Friday, May 13, 2011

Valve trains for OHV engines

Valve trains for OHV engines

The various components used to operate the valves are referred to as the valve train. Generally, and overhead-valve
(OHV) engine with its camshaft in the crankcase has a greater number of moving parts then an overhead-camshaft engine.
A basic arrangement of an in-line OHV engine is shown in figure 2.16. The valve train consists of:
1. The camshaft
2. Valve lifter
3. Push-rod
4. Rocker arm
  •        Valves lifters are also referred to as cam followers or tappets.

In OHV engines the camshaft is located in the bearings in the cylinder-block or crankcase. It is driven from the crankshaft at half the engine speed. It has a cam for each valve and for Carburettor engines, it as an additional cam to operate the mechanical fuel pump. As the cams rotate the moves the valve lifters up and down and this movement is transferred through the other parts of the valve train to the valves in the Cylinder-Head.
There are destitute or is used as a part of the ignition system, there can be a gear on the camshaft for the distributor drive. This gear is also used to drive the oil pump.

Valve lifters
Fig. 2.16
The cams and valve lifters change the rotate motion of the camshaft into linear or trade straight-line motion of the push rods. The rotation of the camshaft moves the valve lifters up and down, and this movement is transferred by the push rods to the rocker arms on the top of the Cylinder-Head.
There are two general types of valve-lifters: solid lifters and hydraulic lifters, although most OHV engines know use hydraulic lifters. (These are discussed below, see section’ hydraulic valve-lifters for OHV engines), solid lifters are actually small hollow cast iron cylinders. There are mounted in bores in the crankcase and are free to rotate. The slow rotation that occurs distributes the wear from the cam over the face of the lifter.

Push rods and rocker arms
Some rocker arms are made of cast steel, some are a steel pressing and others are of aluminum alloy.  The engine in figure 2.16 has cast rocker arms that are mounted on a rocker shaft.  The rocker arms are lubricated by Oil supplied through the hollow shaft.
Fig. 2.17
Figure 2.17, which is one bank of a V-type engine, has pressed steel rocker arms that are supported by ball pivots mounted on studs in the cylinder-head.  The ball lubricated from an oil gallery in the cylinder head through a drilling in the stud.  The push rod is also hollow and this Carries oil to lubricate the end of a rocker arm.  Valve trains with solid valve-lifters have a small clearance, referred to as valve clearance, valve lash or tappet clearance. This is provided between the end of the Rocker arm and the tip of the valve stem. If there was no clearance in the valve train, the valve would not close properly and it would quickly brunt out.
The cast rocker arms were (figure 2.16) have a screw adjustment at the push rod end to adjust the clearance. The pressed Steel rocker arms (figure 2.17) have hydraulic valve lifters and do not have clearance at the valve tip. However, they can be adjusted for certain conditions by turning the nut on the ball stud to raise are lower the rocker arm.

Hydraulic valve lifters for OHV engines
Hydraulic valve lifters are quiet in operation because there is zero lash. That is, there is no free movement in the valve train. There is no need the clearance between the rocker arm and the valve-stem because the hydraulic action takes care of any changes in the valve train due to wear or temperature.
Fig. 2.18 (Left "a" & right "b')
Figure 2.18 shows the construction and operation of a hydraulic lifter for an OHV engine. There are two main parts: a hollow body and a plunger has the cup for the push rod, and there is a spring under the plunger which holds it upwards.
The lifter is supplied with pressure oil from the engine’s lubricating system. A ball valve under the bottom of the plunger allows oil into the chamber beneath the plunger and this holds the plunger against the push-rod.

The hydraulic lifter operates as follows:
  1. With the engine valve closed, there is no load the on the lifter, and the plunger is held upwards by the spring.
  2. Oil from the oil gallery enters the lifter through holes in the lifter body and plunger.
  3. They oil pressure forces the check valve open to keep the chamber below the plunger full (figure 2.18 (a)). This removes lash from the valve train.
  4. As the cam rotates and raises the lifter, the check valve closes to trap the oil in the chamber below the plunger.
  5. The lifter moves upward as an assembly, as shown in the figure 2.18(b), to open the engine valve in the usual way.
  6. As the cam continues to rotate, the lifter moves down words to close the engine valve, and the force on the plunger is relieved. Any oil lost from the chamber is replaced by oil through the check valve.
  7. The chamber is designed to have a slight leak past the plunger. This acts as a lubricant between the plunger and body, and also enables the air to be bled from the lifter. 
See Valve trains for OHC engines>>>

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