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Friday, December 10, 2010

Engine Components : Camshafts & drives

Camshafts & drives:
  • Camshafts & drives
  • Overhead camshaft
  • Cam lobes
  • Timing belts & chains
  • Timing belts & tensioners

Camshafts & drives

The position of the camshaft depends on the design of the engine. It can be in the engine block close to the crankshaft - this is a called a pushrod or overhead valve system. Or there can be one or two camshafts mounted in the cylinder head.
But in both designs it does much the same job - driving the valves and the distributor, and sometimes the fuel pump, and the oil pump.
The camshaft is made of hardenable iron alloy or steel, and it can be cast or machined.
The cam lobes are ground to the proper shape and position in relation to one another.
Accuracy is crucial. If the cam is not exactly the required shape, or if it becomes worn, there can be impacts, fast valve wear, or noisy operation.
The bearing surfaces on the camshaft are ground smooth, and the distributor drive gear is machined into the shaft.
The cam lobes are then flame or induction hardened.
The camshaft has a cam for each valve. In some cases, there is an additional cam known as an eccentric, to operate the fuel pump.
A gear on the camshaft drives the ignition distributor, and, often, an oil pump.

Overhead camshaft

Overhead camshaft
In modern engines, the pushrod system is being replaced by the simpler overhead camshaft arrangement.
The overhead camshaft is located in the cylinder head. There can be 1 or 2 camshafts. Let’s look at a single overhead camshaft arrangement.
Single overhead camshafts can use rocker arms. The cam can lift one end of the rocker arm, or it can press down on the rocker arm.
On double overhead camshaft systems, the most common arrangement is to use a bucket tappet or lifter. It operates in a guide that protects the valve against side thrusts which it would receive if the cam operated directly against the valve.
The adjustment of valve clearance is usually done by changing accurately machined spacers. Spacers are available in a range of thicknesses, and they’re exchanged to obtain the correct clearance.
Some overhead cam engines use a hydraulic lash adjuster to reduce lash in the valve train. They have zero clearance at the valve stem so there’s no need for tappet adjustment.
It can be put in the valve end of the rocker arm. Like the hydraulic valve lifter, it has a body with plunger held against the valve stem by a spring.
Oil supplied to the adjuster keeps the plunger in contact with the valve and eliminates lash.
Lash adjusters can be put in the cylinder head at the end of the rocker arm. The lash adjusters are stationary and have a pivot for the end of the rocker arm. The plunger in the adjuster holds the rocker up against the cam.
In the lash adjuster inside the bucket tappet, the plunger’s hydraulic action holds the bucket body against the cam on the camshaft and also against the tip of the valve stem so that there is zero clearance.

Cam lobes

The cam lobe performs 3 jobs. It opens a valve at the proper time and gives it proper lift. It lets it stay open for a sufficient time. Then it lets it close at the proper time. Accurate valve timing is crucial.
Valve timing diagram
Valve timing can vary from engine to engine, as set out in manufacturers’ specifications, in the valve timing diagram.
Valve lifter resting on base circle
The shape of the cam is called the cam profile or contour. With the valve lifter resting on the base circle, shown as A, the valve is fully closed and there is clearance between the rocker arm and the valve stem. The cam rotates.
Valve fully open
The nose of the cam, B, reaches the valve lifter - and the valve is fully open.
The closing flank
The closing flank , C, closes the valve gradually so that it doesn’t pound against its seat.
On engines without valve lash adjusters, a quietening ramp is built into the shape of the cam. This makes for quieter operation during the opening of the valve. The shape of the nose determines how long it stays open.
The camshaft must always be synchronised to run in time with the crankshaft. This can be done by gears, chains, or toothed, timing belts.
Gear drives are most common in engines with the camshaft in the cylinder block, and in heavy-duty diesels.

Timing belts & chains

Timing belts and chains are used on overhead camshaft engines, because the camshaft is further from the crankshaft.
This is a typical chain drive system. It uses a hydraulic tensioner which is fed by oil under pressure from the lubrication system. The chain also uses guides to reduce noise and vibration.
The toothed timing belt is made of fibreglass or wire- reinforced synthetic rubber. Its teeth match those on the crankshaft and camshaft pulleys.
Timing belts are quieter than chains but usually require regular manual tensioning.
They also have a shorter life than chains. They need regular replacing around 50,000 to 60,000 miles or 80,000 to 100,000 kilometers.
If a belt breaks, it is not only inconvenient but on some engines it can cause a lot of damage

Timing belts & tensioners

Timing belts & tensioners
The toothed, or synchronous timing belt is used for driving camshafts, balance shafts, water pumps and diesel injection pumps.
It has an inner woven core made from fiberglass, Kevlar, or steel braid, coated with synthetic rubber or neoprene. The teeth, which may be square or curved, are molded to close tolerances to match the drive teeth on the crankshaft and timing gears. A molded plastic cover protects the belt from oil or water contamination.
Timing belts have a high working efficiency due to the low friction properties of their construction. This means they require no lubrication and are silent in operation. Timing belts are inexpensive to manufacture, they're heat and wear resistant and have a service life of 50,000 to 100,000 kilometers or 30,000 to 60,000 miles.
Although it stretches little in use, the tension of the timing belt is important. This is normally set with an adjustable idler pulley that applies tension via a spring. This pulley is fixed to the engine by a fastener. Adjustment is performed manually after the timing belt is installed.
Some manufacturers use a spring and oil damper as an automatic belt tensioner. This type of tensioner is effective at reducing timing belt chatter noise as the belt is always under pressure, even as it stretches. A heavy spring acts against a piston attached to a tensioner pushrod. This is mounted so that the tension pulley can apply pressure perpendicular to the back face of the belt. The cylinder is filled with silicone oil, and ball valves allow the piston to be forced out by the spring but prevent the piston from moving rapidly inwards. In operation the spring provides the force that keeps the timing belt tensioned, and the piston valves prevent loss of tension.
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