Shaft alignment & belt tension
At 1450 rpm, SKF's table allows 0.10 mm of offset at the coupling centre and 0.08 mm per 100 mm of angularity, each way, vertical and horizontal, when the machine maker gives no tighter limit.
Enter the speed and the four readings from the laser or the dial; the shafts are drawn with the limits around them. Switch to belt tension for a V-belt's deflection and force: a new SPA belt on a 125 mm pulley at 500 mm centres is pushed in 7.9 mm with 38 N.
Over the limit: horizontal offset. Correct and read again.
- Offset limit
- Angularity limit
- Vertical offset
- 0.06mm
- Vertical angularity
- -0.04mm/100 mm
- Horizontal offset (over)
- 0.12mm
- Horizontal angularity
- 0.03mm/100 mm
How it’s calculated
Offset: the distance between the shaft axes at the coupling centre. Angularity: the gap difference across the coupling over its diameter, × 100 (mm per 100 mm); a 0.04 mm difference across a 200 mm coupling is 0.02 mm/100 mm. Each reading, either sign, against SKF's limit for the speed band.
SKF acceptable maximum misalignment (TKSA 20, table 1)
| Speed, rpm | Offset, mm | Angularity, mm/100 mm | Offset, thou | Angularity, thou/in |
|---|---|---|---|---|
| 0–1000 | 0.13 | 0.10 | 5.1 | 1.0 |
| 1000–2000 | 0.10 | 0.08 | 3.9 | 0.8 |
| 2000–3000 | 0.07 | 0.07 | 2.8 | 0.7 |
| 3000–4000 | 0.05 | 0.06 | 2.0 | 0.6 |
| 4000–6000 | 0.03 | 0.05 | 1.2 | 0.5 |
What are offset and angularity?
Offset is how far apart the two shaft axes are at the coupling centre. Angularity is the angle between them, given as the gap difference across the coupling per 100 mm of its diameter. Both are read twice: vertically (corrected with shims under the feet) and horizontally (corrected by moving the feet sideways). All four must be inside the limit for the speed; the faster the shafts turn, the tighter it is.
Why check the soft foot first?
A foot that doesn't sit flat springs the frame when its bolt is tightened, and the alignment moves with it. Loosen each bolt in turn with a dial on the foot: more than 0.05 mm (0.002 in) of lift is shimmed out before the alignment is read.
How do you check V-belt tension?
Measure the span between the pulleys, push the middle of it in by 16 mm per metre of span with a tension gauge, and read the force. Below the table's force the belt is slack and will slip; well above it loads the bearings. A new belt is set 25 % higher and reset after it has run in.