1 March 1997
6-4. DRIVE MECHANISMS.
a. Direct drive. Direct drive, with the shaft of the drive unit directly connected to the pump
shaft, is the most common configuration. This connection can be either close-coupled or
flexible-coupled. When using a close-coupled connection, the pump is mounted directly on the
drive shaft. This is the normal arrangement for a vertical pump driven by an electric motor. A
horizontal pump will usually have a flexible connection, with the engine mounted adjacent to the
pump. A vertical motor mounted above, and at a distance from a vertical pump, will be
connected to the pump with one or more lengths of flexible shafting. Direct drive offers the most
efficient operation because no power is lost between the drive unit and the pump.
b. Belt drive. Belt drives may be utilized if the pump speed is different from those available
with standard drive units, or if speed adjustment is required. Speed adjustment is accomplished
by changing pulley or sheave ratios. Belt drives used with horizontal pumps require more floor
space than a direct drive unit. There is power loss through the belt, which results in lower
efficiency, and belt wear increases maintenance requirements. Belt drives will be used only
when it is not possible to choose single speed equipment to cover service conditions, or where
pump speed adjustments may be required, but variable speed operation is not.
c. Right angle drive. Right angle drives will be used on vertical pumps being driven by
horizontal engines. If the engine serves as emergency standby, a combination gear box will be
installed on the angle drive to allow operation of the pump by the primary drive unit, which is
normally an electric motor. A clutch or disconnect coupling disengages the right angle gear
when the motor drives the pump. When the engine drives the pump, the clutch is engaged and
the motor rotates freely. In case of a power failure the engine is automatically started, and after
reaching partial operating speed is engaged to drive the pump.
a. Mode of operation. Wastewater pumps will be designed to operate in one of the following
modes: (1) constant speed, (2) adjustable speed, or (3) variable speed. The type of speed
control system will be selected accordingly. As indicated in paragraph 6-3a, the type of speed
control required will influence the type of electric motor to be used.
(1) Constant speed. Constant speed drive is the simplest, most reliable, and most
economical mode of operation, and will be suitable for the majority of wastewater pumping
applications at military installations. However, where there is a need to match pumping rates
with the incoming wastewater flowrates, a variable speed drive will usually be more appropriate.
(2) Adjustable speed. By changing pulley or sprocket ratios on a belt driven pump, the
speeds can be adjusted to accommodate several constant speed pumping rates. This type of
system will be used mainly in sludge pumping, but can be a good alternative to variable speed
control in wastewater pumping when speed adjustment is not required too often. Where
automatic operation is needed pulleys or sheaves can be positioned through the use of
pneumatic, hydraulic or electric devices.
(3) Variable speed. Variable speed operation will usually be required at large pumping
stations where the entire wastewater flow, or major portions thereof, must be pumped to the
treatment facility, and where it is desired to match the incoming flowrates in order to maintain a
smooth, continuous flow into the plant. Pumping stations will normally require more pumps
under a constant speed system than one utilizing variable speeds. Also, the size of the wet well