Everything about bearings is like witchcraft to me. Or quantum physics - i.e. it's "all complicated" and no one can tell anything for sure. Take the bearing L10 life, for example. Pure statistics. In this amount of hours, 90% of your bearings will survive, and 10% will (begin to) fail due to fatigue. To top it off - some of the surviving bearings will last 10 times as long as their siblings, all working under the exact same conditions. Pure witchcraft!
Joking aside, knowing how to treat bearings right is very important for us pump guys and gals, because when we don't - things get ugly... I am talking thousands of dollars ugly.
Now, while ball, needle, and cylindrical roller bearings are usually "fit with care and forget", tapered-roller bearings present a certain challenge because they require the correct setting, which is exactly what I will be touching on in this post.
And I will use a Rexroth A10VO100 as an example. Why this particular model? First of all - because it is so ubiquitous, and second - because more often than not this model gets a new set of bearings on every overhaul, and many shops limit themselves to replacing the bearings without giving any attention to the preload. This practice works, but it's not ideal. Of course, not all shops cut the "tapered roller bearing corners", but I've seen enough to be willing to write about it.
But why is this even important?
Can a pair of tapered bearings survive a small axial play? Absolutely, it can. Can it survive an "aggressive" preload? Sure! Tapered roller bearings were created with axial loads in mind, remember? So.. what is the problem then? The problem, my friends, is service life - which is something that we, as mechanics, are trying to maximize. And if you look at the service life of a pair of tapered bearings plotted against the preload, you will see a bell-shaped curve, with the highest point (longest service life) "hovering" closely above the zero preload:
There's no correct scale (or shape) in this graph, it just serves to give you a general idea.
In industrial "grease-lubricated situations", pairs of tapered bearings are often assembled with small play, to account for their thermal expansion, but since hydraulic pump bearings are submerged in case oil, which keeps their temperature close to that of the rest of the parts, it is usually recommended to assemble such bearing systems with preload.
Furthermore, bearings run in (as well as the spacers), so the preload compensates for that, and of course, we also need to remember that the rotary group components inside of our axial piston pump are exerting tons of force "stretching" the housing, so the bearing preload accounts for that as well.
Ok, so now we determined that we want our tapered roller bearings preloaded, but by exactly how much? How can we even tell when it's "too much" and when it's "not enough"? And my answer to this is - "This is exactly why I call setting tapered roller bearings an art ".
My advice then is - first, as always, look up the OEM recommendations, and then not only set the bearings using "correct practices", but also try out "playing" with the preload to see how the shaft "feels" when you set the bearings too loose or too tight by a measured amount (because "putting numbers on things" is important for learning).
I am not sure how to describe this with words, so I made a small video that demonstrates how Rexroth A10VO100 shaft bearings "respond" to changes in the preload. This is what I mean by saying "playing to learn".
A couple of additional "bearing" thoughts now: