Hardware · 2026

ForteGrip

A parallel gripper whose stroke is longer than its folded width, with enough force for a heavy object and a low enough backdriving force for a fragile one.

Mechanical design · Manipulation · Backdrivability · FEA

Co-author · Experimental setup · Manuscript review · Figure preparation

Paper · ICRA 2027, under review

Four rows of video frames: a chip picked from a can, a chip held between the fingers, a cup of liquid lifted and poured, and a heavy book held up — The same gripper picks a chip from a can, lets the chip be pulled free, pours from a soft cup, and holds a 2.1 kg book.

Key results

Folded gripper

finger stroke: 7.8cm
mass: 0.43kg
stroke divided by folded width: 1.26

Bench measurements

maximum grasp force: 100–120N
continuous grasp force: 50N
backdriving breakaway: 6.3N
steady backdriving force: 3N

Folded size is 6 × 6.2 × 13.5 cm, so the 7.8 cm stroke is longer than the folded width. Backdriving values are the mean of 10 pulls. The continuous force is the load the motor can hold for one minute while the winding stays under 80°C.

Goal: Make one parallel gripper that can enter a narrow opening, hold a heavy object, and still yield when something pushes the fingers open.
Approach: Invert the usual rail so the guides fold with the fingers, and share the grasp load between those rails and crank arms with roller bearings. A quasi-direct-drive actuator keeps the transmission backdrivable.
Outcome: Among ten commercial grippers of similar size, ForteGrip is the lightest and the easiest to backdrive, and it is second on both force density and stroke-to-folded-width.

What I worked on

Helped set up and run the experimental evaluation
Reviewed the manuscript and provided technical and editing feedback
Prepared figures and visual material for the paper

Mechanism

A parallel gripper usually stays about as wide as its stroke, even when the fingers are closed. That width is what keeps it out of a narrow shelf, a can, or a sleeve.

A rail that folds with the fingers

On grippers such as the WidowX, Franka Hand, and Fetch, the linear rails are fixed to the body and the fingers ride along them. The finger module therefore has to be at least as wide as the stroke. Closing the fingers does not make the gripper narrower.

ForteGrip turns that arrangement around. The carriages stay fixed to the housing, and the rails move with the fingers. When the gripper closes, the rails collapse into the body instead of reserving space for the open position. The stroke, 7.8 cm, is longer than the folded width, 6.2 cm.

A cantilever finger on a fixed rail, as on the I2RT Linear 4310, can also look narrow when closed. The cost is bending moment on the guide, which pushes the design toward a larger rail. ForteGrip avoids that by sending a large share of the grasp force through the connecting rods. The fingers stay in plane, and the rail and carriage carry less of the load.

ForteGrip beside a water bottle, with its folded size, 7.8 cm stroke, 0.43 kg mass, and force figures, plus three object demonstrations — Folded size 6 × 6.2 × 13.5 cm. The fingers pass a 7.3 cm opening, and the open gripper is 13.5 cm wide around a book.

Linkage

The link lengths were chosen for three targets at once:

a grasp force of 100 N
an opening greater than 7.5 cm, in the upper range of grippers this size
a folded width small enough for a constrained opening

A crank–slider does not turn torque into force at a constant ratio, so the peak grasp depends on how far the fingers are open. With the actuator’s 6 Nm peak torque, the model’s weakest point is 92.4 N. On the bench the average maximum is about 100 N, across the opening range.

Exploded view of ForteGrip, and a plot of maximum grasping force against opening distance for the crank-slider linkage — Carriages are fixed. Rails, crank, and connecting rods move with the fingers. The curve is the maximum grasp force from a 6 Nm actuator.

A 5 mm rail, checked at 120 N

The guide is only 5 mm wide, narrow enough that the linkage does not have to be cut away to clear it. On its own, that carriage is not rated for a 100 N grasp. A needle roller bearing on the crank, 7 × 11 × 9 mm and rated near 3 kN, takes bending load that would otherwise go into the rail.

A transient structural simulation treats the steel rail as flexible and the other parts as rigid, with deformable joints where the rods meet the fingers and where the carriages slide. The worst case is a 120 N grasp applied 15 mm off the centerline, at the fingertip rather than mid-finger, while the crank is still closing.

Peak von Mises stress in the rail is 136.86 MPa, against a 448 MPa yield strength for the 440 stainless carriage. Deflection of the guide is on the order of microns. At the carriage, the yaw moment is 1.78 Nm (rated 1.898 Nm) and the pitch moment is 0.052 Nm (rated 2.17 Nm).

Structural simulation of the gripper closing under a 120 N offset load, with von Mises stress and rail deflection — Worst-case closing load: 120 N, 15 mm off center. Stress stays below the carriage yield strength, and the rail deflection is a few microns.

Why the rail stays small

A larger guide would raise the safety margin, and it would also force the linkage to be redrawn around it. The 7 mm rail drops the stroke-to-width ratio from 1.26 to 1.20 and the grasp from 100 N to 75 N. The 9 mm rail drops the ratio further, to 1.18, at the same 75 N. Shaft-style linear bearings were no stronger at a comparable size. The 5 mm rail is the one that keeps both the force and the folded width.

The crank also has to stay thin. Thrust bearings on both ends of the link would preload it and limit side motion, and they would thicken the whole assembly. The crank instead uses thin dry-running bearings together with thrust ball bearings. Friction is a little higher. It is still low enough for the chip grasp below.

Three ForteGrip variants drawn with 5 mm, 7 mm, and 9 mm guide rails, with overall length and grasp force for each — The selected 5 mm rail (top) is the shortest and the strongest of the three. Wider rails make the linkage longer and cut the grasp force to 75 N.

Comparison

The paper compares ForteGrip with ten commercial grippers of similar size. Dimensions come from published CAD and spec sheets. Where those were missing, the Fetch, Rethink PSG, and DH Robotics PGIA were measured directly.

Force density is the maximum grasp force divided by the volume of the smallest box around the fully closed gripper. Stroke-to-width is the stroke divided by that closed width. Grippers with a worm gear are left out of the backdriving column: the gear locks, so an external force cannot open them.

Ten commercial grippers, each inside its closed bounding box, with dimensions in centimeters — Closed bounding boxes used for the volume in the force-density column. Dimensions are in centimeters.

Specifications from the paper. Backdriving force is the breakaway force, then the steady force once the fingers are moving. ForteGrip is in the top two on backdriving, weight, force density, and stroke-to-width.
Product	Stroke	Grasp force	Backdriving force	Weight	Force density	Stroke / width
WidowX	95 mm	100 N	28.4 / 26.37 N	0.65 kg	61.5 kN/m³	0.52
Franka Hand	80 mm	70 N	21.4 / 18.27 N	0.73 kg	42.7 kN/m³	0.39
I2RT Linear 4310	95 mm	100 N	10.9 / 9.96 N	0.70 kg	97.1 kN/m³	0.93
Fetch	100 mm	245 N	48.2 / 48.1 N	1.5 kg	155.6 kN/m³	0.80
Rethink PSG*	52 mm	—	12.2 / 9.7 N	1.1 kg	53.6 kN/m³	0.30
Ufactory xArm	84 mm	30 N	Not backdrivable	0.8 kg	19.6 kN/m³	0.73
Robotiq 2F-85	85 mm	235 N	Not backdrivable	0.9 kg	152.3 kN/m³	0.67
OnRobot RG2-FT	100 mm	40 N	Not backdrivable	0.78 kg	33.3 kN/m³	1.53
Robotiq HAND-E	50 mm	185 N	Not backdrivable	1.07 kg	225.3 kN/m³	0.67
DH Robotics PGIA	80 mm	140 N	Not backdrivable	1.0 kg	145.5 kN/m³	0.82
ForteGrip	77.6 mm	100 N	6.3 / 3.0 N	0.43 kg	215.1 kN/m³	1.26

* The PSG was weighed and measured without its pneumatic pump, unlike the other grippers, whose actuators are included. Its grasp force is not listed. Worm-gear grippers are marked not backdrivable.

OnRobot’s RG2-FT has a higher stroke-to-width ratio, 1.53, and Robotiq’s HAND-E has a slightly higher force density, 225 kN/m³. Neither can be backdriven. ForteGrip is the gripper that is high on both axes and still opens under a few newtons.

It is also the lightest in the set, at 0.43 kg. The next lightest, the WidowX, is 0.65 kg.

Scatter plot of stroke-to-width ratio against force density. ForteGrip sits toward the top right of ten commercial grippers. — Stroke-to-folded-width against force density. ForteGrip is the point at the upper right of the backdrivable grippers.

Experiments

Three checks: how hard the fingers are to push open, how hard they can grasp, and whether one torque command can cover a chip, a soft cup, and a book.

Backdriving force

A motor winds a stiff nylon line at constant speed and pulls the gripper open through a force gauge. Both the winch and the gripper lie horizontal, so gravity does not add to the reading. Each gripper was pulled 10 times. The band on the plot is the mean and a 0.68 confidence interval.

The first peak is the breakaway force. The steady value is the average over the one-second window that starts half a second after that peak. ForteGrip breaks away at 6.3 N and then holds near 3 N. The I2RT Linear 4310, the other rail gripper in the test, needs about 10 N to start and 9 N to keep moving. Worm-gear grippers did not open even at roughly 100 N.

Backdrive test rig with a winch, force gauge, and gripper, and force-time curves for ForteGrip and five commercial grippers — ForteGrip stays near 3 N after the opening peak. The other five traces climb or level off well above it.

Grasping force

One finger is fixed. The other is held at a commanded opening with high feedback gains, then pulled by a force gauge until the actuator reaches its 6 Nm limit.

Measured peaks follow the kinematic curve: highest near closed, lowest through the middle of the stroke, and still around 100 N at the weak point. For a continuous hold, the gripper tracks a sinusoidal position while hanging a weight, and the motor winding temperature is recorded. Up to 5.1 kg, which is a continuous grasp of 50 N, the winding stays under 80°C for a full minute.

Grasp-force bench with a hanging force gauge, the measured force against opening distance, and motor temperature while holding weights — Left and top: maximum grasp versus opening, measured against the model. Bottom: winding temperature over one minute at 7.5, 10, 11.25, and 12.5 lb.

A chip, a cup, and a book

The opening figure is this test. A person only aims the gripper. Finger width is a position command, and grasp force is a torque command. There is no force sensor.

To enter a Pringles can, the fingers are set partly open so the gripper fits a 7.3 cm mouth. A 0.2 Nm torque then closes on the chip. Because the mechanism backdrives, the contact force stays low enough that the chip does not break, and it can afterwards be pulled out of the fingers by hand.

The cup is held the same way: a constant torque, with the opening left wider than the cup. The fingers conform to the cup instead of crushing it, and the cup can be tilted to pour. The book, 2.1 kg, uses a higher torque. The difference between the chip and the book is the torque command, not a change of hardware.

Paper

ForteGrip: A Compact, High-Force, Backdrivable Parallel Gripper

Shangqun Yu¹, Nisal Perera¹, Anh Duc Tran¹, Jeyoun Dong², Jiyong Oh², Donghyun Kim¹

¹ College of Information and Computer Sciences, University of Massachusetts Amherst. ² Electronics and Telecommunications Research Institute (ETRI), Daegu.

IEEE International Conference on Robotics and Automation (ICRA), 2027. Under review.

Supported by the National Science Foundation (2220924, 2427788, and 2541914), ETRI, and RLWRLD.

Cite

@inproceedings{yu2027fortegrip,
  title     = {ForteGrip: A Compact, High-Force, Backdrivable Parallel Gripper},
  author    = {Shangqun Yu and Nisal Perera and Anh Duc Tran and Jeyoun Dong and Jiyong Oh and Donghyun Kim},
  booktitle = {IEEE International Conference on Robotics and Automation (ICRA)},
  year      = {2027},
  note      = {Under review}
}

ForteGrip

Key results

What I worked on