How it Works
Early Lobe Pumps
Looking back briefly at the development history of the lobe pump will give an understanding of how the new pump design evolved.
The very first pumps were really positive displacement blowers that had two counter rotating shafts, each with a flat sided rotor shape. The mirror image lobes transported a fixed volume of air captured in what is best described as a transport chamber that is created between the lobes and the inner housing.
This simplified illustration shows a cutaway of an early three lobe air blower. Notice the transport chamber in the upper rotor is isolated through the 120 degree span of the two lobes.
The maximum inlet and discharge openings of the machine are dictated by this 120 degree lobe-to-lobe isolation region. If the upper rotor turns in a counter clockwise direction, the six transport chambers will ‘positively displace’ six discrete quantities of gas or liquid from right to left with a distinct pulsation.
Historically, this type of pump is usually run at low speed and has had limited applications. Cement pumping
is just one of the most recognized applications
Continuous Flow Concept
With the advancement of sophisticated milling machines, it became possible to create a helical lobe profile with the purpose of generating continuous flow.
The rotors shown in this illustration have the same cross section shape of the straight lobe rotor shown above except the longitudinal body of each lobe is twisted 120 degrees so that three separate (identical) transport chambers are created to span the 360 degree arc of one rotation.
To have continuous flow, each transport chamber must be isolated in the arc region between the inlet and discharge openings. Since the cross section of transport chamber spans a 120 degree arc and the lobe is twisted 120 degrees, the angular isolation arc has to be 240 degrees.
It's obvious from the three lobe twisted rotor illustration that the identical isolation regions of the opposing rotors doesn't leave very much room in the housing for either the inlet opening or the discharge opening.
On the other hand, a four lobe twisted lobe rotor only requires an isolation arc region of 180 degrees which makes it possible to have inlet an discharge openings the approximate width as the distance between shaft centerline axis. Unfortunately, by basic design limitation, the volume flow of the four lobe rotor is approximately half of the volume flow of the equivalent sweep diameter of a three lobe (segmented) rotor design.
Multiple Segment Concept
OCOR Corporation patented a version of the helical lobe rotor capable of continuous non-pulsating flow where the rotor arrangement provides large inlet and discharge openings in the containment housing.
In graphical terms, cutting a three lobe helical the rotor, shown in the adjacent illustration into three equal segments results in individual transport chambers that only require an isolation arc region of 160 degrees. Breaking down that arc region; with three lobes on each rotor, and three segments on each shaft, the capture arc angle becomes: 120 degrees plus the additional one third of the arc between lobes for a total of 160 degrees,
In actuality, there are three separate lobe pumps stacked against each other along the shaft rotational axis. The inlet and discharge side of each pump is aligned sequentially so that the resulting flow is continuous and non-pulsating.
Elimination of cross flow between the individual pump segments is done with a series of propriety separation plates, shown in green color in the adjacent illustration, and are fixed in position by the end plate(s).
If the pump is to be used to transport and pressurize a media, there exists a potential leak path along the interface of the housing bore and the exterior surface of the separator plates. OCOR has designed a proprietary means to restrict the leakage in this region.
As the adjacent illustration demonstrates, the multiple segment design enlarges the inlet and discharge flow port areas which in turn, reduces the media velocity allowing the rotors to run at a much higher speed, compared to other lobe pump designs. If the system is pumping a low viscosity liquid, the problem of cavitation is virtually eliminated. The new design achieves a compact, high flow, high pressure pump system.
Multiple Segment Pump - Simplified
To reduce fabrication cost, OCOR has designed as simpler pump where the individual rotors have no a helical twist. In other words, straight sided. As with the helical lobe design, the lobes themselves have a modified cycloid’ contour that establishes a constant clearance through a full rotation cycle between the lobe and nesting cavity in the opposing rotor.
With the non-helical, sequentially staggered nesting rotor pairs, it is possible to have the inlet and discharge port openings enlarged to a point comparable with the pump configuration shown in 'The Early Pump Design' section above.
However, the sequentially staggered rotors discussed in this section, do generate a slight pulsation in the flow of approximately 1.4%.
This ‘defect’ is offset by the fact that the transverse forces created by the helical rotors is eliminated allowing a higher load bearing roller type bearing to be used on the pump power input end where the primary timing gear meshes with the slave shaft timing gear creating a transverse side load on the bearings..
