Tappet's function is to transfer the thrust of cam to push rod or valve rod, push push rod or valve to overcome the force of valve spring and move, while bearing the lateral force exerted by camshaft rotation. The installation position is the guide hole bored at the corresponding position of the cylinder block or cylinder head, which is usually made of nickel chromium alloy cast iron or chilled alloy cast iron.
Tappet can be divided into three types: common tappet, hydraulic tappet and roller rocker tappet.
1) There are three types of common tappet: bacterial tappet, cylindrical tappet and roller tappet. Bacterial and cylindrical tappets can reduce their weight because they are hollow. Roller tappets can roll freely because of linear contact, which can reduce wear and tear. The common tappet is rigid structure, which can not automatically eliminate the valve clearance. Therefore, the engine using the common tappet must adjust the valve clearance.
2) Hydraulic tappet has the greatest advantage over common tappet because it can eliminate valve clearance of engine without adjusting valve clearance. At the same time, hydraulic tappet can also reduce the transmission noise of engine valve train.
3) Hydraulic tappet structure, the tappet body is welded into a whole from the top and cylinder, and can move up and down in the tappet hole of the cylinder head. The inner hole and outer circle of sleeve are polished and ground. The outer circle matches the guide hole in tappet, while the inner hole matches the plunger. Both of them can move relatively. A compensation spring is installed at the bottom of the hydraulic cylinder block to press the ball valve against the valve seat of the plunger. It also keeps the top of the tappet in close contact with the surface of the cam to eliminate valve clearance. When the ball valve closes the middle hole of the plunger, the tappet can be divided into two oil chambers, the upper low pressure oil chamber and the lower high pressure oil chamber; after the ball valve is opened, a through chamber is formed.
4) The working principle of hydraulic tappet. When the circular oil groove on the tappet is aligned with the inclined oil hole on the cylinder head, the oil in the engine lubrication system flows into the low pressure oil chamber through the inclined oil hole and the circular oil groove. The key groove on the back of the tappet body can lead the oil into the low pressure oil chamber above the plunger. When the cam rotates and the tappet and plunger move downward, the oil in the high-pressure oil chamber is compressed, the oil pressure rises, and the function of compensation spring makes the ball valve press tightly on the lower end seat of the plunger. At this time, the high-pressure oil chamber is separated from the low-pressure oil chamber. Because the liquid is incompressible, the tappet moves down like a cylinder block, pushing the valve stem open. At this time, the tappet ring oil groove has been staggered with the oblique oil hole, stopping oil feeding. When the tappet reaches the bottom dead-end point and starts to go up, under the action of the valve spring top and cam down-pressure, the high-pressure oil chamber is closed and the ball valve will not open. The hydraulic tappet can still be regarded as a rigid tappet until the cam is in the base circle and the valve closes. At this time, the pressure oil in the main oil passage of the cylinder head enters the low-pressure oil chamber of the tappet through the oblique oil hole. At the same time, the oil pressure in the high-pressure oil chamber decreases, and the compensating spring pushes the plunger upward. The pressure oil from the low pressure oil chamber pushes the ball valve into the high pressure oil chamber, so that the two chambers are connected and filled with oil. At this time, the top of the tappet is still close to the cam. When the valve is heated and expanded, the plunger and the hydraulic cylinder move axially relative, and the oil in the high-pressure oil chamber can be squeezed into the low-pressure oil chamber through the gap between the hydraulic cylinder and the plunger. Therefore, when using hydraulic tappet, valve clearance can not be reserved.
2. The function of push rod is to transfer the thrust from camshaft through tappet to rocker arm in the valve mechanism of top valve and bottom camshaft. Push rod is the most flexible slender part of valve train. Its general structure includes three parts: upper concave ball head, lower convex ball head and hollow rod. Push rods are usually made of cold-drawn seamless steel tubes, and some of them are made of hard aluminium. Steel solid push rod is generally made into a whole with spherical support, then heat treatment; hard aluminium solid push rod is equipped with steel support at both ends, and the upper and lower ends are integrated with the rod body; the former is forged as a whole; the latter is combined with the welding and pressing methods of the rod body. Although there are some differences in structural forms, the requirements for push rods are the same, that is, light weight and high stiffness. Generally, in order to ensure the correct matching of the tappet with the rocker arm and the tappet, the upper end of the tappet is welded with a steel concave spherical joint and the ball head of the rocker arm adjusting screw; the lower end is welded with a spherical joint, which is supported in the concave spherical bearing of the tappet.

3. The main function of rocker arm is to change the direction of force transmission. The rocker arm is equivalent to a lever structure, which transfers the force of the push rod to the end of the valve rod to push the valve to open. The lift of the valve is changed by using the ratio of the arm lengths on both sides (called the rocker arm ratio). The rocker arm of the valve is usually made into a form of unequal lengths. The arm on one side of the valve is 30%-50% longer than that on the other side of the push rod. Obtain larger valve lift.
Rocker arm can be divided into ordinary rocker arm and noise-free rocker arm.
1) The long arm end of an ordinary rocker arm contacts the tail end of the valve with a circular working face to push the valve. The end of the short arm has a screw hole, which is used to install adjusting screw and locking nut to adjust valve clearance. The ball head of the screw is connected with the concave ball seat at the top of the push rod. The connection has high contact stress, relative slip and severe wear, so cemented carbide is often surfacing in this part. Because of the arm length at one end of the valve, the motion distance and acceleration of the moving parts such as push rod and tappet can be reduced under a certain valve lift, thus the inertia force can be reduced. There is usually an oil passage in the rocker arm, which is connected with the center of the rocker arm shaft. Press oil fills the center of rocker arm shaft and flows out of rocker arm oil hole to lubricate tappets and valve rod ends.
2) Noise-free rocker arm is used in some foreign engines. Its main purpose is to eliminate valve clearance and reduce the impact noise. The main structure is convex ring. The convex ring is supported by one end of the rocker arm and rests on the end face of the valve stem. When the valve is in the closed position, under the action of the spring, the plunger pushes the convex ring to swing outward, thus eliminating the valve clearance; when the valve is opened, the push rod moves upward to push the rocker arm, which is in contact with the end face of the convex ring and the valve rod, thus eliminating the valve clearance.
3) Rocker arm assembly mainly includes rocker arm shaft, rocker arm bearing, rocker arm bushing, rocker arm, limit spring, fastening bolt, locking nut and adjusting screw, etc.