You are probably referring to Hollow Shaft motors used on vertical turbine pumps.
That type of motor allows the line shaft to go through the motor and out the top where a nut is installed. Tightening or loosening that nut will raise and lower the line shaft, thus adjusting the impellers to the proper place in the bowls.
You calculate the stretch of the line shaft caused by the weight of the impellers and length of shafting from tables. Then you tighten the nut per the tables to lift the impellers up from the bottom of the bowls plus the stretch of the line shaft.
The Hollow Shaft motor therefore allows quick easy adjustment of the impellers in the bowls, as well as convenient removal and servicing of the motor with no coupling required.
Since the motor bears the weight of the shafting, the motor incorporates the thrust bearing to support the line shaft weight and impeller thrust.
If you really want to see all the advantages of hollow shaft motors on these pumps just think through what you would have to do if the motor was a typical motor with a shaft of its own and coupled to the line shaft. Now you would have to have a coupling, line shaft adjustment built into the turbine head, thrust bearing built into the turbine head, as well as the seal, lubrication for the bearing, and the fluid discharge porting, flanges, and then you have a much larger and taller pump head, and still all that is harder to work on than a hollow shaft.
Now, does all that extra size and height and inconvenience in the pump head save money? No, the pump head costs far more, probably triple the cost, and you save almost no money on the motor.
That makes sense. But what about runout or shaft whip? With the bearings so far away from the impeller isn't the task of centering the shaft is left up to the bushings and the seal? Is that the trade off?
I've dealt with a few seals where we've incorporated bearings to help control shaft whip or deflection, usually agitators.
to longeron
PUMPDESIGNER has given a good overview on the use hollow shaft motors which are used on line shaft vertical pumps, however on reading your reply it seems that you have something else under consideration. It would be really nice to know what you are trying to find out so that we know how best to answer you.
International College
Naresuan University
Phitsanulok
Thailand
You have focused on the problem that occurs when the spider bearings fail down the hole.
If the spider bearings fail, the shaft starts wobbling, then indeed you will not be able to keep a tight adjustment on the packing or seal. By installing your own bearings by machinging them into the pump head you will perhaps help a little, but that excessive leakage should be warning to you that the unit must be pulled and re-conditioned or replaced, with new spider bearings. If you allow the unit to run then the vibration will damage the motor. We often see pumps that burn up motors because the vibration is not dealt with. Motor fails, is replaced. Burns out again, is replaced. Good for the motor shops.
Best way to monitor is to use a vibration monitor. They are affordable. Just mark a spot on the motor and fasten the monitor on each time you take a measurement. Do it first when new, then monitor and graph out the increase in vibration. When vibration starts to rise rapidly, pull the pump and install new spider bearings.
There are "product" lubricated spider bearings where the pumpage lubes the bearings, and there are oil fed spider bearings where oil from a reservoir goes down the hole to lube the bearings. Obviously, if your pumpage destroys bearings then you may want to change to oil lubed spider bearings down hole.
Also, do not use graphite rope type packing, change over to teflon rope packing, and someone somewhere on this thread recommended another type also. Make sure to always leave some water dripping through the packing.
You mentioned bearing distance from the motor. There are typically two spider bearings per 10 foot of shafting, one at each end. If the shafting is high quality and balanced, this works fine and can last for 20 years easily.
However, speed and shaft length is an issue. Most of these pumps are 1750 rpm or slower. 3500 rpm works but is less common and probably will not last as long.
