The Sanderson Mechanism

Robert Sanderson and John Fox
Sanderson Engine Development, LLC

(This article originally appeared in the November 1999 edition of Fluid Handling System Magazine.)

It might be worth your time to examine the Sanderson mechanism for plunger and piston pump design and to compare it to conventional crankshaft and connecting rod technology. You might appreciate an explanation of how the mechanism works and a brief discussion of the design and the reasoning behind it.

Basic Operation and Configuration

Figure 1 shows a perspective drawing of an engine to explain how the mechanism works. The figure has two double-ended pistons, which is actually a four-cylinder device. ONly the moving parts are shown for clarity.

The central piece of the mechanism – the transition arm – is centrally supported by a U-joint. Pins project radially outward from the center of the U-joint and are located 180 degrees apart. These pins project through the joints between the pistons and transmit linear motion to the pistons.

The nose pin through which rotary motion is delivered to the transition arm is perpendicular to a line between the piston pins. The nose pin is about the same distance from the central pivot as are the piston pins and is supported by the transition arm. An offset bushing on the flywheel connects the nose pin t the rotating motion of the input shaft that in turn produces the reciprocating motion of the piston pins moving the pistons (or plungers). There can be anywhere from two to seven piston pins extending outward radially from the central pivot.

Double-ended applications can only have even numbers of pistons. Three double-ended pistons are equivalent to a conventional six-cylinder pump with 60-degree timing and six strokes per revolution. The maximum number fo single-sided cylinders is limited only by the space available. Seven cylinders would seem to the the practical limit although more are possible.

From the seals on the pony rod (connecting from the crosshead to the plunger or piston) outward, the design requires no change in the plungers, stuffing boxes or valves, although some port distances will be longer.

Variable Displacement Operation and Configurability

The single drive from the flywheel permits the stroke to be varied by changing the swing angle of the transition arm. Moving the main shaft and the flywheel laterally closer or further from the U-joint pivot changes the stroke of the plungers. This method was used on the prototype engine to vary the stroke from 6-1 to 12-1 while operating.

There are two additional methods for changing the stroke without lateral movement of the main shaft. The first duplicates the small stroke change needed for an engine; the second gives pumps a turndown to zero stroke. Adding this feature to a pump completely eliminates the need for a variable speed drive.

The single point drive concept means that several versions of the same pump can achieve different strokes by changing only one part. A change in the offset distance of the bushing on the flywheel is all that is needed for a change in the stroke. This permits having one pump and several flywheels with different offset distances for handling a range of applications.

Cost Savings

The Sanderson mechanism offers benefits, the first and most obvious being a reduction in size, weight and cost. The volume of the pump is only 40 percent and the footprint is one-third that of a conventional pump. This compactness derives from mounting the cylinders about the output shaft and having a mechanism that delivers straight-line motion to the rods thus avoiding the need for crank connecting rods and crossheads. The weight reduction comes from eliminating extraneous parts and using a smaller frame. Not having to machine so much metal produces cost savings.

A six-throw crankshaft for a 750-hp pump is approximately 7 ft. long and costs between 15 and 20 percent of the total pump cost (see Figure 2). Most of this cost is for machining, but material cost is high since torque is transmitted to one end through the journals and throws where excess material must be provided to carry the torque.

The force path in the Sanderson mechanism is the same from the drive shaft through the transition arm (see Figure 3) to each rod and plunger, requiring no added material to pass the torque to the other pistons or plungers. The nose pin of the transition arm must carry the torque for two plungers with the strength added only to the nose pin of the transition arm. The cost of the two-piece transition arm is about one-forth that of a crankshaft. The cost of the main shaft and flywheel with its bushing must be added, but it is merely a straight turning and just 2 ft. long. Add one low-cost item, the U-joint, and the total cost is still one-half of the cost of a crankshaft.

A standard pump unit needs split-sleeve bearings for each connecting rod journal. The end journals may still need to be tapered roller bearings, but eight high-load split bearings would be needed, plus two end bearings. In contrast, the Sanderson mechanism uses no split bearings. All it uses are off-the-shelf bearings that press-fit in place.

Reduction In Life Cycle Cost

The reduction in friction has not yet been mentioned. The three roller bearings on the main shaft and flywheel rotate completely, but the remaining bearings oscillate only 30 degrees to reduce frictional drag.

The design of the center joint between the pistons where the drive pins connect further reduces friction. The force that drives the plungers through the rods does not generate a lateral resultant force like conventional connection rods. Lacking any side load, a crosshead is not needed. The remaining friction is from the weight of the rod assemblies supported by linear bushings that replace the crossheads. A connecting rod for a 750-hp, six-plunger pump would induce over 3,000 lbs. of side load on a crosshead as compared with the 75 lb. assembly that generates friction in the Sanderson mechanism. Although this reduction in friction is significant and desirable, it has only a small effect on the power needed to drive the pump. The more important benefit for large horsepower mechanisms is the reduced operating temperature that extends the life of the bearings significantly.

Vibration is another factor that affects bearing life to the extent that it adds to bearing loads. Well-balanced machinery lasts linger. The motion of standard crankshaft mechanisms is only within 7 to 13 percent of pure sinusoidal and cannot be balanced perfectly. TIn the Sanderson mechanism, balancing is nearly perfect because the reciprocating motion of the transition arm and the plungers is within 1 percent of being pure sinusoidal. This permits a single flywheel counterweight to generate sufficient balancing force. The ability to achieve nearly perfect balancing in a slow turning pump is not as significant as for higher speed machines.

Another feature of the mechanism is its ability to generate longer strokes and operate at lower speeds in producing the same output. A 750-hp unit has an operating speed 20 percent lower than expected, due to its six-inch stroke. This will also extend the life of the bearings and the valves.

The Sanderson mechanism, as applied to a 750-hp pump, reduces the footprint by 66 percent, the volume by 40 percent, eliminates 2,000 lb. in weight, saves thousands of dollars in manufacturing and facilities costs and provides a long-lasting low-vibration pump as compared with conventional reciprocating plunger pumps.