Special Curves: Lobe Pumps

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Special Curves: Lobe Pumps


Epicycloid and Hypocycloid Curves to design a Lobe Pump.

You can design a Lobe Pump rotor by combining Epicycloid and Hypocycloid Curves.

The number of Lobes is given by the ratio of gear teeth in the Gear-Pair. Two and three lobes are very common.

Lobe Pumps are sometimes called Roots Blowers - after the inventor - Roots - and 'Blower' because they were originally designed to pump air.

They are often used to pump liquids with soft solids....for example fruit.


You must know how to:

Add 2× Gear-Pairs - an Internal and an External Gear-Pair

Add Point-Data FB

Export data from a Graph FB to Excel®


Import a TXT file from Excel as a XYZ Curve File

Convert the Curve to a Sketch and add a circular pattern to Extrude - one Rotor of the Lobe-Pump

Import it to MD, one on each Rotor

We will first add the Gears to see the complete Path.

Then reduce their motion-range and their start-position.

Add Point-Data FBs to measure the XY Coordinates of the External and Internal

We construct one of the Rotors of the Lube-Pump and re-import to


STEP 1: Add a Gear-Pair #1 - Orbiting External gear.

The input is a Crank. Use these settings:

Mesh = External

Module  = 1

Gear 1 = 180

Gear 2 = 45


STEP 1: Add Gear-Pair #2 with an Orbiting Internal gear.

You must add a new Rocker/Crank. It rotates in the same direction as the Rocker/Crank for Gear-Pair #1

Use these settings:

Mesh = Internal ; Module  = 1 ; Gear 1 = 180; Gear 2 = 45


STEP 3: Edit the Motion-Dimensions for the 2 x Cranks.

Make the Base-Value =0º

The image shows the Cranks when the MMA=0

Add a Linear-Motion FB

Connect its output to the input of the two Motion-Dimension FBs.


STEP 4: Edit the Geared-Rockers.

Edit the Geared-Rocker of Gear-Pair # 1 (External Mesh).

Edit the Length of the CAD-Line to equal 22.5mm - its end-Point is on the Pitch-Circle of the Gear.

Edit the Part that is the Geared-Rocker for the Gear-Pair # 2 (Internal Mesh).

Add a Line . Dimension its start-Point = 0,0, and end-Point = -22.5, 0.

The end-Point is 180º out-of-phase with the CAD-Line in the Geared Rocker.


STEP 5: Add a Trace-Point to the two Geared-Rockers.

The Trace-Point shows the basic shape of a Two-Lobe Roots Blower.

However, each Trace-Point shows different sections of each Lobe.

The actual shape of each rotor changes from that of the Epicycloid to the Hypocycloid path.

How do we make the shape? It is easier to export the X,Y Coordinates of each section of the Lobe to SolidWorks , and use its sketch tools to construct the shape.


STEP 5: Add a Gearing FBs

We will rotate each Crank (Driving-Gear) by 0.25 of the total rotation.

Add a Gearing FB. Edit the Gearing-Ratio = 0.25

Connect it to the input of the two Motion-Dimension FB.

Edit the Motion-Dimension FB for the Internal Gear-Pair.

Edit its Base-Value = 90.

The image shows the Trace-Points with the new settings.


STEP 5: Measure the motion of the 2×Trace-Path

Add 2 × Point-Data FBs

Edit each Point-Data FB, select the Points you selected for the Trace-Point

Add 2 × Graph FBs

Connect the X-axis and Y-axis output-connectors from each Point-Data FB to  Y-axis input-connecters of each Graph FB

Note: the image shows one graph with 4 inputs. It is probably better to export from


STEP 5: Export the XY motions of each Point to Excel

Edit Machine Settings dialog-box > No. of Steps = 360

Click the icon at the RIGHT-SIDE of the Graph toolbar.

A list of the graph data, at each Machine Step (see Machine Settings dialog-box), shows to the RIGHT-SIDE of the graphs (below the X and Y-axis Data-Channel selection).



STEP 5: Paste the Data into Excel

You may need to use, in Excel : Data menu > Data Tools group > Text to Columns


STEP 10: Re-arrange the Data

A Curve feature needs columns of XYZ in a TXT or SLDCRV file.

C column (which is the Z-data) is set to all Zeros (0).

(This assumes you want the to put the Rotor on the Front-Plane in SolidWorks

Save the File as a TXT file.

Note: I use two(2) Graphs to give two(2) TXT files.

Ready to Save as a TXT file

Ready to Save as a TXT file


STEP 11: In SolidWorks

Start a new Part. For the

Do Insert menu > Curve > Curve through XYZ Points...



STEP 11: In SolidWorks

Start a new sketch on the Front-Plane.

Do Convert Entities - select the two Curve features

Do Tools menu > Sketch-Tools > Circular Pattern

- Origin as the Rotational Center

- 2 copies, Equal Spacing, 360

- Entities to Pattern : select the two Splines of the sketch.


STEP 11: In SolidWorks

Insert menu > Boss/Base - Extrude

Select the Sketch

Extrude by 40mm


STEP 11: In SolidWorks

Insert menu > Reference Geometry > Coordinate System

(This is not really needed as the Part is in the coordinate position.


STEP 11: In MechDesigner

Edit the CAD-Line of the Rocker/Crank that is at 0º.

CAD-Line dialog-box > SOLIDWORKS tab > Read file button

CAD-Line dialog-box > SOLIDWORKS tab > Import file button



STEP 11: In MechDesigner

Add a second Crank for the center of the other Lobe of the Pump.

Make it 180mm from the center of the Crank used for the Gear-Pair

Use a Gearing FB, and set the Gearing Ratio to -1. This lobe rotates in the opposite direction.

Import the SOLIDWORKS Part on to this Crank also.

This is the basis for the Lobe Pump.

You can add the casing etc.

Usually Lobe Pumps have a simple 1:1 Gear Pair external to the casing, to drive the Lobe shafts.

You can add the gears if you like.

When you Cycle the model, you can see there is rolling/sliding contact between the Lobes at all points in the machine cycle.

In reality there should be some clearance.

A very small clearance prevents the fluid leaking back through the center of the lobes.

Any leaking reduces the Volumetric Efficiency of the Pump.