Helical Camshaft - The Mechanism

The Mechanism

A helical camshaft effects its duration change basically by rotating the opening and closing flanks away from each other starting at a split line on the nose of the lobe. As the flanks move apart the nose region is “filled-in” with an area of constant radius about the centre of rotation of the camshaft. An equal angular amount is removed from the constant radius base circle as the nose constant radius increases.

For the camshaft shown, the minimum duration is the same as the original camshaft's, about 260°, and the maximum duration is 345°.

The mechanism is a coaxial shaft arrangement where the outer shaft carries the main body of the cam lobe. The main body of the cam lobe is in its maximum duration form.

Typically the main lobe body would have a duration of about 450°. The lobe is very long axially, about 45 millimetres (1.8 in), and its profile consists of conventional opening and closing flanks separated by about 170° of constant radius over the nose of the lobe. The lobe has a helical slot machined into it that has a helix angle of about 35° relative to the rotational axis of the camshaft.

The width of the slot is equal to the angular extent of the closing flank of the lobe. One edge of the slot extends diagonally the full length of the lobe across the 170 degree constant nose radius. The other edge is ground so that it is all at base circle level. The slot in fact replaces the closing flank on the main body of the cam lobe. Bridging the slot is a segment of lobe (about 10 mm in thickness) which is ground to the profile of the closing flank. The segment is attached to the inner shaft. One edge of the slot has a constant cylindrical radius, the same radius as the lobe’s nose radius. The other edge has the radius of the lobe’s base circle. A small region along each edge of the closing flank segment has the same constant radius as the edge of the slot that it is adjacent to.

This means the segment can be positioned anywhere along the helical slot and there will always be a smooth transition for the follower to and from the segment. The lobe segment is fixed to the inner shaft so any relative axial movement has the effect of changing the valve opening duration. The follower is arranged so that it always remains aligned with the segment which remains stationary axially.

As the slot has a helix angle of about 35°, any axial movement of the outer shaft causes the segment to rotate, exposing more or less of the nose constant radius and thus changing the duration.