Tom Bishop, P.E.
EASA Senior Technical Support Specialist
Two common scenarios that service centers deal with regarding belt drive applications are failure of a motor drive end ball bearing or breakage of the shaft at the drive end bearing shoulder. The cause of these failures often is over-tensioning of the v-belts. However, there are many other faults or undesirable practices that can lead to premature bearing failure, belt wear and sheave wear.
Due to practical space limitations, this article won’t be exhaustive in its coverage but will deal with common scenarios other than motor bearing failure and shaft breakage.
Worn or damaged sheaves
Worn or damaged sheaves can lead to rapid belt wear, belt slippage and possibly vibration. Badly worn sheaves can result in over-tensioning of the belts to prevent slippage. That in turn can result in overheating and overloading of motor and driven equipment bearings. When some sheave grooves are worn more than others, the effect is that the belts may appear to be mismatched. It also causes “differential driving,” where not all of the belts carry the same load (stress). In the case of banded belts, worn sheave grooves can cause the belts to ride too low in the grooves, thus causing the tie-band to wear against the sheave flanges between the grooves. In severe cases the sheave flange may cut the tie-band and thus separate the belts.
If a belt-to-sheave groove mismatch is suspected, English and metric belt contour and sheave groove gauges available from belt and sheave suppliers can be used to check dimensions. These also are handy for identifying belt cross sections for replacements and checking sheave grooves for wear.
If more than 1/32” (0.8 mm) of wear is visible (see Figures 1 and 2), v-belt life will be measurably reduced; thus a worn sheave should be replaced.
Misaligned sheaves
Sheaves that are not aligned can cause vibration, rapid and uneven wear of v-belt sidewalls and uneven wear on sheave groove walls, considerably shortening the service life of both belts and sheaves. Misalignment can also lead to separation of the tie-band on banded belts, or apparent mismatching of individual belts in multiple belt drives. V-belt sheave alignment should be within 1/16” per 12” (~ 5 mm per m) of drive center distance in any or all of horizontal angular, vertical angular or parallel types of shaft and sheave orientation (see Figure 3).
Insufficient belt tension
This condition along with worn sheave grooves is the leading cause of v-belt slippage issues. Figure 4 illustrates the effects of insufficient tension and over-tension on belt life. Problems associated with insufficient belt tension include:
- Rapid sidewall wear on belts, with typically even wear on both sides
- Belts that turn over in the sheave groove, or jump off the sheave
- Belts that slip and squeal
- Belts that appear to be glazed and are hardened due to “spin burn” (slippage)
Belt squeal with load often indicates insufficient tension. However, belt squeal or “chirping” at startup may be normal. In either case, with load or at startup, the best practice is to check belt tension.
Belt tension can be checked using a belt deflection test gauge available from belt suppliers, checking belt temperature with an infrared thermal imager, or by use of a sonic tension meter. Belt tension should be set to the belt manufacturer specified value. Newly installed belts require about 24 hours to become fully seated in the sheave grooves, thus deliberate initial over-tension per belt manufacturer instructions and a recheck and possible adjustment the next day are necessary for a proper installation.
V-belts don’t always squeal when they are slipping. A method to determine if belt slippage is occurring is to check the temperature of the grooves in the sheave. If the sheave grooves are visible within the belt guard, and an infrared thermal imager is available, the sheave groove temperature can be checked while the machine is in operation. If this cannot be done, the drive needs to be off and all applicable safety procedures followed before checking the sheave groove temperature with a temperature detector. If slippage was present, the groove temperature will be at least 40°F (~20°C) above ambient. If this is the case, and there is no external heat source, then the drive is probably slipping. Belt material dust in the bottom of the belt guard is another indicator of belt slippage.
In addition to checking for slippage, the above mentioned drive outage to check for slippage can be used to check belt temperature. Tests have found that the service life of a v-belt is cut in half for every 18°F (10°C) increase in belt temperature. Further, belt life is reduced if the v-belt temperature is over 140°F (60°C), or 185°F (85°C) for a synchronous belt. Belts that are running hot will harden and develop cracks from the bottom of the belt upwards.
Note: Do not use “belt dressing” to reduce or eliminate belt squeal. Belt dressing compounds treat the symptom; not the cause. Further, these compounds are usually made from a petroleum derivative that can have a destructive effect on the materials in the belts.
Excessive belt tension
Over-tensioning of v-belts can be even more detrimental than insufficient tension, affecting not only the belts, but also bearings and shafts – as mentioned at the beginning of this article. Figure 4 illustrates the effects of over-tension and insufficient tension on belt life. The best practice is to set belt tension to the level prescribed by the belt manufacturer. Common indicators of excessive belt tension include:
- Repeated belt breakage
- Overheated motor and driven equipment bearings
- Excessive vibration
- Shaft whip, bent or broken shaft.
Mixed brands or mismatched belts
Belts from different manufacturers cannot be matched together and will not deliver the service life they should. The same applies to mismatched belts that are inherently different. Although all manufacturers use similar belt numbering systems, different brands with the same number will differ slightly in dimensions and thus should not be mixed in a set. Also, construction differences cause them to ride differently in the grooves and to stretch differently. It has been found that the majority of complaints regarding belt matching were due to other causes, such as misalignment and worn sheaves. These factors should always be checked when there is a suspicion of mismatched belts. Note: Banded belts are an alternative to multiple individual v-belts. Being banded together results in a matched set of belts.
Similar specification numbers for v-belts manufactured to different international standards [e.g., RMA (Rubber Manufacturers Association) and ISO] and with different units of measure (English-inch and metric-mm) may suggest that they are interchangeable. However, the actual belt dimensions can differ enough to result in unacceptable performance. Further, because of differences in groove spacing between English and metric dimension-base sheaves, interchanging joined inch belts with joined millimeter belts results in the belts not properly seating in the sheaves. The consequences are significantly reduced belt life and rapid sheave wear.
Noisy belt drive
When a belt drive is excessively noisy, the belts are often suspected as the cause. To determine if the belts are the source of the noise, spray them with soapy water while the drive is operating. If the noise goes away or decreases, the belts are at least part of the problem. If the same noise remains unchanged after spraying, the problem is probably associated with other drive components.
Inadequate drive selection/design
A drive that has been maintained properly but does not satisfactorily perform may be under-designed, or the suitability of the drive components may not be adequate. Relatively simple verification of adequate drive components can be achieved by using software available from many belt manufacturers. Typically the software is free and available online.
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