Belt alignment, or more accurately sheave alignment, is the process of achieving proper radial and axial alignment of the centerlines of sheave grooves, onto which belts run. Historically, belt or sheave alignment has not been considered a major concern in maintenance, due to belt costs being relatively small. However, if sheaves are not properly aligned and tensioned, the following problems occur:
- Greatly reduced life of the belts
- Increased radial and axial loading of the shafts, and the bearings that support them
- Increased wear and decreased life of the sheaves
- Increased noise, vibration, dust, and heat due to belt wear.
In the past, sheaves were aligned using a straightedge or string, but this method can not accurately measure parallelism of the shafts. A more accurate method is the use of a laser sheave alignment tool.
Radial Runout of the Shaft
Radial runout of the shaft normally indicates a bent shaft. If the shaft bend is out of tolerance, it should be replaced. Long term operation of a machine with a bent shaft can cause excessive vibration, reduced bearing and seal life, and reduced belt life.
Radial Runout of the Sheave
Radial runout of the sheave should be checked after verifying that there is no radial runout of the shaft. If the sheave has runout, it is probably due to an eccentrically bored sheave or bushing. Replace or re-machine the sheave or busing prior to aligning sheaves.
Axial Runout of the Sheave
Axial runout of the sheave usually indicates:
- The taper lock bushing is not mounted correctly. Tighten the bolts of the taper lock bushing using a bolt pattern, and multiple passes.
- The shaft is undersized, causing the sheave or bushing to “cock up” on the shaft.
- The sheave or bushing is bored eccentrically.