OHV PUSHROD DRIVE
The picture section 1 shows this calledd OHV pushrod drive with the camshaft located underneath. Many linking parts were required in order to transmit the cam stroke to the valve - tappet, pushrod, rocker arm and rocker arm bearing support.Further development involved ever-increasing speeds, but the engines were also required to give higher performance within a logter, more compact design. Dueto its only moderate overall rigidly, the OHV pushrod drive soon reached the limits of its speed range. It was therefore necessary to reduce the number of moving parts in the valve train.
Picture section 2 : The camshaft was relocated to the cylinder head, thus eliminating the need for pushrods.
OHC VALVE TRAIN
The came OHC (Overhead Camshaft) valve trains - these are valve trains in which the camshaft is located overhead in the cylinder head.Picture section 3 : In this OHC valve train, there is no tappet, the camshaft is positioned higher up and the valve stroke can be transmitted direct via roller/rocker arms.
Picture section 4 : This fingers follower valve train is the most rigid design of lever based valve train.
Picture Section 5 : OHC valve trains in which the valves are directly actuated by means of tappets are suitable for very high speeds. There is no need for rocker arms or fingers followers in the design.
All types of primary valve trains (picture sections 1 to 5) are widely used in engines maufactured in high volumes. The engineers must consider the main focus of the design - power, torque, displacement, packaging, manufacturing, costs, rtc. And weigh the advantages and disadvantages before deciding on a design. All valve trains from the pushrod drive to compact OHC valve train with directly actuated valves coexist for good reasons.
HYDRAULIC VALVE LASH ADJUSTMENT
Formerly, it was necessary to adjust the valve lash when the valve train was first installed and subsequently at defined maintenance intervals by mechanical means using adjustment screws or shims. Today, automatic hydraulic valve lash adjustment has become well established. This means little variation in overlap of valve lift curves over all operating cycles during the whole life of the engines, resulting in uniformly low exhaust emissions.It was not untill the early of 1903s that the idea of Frenchman Amedee Bolle (the 1911 patent) reached volume production and interestingly this was not in the homeland of its inventor, but the pierce Arrow in USA. By the end of the 1950s, 80% of car engines there were already fitted with hydraulic valve lash adjustment. In Europe, economic reasons dictated that engine design at the time tended to smaller displacement, high speed enginess. As a result, volume production of hydraulic valve lash adjusters in Erope began some 20 years later.
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