Information Age & Silicon Revolution (1960–1990)Industrial Robotics & Machine Tending

Slocum–Jurgens Double-Handed Robot End Effector

US 4,765,668

Opposed-Thread Ball Screws, Interchangeable Fingers, and Symmetric Machine Tending

Inventor(s)Alexander H. Slocum, Peter A. Jurgens

Grant DateAugust 23, 1988

Filing DateJune 26, 1987

LocationMcLean, Virginia; Kirkland, Washington

US 4,765,668 claims a robot-mounted end effector with opposed left- and right-hand ball-screw threads that move paired hands symmetrically about a fixed midpoint. Its principal forms add a second, opposite-side hand pair, enclosed motors and gears, removable dovetail-mounted fingers, a transverse connector axis, and rotation about the frame's longitudinal axis. The preferred embodiment describes a double-handed gripper for tending machine tools, including a 5 mm screw lead, 43 mm/s maximum hand travel, 0.05 mm reported repeatability, and a typical 6-inch jaw opening.

USPTO PDF

Engineering Analysis & Physical Principles

How It Works: Step-by-Step Mechanical & Physical Breakdown

The practical problem is not simply to close a pair of jaws. A machine-tending robot may need to extract a finished part, present the other side, load a blank, use a different contact shape for another tool, and put each object back on the same center line. Slocum and Jurgens answer with an elongated double hand. On either side of its symmetric frame, a left/right-hand ball screw moves two hands by equal and opposite amounts. Removable dovetail fingers turn the hand into an exchangeable interface, while the connector supplies source-described rotation and transverse translation. The legal scope is the claimed combination of those organs, not all robotic grippers.

The Core Breakthrough Mechanism

For one opposed-thread screw with lead $\ell=0.005\ \mathrm{m/rev}$ , a screw rotation $\theta$ moves one hand by $x=\ell\theta/(2\pi)$ and the opposed hand by $-x$ . Their relative opening is therefore $g=2x=\ell\theta/\pi$ while their midpoint remains fixed in the ideal kinematics. The preferred embodiment reports a 5 mm lead, a 43 mm/s maximum travel along the screw, a typical 0.1524 m jaw opening, and repeatability no worse than 0.00005 m over its grip-force range. It also prints a maximum gripping-force pair with inconsistent SI/imperial values—"2000 N (1555 lbs.)"—so the exhibit preserves the explicit 2,000 N value as a labeled source datum and never silently converts or reconciles the conflicting parenthesis. Contact pressure, payload capacity, and pressure-to-force transfer are not printed and are refused rather than fabricated.

Interactive Real-Time Physical Simulation

Drag to rotate · Scroll to zoom · Shared controls update the displayed modelDrag to rotate · Pinch to zoom · Shared controls update the displayed model

INITIALIZING THREE.JS WEBGL SIMULATION...

Host-Model Telemetry/Computed Readout

Opposed-Thread Gripper Kinematics

Jaw Opening

Source

79.2mm · source typical[1]

Per-Hand Offset

Source

39.6mm · 5 mm/rev lead[1]

Encoder Phase

Source

6.02of 8 pegs[1]

Requested Grip

Source

900N · setpoint only[1]

Reported Repeatability

Source

0.05mm · source report[1]

Typical jaw-opening fraction0.52 of 152.4 mm

Source-labelled grip setpoint900 N (not contact force)

Claim 17 frame rotation0 °

Claims 13–15 finger change0 retained → fixture

Detailed Component Architecture

1Opposed-Thread Ball Screw

A right-hand and left-hand portion on one screw drive two nuts in equal and opposite longitudinal directions around an unthreaded center section.

With screw lead $\ell$ , one hand has displacement $x=\ell\theta/(2\pi)$ and the pair's gap changes by $g=\ell\theta/\pi$ . The cancellation in the mean position, $(x+(-x))/2=0$ , is why symmetric thread and hand placement can hold the gripping midpoint. The source supplies $\ell=5\ \mathrm{mm/rev}$ for one prototype; it does not give backlash, stiffness, acceleration, or a contact model.

19th-C. Term: left and right hand threaded ball screwModern: dual-opposed recirculating-ball linear actuator

2Double-Handed Symmetric Frame

Upper and lower cylindrical members, joined by a central web, carry a pair of independently driven hand sets on opposite transverse sides.

Claim 3 adds the second ball screw, second pair of hands, and second pair of removable fingers. In the specification, symmetry lets one workpiece be removed while another is already available on the opposite hand. The patent gives the structural layout but no source-verified overall length, mass, robot payload, or center-of-mass position, so the 3D model's proportions are explicitly illustrative.

19th-C. Term: upper and lower cylindersModern: parallel tubular structural rails around a central web

3Motor, Spur Gears, and Eight-Count Encoder

Each motor turns a screw through a two-spur gear train; eight pegs and an inductive switch provide a coarse rotational count.

The printed prototype gives 35.6 mm and 48.3 mm pitch diameters, so its ideal screw-to-motor angular-speed ratio is $35.6/48.3$ . Eight pegs give eight counts per motor-gear revolution. The source reports a separate maximum travel figure, but its motor-speed and travel figures do not establish a single verified dynamic operating point; the visual shows deterministic kinematics and labels source data instead of pretending to solve a pneumatic drive model.

19th-C. Term: inductive proximity switchModern: non-contact magnetic/inductive rotational encoder sensor

4Dovetail Finger Change

A projecting dovetail tenon, hand channel, fixed bosses, and spring-loaded detent make each grasping finger a slide-in mechanical tool interface.

The bosses locate the inserted finger, the detent retains it, and the tapered dovetail centers it under load. The source describes changing fingers by closing into an auxiliary fixture, then opening to leave them behind. This defines a process and retention geometry, not a claim that any arbitrary object will be securely held without knowing finger shape, material, coefficient of friction, or object mass.

19th-C. Term: tenonModern: projecting dovetail slide key

5Rotation and Transverse Connector

The robot connector may rotate the frame about its long axis and a double piston assembly may move it transversely, with a linear transducer for position feedback.

Claims 16 and 17 separately protect transverse reciprocation and at least partial axial rotation. The connector's actual robot-side form, stroke, bore, supply pressure, bearing preload, and sensor resolution are not supplied. The teaching visual therefore lets the visitor select source-described axial rotation but does not claim a historical 6-DOF arm or calculate unprinted piston force.

Engineering Principles & Equations

Governing Equations & Engineering Principles

Authored explanation paired with its stated mathematical relation

Opposed-Thread Screw Gap and Fixed Midpoint

Source-Bounded End-Effector KinematicsClaim 1

Mathematical Governing Law

\htmlClass{eq-term eq-term-jaw_gap eq-term-amethyst}{\htmlData{var=jaw_gap}{\textcolor{#9333ea}{g}}}=\htmlClass{eq-term eq-term-screw_lead eq-term-emerald}{\htmlData{var=screw_lead}{\textcolor{#059669}{\ell}}}\textcolor{#0891b2}{\theta}/\pi,\qquad\textcolor{#0891b2}{x_L}=+\htmlClass{eq-term eq-term-jaw_gap eq-term-amethyst}{\htmlData{var=jaw_gap}{\textcolor{#9333ea}{g}}}/2,\quad\textcolor{#d97706}{x_R}=-\htmlClass{eq-term eq-term-jaw_gap eq-term-amethyst}{\htmlData{var=jaw_gap}{\textcolor{#9333ea}{g}}}/2,\quad\htmlClass{eq-term eq-term-midpoint eq-term-teal}{\htmlData{var=midpoint}{\textcolor{#0d9488}{m}}}=0

Terms:

Plain English DecoderHover or tap any highlighted phrase

The live follows the disclosed and screw rotation; left and right hands move equally in opposite directions so the ideal stays fixed.

g

Jaw Opening

Distance between the opposed fingers in the source-bounded teaching model

mm[L]

The preferred embodiment says a jaw opening can typically be 6 inches. The control scales that printed 0.1524 m value; it is not an unqualified capability of every claimed gripper.

Live Physical Value:

79.2 mm · source typical

Physical Principle & Engineering Insight

This is a direct kinematic reading of the printed 5 mm opposed-thread prototype. It intentionally does not treat the reported force or repeatability as a derived contact, payload, stiffness, or pneumatic calculation.

Historical Context: Claim 1 makes symmetric hand motion around a screw midpoint and removable fingers the legal core of the gripper combination.

Eight-Peg Gear Encoder Quantization

Source-Bounded Rotation SensingClaim 8

Mathematical Governing Law

\textcolor{#d97706}{\Delta\theta_m}=2\pi/\textcolor{#0891b2}{8},\qquad\htmlClass{eq-term eq-term-repeatability eq-term-amethyst}{\htmlData{var=repeatability}{\textcolor{#9333ea}{n}}}=\textcolor{#0891b2}{8}\textcolor{#059669}{N_m}

Terms:

Plain English DecoderHover or tap any highlighted phrase

Eight source-described pegs make the advance in eighth-turn events; this indicates gear rotation without claiming that it alone created the printed .

n\bmod 8

Encoder Peg Phase

Continuous teaching phase of the eight pegs mounted on the motor spur gear

of 8 pegs[1]

The specification says the inductive switch senses eight pressed-in pegs to provide an 8-count encoder. It does not state interpolation, controller bandwidth, or a complete feedback chain.

Live Physical Value:

6.02 of 8 pegs

Physical Principle & Engineering Insight

The relation renders the source's eight-pulse encoder count, not a modern high-resolution servo. The reported repeatability remains a printed result rather than an inferred consequence of peg count.

Historical Context: Claim 8 brings a spur-gear rotation signal into the mechanically driven end-effector combination.

Screw lead kinematicsAuthored Principle 1

Stated relation

$x=\ell\theta/(2\pi),\qquad g=\ell\theta/\pi$

A screw lead ℓ is linear travel per revolution. Opposite thread hands translate in opposite directions, doubling jaw-gap change while keeping an ideal midpoint stationary. The live exhibit uses the disclosed 5 mm prototype lead.

Ideal screw force / torque relationAuthored Principle 2

Stated relation

$F=2\pi\eta T/\ell$

For an idealized screw, torque T and efficiency η determine axial force F for lead ℓ. The patent reports up to 90% screw efficiency, but it does not provide the full torque at the screw under the quoted grip measurement, contact geometry, or loss chain, so the exhibit does not promote this relation into an asserted historical grip-force calculation.

Symmetric-midpoint errorAuthored Principle 3

Stated relation

$m=(x_L+x_R)/2$

If equal-and-opposite hand displacements are x and −x, their midpoint m remains at zero. Backlash, unequal compliance, and unequal loading would perturb that ideal; the grant reports a repeatability result but does not provide enough measurements for a stiffness or error-budget model.

Eight-count encoder quantizationAuthored Principle 4

Stated relation

$\Delta\theta=2\pi/8$

Eight pegs sensed by one inductive switch correspond to eight count positions per sensed gear revolution. The source does not describe interpolation, control bandwidth, or whether this count alone supplied final position accuracy.

Interactive Schematic Sheet (1)

Source Figure 1: frame 12, opposing upper/lower hand pairs, removable fingers, gears 66 and 68, pegs 72, switch 74, and connector-side features.

1.00x

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Why It Still Matters

Industrial end effectors remain where a robot's abstract motion becomes a physical commitment to a workpiece. This grant is a lucid museum object because it separates the jobs that are often blurred together: symmetric width adjustment, exchangeable contact tooling, simultaneous handling on opposite sides, axial rotation, and transverse positioning. Modern grippers may use different drives, sensors, safety systems, and software, but an engineer can still trace why a fixed midpoint and a fast finger-change interface matter in a machine-tending cell.

Legal Claims Decoder (20 Numbered Claims)

Compare dense legalistic claims directly with decoded plain-English functional specifications.

Claim #1Independent Master Claim

1/20

Verbatim Historical Legal Text

“A robot end effector or gripper comprising: a manipulator having an elongate frame that defines a longitudinal frame axis; a motor mounted on said frame; a ball screw rotatably mounted on said frame generally parallel to said longitudinal frame axis, said screw being rotatable by said motor and having a mid portion, a left hand threaded portion on one side of said mid portion, and a right hand threaded portion on the other side of said mid portion; a pair of hands slidingly mounted on said frame for reciprocal movement in said longitudinal frame axis, each hand having inner sides facing the other hand and opposed outer sides, one hand threadedly engaged by said screw left hand portion and the other hand threadedly engaged by said screw right hand portion, and both hands being slidingly mounted on said frame such that upon rotation of said ball screw in one direction said hands are moved relatively apart and upon rotation of said ball screw in the other direction said hands are moved relatively together, all such movement being substantially symmetrical to said screw mid portion; a pair of fingers used to grasp an object; and means for removably mounting each said finger on a corresponding one of said hands such that said fingers can be automatically mounted and dismounted on said hands.”

Plain English Engineering Translation

Claim 1 defines the core single-sided hand: an elongate frame, a motor-driven ball screw with left- and right-hand portions, two guided hands that move symmetrically around the screw midpoint, two grasping fingers, and removable mounting for each finger. The symmetrical pairing and automatic finger mount/dismount provision do the central legal work. Its legal boundary is the stated mechanical combination, not the general idea of a robot hand, a ball screw, a parallel gripper, or interchangeable tooling. The grant supplies the preferred embodiment's topology and several prototype values, but not a general workpiece shape, friction coefficient, payload, contact-pressure law, or robot-arm geometry.

Key Protected Innovations:

Opposed-thread ball screwSymmetric paired handsAutomatically exchangeable fingers

Historical Legal Impact:

Principal independent claim for the symmetric ball-screw hand and removable-finger combination.

The Historical Bottleneck

Machine-tool tending demands repeatable grasping, part turning, and rapid change of the contact geometry without replacing a complete end effector or leaving the cell idle for a lengthy adjustment.

Why Prior Art Failed

•The specification says ordinary parallel-jaw grippers can be mechanically and electrically simple, but cannot reorient a gripped part and require a known presentation orientation.
•The source says different part shapes often require different hands, with adjustment work disabling the robot, while very flexible accurate manipulators can become mechanically/electrically complex and lose grip-force-to-weight advantage.

The Breakthrough Insight

“Use opposed screw threads to hold a symmetric gripping midpoint, duplicate that mechanism on the other side of a narrow structural frame, and make the fingers—not the whole hand—the exchangeable machine-tending interface.”

After the Grant

No priority contest or infringement dispute is asserted for this record. The archival account stays with the published grant, its continuation statement, its twenty printed claims, and the preferred machine-tending embodiment rather than inferring later commercial use from a citation list.

Civilizational Impact

The grant makes industrial end-effector design inspectable at the level that matters in a workcell: a machine needs not only a robot arm but a repeatable way to meet varied parts, transfer them, turn them, and exchange contact tooling. This record does not claim that this particular design dominated the market; it preserves the document's concrete design response to that engineering problem.

Further Context

The preferred-embodiment sentence prints “2000 N (1555 lbs.)”; those quantities are not equivalent. The source face preserves the text, and the model does not use the parenthetical conversion as an independent performance fact.
The grant identifies the original assignee as the United States of America represented by the Secretary of Commerce, not a private robot manufacturer.