Functions of Automobile Tappets, Push Rods, And Rocker Arms

1. The function of the tappet is to transmit the thrust of the cam to the push rod or valve stem, push the push rod or valve to overcome the force of the valve spring and move, and at the same time bear the lateral force applied by the camshaft when it rotates. Its installation position is the guide hole bored out at the corresponding part of the cylinder block or cylinder head, and is usually made of nickel-chromium alloy cast iron or cold-shocked alloy cast iron.

Tappets can be divided into three types: ordinary tappets, hydraulic tappets, and roller rocker arm tappets.

1) Ordinary tappets Ordinary tappets have three forms: mushroom-shaped tappets, barrel tappets, and roller tappets. Mushroom-shaped and barrel tappets are hollow in form, which can reduce their own weight; roller tappets have line contact, and the roller can roll freely, which can reduce wear. Ordinary tappets are all rigid structures and cannot automatically eliminate valve clearance. Therefore, engines using ordinary tappets must adjust the valve clearance.

2) Characteristics of hydraulic tappets. The biggest advantage of hydraulic tappets over ordinary tappets is that they can eliminate the engine valve clearance without adjusting the valve clearance; at the same time, hydraulic tappets can also reduce the transmission noise of the engine valve mechanism.

3) Structure of hydraulic tappets. The tappet body is welded into one piece by the upper cover and the cylinder, and can move up and down in the tappet body hole of the cylinder head. The inner hole and outer circle of the sleeve are finely ground. The outer circle matches the guide hole in the tappet, and the inner hole matches the plunger. Both can move relative to each other. A compensation spring is installed at the bottom of the hydraulic cylinder body to press the ball valve against the valve seat of the plunger. It can also keep the top surface of the tappet and the surface of the cam in close contact to eliminate the 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; when the ball valve is opened, a through chamber is formed.

2. The function of the push rod is to transmit the thrust transmitted from the camshaft through the tappet to the rocker arm in the valve mechanism of the overhead valve and the lower camshaft. The push rod is the most bendable and slender part in the valve train. Its general structure includes three parts: the upper concave ball head, the lower convex ball head and the hollow rod. The push rod is usually made of cold-drawn seamless steel pipe, and some are made of hard aluminum. The solid steel push rod is generally made into a whole with the spherical support and then heat treated; the two ends of the solid push rod made of hard aluminum are equipped with steel supports, and its upper and lower ends are made into one piece with the rod body; the ball head and the rod body of the former are forged as a whole, and the two ends of the latter are welded and pressed together with the rod body. Although there are certain differences in the structural form, the requirements for the push rod are the same, that is, light weight and high rigidity. In general, in order to ensure the correct matching of the push rod with the rocker arm and the tappet, a steel concave spherical joint is welded on the upper end of the push rod to match the ball head of the rocker arm adjustment screw; a spherical joint is welded on the lower end and supported in the concave ball seat of the tappet.

3. The function of the rocker arm is mainly to change the direction of force transmission. The rocker arm is equivalent to a lever structure, which changes the direction of the push rod force and transmits it to the tail end of the valve stem to push the valve 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 valve rocker arm is generally manufactured in unequal lengths, with the arm on the side close to the valve being 30% to 50% longer than the arm on the side close to the push rod, so that a larger valve lift can be obtained.

Rocker arms can be divided into ordinary rocker arms and noiseless rocker arms.

1) Ordinary rocker arms, whose long arm ends contact the tail end of the valve with an arc-shaped working surface to push the valve. There are screw holes at the end of the short arm for installing adjustment screws and locking nuts to adjust the valve clearance. The ball head of the screw is connected to the concave ball seat at the top of the push rod. The contact stress of this connection part is high, and there is relative slippage and severe wear, so hard alloy is often welded on this part. Because the arm on the end close to the valve is long, the movement distance and acceleration of moving parts such as push rods and tappets can be reduced under a certain valve lift, thereby reducing the inertia force. There is usually an oil channel in the rocker arm, which is connected to the center of the rocker arm shaft. The pressurized oil fills the center of the rocker arm shaft and flows out from the rocker arm oil hole to lubricate parts such as the tappet and the valve stem end.

2) Noiseless rocker arm. Some foreign engines use noiseless rocker arms. The main purpose is to eliminate valve clearance and reduce the impact noise generated. The main structure is the convex ring. The convex ring uses one end of the rocker arm as a fulcrum 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, thereby eliminating the valve clearance; when the valve is opened, the push rod moves upward to push the rocker arm, and the rocker arm has been in contact with the end face of the valve stem through the convex ring, so the valve clearance is eliminated.

3) The rocker arm assembly mainly includes a rocker arm shaft, a rocker arm shaft support, a rocker arm bushing, a rocker arm, a limit spring, a fastening bolt, a locking nut and an adjusting screw.