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High thrust motors also have optional high thrust design available, by changing the thrust bearings. Typical thrust ratings are:
100% Thrust
175% Thrust
250% Thrust
300% Thrust
500% Thrust
Generally smaller motors are only offered with 100% thrust, while larger motors are available with more thrust options.
The VSS HT motor is essentially a VHS motor with the motor hollow shaft replaced with a solid shaft. This effectively moves the coupling from the top of the motor to the bottom of the motor. The axial adjustment of the pump depends on the coupling supplied by the pump manufacturer. The pump also accommodates motor Shaft thermal growth. NEMA does not define this motor and there are different frame, flange and shaft versions supplied by motor manufactures for any rating.
VSS Medium thrust motors are actually a hybrid High Thrust motor developed for In-Line pumps. These motors are commonly called In-Line (IL) motors. These pumps have the pump impeller mounted directly on the motor shaft. This motor is a VSS motor with a reduced shaft and flange run-out tolerance. NEMA MG 1, Part 18, Definite Purpose Motors defines the shaft dimensions and tolerances. The frame designation has a HP or a LP suffix, i.e. 444LP. Initially the motors were designed with a thrust bearing in the P-base to eliminate external shaft thermal growth and provide increased radial loading in addition to thrust. However, the petro-chemical industry, which uses the majority of these motors, decided that the thrust bearing should be placed at the top of the motor and now the pump manufactures pump design must accommodate thermal shaft growth and minimize the radial loading on the smaller bearing, typical to the P-Base motor. VSS IL motors are generally used for booster pumps.
VSS Normal thrust (NT) motors are generally used in zero thrust applications. NEMA MG 1, Part 18, Definite Purpose Motors defines the shaft dimensions and tolerances. Normally flange mounted, zero thrust motors have a C or D flange. P-base motors differ from C and D-flange motors that P-base motors usually have a small thrust bearing. Unfortunately some manufacturers also supply P-base, normal thrust motors without the thrust bearing – the Conrad type bearing is capable of carrying some thrust. VSS NT motors are normally used for turbine, mix flow and propeller pumps
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