The Sanderson Rocker Arm Mechanism (S-RAM) is an elegantly simple mechanism that converts reciprocating to rotary motion, producing high efficiency in both directions without the energy-robbing side forces on the pistons or crossheads common to crankshaft, swash plate or wobble plate drive mechanisms. The S-RAM drive mechanism can vary piston stroke while maintaining a fixed head clearance, which is not possible with other drive mechanisms. The S-RAM can also be configured with double-ended pistons dramatically increasing power density.  The unique S-RAM advantages create engines, pumps and compressors that are more efficient, smaller and lower in cost.

How it works

A simple form of the S-RAM is shown in the adjacent animation. The animation shows the four main components: rocker arm, universal joint, nose pin, and piston joint. A piston is attached to one arm of a 90-degree rocker arm, through a patented joint designed to allow only a force parallel to the piston axis to reach the piston. The piston runs true in the cylinder and the piston side load forces are therefore zero by definition.

Ordinarily, piston side load is necessary to support the shaft torque load, but here the torque is supported by the connection to a modified universal joint (three options), which is grounded on one side and attached to the rocker arm on the other. This ground supports all of the output torque; none of it goes to piston side load. The rocker arm mounted this way cannot rotate about the shaft axis, but the nose pin can move in a circle without rotating, and the drive pin can swing back and forth in a reciprocating motion. The piston motion follows the vertical component of the nose pin motion, and ignores the horizontal component. This is the recipe for simple harmonic motion and the piston moves with nearly sinusoidal motion. The nose pin drives the crank to complete the mechanism.

This kinematic description applies to one piston, but as many pistons can be arrayed in a circle as will physically fit. Additional pistons require one additional drive arm apiece, but all share the same nose pin arm. Transfer of forces through a rocker arm is extremely efficient, works equally well in both directions and exhibits no tendency toward slip-stick behavior. Rocker arms are far more efficient than crankshaft, swash or wobble plates at transferring energy.

The S-RAM drive can be configured as a fixed or variable stroke mechanism. The stroke of the S-RAM can be varied by moving the S-RAM ZSL joint, changing the nose pin angle, or a combination of both. The animation shows both of these movements, including running the S-RAM at zero stroke.

The unique S-RAM advantages allow engine, pump or compressor designers to optimize machine performance in ways not possible by crank, swash plate, wobble plate and radial piston drives. Below are some of the key features and benefits of the S-RAM drive. Please refer to the engine, pumps, or compressor pages to learn about specific benefits.

Reduced friction

The S-RAM converts linear to rotary motion and vice versa without the energy-robbing side forces on the pistons or crossheads common to crankshaft, swash plate or wobble plate mechanisms.

Variable stroke - Fixed Head clearance

Unlike crankshaft, swash plate, radial piston and wobble plate mechanisms, the S-RAM can vary stroke while maintaining the same head clearance.

Double-Ended operation

The S-RAM can be easily configured with double ended pistons that will operate 180 degrees out of phase with the first. This takes up little space and can dramatically increase power density.

Compatibility with existing designs

The S-RAM is a new way to drive pistons and no new requirements are placed above the piston. This open architecture allows a designer to use existing valve, cylinder, injection and combustion arrangements.  Conversion from any current design can be very straightforward.

Low heat generation

Low piston friction in the S-RAM leads results in minimal heat being generated within the body of the operating mechanism. Measurements taken from a working 20 HP hydraulic motor showed a temperature rise of only 40F (20C) at full load.  All of our pump and compressor machines run this cool or cooler. Reduced heat generation can reduce the size of the required cooler.

Near perfect balance

The motion of the S-RAM pistons is near sinusoidal and the harmonic components are unusually small. There are no even harmonics since the piston motion is symmetrical about the midpoint of travel.  Any number of pistons three or greater and evenly spaced can be nearly perfectly balanced.  The S-RAM with variable stroke can also be balanced.

Fixed or Rotating-Barrel designs

The S-RAM drive can also be used in rotating and fixed-barrel configurations. We have built both.

Wear and Life expectancy

Tests on our prototype S-RAMs have shown very low wear, which one would expect as a result of low friction. Bearing surfaces on the S-RAM 500 hp pump showed no measurable wear after several hundred hours of operation.

Reduce parts count and weight

The S-RAM uses significantly fewer parts and lower weight than crankshaft mechanisms and similar parts count as compared to swash plate and bent axis mechanisms.

Scaling to higher horsepower

The cool running of the S-RAM allows higher horsepower to be achieved. Our 500 HP pump prototype ran with a low temperature rise such that a 3,000 HP pump was deemed feasible.

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