), where the camshaft is located below the combustion cha ...
(SOHC) engine, the tappets are integrated into the design of the rocker arms as one piece, since the camshaft interacts with the rocker arm directly.
Mass-production of SOHC engines for passenger cars became more common in the 1970s, in the form of
crossflow cylinder head
A crossflow cylinder head is a cylinder head that features the intake and exhaust ports on opposite sides. The gases can be thought to flow across the head. This is in contrast to reverse-flow cylinder head designs that have the ports on the same ...
s with
overhead rockers located directly above a single overhead camshaft, as a more efficient design which could be cost-effectively manufactured. The 1970-2001
Ford Pinto engine
The Ford Pinto engine was the unofficial name for a four-cylinder internal combustion engine built by Ford Europe. In Ford sales literature, it was referred to as the EAO or OHC engine and because it was designed to the metric system, it was som ...
was one of the first mass-production engines to use an SOHC design with a toothed cambelt. In this configuration, the rockers combine the function of sliding tappet, rocker and adjustment device. Adjustment of the valve clearance was usually by a threaded stud at the valve end of the rocker. The linear sliding tappet side often had a high rate of wear and demanded careful lubrication with oil containing zinc additives.
A relatively uncommon design of an SOHC camshaft with four valves per cylinder was first used in the 1973-1980
Triumph Dolomite Sprint
The Triumph Dolomite small saloon car was produced by the Triumph Motor Company division of the British Leyland Corporation in Canley, Coventry between October 1972 and August 1980.
Background
The Dolomite was the final addition to Triumph's ...
inline-four engine, which used a camshaft with 8 lobes that actuated the 16 valves via a clever arrangement of rocker arms.
Double overhead camshaft engines
Double overhead camshaft
An overhead camshaft (OHC) engine is a piston engine where the camshaft is located in the cylinder head above the combustion chamber. This contrasts with earlier overhead valve engines (OHV), where the camshaft is located below the combustion cha ...
(DOHC) 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'. Most engines used a crossflow cylinder head with the valves in two rows in line with their corresponding camshaft.
The tappet clearance adjustment is typically set using a small
shim
Shim may refer to:
* Shim (spacer), a thin and often tapered or wedged piece of material
** CPU shim, a spacer for a computer heat sink
** Shim (fencing), a device used in the sport fencing
** Shim (lock pick), a tool used to bypass padlocks
* Sh ...
, located 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. In early DOHC engines, the engine would first be assembled with a default shim of known thickness, then the gap measured. This measurement would be used to calculate the thickness of shim that would result in the desired gap. After installation of the new shim, the gaps would then be measured again to verify that the clearance was correct. As the camshaft had to be removed to change the shims, this was a very time consuming operation (especially since the position of the camshaft could vary slightly each time it was re-installed).
Later engines used an improved design where the shims were located above the tappets, which allowed each shim to be changed without removing either the tappet or camshaft. A drawback of this design 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 1966-2000
Fiat Twin Cam engine
Designed by ex Ferrari engineer Aurelio Lampredi, the Fiat Twin Cam (also known as the Lampredi Twin Cam) was an advanced inline-four automobile engine produced from 1966 through 2000 as a Fiat/Lancia engine until it was replaced by the "family B ...
, followed by engines from Volvo and the water-cooled Volkswagens.
Other uses
![Arc valve, tappet and valve chest (Rankin Kennedy, Modern Engines, Vol VI)](https://upload.wikimedia.org/wikipedia/commons/0/07/Arc_valve%2C_tappet_and_valve_chest_%28Rankin_Kennedy%2C_Modern_Engines%2C_Vol_VI%29.jpg)
The term 'tappet' is also used, obscurely, as a component of valve systems for other machinery, particularly as part of a
bash valve
A bash valve is a valve within a piston engine, used to control the admission of the working fluid.[B ...](_blank)
in
pneumatic cylinder
Pneumatic cylinders (sometimes known as air cylinders) are mechanical devices which use the power of compressed gas to produce a force in a reciprocating linear motion.
Like hydraulic cylinders, something forces a piston to move in the desire ...
s. Where a reciprocating action is produced, such as for a pneumatic drill or
jackhammer
A jackhammer (pneumatic drill or demolition hammer in British English) is a pneumatic or electro-mechanical tool that combines a hammer directly with a chisel. It was invented by William Mcreavy, who then sold the patent to Charles Brady Kin ...
, the valve may be actuated by
inertia
Inertia is the idea that an object will continue its current motion until some force causes its speed or direction to change. The term is properly understood as shorthand for "the principle of inertia" as described by Newton in his first law ...
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.
References
{{Automotive engine , state=expanded
Beam engines
Engine valves
Engine components
Valvetrain