FEBRUARY 1999

Problem: When mounting a large spherical roller bearing on a tapered journal, the usual method is to measure the radial clearance of the bearing before mounting and then drive the bearing up the taper until the prescribed mounted clearance is achieved.

The alternative to this method is to measure the inner ring drive-up (values are usually listed in the manufacturers' mounting tables).

The problem we encountered involved a 260 mm bore double row spherical roller bearing which was to be mounted on a solid tapered journal (not an adapter sleeve). The installation also required the bearing to be installed in a solid housing so that the outboard side face of the bearing was about 3 in. (75 mm) inside the housing from the outboard housing face side. The shaft (weighing 64,000 lb) had to be jacked up so the impact on the slug wrench would not be transferred to the rollers and raceways.

We could not check the radial clearance during mounting as the bearing was jacked up against the top of the housing bore and could not be properly positioned for measuring the radial clearance accurately. The shaft jacks could not be let down, since their position inside the crusher box was not a proper jacking point and it required some skill to get them set up so it would be safe and secure. Also, the drive-up could not be measured since there were no reasonable reference points on the journal close enough to provide an accurate measurement.

For a bearing with a 260 mm bore, the clearance reduction requirement is 0.0047 in. to 0.0067 in. with a drive-up of 0.075 in. to 0.102 in. Our aim was to attain between 0.005 in. to 0.006 in. clearance reduction with a drive-up of 0.090 in. to 0.108 in. (We used 0.097 in.)

Solution: The method we used was "nut rotation." The lock nut used was an N052 with a thread pitch of 6 t.p.i. The advance per rotation for this thread pitch is 0.166 in., which is more than we required, so a little math had to be done. The formula is quite simple: D2 = D1 / A1 x A2, where D1 = 360°, D2 = degrees of rotation required, A1 = axial displacement per nut rotation, and A2 = axial displacement required for drive up. The result is 360 / 0.166 x 0.097 = 210° rotation. We tightened the nut firmly against the bearing inner ring, ensuring it was seated solidly on the taper seat.

The starting point of the nut was marked on the nut and the journal. A second mark was made 210° clockwise around the shaft. This was quite accurate, since we measured the circumference of the shaft next to the thread to be 28.5 in.

The formula is again simple: C2 = C1 / D1 x D2, where D1 = 360°, D2 = degrees of rotation required, C1 = circumference of the journal adjacent to the end of the thread (this applies to both sleeve and tapered journal arrangements), and C2 = nut rotation in inches from the starting point mark. Here's the calculation: 28.5 / 360 x 210 = 16.625 in.

We used a tape measure to put a mark on the journal 16 5/8 in. clockwise from the start point, then tightened the lock nut till the marker on the nut lined up with the second mark on the shaft. We checked the positioning of the lock nut locking device and found we had to tighten the nut a few more degrees to get the key tap plate to set into the shaft key slot position.

The jacks were removed, allowing the bearing to settle on the bottom of the housing bore. Then the shaft was rotated several times to set the rolling elements in the centre of the outer ring raceway, so that an accurate mounted radial clearance could be measured. For this bearing, the clearance before mounting was 0.013 in., while the mounted radial clearance was 0.008 in. This gave us a clearance reduction of 0.005 in.

Advantage: The major advantage of this method is the fact that we did not have to let the shaft down, check the clearance, jack it back up, and so on. The drive-up was a straightforward operation and was very accurate.

Over the years, we have found drive-up measurements to be efficient and precise.

NOTE: If journal is hot--in the 250°F range, as soon as you place the bearing on the journal the inner ring begins to absorb heat from the journal and expand. The outer ring is not affected by the heat, so it does not expand, resulting in a radial clearance reduction without tightening the lock-nut. ("Ro-Ball has a system for this one that you may want to pass on to readers at some later date," the author notes.)

Mr. O thanks Ed Miles of Ro-Ball Services in Stratford, Ont., for this tip. He can be reached at (519) 271-5424, fax (519) 271-2588, or by e-mail: edmiles@granite.cyg.net.