Demystifying the Camshaft: Part 2 – Terminology

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The anatomy of a camshaft lobe. Some manufacturers may use slightly different terms, but these are the 6 primary areas of the lobe.

Opening Flank:

This is the portion of the lobe following the opening ramp. At this point the valve starts to open rapidly, and will accelerate based on the shape of the flank.

Note: Some cam manufacturers refer to the opening ramp as the “lash ramp” and refer to the “opening flank” as the “opening ramp.”

Nose:

This is the rounded profile at the top of the lobe. As the follower reaches the nose, the valve motion decelerates as the valve reaches maximum lift. The shape of the nose determines how long the valve spends near maximum lift – a bigger radius on the nose means the valve spends more time near that maximum.

Closing Flank:

Following the nose, the closing flank is where the valve begins its journey back down to the seat. The profile of the flank determines how quickly this happens.

Closing Ramp:

This is the section that is tangent to the base circle on the closing side of the lobe, where the valve returns to the seat. Once again, this transition area is usually gradual so that the valve doesn’t slam shut in an uncontrolled fashion.

As is the case on the opening side, some cam manufacturers refer to the “closing flank” as the “closing ramp,” and refer to the closing ramp again as a “lash ramp.”

 

If you’re trying to identify an unknown cam, lift is easily measured. In order to determine actual lift at the valve, take the measured lift at the cam, subtract the valve lash, and multiply the result by the rocker arm ratio.

Lift:

We all know this term. Lift is simply the distance that the valve is raised from the valve seat. The advertised number is always measured at the valve at the point of maximum lift. Some manufacturers will also give you the amount of lift measured at the camshaft, which is easily determined by measuring the maximum height of the lobe and subtracting the diameter of the base circle. If you don’t have this number, it’s easy to determine mathematically – divide the amount of advertised lift at the valve by the follower/rocker arm ratio, and subtract from that the amount of valve lash, and you should have the amount of lift at the cam.

 

This is a graph of valve lift vs. crankshaft rotation. Remember that the crankshaft completes two rotations for every single revolution of the camshaft – hence the 720 degrees of total rotation. This chart begins with the 0 degree origin at TDC of the compression stroke.

Advertised Duration:

A general term, this is the amount of time, measured in degrees of crankshaft rotation, that the valve is open. Remember that the camshaft rotates at ½ the rate of the crankshaft, so duration is always measured in crankshaft degrees to avoid confusion. Different manufacturers may measure this in different ways – some may measure it at a specific amount of lift (the SAE standard is 0.006” measured at the cam follower, for example), while some may measure between the points where the valve actually lifts from the seat and returns to the seat (sometimes referred to as “seat-to-seat” duration).

 

 

Sketches of different cam lobe profiles. Some of these may be slightly exaggerated, but what is shown are 4 different profiles with the same lift. Obviously, each lobe profile determines a very different set of valve events with regards to duration and timing. Note that the cam profile on the far left is asymmetrical – many camshaft designers produce profiles that open and close the valves at different rates, depending on the application and desired power curve characteristics.

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