Camshaft overview, role, classification and maintenance knowledge
作者： Release Date: 2017-02-16 Author: Click:
The main body of the camshaft is a cylindrical rod with the same length as the cylinder group. There are several cams on the top for driving the valves. One end of the camshaft is a bearing support point, and the other end is connected to the driving wheel. The side of the cam is egg-shaped. The purpose of its design is to ensure sufficient intake and exhaust of the cylinder, specifically to complete the opening and closing of the valve in the shortest possible time. In addition, considering the durability of the engine and the smoothness of the operation, the valve should not have too much impact due to the acceleration and deceleration during the opening and closing action, otherwise it will cause severe valve wear, increased noise, or other serious consequences. Therefore, the cam and the engine's power, torque output and smooth running are directly related. In addition, the work of the camshaft must ensure the intake and exhaust of the cylinder in an accurate time. Each cylinder of the four-stroke engine has one intake stroke and one exhaust stroke within two weeks of crankshaft rotation. Therefore, the speed of the camshaft must be half of the speed of the crankshaft.
The function of the camshaft is to control the opening and closing actions of the valve. Although the speed of the camshaft in a four-stroke engine is half that of the crankshaft (the speed of the camshaft is the same as that of the crankshaft in a two-stroke engine), it usually still has a high speed and needs to withstand a large amount of torque. Camshaft has high requirements on strength and support. Its material is generally special cast iron, and occasionally it uses forgings. Because the valve motion is related to the power and running characteristics of an engine, the camshaft design occupies a very important position in the engine design process.
According to the number of camshafts, it can be divided into two types: single overhead camshaft (SOHC) and double overhead camshaft (DOHC).
The single overhead camshaft is a design with only one camshaft in the cylinder head. An in-line cylinder engine using this design requires only one camshaft placed above the cylinder head, while a V-cylinder engine requires two camshafts, one on each cylinder bank.
Double overhead camshaft is a valve arrangement with two camshafts in the cylinder head. The two camshafts control the intake and exhaust valves respectively. Depending on the structure of the engine (mainly the arrangement of the cylinders), a general double overhead camshaft automobile engine can have two to four different camshafts.
Advanced manufacturing process:
At present, the main methods of producing camshafts at home and abroad are: the process of forming a martensite layer by high-frequency quenching of the peach peach part after high-frequency quenching after the steel forging blank is cut. In the late 1970s, Germany and France successively developed new argon arc remelting processes for camshafts; hardenable cast iron camshafts mainly based in the United States; cold hardened cast iron camshafts mainly based in Japan and France; and cams Cr-Mn-Mo alloy coatings are used on the surface for casting surface alloying.
Common problems with camshafts and how they affect the engine.
(1) The swing of the cam journal (radial motion) should not be greater than 0.05 ^ -0.10 mm, and the excess should be corrected, otherwise the gap between the bearing pads and the valve foot will be inaccurate, which will cause the oil pressure to decrease. If the engine is running at a high speed, the engine will be unstable at high speeds, the camshaft will run frequently and the exhaust valve foot gap will be low, and the engine will make a sudden noise.
(2) The camshaft is worn out, and its out-of-roundness and taper should not be greater than 0.05 mm. Those exceeding it should be ground to the repair size. After polishing, the diameter of the journal should be greater than the total height of the cam by 1.0-1.5 mm, otherwise the oil pressure will be abnormal and the bearing surface of the bearing pad will be poor. Those below the cam lift height will cause the camshaft to fail to fit on the cylinder block, and The disadvantage of camshaft runout can occur.
(3) If the cam surface of the camshaft has hits, rough and uneven wear, the top of the cam should be overlaid and polished, otherwise it will affect the work of the tappet, exacerbate the wear of the spherical surface, and is very unfavorable to the engine power. As the valve tappet spherical surface and the top of the cam wear more and more, the greater the gap between the valve feet, the ball surface of the tappet will inevitably be broken.
(4) When the difference between the base circle of the cam and the top of the cam is less than the allowable tolerance of 0.45 mm, the top of the cam should be overlaid and repaired, otherwise the inflated amount will be insufficient, the exhaust will be restricted, and the engine power will be affected. It is possible for the exhaust cam lift height to be less than the tolerance, but if the intake cam lift height is lower than the tolerance, the intake valve opening is small, which has a great impact on the thermal efficiency of the engine.
(5) When the swing of the cam journal intermediate journal is not greater than specified, the misalignment between the cam base circle and the shaft whisker shall not be greater than 0.05 mm, and shall be trimmed when exceeding. Otherwise, due to the beating of the cam base circle, the valve foot clearance may be inaccurate or the valve foot may be leaked. It is also easy to miss the valve face working surface, causing the carburetor to temper and the exhaust pipe to fire, which will reduce the engine power. If the deviation of the full base circle is found and the valve foot gap is excessively enlarged, it will not only cause abnormal sound of the valve foot, but also advance the ignition time. More importantly, it will cause abnormal valve lifters or abnormal wear on the top of the cam.