Sanderson Mechanism

Goal

Reduce size, weight, cost, friction and vibration of reciprocating pumps/engines while providing variable displacement and near-perfect balancing.

Problem

Conventional crankshaft and connecting-rod systems are bulky, heavy, expensive, generate high side loads, friction and vibration, and require complex bearing arrangements.

Concept Summary

The Sanderson Mechanism replaces the crankshaft with a centrally supported U-joint and a transition arm that transmits rotary motion from a flywheel to multiple double-ended pistons via radial pins. Stroke is varied by changing the offset of a bushing on the flywheel or by moving the main shaft laterally. The design eliminates split-bearing assemblies, reduces side loads, and achieves near-sinusoidal motion, resulting in a compact, low-vibration, low-cost pump/engine.

Detailed Description

A perspective drawing shows a four-cylinder device formed by two double-ended pistons. The transition arm is supported by a U-joint; pins project radially 180 deg apart to drive the pistons. A nose pin on the arm receives rotary motion from an offset bushing on the flywheel. By moving the flywheel or adjusting the bushing offset, the stroke can be changed (prototype demonstrated 6:1 to 12:1). The mechanism uses off-the-shelf press-fit bearings, eliminates split-load bearings, and reduces friction by minimizing side forces. The balanced motion stays within 1 % of a pure sinusoid, allowing a single counterweight to balance the assembly. A 750-hp pump built with this mechanism is 66 % smaller in footprint, 40 % smaller in volume, 2,000 lb lighter, and operates at 20 % lower speed than a comparable conventional pump.

Principles

Conversion of rotary to linear motion via a transition arm
Variable displacement through adjustable bushing offset
Balanced sinusoidal motion
Reduction of side loads and friction
Use of U-joint for central support

Scientific Domains

Materials

Steel (for pistons, arm, flywheel)
Bearing steel
Cast iron (possible housing)
Standard off-the-shelf bearings

Mechanisms of Action

Rotary motion of flywheel drives transition arm
Transition arm pushes radial pins attached to pistons
Adjustable offset changes lever arm length, altering stroke
Elimination of crankshaft and connecting rods reduces mass and friction
Balanced motion reduces vibration and bearing wear

Energy Sources

Applications

Hydraulic pumps
Vehicle powertrain (hydraulic hybrid)
Industrial compressors
Marine and aerospace pumps

Claimed Performance

60 % reduction in size & weight, footprint reduced by 66 %, volume reduced by 40 %, weight reduction of ~2,000 lb for a 750-hp pump, 20 % lower operating speed for same output, near-perfect balancing (within 1 % sinusoidal), variable stroke from 6:1 to 12:1 demonstrated on prototype.

Experimental Evidence

Prototype engine varied stroke from 6:1 to 12:1 while operating. A 750-hp pump built with the mechanism showed 66 % smaller footprint, 40 % smaller volume, and 2,000 lb weight reduction compared with a conventional pump.