The equalizing-thrust arrangement connects a rocking pump beam to a guided pump rod through crossed links. The beam end follows an arc, while the rod moves vertically, so their paths do not remain aligned. Explore that mismatch in the schematic and calculate beam-end travel, lateral sweep and a reference pivot moment.
Equalizing Thrust Interactive Calculator
Enter beam radius, rocking angle, rod guide length and load to compare beam-end travel, lateral sweep, offset relative to rod length and a reference pivot moment.
Equation Used
- Rigid rocking beam about fixed pivot O.
- Lower yoke constrained to a vertical guide.
- Cross-link deflection is exaggerated to expose incompatibility.
- No pin reactions, stress, friction, strength or fatigue model.
The crossed links connect a rocking beam to a vertically guided pump rod. The schematic illustrates why the different paths require deflection. The calculator reports ideal beam-arc dimensions and a reference load moment; it does not predict rod stress.
Cross-link pump motion
A rocking beam turns about its central frame pivot. At the pump end, crossed links connect a beam-attached upper pin pair to a lower pin pair on the guided vertical pump rod.
Those attachment paths are incompatible with two perfectly rigid cross links. Hiscox explicitly calls the arrangement not a true parallel motion and warns that the cross connection strains the pump rod. The animation therefore exaggerates the resulting link-path mismatch as visible deflection; it is a qualitative explanation, not an exact closed-link or stress solution.
Historical pump connection
This historical arrangement transfers motion from a rocking pump beam to a vertically guided rod. Because the crossed links do not provide true parallel motion, a practical design must account for the resulting deflection and side loading.
Geometry calculated here
For beam radius R and half-angle theta, peak-to-peak end travel is 2R sin(theta), while the one-sided lateral arc sweep is R(1-cos(theta)). The offset index is 100 times that lateral sweep divided by entered rod length L. The reference pivot moment is W R, with R converted to metres: it assumes the load acts perpendicular to the beam radius. For a vertical force on a beam at angle alpha to horizontal, its moment would instead be W R cos(alpha). The displayed W R is a reference value, not a changing linkage-force solution. The travel and offset quantities describe the path mismatch; they do not calculate bending stress.
600 mm beam-radius example
At an 8 degree half-angle, a 600 mm beam radius gives about 167 mm peak-to-peak vertical end travel and about 5.84 mm one-sided lateral sweep. A 1,000 N load acting perpendicular to that radius gives the displayed reference moment of 600 N·m. With the default 800 mm rod guide length, the offset index is about 0.73%.
Scope and limitations
The crossed connection is compact and historically recognizable, but it is not true parallel motion. This explanatory model does not calculate link-pin reactions, pump-rod bending stress, clearances, friction, wear, strength or fatigue.
Questions
What do the crossed links connect?
They connect pins on the rocking beam to pins on the vertically guided pump rod.
Does it keep the pump rod perfectly straight?
No. Hiscox calls it not a true parallel motion and notes rod strain from the cross connection.
Primary reference
Building or designing a mechanism like this?
Explore the precision-engineered motion control hardware used by mechanical engineers, makers, and product designers.
