Bucket tappets
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Tappets are a type of plungers in the valve gear of internal combustion engines with overhead camshafts. When the valve is opened, tappets transmit the force from the cam to the valve stem and keep the transverse force away from the valve. They are an essential element of the valve train and are therefore responsible for the gas exchange in the engine. Bucket tappets have the shape of a short hollow cylinder and enclose the valve stem and spring. Therefore the valve train is more compact than one with mushroom tappets or rocker arms . They are also lighter than other designs and are suitable for high speeds. The inventor is the Swiss designer Ernest Henry , who first used bucket tappets for the manufacturer Établissements Ballot in 1919 . However, the Hispano-Suiza 8 aircraft engine from 1915 had a similar design .
Working method
Bucket tappets slide in a cylindrical guide in the cylinder head in the axial direction of the valve above the inlet and outlet valves. They absorb the force from the camshaft in the direction of the valve and transfer it to the valve. The cam slides on the tappet surface as it rotates, so that transverse forces perpendicular to the valve stem are reliably kept away. The valve is closed after its maximum opening by a valve spring .
Designs
In the original shape of the bucket tappet, the valve clearance is checked and adjusted as part of the maintenance work. For this purpose, the effective length of the valve stem can be changed, e.g. B. depending on the design by grinding the shaft or regrinding (lowering) the valve seat, by inserting another spacer between the end of the shaft and the tappet or using a different tappet. To adjust the play, the tappet and the associated camshaft must be removed. This disadvantage is avoided by the bucket tappets developed by Aurelio Lampredi in the mid-1960s and patented for Fiat, which in their upper part receive a flat-ground spacer plate - usually with a circular cross-section - within a small raised edge. Such plates, also called “shim” after the English term, are available in various thicknesses, for example between 3 mm and 5 mm in 50 µm increments.
In order to bypass the Fiat patents for this design, other automobile manufacturers developed their own adjustment methods. For example, Audi developed bucket tappets into which a screw that was beveled longitudinally on one side was screwed transversely, which pressed against the valve stem with this bevel. The valve clearance could then be changed by turning this screw. Alfa Romeo developed a bucket tappet for the Alfasud in which a hexagon socket screw was screwed into the axis of the valve stem, which was held tightly in the tappet by a clamping disc. The valve clearance was adjusted by simply turning this screw with an Allen key. Ultimately, however, the Fiat solution generally prevailed until the hydraulic backlash compensation was established across the board.
Bucket tappets with hydraulic valve clearance compensation were developed in order to achieve greater freedom from maintenance, even in engines with overhead camshafts . There have been solutions for hydraulic valve clearance compensation in the USA since the 1930s.
With "hydraulic valve lifters", the oil pump presses engine oil through a bore with a check valve into a hydraulic cylinder in the tappet, the piston of which extends between the cam base circle and the valve until the valve clearance disappears. If force is exerted by the cam during the next cycle, the check valve in the tappet closes, the tappet presses on the valve stem and the valve opens. This also applies to further engine operation: there is no valve clearance as long as the reservoir in the cylinder is repeatedly exposed to the pressure of the engine oil. Since a small amount of oil always flows off through the gap between piston and cylinder and the closing force of the valve spring generates a pressure that is orders of magnitude higher than the oil pump, the valve is safely closed.
After the engine has been switched off, it takes a time, which depends on the viscosity of the oil, until all the tappets have released the oil again. There is then again a play between the tappet and the cam base circle which disappears during the first revolutions after the next engine start.
See also
literature
- Peter Gerigk, Detlev Bruhn, Dietmar Danner: Automotive engineering. 3rd edition, Westermann Schulbuchverlag GmbH, Braunschweig, 2000, ISBN 3-14-221500-X
Individual evidence
- ↑ Ludwig Apfelbeck: Paths to the high-performance four-stroke engine. Motorbuch Verlag. 1st edition 1978. ISBN 3-87943-578-2 . P. 16
- ↑ Abell, R. F .: The Operation and Application on Hydraulic Valve Lifters, SAE Paper 690347