Many people are like auto mechanics.
Just remove and replace identical parts.
They make the assumption that the manufacturer
specified these parts for a reason. An assumption
made in blind faith. And for somewhat good reason.
If they replace the worn part with an exact replacement,
it resolves them of having responsibility of exercising
any of their own judgement on the suitability of said part.
Follower, not responsible.
Now if an exact replacement part is not available, someone
has to have enough knowledge of the part or application to
be able to specify a new similar part. There technical knowledge
and understanding of fit and function are involved. So he
who recommends an alternative replacement is now liable in some
part for fit and potential failure if things don't work out.
Yet people do this every day.
Think auto parts clerk who has some automotive knowledge.
Individual contributor and some responsibility if things go south.
Then there is the case where the exact replacement part is not available
and someone has to find a replacement part that might not fit without
designing an adaptation, and may have a different set of specifications,
whether it be in strength, material, tolerance, physical size, electrical
characteristics, whatever. In this person has to have an almost
engineering knowledge of the machine and the forces and other physical
attributes acting on the part, and how the part will function in the
system where it resides. This person assumes total responsibility
for the part being suitable and perhaps the results if something
should happen if this part fails. This person is a leader in his field.
Sometimes this person may know more about the machine or application
than the original designer of the machine in question.
And with a high specification bearing in a wide open tolerance pulley,
maybe something with less precision can be tolerated. One prime consideration
with an idler pulley is speed. The person making the bearing substitution
should know or be aware of this. Because idler pulleys are generally
smaller than drive pulleys, their surface speed and therefore rotational
rpms is generally quite high. So a lower precision bearing may be just fine
as long as the speed rating of the bearing, in combination with the type
of grease, and the quantity of grease fill all will factor into this.
All this is a consideration the person specifying a replacement should
know, and be thinking about. Be honest with yourself in how much you know
about your machine and machine design. No shame in consulting someone else,
like posting here. You can't know what you don't know. But if you are well
versed in the subject at hand, then you should feel confident in specifying
an engineering equivalent substitution. If not, replace the part with one
as exact as you can find to original.
It is not about good enough. It is not about perfection.
It is about taking all variables into account.
That is the definition of good engineering.