Some design considerations for timing gearsTags : valvetrain
Although the North American passenger car market still uses overhead valve (pushrod) valvetrains, they are very much out of vogue in the rest of the world. While big-capacity, lazy, low-revving engines are common in the US, where gasoline is relatively inexpensive, small-capacity, higher-revving engines are used where gasoline is more costly. Small engines power small cars, and these require less fuel.
Higher-revving engines are a challenge for the valvetrain, and the overhead cam engines used in these smaller engines are much less problematic than their pushrod counterparts. The connection between the cam lobe and the head of the valve is far stiffer, and has fewer components, so controlling the valve at high speed is easier.
Race engines are even more of a challenge. As engineers, we want to open the valves further and for them to open and close very rapidly, and we want our engines to operate at higher speeds than an engine from a typical passenger vehicle. So, where the camshafts in a passenger car are driven by a belt – or by chain on a motorcycle – most bespoke race engines use a geartrain to drive the camshafts. The advantages of gears over belts and chains are increased stiffness and greater accuracy, particularly over extended periods of time. Belts and chains stretch over time; gears do not.
Cam drive gears are made from steel, but their loading is complex as the torque transmitted is far from uniform through the engine’s operating cycle. Despite this, some of the gears in a race engine geartrain can have impressively small face widths (the term given to the width of the gear tooth between the faces of the gear at the pitch circle).
Moving inwards from the ‘working’ portion of the tooth, the root radius needs to be considered. While these radii are usually generated by the action of the gear cutter, some people advocate producing a more optimal form in this area to reduce stress concentrations.
It is common to reduce the width of the gear between the hub and the teeth, but the designer needs to be careful to provide sufficient ‘depth’ of material to support the teeth. The narrowed area, which reduces the mass and inertia of the gear, is often provided with holes and slots to further reduce mass, but if too many holes are used then cracks can develop. There may also be a penalty in terms of oil drag and shear from using holes rather than simply making the web between the hub and rim of the gear thinner.
The number of teeth chosen for the gears is dictated to some extent by the 2:1 speed ratio required between crankshaft and camshafts, but the choice is made so that the same highly loaded teeth are not always in contact with each other. So, we find that prime numbers are often used here.
It is not always possible to find gears that will fit in the allowable space and operate at the exact centre distances required. In such cases, it is necessary to adjust the profile of the gears in a design process called ‘profile shifting’, which allows a gear to operate properly but with a slightly larger or smaller diameter than its tooth size and number of teeth suggest. Profile shifting needs to be done carefully, however, so as not to compromise the strength of one or both gears.
Written by Wayne Ward