Bearing Down

Maintenance Q & A — August 2004
By Capt. Ken Kreisler

Bearing Down
Bearing damage in a diesel engine, making better electrical connections, and more.
 More of this Feature
• Bearing damage, and more
• Better connections, and more
• PMY Tries... The FilterGlove

 Related Resources
• Maintenance Q&A Index

Why would a bearing farthest from the oil pump on a diesel show the most damage? J .S., via e-mail
Bearing failure usually occurs because of a lack of proper lubrication, due to either contaminated oil or oil starvation, perhaps stemming from a clogged filter, faulty oil pump, restricted oil line, or oil leak.

In a diesel, oil is pumped through the oil cooler and then through one or more filters. Bypass valves ensure there is a sufficient supply of lubricant if a filter or cooler becomes clogged. From the filter, oil flows through various passages to components requiring lubrication and cooling, then returns to the oil pan.

Excessive bearing wear can occur if the oil system operates in bypass mode for an extended period of time, bypassing the filter and allowing contaminants to accumulate in the oil. The accompanying diagram shows a normal, lubricated crankshaft journal and bearing. Catastrophic bearing damage results when a crankshaft journal and bearing receive an insufficient supply of oil. The result is bearing material displaced by the crankshaft journal, the amount of damage depending on the duration of the oil starvation.

A “smeared” bearing occurs when the top lead-tin overlay is relocated, usually in the center of the bearing. A “scuffed” bearing occurs when the deeper aluminum layer is exposed, also usually in the center. The most severe damage is a “seized” bearing, in which bearing material melts and becomes welded to the crankshaft surface.

In cases where the problem is oil starvation, the bearing farthest from the oil pump usually shows the most damage because it received the least amount of oil. In cases where oil is bypassed around the filters, contaminated oil usually causes excessive wear that shows up equally on all bearings.

How long after installing a new V-belt should it be checked, and are there any other tips for maintaining them? A.N., via e-mail
Most new V-belts stretch after the first few hours of operation, so you will need to check their tension frequently at first. To do this, run your engine in neutral at 1200 rpm for about 15 minutes, after which you can shut it down. Check the belt tension, and adjust it as necessary. The rule of thumb for doing this is between one-half and three-quarters of an inch of give on the longest stretch when you firmly depress the area with your thumb. Too much tension will impose excessive loads on bearings, and too little tension will cause slippage and excessive belt wear. Give them one more look after eight hours of operation, and check them next when your engine reaches 150 hours.

Preventive maintenance calls for regular inspection for nicks, cuts, abrasion, and glazing. If you find a belt that needs to be changed, it’s most likely a good idea to change all your belts at the same time. And keep several spares on board just in case you need one when you are away from the dock.

Next page > Making better connections, and more > Page 1, 2, 3

This article originally appeared in the July 2004 issue of Power & Motoryacht magazine.

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