Some thoughts on camshaft lobes

From previous articles I've written on inspecting camshaft lobes, one point that possibly wasn't stated clearly is that the typical camshaft lobe failure is not a lubricant failure. Signs of lubricant failure show surface sliding damage such as scuffing, scoring, and metal transfer across the surfaces. Not that this doesn't occur, especially, in later stages of lobe disintegration, but often the primary failure that starts the whole process is not related to lubrication.
If you look at the initial damage on a camshaft lobe what you see is a good engineered surface with irregular shaped potholes. A pothole is a good analogy as both a camshaft with a follower rolling on it and a road surface are both non-conforming surfaces with Hertizian type stresses. Potholes in roads release chunks of asphalt with top road surface in good condition. Same with a camshaft lobe, the surface doesn't look bad in the early stages of failure. It's just that small flakes are missing. There is a simple reason for this, the highest stress is below the surface.
Whenever you have rolling or non-flat surfaces, the highest stress occurs below the surface. This is called hertzian stress. Fatigue cracks start off below the surface, enlarge and eventually a chunk of material is released. The wear scar often clearly shows crack formation and crack growth.
Lubricants are not involved in this process. Not that a lubricant isn't necessary for preventing surface wear, but subsurface fatigue cracks are not affected by lubricants. Failure is determined by the magnitude of the stress and the number of stress cycles. So whenever you add one of those calcium fortified camshaft lubricants to your engine oil, do not expect miracles. Given enough time and hours, all camshaft lobes fail through Hertzian fatigue.

Rivet Edge Distance

Rivet Edge Distance

When a load is transmitted from one part to another through a rivet, bolt, or pin, the fastener exerts compressive forces against the edges of the hole, tending to crush the metal ahead of the fastener or tear the hole out if it is near the edge of the piece.  Edge distance is the distance from the center of the rivet or bolt to the edge of a plate or shape toward which the pressure of the rivet or bolt is directed. Consult the maintenance manual as edge distance will vary with manufacturer and with the particular aircraft. Edge distance applies not only to sheet edges but to bends.

What happens when you place rivets (or other fasteners) too close to the edge in primary aircraft structures?

Rivet tearing (bearing critical failure)

"The design of mechanically fastened joints has always been guided by the principle that the material being joined should fail before the fastener, and this is the practice with composits." MIL-HDBK-17-3F.

Fatigue crack in aircraft skin from rivet hole

Strength starts to decrease when the distance from the center of the hole to the edge gets below 2 times the hole diameter (2D)

If your edge margin is below 1.5 then the joint is critical for tear-out of the material or fatigue cracking.

Rivet Edge Distance -- Yield Strength

There is a large loss in bearing strength when one goes from 2.0 X down to 1.5 X. Source T.O.1-1A-9, Table3-20.

When a load is transmitted from one part to another through a rivet, bolt, or pin, the fastener exerts compressive forces against the edges of the hole, tending to crush the metal ahead of the fastener or tear the hole out if it is near the edge of the piece.  Edge distance is the distance from the center of the rivet or bolt to the edge of a plate or shape toward which the pressure of the rivet or bolt is directed. Consult the maintenance manual as edge distance will vary with manufacturer and with the particular aircraft. Edge distance applies not only to sheet edges but to bends.

Composites:

For composite structures 2.5D + 0.05 inch is used by several manufacturers. Composites are more sensitive to edge distances and hole spacings than metal joints. The brittle nature of composites and the absence of local yielding creates higher peak stresses.

History of Edge Distance:

As early as 1940 edge distance was 2D. "The Aluminum Company of America has demonstrated by test that this distance should be not less than twice the diameter of the rivet, measured from the center of the hole." Aircraft Maintenance for the Airplane Mechanic 1940.  By the 1950's edge Distance had drifted toward the edge; typically 1.5D or 1.5 times the hole diameter when measured from the center of the hole to the edge of the material. Wartime requirements might have justified a closer edge distance.  Some old Boeing military aircraft used an edge distance of 1.7 +0.030 - 0.06 inch.

After the Comet-disasters (1954), it was changed in Great-Britain to a minimum of 2D. Many manufacturers adopted this standard based on engineering and fear. Alcoa Structural Handbook dated 1956 has the strength values based on 2D.

To add confusion, some manufacturers used the term "Edge Margin". Typically, Douglas often used the term "Edge-Margin" while others used the term "Edge Distance". After Boeing purchased Douglas, the integration caused some confusion. To further the confusion, some sources used Edge Distance to mean the distance from the edge of the hole instead of the usual center of the hole distance. Currently, most references use the term Edge Distance and measure from the center of the hole. Many engineering standards call for 2D +0.05 for metals. The 0.05 allows for maintaining the 2D distance after repair using oversize fasteners, and allowance for manufacturing and repair tolerances.

FAA AC65-15A page 73 states a Edge-Distance of at least 2D minimum, with a recommended of 2-1/2 times rivet diameter. Interesting strength data is contained  MIL-HDBK-5 that shows significant strength reductions below 2D. Below 2D is sometimes used depending on stress engineering data. It all depends on the loads.