Reply To: Bearing clearances

[Randy Roberson]

I wondered why, when the option of automatic transmissions appeared, it was not unusual for the engine in the automatic-equipped car to be rated a little higher in HP. I concluded that perhaps it was desirable to have a little more output to compensate for the effect of the automatic, and this was possible because the firing pressure could be increased without compromising engine life due to the fact there would be no lugging possible with an automatic.

We operate many old, large-bore gas engine-compressor units, which are equipped with relatively thick Babbitted bearings. Our experience is that thick Babbitt has less fatigue life and will crack and flake out. Bonding when the Babbitt is poured is also critical. Newer machines with higher speeds and firing pressures went toward thinner Babbitt, and even solid aluminum bearings. The latest higher-speed engines are equipped with steel-backed trimetal bearings with almost no Babbitt. They have to be just about perfect but will run very reliably. They wreck fast and hard, though.