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Posted by David Sherman [72.47.9.228] on Wednesday, December 22, 2010 at 14:03:13 :
In Reply to: Installing or not an hour meter??? posted by junior [64.136.27.226] on Wednesday, December 22, 2010 at 12:53:30 :
On the military trucks, the "hour meter" is driven off of the tachometer exactly like how the odometer is driven off the speedometer. So, what you're really measuring is the total number of revolutions of the engine. That seems close to ideal for engine maintenance since the amount of wear on pistons rings and cylinders is pretty closely proportional to the number time they've gone up and down. I seem to remember calculating once that army truck "hours" match "real" hours if driving at 30 mph in 4th gear the whole time.
To a first order approximation, a maintenance schedule based on miles would be best for the drive train downstream of the transmission (u-joints, axles, etc) and maintenance based on engine revolutions would be best for the engine and driven accessories.
A true hour meter would be best for equipment that normally runs at a constant engine speed such as stationary power plants and construction equipment. I'm not sure how useful it would be for a truck where the engine speed and load vary all over the place. The military system, with "hours" that are really total engine revolutions, seems like a more accurate measure of wear.
Obviously, not all miles are equal in terms of vehicle wear, but not all hours are equal either. Both measurement are a compromise. With a lot of larger engines these days, they change the oil based on lab an.alysis of the oil rather than on either miles or hours.
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