hands-on - ipd | quality parts for caterpillar, … · analysis, but often a more “hands-on”...

70 JULY-SEPT 2016 engine professional

Hands-on Examples of Reported Valve FailuresBY STEVE SCOTT

Engines don’t typically fail at the most opportune time, or in the most convenient locations, so gathering facts, recording conditions, preserving parts, and analyzing evidence can be a real challenge. Lab analysis, material analysis, dimensional specifications, and material testing are important in failure analysis, but often a more “hands-on” approach can help to identify the root cause. Attention to small details can make a difference in finding the true root cause of a failure.

Some common reasons for misdiagnosing an engine failure include:

• Not clearly understanding the complaint or problem• Preconceived opinions• Evidence being discarded or destroyed• Not gathering and organizing the facts• Not recording and observing the facts• Not taking time to logically go through the evidence

One important area in an engine that can pose a challenge in the diagnosis of an engine failure is the valve train. For example, following are some areas you can inspect that can help you understand how a valve was operating.

The valve tip can give you an idea of how the valve lash was set.

• Tight valve lash holds valves open and wears the valve tip• Loose valve lash increases the closing speed and stress on the

valve• Can also give insight into

the condition of the rocker assembly

Wear patterns on the stem and guides can give you an idea of:

• Side loading• Clearances and lubrication• Overheating

On the valve face you’re looking for:• Proper seating• Impact damage• Cupping of the valve face or

seat

Two high stress areas on the valve are the fillet radius and the keeper (lock) grooves.

Concentricity between the valve train components is critical!

Following are some examples of valve failures that can be diagnosed with a more hands on approach.

• When a valve breaks in the fillet radius, it is often thought that “the weld failed”. However, the weld is typically higher up the valve stem, and not in the fillet radius. The photo below shows a valve that separated at the weld. The pin magnet on the complete valve indicates the line between the magnetic stem and the non-magnetic valve head section. The pin magnet is a simple way to find the weld line, but it does not work on all valves, and not all valves are welded. In most cases, the failure of the weld is very rare.

• Having all of the parts involved in a failure is very important. Don’t focus solely on the broken part.

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The following failure was reported to have been caused by a defective intake valve.

The photos show:

• Intake valve head lodged in the intake port• Multiple impact impressions in the piston crown• Secondary impact damage to the fractured surface of

the stem, thus any evidence of bending fatigue has been destroyed

• Valve seat broken out of the cylinder head• At the 2 o’clock position, there is a valve impression in

the piston crown• For reference, the * indicates where the stem of the valve

would have approximately been• Notice the locations of the circled impact impressions on

the piston crown:

Closer inspection of these impact impressions shows distinct straight sides, not the round profile of a valve stem.

The impressions are straight down into the piston crown.

(continued)


VALVE FAILURESBY STEVE SCOTT

A valve failure would not loosen the head of a bolt or a nut, but everything in this failure can be directly contributed to damage caused by an engine ingesting this type of an object.

This example of a reported valve failure is from a 3306 Cat engine that failed shortly after being rebuilt.

They are not on an angle, and are too close to the OD of the piston to have been created by the stem of the loose valve head.

These photos show a valve head stuck in the top of an aluminum piston, and some miscellaneous parts laying in the valve cover. Sometimes it’s those small abnormal details that can lead to discovering the true root cause of a failure. In this case, it was the broken rocker arm shaft and stanchion since they are not normally damaged from a valve failure.

Further inspection found that the rocker arm shaft broke directly through the oil passage that lubricates the rocker arm, resulting in seizure of the rocker bushing.

In this application the bottom of this stanchion has an alignment dowel that fits into a hole in the cylinder head. Inspection of that dowel found that it had not aligned properly, and was trapped between the stanchion and cylinder head. With the stanchion not sitting flat on the cylinder head, it created enough force to break the rocker arm shaft. And with the break being inside the bore of the rocker arm, the arm could not move correctly resulting in the valve contacting the piston. The dowel in the stanchion was the root cause of the failure.

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Logically reviewing all damage, impressions, and signs of wear can help determine the timeline and mode of failure. The photos below show the bottom (spring side) and top (bridge side) of a rotocoil. Note that there is impact damage on both surfaces near the center.

Under magnification, the damages to the bottom of the rotocoil have distinct curved impressions. These impressions closely match the radius and diameter of the valve tip.

The valve tip, shown in the photos below, has impact damage from contact with the bottom of the rotocoil.

Closer views of the damage to the top of the rotocoil are shown in the photos below. The large diameter marks would be consistent from contact between the rotocoil and valve bridge. The smaller diagonal impressions are likely to have been caused

by the valve lock (keeper) being trapped between the rotocoil and valve bridge. The damage present on the tip of the valve, and to the bottom and the top of the rotocoil, are evidence that the valve dropped (i.e., the only mode of failure that supports the damage found on these components). Once the valve locks (keepers) are out of position, the rotocoil is allowed to contact the valve bridge above. The valve drops into the cylinder and is driven back upward by the piston, causing the impact to the bottom of the rotocoil and the damage to valve tip. Had the valve head fractured first, this damage would not have happened since the valve stem would still have been held in place.

Another component that should be inspected when investigating a valve failure is the valve springs. Testing the load force of each spring will confirm if the spring tension is properly closing the valve. You should also look for signs of “coil bind”.

Coil bind occurs when the spring is compressed so far that the coils crush each other. Unfortunately this condition is difficult to photograph, but in the photos below, the green arrows point to the impact damaged caused by the coil bind. This condition can force the valve too far into the cylinder and



result in the valve breaking from contacting the piston. It can also cause spring failures. Over speed, or something dramatically wrong in the valve mechanism could cause this condition.

Sometimes it’s the smallest part or impression that has the evidence of what caused a failure. A broken valve tip (indicated by the red circle) was suspected of causing this failure.

However, close examination of the damage to the tip of the valve and valve lock supports a different mode of failure. Since the valve lock grooves in the valve are intact and below the fracture of the tip, this confirms that the lock was out of its

correct position before the tip broke. There is no impact damage to the top of the lock that would have displaced it, and breakage of the tip would not have displaced this lock. There is also no impact to the fractured surfaces of the stem or tip that would have delivered a force to make the impression



in the lock, which confirms that the stem (tip included) were intact when the impact occurred. The damage to the intake valve lock supports that the lock came out of position prior to the tip of the valve breaking.

Overheating can also weaken a valve and cause it to fail. If the valve is even slightly held open when it should be closed,

then hot combustion gasses are allowed to pass into the intake or exhaust port, resulting in overheating the valve. This reduces clearances and can deplete the oil film between the stem and guide causing the valve to stick. Likewise, the majority of the valves’ heat is transferred through to the seat, so anything trapped between the seat and the valve face reduces the valves

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ability to dissipate heat. This can also cause guttering or torching of the valve or seat (insert).

Everyone involved in an engine failure should understand the initial inspection, how the information is gathered, and how the parts & evidence is preserved is vital to failure analysis. Too often critical information is thrown out or unprotected in a rush to get the engine back up and running.Developing a process where you systematically:• Inspect• Organize• Record • Preserve all components associated with the failure

Setting aside time to logically review all findings is vital to failure analysis. A hands-on approach, combined with your engine knowledge and attention to detail, can often lead you to the root cause of a failure.n

AERA Automotive Machining Online Training

Go online for more information about the AERA Cylinder Head and Machinist Certificate

Program. To get started, call Karen at 815-526-7600, ext. 202 or email [email protected].

www.aera.org/training

Steve Scott joined the service department at IPD in 1982, working with parts, service and sales for a variety of equipment, diesel, and natural gas engines. Since 2004, he has been the director of product development and technical support for IPD. For more information, email [email protected].

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