Hubs

The attachment of the coupling to the shaft has been the source of many problems in the industry. The following practices are recommended for critical equipment.

1. If a coupling hub does not have to be removed for maintenance, seal removal, or impeller removal, it should be integral with the shaft.

2. On slow-speed equipment, where the hub does not have to be removed for maintenance and integral hubs are not available from the supplier, a straight fit can be used with a reasonably heavy shrink—1.25 mil/ in. of diameter. Tolerances may allow this value to vary from 1.0 to 1.5 mil/in. of diameter; however, 1.0 mil/in. may be too loose for some applications, while 1.5 mil/in. over-stresses hubs at maximum bore sizes. In light of this, each application should be carefully evaluated if there is no experience at hand to use as a guide. Of course, field removal at the higher shrink fit is more difficult. Keys should not be used.

Where removal is necessary, keyed hubs on straight shafts should use 0.5 mil/in. to 0.75 mil/in. shrink fit. Caution should be exercised in these applications.

3. If the hub removal is necessary, such as required on compressor with non-split seals, a tapered hub fit on the shaft should be used. The removable hubs should have tapped puller holes. The shaft should be keyless with the preferred method of installation and removal by use of hydraulic dilation. Two injection ports 180° apart should be used whether injection is through the shaft or through the hub. Shrink fits should be 2 to 2.5 mil/in. of diameter. API 671 recommends 1.5 mil/in. minimum, but experience indicates the heavier shrink may be required. For the juncture rating calculation, a friction value of .12 is recommended.

In the NEMA standard, % in./ft taper has been used for keyed tapered hubs. However, API 671 recommends a Vz in./ft taper for hydraulic release, keyless hubs (see API 671, Appendix E). The [A in./ft seems a reasonable compromise, but there is still a strong following for the NEM A taper.

Contact area for hydraulically removable, keyless hubs should be a minimum 85% when using bluing and a taper gauge. It is recommended that a set of gauges, ring and plug, be purchased with a new unit for maintenance and replacement purposes. A male and female cast iron lapping block set should also be made at the same time to help achieve the required fits by lapping.

For installation, care should be used to install the hub at the proper location on the shaft taper to provide the interference needed. A jig should be used and allowance made for hub shrinkage on cooling.

4. If keyed tapered shafts are furnished, as may be true with some installations, particularly for machines under 1,000 hp with small shafts, two keys are recommended. A shrink fit of 1 to 1.5 mil/in.

should be used. Unfortunately, heavier shrinks are impractical because of hub stress.

For maintenance and stocking, it is sometimes desirable for plants with a large number of pieces of equipment to minimize the number of coupling sizes. Also, plant-to-plant coupling exchanges may be made if the size permits. However, caution should be exercised because what appears interchangeable may not be. Because of the tuning of torsional criticáis, the original equipment vendor may have had a coupling custom-made for a system. In the fine tuning of torsional criticáis, couplings are the logical focus as the other components must be established before a system torsional study can even be made. A coupling tuned for one system may not be a logical choice for another. Therefore, before arbitrary trades are made, this aspect should be reviewed. The same comments apply to lateral criticáis; however, a new critical speed analysis may not be necessary if the replacement coupling weight is within 10% of the original . If heavier, a new lateral analysis should be performed.

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