tonearm bearings. How do we attach cartridge without ruining bearings?

I’ve never given it much thought. I’d think that you would have to be pretty ham fisted to do any kind of damage. 

If the bearings were that fragile, tonearms would wear out after a year or two of regular use.  In most cases, they are not.  I am more worried about the snowstorm we are now having.  Obviously, this assumes you take normal precautions not to apply too much rotational torque to the bearings whilst tightening the headshell screws.  Since the screws that hold the cartridge to the headshell rotate in the horizontal plane, if you just immobilize the arm wand while tightening them, I cannot imagine doing any harm.

Many times arm bearing problems are due to the relaxation of the metals of the given pillars, over time.
The metal relaxing will either tighten, or loosen the bearings.

This is so pervasive, that with Dual turntables, the bearing tightening or adjusting procedure is plainly spelled out in the service manuals.  ALL dual turntables need this. When done, and it is fairly simple to do..when done, they go back to being superb budget turntables.

Not all arms suffer from it, but some have a little bit.

Other than that, they can be fairly robust, but one can flat spot the bearings, if not careful.

But, yes, indeed, in most cases they are fairly tough.

VPI arms can be unplugged and lofted off......easier than easy...no damage.

That's why I like my Dynavector 501 arm, with the removeable headshell. Myself, I'm always worried about damaging the stylus/cantilever when mounting a cartridge. Adds to the challenge, I'm glad I don't have to worry about mounting a $15,000 one!

Regards,
Dan

Tightening bearings on a tone arm, to as close to perfect as possible, from notably loose to just right, will generally happen in less than 5 degrees of turn of a given adjustment screw. In most cases you will not even see the degrees of turn, but feel it.

When perfecting the last bit of it--it will be the level of pressure from your twisting the given screwdriver, but not visually seeing that screw move.

When attempting to do so, then...move slowly, Help is on the way....