Camshaft drive gears

Tuesday, August 17, 2010

Tags :  crankshafts

crankshaftsWhile the main purpose of a crank is obvious, it has many other demands placed upon it. Rotating constantly as it does, it is ideal for taking drives to other assemblies such as pumps. While it is possible to drive pumps electrically - and there are some advantages to doing this - it is banned in some forms of motorsport, and the vast majority of series-production engines drive their pumps mechanically.

With very few exceptions, four-stroke engines use camshafts to open poppet valves, and the cams need to be driven at a fixed speed ratio to the crankshaft, and timed to the motion of the piston very precisely. While it would perhaps be convenient to do so, nobody drives the cams electrically, and so a mechanical connection between the crankshaft and the camshaft(s) is necessary.

In the case of belt-driven cams, there is the simple solution of bringing a plain shaft out of the engine through a seal and fitting a pulley. For high-speed overhead cam race engines, however, the general preference is to drive the cams via gears or chains. While chains are still in wide use, gears have largely supplanted them for bespoke applications but, in both cases, we need to have a driving gear or sprocket on the crankshaft.

It is of course possible to cut a gear or sprocket integral with the crankshaft, and in many cases this is exactly what is done. In the case of motorcycles, gears are often cut integral with the crank in production engines, and when these are used as the basis for a racing engine there is no choice but to follow the production manufacturing methods.

In some other bespoke engines, the cam and auxiliary drive gear is cut integrally on one end of the crankshaft. There are advantages to this - fewer components are involved, no risk of the gear becoming loose and it is ultimately a lighter and neater solution. But the manufacture of the crankshaft is necessarily more complex, and the parts are therefore more expensive and take longer to manufacture.

In many applications, a gear or sprocket is separately made and then fixed to the finished crankshaft. This is quite common, but the number of components is increased, with often multiple fasteners and dowels locating and holding the gear in position.

This method, however, has its advantages. The gear, if worn, can be easily replaced and there is the option of making the gear from a different material from the crankshaft. Nitride-hardened steels are common for racing crankshafts, but some people have reservations about nitrided gears, preferring to use a carburised gear instead.

I spoke to UK crankshaft manufacturer Arrow Precision regarding integral gears, and asked managing director Ian Arnold about the popularity of integral gears. "About 20% of our race cranks have integral gear or sprockets," he says.

It is clear that, from a design point of view, this is an attractive solution, but in terms of manufacture, the extra complexity costs time and, of course, money. There is about an extra week's worth of work in having an integral gear or sprocket cut, and the extra cost depends to a large extent on the number of parts being made. For a one-off special, the increase can be as much as 10%.

Fig. 1 - This nitrided V6 crankshaft has an integral gear (Courtesy of Arrow Precision)

Written by Wayne Ward

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Comments

Making a gear integral with the crank is definitely lighter and more reliable.  But if the gear needs to be located close to the main journal, it cannot be finish ground unless the root diameter is larger than the adjacent journal.  Making the crank gear with a small diameter is usually beneficial, due to the fact that a 4-stroke cam drive requires a 2:1 reduction.  Keeping the crank gear small negates the need for compound idlers, and keeps the cam gear small also.

A nitrided gear will have less heat treat distortion than a carburized gear.  But it will also have less case thickness, and less contact load capability.  The carburized gear will need a finish grind operation, while the nitride gear may only require a rotary honing operation for clean up.