What ABEC 7 and ABEC 9 mean, and why bearing origin matters
By Northern ServicesPublished on Reading time: 3 min read
Ask a spindle rebuilder what they put back in and you learn most of what you need to know about the shop. Here is our answer, and the reasons behind it.
ABEC is a tolerance grade
ABEC (from the Annular Bearing Engineers' Committee) grades how precisely a bearing is made: the roundness of the races, the accuracy of the bore and the outside diameter, and how true the bearing runs. The scale goes 1, 3, 5, 7, 9; the higher the number, the tighter the tolerances. ISO uses P-classes for the same idea (ABEC 7 corresponds to ISO P4, ABEC 9 to P2).
A bearing turning at 18,000 or 24,000 rpm with a cutter on the end of its shaft has no room for a race that is a few microns out of round. That is why spindles are built with ABEC 7 and better, and why a rebuild has to use the same.
Angular contact, in matched pairs
Spindle bearings are angular-contact ball bearings: the balls run on races cut at an angle, so one bearing carries thrust in one direction as well as radial load. They are fitted in pairs, face to face or back to back, so the pair carries thrust both ways and holds the shaft stiff.
The pair has to be a matched duplex set: two bearings made and measured together so that when they are clamped up they share the load evenly and take the right preload, the small internal clamping force that removes play and sets stiffness. Two random bearings of the same size do not do this. Too little preload and the shaft wanders; too much and the set runs hot and dies early.
Why the maker matters
The grade on the box says what the tolerances should be. The maker decides whether they are. We use SKF and the German and Italian spindle-bearing makers because their ABEC 7 and 9 sets are what they claim to be, batch after batch, with the grease and the cage the application needs. A bearing that is cheap because it was made to a looser standard than its label is the most expensive part you can put in a spindle, because it takes the shaft and the housing with it when it goes.
What else goes into a good set
- Grease and quantity. Spindle bearings run on a small, precise charge of high-speed grease, sealed for life. Too much grease runs hot.
- Fit. The bores and the housing seats are measured and the bearings fitted to size; a sloppy fit spins a race and scores the seat.
- Run-in. New sets are run in under control so the grease distributes and the temperature settles before the spindle is tested at speed.
- Balance. The rotor is balanced after the bearings are fitted, because the bearing set is part of what is spinning.
That is the bearing policy on our repair page, and the reason every rebuild carries a six-month warranty.