A two-segment jaw gripper is a clamping end effector with two opposing fingers that move synchronously to grasp a part between them. You see it on the wrist of nearly every UR5e and FANUC LR Mate pick-and-place robot in modern packaging cells. Its job is simple — apply enough normal force on two contact faces to hold the workpiece against gravity, acceleration, and process loads without crushing or slipping. Done right, a 40 mm-stroke pneumatic unit like the SMC MHZ2-16D delivers around 80 N of grip force at 0.5 MPa and cycles millions of times before service.
Two-segment Jaw Gripper Interactive Calculator
Vary air pressure, stroke, finger length, and friction to see grip force, jaw force, holding force, and guide loading.
Equation Used
The calculator uses the article example as the reference point: 80 N of grip force at 0.5 MPa. Pressure scales the available piston force, while longer fingers reduce usable fingertip force roughly in proportion to the reference 40 mm length.
- Reference gripper is the article example: 80 N at 0.5 MPa.
- Grip force reduces approximately linearly as finger length exceeds the 40 mm reference.
- Grip force is total normal force across both jaws.
- Holding force is estimated as mu * F_grip for a vertical friction hold.
Operating Principle of the Two-segment Jaw Gripper
The two-segment jaw gripper takes a single linear or rotary input — air pressure, a servo, or a stepper — and splits it through a symmetric drivetrain so both jaws move equal distances toward the centreline. That symmetry is what makes it a self-centering gripper. A part dropped onto the jaws gets pulled toward the tool's centre regardless of where it landed in the X-axis, which is why two-finger grippers are the default choice for vision-guided pick and place where part position varies by ±2 mm.
Inside a parallel jaw gripper like the SMC MHZ2 or Festo HGPL, the input piston drives a wedge or T-slot that engages both jaw carriers at once. Tolerances on the wedge contact surfaces matter — typically held to ±0.01 mm — because any backlash shows up as the two fingers reaching the part at slightly different times. When that happens you get the classic failure mode: the part rotates as it's clamped, the vision system flags it as misaligned downstream, and your cycle stops. Pneumatic gripper bodies use hardened tool steel jaw guides running on needle bearings or cross rollers to keep that backlash under 0.02 mm over millions of cycles.
Grip force comes from input force multiplied by the mechanical advantage of the wedge or linkage, minus friction losses in the guide. Finger length is the silent killer here — every millimetre you add beyond the manufacturer's reference finger length acts as a moment arm against the jaw guide, and grip force at the fingertip drops roughly linearly. Push fingers too long and you don't just lose force, you also start chewing out the bearings because the side load exceeds what the guide was rated for. The MHZ2-16D, for example, lists a maximum allowable finger length of 40 mm — go to 80 mm and you've halved your usable force and doubled your guide wear rate.
Key Components
- Jaw Carriers (Fingers): The two moving members that contact the workpiece. Machined or 3D-printed custom tips bolt to standard mounting patterns — typically M3 or M4 on small grippers like the Robotiq Hand-E. Parallelism between the two jaw faces should hold to 0.05 mm over the full stroke or you get line contact instead of face contact and grip force drops 30-50%.
- Drive Wedge or T-Slot: Converts the piston's axial motion into transverse jaw motion. The wedge angle, typically 30-45°, sets the mechanical advantage. A 30° wedge gives roughly 1.7× force multiplication; a 45° wedge gives 1.0×. Tolerance on the slot fit is ±0.01 mm — slop here translates directly to non-synchronous closure.
- Jaw Guide: Linear bearing that constrains jaw motion to one axis. Cross-roller guides handle higher moment loads than ball cages and are standard on industrial units like the Schunk MPG-plus. Guide preload must stay above zero across the temperature range — a 20°C swing in a foundry cell can unload a poorly-specified guide and let the jaws cock.
- Actuator (Pneumatic Piston or Servo): The input source. Pneumatic versions run at 0.2-0.7 MPa supply with 0.5 MPa nominal. Servo versions like the Robotiq Hand-E or OnRobot RG2 add force feedback and programmable position, which matters when you handle parts of varying size on the same gripper.
- Sensor Slots: Magnetic reed or solid-state switches detect open and closed positions. Without these the controller has no idea whether the gripper actually grabbed the part. Most failures in production cells trace back to a missing or misaligned closed-position sensor letting an empty gripper continue the cycle.
Who Uses the Two-segment Jaw Gripper
Two-segment jaw grippers show up wherever a robot needs to pick discrete parts of consistent size. They dominate the small-payload end-effector market because they're simple, fast, and easy to integrate. The same gripper architecture scales from 5 N grip force on a Dorna 2 desktop arm picking SMD components to 8000 N on a heavy industrial unit clamping forged crankshaft blanks at a tier-1 automotive supplier. What changes is the actuator size, finger material, and sensor package — the underlying two-finger gripper geometry is identical.
- Consumer Electronics Assembly: Foxconn iPhone assembly lines use Schunk EGP-C electric parallel grippers to place camera modules into housings with ±0.02 mm repeatability.
- Pharmaceutical Packaging: Stäubli TX2-60 robots fitted with SMC MHZ2-16D pneumatic grippers transfer vials between filling and capping stations on Bosch packaging lines at 120 cycles per minute.
- Automotive Tier-1: FANUC R-2000iC robots with Destaco RPL Series large-stroke grippers clamp 6 kg differential gear blanks for transfer between CNC ops at ZF Friedrichshafen plants.
- Food & Beverage: ABB IRB 360 FlexPicker delta robots run silicone-tipped two-finger grippers to pick chocolate truffles into trays at Lindt at 150 picks per minute.
- Lab Automation: Hamilton STAR liquid handlers use miniature parallel grippers to move 96-well plates between thermocyclers and washers in COVID PCR labs.
- Collaborative Robotics: Universal Robots UR5e with Robotiq Hand-E electric grippers handles bin-picked metal parts in machine-tending cells at Trelleborg Sealing Solutions.
The Formula Behind the Two-segment Jaw Gripper
The grip force at the fingertip is what determines whether the part stays in the jaws when the robot accelerates through a transfer move. At the low end of typical operating air pressure — 0.2 MPa — you only get about 40% of the gripper's rated force, which is fine for paper cups but will drop a 200 g aluminium block during a fast move. At nominal 0.5 MPa you hit the catalogue figure. Push to 0.7 MPa and you gain a bit more force but accelerate seal wear and stall the gripper closure speed because the exhaust flow becomes the bottleneck. The sweet spot for almost every production cell sits at 0.45-0.55 MPa with finger length kept at or below the manufacturer's reference value.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Fgrip | Effective grip force at the fingertip | N | lbf |
| P | Supply air pressure (gauge) | MPa | psi |
| Apiston | Effective piston area | mm² | in² |
| μmech | Mechanical advantage of wedge/linkage including friction | dimensionless | dimensionless |
| Lref | Manufacturer reference finger length | mm | in |
| Lfinger | Actual finger length from jaw mounting face to contact point | mm | in |
Worked Example: Two-segment Jaw Gripper in an SMC MHZ2-16D on a UR5e palletizing cell
You are sizing an SMC MHZ2-16D pneumatic parallel gripper on a UR5e collaborative robot to pick a 250 g extruded aluminium heat sink from an injection moulding machine and place it on an outfeed conveyor. The gripper has 16 mm bore, a 40 mm reference finger length, and you've designed custom 60 mm fingers with V-grooves to locate on the heat sink fins. Shop air sits at 0.5 MPa nominal, and the UR5e peak transfer acceleration is 2.5 m/s².
Given
- Pnom = 0.5 MPa
- Apiston = 201 mm² (16 mm bore)
- μmech = 0.85 dimensionless
- Lref = 40 mm
- Lfinger = 60 mm
- mpart = 0.25 kg
- apeak = 2.5 m/s²
Solution
Step 1 — at nominal 0.5 MPa, compute the raw piston force before the wedge:
Step 2 — apply the mechanical advantage and the finger-length penalty for the 60 mm custom fingers:
Step 3 — at the low end of typical shop air, 0.3 MPa (a Friday afternoon when half the plant is running):
That's enough to hold the heat sink statically (μ × Fgrip > m × g needs only ~6 N at μ = 0.4) but the moment the UR5e accelerates at 2.5 m/s² you need Fgrip ≥ m × (g + a) / μ = 0.25 × (9.81 + 2.5) / 0.4 = 7.7 N. Still safe �� but pressure dips below 0.25 MPa will start dropping parts on hard moves.
Step 4 — at the high end, 0.7 MPa, with the same 60 mm fingers:
That extra force is theoretical — at 0.7 MPa the MHZ2 closes faster, and on a thin aluminium fin you'll start to deform the fin tips. The sweet spot for this part sits right at the 0.5 MPa nominal value.
Result
Predicted nominal grip force at the fingertip is 56. 9 N — comfortably above the 7.7 N minimum the dynamic load demands, with a safety factor of about 7×. The low-end (0.3 MPa) result of 34.2 N still works but cuts safety factor to 4×, and the high-end (0.7 MPa) result of 79.7 N risks fin deformation, so 0.5 MPa is the right setpoint. If you measure significantly less than 56.9 N on a force-sensing test fixture, the most likely causes are: (1) air supply regulator drift dropping line pressure below 0.4 MPa during peak plant demand — check with a gauge at the gripper, not at the compressor; (2) finger mounting bolts loosened or finger flex letting the contact point sit further from the jaw than the 60 mm design value, effectively increasing L<sub>finger</sub>; or (3) a worn piston seal in the MHZ2 leaking past the piston, which typically shows up as slow closure time before grip force loss becomes obvious.
When to Use a Two-segment Jaw Gripper and When Not To
The two-finger gripper isn't always the right tool. Three-finger and vacuum alternatives each beat it on specific axes. Pick by part geometry, cycle rate, and force budget — not by what's already on the shelf.
| Property | Two-Segment Jaw Gripper | Three-Finger Centric Gripper | Vacuum Cup End Effector |
|---|---|---|---|
| Grip force range (typical industrial) | 5-8000 N | 10-5000 N | 1-500 N (cup-area limited) |
| Cycle time (open-close) | 30-150 ms | 80-250 ms | 20-100 ms (with venturi) |
| Part shape fit | Prismatic, parallel-faced parts | Cylindrical, irregular, self-centering on round stock | Flat or gently curved sealed surfaces |
| Position repeatability | ±0.02-0.05 mm | ±0.05-0.1 mm | ±0.5 mm (cup compliance) |
| Cost (industrial 100 N class) | $200-800 | $600-2000 | $50-300 plus vacuum generator |
| Service life (cycles) | 10-50 million | 5-20 million | 2-10 million (cup wear) |
| Best application fit | Pick-and-place of machined or moulded parts | Round shafts, bottles, tapered parts | Sheet glass, cardboard, smooth panels |
Frequently Asked Questions About Two-segment Jaw Gripper
The catalogue formula assumes the jaw guide is rigid and the fingers are infinitely stiff. Neither is true. Doubling finger length doubles the moment on the jaw guide, which deflects measurably — typically 0.1-0.3 mm at the fingertip on a 16 mm-class gripper. That deflection lets the contact point shift, often turning face contact into edge contact, and effective grip force drops more than the 1/Lfinger term predicts.
Rule of thumb: stay within the manufacturer's listed maximum finger length, and if you must go longer, machine the fingers from steel or 7075 aluminium rather than 6061 to keep flex under 0.05 mm at rated force.
If your cell handles parts of varying size on the same gripper, go electric. The Hand-E gives you programmable jaw position and force feedback, so you can detect a missed pick or a wrong part by reading position at closure. A pneumatic gripper only knows fully-open or fully-closed via reed switches.
If every cycle picks the same part and you need 50 ms cycle times, stay pneumatic. Electric grippers cap out around 100-150 ms close time, and pneumatic units like the MHZ2 will outlast them on raw cycle count by 3-5×.
Non-synchronous jaw closure. One jaw is reaching the part before the other, pushing it sideways until the trailing jaw arrives. The cause is almost always wear or contamination in the wedge or T-slot drive — backlash above 0.05 mm at the jaw face is enough to produce visible rotation on a 30 mm part.
Diagnostic check: with the gripper off the robot, close it slowly by hand on a piece of carbon paper between flat jaws. If the contact marks aren't symmetric and simultaneous, the wedge is worn or the guide rails are dirty. On an MHZ2 this usually means it's time for a seal kit and a guide clean-out.
Stroke margin doesn't help if the part is sitting near the edge of the gripper's self-centering range. Two-finger grippers self-centre well within ±1 mm of the tool centreline, but beyond that the leading jaw shoves the part before the trailing jaw catches up, and the part walks out of the grasp entirely.
Tighten your vision tolerance or add compliant V-groove tips. The V-groove pulls the part toward centre as the jaws close and recovers most of the lost capture range.
You almost certainly forgot the acceleration term. A 500 g part held statically needs about 12 N of grip at μ = 0.4. The same part on a UR10e doing a 2.5 m/s² transfer needs Fgrip ≥ m × (g + a) / μ = 0.5 × 12.3 / 0.4 = 15.4 N. Add a safety factor of 3-5× for vibration and surface contamination and the real requirement is closer to 60 N.
If you're picking from a CNC and parts have residual cutting fluid on them, μ can drop to 0.15. Plan grip force around the worst-case friction coefficient, not the textbook one.
Use compliance when part-to-part dimensional variation exceeds your gripper's repeatability budget. Injection-moulded parts shrink ±0.5% from shot to shot. A rigid jaw sized to nominal will under-grip the small parts and crush the large ones. A urethane V-tip with 1-2 mm of compliance absorbs that variation and applies consistent force across the batch.
Avoid compliance on high-acceleration moves above 5 m/s² — the spring rate of the tip becomes a dynamic system and the part oscillates inside the jaws, sometimes enough to walk free. For those cases, rigid tips with a tight tolerance band on the part are the right answer.
References & Further Reading
- Wikipedia contributors. Robot end effector. Wikipedia
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