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■ To obtain the minimum pitch diameters for flat- belt, timing-belt, chain, and gear drives, the mul- tiplier given in Table 3 should be applied to the narrow V-belt sheave pitch diameters in 14.41 for alternating-current (ac) general-purpose motors or to the V-belt sheave pitch diameters as deter- mined from NEMA 14.67 for industrial direct current motors:
(The multipliers do not improve or worsen L10 life. The tension in the belt will, but the size of the pulley diameter has little influence in the bearing loading, as the location of the load on the shaft is the same).
IEEE 841
IEEE Standard 841-2001 addresses both bearing life and loading. Section 6 of that standard addresses mechanical features of the motor, namely that horizontal and vertical ball bearing and roller bearing manufactur- ing tolerance limits shall be in accordance with Table 4 of ABMA 20-1996. In addition, ball bearings used in horizontal motors shall have ABMA C/3 clearances.
■ Bearings are to be selected to provide an L10 life of 26,280 h minimum per ABMA 9-1990 or ABMA
11-1990, as applicable. L10 life calculations for vertical motors and horizontal motors mounted in a vertical position shall con- sider applicable thrust loading. L10 life calculations shall be based on external side loads for NEMA belted application lim- its, in accordance with Section
14.42 of NEMA MG 1-2003, and internal loads defined by the manufacturer. For NEMA frame sizes above 445T, mini- mum sheave sizes in centime- ters shall be defined by (1) or
(2) and shall be applied per NEMA requirements for nar- row width belts:
Sheave diameter = 70.0 (P/NR)1/3, (1)
where P is rated is rated motor power (kW) and NR
is rated motor speed (r/min).
Sheave diameter = 63.5 (P/NR)1/3, (2)
where P is rated motor power (hp) and NR is rated motor speed (r/min).
Note that the two-pole belted applications are not recommended for motors above 19 kW (25 hp).
■ Antifriction bearings are acceptable where the dN factor is less than 300,000. (The dN factor is the product of bearing size (bore) in millimeters and the rated speed in r/min).
■ When direct coupled, the stabilized bearing temper- ature rise at rated load shall not exceed 45 ◦C (50 ◦C on two-pole motors) as measured by a thermometer or thermocouple on the surface of the bearing hous- ing as close to the outer race as possible.
■ Bearings shall be suitable for, and supplied with, rust-inhibiting grease compatible with polyurea- thickened grease.
■ Some modifications to motors may be required to accommodate oil mist lubrication. The manufactur- er shall be consulted for oil mist lubrication.
IEEE 841–2001 requires some additional bearing sys- tem mechanical construction requirements:
■ Bearings shall be regreasable without disassembly of the fan or fan covers and shall contain a reservoir equipped with outlet plugs that extend beyond the fan cover for elimination of purged grease.
■ Inner bearing caps shall be provided so that the entry of oil or grease into the motor interior is minimized.
■ Bearing and grease reservoirs shall be protected from entry of contaminants. Most end users request that replaceable rotating noncontact labyrinth seals be used for this.
In section 3.2 of IEEE Standard 841-2001, abnormal axial or side thrust is defined as an unusual service condi-
tion that must be defined by the users for analysis by the motor manufacturer.